Patent Application
20130333066
The present specification teaches the generation of drought tolerant plants. The present disclosure enables manipulation of a phenotypic characteristic referred to herein as “stay-green” to generate drought tolerant plants by recombinant, mutagenic and/or breeding and selection methods. Plant management practice systems to increase crop yield and harvest efficiency in water-limited environments are also taught herein.
Bibliographic details of the publications referred to by author in this specification are collected alphabetically at the end of the description.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.
An increasing human population necessitates improvements in crop productivity. This has been a major goal for plant breeders and geneticists. One approach to improving crop productivity involves the selection of plant traits which facilitate higher grain yield and stability (Springer, Nature Genetics 42:475-476, 2010). This approach has been referred to as the “Green Revolution”. Other approaches include the development of ideal plant architectures which have, for example, led to the identification of a quantitative trait locus (QTL) which encodes squamosa promoter binding protein-like 14 (OsSPL14) in rice and which facilitates improved rice yield (Jiao et al., Nature Genetics 42:541-544, 2010; Miura et al., Nature Genetics 42:545-549, 2010).
Drought is the single most important constraint to cereal production worldwide. Sorghum is a repository of drought resistance mechanisms, which include C4 photosynthesis, deep roots and thick leaf wax which enable growth in hot and dry environments. Drought tolerance makes sorghum especially important in dry regions such as sub-Saharan Africa, western-central India, north-eastern Australia, and the southern plains of the US. With increasing pressure on the availability of scarce water resources, the identification of traits associated with grain yield under drought conditions becomes more important.
The drought adaptation mechanism identified in sorghum which results in the retention of green leaves for longer periods during grain filling under drought is known as ‘stay-green’. Stay-green has been associated with high grain yield under post-anthesis drought in sorghum (Borrell et al., Crop Sci. 40:1037-1048, 2000b; Kassahun et al., Euphytica 72:351-362, 2010), wheat (Triticum aestivum L.) [Spano et al., J. Exp. Bot. 54:1415-1420, 2003; Christopher et al., Aust. J. Agric. Res. 59:354-364, 2008], rice (Oryza sativa L.) [Kashiwagi et al., Plant Physiology and Biochemistry 44:152-157, 2006] and maize (Zea mays L.) [Zheng et al., Plant Breed 128:54-62, 2009]. In addition, it may indirectly affect grain yield under drought by improving charcoal rot (Macrophomina phaseolina [Tassi] Goid.) resistance (Tenkouano et al., Theor. Appl. Genet. 85:644-648, 1993; Garud et al., Int. Sorghum and Millets Newsl. 43:63-65, 2002). This reduces lodging (Reddy et al., Euphytica 159:191-198, 2008), allowing plant breeders to exploit the positive association between plant height and grain yield (Jordan et al., Theor. Appl. Genet. 106:559-567, 2003). Stay-green has been an important selection criterion for sorghum breeding programs targeting drought adaptation in both the US (Rosenow et al., Agric. Water Manag. 7:207-222, 1983) and Australia (Henzell et al., Australia Int. Sorghum and Millets Newsl. 38:1-9, 1997).
A considerable body of physiological evidence is mounting in support of this trait (Borrell et al., Crop Sci. 40:1026-1037, 2000a; Borrell and Hammer, Crop Sci. 40:1295-1307, 2000; Harris et al., J. Exp. Bot. 58:327-338, 2007; Christopher et al., 2008 supra; Van Oosterom et al., Field Crops Res. 115:19-28, 2010a and Van Oosterom et al., Field Crops Res. 115:29-38, 2010b). Although this drought resistance mechanism has been utilized by sorghum breeders in the US and Australia for over 25 years, and the broad physiological basis of the trait is becoming better understood, the causal mechanisms and the genetic loci involved have hitherto been unknown.
Under water limiting conditions, grain yield is a function of transpiration (T), transpiration efficiency (TE), and harvest index (HI) [Passioura, J. Aust. Inst. Agric. Sci. 43:117-120, 1977]. Within this framework, grain yield is linked to post-anthesis T (Turner, J. Exp. Bot. 55:2413-2425, 2004; Manschadi et al., Funct. Plant. Biol. 33:823-837, 2006), because HI increases with the fraction of total crop T used after anthesis (Passioura, 1977 supra; Sadras and Connor, Field Crops Res. 26:227-239, 1991; Hammer, Agric. Sci. 19:16-22, 2006). Increased post-anthesis T is associated with reduced drought stress around anthesis, which can positively affect crop growth rate at anthesis of cereals and hence grain number (Andrade et al., Crop Sci. 42:1173-1179, 2002; Van Oosterom and Hammer, Field Crops Res. 108:259-268, 2008). If the total amount of available water is limited, post-anthesis T can be increased by restricting pre-anthesis T. This can be achieved by restricting canopy size, either genetically or through crop management. However, a smaller canopy will only reduce total T if its TE is not compromised. Significant genotypic differences in TE have been reported for sorghum (Hammer et al., Aust. J. Agric. Res. 48:649-655, 1997; Henderson et al., Aust. J. Plant Physiol. 25:111-123, 1998; Mortlock and Hammer, J. Crop Prod. 2:265-286, 1999; Xin et al., Field Crops Res. 111:74-80, 2009). Alternatively, post-anthesis water use can be increased by increasing the total amount of water accessed by the crop, either through deeper rooting or reduced lower limit of water extraction (Manschadi et al., 2006 supra).
The stay-green trait affects a number of the above processes in sorghum. First, stay-green reduces water use during the pre-anthesis period by restricting canopy size (via reduced tillering and smaller leaves).
Second, stay-green improves water accessibility by increasing the root:shoot ratio. There is some experimental evidence for better water extraction in stay-green lines, although more research is required. These root responses could also be explained by enhanced auxin transport (Wang et al., Molecular Plant 2(4):823-831, 2009). Third, stay-green increases the greenness of leaves at anthesis, effectively increasing photosynthetic capacity, and, therefore, TE (providing that photosynthesis increases proportionately more than conductance). The increase in leaf greenness is an indirect affect of reduced leaf mass, i.e. nitrogen is concentrated in the leaf.
Producing more food with less water is one of the major challenges currently facing humanity. There is a real and urgent need in both developing and developed countries to identify the genes and gene networks controlling drought adaptation in crop plants. This enables increased drought adaptation in a wide range of crop species grown in water-limited environments worldwide.
The present disclosure teaches quantitative trait loci (QTLs) associated with and/or which otherwise facilitate the stay-green phenotype. The QTLs are referred to herein as “stay-green (Stg) X” wherein X is a numeral increasing from 1 which represents the region on a chromosome associated with the stay-green phenotype.
As taught herein, the QTLs identify genetic regions on sorghum which carry loci which encode one to a number of proteins or regulatory agents such as microRNAs which facilitate the stay-green phenotype. Modulation of expression of one or more these loci in a crop plant generates a canopy architecture which facilitates a shift in water use by the plant to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency thereby increasing harvest index (HI) and grain yield under water-limited conditions.
In an embodiment, the loci encode a PIN protein which is associated with an auxin. PIN proteins are auxin efflux carriers which contain transmembrane domains and are mainly localized in the plasma membranes. The term “PIN” is derived from “pin-like inflorescence”.
The term “SbPINn” is used to describe such a gene in sorghum wherein n is a numeral defining the auxin efflux carrier component and n is 1 through 11. Reference to “SbPINn” includes its homologs and orthologs in other plants. Examples of SbPINn loci in sorghum include those listed in Table 1A such as but not limited to SbPIN4 and SbPIN2 or their equivalents in other plants. Modulation of expression of a PIN or expression of a PIN with a particular polymorphic variation is taught herein to facilitate exhibition of the stay-green phenotype. The present disclosure teaches PINs from other plants such as rice. The letter prefix has a PIN specifies its source (eg Sb, sorghum; Os, rice; etc). The location of a SbPIN in sorghum is determined by gene ID number (See Table 1A). As examples, SbPIN4 corresponds to OsPIN5 and SbPIN2 corresponds to OsPIN3a.
SbPIN4 and SbPIN2 are examples of SbPINs taught herein to be responsible for the stay-green trait in sorghum associated with Stg1, and Stg2, respectively, resulting in a range of phenotypes that confer drought adaptation via decreased water use until anthesis (due to reduced tillering and smaller leaves), increased water accessibility (due to enhanced root:shoot ratio), increased transpiration efficiency under mild water deficit (due to higher leaf nitrogen concentration), increased biomass per leaf area under terminal water deficit (due to increased transpiration per leaf area) and increased grain yield, grain size and lodging resistance. Other examples are listed in Table 1A and include their equivalents in other plants.
The present disclosure teaches a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising modulating the level of expression of an existing or introduced pin-like inflorescence (PIN) locus in all or selected tissue in the plant to facilitate a stay-green phenotype, which phenotype includes a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water-limited conditions.
Enabled herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent encoding a PIN protein selected from SbPIN1 to 11 or an equivalent thereof from another plant or functional homolog or ortholog thereof; or which modulates expression of an indigenous PIN protein; wherein the level and location of PIN expression facilitate a stay-green phenotype, which phenotype includes, inter alia, a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth resulting in increased harvest index and grain yield under water limiting conditions.
The present disclosure further teaches a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent which encodes a sorghum PIN protein selected from SbPIN4 and SbPIN2 or an equivalent in another plant or which modulate expression of an indigenous PIN. Reference to modulating includes increasing and decreasing the level of expression. Furthermore, a PIN may be selected having a desired expression profile in all or selected tissues in a plant.
Enabled herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into the plant or a parent of the plant a genetic agent which encodes a product which is associated with or facilitates a stay-green phenotype which phenotype includes a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water-limited conditions, and wherein the product is selected from the list consisting of SbPIN1 to 11 or an equivalent thereof in another plant or which agent modulates expression of an indigenous PIN to the plant.
Also taught herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent which encodes a protein selected from the list consisting of SbPIN4 and SbPIN2 or an equivalent thereof in another plant or which agent modulates expression of an indigenous PIN in the plant. By “introducing” includes via recombinant intervention as well as by mutagenesis or breeding protocol followed by selection.
The instant disclosure is instructional for a method of generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant and/or parent of the plant a genetic agent which encodes two or more PIN proteins or which modulates expression of two or more indigenous PIN proteins in a plant. Examples of PINs in sorghum include SbPIN1 to 11 such as SbPIN4 and SbPIN2 or an equivalent thereof from another plant. For example SbPIN4 corresponds to OsPIN5 and SbPIN2 corresponds to OsPIN3a. SbPINs and OsPINS are defined in Table 1A.
Genetically modified plants and their progeny exhibiting the stay-green trait are also enabled herein as well as seeds, fruit and flowers and other reproductive or propagating material.
Genetic material which encodes a PIN protein or functional homolog or ortholog thereof which is associated with or facilitates a stay-green phenotype, which phenotype includes a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water limiting conditions and an agent which modulates expression of the PIN; are both enabled herein.
In an example, the genetic material is selected from (i) an agent which encodes SbPIN4; and (ii) an agent which modulates levels of expression of SbPIN4 or its equivalent in another plant. In another example, the genetic material selected from (i) an agent which encodes SbPIN2; and (ii) an agent which modulates expression of SbPIN2 or its equivalent in another plant.
The genetic material includes an agent which encodes a SbPIN4 or SbPIN2 protein or functional homolog or ortholog thereof or an equivalent thereof in another plant which is associated with or facilitates a stay-green phenotype, which phenotype includes a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water limiting conditions; and an agent which modulates expression of SbPIN4 or SbPIN2, or an equivalent thereof in another plant.
Taught herein is a plant management system to reduce crop reliance on water or to otherwise improve water use efficiency and to enhance grain or product yield. The plant management system includes the generation of drought adapted crop including cereal plants using the selection and modulation of expression of a PIN locus or its functional equivalent as herein defined alone or in combination with the introduction of other useful traits such as grain size, root size, salt tolerance, herbicide resistance, pest resistance and the like. Alternatively or in addition, the plant management system comprises generation of drought adapted plants and agricultural procedures such as irrigation, nutrient requirements, crop density and geometry, weed control, insect control, soil aeration, reduced tillage, raised beds and the like. Examples of a PIN locus include SbPIN1 to 11 (Table 1A) such as SbPIN4 and SbPIN2 and an equivalents thereof in another plant.
A business model is also taught herein for improved economic returns on crop yield, the model comprising generating crop plants having a canopy architecture which facilitates a shift in water use by the plant to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency thereby increasing HI and grain yield under water-limited conditions, obtaining seed from the generated crop plant and distributing the seed to grain producers to enhance yield and profit.
The following abbreviations are used in the subject specification:
CI, confidence interval
CWU, crop water use
DW, dry weight
GLA, green leaf area
HD, high density
HI, harvest index
HT, high tillering
HW, high water
HWHD, high water, high density (intermediate water stress)
HWLD, high water, low density (least water stressed)
LA, leaf area
LD, low density
LT, low tillering
LW, low water
LWHD, low water, high density (most water stressed)
LWLD, low water, low density (intermediate water stress)
NIL, Near-isogenic line
OsPIN, PIN from rice
PAB, post-anthesis biomass
PASM, post-anthesis stem mass
PIN, pin-like inflorescence
PPBR, pre:post anthesis biomass ratio
QTL, quantitative trait locus
ROS, rain-out shelter
RWC, relative water content
SbPIN, PIN from sorghum
SLW, specific leaf weight
SML, statistical machine learning
Stg, stay-green
T, transpiration
T2, tiller in the axil of leaf 2
T3, tiller in the axil of leaf 3
T4, tiller in the axil of leaf 4
T5, tiller in the axil of leaf 5
T6, tiller in the axil of leaf 6
TE, transpiration efficiency
TS, terminal stress
VPD, vapor pressure deficit
WW, well watered
Table 1A provides information on PINs from sorghum and rice.
Sorghum Stg QTL details
Sorghum
Sorghum
Some figures contain color representations or entities. Color photographs are available from the Patentee upon request or from an appropriate Patent Office. A fee may be imposed if obtained from a Patent Office.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element or integer or method step or group of elements or integers or method steps but not the exclusion of any other element or integer or method step or group of elements or integers or method steps.
As used in the subject specification, the singular forms “a”, “an” and “the” include plural aspects unless the context clearly dictates otherwise. Thus, for example, reference to “a locus” includes a single locus, as well as two or more loci; reference to “an auxin” includes a single auxin, as well as two or more auxins; reference to “the disclosure” includes a single and multiple aspects taught by the disclosure. Aspects taught herein are encompassed by the term “invention”. All aspects taught, described or claimed herein are enabled within the width of the disclosure.
The present disclosure teaches QTLs associated with and which facilitate the stay-green phenotype in crop including cereal plants. The QTLs are referred generically as StgX wherein X is a numeral from 1 and above corresponding to a genetic locus or genetic loci region on a particular chromosome in a crop plant. A sub-region is referred to as StgXm where m is an alphabetical designation of a region within StgX. Modulation of expression of an StgX in all or selected tissue in a plant is taught herein to facilitate a physiological and genetic network which induces or promotes a shift in water use by the crop plant to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency, thereby increasing harvest index (HI) and grain yield under water-limited conditions. “Expression” of an StgX includes up-regulating or down-regulating expression levels (i.e. modulation of expression) of a locus as well as selection of a polymorphic variant which is expressed at a higher or lower level or which encodes a more or less active product in all or selected tissue in a plant. The locus may itself confer this phenotype or a functional equivalent thereof such as a cDNA encoding the same protein encoded by the locus.
The QTL's identify loci encoding PIN proteins. PIN proteins are auxin efflux carriers which contain transmembrane domains and are mainly localized in the plasma membranes. The term “PIN” is derived from “pin-like inflorescence”. A pin for sorghum is “SbPIN”. The instant disclosure teaches a PIN from any plant (e.g OsPIN from rice). The genomic location of an SbPIN from sorghum is described in Table 1A.
Enabled herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent encoding a PIN protein or functional homolog or ortholog thereof; or which modulates expression of an indigenous PIN protein; wherein the level and location of PIN expression facilitate a stay-green phenotype, which phenotype includes, inter alia, a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth resulting in increased harvest index and grain yield under water limiting conditions.
The present disclosure further teaches a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent which encodes a sorghum SbPIN protein selected from SbPIN1 through SbPIN11 or an equivalent in another plant or which modulate expression of an indigenous PIN. Examples of PINs include SbPIN4 and SbPIN2 and other SbPINs listed in Table 1A and their equivalents in another plant as well as a PIN having a particularly desired polymorphic variation which, for example, permits an altered expression profile of elevated levels of the PIN protein. Examples of PINs in other plants OsPIN5 which corresponds to SbPIN4 and OsPIN3a which corresponds to SbPIN2.
The genetic agent may be a locus or genomic region or its functional equivalent such as cDNA or genomic DNA fragment. Alternatively the agent may modulate expression as an indigenous PIN locus in a particular plant. By “introducing” means by recombinant intervention, or by mutagenisis or breeding followed by selection.
Without intending to limit the technology of the present specification to any one theory or mode of action, modulation expression of a PIN alone or in combination with a genetic or physiological network alters plant architecture to enhance or otherwise promote efficient water use. In one aspect, the modified architecture is modified plant canopy architecture.
The term “progeny” includes immediate progeny as well as distant relatives of the plant, as long as it stably expresses the stay-green trait first introduced to an earlier parent.
Reference to a “crop plant” includes a cereal plant. The crop plants enabled herein include sorghum, wheat, oats, maize, barley, rye, rice, abaca, alfalfa, almond, apple, asparagus, banana, bean-phaseolus, blackberry, broad bean, canola, cashew, cassaya, chick pea, citrus, coconut, coffee, corn, cotton, fig, flax, grapes, groundnut, hemp, kenaf, lavender, mano, mushroom, olive, onion, pea, peanut, pear, pearl millet, potato, ramie, rapseed, ryegrass, soybean, strawberry, sugarbeet, sugarcane, sunflower, sweetpotato, taro, tea, tobacco, tomato, triticale, truffle and yam. In an example, the drought tolerance mechanisms of sorghum are used to promote drought tolerance in sorghum as well as other crop plants. In an example, the genetically modified plant uses water more efficiently than a non-genetically modified plant of the same species. Existing PIN loci in each of the above plants are referred to as “indigenous” PINs. The instant disclosure teaches up- and down-regulating an indigenous PIN or selecting a PIN having a particular expression profile. An “indigenous” PIN is a PIN locus in a parent plant prior to any manipulation (recombinant, mutagenisis or breeding).
By “drought tolerance” includes drought escape, drought adaptation, drought resistance, reduced sensitivity to drought conditions, enhanced water use efficiency as well as an ability to shift water use to the post-anthesis period or increased accessibility of water during crop growth, thereby increasing HI and grain yield under water-limited conditions. Plants exhibiting drought tolerance are described as “drought adapted plants” or “plants exhibiting reduced sensitivity to water-limited conditions”. Taught herein is that drought tolerance is induced, facilitated by or otherwise associated with the stay-green phenotype.
By “genetically modified”, in relation to a plant, includes an originally derived genetically modified plant as well as any progeny, immediate or distant which stably express the stay-green trait. Hence, the present disclosure teaches both classical breeding techniques to introduce the genetic agent, i.e. the stay-green QTL or a functional equivalent thereof such as cDNA or a genomic fragment or an agent which up-regulates or down-regulates (i.e. modulates) expression of the QTL or the protein encoded therefrom as well as genetic engineering technology. The latter is encompassed by the terms “genetic engineering means” and “recombinant means”. Markers defining stay-green can also be screened during breeding protocols to monitor transfer of particular genetic regions. Furthermore, a specific stay-green region is genetically inserted by recombinant means into a plant cell or plant callus and a plantlet regenerated. A “genetically modified” plant includes a parent or any progeny as well as any products of the plant such as grain, seed, propagating material, pollen and ova. In addition, a PIN locus may be expressed in one particular plant tissue but not expressed or its expression reduced in another tissue. Furthermore, a plant may be subject to mutagenisis such as genetic, radioactive or chemical mutagenisis and mutated plants selected with a PIN having a desired phenotype.
Reference to the “stay-green phenotype” includes characteristics selected from enhanced canopy architecture plasticity, reduced canopy size, enhanced biomass per unit leaf area at anthesis, higher transpiration efficiency, increased water use during grain filling, increased plant water status during grain filling, reduced pre:post anthesis biomass ratio, delayed senescence, increased grain yield, larger grain size, and reduced lodging.
Enabled herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into the plant or a parent of the plant a genetic agent which encodes a product which is associated with or facilitates a stay-green phenotype which phenotype includes a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water-limited conditions, and wherein the product is selected from the list consisting of SbPIN1 to 11 including SbPIN4 and SbPIN2, and other SbPINs listed in Table 1A or an equivalent thereof in another plant or which agent modulates expression of an indigenous PIN in the plant.
Hence, taught herein is the use of genetic material encoding a PIN or genetic material which modulates levels of an indigenous PIN to facilitate the stay-green phenotype.
Enabled herein are genetically modified plants which exhibit the stay-green phenotype as a result of the genetic modification as well as seeds, fruit, flowers and other reproductive or other propagating material. Also enabled are root stock and propagating stock. This is based on the premise that the seeds, fruit, flowers, reproductive and propagating material exhibit or can pass on the stay-green phenotype introduced into the ultimate parent(s).
Reference to an “agent which modulates levels of expression of a PIN” includes promoters, microRNAs, genes and chemical compounds which facilitate increased or decreased expression of the gene in all or selected tissue or increased or decreased activity of a gene product as well as cDNA and genomic fragments. An agent may also be an intron of a genomic gene which is part of a natural genetic network to facilitate modulation of expression.
A PIN protein produces an auxin gradient in cells and contains transmembrane domain and is mainly localized in the plasma membrane. PIN proteins are the rate limiting factors of auxin transport and provide vectorial direction for the auxin flows. It is taught herein that at least one of Stg1 or Stg2 encodes a PIN protein. Introduction of Stg1 or Stg2 de novo in a plant or elevation of its modulation or expression of an indigenous Stg1 or Stg2 is taught to facilitate exhibition of one or more features or sub-features associated with the stay-green phenotype.
As indicated above, PIN proteins are efflux carriers of auxin which mediate polar auxin transport (PAT) from cell to cell as opposed to the transport of auxin through the xylem (Rashotte et al., Plant Cell 13:1683-1697, 2000; Friml et al., Current Opinion in Plant Biology 6:7-12, 2003). The term ‘PIN’ is derived from the pin-like inflorescence which develop in Arabidopsis when auxin transport is defective. Enabled herein is a SbPINn from sorghum where n is a numeral from 1 through 11 as well as a PIN from any other plant.
Also taught herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent which encodes a protein selected from the list consisting of SbPIN1 to 11 such as SbPIN4 and SbPIN2 or other SbPINs listed in Table 1A or an equivalent thereof in another plant or which agent modulates expression of an indigenous PIN in the plant.
It is taught by the present disclosure that sorghum SbPIN4 and SbPIN2 are major drought adaptation genes along with other SbPINs as well as their equivalents in other plants. Differences in auxin signalling explain all of the multiple phenotypes observed in plants with a PIN expression profile. Phenotypes exhibited by SbPIN4 or SbPIN2 plants for example are explained by changes in auxin efflux and include reduced tillering, smaller leaves (both length and width), reduced leaf mass and increased root:shoot ratio. Phenotypes exhibited by SbPIN4 or SbPIN2 plants for example can also be explained indirectly (or as emergent consequences of these direct effects) and include increased availability of water at anthesis, higher leaf N concentration at anthesis, increased transpiration and biomass per unit leaf area, higher transpiration efficiency, retention of green leaf area during grain filling, increased harvest index, higher grain yield, larger grain size and increased lodging resistance. Enabled herein is that equivalents of SbPIN4 and SbPIN2 and other SbPINs are operative across other major cereal and crop species to enhance drought adaptation in localities worldwide where water limits crop growth post-anthesis.
In accordance with the teachings of the present specification, modulation of expression of a PIN selected from SbPIN1 to 11 such as SbPIN4 (Stg1) and SbPIN2 (Stg2) or their equivalent in another plant in all or selected tissue confers drought adaptation both directly, and indirectly, ultimately leading to higher grain yield, larger grain size, and lodging resistance under water-limited conditions.
In an example, the stay-green phenotype involves the presence of multiple proteins such as two or more of SbPIN1 to 11 such as SbPIN4 and SbPIN2.
Taught herein is a method for generating a genetically modified plant which uses water more efficiently than a non-genetically modified plant of the same species, the method comprising introducing into a plant or parent of the plant a genetic agent which encodes two or more PINs or functional homolog or ortholog thereof which are associated with or facilitates a stay-green phenotype, which phenotype includes a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water limiting conditions; or an agent which modulates levels of expression of the two or more PINs.
By “two or more” means 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11. A single or multiple PIN loci may also be expressed or inhibited.
In an example, the genetic material is selected from (i) an agent which encodes SbPIN4; and (ii) an agent which modulates levels of expression of SbPIN4 or its equivalent in another plant. In another example, the genetic material selected from (i) an agent which encodes SbPIN2; and (ii) an agent which modulates expression of SbPIN2 or its equivalent in another plant.
Increased water availability at anthesis is achieved via reduced water use due to two mechanisms (reduced tillering and smaller leaves) in plants containing the Stg1 region. Both mechanisms, individually, appear to reduce canopy size by about 9%, on average. The ‘low-tillering’ mechanism dominates in low density environments when tillering potential is high. The ‘small-leaf’ mechanism dominates in high density environments when tillering potential is low. Combined, these two mechanisms provide crop plants with considerable plasticity to modify canopy architecture in response to the severity of water limitation.
Stay-green enhances canopy architecture plasticity via constitutive and adaptive responses. Canopy size in Stg1 or Stg2 is reduced by about 5%, even when water is not limiting (constitutive response). Canopy size is further reduced (adaptive response) in a mild drought (˜10%) and more severe drought (˜15%). Low tillering is primarily a constitutive response. Small leaf size is both a constitutive and adaptive response.
Furthermore, it is taught herein that the Stg1 or Stg2 region confers drought adaptation by reducing canopy size (via reduced tillering and smaller leaves) and reducing crop water use at anthesis. This is shown by a high correlation (r2=0.9) between canopy size and crop water use in artificial drought (rain-out shelter [ROS]) and lysimeter studies.
Increased water availability at anthesis is also achieved via increased water accessibility (better water extraction and deeper or greater lateral spread).
Stay-green enhances biomass per unit leaf area at anthesis. Assuming root mass is equivalent (or at least not significantly less), these differences could be explained by differences in transpiration (T) per unit leaf area [LA] (T/LA) and/or transpiration efficiency (TE). Lysimetry studies indicate that increases in T/LA, rather than TE, drive the observed increases in biomass per leaf area. Note that increased T/LA only occurred when water deficit was sufficient to reduce leaf area. When water deficit was less severe (i.e. not enough to reduce leaf area), then T/LA decreased, resulting in higher TE.
Higher TE in StgX lines such as Stg1 or Stg2 lines is also observed when water deficit is less severe. Increased TE via introgressing Stg1 or Stg2 is proposed to be due to a) proportionally higher photosynthetic capacity compared with stomatal conductance, due to smaller, thinner and greener leaves, and/or b) a decrease in transpiration while maintaining biomass. Lysimetry studies indicate that both of these mechanisms contribute to higher TE in Stg1 lines, with the reduction in transpiration the primary mechanism.
Changes in transpiration per unit leaf area is taught herein to be due to a) number of stomata, b) size of stomatal aperture, c) changes in the timing of stomatal opening and closing relative to VPD, and/or d) the number of hair base cells (which affects the boundary layer and hence T/LA). Introgressing Stg1, for example, into RTx7000 modified leaf anatomy by increasing the number of bundle sheath cells surrounding the vascular bundle.
Differences in the morphology of leaves are apparent between RTx7000 and Stg1 or Stg2. In this case, there were more and smaller bundle sheaths surrounding the vascular bundle in Stg1 or Stg2. Also, there were fewer stomata and more hair base cells per unit leaf area (leaves 7 and 10) in Stg1 compared with RTx7000.
Increased water use during grain filling is achieved via (i) increased water availability at anthesis and (ii) increased water accessibility (better water extraction and deeper or greater lateral spread) during grain filling.
Crop water use (CWU) before anthesis was negatively correlated with CWU after anthesis in an artificial drought (rain-out shelter [ROS]) experiment. For example in one experiment, a 25% increase in water use after anthesis (80 vs 60 mm) resulted in a 25% increase in grain yield (400 vs 300 g/m2). This translated to 50 kg/ha of grain for every additional mm of water available.
Increased water use during the grain filling period was exhibited by Stg1 or Stg2 under both low and high density treatments in a rain-out shelter (ROS) experiment. This was due primarily to (i) reduced water use at anthesis under high density, and (ii) increased water accessibility during grain filling under low density.
It is taught herein that StgX such as Stg1 or Stg2 confers drought adaptation by being associated with pre- and post-anthesis biomass production. The Stg1 or Stg2 region, for example, reduces the pre:post anthesis biomass ratio below a critical level, increasing grain yield and lodging resistance.
In accordance with teachings of the present disclosure, expression of StgX such as Stg1, Stg2, Stg3 and/or Stg4 facilitates one or more of the following phenotypes:
(i) delayed leaf senescence (stay-green), higher grain yield and lodging resistance are consequences of higher plant water status during grain filling (due to increased water use during grain filling);
(ii) introgressing StgX into a, for example, RTx7000 background increases plant water status at mid-grain filling, as indicated by a) higher relative water content (RWC), and b) lower leaf water potential (LWP);
(iii) higher grain yield and larger grain size are consequences of increased water availability during grain filling;
(iv) higher grain yield, larger grain size and increased lodging resistance are not mutually exclusive (i.e. all three traits are exhibited by StgX);
(v) yield and grain size advantage are relatively higher under severe terminal drought than mild terminal drought;
(vi) the benefit of the stay-green genes in a, for example, RTx7000 background (inbreds) occurs in the yield range of 1-3 t/ha (12-22%), followed by a lesser but still significant benefit in the 3-4 t/ha yield range (8-10%). There was, however, a small penalty associated with these regions (2-4%) at higher yield levels (5-8 t/ha) due to wetter conditions. Note that these yield ranges would be considerably higher in hybrids. Since the average sorghum grain yield for hybrids in the northern grain belt is about 2.5 t/ha, the benefit of the stay-green genes should be significant. No reduction in grain yield under wetter conditions (water not limiting) due to stay-green has been observed in hybrids;
(vii) introgressing StgX into, for example, RTx7000 also increases grain size by 11%, on average, under severe terminal drought. There was no impact of the StgX QTL on grain size under a mild terminal drought or under no drought; and
(viii) each of the key StgX mechanisms maps to a defined region, suggesting that the action of a single gene has multiple pleiotrophic effects.
The present disclosure further teaches a business model to enhance economic returns from crop production. According to these teachings, provided is a business model for improved economic returns on crop yield, the model comprising generating crop plants having a PIN expression profile resulting in the crop plant having a shift in water use by the plant to the post-anthesis period or increased accessibility of water during crop growth thereby increasing HI and grain yield under water-limited conditions, obtaining seed from the generated crop plant and distributing the seed to grain producers to yield enhanced yield and profit. Reference to a PIN includes SbPIN1 to 11 such as SbPIN4 and SbPIN2, as well as their equivalents in other plants.
Taught herein is a plant management system to reduce crop reliance on water or to otherwise improve water use efficiency and to enhance grain or product yield. The plant management system includes the generation of drought adapted crop including cereal plants using the selection and modulated expression of PIN locus or its functional equivalent as herein defined alone or in combination with the introduction of other useful traits such as grain size, root size, salt tolerance, herbicide resistance, pest resistance and the like. Alternatively or in addition, the plant management system comprises generation of drought adapted plants and agricultural procedures such as irrigation, nutrient requirements, crop density and geometry, weed control, insect control, soil aeration, reduced tillage, raised beds and the like.
The present specification is instructional as to a means to induce or enhance drought adaptation capacity in a plant by introducing do novo one or more features of the stay-green phenotype or elevating or reducing expression of an existing one or more PIN loci in a plant and/or selecting an PIN polymorphic variant with improved or enhanced expression or product activity. The manipulation of the stay-green phenotype may be done alone or as part of an integrated plant management system which may include further trait selection and/or improved agronomic techniques. The resulting crops use water more efficiently and have a higher yield of grain and increased grain size.
The present teachings include business models to collect seed from drought adapted or enhanced crop plants for distribution to growers to ultimately increase grain yield.
The present disclosure is enabling in the respect of the use of a genetic agent encoding a PIN protein or which modulates levels of expression of an indigenous PIN protein in the manufacture of a drought adapted plant.
QTLs are identified herein as carrying one or more PIN loci wherein the level of expression of which is selected in breeding protocols or by genetic engineering, to promote a stay-green phenotype.
Genetically modified plants and their progeny exhibiting the stay-green trait are also taught herein as well as seeds, fruit and flowers and other reproductive or propagating material.
Genetic material which encodes a PIN protein or functional homolog or ortholog thereof which is associated with or facilitates a stay-green phenotype, which phenotype includes a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water limiting conditions; and an agent which modulates the level of the PIN; are both enabled herein.
In an example, the genetic material is selected from (i) an agent which encodes an SbPIN listed in Table 1A; and (ii) an agent which modulates levels of expression of an SbPIN listed in Table 1A or its equivalent in another plant.
In an example, the genetic material is selected from (i) an agent which encodes SbPIN4; and (ii) an agent which modulates levels of expression of SbPIN4 or its equivalent in another plant. In another example, the genetic material selected from (i) an agent which encodes SbPIN2; and (ii) an agent which modulates expression of SbPIN2 or its equivalent in another plant.
The genetic material includes an agent which encodes a PIN, or functional homolog or ortholog thereof which is associated with or facilitates a stay-green phenotype, which phenotype includes a canopy architecture which facilitates a shift in water use to the post-anthesis period or increased accessibility of water during crop growth or increased transpiration efficiency resulting in increased harvest index and grain yield under water limiting conditions. The active agent includes an agent which up-regulates or down-regulating levels of the PIN. The subject genes may also be used as markers to transfer regions of genomes comprising one or more of the genes, or their equivalents in other plants, to a particular plant in order to include the stay-green phenotype.
Aspects taught and enabled herein are further described by the following non-limiting Examples.
A quantitative trait locus (QTL), which is referred to herein as Stg1 which is an example of an StgX, has been identified which increases or enhances water use efficiency by sorghum plants. Stg1 encodes a sorghum bicolor member of the auxin efflux carrier component 4 family, PIN4 (or SbPIN4).
This major drought adaptation gene has been fine-mapped on the sorghum genome. Changes in auxin efflux could explain all of the multiple phenotypes observed in plants containing SbPIN4. The candidate gene (and promoter region) is sequenced in the two parents of the fine-mapping population (RTx7000 and Tx642) to identify a single nucleotide polymorphism. RNA expression profiling of the Stg1 fine-mapping population is also conducted for a subset of lines, times and organs. Phenotypes exhibited by SbPIN4 plants that could be explained directly by changes in auxin efflux include reduced tillering, smaller leaves (both length and width), reduced leaf mass and increased root:shoot ratio. Phenotypes exhibited by SbPIN4 plants that could be explained indirectly (or as emergent consequences of these direct effects) include increased availability of water at anthesis, higher leaf N concentration at anthesis, increased transpiration and biomass per unit leaf area, reduced pre:post anthesis biomass ratio, higher transpiration efficiency, retention of green leaf area during grain filling, increased harvest index, higher grain yield, larger grain size and increased lodging resistance. It is proposed that SbPIN4 works across other major cereal and crop species to enhance drought adaptation in localities worldwide where water limits crop growth post-anthesis.
Stg1 (SbPIN4) confers drought adaptation both directly, and indirectly, ultimately leading to higher grain yield, larger grain size, and lodging resistance under water-limited conditions.
Increased water availability at anthesis is achieved via reduced water use due to two mechanisms (reduced tillering and smaller leaves) in plants containing the Stg1 region. Both mechanisms, individually, appear to reduce canopy size by about 9%, on average. The ‘low-tillering’ mechanism dominates in low density environments when tillering potential is high. The ‘small-leaf’ mechanism dominates in high density environments when tillering potential is low. Combined, these two mechanisms provide crop plants with considerable plasticity to modify canopy architecture in response to the severity of water limitation.
Stay-green enhances canopy architecture plasticity via constitutive and adaptive responses. Canopy size in Stg1 is reduced by about 5%, even when water is not limiting (constitutive response). Canopy size is further reduced (adaptive response) in a mild drought (˜10%) and more severe drought (˜15%). Low tillering is primarily a constitutive response. Small leaf size is both a constitutive and adaptive response.
There is a link between reduced canopy size (via reduced tillering and smaller leaves) and reduced crop water use at anthesis. High correlation (r2=0.9) between canopy size and crop water use in ROS and lysimeter studies.
Increased water availability at anthesis is also achieved via increased water accessibility (better water extraction and deeper or greater lateral spread).
Stay-green enhances biomass per unit leaf area at anthesis. Assuming root mass is equivalent (or at least not significantly less), these differences could be explained by differences in transpiration per unit leaf area (T/LA) and/or transpiration efficiency (TE). Lysimetry studies indicate that increases in T/LA, rather than TE, drive the observed increases in biomass per leaf area. Note that increased T/LA only occurred under low VPD conditions; T/LA was actually reduced under high VPD conditions, presumably as a water conservation mechanism.
Higher TE in Stg1 lines was also observed under higher VPD conditions. Increased TE via introgressing Stg1 may be due to a) proportionally higher photosynthetic capacity compared with stomatal conductance, due to smaller, thinner and greener leaves, and/or b) a decrease in transpiration while maintaining biomass. Lysimetry studies indicate that both of these mechanisms contribute to higher TE in Stg1 lines, with the reduction in transpiration the primary mechanism.
Changes in transpiration per unit leaf area could be due to a) number of stomata, b) size of stomatal aperture, c) changes in the timing of stomatal opening and closing relative to VPD, and/or d) the number of hair base cells (which affects the boundary layer and hence T/LA). Introgressing Stg1 into RTx7000 reduced the number of stomata and increased the number of hair base cells per unit leaf area in leaves 7 and 10; both mechanisms can conserve water by reducing T/LA.
Introgressing Stg1 into RTx7000 modified leaf anatomy by increasing the number of bundle sheath cells surrounding the vascular bundle. The increased number of cells in the bundle sheath might also contribute to increased photosynthetic assimilation and hence TE.
Differences in the morphology of leaves (e.g. Leaves 7 and 10) are apparent between Tx7000 and Stg1. In this case, there were more and smaller bundle sheaths surrounding the vascular bundle in Stg1. The increased number of cells in the bundle sheath might also contribute to increased photosynthetic assimilation and hence TE.
Increased water use during grain filling is achieved via (i) increased water availability at anthesis and (ii) increased water accessibility (better water extraction and deeper or greater lateral spread) during grain filling.
Crop water use (CWU) before anthesis was negatively correlated with CWU after anthesis in an ROS experiment. For example, in one experiment a 25% increase in water use after anthesis (80 vs 60 mm) resulted in a 25% increase in grain yield (400 vs 300 g/m2). This translated to 50 kg/ha of grain for every additional mm of water available.
Increased water use during the grain filling period was exhibited by Stg1 under both low and high density treatments in an ROS experiment. This was due primarily to (i) reduced water use at anthesis under high density, and (ii) increased water accessibility during grain filling under low density.
Stg1 region confers drought adaptation via a link between pre- and post-anthesis biomass production. The Stg1 region reduces the pre:post anthesis biomass ratio below a critical level, increasing grain yield and lodging resistance.
Delayed leaf senescence (stay-green), higher grain yield and lodging resistance are consequences of higher plant water status during grain filling (due to increased water use during grain filling).
Introgressing Stg1 into a RTx7000 background increased plant water status at mid-grain filling, as indicated by a) higher relative water content (RWC), and b) lower leaf water potential (LWP).
Higher grain yield and larger grain size are consequences of increased water availability during grain filling.
Higher grain yield, larger grain size and increased lodging resistance are not mutually exclusive (i.e. all three traits are exhibited by Stg1).
Yield and grain size advantage are relatively higher under severe terminal drought than mild terminal drought.
Studies indicate that the greatest benefit of the stay-green genes in a RTx7000 background (inbreds) occurs in the yield range of 1-3 t/ha (12-22%), followed by a lesser but still significant benefit in the 3-4 t/ha yield range (8-10%). There was, however, a small penalty associated with these regions (2-4%) at higher yield levels (5-8 t/ha) due to wetter conditions. Note that these yield ranges would be considerably higher in hybrids. Since the average sorghum grain yield for hybrids in the northern grain belt is about 2.5 t/ha, the benefit of the stay-green genes should be significant. Overall, no reduction in grain yield under wetter conditions (water not limiting) due to stay-green has been observed in hybrids.
Introgressing Stg1 into RTx7000 also increased grain size by 11%, on average, under severe terminal drought. There was no impact of this QTL on grain size under a mild terminal drought or under no drought.
Each of the key Stg1 mechanisms maps to the same region, indicating the action of a single gene with multiple pleiotrophic effects.
Data show the impact of Stg1 on tillering under both high water (HW) and low water (LW) conditions. Differences in canopy development before flowering were largely a consequence of variation in tillering among lines. Culm number per m2 at anthesis was the best overall measure of the effect of tillering on canopy dynamics. Culm numbers per m2 were equivalent under both water regimes (12.89), indicating that reduced tillering is a constitutive trait. Genotypes varied significantly (P<0.001) in this parameter, ranging from 8.59 to 16.67. However, genotype and treatment did not interact significantly for this parameter.
Culm numbers per m2 were analyzed in terms of their Stg status and category means are presented in Table 1. RTx7000 produced 41% more (P<0.05) culms/m2 than B35 (14.07 vs. 10.00). Introgression of the Stg1 region alone into RTx7000 (6078-1) reduced culms/m2 significantly (P<0.05) compared with RTx7000 (9.40 vs. 14.07). Compared with Stg1 only, additional introgressions of either Stg2 or Stg4 increased culm numbers to 10.49 (1,2 combination) and 10.74 (1,4 combination). Note that the three near-isolines containing no Stg regions (2212-3, 2235-11 and 6120-16) also exhibited high tillering equivalent to Tx7000. Hence the overall ranking of tillering in these lines is Stg1<B35<Stg4<Stg2<Stg3<none<RTx7000.
At anthesis, culm numbers were highly correlated (r2=0.71) with total green leaf area (GLAA;
Culms per m2 in the recurrent parent (RTx7000) and various lines containing introgressions of the Stg1 QTL, both alone and in combination with other Stg QTL.
Differences in green leaf area at anthesis (GLAA) were primarily due to differences in tiller green leaf area at anthesis (GLAAt), since GLAAt was highly correlated with GLAA (r2=0.78), yet mainstem leaf area was not.
Tiller green leaf area at anthesis was analyzed in terms of Stg status and category means are presented in Table 2. RTx7000 produced almost eight-fold more (P<0.05) GLAAt than B35 (15460 vs. 1980). Introgression of the Stg1 region alone into RTx7000 (6078-1) reduced GLAAt significantly (P<0.05) compared with RTx7000 (3121 vs 15460). Compared with Stg1 only, additional introgressions of either Stg2 or Stg4 increased GLAAt to 4187 (1,2 combination) and 4797 (1,4 combination). All lines containing Stg1 (in any combination) were not significantly different (P<0.05) in GLAAt from Stg1 alone. Note that GLAAt in the three near-isolines containing no Stg regions (2212-3, 2235-11 and 6120-16) was not significantly different from RTx7000. Hence the significantly different (P<0.05) rankings of GLAAt in these lines are B35=Stg1=Stg4<Stg2=Stg3<none=RTx7000.
Tiller green leaf area at anthesis in the recurrent parent (RTx7000) and various lines containing introgressions of the Stg1 QTL, both alone and in combination with other Stg QTL.
Green leaf area and culms per m2 at anthesis were highly correlated under high (HD) and low (LD) density treatments in the rainout shelter experiment (
A Stg1 fine-mapping population was grown in the field under high and low density conditions in three consecutive years. The number of culms per plant was measured at anthesis in each year and a combined analysis was undertaken across years. Overall, RTx7000 produced 47% more culms per plant than 6078-1 (1.85 vs. 1.26;
In these field studies, the trait (culm number per plant) was mapped. An arbitrary value of 1.54 culms per plant gave the optimal separation between high and low tillering for mapping purposes (i.e. recombinants with less than 1.54 culms were BB genotypes while those with more than 1.54 culms were TT genotypes). Stepping down through the markers reveals that gain-of-function (low tillering) was achieved in three genotypes (10564-2, 10704-1 and 10620-4). One recombinant (10568-2) exhibited a high tillering phenotype, while three others (10620-4, 10704-1 and 10564-2) exhibited low tillering phenotypes. An auxin efflux carrier component 5 gene was proposed to be the candidate gene.
A subset of the Stg1 fine-mapping population was grown in the field at the Rain-Out Shelter (ROS) under high and low water conditions, with each water treatment split for high and low density. This created four water regimes with increasing levels of water deficit: HWLD (least stressed)<HWHD<LWLD<LWHD (most stressed). The number of culms per plant was measured at 44 days after emergence in each plot. Differences were most obvious in the Low Density (LD) treatment, since expression of tillering is maximized in this treatment. On average, RTx7000, 10568-2 and 10709-5 produced 27% more culms per plant than 6078-1 and 10604-5 under LWLD conditions (2.05 vs. 1.62;
Three additional fine-mapping studies were undertaken on the Stg1 population under controlled conditions in an igloo. In these studies, tillering was analyzed in more detail compared with the earlier field studies. The total number of tillers was counted and, more specifically, the number of tillers emerged from the axil of leaves 2 (T2), 3 (T3) and 4 (T4) were counted. Presence or absence of T2 was the best indicator of overall tillering potential for a given recombinant. T2 was also the best trait to use for fine-mapping the gene.
In this experiment, the total number of tillers was the sum of T2, T3 and T4, where T2 was the tiller emerging from the axil of leaf 2 (and so on for T3 and T4), including secondary tillers. Significant genotypic variation was observed for all of the traits relating to tillering in this study (Table 34), with heritabilities generally above 30.
Table 3 provides a summary of predicted means, P-value and heritability of tillering traits measured at the L11 harvest. Significant differences (P<0.05) are shaded in yellow while heritabilities >20 are shaded in green.
Separate analysis of the T2, T3 and T4 data found that 6078-1 produced no T2 tillers in any of the four replicates, while RTx7000 produced T2 tillers in 2 of 4 replicates (
Differences were also apparent in T3 numbers. 6078-1 produced a T3 tiller in 1 of 4 replicates, while RTx7000 produced a T3 tiller in all 4 replicates (
T4 tiller numbers also varied among genotypes. 6078-1 produced a T4 tiller in 3 of 4 replicates, while RTx7000 produced a T4 in all 4 replicates. Hence the Stg1 introgression essentially prevented the growth of T2 and T3 tillers in a RTx7000 background.
Lines were separated which produced a T2 tiller in 0-2 replicates (low tillering group; 8 recombinants) from those that produced a T2 tiller in 3-4 replicates (high tillering group; 8 recombinants) [Table 4,
Table 4 shows the presence of tillers (T1-T3) and total tiller number for eight high-tillering recombinants (brown shading) and eight low-tillering recombinants (green shading) from the Stg1 fine-mapping population.
Gain-of-function (low tillering) is achieved in recombinant 10604-1-157-5. Gain-of-function (low tillering) is also achieved in three recombinants (10604-1-195-5, 10604-1-56-7, 10604-1-477-4).
A subset of lines was used in this experiment to validate the tillering region. More replicates per recombinant (20) were used to further reduce the error variance and increase the power of discrimination among lines. Results indicated the presence of an auxin efflux corner gene.
A breakpoint analysis of those lines which, according to their genotype (BB or TT), ‘step up’ or ‘step down’ through the region of interest, was undertaken to further pinpoint the low-tillering gene. A clear break was apparent, separating lines that produced a total tiller number >2.5 (high tillering group; 5 recombinants) from those that produced a total tiller number <2.5 (low tillering group; 3 recombinants) [Table 5,
Table 5 shows the presence of tillers (T1-T4), including secondary tillers, and total tiller number for five high-tillering recombinants (brown shading) and three low-tillering recombinants (green shading) from the Stg1 fine-mapping population.
Gain-of-function (low tillering) is achieved in recombinant 10604-1-157-5. The PIN4 gene in Sorghum bicolor (designated herein “SbPIN4”) is therefore a strong candidate for the Stg1 low tillering gene.
Overall, introgressing the Stg1 region into RTx7000 reduced leaf size (length and width) under well-watered and water-limited conditions, indicating a constitutive gene action. However, the reduction in leaf size was generally greater under water-limited conditions indicating, to some extent, an adaptive (inducible) response in addition to the constitutive response. Hence Stg1 confers two mechanisms for reducing canopy size: a) reduced tillering, and b) reduced leaf size. Combined, these two mechanisms provide a fair degree of plasticity for the plant to modify canopy architecture in response to environmental and/or management factors.
A series of lysimeter studies is particularly instructive in assessing leaf size patterns under varying levels of vapor pressure deficit (VPD) (
However for the remaining tillers (T4-T6), the leaf size distributions differed markedly between experiments. While there was no difference in leaf size between RTx7000 and 6078-1 under high VPD, the leaves of 6078-1 were significantly smaller under low VPD. This indicates an adaptive (inducible) response to leaf size reduction under certain environmental conditions for tillers T4-T6.
Hence, under low water stress, introgressing Stg1 into RTx7000 resulted in smaller leaves in the mainstem and largest tiller (T3), but not in T4-T6. This response would enable the plant to maximise light interception in the later tillers when conditions are favorable. Under high water stress, introgressing Stg1 into RTx7000 resulted in smaller leaves in all culms. This response would enable the plant to dramatically reduce its canopy size under water-limited conditions.
Experiments were conducted under the Rain-Out Shelter (ROS) to assess the impact of the Stg1 region under two crop densities, thereby creating two levels of water deficit (high density=high stress; low density=low stress). In general, tillering was low or absent under HD (20 plants/m2) and normal under LD (10 plants/m2).
Canopy size was smaller in both years under the high density (HD) treatment, reflecting the greater water deficit generated by this treatment. In both years under the milder (LD) and more severe (HD) water deficits, leaf sizes were generally smaller in 6078-1 (Stg1) compared with RTx7000. The exception was where the leaf size distribution pattern was similar for 6078-1 and RTx7000 in the milder water deficit (LD), yet leaves were significantly smaller in 6078-1 (up to 18% smaller) under greater water deficit (HD), suggesting an adaptive response by Stg1 plants to increasing water deficit. In fact, introgressing the Stg1 region into RTx7000 reduced the size of the four largest leaves (L10-L13) by an average of 16.5% in the more severe water deficit (HD). Since there was little tillering in either genotype in this treatment, reduced leaf size in 6078-1 should have markedly decreased canopy size and hence crop water use (assuming similar transpiration per unit leaf area).
Note that the leaf size reduction mechanism associated with Stg1 appears to operate in both the presence (LD) and absence of tillering (HD), but appears to be best expressed under HD where uniculm and high water deficit conditions generally occur.
A subset of the Stg1 fine-mapping population was grown in the field at the Rain-Out Shelter (ROS) under high and low water conditions, with each water treatment split for high and low density. This created four water regimes with increasing levels of water deficit: HWLD (least stressed)<HWHD<LWLD<LWHD (most stressed). The area of each fully-expanded mainstem leaf was measured for all genotypes in all treatments.
Introgressing the whole Stg1 region (6078-1) and, more specifically, the smaller region designated 10604-5, resulted in a reduction in the size of leaves 9-13 under low-water and high-density conditions (
Under low density conditions, leaf size distributions were affected by tillering, resulting in some crossovers compared with the high density treatment (see
The “small leaf size” gene mapped to the candidate gene, auxin efflux carrier component 5.
Leaf area varied significantly (P<0.001) among genotypes with a heritability approaching 60 for leaves 4 and 5. Introgressing the Stg1 region into RTx7000 reduced the area of leaves 1-6 (
Differences in leaf area were due more to differences in leaf length (
The allometric relationship in the Stg1 fine-mapping population between the area of leaf (n) and the area of leaf (n+1) indicates a significant change at about Leaf 8 (concurrent with floral initiation). Thereafter, increases in leaf size occurred at a lesser rate.
Introgressing the Stg1 region into RTx7000 reduced the area of leaves 9-11 (
Table 6 shows a summary of predicted means, P-value and heritability of leaf size traits measured at the L11 harvest. Significant differences (P<0.05) are shaded in yellow while heritabilities >20 are shaded in green. GLA=green leaf area. DW=dry weight. SLW_L9_L11=specific leaf weight.
Most of the variation in green leaf area at the Leaf 11 harvest was due to differences in tillering. However, leaves 9-11 were smaller in 6078-1 compared with RTx7000. These differences were significant (P<0.05). Note that ‘low tillering’ and ‘small leaves’ were both associated with the same region, indicating the possibility of a single gene controlling both canopy architecture traits.
Leaf number and length were linearly correlated for the parents of the Stg1 fine-mapping population (
For mapping purposes, the ‘tails’ of the Stg1 fine-mapping population were selected. Two genotypes exhibited particularly long leaves (10604-1-157-5 and 10604-1-318-1) and three genotypes exhibited particularly short leaves (10604-1-222-1, 10604-1-501-327-3 and 6078-1).
Gain-of-function (short leaf) is achieved in recombinant 10604-1-222-1. The ‘small leaf’ gene is proposed to map to the same region as the ‘low tillering’ gene.
For mapping purposes, the ‘tails’ of the Stg1 fine-mapping population were selected (
Stepping up through the markers, gain-of-function (short leaf) is achieved in recombinant 10604-1-222-1. Hence, once again, the ‘small leaf’ gene maps to the same region as the ‘low tillering’ gene. Note that leaf length in L9 and L10 map to the same region.
An explanation is a single gene with multiple pleiotrophic effects. Enhanced availability of auxin would explain both the low tillering and small leaf size phenotypes observed in plants containing this region. An auxin efflux carrier component 5 gene is located in the target region and is therefore identified as a candidate.
Increased water availability at anthesis is achieved via reduced water use due to two mechanisms (reduced tillering and smaller leaves) in plants containing the Stg1 region
Stay-green exhibits both constitutive and adaptive responses (
Reduced crop water use at anthesis can be caused by a) a smaller canopy size with equivalent transpiration per unit leaf area, b) an equivalent canopy size with lower transpiration per unit leaf area, or c) a smaller canopy size and lower transpiration per unit leaf area. ROS and lysimetry studies indicate that under high water stress conditions, the Stg1 region, and in particular the recombinant containing the Stg1 candidate gene (10604-5), exhibited lower crop water use due to a smaller canopy size rather than lower transpiration per unit leaf area. High correlations (r2=0.9) between canopy size and crop water use were observed in ROS and lysimeter studies.
Tillering was negligible in this experiment due to the high crop density. Hence differences in canopy size were due to differences in leaf size (
In turn, green leaf area at anthesis was highly correlated with crop water use at anthesis (
Transpiration (T) is the product of leaf area (LA) and transpiration per leaf area (T/LA). Under high VPD conditions, LA was similar between Stg1 and RTx7000 (11795 vs 11628 cm2), yet T/LA was less in Stg1 than Tx7000 (2.60 vs 2.85), resulting in less water use per plant (T) in Stg1 than RTx7000 (30.7 vs 32.8 1). Hence water savings in Stg1 (in a high VPD environment) were achieved entirely by a reduction in T/LA, suggesting that this is a constitutive water conservation strategy conferred by Stg1. In this case, higher transpiration efficiency (TE) in Stg1 was a consequence of equivalent biomass and lower transpiration.
A broader analysis comparing the four Stg QTL (Stg1, Stg2, Stg3 and Stg4) with RTx7000 helps to put the Stg1 response in perspective. Under high VPD conditions, T/LA was positively correlated with T. However under low VPD conditions, T/LA was negatively correlated with T (r2=0.52). Green leaf area and transpiration were positively correlated under both low and high VPD conditions (
Transpiration (T) is the product of LA and T/LA. Under low VPD conditions, LA was 31% less in Stg1 than RTx7000 (4898 vs 7082 cm2). This was offset slightly by a 9% increase in T/LA in Stg1 compared with RTx7000 (5.15 vs 4.70). The net result was a 22% reduction in water use per plant (T) in Stg1 compared with RTx7000 (25.6 vs 32.7 1), primarily due to reduced canopy size. The increase in T/LA exhibited by Stg1 may itself be a drought adaptation mechanism, cooling the leaf and enabling photosynthesis to continue.
The plasticity in T/LA appears to be particularly important in the regulation of plant water status. Under high VPD conditions, reduced T/LA in Stg1 was the key mechanism for reducing T and increasing TE. Under low VPD conditions, increased T/LA in Stg1 may have contributed to maintenance of leaf function via cooling.
Lysimetry studies on a Stg1 fine-mapping subset provide additional insight into this region. Under high VPD conditions, LA per plant was less in 10604-5 (location of Stg1 candidate gene) than RTx7000 (10283 vs 11628 cm2), yet T/LA was equivalent in 10604-5 and RTx7000 (˜2.86), resulting in less water use per plant in 10604-5 than RTx7000 (28.0 vs 32.8 1). Hence water savings in 10604-5 were achieved entirely by a reduction in canopy size.
Under low VPD conditions, LA per plant was less in all of the Stg1 lines compared with RTx7000, resulting in water savings in all Stg1 lines. Therefore it was difficult to fine-map this region, since all recombinants responded similarly to 6078-1.
Buster planted at 5 plants/m2 with 1 m row spacing. Soil depth=1800 mm; soil PAWC=324 mm; N non-limiting. Results are shown in
Root mass and root:shoot ratio (
Root mass per leaf area ratio can be used as a drought adaptation index at the seedling stage since it integrates the capacity of the plant to access water (root mass) with the capacity of the plant to utilise water (leaf area). A higher index indicates a greater capacity to access water per unit leaf area. Stg1 exhibited a higher root mass per leaf area ratio relative to RTx7000 due to both a higher root mass and a smaller leaf area.
The higher root mass per leaf area ratio exhibited by Stg1 at the L6 stage may explain why it used more water early in crop growth (20-50 DAE) compared with RTx7000 under the LWLD treatment (
In a root chamber experiment at Gatton, Queensland, Australia (Van Oosterom et al., 2010 supra), the gravimetric lower limit of water extraction was 0.26% lower for A35 (stay-green) than AQL39 (senescent) hybrids. A35 contains the Stg1 region whereas AQL39 does not. Assuming a bulk density of 1.3 g cm-3 and a soil depth of 150 cm, this could potentially increase available water in the field by >5 mm throughout the life cycle of the crop.
Mainstem biomass per unit leaf area (B/LA) at anthesis was −24% higher in Stg1 than RTx7000 under low water stress (35.2 vs 26.2 g/m2/cm2) and high water stress (40.6 vs 31.4 g/m2/cm2) conditions (Table 7). Mainstem B/LA at anthesis was −14% higher under high water stress than low water stress conditions for both Stg1 and RTx7000, i.e. B/LA increased with water deficit. Note that tiller B/LA was equivalent in Stg1 and RTx7000 under low and high water stress conditions.
Table 7 shows mainstem, tiller and total biomass per leaf area for RTx7000 (recurrent parent) and a number of near-isogenic lines containing various Stg1 introgressions grown under high and low water stress at Biloela, Queensland, Australia.
The detailed water use measurements suggest that the higher biomass per unit leaf area observed in Stg1 lines at Biloela was probably be due to higher transpiration per unit leaf area rather than TE.
Stg1 and RTx7000 displayed equivalent B/LA under low water stress. However, T was ˜7% lower in Stg1, due to ˜10% lower T/LA which, in turn, increased TE by ˜9% (
Under high water stress, B/LA was ˜6% higher in Stg1 compared with RTx7000. B/LA was positively correlated with T/LA but not with TE. Hence, the higher B/LA displayed by Stg1 was due to higher T/LA. In general, B/LA was positively correlated with T/LA and negatively correlated with TE under high water stress.
In this case, Stg1 used ˜22% less water than RTx7000 during the pre-anthesis period. Therefore, Stg1 would have significantly more water available to fill grain, despite lower biomass at anthesis.
In the Stg1 fine-mapping population, the length and greenness (SPAD) of Leaf 10 were highly negatively correlated (r2=0.72,
Greener leaves may increase photosynthetic capacity and therefore water use efficiency. In a subset of the Stg1 fine-mapping population, photosynthesis increased with SPAD value until reaching a plateau at a SPAD of ˜48.5 (
Leaf greenness (SPAD) and WUE (based on an index calculated for Licor software) were positively correlated in a subset of the Stg1 fine-mapping population (
Compared with RTx7000 and other Stg QTLs, Stg1 exhibited a greener leaf (higher SPAD value) and higher WUE (based on an index calculated for Licor software) [
Transpiration efficiency (TE) was negatively correlated with transpiration per leaf area (T/LA) under low and high VPD conditions (
Under high VPD conditions, the slope of the negative correlation between T/LA and TE was steep, such that a slight decrease in T/LA from 2.9 mm/cm2 (Stg4) to 2.6 mm/cm2 (Stg1) resulted in a significant increase in TE from 4.2 g/m2/mm (Stg4) to 5.1 g/m2/mm (Stg1) [
Introgressing Stg1 into RTx7000 variously affected T/LA, depending on VPD conditions. Relative to RTx7000, Stg1 increased T/LA by ˜9% under low VPD and decreased T/LA by ˜10% under high VPD. T/LA, inter alia, can be regulated by a) the number of stomata per unit leaf area, b) the size to the stomatal aperture, c) the timing of stomatal opening and closing, and/or d) the number of hair base cells (which affects the boundary layer and hence T/LA). Measurements of two of these four components (a and d) have been made. In one rainout shelter experiment, individual leaves were harvested from the high density treatment within the irrigated control, cuticles removed, and images taken of the cuticle surface. These images were used to determine a) the number of stomata per unit leaf area, b) the number of epidermal cells per unit leaf area, and c) the number of hair base cells per unit leaf area.
At the same time, transverse leaf sections were taken. Preliminary analysis of these data indicate that introgressing Stg1 into RTx7000 modified leaf anatomy. Differences in the morphology of leaves (e.g. Leaves 7 and 10) are apparent between RTx7000 and Stg1. In this case, there were more and smaller bundle sheaths surrounding the vascular bundle in Stg1. The increased number of cells in the bundle sheath might also contribute to increased photosynthetic assimilation (PNAS 2007) and hence TE.
Crop water use (CWU) before anthesis was negatively correlated with CWU after anthesis in the ROS experiment (
Increased water use during the grain filling period was exhibited by Stg1 under both low and high density treatments in the ROS experiment. This was due primarily to (i) increased water availability at anthesis under high density, and (ii) increased water accessibility during grain filling under low density (
In a study of RTx7000 and four Stg NILs (Stg1, Stg2, Stg3 and Stg4), CWU before and after anthesis were negatively correlated in an ROS experiment under low density conditions (
Under low density (LD), reducing pre-anthesis biomass by 23% (from 700 to 640 g/m2) increased post-anthesis biomass more than twofold (from ˜200 to 425 g/m2). Under LD, Stg1 produced similar pre-anthesis biomass to RTx7000 (˜610 g/m2), yet produced less post-anthesis biomass (265 vs 327 g/m2). However under HD, Stg1 and RTx7000 produced similar pre-anthesis biomass (˜840 g/m2), yet Stg1 produced more post-anthesis biomass (195 vs 17 g/m2).
The relation between GLAA and the pre:post anthesis biomass ratio is critical in the Stg1 story. GLAA must be cut back to <3 to ensure the availability of adequate water for grain filling, and this is the critical role of the Stg1 gene. In this experiment, Stg1 reduced GLAA adequately to achieve a pre:post anthesis biomass ratio of <3 under LD, but not HD. Under HD, note that introgressing Stg1 into RTx7000 reduced the GLAA from 31200 to 29300 cm2/m2, reducing the pre:post anthesis biomass ratio from 8.2 to 6.5 (but still not to <3). This highlights the importance of appropriate management strategies such as crop density in maximising limited water resources.
The negative relation between GLAA and post-anthesis stem mass is also critical to the Stg1 story. Lower GLAA, and hence reduced water use at anthesis, was associated with higher post-anthesis stem mass (a component of lodging resistance). Introgressing Stg1 into RTx7000 increased post-anthesis stem mass under both LD (marginal increase) and HD (significant increase) conditions.
The relation between the pre:post anthesis biomass ratio (PPBR) and post-anthesis biomass is instructive. The two density treatments provide a continuum in the range of PPBR from <2 to >8. Reducing PPBR from >8 to ˜3 resulted in a gradual increase in post-anthesis biomass. However, further reducing PPBR below 3 resulted in a relatively sharp increase in post-anthesis biomass, presumably because more water was available during grain filling when the PPBR ratio fell below 3. Introgressing Stg1 into RTx7000 increased post-anthesis biomass under HD but not LD.
The relation between the pre:post anthesis biomass ratio (PPBR) and post-anthesis stem mass is equally instructive. Post-anthesis stem mass is a component of lodging resistance. Analysis of this component provides some understanding of how Stg introgressions affect lodging resistance. Reducing PPBR from >8 to ˜4 resulted in a gradual increase in post-anthesis stem mass. However, further reducing PPBR below 4 resulted in a relatively sharp increase in post-anthesis stem mass. Introgressing Stg1 into RTx7000 increased post-anthesis stem mass under both LD (marginal increase) and HD (significant increase) conditions.
The relation between PPBR and grain yield was less clear in this experiment. While grain yield was higher in Stg1 than RTx7000 under both densities, the higher yield could only be explained by lower PPBR in Stg1 under HD.
Two Stg1 introgressions were examined in this experiment: a) 6078-1 (the whole Stg1 region), and b) 10946-5 (a recombinant covering about ⅓ of the Stg1 region between markers Sb03QGM116 and Sb03QGM106).
Reducing pre-anthesis biomass by 20% (from 920 to 735 g/m2) increased post-anthesis biomass by about 100% (from 200 to 400 g/m2) [
Canopy size, as evidenced by GLAA, largely determined the ratio of pre:post anthesis biomass (
Canopy size, as evidenced by GLAA, was a determinant of post-anthesis stem mass (PASM) [
The relation between the pre:post anthesis biomass ratio (PPBR) and post-anthesis biomass (PAB) was strong (
The pre:post anthesis biomass ratio (PPBR) also affected lodging resistance
(
Grain yield remained low (at a benchmark of ˜4.2 t/ha) until the pre:post anthesis biomass ratio fell below ˜3 (HD) or ˜2.5 (LD) [
Four Stg1 introgressions were examined in this experiment: a) 6078-1 (the whole Stg1 region); b) 10709-5 (a recombinant covering about ⅓ of the Stg1 region); c) 10604-5 (a recombinant covering about ¾ of the Stg1 region); and d) 10568-2 (a recombinant covering almost ½ of the Stg1 region).
Under low density (LD), pre-anthesis biomass varied by only ˜5% (from 522 to 552 g/m2) among genotypes, yet post-anthesis biomass varied almost twofold (from 173 to 313 g/m2). This suggests that the considerable differences in post-anthesis biomass were affected by something other than pre-anthesis biomass, e.g. differences in water accessibility. For example, 10709-5 and RTx7000 both produced ˜550 g/m2 of pre-anthesis biomass, yet the Stg1 recombinant (10709-5) produced ˜60% more post-anthesis biomass.
Under high density (HD), pre- and post-anthesis biomass were highly negatively correlated. Introgressing Stg1 into RTx7000 reduced pre-anthesis biomass by 9% and increased post-anthesis biomass by 23%.
Crop water use (CWU) at anthesis better discriminated between genotypes than did pre-anthesis biomass. Combining the HD and LD data, post-anthesis biomass (PAB) remained low (at a benchmark of ˜150 g/m2) until the CWU at anthesis fell below ˜180 mm Below this critical value, PAB increased for each incremental reduction in CWU down to a level of 175 mm, with PAB plateauing at about 310 g/m2. Further reductions in CWU at anthesis below 175 mm did not result in additional PAB.
Canopy size, as evidenced by GLAA, largely determined the ratio of pre:post anthesis biomass (PPBR). Under both high and low density treatments, introgressing Stg1 (or particular recombinants such as 10709-5) into a RTx7000 background reduced GLAA which, in turn, reduced the ratio of pre:post anthesis biomass, thereby increasing water availability for grain filling under these water-limited conditions. The PPBR value for 6078-1 appears anomalous (too high) since this genotype is placed well above the GLAA/PPBR regression line.
Canopy size, as evidenced by GLAA, was a determinant of post-anthesis stem mass (PASM). Under the high density treatment, introgressing Stg1 (or Stg1 recombinants such as 10604-5 and 10709-5) into a RTx7000 background reduced GLAA which, in turn, increased PASM. Approximately, 60 g/m2 more stem reserves were utilized under HD compared with LD, reflecting the higher stress imposed by this treatment.
The relation between the pre:post anthesis biomass ratio (PPBR) and post-anthesis biomass (PAB) was strong. The two density treatments provide a continuum in the range of PPBR from <2 to >5, although the slope of the regression was greater for LD than HD. Under HD, reducing PPBR from ˜6 to ˜3.5 resulted in a gradual increase in PAB from ˜130 g/m2 (RTx7000) to ˜180 g/m2 (10604-5). Further reducing PPBR below ˜3 under LD resulted in a steeper increase in PAB, presumably because more water was available during grain filling when the PPBR ratio fell below 3.
The pre:post anthesis biomass ratio (PPBR) also affected lodging resistance. In this case, post anthesis stem mass (PASM) is used as a surrogate for lodging resistance. Under high and low densities, PPBR was negatively correlated with post-anthesis stem biomass. That is, a high pre:post anthesis biomass ratio increased the amount of stem reserves remobilized during grain filling, thus reducing stem biomass and increasing the likelihood of lodging. Compared with RTx7000, Stg1 significantly reduced the amount of stem reserves mobilized under HD (˜100 vs 160 g/m2). The extent of stem reserves mobilized was greater under HD than LD, reflecting the greater water deficit under HD. For example, the difference in stem reserve mobilization between HD and LD was more than twofold in RTx7000 (about 160 vs 60 g/m2). Interestingly, PASM increased with decreasing PPBR over the whole range of PPBR (1.5-6), whereas grain yield, and to a lesser extent PAB, only increased when PPBR fell below ˜3. This suggests that relatively small water savings before anthesis were still able to improve lodging resistance, although greater water savings were required before grain yield responded.
Grain yield remained low (at a benchmark of ˜3.1 t/ha) until the pre:post anthesis biomass ratio fell below ˜3. Below this critical value, grain yield increased significantly for each incremental reduction in this ratio. Since none of the Stg1 introgressions reduced the PPBR to <3 under HD, no yield benefits were realised from Stg1 in this treatment. Under LD, some of the Stg1 introgressions reduced PPBR below the critical level, resulting in yield increases of 12% (10568-2) and 5% (10709-5), relative to RTx7000. These data provide a critical link between Stg1 gene action (reduced canopy size at anthesis) and grain yield under terminal drought.
CWU during grain filling remained low (at a benchmark of ˜60 mm) until the pre:post anthesis biomass ratio fell below ˜3.5 (
The relation between CWU during grain filling and grain yield was positive (
Plant water status was determined on FL-2 (two leaves below the flag) at mid-grain filling using two methodologies: leaf water potential (LWP) and relative water content (RWC). LWP was measured in the field with a pressure bomb. Following determination of LWP in the field, a sample of the same leaf was placed on ice and, within a few minutes, taken to a laboratory some 300 m away for determination of RWC by standard methods.
The RWC of FL-2 was negatively correlated with the relative rate of leaf senescence at mid-grain filling under both high and low densities in a set of Stg NILs including the recurrent parent. Correlations for HD and LD were parallel, but offset by about 0.35 units of leaf senescence, i.e. for a given level of RWC, say 70, the relative rates of leaf senescence were 2.1 and 2.45 for LD and HD, respectively. Introgressing the Stg1 region into RTx7000 increased RWC at mid-grain filling (FL-2) and decreased the relative rate of leaf senescence under both HD and LD, although the impact was greater under HD.
In turn, the relative rate of leaf senescence was highly negatively correlated with green leaf area at maturity (GLAM) under HD and LD (
Higher stem mass at maturity is a component of lodging resistance. RWC at mid-grain filling (FL-2) was highly negatively correlated with stem mass at maturity in a set of Stg NILs grown under water-limited conditions at two crop densities (
Post-anthesis biomass is mainly comprised of a) post-anthesis stem mass (PASM), a measure of stem reserve mobilization and a component of lodging resistance, and b) grain yield. Grain-growers require that both grain yield and lodging resistance be maximized, i.e. they do not want one at the expense of the other. Post-anthesis stem mass was highly linearly correlated with PAB under HD and LD conditions (
While the correlations between PASM and PAB were high under HD and LD (
The potential trade-off between PASM and grain yield is highlighted in
The relation between PASM and stem mass at maturity was relatively flat for the various Stg introgressions under HD and LD, although RTx7000 fell below the regression line in both cases. For a given level of stem reserve utilization (e.g. ˜140 g/m2 under HD or 50 g/m2 under LD), introgressing a Stg region into RTx7000 significantly increased stem mass at maturity, suggesting that some other factor (e.g. stem strength) in addition to the amount of stem reserves utilized was important.
Post-anthesis stem mass (PASM) was highly linearly correlated with PAB under HD and LD conditions (
Grain yield was positively correlated with PAB under HD and LD. Under HD, Stg1 outyielded RTx7000 by 24% although Stg1a was equivalent to RTx7000 in grain yield. Under LD, Stg1 and Stg1a outyielded RTx7000 by 42% and 20%, respectively.
In this experiment, there was no trade-off between PASM and grain yield, since the correlation between these parameters was positive and linear for both crop densities (
The relation between PASM and stem mass at maturity was positively correlated under HD and negatively correlated under LD. Under HD, PASM and stem mass at maturity were both significantly higher in Stg1 than RTx7000. Under LD, stem mass at maturity was higher in Stg1 than RTx7000 (314 vs. 271 g/m2), although Stg1 utilized more stem reserves compared with RTx7000 (87 vs 66 g/m2). Overall, Stg1 increased stem mass at maturity by 22% (HD) and 16% (LD) relative to RTx7000. Also, Stg1a utilized significantly less stem reserves than RTx7000 under both crop densities.
Post-anthesis stem mass (PASM) was highly linearly correlated with PAB under HD and LD conditions (
Grain yield was positively correlated with PAB under LD and negatively correlated under HD, although overall (combining HD and LD), the relationship was positive, with RTx7000 (HD) as an outlier.
In this experiment, there was no trade-off between PASM and grain yield, since the correlation between these parameters was positive and linear for both crop densities. Under HD, the Stg1 parent (6078-1) exhibited high stem mass and grain yield compared with the other Stg1 introgressions. RTx7000 was an anomaly under HD, exhibiting low stem mass and high grain yield. Under LD, 10709-5 and 10568-2 exhibited high stem mass and grain yield relative to RTx7000.
The relation between PASM and stem mass at maturity was positively correlated under both densities. Under HD, PASM and stem mass at maturity were both significantly higher in 6078-1 and 10709-5 than RTx7000. Under LD, only one Stg1 recombinant (10568-2) exceeded RTx7000 in PASM and stem mass at maturity.
Introgressing Stg1 into a RTx7000 background increased plant water status at mid-grain filling, as indicated by a) higher relative water content (RWC) in FL-2 under LD and HD lower leaf water potential (LWP) in FL-2 under LD and HD. Overall, plants were more stressed under LD than HD in this experiment, evidenced by lower RWC under LD. However, the beneficial impact of Stg1 on plant water status was more dramatic under HD. where RWC was 26% higher in Stg1 than RTx7000.
RWC at mid-grain filling in FL-2 was positively correlated with grain yield under HD and LD. At higher levels of plant water stress (RWC<73), grain yield was higher under LD than HD for a given level of RWC. RWC and grain yield were higher in Stg1 than RTx7000 under both crop densities. For example in Stg1 under HD, a 26% increase in RWC was associated with a 58% increase in grain yield, relative to RTx7000.
The leaf water potential (LWP) of FL-2 at mid-grain filling was negatively correlated with grain yield under HD and LD conditions (
Critical to the hypothesis on Stg1 function is the link between pre- and post-anthesis water use and, subsequently, the link between post-anthesis water use and grain yield. The Stg1 gene is of little value at the field level is there is no link between increased water availability during grain filling and either grain yield or grain size.
First, it is important to establish the link between the pre:post anthesis biomass ratio (PPBR) and crop water use (CWU) during grain filling. CWU during grain filling remained low (at a benchmark of ˜85 and 95 mm for LD and HD, respectively) until the pre:post anthesis biomass ratio fell below ˜3 and 2.5 for LD and HD, respectively (
Second, it is important to show the link between CWU during grain filling and grain yield. In general, these parameters were positively associated in a ROS experiment, apart from two distinct outliers under LD (Stg2 and Stg3) [
Finally, the link between PPBR and grain yield under water-limited conditions completes the picture (
CWU during grain filling was positively correlated with grain size under both HD and LD treatments (
The relation between PPBR and grain size under water-limited conditions highlights the importance of water conservation before anthesis as a determinant of grain size (
CWU during grain filling remained low (at a benchmark of ˜58 mm for HD) until the pre:post anthesis biomass ratio fell below ˜3.5. Note that the PPBR did not drop below the critical threshold in any genotype under HD, hence CWU during grain filling remained relatively low for all genotypes in this treatment. However, as genotypes fell below this critical value in the LD treatment, CWU during grain filling increased significantly for each incremental reduction in this ratio. Only one Stg1 recombinant (10709-5) increased CWU during grain filling relative to RTx7000.
In general, CWU during grain filling and grain yield were positively correlated using a combined data set from the HD and LD treatments, with genotypes using more water and producing more grain under LD. However, the relation between these parameters was not very strong. Nor did particular Stg1 recombinants consistently out-yield RTx7000 in HD and LD treatments. See
QTL analysis
Stay-green QTL data were collected from 7 studies (Crasta et al., 1999; Hausmann et al., 2002; Kebede et al., Theoretical and Applied Genetics 103:266-276, 2001; Srininvas et al., Theor Appl Genet 118:703-717, 2009; Subudhi et al., Theor App Genet 101:733-741, 2000; Tao et al., Theor Appl Genet 100:1225-1232, 2000; Xu et al., Genome 43:461-469, 2000). From the 7 studies, 47 individual QTL were identified and projected onto the sorghum consensus map (Mace et al., BMC Plant Biol. 9:13, 2009). Where estimated Confidence Intervals (CI) of QTL for the same trait overlapped, those QTL were grouped into a meta QTL. Nine meta-QTL for stay-green were identified in this way. QTL for the same trait were classified as separate QTL if their CI had no region in common and mean QTL location were less than or equal to 15 cM away from each other.
Statistical Machine Learning (SML) QTL analysis (Bedo et al., BMC Genetics 9:35, 2008) was conducted on a set of over 500 entries on the DEEDI sorghum pyt males trial. 23 QTL identified with a probability <0.05 were also plotted onto the consensus map.
All available PIN genes in rice and Arabidopsis were searched for via NCBI (www.ncbi.org). In total, sequence for 9 rice PIN genes (OsPIN1, OsPIN1b, OsPIN1c, OsPIN2, OsPIN3a, OsPIN3b, OsPIN4, OsPIN5, OsPIN6) and 3 Arabidopsis PIN genes (AtPIN1, AtPIN2, AtPIN4) were identified. All genes (protein sequence) were BLASTed against the sorghum WGS (www.gramene.org) and the top 100 hits were identified. The score (S value: a measure of the similarity of the query to the sequence shown), E-value (the probability due to chance, that there is another alignment with a similarity greater than the given S score), % ID and length of sequence homology for each of the 1200 hits were collated. The relationship between the 4 measures was analyzed and the S score was selected as the main measure to assess likelihood of sequence similarity. Following an analysis of the distribution of S score values, 3 S score categories were identified (>1000; >499 and <1000; <499) and a list of 11 sorghum genes with scores >499 (i.e. in the first 2 categories) was produced (Table 8).
Sorghum Gene
Of the 11 PIN orthologues identified, 10 (90.9%) aligned with known QTL for stay-green. Only one of the 11 sorghum PIN genes (Sb03g043960 on SBI-03, did not align to any reported QTL. Of the 79 QTL plotted (23 SML-QTL and 56 from the literature/meta-QTL), 30 (38%) aligned with the PIN orthologs.
The stay-green source BTx642 (B35), and near-isogenic lines (NILs) containing the Stg1, Stg2, Stg4 QTLs, as well as the contrasting senescent line Tx7000 were grown in root pipes in a glasshouse.
The aim of the experiment was to measure expression levels of genes that are identified herein as stay-green gene candidates under well-watered conditions and after a drought stress has been imposed on the plants to see whether there were any differences in expression in the stay-green compared with the senescent plants.
The experiment was divided into two parts: an early drought stress (Exp1) and a late drought stress (Exp2).
Results from the early drought stress experiment are shown in Table 9.
The main differences in expression of these PIN genes are summarised below (Table 10 and in
Emerging patterns were identified for SbPIN4 and SbPIN2, Stg1 and Stg2, respectively.
In both cases the expression of these genes was higher in stay-green lines compared to the senescent line in response to water deficit.
These two PIN genes showed differences in tissue specificity. SbPIN4 was generally (across all conditions) more highly expressed in roots and stems and less expressed in leaves, while SbPIN2 generally showed higher expression in leaves and stems and lower expression in roots).
Those skilled in the art will appreciate that aspects described herein are susceptible to variations and modifications other than those specifically described. It is to be understood that those aspects include all such variations and modifications. Aspects herein described also include all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of the steps or features.
Those skilled in the art will appreciate that aspects described herein are susceptible to variations and modifications other than those specifically described. It is to be understood that these aspects include all such variations and modifications. Such aspects also include all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of the steps or features.
This application is associated with U.S. Provisional Application No. 61/413,902, filed 15 Nov. 2010, the entire contents of which, are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU2011/001478 | 11/15/2011 | WO | 00 | 8/26/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61413902 | Nov 2010 | US |
