Patent Application
20130305415
The present application relates to methods for increasing tolerance of plants and/or their offspring to one or more stresses, in particular relating to the generation of stress tolerant plants and seeds/propagules thereof.
The ability to provide plants that are tolerant to usually unfavourable environmental conditions is highly desirable. For example, seeds that germinate into crop plants that show enhanced tolerance to drought or other water stresses than their parent(s) could be particularly useful. Moreover, it would be desirable to produce tolerant seeds/propagules for use in climates where yields are currently restricted by limited water availability.
According to one aspect of the present invention, there is provided a method for the production of a stress tolerant plant or precursor thereof, the method comprising:
(i) subjecting one or more parental plants to one or more stress conditions selected from unfavourable conditions relating to relative humidity, water availability, periodic drought, nutrients, sunlight, wind, temperature, pH, exogenous chemicals, chemical toxins such as salt, herbivory, prophylactic chemicals, fertilizers, pathogen attack such as bacterial, fungal, or virus infection and pest infestation; and
(ii) generating offspring from said one or more parental plants,
wherein said offspring show enhanced tolerance relative to the one or more parental plants to one or more stress conditions selected from unfavourable conditions relating to relative humidity, water availability, periodic drought, nutrients, sunlight, wind, temperature, pH, exogenous chemicals, chemical toxins such as salt, herbivory, prophylactic chemicals, fertilizers, pathogen attack such as bacterial, fungal, or virus infection and pest infestation.
Preferably, the offspring are adult plants or a precursor thereof such as seeds or vegetative propagules.
Remarkably, it has been found that by subjecting a parent plant to one or more stress conditions, the seed or vegetative offspring produced from that parent plant can exhibit increased tolerance to the same one or more stress conditions. It has also, remarkably, been found that the offspring may be tolerant to one or more stress conditions which differ from those experienced by the one or more parental plants. For example Arabidopsis plants exposed to slightly reduced relative humidity stress nevertheless exhibited increased tolerance to periodic drought stress (See example 2 below). Accordingly, in one embodiment, it is preferred that the offspring are tolerant to one or more stress conditions not experienced by the one or more parental plants.
It will be appreciated that the term “unfavourable” is relative to the plant in question and is a relative term. For example, for plants which usually thrive in medium or high levels of humidity, a low relative humidity may be seen as “unfavourable” and therefore as a stress condition. For example, in the context of a food crop, any condition that leads to reduction in yields, harvestable yields or in sustainable harvests may be viewed as “unfavourable”.
Preferably, the one or more stress conditions are selected from low relative humidity, periodic drought and infection with Botrytis (e.g. Botrytis cynerea).
Preferably, the one or more parental plants are subjected to the one or more stress conditions under semi-controlled or more preferably, controlled conditions.
Preferably, the one or more parental plants are selected from a higher plant, a flowering plant and a dicotyledonous plant.
Preferably, the one or more parental plants are crop plants.
Preferably, the one or more parental plants belong to the Eudicotyledons. Preferably, the one or more parental plants are a member of the Brassicacea or the Malvaceae. Preferably, the one or more parental plants are selected from Arabidopsis plants and a Theobroma plants, for example selected from Arabidopsis thaliana and Theobroma cacao.
Preferably, the one or more parental plants are flowering plants (Magnoliophyta), and the one or more stress conditions are likely to impact on harvestable yield; for example including stresses associated with water availability (low relative humidity or periodic drought), to toxic chemicals (such as salt), exogenous chemicals or to exposure to pathogens (such as Botrytis) or pests.
Preferably, the methods of the present invention are for producing plants capable of generating higher yields under one or more stress conditions experienced and/or not experienced by the one or more parental plants. For example, it is preferred that the plants produced by the methods of the present invention show increased production of biomass, flower number, seed number, seed weight at any chosen time of harvest.
As described above, the methods of the present invention can be used to produce a precursor of a stress tolerant plant such as a seed or a vegetative propagule. For example, in one aspect of the present invention there is provided a method which comprises:
(i) subjecting one or more parental plants to one or more stress conditions selected from unfavourable conditions relating to relative humidity, water availability, periodic drought, nutrients, sunlight, wind, temperature, pH, exogenous chemicals, chemical toxins such as salt, herbivory, prophylactic chemicals, fertilizers, pathogen attack such as bacterial, fungal, or virus infection and pest infestation;
(ii) generating a precursor for offspring from said one or more parental plants, wherein said precursor is capable of developing into a plant which shows increased tolerance relative to the one or more parental plants to one or more stress conditions selected from unfavourable conditions relating to relative humidity, water availability, periodic drought, nutrients, sunlight, wind, temperature, pH, exogenous chemicals, chemical toxins such as salt, herbivory, prophylactic chemicals, fertilizers, pathogen attack such as bacterial, fungal, or virus infection and pest infestation and/or is capable of generating higher yields relative to the one or more parental plants under said stress conditions.
Preferably, the precursor is a seed or a vegetative propagule such as a cutting. For example, the precursor may be an Arabidopsis seed which is capable of growing into a plant tolerant to low relative humidity and/or to periodic drought. In other examples, the precusor is a cutting or a somatic embryo of cocoa (Theobroma cacao L.) which is capable of growing into a plant tolerant to low relative humidity and/or to periodic drought. Further examples include an Arabidopsis seed which is capable of growing into a plant with enhanced resistance to Botrytis.
Preferably, the methods of the invention comprise crossing (i.e. cross-pollinating) two parental plants or self-pollinating a single parental plant. In other examples, a vegetative propagule is created from a parental plant that has been exposed to one or more stress conditions.
Preferably, the methods of the invention comprise generating seed offspring from a single parent genotype, for example by self-pollination or by cross-pollinating one of the treated parental plants with a second (untreated) parental plant.
It will be appreciated that subjecting a parental plant to one or more stress conditions includes subjecting all or a part of the plant to one or more stress conditions. For example, in the case of low relative humidity, all of the plant could be exposed. In the case of infection with Botrytis, a single leaf or portion thereof could be exposed.
According to another aspect of the present invention, there is provided a plant, or precursor thereof, produced by a method as described herein.
As such, the present invention provides a plant or precursor thereof which is tolerant to one or more stress conditions.
Another aspect of the present invention relates to an assay for identifying a plant, or precursor thereof, produced by the methods described herein, the assay comprising analysing a plant, or precursor thereof, suspected of being produced by the method for the presence or absence of one or more sites of genomic methylation, wherein the presence or absence of methylation at said one or more sites is indicative of a plant, or precursor thereof, produced by the method.
Preferably, the method is for producing a low relative humidity and/or periodic drought-tolerant plant (for example an Arabidopsis plant), or seed thereof, and the presence of a methylation state at or within about 10 kb, preferably about 5 kb, preferably about 2 kb of a SPEECHLESS or FAMA gene, or a functional homolog of either gene, is indicative of the acquired stress-tolerance in a plant or seed produced by the methods described herein.
Another aspect of the present invention provides an assay for identifying a plant, or precursor thereof, which is tolerant to one or more stress conditions selected from unfavourable conditions relating to relative humidity, water availability, periodic drought, nutrients, sunlight, wind, temperature, pH, exogenous chemicals, chemical toxins such as salt, herbivory, prophylactic chemicals, fertilizers, pathogen attack such as bacterial, fungal, or virus infection and pest infestation, wherein the assay comprises analysing a plant, or precursor thereof for the presence or absence of one or more sites of genomic DNA methylation, wherein the presence or absence of methylation at said one or more sites is indicative of a plant, or precursor thereof, which is tolerant to said one or more stress conditions. Preferably, the presence of genomic methylation in or within about 10 kb, preferably about 5 kb, preferably about 2 kb, of a SPEECHLESS or FAMA gene, or a functional homolog of either gene, is indicative of a plant, or precursor thereof, which is tolerant to low relative humidity and/or periodic drought.
It will be seen that, according to the present invention, there is provided a method for changing the stress response of the offspring of a plant by previously exposing (prior to conception/zygote formation) one or more of its parents to the same stress(es) or a different stress (hereafter also referred to as conditioning stress[es]).
As detailed herein, in some embodiments the change to stress response in the offspring relates to a different stress type to that experienced by the parents. That is, where exposure to the conditioning stress evokes a changed response to another stress in the offspring.
Preferably, the offspring are clonal propagules of a parental plant. Put another way, it is preferred that the change in stress response is induced in a clonal propagule of the parental plant exposed to the conditioning stress(es).
As will be appreciated, the seeds of crop plants in which either or both parents have been exposed to one or more conditioning stresses are produced for the purpose of improving the stress tolerance of the plants derived from said seeds.
As will be further appreciated, in accordance with the methods of the present invention, plants which have been exposed to one or more conditioning stresses can be used to produce vegetative propagules, for example cuttings, micropropagation, callus-mediated adentitious shooting or somatic embryogenesis, with changed, preferably improved, tolerance to one or more stress conditions.
Particularly preferred examples of the invention include the following.
Preferably, the seeds of plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of changing (preferably improving) the tolerance of the plants derived from said seeds to water stress (examples include but are not limited to low relative humidity stress and periodic drought).
Preferably, the seeds of plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of changing (preferably improving) the tolerance of the plants derived from said seeds to low relative humidity stress.
Preferably, the seeds of Eudicotyledonous plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of improving the tolerance of the plants derived from said seeds to water stress (examples include but are not limited to low relative humidity stress and periodic drought).
Preferably, the seeds of Eudicotyledonous plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of changing (preferably improving) the tolerance of the plants derived from said seeds to low relative humidity stress (examples include but are not limited to low relative humidity stress and periodic drought).
Preferably, the seeds of Brassicacea or Malvaceae plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of improving the tolerance of the plants derived from said seeds to water stress (examples include but are not limited to low relative humidity stress and periodic drought).
Preferably, the seeds of Brassicacea or Malvaceae plants in which either or both parents have been exposed to low relative humidity stresses are produced for the purpose of changing (preferably improving) the tolerance of the plants derived from said seeds to low relative humidity stress.
Preferably, the plants which have been exposed to low relative humidity stresses are used to produce vegetative propagules with changed (preferably improved) tolerance to water stress (examples include but are not limited to low relative humidity stress and periodic drought).
Preferably, the plants which have been exposed to low relative humidity stresses are used to produce vegetative propagules with changed (preferably improved) tolerance to low relative humidity stress.
Preferably, the seeds of plants in which either or both parents have been exposed to biotic stress (examples include but not limited to exposure to pathogenic fungi) are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to the same biotic stresses.
Preferably, the seeds of eudicotyledonous plants in which either or both parents have been exposed to biotic stress (examples include but not limited to exposure to pathogenic fungi) are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to the same biotic stresses.
Preferably, the seeds of plants of the Brassicacea or Malvaceae in which either or both parents have been exposed to biotic stress (examples include but not limited to exposure to pathogenic fungi) are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to the same biotic stresses.
Preferably, the seeds of plants in which either or both parents have been exposed to Botrytis fungi are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to infection by Botrytis fungi.
Preferably, the seeds of eudicotyledonous plants in which either or both parents have been exposed to Botrytis fungi are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to infection by Botrytis fungi.
Preferably, the seeds of plants of the Brassicacea or Malvaceae in which either or both parents have been exposed to Botrytis fungi are produced for the purpose of changing (preferably improving) the resistance of the plants derived from said seeds to infection by Botrytis fungi.
Preferably, the changed stress responses of plants (preferably crop plants) whose parents have been exposed to conditioning stresses (as identified above) leads to changed (preferably enhanced) production of biomass, flower number, seed number, seed weight at any chosen time of harvest.
Preferably, plants with changed tolerance to water stress are produced according to the methods described herein are detected according to changed methylation status of the DNA (measured using standard techniques including but not limited to bisulfite treatment followed by Sanger or NextGen sequencing, High Resolution Melt Analysis or Methyl capture and pPCR) encoding for the SPEECHLESS and/or FAMA genes (or functional homologue thereof) and/or of the DNA sequence immediately flanking said gene, where flanking sequence is preferably <10 kb, more preferably <3 kb and most preferably <1.5 kb of start or stop codons.
Example embodiments of the present invention will now be described with reference to the accompanying figures in which:—
The invention relates to methods for the production of plants and precursors thereof that are tolerant to one or more stress conditions. In particular, the invention relates to methods for producing seeds and/or vegetative propagules that have an enhanced ability to survive, grow and/or produce harvestable products when placed under one or more sub-optimal growing conditions (stresses) that in ‘parental’ plants and untreated lineages cause a significant drop in growth, survivorship, biomass, seed production and/or harvestable yield (for crops).
The methods used in the invention and detailed examples of the invention are set out below.
Within this specification, embodiments have been described in a way that enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the invention.
Within this specification, the terms “comprises” and “comprising” are interpreted to mean “includes, among other things”. These terms are not intended to be construed as “consists of only”.
Within this specification, the term “about” means plus or minus 20%, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2%.
Within this specification, the term “homolog” may mean a gene related to a second gene by descent from a common ancestral DNA sequence. The term may mean a gene similar in structure and evolutionary origin to a gene in another species.
Within this specification, the term “propagule” means any plant material which can be used for the purpose of plant propagation. In asexual reproduction, a propagule may be a woody, semi-hardwood, or softwood cutting, leaf section, or any number of other plant parts. In sexual reproduction, a propagule is a seed or spore. In micropropagation, a type of asexual reproduction, any part of the plant may be used, though it is usually a highly meristematic part such as root and stem ends or buds.
Within this specification, the term “vegetative propagule” means offspring which is the clonal (i.e. genetically identical) descendant of a single parental plant derived via plant materials other than a biological seed. This is in contrast to seed which is usually the result of sexual reproduction, i.e. the decendant of two or more parental plants.
Within this specification, the term “tolerant” means that the offspring/vegetative propagule(s) show an increased tolerance to one or more stresses than do the parental plant(s). It is preferred that this increase is statistically significant.
It has been found that Arabidopsis plants exposed to one form of water stress (low relative humidity LRH), respond in the short term by reducing the number of stomata (pores) in the leaves so that they do not lose excessive amounts of water and survive. The resultant plants are small but do nevertheless survive to set some seeds. Quite remarkably, however, seeds collected from these plants perform better when placed in identical conditions. The plants are also larger and produce far more seed. The plants used are so inbred that the offspring are to all intents and purposes genetically identical to the parent. Thus, it has remarkably been shown that stressing the parent plant pre-adapts the offspring to the same stress in the next generation. A similar phenomenon has been seen with other stresses (i.e. cold and heat stress (Whittle et al, 2009), UV-C light (Molinier et al, 2006), and pathogens such as bacteria (Molinier et al, 2006). The invention described herein is particularly applicable to commercial seed production in crops. Moreover, growing the parental clones/populations used to produce commercial seed lots under appropriate stressed conditions should pre-programme the epigenetic profiles of the seeds to increase the potential for adaptation to the same stresses when germinated. Importantly, such effects do not persist over many generations and rapidly fade. Thus, the present invention provides a means of improving plant production without changing the genetic code.
The density and operation (opening) of stomatal pores on the surface of leaves are both heavily influenced by environmental cues; together they control stomatal conductance of the leaf to water vapour (gs) over short (minute to hour) and long (seasonal to lifetime) timescales. Such plasticity allows the plant to balance the conflicting needs to capture atmospheric carbon dioxide (CO2) for photosynthesis and to minimise water loss through transpiration and water use efficiency (wue) is inversely correlated with leaf stomatal density over a plant's lifetime. There have been recent advances in our understanding of the genetic regulation of stomatal development. A pathway governing stomatal guard cell development involves a “default” fate of protoderm epidermal cells to form stomata but expression of a series of patterning genes blocks entry into the stomatal lineage (and so guard cell formation) and consequently sets stomatal density. Positive regulators determine entry into the stomatal lineage and asymmetric divisions forming stomatal guard cells. Mechanisms allowing plants to maintain plasticity for water conservation and carbon fixation in response to the environmental cues they receive are less clear. The possibility that plastic responses to environmental stress experienced early in the life of the plant could provide adaptive conditioning in anticipation for similar stresses later in development or even in the seminal generation was investigated.
The response of the stomatal pathway to different levels of ambient humidity was analysed. Arabidopsis thaliana ecotypes Landsberg erecta and Columbia were grown under constant low relative humidity (LRH; 45%±5) or under experimental control (65%±5) humidity from seed to seed harvest. Stomatal frequency (index of stomata as a percentage of epidermal cells (SI)) was influenced by LRH stress (
DNA methylation of loci for genes in the stomata pathway was investigated to see whether it was imposed differentially with environment. Differences in DNA methylation were screened for under LRH compared with the control environment in 11 stomata patterning and formation genes (Table 1). Differential methylation associated with RH treatment was found in both regulatory and 5′ coding regions of the SPEECHLESS(SPCH) and FAMA genes (
The role of methylation in regulating stomatal frequency was further investigated using methyltransferase mutants. In the mutant for the maintenance cytosine methyltransferase MET1 (Decreased Methylation 2DNA) (met1)) SI was increased in LRH and differential methylation with treatment was reduced for both SPCH and FAMA. Expression of SPCH was no longer reduced by LRH treatment and FAMA expression increased (
DRM2-mediated transcriptional gene silencing (TGS) by uni-directional methylation of gene promoter sequences in Arabidopsis is directed by 24 nt short-interfering RNAs (siRNAs): Post-transcriptional gene silencing (PTGS) by 21-22 nt secondary siRNAs has also been associated with bi-directional methylation of transcribed regions. A range of mutants for RNA-directed DNA methylation (RdDM) was grown in the control and LRH environments to investigate the role of siRNA direction in the observed DNA methylation and physiological responses. In TGS, RDR2 (ma dependent rna polymerase 2) is required for the synthesis of double-stranded short RNAs. Both maintenance and transitivity of PTGS require RDR6 and depend on transcription of the target gene. Dicer-like RNA III proteins process dsRNA or hairpin RNAs with DCL3 primarily acting on RDR2-produced RNAs and DCL4 on RDR6-produced RNAs; there is, however, some overlap and compensatory processing by the four Arabidopsis DCLs in single dcl mutants. No true rdr2 or dcl3 mutants germinated under LRH stress; both genes were expressed (data not shown), total small RNA content was increased compared with the WT and 24 nt siRNAs were present in seedlings although at much reduced levels (
Small RNA reads from high-throughput sequencing data of A. thaliana show seven small RNAs located within 300 bp upstream of the FAMA gene (accessed in TAIR 9 http://gbrowse.arabidopsis.org). Expression of these small RNAs was quantitatively assayed together with FAMA and surprisingly, given the increased expression of FAMA in rdr2 plants, was positively correlated with expression of FAMA (P=0.014, R2 0.97). Upstream of SPCH (and a predited 177 bp gene At5g53205 for an unknown protein) is a cluster of rolling-curve-type helitron family transposons corresponding with 42 small RNAs and a 40 bp tandem repeat within a 427 bp dispersed repeat region. Small transposons like these are believed to be preferentially dependent on RdDM via DRM1/2 for silencing. It was hypothesised whether these small RNAs could direct non-CG methylation that spread beyond the transposable elements (TEs) to affect transcription of SPCH as has been shown for the seven tandem repeats of the F-box protein encoded by SUPPRESSOR OF drm1 drm2 cmt3 (SDC) and for RdDM arising from tandem direct repeats around the transcription start site of FWA. Expression of SPCH was measured together with expression of a subset of these siRNAs. It was found that it was inversely correlated (P=0.004, R2 0.87) so that SPCH was downregulated when expression of these siRNAs was upregulated and SPCH was methylated in LRH. On exposure to LRH stress, 24 nt siRNAs corresponding the TEs upstream of the SPCH locus were induced and assayed DNA methylation spread into the regulatory and genic regions of SPCH.
Methylation of SPCH and FAMA in isogenic progeny from single parents exposed to LRH was next examined to see whether it correlated with gene expression and SI. LRH-control progeny retained parental methylated status (
There are several plausible causes for the loss of methylation at SPCH in LRH-LRH plants, including loss of RdDM. In equal quantities of total RNA from progenies, the entire complement of small RNA duplexes was reduced in LRH-LRH plants (
Here, loss of siRNAs could trigger active demethylation. Loss of inherited methylation at SPCH in LRH-LRH plants and in the cmt3 mutants (LRH-control) was apparent at symmetric sequences (
Heritable methylation of SPCH was lost in progeny of transgenerationally methylated parents (LRH-control-control) (
In contrast with SPCH, in FAMA the differential methylation pattern exhibited by parental plants was replicated in the progenies so that FAMA was methylated in all LRH plants (
It is proposed that the subtle interplay of both de novo and inherited methylation and demethylation at SPCH effectively “immunised” the progeny against the same stress on stomatal development experienced by their parents. This can be explained as a transgenerational “adaptive imprinting” response that is mediated by targeted DNA methylation.
LRH-LRH and LRH-control progenies apparently benefited in terms of increased biomass and seed production (
Supplied Arabidopsis thaliana (L.) Heynh Landsberg erecta seeds were grown in low (45%) and control (65%) relative humidity (RH) growth chambers and seed collected. Collected seeds from individual parents in both treatments were grown in low and control RH alongside untreated seeds and supplied seeds for several known methyltransferase mutants. This experiment was repeated four times; supplied seeds for known RNAi mutants were grown in the fourth repeated experiment. Each time, stomatal density (stomata mm−2) and index (percentage of epidermal cells forming stomata) were assessed at the same stage of growth by microscopic examination of impressions of the abaxial leaf surface. Plant dry weight, seed weight and seed number were assessed following senescence. Differential methylation with treatment and parentage was screened by high resolution melt (HRM) analysis of PCR products from known genes in the stomatal formation pathway, following bisulfite conversion of sample DNA. Full lengths and upstream of target genes were analysed for differential methylation by capturing the methylated portion of the sample genome and performing qPCR of resulting DNA for 300 bp fragments of the genes of interest. Single base-resolution methylation profiles were confirmed by bisulfite sequencing of ≧32 cloned PCR fragments for target gene regions studied. SPCH and FAMA expression levels were measured in seedling RNA by multiplexed-tandem qPCR (MT-qPCR). MT-qPCR data were analysed in comparison with housekeeping genes of equal efficiencies to target genes by two standard curve analysis. Multiple siRNAs expression was analysed by in solution hybridization and RNase digestion of the enriched small RNA fraction with custom synthesized probes, followed by electrophoretic separation and quantification of the protected probes.
Seeds of Arabidopsis thaliana (L.) Heynh. ecotypes Landsberg erecta (Ler ref. NW20) and Columbia (Col-0 ref. N1092), methyltransferase mutants for MET1 ((Decreased Methylation 2DNA, met1 ref. N854300), Chromomethylase (cmt3 ref. N6365) and Domains rearranged methyltransferase 1/2 (drm1/drm2 ref. N6366) and for RNAi mutants RNA dependent rna polymerase 2 (rdr2 ref. N850602), RNA dependent rna polymerase 6 (rdr6 ref. N24285), Dicer-like 3 (dcl3 ref. N505512) and Dicer-like 4 (dc14 ref N6954) were supplied by NASC (Nottingham, UK). Seeds were sown in seedling compost (Sinclair, Lincoln, U.K.), germinated and grown in controlled environment growth cabinets (Saxcil, R. K. Saxton, Bredbury, Cheshire, U.K.) until harvest, according to ARBC guidelines except that the relative humidity of one cabinet was controlled at 45%±5 whilst the other was maintained at 65%±5. After 64 d, stage 9.70, seeds were harvested from each individual. Harvested Ler seeds, supplied Ler seeds (as before) and supplied seeds for mutants (as before) were sown, germinated and grown as before except that growth cabinets were swapped and no stratification was applied. Different (rotated) growth chambers were used in each of the 4 repeated experiments to accommodate for growth chamber effects (Sanyo Gallenkamp, Loughborough, U.K.). The complete dry biomass and seed mass of individual harvested plants were weighed and seeds counted, following threshing through a series of graded meshes, by capturing a digital image of collected seeds using an Epson Perfection 3170 scanner (Epson (U.K.), Hemel Hempstead, U.K.) then particle analysis using ImageJ software version 1.37 (freeware NIH, USA).
Stomatal density (stomata mm−2) and index (percentage of epidermal cells forming stomata) were determined by making impressions of the entire abaxial surface of one mature rosette leaf (insertion 6-8, length approximately 40 mm) and one cauline leaf (insertion 13-15, length approximately 15 mm) from 48 plants (each of 16 replicate plants from each of 3 individual parents in the treatment*parent experiments) at the same physiological stage (6.50) in each repeated experiment. Digital images were then captured from an Axioscope 2 microscope with an Axiocam camera attached (Carl Zeiss Ltd), using Axio Vision 3.1 (Image Associates, Oxfordshire, UK) software and the number of stomata and other epidermal cells per unit area counted using ImageJ software (as before). Gas exchange (stomatal conductance to water vapour and instantaneous leaf-level water use efficiency) was measured using the Lcpro+ infra-red gas analyzer with Arabidopsis leaf chamber (ADC BioScientific, Great Amwell, U.K.) in 6 replicate plants pre-conditioned in ambient RH in the dark for 12 hr.
Whole seedlings (first true leaf stage), mature and immature leaves from ≧12 replicate plants were snap frozen in liquid nitrogen and stored at −80° C. DNA was extracted using the Dneasy plant mini-kit (Qiagen, U.K.) according to the manufacturer's instructions. 2 μg genomic DNA was then modified by bisulfite treatment using the EZ DNA methylation kit (Zymo Research, Orange, Calif.) according to the manufacturer's instructions. Desulphonated DNA was diluted 1 in 5. High resolution melt (HRM) analysis was used to analyse differential methylation with treatment as in Wojdacz, T. K. & Dobrovic, A. Methylation-sensitive high resolution melting (MS-HRM): a new approach for sensitive and high-throughput assessment of methylation. Nucleic Acids Res. 35, No. 6 e41 (the content of which is incorporated herein by reference in its entirety) except that each 20 μl reaction mix contained 1× Biomix (Bioline, London, U.K.), 25 μM Syto9 dye (Invitrogen, Carlsbad, Calif.) and 300 nM each forward and reverse bisulfite-specific primer for the gene of interest. PCR amplification conditions used were: 2 min at 95° C., then 50 cycles of 95° C. for 15 s and 50° C. for 30s, 60° C. hold for 1 min and HRM from 58-80° C. at 0.5° C. s−1. For each gene, untreated genomic DNA (diluted 1 in 1000) was included as a positive control using the equivalent (but not bisulfite-specific) primer. Differential methylation with treatment was identified using the RotorGene™ 6000 Series Software version 1.7 (Corbett Research UK Ltd., Cambs., U.K.) at an 80% confidence level. Assays were repeated 6-8 times for genes putatively identified as differentially methylated.
Positive results indicating differential methylation in the SPCH and FAMA genes were validated by capturing the methylated portion of genomic DNA using the Methylamp Methylated DNA Capture kit (Epigentek, Cambridge Bioscience, Cambridge, U.K.) and performing comparative qPCR analysis using negative controls provided in the kit (Ig mouse antibody). Subsequently, primers were designed to target every 300 bp of the coding regions, for 600 bp of (5′) upstream regions of the SPCH and FAMA genes and for the 2.3 kb upstream genomic region of SPCH. qPCR and HRM conditions were as described above except that 15 ng of template DNA were used, Ta was 56° C. and an extension phase of 66° C. for 6 min replaced the 1 min hold; HRM was from 68-90° C.
Base-pair resolution methylation profiles were obtained by sequencing ≧32 cloned amplicons (vector pCR2.1; Invitrogen, Carlsbad, Calif.) per sample of three, pooled replicate plants (Geneservice, Source Bioscience PLC, Nottingham, U.K.) following bisulfite treatment and PCR, as described above except that 5 nM labelled, synthetic DNA with methylated and unmethylated cytosines for each PCR product (Sigma-Aldrich Ltd., Gillingham, U.K.) was added to the 2 μg sample DNA prior to bisulfite treatment as a positive control for complete bisulfite conversion. Differential methylation was assessed with reference to the unmodified genomic DNA sequence and comparison of cytosine to thymine conversion between treatments. Sequences were aligned using ClustalW2 and predictability calculated as the inverse of entropy using BioEdit v. 7.0.9.0.
Following each round of FIRM, qPCR and PCR, a sample of products was analysed for size accuracy and purity using the Agilent Bioanalyzer Series II DNA 1000 chip (Agilent, Winnersh, U.K.).
Total RNA was isolated from frozen leaf material using the RNeasy Plant Mini kit (Qiagen, U.K.) according to the manufacturer's instructions. Primers for Multiplexed Tandem PCR (MT-PCR) were designed for the target genes SPCH and FAMA and for the internal control genes PP2A and SAND. MT-PCR was performed as in Stanley, K.K. & Szewczuk, E. Multiplexed tandem PCR: gene profiling from small amounts of RNA using SYBR green detection. Nucleic Acids Res. 33, 20 e180 (2005) (the content of which is incorporated herein by reference in its entirety) using 500 ng starting RNA, except that Sensimix (Quantace, London, U.K.) reverse transcriptase and buffer were used, and reverse transcription was executed at 45° C. for 15 min followed by 70° C. for 15 min. First round multiplexed amplification was carried out in the ABI9700 thermal cycler using Sybr Premix Ex Taq polymerase (Takara Bio Europe, Saint-Germain-en-Laye, France) and final volumes of 200 nM for each primer. PCR was performed under the following conditions: 1 min at 95° C., 10-15 cycles of 95° C. for 15 s, 58° C. for 20 s and 72° C. for 15 s then 72° C. for 7 min. Pre-amplification products were diluted 1:1 and second-round PCRs prepared with Sybr Premix Ex Taq (as before) and internal primers and 1 μA template cDNA. qPCR was carried out in the RotorGene™ 6000 thermal cycler (Corbett Research UK Ltd., Cambs., U.K.) using the following conditions: 95° C. for 1 min, then 40 cycles of 95° C. for 10 s, 60° C. for 20 s and 72° C. for 8 s and HRM from 70-96° C. at 0.5° C. s−1. All reactions were prepared in triplicate and serial dilutions completed for genes of interest and controls. RotorGene™ 6000 Series software version 1.7 was used to determine gene amplification efficiencies and RNA quantification (as before) employing the two standard curve method.
Following each round of PCR and qPCR, a sample of products was analysed for size accuracy and purity using the Agilent Bioanalyzer DNA 1000 chip and kit (as before).
Methods—siRNA Analyses
Total RNA was isolated from seedling samples using the mirVana miRNA isolation kit (Ambion, Warrington, U.K.) according to the manufacturer's instructions, checked and quantified using the Agilent Bioanalyzer RNA 6000 Nano and Small RNA chips and kits (Agilent, Winnersh, U.K.) against small dsRNA standards (New England Biolabs, Hitchin, U.K.). 200 ng total RNA from each sample was enriched for the small RNA fraction using the isolation kit (as before). Unlabelled antisense RNA probes of differing nt lengths were designed and constructed using the mirVana probe construction kit (Ambion, Warrington, U.K.) for SPCH, FAMA and local smRNAs; <four probes were detected in each reaction using the mirVana detection kit (Ambion, Warrington, U.K.) according to the manufacturer's instructions. Probes were post-labelled and visualised fluorescently using the Agilent Bioanalyzer Small RNA chip (as before) and small dsRNA standards ladder (as before).
Primer designs for DNA methylation, RNA and siRNA analyses are included as Tables 1, and 3-5. All primers were designed using Primer3 software; bisulfate-specific primers were based on the returned, bisulfite-specific sequence from MethPrimer software.
When parental plants are grown under control relative humidity, the imposition of a short period of drought caused a marked and significant reduction in chlorophyll content (
Seeds of Arabidopsis thaliana (L.) Heynh. ecotypes Landsberg erecta (Ler ref. NW20), Chromomethylase (cmt3 ref. N6365) and Domains rearranged methyltransferase 1/2 (drm1/drm2 ref. N6366) were supplied by NASC (Nottingham, UK). Seeds were sown in seedling compost (Sinclair, Lincoln, U.K.), germinated and grown in controlled environment growth cabinets (Saxcil, R. K. Saxton, Bredbury, Cheshire, U.K.) until harvest, according to ARBC guidelines except that the relative humidity of one cabinet was controlled at 45%±5 whilst the other was maintained at 65%±5. Harvested Ler seeds, supplied Ler seeds (as before) and supplied seeds for mutants (as before) were sown, germinated and grown under control RH before except that after 40 d half of the plants from each genotype were subjected to a drought treatment by withholding watering for 4 d. After this treatment, watering was restarted until seeds were harvested from each individual on day 64 (stage 9.70). Growth cabinets were swapped and no stratification was applied. Different (rotated) growth chambers were used in each of the 4 repeated experiments to accommodate for growth chamber effects (Sanyo Gallenkamp, Loughborough, U.K.). Each time, stomatal density (stomata mm−2) and index (percentage of epidermal cells forming stomata) were assessed at the same stage of growth by microscopic examination of impressions of the abaxial leaf surface (as described above). The complete dry biomass and seed mass of individual harvested plants were weighed and seeds counted, following threshing through a series of graded meshes, by capturing a digital image of collected seeds using an Epson Perfection 3170 scanner (Epson (U.K.), Hemel Hempstead, U.K.) then particle analysis using ImageJ software version 1.37 (freeware NIH, USA).
It has been found that Arabidopsis plants when innoculated with a pathogenic strain of Botrytis cynerea, respond in the short term by activating the expression of specific disease resistance genes. At the same time the global methylation pattern of the plant genome also changes. Quite remarkably, however, seeds collected from these plants present a higher resistance to the pathogen when innoculated. The plants used are so inbred that the offspring are to all intents and purposes genetically identical to the parent.
Second generation wild type plants were sown to compare whether any changes at methylation level are transmitted to next generation. Morphological data revealed existence of a transgenerational acquired increased resistance to Botrytis cynerea on the wild type Langsberg erecta genotype while none of the methylation mutants showed such increase in resistance (
Analysis of global methylation changes induced by infection with Botrytis cynerea using MSAP (with the enzyme combination MspI/EcoRI that is sensitive to methylation on the CpHpG motif) did not show significant differences between infected and non infected plants (
Seeds of Arabidopsis thaliana (L.) Heynh. ecotypes Landsberg erecta (Ler ref. NW20) and mutants Chromatin-remodeling ATPase (CHR1 ref. N30937) Chromomethylase (cmt3 ref N6365) and Domains rearranged methyltransferase 1/2 (drm1/drm2 ref. N6366) and Kryptonite-2 (KYP-2 ref. N6367) were acquired from the European Arabidopsis Stock Centre.
Plants were grown in seedling compost (Sinclair, Lincoln, U.K.) in 24 cell trays with 1 plant in each 4 cm×4 cm cell, germinated and grown in controlled environment growth cabinets (Saxcil, R. K. Saxton, Bredbury, Cheshire, U.K.) until harvest, according to ARBC guidelines. One cell was removed to allow for bottom watering. Seeds were germinated at 4° C. and grown for 1 week under glass before being transferred to experimental conditions in a controlled-environment growth room. The plants were grown at 22° C. under an 8 hour photoperiod (approx. 70 μmol/m2/s) to inhibit flowering. After 64 d, stage 9.70, seeds were harvested from each individual. The complete dry biomass and seed mass of individual harvested plants were weighed and seeds counted, following threshing through a series of graded meshes, by capturing a digital image of collected seeds using an Epson Perfection 3170 scanner (Epson (U.K.), Hemel Hempstead, U.K.) then particle analysis using ImageJ software version 1.37 (freeware NIH, USA).
To homogenize and standardize the level of methylation across the plant material a Generation 0 (Five plants per genotype) was grown in standard conditions: 24° C. short days (8 h light/16 h darkness), under light intensity of 100 mol m−2 s−1. It allowed excluding any possible epigenetic variation which could exist due to variable seed storage conditions. Seeds obtained from each single plant of each genotype of Generation 0 were used in the subsequent part experiment—growing Generation one (G1). Seeds were collected from a single individual to insure the maximum level of genetic homogeneity across the plant material. Harvested Ler seeds, supplied Ler seeds and harvested mutant seeds supplied seeds for mutants. Seeds were sown, germinated and grown as before except that growth cabinets were swapped.
Four trays were planted (92 plants). Plant trays were randomly assigned for two different treatments (innoculation with Botrytis cynerea and control). Plants were inoculated with the necrotrophic gray mold fungus Botrytis cynerea (strain iMi 169558, International Mycological Institute, Kew, U.K.) five weeks after germination. Plants were treated with 1×105 spores mL−1 suspension, by placing 2 droplets directly on the upper side of leaf number five (in order to ensure that they were at the same developmental stage) using a pipette. Seven days after inoculation, leaf six was sampled from half of the plants from each treatment and sampled plants were discarded. Seeds were collected from five of the remaining individuals and pooled to obtain a significant representation of the epigenetic variability induced by the treatments. Harvested Ler seeds, supplied Ler seeds and harvested mutant seeds supplied seeds for mutants. Seeds were sown, germinated and grown as before except that growth cabinets were swapped in the subsequent part experiment—growing Generation one (G2).
Eight trays were planted (184 plants). Plant trays were randomly assigned for two different treatments (innoculation with Botrytis cynerea and control) as described above. Plants arising from seeds obtained from treated and untreated plants were inoculated again (see Table 6) as described above. Five days after inoculation pictures were taken from the whole plant and inoculated leaves for each of the genotype-G1-G2 treatment groups for documentation and image analysis. Susceptibility or resistance to fungal infection was assessed by measuring the size and intensity of the lesions resulting from Botrytis cynerea inoculation. Seven days after inoculation, leaf six was sampled from half of the plants from each treatment and sampled plants were discarded. Seeds were collected from five of the remaining individuals and pooled to obtain a significant representation of the epigenetic variability induced by the treatments. Generation 2 plants were looked at, specifically morphological changes associated with different background (G1treatment) within the same G2 treatment group.
All DNA extractions were carried out using kits from Qiagen following the manufacturer's instructions. The DNeasy® plant mini kit was used for extracting DNA from A. Thaliana samples from 23 out the 46 plants per treatment. Reagents discussed below all derive from this kit.
Approximately 100 mg of plant tissue was disrupted in liquid nitrogen in a 1.5 ml microcentrifuge tube using a pair of scissors. Immediately, without allowing the tissue to thaw, 400 μl of lysis buffer AP1 preheated to 65° C. and 4 μl of RNase A were added to each tube. The contents were mixed by inversion and incubated at 65° C. for 10 min with occasional mixing every 2-3 min.
Following this, 130 μl of AP2 buffer was added to each sample and the tubes were incubated on ice for 5 min to precipitate the proteins and polysaccharides. Tubes were then subjected to centrifugation for 5 min at 13,000 rpm to precipitate viscous lysates and other solids.
The supernatant was then transferred to a QIAshredder™ column (with silica gel matrix) and centrifuged at 13,000 rpm for 2 min to remove precipitates and cell debris. The column flow-through was collected and transferred into a fresh tube and mixed with 0.5 volumes of wash buffer and 1 volume of ethanol. This mixture was transferred into a second DNeasy mini spin column and subjected to centrifugation at 8,000 rpm for 1 min. The flow-through was discarded since DNA molecules are retained on the column. The bound DNA was washed twice by passing 500 μl of wash buffer AW through the column by centrifugation at 8,000 rpm for 1 min.
Subsequently, the membrane was dried by centrifugation at 13,000 rpm for 1 min after the addition of 100 μl of buffer AE preheated to 65° C. and incubation for 5 min at room temperature.
Aliquots of DNA (1-5 μl) were subjected to 1% (w/v) agarose gel electrophoresis to determine quality and quantity of DNA present. The gel was prepared by dissolving of appropriated quantity of agarose in the appropriate volume of 1×TAE buffer (40 mM tris-acetate, 1 mM EDTA) followed by heating in a microwave oven until all the agarose had melted. The gel solution was cooled to ˜50° C. before adding 10 mg/ml ethidium bromide solution to a final concentration of 0.35 μg/ml and then poured into a casting tray with an appropriate comb in place to create the loading well.
When set, the gel was transferred into a horizontal electrophoresis apparatus with the gel comb at the cathode end. The gel comb was removed and sufficient 1×TAE buffer was added to the electrode chamber to cover the gel by approximately 1 mm. Prepared DNA samples (5 μl DNA: 1 μl blue loading dye [0.23% (w/v) bromophenol blue, 60 mM EDTA, 40% (w/v) sucrose]) were then loaded into the gel wells. HyperLadderII (Bioline, BIO-33040) size markers were loaded into the flanking lanes. The gels were subjected to electrophoresis at constant voltages ranging from 3-5 V/cm for 15-60 min. The DNA was visualized using a UV transilluminator (320 nm wavelength).
Methylation-Sensitive Amplified fragment length polymorphism (AFLP) was performed on a randomly selected eight DNA samples per treatment and was based on the AFLP protocol described by Vos et al (1995) but using isoschizomers targeting the same recognition motif.
The basis of the technique is the detection of restricted fragments of genomic DNA through polymase chain reaction (PCR) amplification. It allows the creation of fingerprints from DNA of any origin or complexity using a limited set of generic primers and needs no prior knowledge of sequences. The use of restriction enzymes sensitive to methylation adapts this method for detection of methylation.
The DNA was restricted with 2 restriction enzymes, one rare and one common cutter sensitive to cytosine methylation. Two different restrictions were carried out with isoschizomers of the common cutter sensitive to different types of cytosine methylation. All enzymes were obtained from Fermentas, Canada.
MspI enzyme: Cuts between the two cytosines of the sequence 5′CCGG 3′ and its action is prevented by methylation on the first C but not by methylation on the second C.
HpaII enzyme: Cuts between the two cytosines of the sequence 5′CCGG 3′ and its action is prevented by methylation on the second C but not by methylation on the first C.
Adaptors specific to the restriction sites are ligated onto the DNA to allow for the amplification of fragments with generic primers and without the need for sequence information to be obtained first. All enzymes were from Fermentas and the adaptors were from Sigma-Genosys Ltd.
An adaptor mix was created by combining 1 nM of the EcoRI adaptor and 10 nM of the MspI/HpaII adaptor.
The amplification rounds were carried out using one oligonucleotide primer that corresponded to the EcoRI ends and one oligonucleotide primer that corresponded to the MspI/HpaII ends. The first round of amplification reduces the number of possible fragments by the addition of one extra base at the 3′ end of the primer, while the second round of amplification further reduces the amount of possible fragments by the addition of one or two addition bases at the 3′ end of the primer. The second round EcoRI primers were labelled 6-Fam (Carboxyfluorescein) to allow visualisation of the products.
The products of the selective amplification step were run on Applied Biosystems Genetic Analyzer. The results were visualised and interpreted using GeneMapper analysis software and exported into Microsoft Excel for further analysis.
Each band within the AFLP protocol was considered to be a single allele of a single locus. For each treatment, the allele identity for each locus was first assigned in a simple qualitative manner 1 (present) or 0 (absent) for each replicate individual. A locus was considered to differ between pairs of stress treatments or between the control and a stress treatment if the allelic profile of individuals for the locus differed by three or more individuals (e.g. 11111111 versus 11111000 would be considered to differ whereas 11111111 vs 00111111 would not). Multivariate analysis (Principal Co-ordinate analysis) was carried out using GenAlex (http://www.kovcomp.co.u/mvsp/).
As a result of the dominant nature of AFLP markers, part of the epigenetic variation between individuals is not captured in presence absence scores (for instance, because of cell type-specific methylation changes). However, these changes might contribute to meaningful variation in fragment peak intensities. Although the relationship between initial fragment copy number and peak height is not linear (for instance, because of PCR steps in the AFLP protocol) (Rodriguez Lopez et al 2004, Verhoeven et al 2009), intensity data may contain at least some biological information on epigenetic variation that can be captured using quantitative analysis (Castiglioni et al., 1999; Klahr et al., 2004). A second approach was therefore used to analyze quantitatively a smaller set of MS-AFLP markers (monomorphic in presence/absence scoring) for which fragment intensity scores were obtained using GeneMapper_software. Raw intensity scores were normalized by dividing each fragment peak height score by the total fluorescence value of all fragments obtained from each sample. This normalization accounts for overall differences in intensity scores between samples, for instance as a result of slight differences between samples in initial DNA concentrations. Normalized intensities were subjected to principal component analysis using Minitab 15 (http://www.minitab.com/en-GB/default.apsx?WT.srch=1&WT.mc_id=SE004815).
It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1009945.5 | Jun 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB11/00904 | 6/14/2011 | WO | 00 | 4/19/2013 |
