Patent Application
20080039328
A mixed population consisting of wild-type, non-transgenic plants (gray color) and transgenic plants comprising the DAAO marker (black color) is treated with either D-alanine or D-isoleucine. While the toxic effect of D-alanine on non-transgenic plants is detoxified by the transgene-mediated conversion (thereby selectively killing the wild-type plantlets), the non-toxic D-isoleucine is converted by the same enzymatic mechanism into a phytotoxic compound (thereby selectively killing the transgenic plantlets).
Wild type Arabidopsis thaliana plantlets were grown on half-concentrated Murashige-Skoog medium (0.5% (wt/vol) sucrose, 0.8% (wt/vol) agar) supplemented with the indicated D-amino acid at either 3 mM (Panel A) or 30 mM (Panel B). While D-alanine and D-serine are imposing severe phytotoxic effects even at 3 mM concentrations no significant effects can be observed for D-isoleucine.
(e-l) Photographs of dao1 transgenic line 10:7 (e-h) and wild-type plants (i-l), grown for 10 d on the highest concentrations of the D-amino acid shown in the respective graphs above. All pictures have the same magnification. FW, fresh weight.
Multiple alignment of the catalytic site of various D-amino acid oxidases allows for determination of a characteristic sequence motif [LIVM]-[LIVM]-H*-[NHA]-Y-G-x-[GSA]-[GSA]-x-G-x5-G-x-A which allows for easy identification of additional D-amino acid oxidases suitable to be employed within the method and DNA-constructs of the invention.
Agrobacterium right border
R. gracilis DAAO
Agrobacterium left border
Furthermore, important restriction sites are indicated with their respective cutting position. The GUS gene is comrpising an intron (int).
Agrobacterium right border
R. gracilis DAAO
Agrobacterium left border
Furthermore, important restriction sites are indicated with their respective cutting position. The GUS gene is comrpising an intron (int).
General methods:
The chemical synthesis of oligonucleotides can be effected for example in the known manner using the phosphoamidite method (Voet, Voet, 2nd edition, Wiley Press N.Y., pages 896-897). The cloning steps carried out for the purposes of the present invention, such as, for example, restriction cleavages, agarose gel electrophoresis, purification of DNA fragments, the transfer of nucleic acids to nitrocellulose and nylon membranes, the linkage of DNA fragments, the transformation of E. coli cells, bacterial cultures, the propagation of phages and the sequence analysis of recombinant DNA are carried out as described by Sambrook et al. (1989) Cold Spring Harbor Laboratory Press; ISBN 0-87969-309-6. Recombinant DNA molecules were sequenced using an ALF Express laser fluorescence DNA sequencer (Pharmacia, Sweden) following the method of Sanger (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467).
DNA and RNA manipulation were done using standard techniques. The yeast R. gracillis was grown in liquid culture containing 30 mM D-alanine to induce dao1, the gene encoding DAAO. Total RNA was isolated from the yeast and used for cDNA synthesis. The PCR primers
were used to amplify the dao1 gene from the cDNA template by PCR. The PCR fragments were sub-cloned into the pGEM-T Easy vector (Promega) and subsequently ligated into the BamHl site of the CaMV 35S expression cassette of the binary vector pPCV702kana17 giving pPCV702:dao1. The vectors were subjected to restriction analysis and sequencing to check that they contained the correct constructs.
A. thaliana plants (ecotype-Col-0) were grown in soil until they flowered. Agrobacterium tumefaciens (strain GV3101:pMP110 RK) transformed with the construct of interest was grown in 500 mL in liquid YEB medium (5 g/L Beef extract, 1 g/L Yeast Extract (Duchefa), 5 g/L Peptone (Duchefa), 5 g/L sucrose (Duchefa), 0,49 g/L MgSO4 (Merck)) until the culture reached an OD600 0.8-1.0. The bacterial cells were harvested by centrifugation (15 minutes, 5,000 rpm) and resuspended in 500 mL infiltration solution (5% sucrose, 0.05% SILWET L-77 [distributed by Lehle seeds, Cat. No. VIS-02]).
Flowering A. thaliana plants were then transformed by the floral dip method (Clough S J & Bent A F (1998) Plant J. 16, 735-743 (1998) with the transgenic Agrobacterium tumefaciens strain carrying the vector described above by dipping for 10-20 seconds into the Agrobacterium solution. Afterwards the plants were kept in the greenhouse until seeds could be harvested. Transgenic seeds were selected by plating surface sterilized seeds on growth medium A (4.4 g/L MS salts [Sigma-Aldrich], 0.5 g/L MES [Duchefa]; 8 g/L Plant Agar [Duchefa]) supplemented with 50 mg/L kanamycin for plants carrying the nptII resistance marker, or 0.3 to 30 mM D-amino acids (as described below) for plants comprising the dual-function marker of the invention. Surviving plants were transferred to soil and grown in the greenhouse.
Lines containing a single T-DNA insertion locus were selected by statistical analysis of T-DNA segregation in the T2 population that germinated on kanamycin or D-amino acid -containing medium. Plants with a single locus of inserted T-DNA were grown and self-fertilized. Homozygous T3 seed stocks were then identified by analyzing T-DNA segregation in T3 progenies and confirmed to be expressing the introduced gene by northern blot analyses.
Agrobacterium tumefaciens strain GV31 01 transformed with the plasmid of interest was grown in 50 mL YEB medium (see Example 4a) at 28° C. overnight. The Agrobacterium solution is mixed with liquid co-cultivation medium (double concentrated MSB5 salts (Duchefa), 30 g/L sucrose (Duchefa), 3.75 mg/l BAP (6-benzylamino purine, Duchefa), 0.5 g/l MES (Duchefa), 0.5 mg/l GA3 (Gibberellic Acid, Duchefa); pH 5.2) until OD600 of 0.5 is reached. Petiols of 4 days old seedlings of Brassica napus cv. Westar grown on growth medium B (MSB5 salts (Duchefa), 3% sucrose (Duchefa), 0.8% oxoidagar (Oxoid GmbH); pH 5,8) are cut. Petiols are dipped for 2-3 seconds in the Agrobacterium solution and afterwards put into solid medium for co-cultivation (co-cultivation medium supplemented with 1.6% Oxoidagar). The co-cultivation lasts 3 days (at 24° C. and ˜50 μMol/m2s light intensity). Afterwards petiols are transferred to co-cultivation medium supplemented with the appropriate selection agent (18 mg/L kanamycin (Duchefa) for plants comprising the nptII marker kanamycin for plants carrying the nptII resistance marker; or 0.3 to 30 mM D-amino acids, as described below) for plants comprising the dual-function marker of the invention) and 300 mg/L Timetin (Duchefa)
Transformed petioles are incubated on the selection medium for four weeks at 24° C. This step is repeated until shoots appear. Shoots are transferred to A6 medium (MS salts (Sigma Aldrich), 20 g/L sucrose, 100 mg/L myo-inositol (Duchefa), 40 mg/L adeninesulfate (Sigma Aldrich), 500 mg/L MES, 0.0025 mg/L BAP (Sigma), 5 g/L oxoidagar (Oxoid GmbH), 150 mg/l timetin (Duchefa), 0.1 mg/L IBA (indol butyric acid, Duchefa); pH 5,8) supplemented with the appropriate selection agent (18 mg/L kanamycin (Duchefa) for plants comprising the nptII marker kanamycin for plants carrying the nptII resistance marker, or 0.3 to 30 mM D-amino acids; as described below) until they elongated. Elongated shoots are cultivated in A7 medium (A6 medium without BAP) for rooting. Rooted plants are transferred to soil and grown in the greenhouse.
Seeds of certain corn inbred lines or corn hybrid lines are germinated, rooted, and further grown in greenhouses. Ears from corn plants are harvested 8 to 14 (average 10) days after pollination (DAP) and immature embryos are isolated therefrom. Timing of harvest varies depending on growth conditions and maize variety. The optimal length of immature embryos for transformation is about 1 to 1.5 mm, including the length of the scutellum. The embryo should be translucent, not opaque. The excised embryos are collected in MS based liquid medium (comprising 1.5 mg/L 2,4-D). Acetosyringone (50 to 100 μM) is added to the medium at either the same time as inoculation with Agrobacterium or right before use for Agroinfection.
Preparation of Agrobacteria: Agrobacteria are grown on YEP medium. The Agrobacterium suspension is vortexed in the above indicated medium (comprising 100 μM acetosyringone media for preferably 1-2 hours prior to infection).
Inoculation/Co-cultivation: The bacterial suspension is added to the microtube (plate) containing pre-soaked immature embryos and left at room temperature (20-25° C.) for 5 to 30 min. Excess bacterial suspension is removed and the immature embryos and bacteria in the residue medium are transferred to a Petri plate. The immature embryos are placed on the co-cultivation medium with the flat side down (scutellum upward). The plate is sealed, and incubated in the dark at 22° C. for 2-3 days. (Co-cultivation medium: MS-base, 1.5 mg/l 2,4-D, 15 μM AgNO3, 100 μM acetosyringone). Alternatively, excised immature embryos are directly put on the co-cultivation medium with the flat side down (scutellum upward). Diluted Agrobacterium cell suspension is added to each immature embryo. The plate is sealed, and incubated in the dark at 22° C. for 2-3 days.
Recovery: After co-cultivation the embryos are transferred to recovery media (MS-base comprising 1,5 mg/l 2,4-D, 150 mg/l Timentin), and incubate the plates in dark at 27° C. for about 5 to 7 days the scutellum side up.
Selection of transformed calli: The immature embryos are transferred to selection media (recovery medium further comprising the selective agent e.g., D-alanine in concentration of 0.3 to 30 mM) (scutellum up) and incubated in the dark at 27° C. for 10-14 days (First selection). All immature embryos that produce variable calli are subcultured to 2-3rd selection media. At this stage, any roots that have formed are removed. Incubation occurs for 2 weeks under the same conditions for the first selection (Second selection). The regenerable calli is excised from the scutellum (the regenerable calli is whitish in color, compact, not slimy and may have some embryo-like structures) and transferred to fresh 2-3rd selection media. Plates are wrapped and incubate in the dark at 27° C. for 2 weeks (3rd selection may not be necessary for most of the genotypes, regenerable calli can be transferred to Regeneration medium).
Regeneration of transformed plants: Proliferated calli (whitish with embryonic structures forming) are excised in the same manner as for 2nd/3rd selection and transferred to regeneration media (like selection medium but without 2,4-D). Plates are wrapped and put in the light (ca. 2,000 lux) at 25 or 27° C. for 2 weeks, or until shoot-like structures are visible. Transfer to fresh regeneration media if necessary. Calli sections with regenerated shoots or shoot-like structures are transferred to a Phytatray containing rooting medium and incubate for 2 weeks under the same condition as above step, or until rooted plantlets have developed. After 2 to 4 weeks on rooting media (half-concentrated MS medium, no 2,4-D, no selective agent), calli that still have green regions (but which have not regenerated seedlings) are transferred to fresh rooting Phytatrays. Rooted seedlings are transferred to Metromix soil in greenhouse and covered each with plastic dome for at least 1 week, until seedlings have established. When plants reach the 3-4 leaf-stages, they are fertilized with Osmocote and then sprayed with selective agent (e.g., D-alanine or D-serine), and grown in the greenhouse for another two weeks. Non-transgenic plants should develop herbicidal symptoms or die in this time. Survived plants are transplanted into 10″ pots with MetroMix and 1 teaspoon Osmocote.
T1 seeds of transgenic Arabidopsis plants were surface-sterilized and sown in Petri plates that were sealed with gas-permeable tape. The growth medium was half strength MS19 with 0.5% (wt/vol) sucrose and 0.8% (wt/vol) agar, plus 3 mM D-alanine, 3 mM D-serine or 50 μg/ml kanamycin as the selective agent. Plants were grown for 5 d after germination with a 16 h photoperiod at 24° C. To evaluate the selection efficiency on different substrates, 2,074, 1,914 and 1,810 T1 seeds were sown on D-alanine-, D-serine- and kanamycin-selective plates, respectively, and the number of surviving seedlings was counted (44, 32 and 43, respectively).
Soluble proteins were extracted by shaking 0.1 g samples of plant material that had been finely pulverized in a 1.5 ml Eppendorf tube in 1 ml of 0.1 M potassium phosphate buffer, pH 8. DAAO activity was then assayed as follows. Reaction mixtures were prepared containing 2,120 μl of 0.1 M potassium phosphate buffer, pH 8, 80 μl of crude protein extract and 100 μl of 0.3 M D-alanine. The samples were incubated for 2 h at 30° C. The enzyme activity was then assessed, by measuring the increase in absorbance at 220 nm (E=1.090 M−1 cm−1) associated with the conversion of D-alanine to pyruvate, after transferring the test tubes to boiling water for 10 to stop the reaction. In control reactions, D-alanine was added immediately before boiling. One unit of DAAO activity is defined as the turnover of one micromole of substrate per minute, and activity was expressed per gram plant biomass (fresh weight). The breakdown of D-isoleucine and D-valine in DAAO incubations, and the associated production of 3-methyl-2-oxopentanoate and 3-methyl-2-oxobutanoate, were analyzed by high-performance liquid chromatography. In other respects the reactions were carried out as described above.
The qualification of the DAAO enzyme as a dual-function selection marker was demonstrated by testing germinated T1 seeds on different selective media. The T-DNA contained both 35S:dao1 and pNos:nptII, allowing D-amino acid and kanamycin selection to be compared in the same lot of seeds.
T1 seeds were sown on medium containing kanamycin (50 μg/ml), D-alanine (3 mM) or D-serine (3 mM), and the transformation frequencies found on the different selective media were 2.37%, 2.12% and 1.67%, respectively. D-alanine had no negative effect on the transgenic plants, even at a concentration of 30 mM, but at this concentration, D-serine induced significant growth inhibition. Fewer transgenic plants were found after selection on 3 mM D-serine because the compound slightly inhibited the growth of the transgenic plants at this concentration.
Further studies using lower concentrations corroborated this conclusion, and efficient selection using D-serine was achieved on concentrations lower than 1 mM (
Selection of seedlings on media containing D-alanine or D-serine was very rapid compared to selection on kanamycin. These D-amino acids inhibited growth of wild-type plants immediately after the cotyledons of wild-type plants had emerged. Therefore, transformants could be distinguished from non-transformed plants directly after germination. The difference between wild-type and transgenic plants after D-amino acid selection was unambiguous, with no intermediate phenotypes. In contrast, intermediate phenotypes are common when kanamycin resistance is used as a selection marker. Furthermore, wild-type seedlings were found to be sensitive to sprayed applications of D-serine and D-alanine. One-week-old seedlings were effectively killed when sprayed on three consecutive days with either 50 mM. D-serine or D-alanine, although the sensitivity of wild-type plants rapidly decreased with age, presumably because as the cuticle and leaves became thicker, uptake by the leaves was reduced. Transgenic seedlings were resistant to foliar application of D-alanine or D-serine, so selection on soil was possible.
Transgenic plants grown under D-alanine and D-serine selection conditions developed normally. Early development of transgenic plants from line 3:7, 10:7 and 13:4 was compared with that of wild-type plants by cultivation on vertical agar plates. No differences in biomass, number of leaves, root length or root architecture were detected for the different sets of plants. Furthermore, soil-cultivated wild-type and transgenic plants (line 10:7) showed no differences in the total number of rosette leaves, number of inflorescences and number of siliqua after 4 weeks of growth.
Also, the phenotypes of 17 individual T1 lines, which were picked for T-DNA segregation, were studied and found indistinguishable from that of wild type when grown on soil. A problem sometimes encountered after selection on antibiotics is the growth lag displayed by transformants. This phenomenon is explained as an inhibitory effect of the antibiotic on the transgenic plants (Lindsey K & Gallois P (1990) J. Exp. Bot 41, 529-536). However, unlike seedlings picked from antibiotic selection plates, transgenic seedlings picked from D-amino acid selection plates and transferred to soil were not hampered in their growth and development, even temporarily. A possible reason for this difference is that the DAAO scavenging of D-amino acids may effectively remove the D-amino acid in the plants. Furthermore, D-alanine and D-serine may merely provide additional growth substrates, because their catabolic products are carbon and nitrogen compounds that are central compounds in plant metabolism. Quantification of dao1 mRNA from six independent D-alanine- and D-serine-resistant lines showed a range of different expression levels mirrored in a range of different DAAO activities. In spite of these differences in mRNA levels and enzyme activities, no phenotypic variation associated with the D-serine and D-alanine treatment was found, suggesting that the DAAO marker is effective over a range of expression levels. As described above, D-isoleucine and D-valine were found to inhibit growth of the transgenic plants, but not the wild-type plants.
Therefore, plants containing the construct were tested as described above on two sets of media, one containing D-isoleucine and the other containing D-valine at various concentrations, to assess whether DAAO could also be used as a counter-selection marker. Unambiguous counter-selection selection was achieved when seeds were sown on either D-isoleucine or D-valine at concentrations greater than 10 mM (
Thirteen individual lines expressing DAAO were tested for their response to D-isoleucine and all of them were effectively killed, whereas wild-type plants grew well, with no sign of toxicity. Similar results were obtained for D-valine, although this compound was found to have a moderately negative effect on wild-type plants at higher concentrations (
Presumably endogenous transaminase may be specific for the L-enantiomer, so the corresponding D-enantiomer is not metabolized in wildtype plants, but only in plants expressing DAAO. The negative effects of L-isoleucine (but not of the D-form) observed on wildtype plants, supports this speculation. Incubation of cell-free extracts from dao1 transgenic line 10:7 with D-isoleucine and D-valine resulted in 15-fold and 7-fold increases in production of 3-methyl-2-oxopentanoate and 3-methyl-2-oxobutanoate, respectively, compared to extracts of wild-type plants. Further, 3-methyl-2-oxopentanoate and 3- methyl-2-oxobutanoate impaired growth of A. thaliana, corroborating the suggestion that these compounds, or products of their metabolism, are responsible for the negative effects of D-isoleucine and D-valine on the transgenic plants. The toxicity of some D-amino acids on organisms is not well understood, and has only occasionally been studied in plants (Gamburg K Z & Rekoslavskaya N I (1991) Fiziologiya Rastenii 38, 1236-1246). Apart from A. thaliana, we have also tested the susceptibility of other plant species to D-serine, including poplar, tobacco, barley, maize, tomato and spruce. We found all tested species susceptible to D-serine at concentrations similar to those shown to be toxic for A. thaliana. A proposed mechanism for D-serine toxicity in bacteria is competitive inhibition of a-alanine coupling to pantoic acid, thus inhibiting formation of pantothenic acid (Cosloy S D & McFall E (1973) J. Bacteriol. 114, 685-694). It is possible to alleviate D-serine toxicity in D-serine-sensitive strains of Escherichia coli by providing pantothenic acid or a-alanine in the medium, but D-serine toxicity in A. thaliana could not be mitigated using these compounds. A second putative cause of D-amino acid toxicity is through competitive binding to tRNA. Knockout studies of the gene encoding D-Tyr-tRNATyr deacylase in E. coli have shown that the toxicity of D-tyrosine increases in the absence of deacylase activity (Soutourina J et al. (1996) J. Biol. Chem. 274, 19109-19114), indicating that D-amino acids interfere at the tRNA level. Genes similar to that encoding bacterial deacylase have also been identified. in A. thaliana (Soutourina J et al. (1996) J. Biol. Chem. 274, 19109-19114), corroborating the possibility that the mode of toxic action of D-amino acids might be through competitive binding to tRNA.
Two expression constructs are constructed for carrying out the present invention (SEQ ID NO: 15, 16). The backbone of both plasmid constructs (pSUN derivative) contains origins for the propagation in E. coli as well as in Agrobacterium and an aadA expression cassette (conferring spectinomycin and streptomycin resistance) to select for transgenic bacteria cells. The sequences for constructing the DNA constructs are amplified incorporating the appropriate restriction sites for subsequent cloning by PCR. Cloning was done by standard methods as described above. The sequence of the constructs is verified by DNA sequence analysis.
The first DNA construct (SEQ ID NO: 15) comprises an expression cassette for the D-amino acid oxidase (DAAO) from Rhodotorula gracilis under control of the Arabidopsis thaliana Nitrilase promoter (SEQ ID NO: 15; base pair 1866-3677, complementary orientation). Further comprised is an expression cassette for the β-glucuronidase which may function as a substitute for an agronomically valuable trait under control of the Arabidopsis sTPT promoter (i.e. TPT promoter truncated version, WO 03/006660; SEQ ID NO: 27 cited therein), and the CaMV 35S terminator.
The second DNA construct (SEQ ID NO: 16) comprises an expression cassette for the D-amino acid oxidase (DAAO) from Rhodotorula gracilis under control of the Pisum sativum ptxA promoter (SEQ ID NO: 16; base pair 1866-2728, complementary orientation). Further comprised is an expression cassette for the β-glucuronidase which may function as a substitute for an agronomically valuable trait under control of the Arabidopsis sTPT promoter (i.e. TPT promoter truncated version, WO 03/006660; SEQ ID NO: 27 cited therein), and the CaMV 35S terminator.
Transgenic Arabidopsis, Brassica napus, and Zea mays plants are generated as described above using either construct I (SEQ ID NO: 15) or construct II (SEQ ID NO: 16) for Agrobacterium mediated transformation. Transgenic plants are selected using the negative selection marker property of the D-amino acid oxidase on medium comprising 0.3, 3 or 30 mM D-alanine (or D-serine). Resulting transgenic plants are selfed to obtain homozygous plants. Homozygous plants are propagated over 2 to 3 generations to ensure stability of the transgenic insertion.
Seeds of transgenic plants are mixed with seeds of the corresponding non-transgenic line (used for transformation). Various proportions of transgenic versus non-transgenic seeds are used (1:1, 1:10, 1:100).
Seeds are sown on standard soil under green-house conditions. After germination, developing plantlets were sprayed at various developmental steps with preparations of D-isoleucine (final concentration of 10 mM, 20 mM, 30 mM, respectively in isotonic salt solution, pH 7.0).
None of the transgenic plants (detectable by GUS staining) is able to reach maturity under the above described conditions, while non-transgenic plants are unaffected by the treatment. Alternatively solutions of racemic D/L-isoleucine can be employed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04006377.8 | Mar 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/02735 | 3/15/2005 | WO | 00 | 9/15/2006 |
