# Patent

## Title
Disease resistant plants

## Abstract

The present invention relates to a plant, which is resistant to a pathogen of viral, bacterial, fungal or oomycete origin, wherein the plant has an increased homoserine level as compared to a plant that is not resistant to the said pathogen, in particular organisms of the phylum Oomycota. The invention further relates to a method for obtaining a plant, which is resistant to a pathogen of viral, bacterial, fungal or oomycete origin, comprising increasing the endogenous homoserine level in the plant.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a U.S. National Phase application filed under 35 U.S.C. §371 claiming priority to PCT Application Serial No. PCT/EP06/10535, filed Nov. 1, 2006, and PCT/EP06/10535 claims priority to PCT Application Serial No. PCT/EP/011718, filed Nov. 1, 2005, and is incorporated by reference herein.
BACKGROUND
The present invention relates to disease resistant plants, in particular plants resistant to organisms of the phylum Oomycota, the oomycetes. The invention further relates to plant genes conferring disease resistance and methods of obtaining such disease resistant plants for providing protection to Oomycota pathogens.
Resistance of plants to pathogens has been extensively studied, for both pathogen specific and broad resistance. In many cases resistance is specified by dominant genes for resistance. Many of these race-specific or gene-for-gene resistance genes have been identified that mediate pathogen recognition by directly or indirectly interacting with avirulence gene products or other molecules from the pathogen. This recognition leads to the activation of a wide range of plant defence responses that arrest pathogen growth.
In plant breeding there is a constant struggle to identify new sources of mostly monogenic dominant resistance genes. In cultivars with newly introduced single resistance genes, protection from disease is often rapidly broken, because pathogens evolve and adapt at a high frequency and regain the ability to successfully infect the host plant. Therefore, the availability of new sources of disease resistance is highly needed.
Alternative resistance mechanisms act for example through the modulation of the defence response in plants, such as the resistance mediated by the recessive m/o gene in barley to the powdery mildew pathogen Blumeria graminis f. sp. hordei. Plants carrying mutated alleles of the wildtype MLO gene exhibit almost complete resistance coinciding with the abortion of attempted fungal penetration of the cell wall of single attacked epidermal cells. The wild type MLO gene thus acts as a negative regulator of the pathogen response. This is described in WO9804586.
Other examples are the recessive powdery mildew resistance genes, found in a screen for loss of susceptibility to Erysiphe cichoracearum. Three genes have been cloned so far, named PMR6, which encodes a pectate lyase-like protein, PMR4 which encodes a callose synthase, and PMR5 which encodes a protein of unknown function. Both m/o and pmr genes appear to specifically confer resistance to powdery mildew and not to oomycetes such as downy mildews.
Broad pathogen resistance, or systemic forms of resistance such as SAR, has been obtained by two main ways. The first is by mutation of negative regulators of plant defence and cell death, such as in the cpr, lsd and acd mutants of Arabidopsis. The second is by transgenic overexpression of inducers or regulators of plant defence, such as in NPR1 overexpressing plants.
The disadvantage of these known resistance mechanisms is that, besides pathogen resistance, these plants often show detectable additional and undesirable phenotypes, such as stunted growth or the spontaneous formation of cell death.
It is an object of the present invention to provide a form of resistance that is broad, durable and not associated with undesirable phenotypes.
In the research that led to the present invention, an Arabidopsis thaliana mutant screen was performed for reduced susceptibility to the downy mildew pathogen Hyaloperonospora parasitica. EMS-mutants were generated in the highly susceptible Arabidopsis line Ler eds1-2. Eight downy mildew resistant (dmr) mutants were analysed in detail, corresponding to 6 different loci. Microscopic analysis showed that in all mutants H. parasitica growth was severely reduced. Resistance of dmr3, dmr4 and dmr5 was associated with constitutive activation of plant defence. Furthermore, dmr3 and dmr4, but not dmr5, were also resistant to Pseudomonas syringae and Golovinomyces orontii. 
In contrast, enhanced activation of plant defense was not observed in the dmr1, dmr2, and dmr6 mutants. The results of this research have been described in Van Damme et al. (2005) Molecular Plant-Microbe Interactions 18(6) 583-592. This article does however not disclose the identification and characterization of the DMR genes.
SUMMARY OF THE INVENTION
According to the present invention it was now found that DMR1 is the gene encoding homoserine kinase (HSK). For Arabidopsis five different mutant dmr1 alleles have been sequenced each leading to a different amino acid change in the HSK protein. HSK is a key enzyme in the biosynthesis of the amino acids methionine, threonine and isoleucine and is therefore believed to be essential. The various dmr1 mutants show defects in HSK causing the plants to accumulate homoserine. The five different alleles show different levels of resistance that correlate to different levels of homoserine accumulation in the mutants.
The present invention thus provides a plant, which is resistant to a pathogen of viral, bacterial, fungal or oomycete origin, characterized in that the plant has an altered homoserine level as compared to a plant that is not resistant to the said pathogen.
This form of resistance is in particular effective against pathogens of the phylum Oomycota, such as Albugo, Aphanomyces, Basidiophora, Bremia, Hyaloperonospora, Pachymetra, Paraperonospora, Perofascia, Peronophythora, Peronospora, Peronosclerospora, Phytium, Phytophthora, Plasmopara, Protobremia, Pseudoperonospora, Sclerospora, Viennotia species.
The resistance is based on an altered level of homoserine in planta. More in particular, the resistance is based on an increased level of homoserine in planta. Such increased levels can be achieved in various ways.
First, homoserine can be provided by an external source. Second, the endogenous homoserine level can be increased. This can be achieved by lowering the enzymatic activity of the homoserine kinase gene which leads to a lower conversion of homoserine and thus an accumulation thereof. Alternatively, the expression of the homoserine kinase enzyme can be reduced. This also leads to a lower conversion of homoserine and thus an accumulation thereof. Another way to increase the endogenous homoserine level is by increasing its biosynthesis via the aspartate pathway. Reducing the expression of the homoserine kinase gene can in itself be achieved in various ways, either directly, such as by gene silencing, or indirectly by modifying the regulatory sequences thereof or by stimulating repression of the gene.
Modulating the HSK gene to lower its activity or expression can be achieved at various levels. First, the endogenous gene can be directly mutated. This can be achieved by means of a mutagenic treatment. Alternatively, a modified HSK gene can be brought into the plant by means of transgenic techniques or by introgression, or the expression of HSK can be reduced at the regulatory level, for example by modifying the regulatory sequences or by gene silencing.
In one embodiment of the invention, an increase (accumulation) in homoserine level in the plant is achieved by administration of homoserine to the plant. This is suitably done by treating plants with L-homoserine, e.g. by spraying or infiltrating with a homoserine solution.
Treatment of a plant with exogenous homoserine is known from WO00/70016. This publication discloses how homoserine is applied to a plant resulting in an increase in the phenol concentration in the plant. The publication does not show that plants thus treated are resistant to pathogens. In fact, WO00/70016 does not disclose nor suggest that an increase in endogenous homoserine would lead to pathogen resistance.
Alternatively, endogenous homoserine is increased by modulating plant amino acid biosynthetic or metabolic pathways.
In one embodiment, the increased endogenous production is the result of a reduced endogenous HSK gene expression thus leading to a less efficient conversion of homoserine into phospho-homoserine and the subsequent biosynthesis of methionine and threonine. This reduced expression of HSK is for example the result of a mutation in the HSK gene leading to reduced mRNA or protein stability.
In another embodiment reduced expression can be achieved by downregulation of the HSK gene expression either at the transcriptional or the translational level, e.g. by gene silencing or by mutations in the regulatory sequences that affect the expression of the HSK gene. An example of a method of achieving gene silencing is by means of RNAi.
In a further embodiment the increase in endogenous homoserine level can be obtained by inducing changes in the biosynthesis or metabolism of homoserine. In a particular embodiment this is achieved by mutations in the HSK coding sequence that result in a HSK protein with a reduced enzymatic activity thus leading to a lower conversion of homoserine into phospho-homoserine. Another embodiment is the upregulation of genes in the aspartate pathway causing a higher production and thus accumulation of L-homoserine in planta.



DESCRIPTION OF DRAWINGS
FIG. 1 shows orthologous HSK sequences that have been identified in publicly available databases and obtained by PCR amplification on cDNA and subsequent sequencing. FIG. 1 shows the alignment of the amino acid sequences of the HSK proteins of Arabidopsis thaliana and orthologs from Citrus sinensis, Populus trichocarpa (1), Populus trichocarpa (2), Solanum tuberosum (2), Vitis vinifera, Lactuca sativa, Solanum tuberosum (1), Solanum lycopersicum, Nicotiana benthamiana, Ipomoea nil, Glycine max, Phaseolus vulgaris, Cucumis sativus, Spinacia oleracea, Pinus taeda, Zea mays, and Oryza sativa using the CLUSTAL W (1.82) multiple sequence alignment programme (EBI). Below the sequences the conserved amino acids are indicated by the dots, and the identical amino acids are indicated by the asteriks. The black triangles and corresponding text indicate the amino acids that are substituted in the five Arabidopsis dmr mutants [SEQ ID NOs. 39-47, 57, 88-89, 91-98, 100, 103, 110, 112, 144-151, 155, 158, 160, 165, 167, 205-211, 218, 225, 227, 264-271, 278, 280, 287, 324-331, 335, 347-348, 360, 364, 368, 370, 373, 375-376, 378, 380-381, and 384.
Table 2 shows the Genbank accession numbers and GenInfo identifiers of the Arabidopsis HSK mRNA and orthologous sequences from other plant species.
FIG. 2 shows the percentage of conidiophore formation by two Hyaloperonospora parasitica isolates, Cala2 and Waco9, on the mutants dmr1-1, dmr1-2, dmr1-3 and dmr1-4 and the parental line, Ler eds1-2, 7 days post inoculation. The conidiophores formed on the parental line were set to 100%.
FIG. 3 is a graphic overview of the three major steps in the cloning of DMR1. a) Initial mapping of dmr1 resulted in positioning of the locus on the lower arm of chromosome 2 between positions 7.42 and 7.56 Mb. Three insert/deletion (INDEL) markers were designed (position of the markers F6P23, T23A1 and F5J6 is indicated by the black lines). These markers were used to identify recombinants from several 100 segregating F2 and F3 plants. Primer sequences of these INDEL markers and additional markers to identify the breakpoints in the collected recombinants is presented in table 3. b) One marker, At2g17270 (indicated by the grey line), showed the strongest linkage with resistance. The dmr1 locus could be further delimited to a region containing 8 genes, at2g17250-at2g17290. The eight genes were amplified and sequenced to look for mutations in the coding sequences using the primers described in table 4. DNA sequence analysis of all 8 candidate genes led to the discovery of point mutations in the At2g17265 gene in all 5 dmr1 mutants. c) Each dmr1 mutant has a point mutation at a different location in the At2g17265 gene, which encodes homoserine kinase.
FIG. 4 shows a schematic drawing of the HSK coding sequence and the positions and nucleotide substitutions of the 5 different dmr1 mutations in the HSK coding sequence (the nucleotide positions, indicated by the black triangles, are relative to the ATG start codon which start on position 1). The 5′UTR and 3′UTR are shown by light grey boxes. Below the nucleotide sequence the protein sequence is shown. The HSK protein contains a putative transit sequence for chloroplast targeting (dark grey part). The amino acid changes resulting from the 5 dmr1 mutations are indicated at their amino acid (aa) position number (black triangles) in the HSK protein.
FIG. 5 shows the position of the homoserine kinase enzyme in the aspartate pathway for the biosynthesis of the amino acids threonine, methionine and isoleucine.
FIG. 6 shows the number of conidiophores per Ler eds1-2 seedlings 5 days post inoculation with two different isolates of H. parasitica, Waco9 and Cala2. The inoculated seedlings were infiltrated with dH2O, D-homoserine (5 mM) or L-homoserine (5 mM) at 3 days post inoculation with the pathogen. Seedlings treated with L-homoserine show a complete absence of conidiophore formation and are thus resistant.
FIG. 7 shows the growth and development of H. parasitica in seedlings treated with water, D-homoserine (5 mM), or L-homoserine (5 mM) as analysed by microscopy of trypan blue stained seedlings.

    
    
        a: Conidiophore formation after HS treatment on Ler eds1-2 seedlings (10× magnification). No conidiophore formation was detected after L-homoserine infiltration, whereas control plants show abundant sporulation.
        b: Haustorial development is affected by L-homoserine (5 mM) infiltration (40× magnification), but not in plants treated with water or D-homoserine.
    
    


FIGS. 8 and 9 show the nucleotide and amino acid sequence of the homoserine kinase gene (At2g17265, NM—127281, GI:18398362) and protein (At2g17265, NP—179318, GI:15227800) of Arabidopsis thaliana, respectively [SEQ ID NOs. 99-100].
FIG. 10 shows the nucleotide and the predicted amino acid sequence of the homoserine kinase coding sequence (CDS) and protein, respectively, of Lactuca sativa [SEQ ID NOs. 101-102].
FIG. 11 shows the nucleotide and the predicted amino acid sequence of the homoserine kinase coding sequence (CDS) and protein, respectively, of Vitis vinifera [SEQ ID NOs. 103-104].
FIG. 12 shows the nucleotide and the predicted amino acid sequence of the homoserine kinase coding sequence (CDS) and protein, respectively, of Cucumis sativus [SEQ ID NOs. 105-106].
FIG. 13 shows the nucleotide and the predicted amino acid sequence of the homoserine kinase coding sequence (CDS) and protein, respectively, of Spinacia oleracea [SEQ ID NOs. 107-108].
FIG. 14 shows the nucleotide and the predicted amino acid sequence of the homoserine kinase coding sequence (CDS) and protein, respectively, of Solanum lycopersicum [SEQ ID NOs. 109-110].



DETAILED DESCRIPTION
This invention is based on research performed on resistance to Hyaloperonospora parasitica in Arabidopsis but is a general concept that can be more generally applied in plants, in particular in crop plants that are susceptible to infections with pathogens, such as Oomycota.
The invention is suitable for a large number of plant diseases caused by oomycetes such as, but not limited to, Bremia lactucae on lettuce, Peronospora farinosa on spinach, Pseudoperonospora cubensis on members of the Cucurbitaceae family, e.g. cucumber, Peronospora destructor on onion, Hyaloperonospora parasitica on members of the Brasicaceae family, e.g. cabbage, Plasmopara viticola on grape, Phytophthora infestans on tomato and potato, and Phytophthora sojae on soybean.
The homoserine level in these other plants can be increased with all techniques described above. However, when the modification of the HSK gene expression in a plant is to be achieved via genetic modification of the HSK gene or via the identification of mutations in the HSK gene, and the gene is not yet known it must first be identified. To generate pathogen-resistant plants, in particular crop plants, via genetic modification of the HSK gene or via the identification of mutations in the HSK gene, the orthologous HSK genes must be isolated from these plant species. Orthologs are defined as the genes or proteins from other organisms that have the same function.
Various methods are available for the identification of orthologous sequences in other plants.
A method for the identification of HSK orthologous sequences in a plant species, may for example comprise identification of homoserine kinase ESTs of the plant species in a database; designing primers for amplification of the complete homoserine kinase transcript or cDNA; performing amplification experiments with the primers to obtain the corresponding complete transcript or cDNA; and determining the nucleotide sequence of the transcript or cDNA.
Suitable methods for amplifying the complete transcript or cDNA in situations where only part of the coding sequence is known are the advanced PCR techniques 5′RACE, 3′RACE, TAIL-PCR, RLM-RACE and vectorette PCR.
Alternatively, if no nucleotide sequences are available for the plant species of interest, primers are designed on the HSK gene of a plant species closely related to the plant of interest, based on conserved domains as determined by multiple nucleotide sequence alignment, and used to PCR amplify the orthologous sequence. Such primers are suitably degenerate primers.
Another reliable method to assess a given sequence as being a HSK ortholog is by identification of the reciprocal best hit. A candidate orthologous HSK sequence of a given plant species is identified as the best hit from DNA databases when searching with the Arabidopsis HSK protein or DNA sequence, or that of another plant species, using a Blast programme. The obtained candidate orthologous nucleotide sequence of the given plant species is used to search for homology to all Arabidopsis proteins present in the DNA databases (e.g. at NCBI or TAIR) using the BlastX search method. If the best hit and score is to the Arabidopsis HSK protein, the given DNA sequence can be described as being an ortholog, or orthologous sequence.
HSK is encoded by a single gene in Arabidopsis and rice as deduced from the complete genome sequences that are publicly available for these plant species. In most other plant species tested so far, HSK appears to be encoded by a single gene, as determined by the analysis of mRNA sequences and EST data from public DNA databases, except for potato, tobacco and poplar for which two HSK homologs have been identified. The orthologous genes and proteins are identified in these plants by nucleotide and amino acid comparisons with the information that is present in public databases.
Alternatively, if no DNA sequences are available for the desired plant species, orthologous sequences are isolated by heterologous hybridization using DNA probes of the HSK gene of Arabidopsis or another plant or by PCR methods, making use of conserved domains in the HSK coding sequence to define the primers. For many crop species, partial HSK mRNA sequences are available that can be used to design primers to subsequently PCR amplify the complete mRNA or genomic sequences for DNA sequence analysis.
In a specific embodiment the ortholog is a gene of which the encoded protein shows at least 50% identity with the Arabidopsis HSK protein or that of other plant HSK proteins. In a more specific embodiment the homology is at least 55%, more specifically at least 60%, even more specifically at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95% or at least 99%.
After orthologous HSK sequences are identified, the complete nucleotide sequence of the regulatory and coding sequence of the gene is identified by standard molecular biological techniques. For this, genomic libraries of the plant species are screened by DNA hybridization or PCR with probes or primers derived from a known homoserine kinase gene, such as the above described probes and primers, to identify the genomic clones containing the HSK gene. Alternatively, advanced PCR methods, such as RNA Ligase Mediated RACE (RLM-RACE), can be used to directly amplify gene and cDNA sequences from genomic DNA or reverse-transcribed mRNA. DNA sequencing subsequently results in the characterization of the complete gene or coding sequence.
Once the DNA sequence of the gene is known this information is used to prepare the means to modulate the expression of the homoserine kinase gene in any one of the ways described above.
More in particular, to achieve a reduced HSK activity the expression of the HSK gene can be down-regulated or the enzymatic activity of the HSK protein can be reduced by amino acid substitutions resulting from nucleotide changes in the HSK coding sequence.
In a particular embodiment of the invention, downregulation of HSK gene expression is achieved by gene-silencing using RNAi. For this, transgenic plants are generated expressing a HSK anti-sense construct, an optimized micro-RNA construct, an inverted repeat construct, or a combined sense-anti-sense construct, so as to generate dsRNA corresponding to HSK that leads to gene silencing.
In an alternative embodiment, one or more regulators of the HSK gene are downregulated (in case of transcriptional activators) by RNAi.
In another embodiment regulators are upregulated (in case of repressor proteins) by transgenic overexpression. Overexpression is achieved in a particular embodiment by expressing repressor proteins of the HSK gene from a strong promoter, e.g. the 35S promoter that is commonly used in plant biotechnology.
The downregulation of the HSK gene can also be achieved by mutagenesis of the regulatory elements in the promoter, terminator region, or potential introns. Mutations in the HSK coding sequence in many cases lead to amino acid substitutions or premature stop codons that negatively affect the expression or activity of the encoded HSK enzyme.
These and other mutations that affect expression of HSK are induced in plants by using mutagenic chemicals such as ethyl methane sulfonate (EMS), by irradiation of plant material with gamma rays or fast neutrons, or by other means. The resulting nucleotide changes are random, but in a large collection of mutagenized plants the mutations in the HSK gene can be readily identified by using the TILLING (Targeting Induced Local Lesions IN Genomes) method (McCallum et al. (2000) Targeted screening for induced mutations. Nat. Biotechnol. 18, 455-457, and Henikoff et al. (2004) TILLING. Traditional mutagenesis meets functional genomics. Plant Physiol. 135, 630-636). The principle of this method is based on the PCR amplification of the gene of interest from genomic DNA of a large collection of mutagenized plants in the M2 generation. By DNA sequencing or by looking for point mutations using a single-strand specific nuclease, such as the CEL-I nuclease (Till et al. (2004) Mismatch cleavage by single-strand specific nucleases. Nucleic Acids Res. 32, 2632-2641) the individual plants that have a mutation in the gene of interest are identified.
By screening many plants, a large collection of mutant alleles is obtained, each giving a different effect on gene expression or enzyme activity. The gene expression or enzyme activity can be tested by analysis of HSK transcript levels (e.g. by RT-PCR), quantification of HSK protein levels with antibodies or by amino acid analysis, measuring homoserine accumulation as a result of reduced HSK activity. These methods are known to the person skilled in the art.
The skilled person can use the usual pathogen tests to see if the homoserine accumulation is sufficient to induce pathogen resistance.
Plants with the desired reduced HSK activity or expression are then back-crossed or crossed to other breeding lines to transfer only the desired new allele into the background of the crop wanted.
The invention further relates to mutated HSK genes encoding HSK proteins with a reduced enzymatic activity. In a particular embodiment, the invention relates to the dmr1 alleles dmr1-1, dmr1-2, dmr1-3, dmr1-4 and dmr1-5.
In another embodiment, the invention relates to mutated versions of the HSK genes of Lactuca sativa, Vitis vinifera, Cucumis sativus, Spinacia oleracea and Solanum lycopersicum as shown in FIGS. 10-14 [SEQ ID NOs. 101-110].
The present invention demonstrates that plants having an increased homoserine level show resistance to pathogens, in particular of oomycete origin. With this knowledge the skilled person can actively modify the HSK gene by means of mutagenesis or transgenic approaches, but also identify so far unknown natural variants in a given plant species that accumulate homoserine or that have variants of the HSK gene that lead to an increase in homoserine, and to use these natural variants according to the invention.
In the present application the terms “homoserine kinase” and “HSK” are used interchangeably.
The present invention is illustrated in the following examples that are not intended to limit the invention in any way. In the examples reference is made to the following figures.
EXAMPLES
Example 1
Characterization of the Gene Responsible for Pathogen Resistance in Dmr Mutants
Van Damme et al., 2005, supra disclose four mutants, dmr1-1, dmr1-2, dmr1-3 and dmr1-4 that are resistant to H. parasitica. The level of resistance can be examined by counting conidiophores per seedling leaf seven day post inoculation with the H. parasitica Cala2 isolate (obtainable from Dr. E. Holub (Warwick HRI, Wellesbourne, UK or Dr. G. Van den Ackerveken, Department of Biology, University of Utrecht, Utrecht, NL). For the parental line, Ler eds1-2 (Parker et al., 1996, Plant Cell 8:2033-2046), which is highly susceptible, the number of conidiophores is set at 100%. The reduction in conidiophore formation on the infected dmr1 mutants compared to seedlings of the parental line is shown in FIG. 2.
According to the invention, the gene responsible for resistance to H. parasitica in the dmr1 mutants of van Damme et al., 2005, supra has been cloned by a combination of mapping and sequencing of candidate genes.
DMR1 was isolated by map-based cloning. The dmr1 mutants were crossed to the FN2 Col-0 mutant to generate a mapping population. The FN2 mutant is susceptible to the H. parasitica isolate Cala2, due to a fast neutron mutation in the RPP2A gene (Sinapidou et al., 2004, Plant J. 38:898-909). All 5 dmr1 mutants carry single recessive mutations as the F1 plants were susceptible, and approximately a quarter of the F2 plants displayed H. parasitica resistance.
The DMR1 cloning procedure is illustrated in FIG. 3 and described in more detail below. The map location of the dmr1 locus was first determined by genotyping 48 resistant F2 plants to be located on the lower arm of chromosome 2. From an additional screen for new recombinants on 650 F2 plants  ̃90 F2 recombinant plants between two INDEL (insertion/deletion) markers on BAC T24112 at 7.2 Mb and BAC F5J6 at 7.56 Mb (according to the TIGR Arabidopsis genome release Version 5.0 of January 2004) were identified, which allowed to map the gene to a region containing a contig of 5 BACs.
The F2 plants were genotyped and the F3 generation was phenotyped in order to fine map the dmr1 locus. The dmr1 mutation could be mapped to a  ̃130 kb region (encompassing 3 overlapping BAC clones: F6P23, T23A1, and F5J6) between two INDEL markers located on BAC F6P23, at 7.42 Mb and F5J6 at 7.56 Mb (according to the TIGR Arabidopsis genome release Version 5.0 of January 2004). This resulted in an area of 30 putative gene candidates for the dmr1 locus, between
the Arabidopsis genes with the TAIR codes AT2g17060 and AT2g17380. Additionally cleaved amplified polymorphic sequences (CAPS) markers were designed based on SNPs linked to genes AT2g17190, AT2g17200, AT2g17270, At2g17300, At2g17310 and At2g17360 genes.
Analyses of 5 remaining recombinants in this region with these CAPS marker data left 8 candidate genes, At2g17230 (NM—127277, GI:30679913), At2g17240 (NM—127278, GI:30679916), At2g17250 (NM—127279, GI:22325730), At2g17260 (NM—127280, GI:30679922), At2g17265 (NM—127281, GI:18398362), At2g17270 (NM—127282, GI:30679927), At2g17280 (NM—127283, GI:42569096), At2g17290 (NM—127284, GI:30679934). Sequencing of all the 8 genes resulted in the finding of point mutations in the AT2g17265 coding gene in the five dmr1 alleles; dmr1-1, dmr1-2, dmr1-3, dmr1-4 and dmr1-5, clearly demonstrating that AT2g17265 is DMR1. FIG. 3 shows a scheme of dmr1 with point mutations of different alleles.
At2g17265 encodes the homoserine kinase (HSK) enzyme, so far the only Arabidopsis gene exhibiting this function.
In Arabidopsis, HSK is encoded by a single gene, At2g17265 (Lee & Leustek, 1999, Arch. Biochem. Biophys. 372: 135-142). HSK is the fourth enzyme in the aspartate pathway required for the biosynthesis of the amino acids methionine, threonine and isoleucine. HSK catalyzes the phosphorylation of homoserine to homoserine phosphate (FIG. 5).
Example 2
Amino Acid Analysis
Homoserine phosphate is an intermediate in the production of methionine, isoleucine and threonine in Arabidopsis. Since homoserine kinase has a key role in the production of amino acids, free amino acid levels were determined in the parental line Ler eds1-2 and the four different dmr1 mutants. For this amino acids from total leaves were extracted with 80% methanol, followed by a second extraction with 20% methanol. The combined extracts were dried and dissolved in water. After addition of the internal standard, S-amino-ethyl-cysteine (SAEC) amino acids were detected by automated ion-exchange chromatography with post column ninhydrin derivatization on a JOEL AminoTac JLC-500N (Tokyo, Japan).
Amino acid analysis of four different dmr1 mutants and the parental line, Ler eds1-2 showed an accumulation of homoserine in the dmr1 mutants, whereas this intermediate amino acid was not detectable in the parental line Ler eds1-2. There was no reduction in the level of methionine, isoleucine and threonine in the dmr1 mutants (Table 1).







TABLE 1







Concentration (in pmol/mg fresh weight) of homoserine,


methionine, threonine and isoleucine in above-ground parts


of 2-week old seedlings of the parental line Ler eds1-2


and the mutants dmr1-1, dmr1-2, dmr1-3 and dmr1-4.












Homoserine
Methionine
Isoleucine
Threonine















dmr1-1
964
29
12
264


dmr1-2
7128
14
29
368


dmr1-3
466
11
16
212


dmr1-4
6597
11
32
597


Ler eds 1-2
0
7
10
185









Due to the reduced activity of the HSK in the dmr1 mutants, homoserine accumulates. This effect could be further enhanced by a stronger influx of aspartate into the pathway leading to an even higher level of homoserine. The high concentration of the substrate homoserine would still allow sufficient phosphorylation by the mutated HSK so that the levels of methionine, isoleucine and threonine are not reduced in the dmr1 mutants and the parental line, Ler eds1-2 (Table 1).
Example 3
Pathogen Resistance is Achieved by Application of L-Homoserine
To test if the effect is specific for homoserine the stereo-isomer D-homoserine was tested. Whole seedlings were infiltrated with water, 5 mM D-homoserine and 5 mM L-homoserine. Only treatment with the natural amino acid L-homoserine resulted in resistance to H. parasitica. Seedlings treated with water or D-homoserine did not show a large reduction in pathogen growth and were susceptible to H. parasitica. The infiltration was applied to two Arabidopsis accessions, Ler eds1-2 and Ws eds1-1, susceptible to Cala2 and Waco9, respectively. Conidiophore formation was determined as an indicator for H. parsitica susceptibility. Conidiophores were counted 5 days post inoculation with H. parasitica and 2 days post infiltration with water, D-homoserine or L-homoserine. (FIG. 6). L-homoserine infiltration clearly results in reduction of conidiophore formation and H. parasitica resistance. This was further confirmed by studying pathogen growth in planta by trypan blue staining of Arabidopsis seedlings. Plants were inoculated with isolate Cala2. Two days later the plants were treated by infiltration with water, 5 mM D-homoserine, and 5 mM L-homoserine. Symptoms were scored at 5 days post inoculation and clearly showed that only the L-homoserine-infiltrated seedlings showed a strongly reduced pathogen growth and no conidiophore formation (FIG. 7).
Microscopic analysis showed that only in L-homoserine treated leaves the haustoria, feeding structures that are made by H. parasitica during the infection process, are disturbed. Again it is shown that increased levels of homoserine in planta lead to pathogen resistance.
Example 4
Identification of HSK Orthologs in Crops
Screening of Libraries on the Basis of Sequence Homology
The nucleotide and amino acid sequences of the homoserine kinase gene and protein of Arabidopsis thaliana are shown in FIGS. 8 and 9 [SEQ ID NOs. 99-100].
Public libraries of nucleotide and amino acid sequences were compared with the sequences of FIGS. 8 and 9 [SEQ ID NOs. 99-100].
This comparison resulted in identification of the complete HSK coding sequences and predicted amino acid sequences in Citrus sinensis, Populus trichocarpa (1), Populus trichocarpa (2), Solanum tuberosum (2), Solanum tuberosum (1), Nicotiana benthamiana, Ipomoea nil, Glycine max, Phaseolus vulgaris, Pinus taeda, Zea mays, and Oryza sativa. The sequence information of the orthologous proteins thus identified is given in FIG. 1. For many other plant species orthologous DNA fragments could be identified by BlastX as reciprocal best hits to the Arabidopsis or other plant HSK protein sequences.
Identification of Orthologs by Means of Heterologous Hybridisation
The HSK DNA sequence of Arabidopsis thaliana as shown in FIG. 8 [SEQ ID NO. 99] is used as a probe to search for homologous sequences by hybridization to DNA on any plant species using standard molecular biological methods. Using this method orthologous genes are detected by southern hybridization on restriction enzyme-digested DNA or by hybridization to genomic or cDNA libraries. These techniques are well known to the person skilled in the art. As an alternative probe the HSK DNA sequence of any other more closely related plant species can be used as a probe.
Identification of Orthologs by Means of PCR
For many crop species, partial HSK mRNA or gene sequences are available that are used to design primers to subsequently PCR amplify the complete cDNA or genomic sequence. When 5′ and 3′ sequences are available the missing internal sequence is PCR amplified by a HSK specific 5′ forward primer and 3′ reverse primer. In cases where only 5′, internal or 3′ sequences are available, both forward and reverse primers are designed. In combination with available plasmid polylinker primers, inserts are amplified from genomic and cDNA libraries of the plant species of interest. In a similar way, missing 5′ or 3′ sequences are amplified by advanced PCR techniques; 5′RACE, 3′ RACE, TAIL-PCR, RLM-RACE or vectorette PCR.
As an example the sequencing of the Lactuca sativa (lettuce) HSK cDNA is provided. From the Genbank EST database at NCBI several Lactuca HSK ESTs were identified using the tblastn tool starting with the Arabidopsis HSK amino acid sequence. Clustering and alignment of the ESTs resulted in a consensus sequence for a 5′ HSK fragment and one for a 3′ HSK fragment. To obtain the complete lettuce HSK cDNA the RLM-RACE kit (Ambion) was used on mRNA from lettuce seedlings. The 5′ mRNA sequence was obtained by using a primer that was designed in the 3′ HSK consensus sequence derived from ESTs (R1SIa: GCCTTCTTCACAGCATCCATTCC) [SEQ ID. NO 1] and the 5′ RACE primers from the kit. The 3′ cDNA sequence was obtained by using two primers designed on the 5′RACE fragment (Let3RACEOut: CCGTTGCGGTTAATGAGATT [SEQ ID NO. 2], and Let3RACEInn: TCGTGTTGGTGAATCCTGAA) [SEQ ID NO. 3] and the 3′ RACE primers from the kit. Based on the assembled sequence new primers were designed to amplify the complete HSK coding from cDNA to provide the nucleotide sequence and derived protein sequence as presented in FIG. 10 [SEQ ID NOs. 101-102]. A similar approach was a used for Solanum lycopersicum (FIG. 14 [SEQ ID NOs. 109-110]) and Vitis vinifera (FIG. 11 [SEQ ID NOs. 103-104]).
The complete HSK coding sequences from more than 10 different plants species have been identified from genomic and EST databases. From the alignment of the DNA sequences, conserved regions in the coding sequence were selected for the design of degenerate oligonucleotide primers (for the degenerate nucleotides the abbreviations are according to the IUB nucleotide symbols that are standard codes used by all companies synthesizing oligonucleotides; G=Guanine, A=Adenine, T=Thymine, C=Cytosine, R=A or G, Y=C or T,
M=A or C, K=G or T, S=C or G, W=A or T, B=C or G or T, D=G or A or T, H=A or C or T, V=A or C or G, N=A or C or G or T).
The procedure for obtaining internal HSK cDNA sequences of a given plant species is as follows:

    
    
        1. mRNA is isolated using standard methods,
        2. cDNA is synthesized using an oligo dT primer and standard methods,
        3. using degenerate forward and reverse oligonucleotides a PCR reaction is carried out,
        4. PCR fragments are separated by standard agarose gel electrophoresis and fragments of the expected size are isolated from the gel,
        5. isolated PCR fragments are cloned in a plasmid vector using standard methods,
        6. plasmids with correct insert sizes, as determined by PCR, are analyzed by DNA sequencing,
        7. Sequence analysis using blastX reveals which fragments contain the correct internal HSK sequences,
        8. The internal DNA sequence can then be used to design gene- and species-specific primers for 5′ and 3′ RACE to obtain the complete HSK coding sequence by RLM-RACE (as described above).
    
    


As an example the sequencing of the Cucumis sativus (cucumber) HSK cDNA is provided. For cucumber two primer combinations were successful in amplifying a stretch of internal coding sequence from cDNA; combination 1: primer F1Kom (GAYTTCYTHGGMTGYGCCGT) [SEQ ID NO. 4] and M1RC (GCRGCGATKCCRGCRCAGTT) [SEQ ID NO. 5], and combination 2: primer M1Kom (AACTGYGCYGGMATCGCYGC) [SEQ ID NO. 6] and R1Kom (CCATDCCVGGAATCAANGGVGC) [SEQ ID NO. 7]. After cloning and sequencing of the amplified fragments cucumber HSK-specific primers were designed for 5′ RACE (Cuc5RACEOut: AGAGGATTTTTACTAAGTTTATTCGTG [SEQ ID NO. 8] and Cuc5RACEInn: AGACATAATCTCCCAAGCCATCA [SEQ ID NO. 9]) and 3′ RACE (Cuc3RACEOut: TGATGGCTTGGGAGATTATGTCT [SEQ ID NO. 10] and Cuc3RACEInn: CACGAATAAACTTAGTAAAAATCCTCT [SEQ ID NO. 11). Finally the complete cucumber HSK cDNA sequence was amplified and sequenced (FIG. 12 [SEQ ID NOs. 105-106]). A similar approach was a used for spinach, Spinacia oleracea (FIG. 13 [SEQ ID NOs. 107-108]).
Orthologs identified as described in this example can be modified using well-known techniques to induce mutations that reduce the HSK expression or activity. Alternatively, the genetic information of the orthologs can be used to design vehicles for gene silencing. All these sequences are then used to transform the corresponding crop plants to obtain plants that are resistant to Oomycota.
Example 5
Reduction of Homoserine Kinase Expression in Arabidopsis by Means of RNAi
The production of HSK silenced lines has been achieved in Arabidopsis by RNAi. A construct containing two  ̃750 bp fragments of the HSK exon in opposite directions was successfully transformed into the Arabidopsis Col-0 accession. The transformants were analysed for resistance to H. parasitica, isolate Waco9. Several transgenic lines were obtained that confer resistance to H. parasitica. Analysis of HSK expression and homoserine accumulation confirm that in the transformed lines the HSK gene is silenced, resulting in resistance to H. parasitica. 
Example 6
Mutation of Seeds
Seeds of the plant species of interest are treated with a mutagen in order to introduce random point mutations in the genome. Mutated plants are grown to produce seeds and the next generation is screened for increased accumulation of homoserine. This is achieved by measuring levels of the amino acid homoserine, by monitoring the level of HSK gene expression, or by searching for missense mutations in the HSK gene by the TILLING method, by DNA sequencing, or by any other method to identify nucleotide changes.
The selected plants are homozygous or are made homozygous by selfing or inter-crossing. The selected homozygous plants with increased homoserine levels are tested for increased resistance to the pathogen of interest to confirm the increased disease resistance.
Example 7
Transfer of a Mutated Allele into the Background of a Desired Crop
Introgression of the desired mutant allele into a crop is achieved by crossing and genotypic screening of the mutant allele. This is a standard procedure in current-day marker assistant breeding of crops.







TABLE 2







GI numbers (GenInfo identifier) and Genbank accession


number for Expressed Sequence Tags (ESTs) and mRNA


sequences of the Arabidopsis HSK mRNA and orthologous


sequences from other plant species.












Common





Species
name
Detail
GI number
Genbank














Arabidopsis
Thale cress
mRNA
39104571
AK117871


thaliana


Citrus sinensis
Sweet Orange
ESTs
55935768
CV886642





28618675
CB293218





55935770
CV886643





28619455
CB293998


Glycine max
Soybean
ESTs
10846810
BF069552





17401269
BM178051





8283472
BE021031





16348965
BI974560





7285286
AW597773





58024665
CX711406





58017647
CX704389





20449357
BQ253481





16105339
BI893079





37996979
CF808568





37996460
CF808049





6072786
AW102173





26057235
CA800149





6455775
AW186458





6072724
AW102111





9203587
BE329811


Ipomoea nil
Japanese
ESTs
74407098
CJ761918



morning glory

74402449
CJ757269





74402115
CJ756935





74388670
CJ743490


Nicotiana
Tobacco
ESTs
39880685
CK295868


benthamiana


39859026
CK284950





39864851
CK287885





39864855
CK287887





39859024
CK284949





39864853
CK287886





39880683
CK295867





39864849
CK287884


Oryza sativa
Rice
mRNA
50916171
XM_468550





32970537
AK060519


Phaseolus
Common
ESTs
62708660
CV535256


vulgaris
Bean

62710636
CV537232





62708052
CV534648





62709395
CV535991





62710761
CV537357





62708535
CV535131





62708534
CV535130





62711318
CV537914





62707924
CV534520





62710733
CV537329





62709601
CV536197





62709064
CV535660





62708834
CV535430


Pinus taeda
Loblolly Pine
ESTs
70780626
DR690274





67490638
DR092267





48933532
CO162991





34354980
CF396563





67706241
DR117931





17243465
BM158115





34349136
CF390719





66981484
DR057917





48932595
CO162054





66689208
DR011702





48933450
CO162909





34350236
CF391819





67706323
DR118013





48932678
CO162137





66981399
DR057832





34354850
CF396433









Populus
Poplar
Genome v1.0, LG_IX,


trichocarpa 1

149339-148242




Expression confirmed by ESTs


Populus
Poplar
Genome v1.0, scaffold_66,


trichocarpa 2

1415935-1417032




Expression confirmed by ESTs











Solanum
Potato
ESTs
66838966
DR037071


tuberosum 1


61238361
DN588007





39804315
CK251362





39801776
CK250065





9250052
BE340521





39832341
CK275363





21917848
BQ116921





9249876
BE340345





39815050
CK258070





39804985
CK251702





39804987
CK251703





39825384
CK268406





39832342
CK275364





66838967
DR037072





9250394
BE340863





39804317
CK251363





39825385
CK268407





21375072
BQ516203


Solanum
Potato
ESTs
39813353
CK256373


tuberosum 2


39793361
CK246131





39793359
CK246130





39813352
CK256372


Zea Mays
Maize
ESTs
76021237
DT948407





76913306
DV165065





71446162
DR827212





71449720
DR830770





78117576
DV535963





91048486
EB158904





71439095
DR820145





76936546
DV174774





76012246
DT939416





78085419
DV513812





71766843
DR964780





76924795
DV170131





71449067
DR830117





91875652
EB405609





71450175
DR831225





78103551
DV521979





78090555
DV518929





78104654
DV523072





76926251
DV170768





78111568
DV529965





71773353
DR971257





71425952
DR807002





93282458
EB674722





78074199
DV502633





76293328
DV032896





78075462
DV503896





91054750
EB165168





86469295
DY235665





74243218
DT651132





74242899
DT650813





101384764
EB814428





91054750
EB165168





71440426
DR821476





78121780
DV540164





78103550
DV521978





86469294
DY235664





91877777
EB407734





67014441
CO443190





76924794
DV170130





76021236
DT948406





71446161
DR827211





78110960
DV529358





78074736
DV503170





71428043
DR809093





86469052
DY235422





71440425
DR821475





78121779
DV540163





78104653
DV523071





37400920
CF637820





78074198
DV502632





71449719
DR830769


Solanum
Tomato

58213736
BP877213


lycopersicum


7333245
AW621598





4386685
AI482761









Unigene

Sequence described in this patent


SGN-U223239

application


from Sol


Genomics


Network


Lactuca sativa
Lettuce
Sequence described in this patent




application


Vitis vinifera
Grape vine
Sequence described in this patent




application


Spinacia oleracea
Spinach
Sequence described in this patent




application


Cucumis sativus
Cucumber
Sequence described in this patent




application





A GI number (genInfo identifier, sometimes written in lower case, “gi”) is a unique integer which identifies a particular sequence. The GI number is a series of digits that are assigned consecutively to each sequence record processed by NCBI. The GI number will thus change every time the sequence changes. The NCBI assigns GI numbers to all sequences processed into Entrez, including nucleotide sequences from DDBJ/EMBL/GenBank, protein sequences from SWISS-PROT, PIR and many others. The GI number thus provides a unique sequence identifier which is independent of the database source that specifies an exact sequence. If a sequence in GenBank is modified, even by a single base pair, a new GI number is assigned to the updated sequence. The accession number stays the same. The GI number is always stable and retrievable. Thus, the reference to GI numbers in the table provides a clear and unambiguous identification of the corresponding sequence.






Table 3

Primer sequences of insertion/deletion (INDEL, size difference indicated in brackets) markers and cleaved amplified polymorphics sequences (CAP, polymorphic restriction site indicated in brackets) used in the mapping of the DMR1 locus.













TABLE 3





Primer









name: BAC




TYPE
GI number


and/or TAIR
Forward primer
SEQ ID
Reverse primer
SEQ ID
(size/
of TAIR At


At code
sequence
NO.
sequence
NO.
enzyme)
code







T24l12
AATCCAAATTTCTT
SEQ ID
AAACGAAGAGTGAC
SEQ ID
INDEL
18398180





NO. 12

NO. 13


 


(At2g16670)
GCGAGAACACA
SEQ ID
AATGGTTGGAG
SEQ ID
(33)




NO. 14

NO. 15


 


F5J6
CCGTCAGATCAGTC
SEQ ID
CAGAAGCTGATGAT
SEQ ID
INDEL
23506018,




NO. 16

NO. 17


 


(AT2g17370-
CTCATCTTGTT
SEQ ID
CGTGGAAAGTA
SEQ ID
(30)
30679966


80)

NO. 18

NO. 19


 


F6P23
CGGTTTCATGTCGA
SEQ ID
AAGAAGAGAACTGC
SEQ ID
INDEL
22325728




NO. 20

NO. 21


 


(AT2g17060)
GGAAGATCATA
SEQ ID
GTCAACCTTCC
SEQ ID
(37)




NO. 22

NO. 23


 


T23A1
TCCTTCCATGTCCG
SEQ ID
AACAAATTTGCTTC
SEQ ID
INDEL
42570808,




NO. 24

NO. 25


 


(AT2g17220-
AAACCA
SEQ ID
CAGCCTTT
SEQ ID
(26)
30679913


30)

NO. 26

NO. 27


 


AT2g17190
GAATAGAGGTTGAT
SEQ ID
CTCTTGTATGTTTT
SEQ ID
CAP
30679898




NO. 28

NO. 29


 



GGAAATCAAGA
SEQ ID
ACTGGGCTGAT
SEQ ID
(MseI)




NO. 30

NO. 31


 


AT2g17200
CCTCTCCACCCATT
SEQ ID
CGATCCATTTCGTC
SEQ ID
CAP
30679902




NO. 32

NO. 33


 



TCTAATTTCG
SEQ ID
AAGCAATCTAC
SEQ ID
(MboII)




NO. 34

NO. 35


 


AT2g17270
GATGCAGCTAAATT
SEQ ID
ACGAAAATATCAAA
SEQ ID
CAP
30679927




NO. 36

NO. 37


 



ATCAGTGTGAA
SEQ ID
AAGCTCCTTC
SEQ ID
(NlaIII)




NO. 38

NO. 39


 


AT2g17300-
AGGTAGGATGGTAT
SEQ ID
GCATGTTTTCTCTA
SEQ ID
CAP
30679937,


05

NO. 40

NO. 41


 



TATGTTTGAACT
SEQ ID
AGCGATAGAAG
SEQ ID
(EcoRI)
22325732




NO. 42

NO. 43


 


AT2g17310
ATGGGTAACGAAAG
SEQ ID
CACATGTATAAGGT
SEQ ID
CAP
42569097




NO. 44

NO. 45


 



AGAGGATTAGT
SEQ ID
CTTCCCATAGA
SEQ ID
(MseI)




NO. 46

NO. 47


 


AT2g17360
CCAACAAGTATCCT
SEQ ID
CCACATCAAACTTA
SEQ ID
CAP
30679959




NO. 48

NO. 49


 



CTTTTGTTGTT
SEQ ID
ATGAACTCCAC
SEQ ID
(MaeIII)




NO. 50

NO. 51

















TABLE 4







Primer sequences used for amplifying and sequencing of



eight candidate DMR1 genes for which the TAIR and


GI codes are indicated











Primer name
Primer sequence
SEQ ID NO.
TAIR codes
GI codes















MvD17230_F
TTCCCGAAGTGTACATTAAAAGCTC
SEQ ID NO. 52
At2g17230
30679913



 


MvD17230_R
TATGTCATCCCCAAGAGAAGAAGAC
SEQ ID NO. 53
At2g17230
30679913


 


MvD17240-F
CAATAAAAGCCTTTAAAAGCCCACT
SEQ ID NO. 54
At2g17240
30679916


 


MvD17240_R
TAGCTTCTGAAACTGTGGCATTACA
SEQ ID NO. 55
At2g17240
30679916


 


MvD17250_1F
CATGATTTGAGGGGTATATCCAAAA
SEQ ID NO. 56
At2g17250
22325730


 


MvD17250_1R
GGAGGTGGGATTTGAGATAAAACTT
SEQ ID NO. 57
At2g17250
22325730


 


MvD17250_2F
TAGCCTAGAACTCTCTGTTCGCAAG
SEQ ID NO. 58
At2g17250
22325730


 


MvD17250_2R
CATTATTTTGCGTAGTTGTGAGTGG
SEQ ID NO. 59
At2g17250
22325730


 


MvD17250_3F
CGAAGAAATCCTACAATCAACCATC
SEQ ID NO. 60
At2g17250
22325730


 


MvD17250_3R
TCTCACAATTCCCATCTCTTACTCC
SEQ ID NO. 61
At2g17250
22325730


 


MvD17260_1F
TTACTCATTTGGGTGAACAGAACAA
SEQ ID NO. 62
At2g17260
30679922


 


MvD17260_1R
ATCATCCCTAATCTCTCTGCTTCCT
SEQ ID NO. 63
At2g17260
30679922


 


MvD17260_2F
GATTAAGATACGGGGAATGGAGTCT
SEQ ID NO. 64
At2g17260
30679922


 


MvD17260_2R
ATGCAGACAAATAAGATGGCTCTTG
SEQ ID NO. 65
At2g17260
30679922


 


MvD17260_3F
GTTGTTGCTCCTGTCACAAGACTTA
SEQ ID NO. 66
At2g17260
30679922


 


MvD17260_3R
GAACAAAGACGAAGCCTTTAAACAA
SEQ ID NO. 67
At2g17260
30679922


 


MvD17265_F
GAGGACTGCATCTAGAAGACCCATA
SEQ ID NO. 68
At2g17265
18398362


 


MvD17265_R
TGGGCTCTCAACTATAAAGTTTGCT
SEQ ID NO. 69
At2g17265
18398362


 


MvD17270_F1
TAACGGTAAAGCAACGAATCTATCC
SEQ ID NO. 70
At2g17270
30679927


 


MvD17270_R1
TCAAACTGATAACGAGAGACGTTGA
SEQ ID NO. 71
At2g17270
30679927


 


MvD17270_F2
TTGCGTTCGTTTTTGAGTCTTTTAT
SEQ ID NO. 72
At2g17270
30679927


 


MvD17270_R2
AAACCAGACTCATTCCTTTGACATC
SEQ ID NO. 73
At2g17270
30679927


 


MvD17280_F1
TTTAGGATCTCTGCCTTTTCTCAAC
SEQ ID NO. 74
At2g17280
42569096


 


MvD17280_R1
GAGAAATCAATAGCGGGAAAGAGAG
SEQ ID NO. 75
At2g17280
42569096


 


MvD17280_F2
GCTTAAATAGTCCTCCTTTCCTTGC
SEQ ID NO. 76
At2g17280
42569096


 


MvD17280_R2
TCTGCTGGTTCTCATGTTGATAGAG
SEQ ID NO. 77
At2g17280
42569096


 


MvD17290_F1
CTCTCCTTCATCATTTCACAAATCC
SEQ ID NO. 78
At2g17290
30679934


 


MvD17290_R1
TTCCTCTCGCTGTAATGACCTCTAT
SEQ ID NO. 79
At2g17290
30679934


 


MvD17290_F2
TGCCACAGGTGTTGACTATGC
SEQ ID NO. 80
At2g17290
30679934


 


MvD17290_R2
TGCTCTTAAACCCGCAATCTC
SEQ ID NO. 81
At2g17290
30679934


 


MvD17290_F3
GAAGATGGCTTTAAAGGTCAGTTTGT
SEQ ID NO. 82
At2g17290
30679934


 


MvD17290_R3
AGCAACAACAACTAAAAGGTGGAAG
SEQ ID NO. 83
At2g17290
30679934

## Claims

1. An isolated lettuce plant which is resistant to Bremia Lactucae, wherein the lettuce plant has an increase endogenous L-homoserine level as compared to a lettuce plant that is not resistant to Bremia Lactucae, wherein said lettuce plant has a mutation in the homoserine kinase gene of SEQ ID NO: 101 lowering the homoserine kinase activity of SEQ ID NO: 102.

2. The plant of claim 1, wherein the mutation in the homoserine kinase gene leads to an amino acid substitution in the encoded protein.

3. A method for obtaining a lettuce plant, which is resistant to Bremia lactucae, comprising increasing the endogenous L-homoserine level in the lettuce plant by a mutation in the homoserine kinase gene of SEQ ID No. 101 lowering the homoserine kinase activity of SEQ ID No. 102 or reducing the expression of SEQ ID No. 101.

4. The method of claim 3, wherein the mutation results in one or more amino acid changes that lead to a lower homoserine kinase activity.

5. The method of claim 3, wherein the mutation is effected by mutagenic treatment of the lettuce plant.

6. The method according to claim 4, wherein the mutagenic treatment is effected with a mutagen or with radiation.