Os04g0622600

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Xa-1 is one of bacterial blight resistance genes in rice. Although at least 38 resistance genes have been identified, Xa-1 is specific to X. oryzae pv. Oryzae. Xa-1 can be considered as a dominant resistance gene[1].

Annotated Information

Function

The Xa1 gene in rice confers resistance to Japanese race 1 of Xanthomonas oryzae pv. oryzae, the causal pathogen of bacterial blight (BB). We isolated the Xa1 gene by a map-based cloning strategy. The deduced amino acid sequence of the Xa1 gene product contains nucleotide binding sites (NBS) and a new type of leucine-rich repeats (LRR); thus, Xa1 is a member of the NBS-LRR class of plant disease-resistance genes, but quite different from Xa21, another BB-resistance gene isolated from rice. [2][3] Xa1 Is a Single-Copy Gene. Southern hybridization analysis was carried out by using the genomic DNAs from resistance line IR-BB1 and susceptible isogenic line IR24. The NBS region of Xa1 hybridized to a single band of DNA from IR-BB1 and IR24, suggesting that Xa1 is a single-copy gene and IR24 also has a sequence homologous to Xa1 at the same locus.[2][3]

Expression

Xa1 gene expression was induced on inoculation with a bacterial pathogen and wound, unlike other isolated resistance genes in plants, which show constitutive expression. The induced expression may be involved in enhancement of resistance against the pathogen.[2]
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So far, there is no report of induced expressed R gene. Interestingly, Xa1 mRNA was detected from rice leaves at 5 days after cutting with water and inoculation of both the compatible and incompatible strains of Xoo, but was not detected in intact leaves. These findings suggested that the Xa1 gene expression may be induced by stimulus of wounding involved in pathogen infection, and accumulation of Xa1 gene product may lead to high efficiency of interaction with avr gene product. This interaction may activate the signal transductions involved in Xa1-madiated BB resistance. Thus, it is likely that Xa1 plays an important role in pathogen recognition, which is supported by the analysis of race-specific resistance of the Xa1 transformants. Which factors regulate the expression of Xa1 remains an interesting question.[2]

Evolution

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Map-based cloning methods have been applied for isolation of Xa-1, one of the bacterial blight resistance genes in rice.Xa-1 was previously mapped on chromosome 4 using molecular markers. For positional cloning of Xa-1, a high-resolution genetic map was made for theXa-1 region using an F2population of 402 plants and additional molecular markers. Three restriction fragment length polymorphism (RFLP) markers, XNpb235, XNpb264 andC600 were found to be linked tightly to Xa-1, with no recombinants, and U08750 was mapped 1.5 cM from Xa-1. The screening of a yeast artificial chromosome (YAC) library using theseXa-1-linked RFLP markers resulted in the identification of ten contiguous YAC clones. Among these, one YAC clone, designated Y5212, with an insert of 340 kb, hybridized with all three tightly linked markers. This YAC was confirmed to possess the Xa-1 allele by mapping the Xa-1 gene between both end clones of this YAC (Y5212R and Y5212L).[1]

Based on the deduced amino acid sequence, the Xa1 gene is a class 1 resistance gene; a cytoplasmic receptor-like protein with NBS and LRR domains. Thus, the structure is quite different from the first cloned rice resistance gene, Xa21 (16), which is a class 4 gene. The most intriguing finding is that the expression of Xa1, unlike any previously studied resistance genes, is induced by pathogen infection and wound.[2]

Structure

  • The composite nucleotide sequence of the Xa1 cDNA encoded a 5,406-bp ORF that was flanked by 59 and 39 untranslated regions of 112 and 392 bp, respectively. The nucleotide sequence of the 13.5-kb EcoRI-BamHI genomic fragment was also determined and compared with that of the cDNA sequence. The Xa1 gene was composed of four exons separated by three introns.[2]
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  • The C-terminal half of Xa1 gene is composed of LRR, the common motif of the several classes of resistance genes.
  • XA1 protein contains 22 potential N-linked glycosylation sites. Six of these sites are at amino acid position 36 of each LRR repeat. These structural characteristics suggest that the Xa1 product interacts with other proteins in a defense reaction–signal transduction pathway[2].

Labs working on this gene

  • Department of Agriculture, Kyushu University, 6-10-1, Hakozaki, Higashiku, Fukuoka 812, Japan
  • Rice Genome Research Program, National Institute of Agrobiological ResourcesySociety for Techno-innovation of Agriculture, Forestry and Fisheries, Kannondai, Tsukuba, Ibaraki 305, Japan
  • National Institute of Agrobiological Resources, 1-2-1, Kannondai, Tsukuba, Ibaraki 305, Japan
  • Plant Breeding Laboratory, Department of Agriculture, Kyushu University, Hakozaki, Higashiku, Fukuoka 812-81, Japan

References

  1. 1.0 1.1 Yoshimura S, Umehara Y, Kurata N, et al. Identification of a YAC clone carrying the Xa-1 allele, a bacterial blight resistance gene in rice[J]. Theoretical and Applied Genetics, 1996, 93(1-2): 117-122.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 oshimura S, Yamanouchi U, Katayose Y, et al. Expression of Xa1, a bacterial blight-resistance gene in rice, is induced by bacterial inoculation[J]. Proceedings of the National Academy of Sciences, 1998, 95(4): 1663-1668.
  3. 3.0 3.1 Sasaki T. The rice genome project in Japan[J]. Proceedings of the National Academy of Sciences, 1998, 95(5): 2027-2028.