Disease resistance (R) genes in plants encode products that specifically recognise incompatible pathogens and trigger a cascade of events leading to disease resistance in the host plant¹. R-gene specificity is dictated by both host R genes and cognate avirulence (avr) genes in pathogens^{2, 3}. However, the basis of gene-for-gene specificity is not well understood. Here, we report the cloning of the R gene Xa27 from rice and the cognate avr gene avrXa27 from Xanthomonas oryzae pv. oryzae. Resistant and susceptible alleles of Xa27 encode identical proteins. However, expression of only the resistant allele occurs when a rice plant is challenged by bacteria harbouring avrXa27, whose product is a nuclear localized type-III effector. Induction of Xa27 occurs only in the immediate vicinity of infected tissue, whereas ectopic expression of Xa27 resulted in resistance to otherwise compatible strains of the pathogen. Thus Xa27 specificity towards incompatible pathogens involves the differential expression of the R gene in the presence of the AvrXa27 effector.

1. Temasek Life Sciences Laboratory, National University of Singapore, 1 Research Link, Singapore 117604, Singapore
2. Department of Plant Pathology, Kansas State University, Manhattan, Kansas 66506, USA
3. Institute of Molecular Agrobiology, National University of Singapore, 1 Research Link, Singapore 117604, Singapore
4. †Present addresses: Laboratory of Plant Molecular Signalling, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China (D.W.); Department of Plant Physiology, National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba, Ibaraki, 305 8602, Japan (C.S.); Institute of Molecular and Cell Biology, 61, Biopolis Drive (Proteos), Singapore 138673, Singapore (F.Y.); Genome Institute of Singapore, 60 Biopolis Street, Singapore 138672, Singapore (Z.C.); and Department of Plant Pathology, Ohio State University, Columbus, Ohio 43210, USA (G.-L.W.)
5. *These authors contributed equally to this work

Correspondence to: Zhongchao Yin¹ Correspondence and requests for materials should be addressed to Z.Y. (Email: yinzc@tll.org.sg). Sequences of O. sativa Xa27 IR24 allele (xa27), Xa27 IRBB27 allele (Xa27), Xa27 full-length cDNA and X. oryzae pv. oryzae PXO99^A avrXa27 locus are deposited in GenBank under accession numbers AY986491, AY986492, AY986493 and AY986494.

Received 2 February 2005; Accepted 6 April 2005

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