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 plant. R-gene specificity is dictated by both host R genes and cognate avirulence (avr) genes in pathogens. 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.
植物的抗病基因（R基因）編碼的蛋白特異識別非親和病原菌觸發(fā)一系列級聯(lián)反應(yīng)引起寄主植物的抗病性。R基因的特異性由寄主的R基因和病原菌的無毒基因（avr基因）共同決定。然而，基因?qū)虻奶禺愋缘姆肿踊A(chǔ)人們了解的并不多。本文我們報道了水稻R基因Xa27以及白葉枯病菌中對應(yīng)的avr基因avrXa27的克隆。Xa27抗病等位基因和感病等位基因編碼相同的蛋白，但是只有抗病等位基因在水稻接種了攜帶有核定位的III型效應(yīng)因子avrXa27的病原菌后才表達(dá)。Xa27只在緊鄰接種組織中被誘導(dǎo)表達(dá)，而Xa27的異位表達(dá)產(chǎn)生對其他親和病原菌菌株的抗性。因此，Xa27對非親和病原菌的特異性與avrXa27效應(yīng)因子存在的情況下R基因表達(dá)的差異有關(guān)。