Xanthomonas oryzae pv. oryzae is the causal agent of rice bacterial blight, a destructive rice disease worldwide. The gene xa5 provides race-specific resistance to X. oryzae pv. oryzae, and encodes the small subunit of transcription factor IIA. How xa5 functions in bacterial blight resistance is not well understood, and its recessive gene action is disputed. Here we show that xa5 is inherited in a completely recessive manner and the susceptible allele Xa5 is fully dominant. In accordance with this, bacterial growth in heterozygous and homozygous susceptible lines is not significantly different. Further, one allele of Xa5 is sufficient to promote disease in previously resistant plants; additional copies are not predictive of increased lesion length. Surprisingly, a resistant nearly isogenic line (NIL) of an indica variety sustains high levels of bacterial populations compared to the susceptible NIL, yet the resistant plants restrict symptom expression. In contrast, in japonica NILs, bacterial population dynamics differ in resistant and susceptible genotypes. However, both resistant indica and japonica plants delay bacterial movement down the leaf. These results support a model in which xa5-mediated recessive resistance is the result of restricted bacterial movement, but not restricted multiplication.
由水稻黃單胞菌引起的白葉枯病是世界上對(duì)水稻生長(zhǎng)最具破壞性的病害。xa5是白葉枯病的小種專化性抗病基因，編碼轉(zhuǎn)錄因子IIA的小亞基。xa5在抗白葉枯病中的功能并不清楚，同時(shí)其顯隱性還有爭(zhēng)議。本文我們的研究結(jié)果表明xa5是完全隱性遺傳，其感病等位基因Xa5則是完全顯性。與此一致的是白葉枯病菌在雜合和純合感病植株上的生長(zhǎng)沒(méi)有顯著差別。另外，一個(gè)Xa5的等位基因就足以使抗病品種發(fā)病，而增加一個(gè)拷貝并不能增加病斑的長(zhǎng)度。令人意外的是，秈稻背景的抗病近等基因系比感病近等基因系植株上的病原菌數(shù)目更多，但并沒(méi)有表現(xiàn)出病斑。相反，粳稻背景的抗病和感病近等基因系的病原菌數(shù)目有很大差異。然而無(wú)論是秈稻和粳稻背景的抗病進(jìn)等基因系，病原菌的轉(zhuǎn)移都被延遲。這些結(jié)果支持了這樣的假說(shuō)：xa5介導(dǎo)的隱性抗病反應(yīng)由于抑制了病原菌的轉(zhuǎn)移而不是限制病原菌的增殖引起。