Xanthomonas oryzae pathovar oryzae (Xoo) uses transcription activator-like effectors (TALEs) to cause bacterial blight (BB) in rice. In turn, rice has evolved several mechanisms to resist BB by targeting TALEs. One mechanism involves the nucleotide-binding leucine-rich repeat (NLR) resistance gene Xa1 and TALEs. Reciprocally, Xoo has evolved TALE variants, C-terminally truncated versions (interfering TALEs or iTALEs), to overcome Xa1 resistance. However, it remains unknown to what extent the two co-adaptive mechanisms mediate Xoo–rice interactions. In this study, we cloned and characterized five additional Xa1 allelic R genes, Xa2, Xa31(t), Xa14, CGS-Xo111, and Xa45(t) from a collection of rice accessions. Sequence analysis revealed that Xa2 and Xa31(t) from different rice cultivars are identical. These genes and their predicted proteins were found to be highly conserved, forming a group of Xa1 alleles. The XA1 alleles could be distinguished by the number of C-terminal tandem repeats consisting of 93 amino acid residues and ranged from four in XA14 to seven in XA45(t). Xa1 allelic genes were identified in the 3000 rice genomes surveyed. On the other hand, iTALEs could suppress the resistance mediated by Xa1 allelic R genes, and iTALE genes were prevalent (～95%) in Asian, but not in African Xoo strains. Our findings demonstrate the prominence of a defense mechanism in which rice depends on Xa1 alleles and a counteracting mechanism in which Xoo relies on iTALEs for BB.
稻黃單胞菌（Xanthomonas oryzae）的致病變種（X.oryzae pv. oryzae, Xoo）和（X. oryzae pv. oryzicola, Xoc）分別引起水稻白葉枯病（Bacterial Blight, BB）和細(xì)菌性條斑?。˙acterial Leaf Streak, BLS）。2016年該合作研究團(tuán)隊曾在Nature Communications首次從稻黃單胞菌中發(fā)現(xiàn)了克服Xa1抗性的iTALE（interfering TAL effector），之前認(rèn)為，其是轉(zhuǎn)錄激活類因子(TALEs)在進(jìn)化重組過程中的碎片，沒有生物學(xué)功能（https://www.nature.com/articles/ncomms13435）。該研究還發(fā)現(xiàn)，水稻白葉枯病抗病基因Xa1（NLR家族），可以被任一典型結(jié)構(gòu)的TALE激活；不含有iTALE的Xoo和Xoc菌株均能激活Xa1抗性；在水稻-稻黃單胞菌無休止的軍備競賽中，絕大多數(shù)菌株（~95%）進(jìn)化產(chǎn)生了iTALE，抑制了Xa1抗性。
隨后兩個研究組發(fā)現(xiàn)，不含iTALE的Xoo菌株，還能在IRBB2 (Xa2)、扎昌龍(Xa31(t))、IRBB14(Xa14)、尼瓦拉野生稻Xa45(t)和Carolina Gold Select(CGS-Xo111)等水稻上激發(fā)過敏反應(yīng)。分離這些R基因發(fā)現(xiàn)，它們均是Xa1的等位基因型，能被任一典型結(jié)構(gòu)的TALE激活抗性（ETI）并被iTALE所抑制（ETS）。這些成員的主要差別在其羧基端由93個氨基酸單一重復(fù)單元串聯(lián)形成的LRR功能域。Xa14是該類成員中抗譜最廣和LRR重復(fù)單元數(shù)最少的成員。
據(jù)此推測，在共進(jìn)化中，病菌的iTALE是TALE的逃逸子(decoy)，避免被識別；相應(yīng)地，在水稻中理論上存在Xa1等位基因型被TALE激活的逃逸子。目前中美兩個研究組正加強(qiáng)合作，試圖發(fā)現(xiàn)水稻中Xa1等位基因型的逃逸子，以便使得這類NLR類R基因在水稻生產(chǎn)上發(fā)生廣譜抗病性的作用。
該研究還提示，種植R基因的水稻品種仍是防控BB和BLS最為經(jīng)濟(jì)、有效和安全的防治措施，但還應(yīng)監(jiān)測和預(yù)警田間病菌的毒力進(jìn)化，科學(xué)指導(dǎo)R基因的應(yīng)用和布局。