Introduction: Xa23 as an executor mediates broad-spectrum resistance to Xanthomonas oryzae pv. oryzae (Xoo), which contains a matching avirulence gene avrXa23, in rice for bacterial leaf blight (BLB). avrXa23 encodes a transcription activator-like effector (TALE) protein which binds to the EBE (effector-binding element) of the Xa23 promoter. It is unclear whether the considerable pressure of Xa23 leads to an emerging Xoo strain that overcomes Xa23 resistance.
Objectives: This study aimed to uncover new Xoo isolate(s) that overcome Xa23-mediated resistance and to investigate how the pathogen evades the resistance.
Methods: Totally 185 Xoo isolates were used to screen possibly compatible strain(s) with Xa23-containing rice CBB23 by pathogenicity test. Genome Sequencing, Southern blot, tal gene cloning, Western blot, qRT-PCR and electrophoretic mobility shift assays (EMSA) were conducted to determine the mechanism of one Xoo isolate being compatible with Xa23-containing rice.
Results: One isolate AH28 from Anhui province is compatible with CBB23. AH28 strain contains an ortholog of avrXa23, tal7b and has 17 tal genes. The 4th RVD (repeat-variable diresidue) in Tal7b are missed and the 5th and 8th RVDs changed from NG and NS to NS and S*, respectively. These alternations made Tal7b unable to bind to the EBE of Xa23 promoter to activate the expression of Xa23 in rice. The ectopic expression of tal7b in a tal-free mutant PH of PXO99A did not alter the virulence of the strain PH, whereas avrXa23 made AH28 from compatibility to incompatibility with Xa23 rice.
植物病害是農(nóng)業(yè)生產(chǎn)的重要影響因素，嚴(yán)重威脅全球糧食安全，其中由稻黃單胞菌(Xoo)引起的白葉枯病是水稻生產(chǎn)中的重要致災(zāi)病害。培育和種植含有抗病基因的抗病水稻品種是生產(chǎn)上防控該病害最經(jīng)濟(jì)、有效的措施，但是隨著抗病品種的大面積推廣種植，Xoo進(jìn)化出新的毒性菌株，克服抗病品種的抗性，使得水稻白葉枯病在過去得到控制的地區(qū)“卷土重來”，日趨嚴(yán)重。Xa23是目前水稻中白葉枯病抗譜最廣的抗病基因，廣泛應(yīng)用于水稻育種中。該基因通過其啟動(dòng)子上的效應(yīng)蛋白結(jié)合元件（effector-binding element, EBE）誘捕（“trap”）Xoo菌株中的TALE類無毒效應(yīng)蛋白AvrXa23，激發(fā)效應(yīng)物誘導(dǎo)的免疫反應(yīng)（effector-triggered immunity, ETI），抑制效應(yīng)物誘導(dǎo)的感病反應(yīng)（effector-triggered susceptibility, ETS），使水稻抗病。Xoo是否進(jìn)化出能夠克服Xa23抗病基因的新型毒性菌株，目前尚不清楚。
該研究從全國各水稻種植區(qū)收集發(fā)病材料，分離得到185個(gè)Xoo菌株?？共⌒栽囼?yàn)顯示，含有Xa23抗病基因的水稻品種CBB23對其中184個(gè)菌株表現(xiàn)抗病性，僅對來自安徽的AH28菌株表現(xiàn)感病性?；蚪M測序和序列比較分析表明AH28菌株不含無毒基因avrXa23，而是含有一個(gè)與avrXa23類似的tal7b基因。基因異源表達(dá)、接種試驗(yàn)和基因表達(dá)分析結(jié)果表明tal7b基因不具有avrXa23基因的無毒基因功能，不能夠激發(fā)Xa23抗病基因的抗性。啟動(dòng)子結(jié)合試驗(yàn)也表明Tal7b蛋白不能與Xa23啟動(dòng)子的EBE結(jié)合。這些結(jié)果揭示了Xoo菌株通過突變TALE類無毒蛋白中間重復(fù)區(qū)序列這一策略來逃避抗病基因識別與結(jié)合的新機(jī)制，為水稻白葉枯病抗性喪失的治理提供了新的見解。