Wang Congying, Chen Shen, Feng Aiqing, Su Jing, Wang Wenjuan, Feng Jinqi, Chen Bing, Zhang Meiying, Yang Jianyuan, Zeng Liexian, Zhu Xiaoyuan
Guangdong Provincial Key Laboratory of High Technology for Plant Protection, Plant Protection Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou, 510640, China.
Rice (N Y). 2021 May 30;14(1):48. doi: 10.1186/s12284-021-00490-z.
The rice (Oryza sativa) gene Xa7 has been hypothesized to be a typical executor resistance gene against Xanthomonas oryzae pv. oryzae (Xoo), and has conferred durable resistance in the field for decades. Its identity and the molecular mechanisms underlying this resistance remain elusive.
Here, we filled in gaps of genome in Xa7 mapping locus via BAC library construction, revealing the presence of a 100-kb non-collinear sequence in the line IRBB7 compared with Nipponbare reference genomes. Complementary transformation with sequentially overlapping subclones of the BACs demonstrated that Xa7 is an orphan gene, encoding a small novel protein distinct from any other resistance proteins reported. A 27-bp effector binding element (EBE) in the Xa7 promoter is essential for AvrXa7-inducing expression model. XA7 is anchored in the endoplasmic reticulum membrane and triggers programmed cell death in rice and tobacco (Nicotiana benthamiana). The Xa7 gene is absent in most cultivars, landraces, and wild rice accessions, but highly homologs of XA7 were identified in Leersia perrieri, the nearest outgroup of the genus Oryza.
Xa7 acts as a trap to perceive AvrXa7 via EBE in its promoter, leading to the initiation of resistant reaction. Since EBE is ubiquitous in promoter of rice susceptible gene SWEET14, the elevated expression of which is conducive to the proliferation of Xoo, that lends a great benefit for the Xoo strains retaining AvrXa7. As a result, varieties harboring Xa7 would show more durable resistance in the field. Xa7 alleles analysis suggests that the discovery of new resistance genes could be extended beyond wild rice, to include wild grasses such as Leersia species.
水稻（Oryza sativa）基因Xa7被推测为抗水稻白叶枯病菌（Xanthomonas oryzae pv. oryzae，简称Xoo）的典型执行抗性基因，数十年来在田间赋予了持久抗性。其特性以及这种抗性背后的分子机制仍不清楚。
在此，我们通过构建BAC文库填补了Xa7定位位点的基因组空白，发现与日本晴参考基因组相比，IRBB7品系中存在一个100 kb的非共线序列。用BACs的连续重叠亚克隆进行互补转化表明，Xa7是一个孤儿基因，编码一种与报道的任何其他抗性蛋白都不同的新型小蛋白。Xa7启动子中的一个27 bp效应子结合元件（EBE）对于AvrXa7诱导表达模式至关重要。XA7锚定在内质网膜上，并在水稻和烟草（Nicotiana benthamiana）中触发程序性细胞死亡。大多数栽培品种、地方品种和野生稻种质中都不存在Xa7基因，但在稻属最近的外类群李氏禾（Leersia perrieri）中鉴定到了XA7的高度同源物。
Xa7通过其启动子中的EBE作为陷阱感知AvrXa7，从而引发抗性反应。由于EBE在水稻感病基因SWEET14的启动子中普遍存在，其表达升高有利于Xoo的增殖，这对保留AvrXa7的Xoo菌株非常有利。因此，携带Xa7的品种在田间将表现出更持久的抗性。Xa7等位基因分析表明，新抗性基因的发现可以扩展到野生稻之外，包括李氏禾等野生禾本科植物。
