Yu Chao, Chen Huamin, Tian Fang, Yang Fenghuan, He Chenyang
State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, 100193, China.
BMC Microbiol. 2017 Aug 9;17(1):171. doi: 10.1186/s12866-017-1083-6.
Bacterial blight of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most important crop diseases in the world. More insights into the mechanistic regulation of bacterial pathogenesis will help us identify novel molecular targets for developing effective disease control strategies. A large flagellar gene cluster is regulated under a three-tiered hierarchy by σ factor RpoN2 and its activator FleQ, and σ factor FliA. A hypothetical protein gene fliTX is located upstream of rpoN2, however, how it is regulated and how it is related to bacterial behaviors remain to be elucidated.
Sequence alignment analysis indicated that FliTX in Xoo is less well conserved compared with FliT proteins in Escherichia coli, Salmonella typhimurium, and Pseudomonas fluorescens. Co-transcription of fliTX with a cytosolic chaperone gene fliS and an atypical PilZ-domain gene flgZ in an operon was up-regulated by RpoN2/FleQ and FliA. Significantly shorter filament length and impaired swimming motility were observed in ∆fliTX compared with those in the wildtype strain. ∆fliTX also demonstrated reduced disease lesion length and in planta growth in rice, attenuated ability of induction of hypersensitive response (HR) in nonhost tobacco, and down-regulation of type III secretion system (T3SS)-related genes. In trans expression of fliTX gene in ∆fliTX restored these phenotypes to near wild-type levels.
This study demonstrates that RpoN2- and FliA-regulated fliTX is indispensible for flagellar motility and virulence and provides more insights into mechanistic regulation of T3SS expression in Xoo.
由稻黄单胞菌稻致病变种（Xoo）引起的水稻白叶枯病是世界上最重要的作物病害之一。深入了解细菌致病机制的调控将有助于我们确定开发有效病害控制策略的新分子靶点。一个大的鞭毛基因簇受σ因子RpoN2及其激活因子FleQ和σ因子FliA的三级调控。然而，一个假定的蛋白质基因fliTX位于rpoN2上游，其调控方式以及与细菌行为的关系仍有待阐明。
序列比对分析表明，与大肠杆菌、鼠伤寒沙门氏菌和荧光假单胞菌中的FliT蛋白相比，Xoo中的FliTX保守性较差。在一个操纵子中，fliTX与胞质伴侣基因fliS和一个非典型PilZ结构域基因flgZ的共转录受RpoN2/FleQ和FliA上调。与野生型菌株相比，∆fliTX中观察到明显更短的丝状体长度和受损的游动能力。∆fliTX在水稻中还表现出病斑长度缩短和体内生长受抑制，在非寄主烟草中诱导过敏反应（HR）的能力减弱，以及III型分泌系统（T3SS）相关基因下调。在∆fliTX中对fliTX基因进行反式表达可将这些表型恢复到接近野生型水平。
本研究表明，RpoN2和FliA调控的fliTX对鞭毛运动性和毒力不可或缺，并为Xoo中T3SS表达的机制调控提供了更多见解。
