The bacterial stringent response is a response to nutrition deprivation and other stress conditions. In Gram‐negative bacteria, this process is mediated by the small signal molecules guanosine pentaphosphate pppGpp and guanosine tetraphosphate ppGpp (collectively referred to as (p)ppGpp), and the RNA polymerase‐binding transcription factor DksA. The (p)ppGpp synthetase RelA and the bifunctional (p)ppGpp synthase/hydrolase SpoT are responsible for cellular (p)ppGpp levels. Here, we investigated the roles of DksA and (p)ppGpp in the virulence traits of Xanthomonas citri subsp. citri (Xcc), the causal agent of citrus canker. ΔdksA and (p)ppGpp‐deficient ΔspoTΔrelA strains caused reduced virulence and compromised growth in host plants, indicating that DksA and (p)ppGpp are required for full virulence of Xcc. To characterize the effect of stringent response regulators on gene expression, RNA‐seq was conducted using ΔdksA and ΔspoTΔrelA mutant strains grown in hrp‐inducing XVM2 medium. Transcriptome analyses showed that DksA and (p)ppGpp repressed the expression of genes encoding tRNAs, ribosome proteins, iron acquisition and flagellum assembly, and enhanced the expression of genes for histidine metabolism, type 3 secretion system (T3SS), type 2 secretion system (T2SS) and TonB‐dependent transporters. Phenotypically, the ΔdksA and ΔspoTΔrelA strains displayed altered motility, enhanced siderophore production and were unable to cause the hypersensitive response on non‐host plants. In conclusion, stringent response regulators DksA and (p)ppGpp play an important role in virulence, nutrition uptake and host adaptation of Xcc.

中文翻译：

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严格的响应调节剂（p）ppGpp和DksA积极调节柠檬黄单胞菌的毒力和宿主适应性。

细菌的严格反应是对营养缺乏和其他压力条件的反应。在革兰氏阴性细菌中，此过程由小信号分子五磷酸鸟苷鸟苷pppGpp和鸟苷四磷酸鸟苷ppGpp（统称为（p）ppGpp）和RNA聚合酶结合转录因子DksA介导。（p）ppGpp合成酶RelA和双功能（p）ppGpp合酶/水解酶SpoT负责细胞（p）ppGpp的水平。在这里，我们调查了DksA和（p）ppGpp在柠檬黄单胞菌亚种的毒力特性中的作用。螨（XCC），柑桔溃疡病的致病原。Δ dksA和（p）ppGpp的缺陷Δ点Δ RELA菌株在宿主植物中引起了降低的毒力并损害了其生长，表明DccA和（p）ppGpp是Xcc完全毒力所必需的。为了表征对基因表达严格应答调节的作用，RNA-SEQ使用Δ进行dksA和Δ点Δ RELA在生长的突变菌株HRP -inducing XVM2平台。转录组分析显示DksA和（p）ppGpp抑制编码tRNA，核糖体蛋白，铁获得和鞭毛装配的基因的表达，并增强用于组氨酸代谢，3型分泌系统（T3SS），2型分泌系统的基因的表达（ T2SS）和依赖TonB的转运蛋白。表型的Δ dksA和Δ现货ΔrelA菌株显示出改变的运动能力，增强的铁载体生成，并且无法在非寄主植物上引起超敏反应。总之，严格的应答调节剂DksA和（p）ppGpp在毒力，营养吸收和宿主对Xcc的适应中起着重要作用。