Ralstonia solanacearum PhcB and PhcA control a quorum-sensing (QS) system that globally regulates expression of about one third of all genes, including pathogenesis genes. The PhcB–PhcA QS system positively regulates the production of exopolysaccharide (EPS) and negatively regulates hrp gene expression, which is crucial for the type III secretion system (T3SS). Both EPS and the T3SS are essential for pathogenicity. The gene rsc2734 is located upstream of a phcBSR operon and annotated as a response regulator of a two-component system. Here, we demonstrated that RSc2734, hereafter named PrhX, positively regulated hrp gene expression via a PrhA–PrhIR–PrhJ–HrpG signalling cascade. Moreover, PrhX was crucial for R. solanacearum to invade host roots and grow in planta naturally. prhX expression was independent of the PhcB–PhcA QS system. PrhX did not affect the expression of phcB and phcA and the QS-dependent phenotypes, such as EPS production and biofilm formation. Our results provide novel insights into the complex regulatory network of the T3SS and pathogenesis in R. solanacearum.

中文翻译：

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PhcB邻近调节因子PrhX对青枯菌III型分泌系统表达及发病机制的正调控

Ralstonia solanacearum PhcB 和 PhcA 控制群体感应 (QS) 系统，该系统全局调节约三分之一的基因（包括发病基因）的表达。PhcB-PhcA QS 系统正向调节胞外多糖 (EPS) 的产生并负向调节hrp基因表达，这对于 III 型分泌系统 (T3SS) 至关重要。EPS 和 T3SS 对于致病性都是必需的。基因rsc2734位于phcBSR操纵子的上游，并被注释为双组分系统的响应调节器。在这里，我们证明 RSc2734（以下称为 PrhX）通过 PrhA-PrhIR-PrhJ-HrpG 信号级联正调节hrp基因表达。此外，PrhX对于青枯菌侵入宿主根部并在植物中自然生长至关重要。prhX表达独立于 PhcB-PhcA QS 系统。PrhX 不影响phcB和phcA的表达以及 QS 依赖性表型，例如 EPS 产生和生物膜形成。我们的结果为青枯菌T3SS 的复杂调控网络和发病机制提供了新的见解。