Xanthomonas citri subsp. citri requires a genus-specific outer membrane protein and TolB to coordinate cell membrane integrity and virulence

Xanthomonas citri subsp. citri (Xcc) possesses a Xanthomonas-specific outer membrane protein XAC1347 (OMP_Xan) that exerts a role in the expression of the type III secretion system for pathogenicity. In this study, we reported that OMP_Xan was required for salt stress tolerance and cell membrane integrity, as well as the expression of the gum genes for the production of extracellular polysaccharides. Pull-down and yeast two-hybrid assays revealed that OMP_Xan interacts with TolB, a substrate of the Tol/Pal membrane protein system. The deletion of tolB resulted in similar phenotypic alterations as the OMP_Xan mutant in salt stress tolerance, cell membrane integrity, and the expression of hrpG, hrpX, hrpD6, and hrcC for pathogenicity. In contrast, the absence of TolB resulted in an increased level of expression of the gum genes and the production of extracellular polysaccharides. These results indicate that the interaction of OMP_Xan and TolB coordinates multi-faceted mechanisms to manage environmental stress and pathogenicity.IMPORTANCEThe gram-negative Xanthomoas citri subsp. citri (Xcc) is a causal agent of citrus canker, a serious bacterial disease on citrus plants. Our previous research reported that a Xanthomonas-specific outer membrane protein XAC1347 (OMP_Xan) is necessary for type III gene expression. This manuscript provided evidence to show that OMP_Xan interacts with Tol/Pal system substrate TolB. Moreover, OMP_Xan and TolB are both required for cell membrane integrity, stress adaption, and virulence. The overall results support that OMP_Xan and TolB coordinate multi-faceted mechanisms to manage environmental stress and pathogenicity.