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

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