AbstractAfter infecting roots of tomato plants, the gram‐negative bacteriumRalstonia pseudosolanacearumstrain OE1‐1 activates quorum sensing (QS) to induce production of plant cell wall‐degrading enzymes, such as β‐1,4‐endoglucanase (Egl) and β‐1,4‐cellobiohydrolase (CbhA), via the LysR family transcriptional regulator PhcA and then invades xylem vessels to exhibit virulence. ThephcA‐deletion mutant (ΔphcA) exhibits neither the ability to infect xylem vessels nor virulence. Compared with strain OE1‐1, theegl‐deletion mutant (Δegl) exhibits lower cellulose degradation activity, lower infectivity in xylem vessels, and reduced virulence. In this study, we analysed functions of CbhA other than cell wall degradation activity that are involved in the virulence of strain OE1‐1. ThecbhA‐deletion mutant (ΔcbhA) lacked the ability to infect xylem vessels and displayed loss of virulence, similar to ΔphcA, but exhibited less reduced cellulose degradation activity compared with Δegl. Transcriptome analysis revealed that thephcAexpression levels in ΔcbhAwere significantly lower than in OE1‐1, with significantly altered expression of more than 50% of PhcA‐regulated genes. Deletion ofcbhAled to a significant change in QS‐dependent phenotypes, similar to the effects ofphcAdeletion. Complementation of ΔcbhAwith nativecbhAor transformation of this mutant withphcAcontrolled by a constitutive promoter recovered its QS‐dependent phenotypes. The expression level ofphcAin ΔcbhA‐inoculated tomato plants was significantly lower than in strain OE1‐1‐inoculated plants. Our results collectively suggest that CbhA is involved in the full expression ofphcA, thereby contributing to the QS feedback loop and virulence of strain OE1‐1.