β-1,4-Cellobiohydrolase is involved in full expression of phcA, contributing to the feedback loop in quorum sensing of Ralstonia pseudosolanacearum strain OE1-1

After infecting roots of tomato plants, the gram-negative bacterium Ralstonia pseudosolanacearum strain OE1-1 activates quorum sensing (QS) to induce production of plant cell wall-degrading enzymes, such as beta-1,4-endoglucanase (Egl) and beta-1,4-cellobiohydrolase (CbhA), via the LysR family transcriptional regulator PhcA and then invades xylem vessels to exhibit virulence. The phcA-deletion mutant (Delta phcA) exhibits neither the ability to infect xylem vessels nor virulence. Compared with strain OE1-1, the egl-deletion mutant (Delta egl) 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. The cbhA-deletion mutant (Delta cbhA) lacked the ability to infect xylem vessels and displayed loss of virulence, similar to Delta phcA, but exhibited less reduced cellulose degradation activity compared with Delta egl. Transcriptome analysis revealed that the phcA expression levels in Delta cbhA were significantly lower than in OE1-1, with significantly altered expression of more than 50% of PhcA-regulated genes. Deletion of cbhA led to a significant change in QS-dependent phenotypes, similar to the effects of phcA deletion. Complementation of Delta cbhA with native cbhA or transformation of this mutant with phcA controlled by a constitutive promoter recovered its QS-dependent phenotypes. The expression level of phcA in Delta cbhA-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 of phcA, thereby contributing to the QS feedback loop and virulence of strain OE1-1.

Automated summary

After infecting tomato plants, the bacterium Ralstonia pseudosolanacearum strain OE1-1 activates quorum sensing to induce the production of plant cell wall-degrading enzymes and invades xylem vessels to cause disease. A phcA-deletion mutant shows loss of virulence and inability to infect xylem vessels. A cbhA-deletion mutant also lacks the ability to infect xylem vessels and exhibits reduced virulence. Our study reveals that CbhA is involved in the full expression of phcA, contributing to the virulence of strain OE1-1.