Hydroxycinnamic Acids Activate RpfB-Dependent Quorum Sensing Signal Turnover in Phytopathogen <i>Xanthomonas campestris</i>

Phytopathogen <i>Xanthomonas campestris</i> pv <i>campestris</i> (Xcc) uses various mechanisms, including the quorum sensing (QS) system, to regulate infection and colonize cruciferous host plants. Hydroxycinnamic acids (HCAs) are widely distributed in the plant kingdom, but their effect on the diffusible signaling factor family QS signal of Xcc remains unknown. In this study, we report that HCAs activate RpfB-dependent QS signal turnover via the sensor protein HepR, which in turn activates the HepABCD resistance-nodulation-division efflux pump to remove HCAs in Xcc. Exogenous addition of three HCAs induced DSF signal turnover during the late growth phase of Xcc in XYS medium. Furthermore, HCAs-induced DSF turnover was dependent on the DSF-degrading enzyme RpfB and the sensor HepR. Additionally, Xcc pumps HCAs through the HepABCD efflux pump. An electrophoretic mobility shift assay demonstrated that HCAs interfere with the binding of HepR to the P_hep promoter. This work provides insight into the molecular interactions between Xcc and cruciferous plants.