Xanthomonas campestris Employs the RND Family Pump HepABCD for Phenolic Acid Efflux and Enhancing Viability and Antioxidant Activity.

Xanthomonas campestris pv. campestris (Xcc) is the causal pathogen of black rot in cruciferous plants. Upon infection, Xcc triggers the accumulation of some phenolic acids in the host plants. The mechanism by which Xcc copes with these defense compounds remains unclear. Here, we revealed that Xcc exports benzoic acid, cinnamic acid, and their monohydroxylated derivatives through the resistance-nodulation-division family efflux pump HepABCD. This efflux system influenced glutathione and catalase levels in the wild-type strain XC1, affecting cell viability and modulating ROS levels. We identified HepR as a sensor for 4-hydroxybenzoic acid (4-HBA). The tryptophan residue W22 is critical for HepR binding to 4-HBA. Binding of 4-HBA caused HepR to dissociate from its promoter Phep and induced hepABCD expression. Additionally, HepR acts as a redox sensor, and cysteine-to-serine mutations at C39 or C77 significantly reduced its binding affinity to Phep. Collectively, these findings highlight the crucial role of phenolic acid efflux in Xcc viability and host colonization.