The two-component system CpxA/CpxR regulates pathogenesis and stress adaptability in the poplar canker bacterium Lonsdalea populi.

Bacteria employ two-component systems (TCSs) to rapidly sense and respond to their surroundings often and during plant infection. Poplar canker caused by Lonsdalea populi is an emerging woody bacterial disease that leads to high mortality and poplar plantation losses in China. Nonetheless, the information about the underlying mechanism of pathogenesis remains scarce. Therefore, in this study, we reported the role of a TCS pair CpxA/CpxR in regulating virulence and stress responses in L. populi. The CpxA/R system is essential during infection, flagellum formation, and oxidative stress response. Specifically, the Cpx system affected flagellum formation by controlling the expression of flagellum-related genes. CpxR, which was activated by phosphorylation in the presence of CpxA, participated in the transcriptional regulation of a chaperone sctU and the type III secretion system (T3SS)-related genes, thereby influencing T3SS functions during L. populi infection. Phosphorylated CpxR directly manipulated the transcription of a membrane protein-coding gene yccA and the deletion of yccA resulted in reduced virulence and increased sensitivity to H₂O₂. Furthermore, we mutated the conserved phosphorylation site of CpxR and found that CpxR^D51A could no longer bind to the yccA promoter but could still bind to the sctU promoter. Together, our findings elucidate the roles of the Cpx system in regulating virulence and reactive oxygen species resistance and provide further evidence that the TCS is crucial during infection and stress response.