Contribution of two-component response regulator OmpR to virulence, motility, exopolysaccharide production, and osmotic stress in Pseudomonas syringae pv. actinidiae

Bacterial canker disease caused by Pseudomonas syringae pv. actinidiae ( Psa ) is the most devastating disease in kiwifruit cultivation. The EnvZ/OmpR two-component system (TCS) has been confirmed to regulate virulence and mediate environmental stress responses in Gram-negative bacteria. However, the functional role of EnvZ/OmpR in Psa has not been fully clarified. In this study, we constructed markerless ompR , envZ , and ompR-envZ mutants, and ompR complementation and overexpression strains using homologous recombination. The deletion of ompR or envZ tremendously reduced the swimming and swarming motility of Psa , as well as tolerance to osmotic stress, while overexpression of ompR impaired its virulence against kiwifruit but enhanced exopolysaccharide production. EnvZ negatively regulated hrpR/S expression in both King’s B and minimal medium, whereas OmpR regulated hrpR/S expression negatively in King’s B and positively in minimal medium. However, OmpR did not regulate the expression of genes gacA , algU , lpxC , fur , and fleQ , which are associated with known virulence functions, despite its binding to their promoters. Additionally, based on bioinformatic prediction, two new OmpR regulons ( envC and tolQ ) related to virulence were identified in Psa . Meanwhile, OmpR directly bound to the promoters of envC and tolQ , and negatively regulated their expression in minimal medium. These findings enrich our understanding of the OmpR-mediated regulatory network and its roles in the pathogenesis of P. syringae .