Pseudomonas syringae histidine kinase BvgS acts as the sensory receptor of plant-derived putrescine to activate the type III secretion system and enhance bacterial virulence

Plant diseases caused by Pseudomonas syringae pathovars pose a substantial threat to global crop production. While plants produce a range of secondary metabolites as part of their defense response, how bacterial pathogens exploit these compounds to facilitate infection remains poorly understood. Here, we report that P. syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker, senses putrescine (Put), a crucial plant metabolite involved in defense and physiological regulation, via the histidine kinase BvgS. We identify BvgS as a Put receptor in bacteria and show that this perception upregulates the expression of the type III secretion system (T3SS), a major virulence determinant. Binding and virulence assays demonstrate that Put sensing through the periplasmic solute-binding protein (PBPb) domains of BvgS is essential for T3SS activation both in vitro and during plant infection. Modulating Put levels in Actinidia plants, either by genetic reduction or exogenous application, correspondingly alters T3SS activity and bacterial invasion. Evolutionary analysis indicates that the PBPb domain is highly conserved across diverse P. syringae pathovars, suggesting a widespread mechanism for virulence potentiation. This work delineates a signaling pathway by which a phytopathogen co-opts a central host metabolite to enhance expression of its core virulence machinery, thereby increasing infectivity.