Pseudomonas syringae senses polyphenols via phosphorelay crosstalk to inhibit virulence

Bacteria use a variety of mechanisms, such as two-component regulatory systems (TCSs), to rapidly sense and respond to distinct conditions and signals in their host organisms. For example, a type III secretion system (T3SS) is a key determinant of the virulence of the model plant pathogen Pseudomonas syringae and contains the TCS RhpRS as a key regulator. However, the plant-derived compound targeting RhpRS remains unknown. Here, we report that RhpRS directly interacts with polyphenols and responds by switching off P. syringae T3SS via crosstalk with alternative histidine kinases. We identify three natural polyphenols that induce the expression of the rhpRS operon in an RhpS-dependent manner. The presence of these three specific polyphenols inhibits the phosphatase activity of RhpS, thus suppressing T3SS activation in T3SS-inducing conditions. The Pro40 residue of RhpS is essential to respond to these polyphenols. In addition, three non-cognate histidine kinases cooperatively phosphorylate RhpR and antagonize the rhpS mutant phenotype. This work illustrates that plant polyphenols can directly target P. syringae RhpRS, which results in bacterial virulence being switched off via a phosphorylation-related crosstalk. SYNOPSIS Polyphenols inhibit the induction of the T3SS of Pseudomonas syringae through the interaction with the Pro40 residue of RhpS. Three non-cognate histidine kinases of RhpR are involved in the signaling pathway of polyphenol-RhpRS-T3SS. Polyphenols directly suppress RhpS and the type III secretion system in P . syringae . The RhpS Pro40 residue is indispensable for polyphenol perception. PSPPH_5115, PSPPH_3736 and PSPPH_3550 are non-cognate histidine kinases of RhpR. Graphical Abstract Polyphenols inhibit the induction of the T3SS of Pseudomonas syringae through the interaction with the Pro40 residue of RhpS. Three non-cognate histidine kinases of RhpR are involved in the signaling pathway of polyphenol-RhpRS-T3SS.