Ca-Induced Two-Component System CvsSR Regulates the Type III Secretion System and the Extracytoplasmic Function Sigma Factor AlgU in Pseudomonas syringae pv. tomato DC3000.

Two-component systems (TCSs) of bacteria regulate many different aspects of the bacterial life cycle, including pathogenesis. Most TCSs remain uncharacterized, with no information about the signal(s) or regulatory targets and/or role in bacterial pathogenesis. Here, we characterized a TCS in the plant-pathogenic bacterium pv. tomato DC3000 composed of the histidine kinase CvsS and the response regulator CvsR. CvsSR is necessary for virulence of pv. tomato DC3000, since Δ and Δ strains produced fewer symptoms than the wild type (WT) and demonstrated reduced growth on multiple hosts. We discovered that expression of is induced by Ca concentrations found in leaf apoplastic fluid. Thus, Ca can be added to the list of signals that promote pathogenesis of pv. tomato DC3000 during host colonization. Through chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq) and global transcriptome analysis (RNA-seq), we discerned the CvsR regulon. CvsR directly activated expression of the type III secretion system regulators, and , that regulate pv. tomato DC3000 virulence in a type III secretion system-dependent manner. CvsR also indirectly repressed transcription of the extracytoplasmic sigma factor and production of alginate. Phenotypic analysis determined that CvsSR inversely regulated biofilm formation, swarming motility, and cellulose production in a Ca-dependent manner. Overall, our results show that CvsSR is a key regulatory hub critical for interaction with host plants. Pathogenic bacteria must be able to react and respond to the surrounding environment, make use of available resources, and avert or counter host immune responses. Often, these abilities rely on two-component systems (TCSs) composed of interacting proteins that modulate gene expression. We identified a TCS in the plant-pathogenic bacterium that responds to the presence of calcium, which is an important signal during the plant defense response. We showed that when is grown in the presence of calcium, this TCS regulates expression of factors contributing to disease. Overall, our results provide a better understanding of how bacterial pathogens respond to plant signals and control systems necessary for eliciting disease.