Blue light perception by epiphytic Pseudomonas syringae drives chemoreceptor expression enabling efficient plant infection

In this work, we have evaluated the effect of light perception on gene expression in epiphytic populations of the model pathogen Pseudomonas syringae pv. tomato DC3000 (PsPto) during the initial contact with the tomato leaf surface. We have found that the contact with the leaf surface triggers a profound genetic reprogramming in PsPto. The initial response is characterized by a significant activation of bacterial motility and chemotaxis genes, regardless of light conditions. However, we have also observed that once on the leaf surface, blue light perception by PsPto is required for optimal colonization. Blue light triggers the activation of metabolic activity and increases expression of five chemoreceptors through the function of PsPto photoreceptors. Chemoreceptors mediate chemotaxis towards compounds released from the leaf interior and from stomata permitting plant entry and the initiation of the virulence process. We show that the inactivation of two of the blue-light responsive chemoreceptors causes a reduction in virulence. In addition, blue light is a signal that was shown to open plant stomata, facilitating bacterial entry into the plant. Our results highlight that during PsPto interaction with tomato plants, light perception, chemotaxis, and virulence are highly interwoven processes.