Probing a Novel Negative Regulatory Cascade in Pseudomonas Syringae Virulence

Pseudomonas syringae is a widespread plant pathogen, causing huge economic losses and presenting a threat to food security worldwide. Like many other Gram-negative pathogens, P. syringae uses its type III secretion system (T3SS) to invade host plants andcause lethal diseases. Although a number of components regulating the T3SS gene expression have been identified, how the bacterium senses the environmental conditions and further transduce the signal(s) to the known regulatory components remain largely unknown. We have recently found a novel two-component system PSPPH3736/3737 that regulates T3SS, motility, and biofilm in P. syringae. Interestingly, PSPPH3736 can phosphorylate RhpR that is a key repressor of T3SS. The PSPPH3736/3737 is regulated by PSPPH0222, which is a leucine-responsive transcription factor. We propose a novel signaling pathway of leucine-PSPPH0222-PSPPH3736/3737-RhpR-T3SS/virulence, whose verification is the focus of the proposal. Genetic and biochemical characterizations of this novel signalling pathway is the first step toward identifying signals to which the T3SS responds, and toward the development of new strategies for disease control based on T3SS manipulation. Our long-term goal is to leverage the outcomes of the herein proposed studies to develop better therapies against P. syringae infection.