Pathogen effector PsAvh109 suppresses plant immunity by targeting salicylic acid-mediated transcriptional regulation through Mediator complex interference
Plant pathogens disrupt host immunity by targeting transcriptional regulators. Here, we elucidate a molecular mechanism whereby the Phytophthora sojae effector PsAvh109 suppresses plant immunity through direct interference with the Mediator complex. PsAvh109 functions by mimicking the host repressor TOPLESS (TPL), competitively binding to MED21 and inhibiting the MED21-MED6 interface, thereby suppressing salicylic acid (SA)-responsive defense gene expression. Notably, PsAvh109 expression is induced by SA, the same signal that dissociates TPL from MED21 to activate immunity. This temporal coordination enables PsAvh109 to disrupt transcription precisely during the host's immune activation window. Our findings reveal a previously unrecognized strategy where pathogens exploit host signaling dynamics to deploy effectors that enforce sustained repression of defense programs. This work highlights a critical layer of pathogen adaptation in host-pathogen interactions and establishes a framework for understanding effector-mediated modulation of transcriptional machinery.