Bacterial Effectors Target the Common Signaling Partner BAK1 to Disrupt Multiple MAMP Receptor-Signaling Complexes and Impede Plant Immunity

Abstract

SUMMARY Successful pathogens have evolved strategies to interfere with host immune systems. For example, the ubiquitous plant pathogen Pseudomonas syringae injects two sequence-distinct effectors, AvrPto and AvrPtoB, to intercept convergent innate immune responses stimulated by multiple microbe-associated molecular patterns (MAMPs). However, the direct host targets and precise molecular mechanisms of bacterial effectors remain largely obscure. We show that AvrPto and AvrPtoB bind the Arabidopsis receptor-like kinase BAK1, a shared signaling partner of both the flagellin receptor FLS2 and the brassinosteroid receptor BRI1. This targeting interferes with ligand-dependent association of FLS2 with BAK1 during infection. It also impedes BAK1-dependent host immune responses to diverse other MAMPs and brassinosteroid signaling. Significantly, the structural basis of AvrPto-BAK1 interaction appears to be distinct from AvrPto-Pto association required for effector-triggered immunity. These findings uncover a unique strategy of bacterial pathogenesis where virulence effectors block signal transmission through a key common component of multiple MAMP-receptor complexes.