Endophytic commensal bacteria capitalize on the AvrPto-FER pathway to enhance proliferation during early stages of pathogen invasion.

Leaves typically harbor a community of pre-existing beneficial and commensal bacteria that contribute to plant health. When pathogens invade, plants employ a series of strategies to response to the invasion, including the modulation of the microbial community structure. However, it remains unclear how commensal bacteria respond to pathogen at the early stage, and whether this response is specifically regulated. Here, we show that infection of Arabidopsis thaliana leaves by the pathogen Pseudomonas syringae pv. tomato DC3000 leads to a significant increase in the population of commensal bacteria, characterized by enrichment of Gammaproteobacteria and Alphaproteobacteria, alongside a reduction in Firmicutes and Betaproteobacteria. This cascade of events specifically occurs because AvrPto, an effector secreted by Pst DC3000, targets and inhibits the host receptor kinase FER, resulting in suppression of FER-mediated pattern-triggered immunity via the previously identified RIPK-RBOHD module. This specific suppression via FER pathway creates a condition that facilitates rapid proliferation of pre-existing commensal bacteria during early pathogen invasion. Our work provides a paradigm for the study of the interaction and ecological generality between commensal bacteria and pathogens with spatiotemporal patterns.