Secreted phospholipase A2α generates a pathogen-derived lysophospholipid to signal local immunity

In plants, effector-triggered immunity (ETI) is initiated when NLR receptors recognize pathogen effectors, yet the molecular signals linking this recognition to downstream defense activation remain poorly defined, in contrast to the well-characterized immunogenic signals of pattern-triggered immunity. Here, we show that a lysophospholipid is a previously unidentified ETI-mediating immune signal generated through host enzymatic conversion of pathogen membrane lipids. Upon recognition of avirulent Pseudomonas syringae , secretory phospholipase A2α (PLA2α) is rapidly induced and secreted into the apoplast, where it hydrolyzes bacterial phosphatidylethanolamine to produce lysophosphatidylethanolamines (LPEs), predominantly LPE18:1. Genetic ablation of PLA2α compromises local immunity, hypersensitive response confinement, and defense gene activation, all of which are largely rescued by exogenous LPE18:1. Mechanistically, LPE18:1 promotes ICS1 -dependent salicylic acid biosynthesis and NPR1 -mediated transcriptional reprogramming. Together, these findings support pathogen-derived LPE18:1 as a newly identified lipid-based immune signal that links NLR activation to spatially confined defense responses. The evolutionary conservation of secretory PLA2 enzymes and lysophospholipid signaling from plants to mammals suggests that host-directed enzymatic remodeling of pathogen membranes into immune-activating lipid signals may constitute a conserved strategy of innate immunity.