Lee, H. Y.; Ling, T.; Jung, J.; Kumar, K.; Kim, S.-C.; Back, K.; Jung, S.; Hwang, I.; Ryu, S. B.

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 A2 (PLA2) is rapidly induced and secreted into the apoplast, where it hydrolyzes bacterial phosphatidylethanolamine to produce lysophosphatidylethanolamines (LPEs), predominantly LPE18:1. Genetic ablation of PLA2 compromises local immunity, hypersensitive response confinement, and defense gene activation, all of which are fully 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 previously unrecognized 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 fundamental and ancient strategy of innate immunity.