To defend against pathogens, plants have developed complex immune systems, including plasma membrane receptors that recognize pathogen-associated molecular patterns, such as chitin from fungal cell walls, and mount a defense response. Here, we identify a chitinase, MoChia1 (Magnaporthe oryzae chitinase 1), secreted by M. oryzae, a fungal pathogen of rice (Oryza sativa). MoChia1 can trigger plant defense responses, and expression of MoChia1 under an inducible promoter in rice enhances its resistance to M. oryzae. MoChia1 is a functional chitinase required for M. oryzae growth and development; knocking out MoChia1 significantly reduced the virulence of the fungus, and we found that MoChia1 binds chitin to suppress the chitin-triggered plant immune response. However, the rice tetratricopeptide repeat protein OsTPR1 interacts with MoChia1 in the rice apoplast. OsTPR1 competitively binds MoChia1, thereby allowing the accumulation of free chitin and reestablishing the immune response. Overexpressing OsTPR1 in rice plants resulted in elevated levels of reactive oxygen species during M. oryzae infection. Our data demonstrate that rice plants not only recognize MoChia1, but also use OsTPR to counteract the function of this fungal chitinase and regain immunity.

Plant pattern recognition receptors (PRRs) perceive pathogen‐associated molecular patterns (PAMPs) to activate immune responses. Medium‐chain 3‐hydroxy fatty acids (mc‐3‐OH‐FAs), which are widely present in Gram‐negative bacteria, were recently shown to be novel PAMPs in Arabidopsis thaliana. The Arabidopsis PRR LIPOOLIGOSACCHARIDE‐SPECIFIC REDUCED ELICITATION (LORE) is a G‐type lectin receptor‐like kinase that recognizes mc‐3‐OH‐FAs and subsequently mounts an immune response; however, the mechanisms underlying LORE activation and downstream signaling are unexplored. Here, we report that one of the mc‐3‐OH‐FAs, 3‐OH‐C10:0, induces phosphorylation of LORE at tyrosine residue 600 (Y600). Phosphorylated LORE subsequently trans‐phosphorylates the receptor‐like cytoplasmic kinase PBL34 and its close paralogs, PBL35 and PBL36, and therefore activates plant immunity. Phosphorylation of LORE Y600 is required for downstream phosphorylation of PBL34, PBL35, and PBL36. However, the Pseudomonas syringae effector HopAO1 targets LORE, dephosphorylating the tyrosine‐phosphorylated Y600 and therefore suppressing the immune response. These observations uncover the mechanism by which LORE mediates signaling in response to 3‐OH‐C10:0 in Arabidopsis.