Structural basis for size-selective perception of chitin in plants.

Plants detect microbes through pattern recognition receptors that perceive conserved microbial surface motifs known as microbe-associated molecular patterns (MAMPs). Lysin motif (LysM) receptors recognize and mediate downstream responses to chitinous MAMPs. In this study, we elucidated the mechanism for the specific recognition of long-chain chitin oligomers and identified a hallmark bridge domain characteristic of the CHITIN PERCEPTION (CHIP) receptor class. Structural analysis of receptor-ligand complexes, biochemistry, and in planta functional studies of inhibitory nanobodies revealed the mechanism of size-selective, high-affinity chitin perception in Lotus japonicus and Medicago truncatula. Additionally, we identified the CHITIN ELICITOR RECEPTOR KINASE (CERK) class as low-affinity, yet essential, co-receptors and propose a mechanistic model for a ligand-induced core signaling heterocomplex. Our findings establish the structural and mechanistic basis of differential chitin perception and receptor complex assembly in chitin-elicited plant immune signaling.