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Chitosan perception in Arabidopsis requires the chitin receptor AtCERK1 suggesting an improved model for receptor structure and function

Chitin, a linear polymer of N-acetyl-D-glucosamine, and chitosans, fully or partially deacetylated derivatives of chitin, are known to elicit defense reactions in higher plants. We compared the ability of chitin and chitosan oligomers and polymers (chitin oligomers with degree of polymerization 3 to 8; chitosan oligomers with degree of acetylation 0% to 35% and degree of polymerization 3 to 15; chitosan polymers with degree of acetylation 1% to 60% and degree of polymerization ~1300) to elicit an oxidative burst indicative of induced defense reactions in A. thaliana seedlings. Fully deacetylated chitosans were not able to trigger a response; elicitor activity increased with increasing degree of acetylation of chitosan polymers. Partially acetylated chitosan oligomers required a minimum degree of polymerization of 6 and at least four N-acetyl groups to trigger a response. Invariably, elicitation of an oxidative burst required the presence of the chitin receptor AtCERK1. Our results as well as previously published studies on chitin and chitosan perception in plants are best explained by a new general model of LysM-containing receptor complexes where two partners form a long, but off-set chitin-binding groove and are, thus, dimerized by one chitin or chitosan molecule, sharing a central GlcNAc unit with which both LysM domains interact. To verify this model and to distinguish it from earlier models, we assayed elicitor and inhibitor activities of selected partially acetylated chitosan oligomers with fully defined structures. In contrast to the initial “continuous groove”, the original “sandwich”, or the current “sliding mode” models for the chitin/chitosan receptor, the here proposed “slipped sandwich” model - which builds on these earlier models and represents a consensus combination of these - is in agreement with all experimental observations.