Arabidopsis immune responses triggered by cellulose‐ and mixed‐linked glucan‐derived oligosaccharides require a group ofleucine‐rich repeat malectinreceptor kinases

Abstract

The plant immune system perceives a diversity of carbohydrate ligands from plant and microbial cell walls through the extracellular ectodomains (ECDs) of pattern recognition receptors (PRRs), which activate pattern-triggered immunity (PTI). Among these ligands are oligosaccharides derived from mixed-linked beta-1,3/beta-1,4-glucans (MLGs; e.g. beta-1,4-D-(Glc)(2)-beta-1,3-D-Glc, MLG43) and cellulose (e.g. beta-1,4-D-(Glc)(3), CEL3). The mechanisms behind carbohydrate perception in plants are poorly characterized except for fungal chitin oligosaccharides (e.g. beta-1,4-d-(GlcNAc)(6), CHI6), which involve several receptor kinase proteins (RKs) with LysM-ECDs. Here, we describe the isolation and characterization of Arabidopsis thaliana mutants impaired in glycan perception (igp) that are defective in PTI activation mediated by MLG43 and CEL3, but not by CHI6. igp1-igp4 are altered in three RKs - AT1G56145 (IGP1), AT1G56130 (IGP2/IGP3) and AT1G56140 (IGP4) - with leucine-rich-repeat (LRR) and malectin (MAL) domains in their ECDs. igp1 harbors point mutation E906K and igp2 and igp3 harbor point mutation G773E in their kinase domains, whereas igp4 is a T-DNA insertional loss-of-function mutant. Notably, isothermal titration calorimetry (ITC) assays with purified ECD-RKs of IGP1 and IGP3 showed that IGP1 binds with high affinity to CEL3 (with dissociation constant K-D = 1.19 +/- 0.03 mu m) and cellopentaose (K-D = 1.40 +/- 0.01 mu M), but not to MLG43, supporting its function as a plant PRR for cellulose-derived oligosaccharides. Our data suggest that these LRR-MAL RKs are components of a recognition mechanism for both cellulose- and MLG-derived oligosaccharide perception and downstream PTI activation in Arabidopsis.