Arabidopsis immune responses triggered by cellulose and mixed-linked glucan oligosaccharides require a group of Leucine-Rich Repeat-Malectin Receptor 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 β-1,3/β-1,4-glucans (MLGs, e.g., β-1,4-D-(Glc)2 -β-1,3-D-Glc, MLG43) and cellulose (e.g., β-1,4-D-(Glc)3 , CEL3). The mechanisms of carbohydrates perception by plants are poorly characterised, except for fungal chitin oligosaccharides (e.g., β-1,4-D(GlcNAc)6 , CHI6) that involves several Receptor Kinase proteins (RK) with LysM-ECDs. Here, we describe the isolation and characterisation 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 and igp2/3 harbour point mutations (E906K and G773E, respectively) 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 CEL3 (Kd=1.19 ± 0.03 μM) and cellopentaose (Kd=1.40 ± 0.01 μM), but not MLG43, supporting its function as a plant PRR for cellulose oligosaccharides. Our data suggest that these LRR-MAL RKs are components of a novel recognition mechanism for both cellulose and MLG-derived oligosaccharides perception and downstream PTI activation in Arabidopsis.