Arabidopsis Leucine Rich Repeat-Malectin Receptor Kinases in immunity triggered by cellulose and mixed-linked glucan oligosaccharides
Authors: Martín-Dacal Marina , Fernández-Calvo Patricia , Jiménez-Sandoval Pedro , López Gemma , Garrido-Arandía María , Rebaque Diego , del Hierro Irene , Torres Miguel Ángel , Kumar Varun , Berlanga Diego José , Mélida Hugo , Pacios Luis F. , Santiago Julia , Molina Antonio 
Published: August 2022

Plant immune system perceives through the extracellular ectodomains (ECDs) of Pattern Recognition Receptors (PRRs) a diversity of carbohydrate ligands from plant and microbial cell walls, 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 perception of carbohydrates by plants are poorly characterized, with the exception of that determining recognition of fungal chitin oligosaccharides (e.g., β-1,4-D(GlcNAc)6, CHI6) that involves several PRRs with LysM-ECDs that function as receptor or co-receptors. Here, we describe the isolation and characterization ofArabidopsis thalianamutantsimpaired inglycanperception (igp), which are defective in PTI activation mediated by MLG43 and CEL3, but not CHI6.igp1-igp4are altered in receptor-like kinases [RLKs: AT1G56145 (IGP1), AT1G56130 (IGP2/3), and AT1G56140 (IGP4)] with Leucine-Rich-Repeat (LRR) and Malectin (MAL) domains in their ECDs.igp4is a T-DNA insertional, loss of function mutant whereasigp1and the allelicigp2/igp3harbour point mutations (E906K and G773E, respectively) in their kinase domains, which impact their structure and surface electrostatic potential as revealed byin silicostructural analyses. Notably, Isothermal Titration Calorimetry assays with purified ECD-RLKs showed that AT1G56145 binds with high affinity CEL3 (Kd = 1.19 ± 0.03 μM) and cellopentaose (Kd = 1.40 ± 0.01 μM), but not MLG43, supporting AT1G56145 function as a plant PRR for cellulose oligosaccharides. Our data suggest that these LRR-MAL RLKs are receptor/co-receptors of a novel mechanism of perception of cellulose and MLG-derived oligosaccharides and PTI activation inArabidopsis thaliana.

Significance Statement

New oligosaccharides that trigger plant immunity have been described recently, but the mechanisms of perception of these glycans are unknown. We describe here threeArabidopsis thalianareceptor kinases (AT1G56130, AT1G56140, and AT1G56145) with Leucine Rich Repeat (LRR) and Malectin (MAL) domains in their extracellular ectodomains (ECDs), which function as Pattern Recognition Receptors (PRRs) triggering immune response mediated by oligosaccharides from cellulose (β-1,4-glucan) and mixed-linked β-1,3/1,4-glucans (MLGs) of plant and microbial cell walls. The ECD-AT1G56145 binds cellulose oligosaccharides, but not MLGs, supporting its function as a novel receptor of carbohydrate ligands in plants. Our data indicate that these LRR-MAL-PRRs control a complex mechanism of oligosaccharides perception and immune activation that differs from that of fungal chitin oligosaccharides recognition which involves LysM-PRRs.