Arabidopsis lysin-motif proteins LYM1 LYM3 CERK1 mediate bacterial peptidoglycan sensing and immunity to bacterial infection
Résumé
Recognition of microbial patterns by host pattern recognition receptors is a key step in immune activation in multicellular eukaryotes. Peptidoglycans (PGNs) are major components of bacterial cell walls that possess immunity-stimulating activities in metazoans and plants. Here we show that PGN sensing and immunity to bacterial infection in Arabidopsis thaliana requires three lysin-motif (LysM) domain proteins. LYM1 and LYM3 are plasma membrane proteins that physically interact with PGNs and mediate Arabidopsis sensitivity to structurally different PGNs from Gram-negative and Gram-positive bacteria. lym1 and lym3 mutants lack PGN-induced changes in transcriptome activity patterns, but respond to fungus-derived chitin, a pattern structurally related to PGNs, in a wild-type manner. Notably, lym1, lym3, and lym3 lym1 mutant genotypes exhibit supersusceptibility to infection with virulent Pseudomonas syringae pathovar tomato DC3000. Defects in basal immunity in lym3 lym1 double mutants resemble those observed in lym1 and lym3 single mutants, suggesting that both proteins are part of the same recognition system. We further show that deletion of CERK1, a LysM receptor kinase that had previously been implicated in chitin perception and immunity to fungal infection in Arabidopsis, phenocopies defects observed in lym1 and lym3 mutants, such as peptidoglycan insensitivity and enhanced susceptibility to bacterial infection. Altogether, our findings suggest that plants share with metazoans the ability to recognize bacterial PGNs. However, as Arabidopsis LysM domain proteins LYM1, LYM3, and CERK1 form a PGN recognition system that is unrelated to metazoan PGN receptors, we propose that lineage-specific PGN perception systems have arisen through convergent evolution. Sensing microbial surface patterns via host-encoded pattern recognition receptors (PRRs) is a prerequisite for the activation of antimicrobial defenses in multicellular organisms (1–6). Microbial signatures triggering host innate immunity are collectively referred to as pathogen or microbe-associated molecular patterns (PAMPs/MAMPs) (7, 8). Several PAMPs, including bacterial lipopolysaccharides, flagellins, and fungal cell wall-derived glucan and chitin fragments, have been shown to possess immunogenic activities in metazoans and plants, thus suggesting evolutionary conservation of pattern recognition systems across lineage borders (1, 9). However, differences in the modular composition and ligand specificities of human (hTLR5) and plant (FLS2) flagellin receptors support the view that host sensors for microbial patterns have arisen independently through convergent evolution in different kingdoms (10). In addition, as flagellin is the only one of the aforementioned patterns for which both metazoan and plant receptors have been identified, sensible propositions on the evolutionary origin of eukaryotic innate immune systems require identification of additional host pattern recognition receptors. Peptidoglycans (PGNs) constitute building blocks of the cell walls of Gram-positive and Gram-negative bacteria that are composed of alternating β(1,4)-linked N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) residues (11, 12). Polymeric heteroglycan chains are bridged by oligopeptides, thereby forming a crystal lattice structure that provides rigidity to the bacterial envelope. Lysine (Lys) and meso-diaminopimelic acid (DAP) residues in the peptide moiety specify bacterial PGNs as either Lys- or DAP-type. Monomeric and polymeric PGNs of both subtypes constitute bacterial PAMPs that trigger antibacterial defenses and promote innate immunity in mammalian hosts as well as in Drosophila melanogaster (13, 14). Recognition of PGNs in animal hosts is mediated through various PRRs, including scavenger receptors, nucleotide-binding oligomerization domain-containing proteins (NOD), peptidoglycan recognition proteins (PGRPs), PGN hydrolases, and TOLL-like receptor TLR2 (13–17). Polymeric PGNs from Gram-positive and Gram-negative bacteria or a mixture of oligomeric muropeptides derived thereof act as PAMPs in the model plant Arabidopsis thaliana (18, 19). Our previous experiments suggested that the carbohydrate backbone of PGN is important for its immunogenic activity and that PGN is recognized in a receptor-mediated manner in Arabidopsis (19). Plant lysin-motif (LysM) domain proteins have been widely implicated in the recognition of GlcNAc-containing glycans. In legumes, establishment of symbiosis with soil-borne rhizobacteria requires LysM domain receptor proteins mediating the recognition of bacterial lipochitooligosaccharide nodulation (Nod) factors (20, 21). Likewise, recognition of the fungal PAMP chitin (an unbranched 1,4-linked GlcNAc homopolymer) and immune stimulation in Oryza sativa or Arabidopsis are dependent on LysM-type PRRs OsCEBiP/OsCERK1 or AtCERK1, respectively (22–25). Importantly, previous studies further suggested that Arabidopsis employs different perception systems for bacterial PGN and fungal chitin (19), despite the fact that the latter is structurally closely related to the carbohydrate moiety of bacterial PGN. Here we report the identification of a tripartite PGN recognition system in the plasma membrane of Arabidopsis with shared functions in PGN sensing and transmembrane signaling. This system comprises two LysM domain proteins implicated in PGN ligand binding (LYM1, LYM3) and a transmembrane LysM receptor kinase (CERK1) that is likely required for conveying the extracellular signal across the plasma membrane and for initiating intracellular signal transduction. Importantly, all three proteins were shown to be indispensable for PGN sensitivity and immunity to bacterial infection.