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.

Many higher plants establish symbiotic relationships with arbuscular mycorrhizal (AM) fungi that improve their ability to acquire nutrients from the soil. In addition to establishing AM symbiosis, legumes also enter into a nitrogen-fixing symbiosis with bacteria known as rhizobia that results in the formation of root nodules. Several genes involved in the perception and transduction of bacterial symbiotic signals named "Nod factors" have been cloned recently in model legumes through forward genetic approaches. Among them, DMI3 (Doesn't Make Infections 3) is a calcium- and calmodulin-dependent kinase required for the establishment of both nodulation and AM symbiosis. We have identified, by a yeast two-hybrid system, a novel protein interacting with DMI3 named IPD3 (Interacting Protein of DMI3). IPD3 is predicted to interact with DMI3 through a C-terminal coiled-coil domain. Chimeric IPD3::GFP is localized to the nucleus of transformed Medicago truncatula root cells, in which split yellow fluorescent protein assays suggest that IPD3 and DMI3 physically interact in Nicotiana benthamiana. Like DMI3, IPD3 is extremely well conserved among the angiosperms and is absent from Arabidopsis. Despite this high level of conservation, none of the homologous proteins have a demonstrated biological or biochemical function. This work provides the first evidence of the involvement of IPD3 in a nuclear interaction with DMI3.

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.