A LysM Receptor-Like Kinase Plays a Critical Role in Chitin Signaling and Fungal Resistance inArabidopsis

Abstract

Chitin, a polymer of N-acetyl-d-glucosamine, is found in fungal cell walls but not in plants. Plant cells can perceive chitin fragments (chitooligosaccharides) leading to gene induction and defense responses. We identified a LysM receptor-like protein (LysM RLK1) required for chitin signaling in Arabidopsis thaliana. The mutation in this gene blocked the induction of almost all chitooligosaccharide-responsive genes and led to more susceptibility to fungal pathogens but had no effect on infection by a bacterial pathogen. Additionally, exogenously applied chitooligosaccharides enhanced resistance against both fungal and bacterial pathogens in the wild-type plants but not in the mutant. Together, our data indicate that LysM RLK1 is essential for chitin signaling in plants (likely as part of the receptor complex) and is involved in chitin-mediated plant innate immunity. The LysM RLK1-mediated chitin signaling pathway is unique, but it may share a conserved downstream pathway with the FLS2/flagellin- and EFR/EF-Tu–mediated signaling pathways. Additionally, our work suggests a possible evolutionary relationship between the chitin and Nod factor perception mechanisms due to the similarities between their potential receptors and between the signal molecules perceived by them.

Chitin and chitin-related compounds in plant–fungal interactions

“…In the Arabidopsis genome, five chitin elicitor LysM receptor-like kinase 1 (LysM RLKs) are encoded, of which only one was implicated in chitin detection and named AtCERK1 for chitin elicitor receptor kinase 1 or LysM RLK1 (Miya et al 2007; Wan et al 2008). Structurally, AtCERK1, an RLK, is composed of a transmembrane domain, three tandem LysMs in its ectodomain and an intracellular serine/threonine kinase domain (Miya et al 2007; Wan et al 2008) in Arabidopsis .…”

“…Structurally, AtCERK1, an RLK, is composed of a transmembrane domain, three tandem LysMs in its ectodomain and an intracellular serine/threonine kinase domain (Miya et al 2007; Wan et al 2008) in Arabidopsis . Chitin-induced AtCERK1 homodimerisation has been proved to be necessary for its activation (Liu et al 2012).…”

“…In the situation of defence responses to pathogenic fungi, LysM RLK1 (Wan et al 2008) or CERK1 (Miya et al 2007)/CERK1 (as in Arabidopsis ) or CERK1/CEBiP receptor dimers (as in rice, Shimizu et al 2010) needs longer chito-oligomers, like octamers to activate its response through heterodimerisation (Hayafune et al 2014; Liu et al 2015). The perception of shorter oligosaccharides such as chitotetraose could instead rely on monomeric receptors (Miyata et al 2014; Shinya et al 2015) as into each receptor’s LysM domain, half of this oligomer molecule (C4) fits (Liu et al 2012; Shinya et al 2015).…”

“…The LysM motif is known to exist in the putative Nod-factor receptor kinases involved in the symbiotic signalling between leguminous plants and rhizobia (Desaki et al 2017). Chitin-oligomers from fungal pathogens are structurally like lipo-chito-oligosaccharide Nod factors produced by rhizobia, and LysM RLK1 (Wan et al 2008) or CERK1 (Miya et al 2007) is homologous to legume Nod factor receptors (NFR) NFR1 and NFR5 (Kaku et al 2006). …”

Structural basis for recognition of an endogenous peptide by the plant receptor kinase PEPR1

“…RKs mediate plant immunity by acting as pattern recognition receptors (PRRs) to recognize pathogen-derived signature components (pathogen-associated molecular patterns, PAMPs) or host-derived danger signals (damage-associated molecular patterns, DAMPs) [2][3][4]. Well-documented PRRs include RKs FLS2, EFR and CERK1 from Arabidopsis, which recognize the conserved N-terminal portion of bacterial flagellin (flg22) [5], elongation factor Tu (EFTu) [6], and fungal cell wall component chitin [7,8], respectively. Recognition of ligands by the ectodomains of their respective receptors activates PAMP-triggered immunity (PTI) that confers resistance to a multitude of pathogens.…”