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In addition to the R gene-mediated pathways of plant resistance to specific pathogens, plants have the capacity to recognize a number of microbial surface-derived molecules, which elicit a general immune response in both host and nonhost plants. These are known as pathogen-associated or microbe-associated molecular patterns (PAMPs/MAMPs), so named because they are presumed to contain a structure or a pattern that is absent from eukaryotic host molecules and allows the host to recognizeamicrobial (and potentiallypatho-genic) invader. Many PAMPs that have been identified are essential for microbial metabolism or for penetration and invasion of a host cell and are therefore broadly conserved among diverse microbial pathogens (Parker, 2003). These include lipopoly-saccharides of Gram-negative bacteria, peptidoglycans from Gram-positive bacteria, eubacterial flagellin, and glucans, chi-tins, and proteins derived from fungal cell walls (Nürnberger and Brunner, 2002).
Chitin is a polymer of N-acetyl-D-glucos-amine that is a major component of fungal cell walls and has been recognized as a general elicitor of plant defense responses for many years (Boller, 1995). Fungal infection induces the expression of chitinases in plant cells, and these chitin-degrading enzymes accumulate at the site of invasion. In addition to the direct effect of limiting fungal invasion by degrading the fungal cell wall, the resulting chitin fragments (chito-oligosaccharides) also appear to function as elicitors of numerous downstream defense response genes. Interestingly, Nod factors, which are produced by rhizobia (symbiotic N^sub 2^-fixing bacteria) and are essential for the induction of the nodulation process in legumes, are chitin-relatedmolecules (lipochi-tooligosaccacharides). The predicted Nod factor receptor proteins NFR1 and NFR5 in legumes are LysM domain-containing www.plantcell.org/cgi/doi/10.1105/tpc.108.058784 receptor-like kinases (LysM RLKs) (Limpens et al., 2003; Radutoiu et al., 2003). The LysM RLKs represent a relatively large plant-specific protein family present in nonle-gumes and legumes (Zhang et al., 2007) and have been considered good candidates for playing a role in fungal chitin perception.
In this issue of The Plant Cell, Wan et al. (pages 471-481) show that LysM RLK1 is required for chitin signaling in Arabidopsis . The authors demonstrate that a mutation in RLK1 blocks the induction of chitooligosaccharide responsive genes and leads to enhanced susceptibility of plants to fungal pathogens. These results are similar to those reported recently by Miya et al. (2007), who also showed that LysM RLK1 (called CERK1...