Abstract
Nitrogen-fixing symbiosis between legumes and rhizobia is initiated by the recognition of rhizobial Nod factors (NFs) by host plants. NFs are diversely modified derivatives of chitin oligosaccharide, a fungal elicitor that induces defense responses in plants. Recent evidence has shown that both NFs and chitin elicitors are recognized by structurally-related LysM-receptor kinases. Transcriptome analyses of Lotus japonicus roots indicated that NFs not only activate symbiosis genes but also transiently activate defense-related genes through NF receptors. Conversely, chitin oligosaccharides were able to activate the symbiosis genes independent of NF receptors. Analyses using chimeric genes consisting of the LysM receptor domain of a Lotus NF receptor, NFR1, and the kinase domain of an Arabidopsis chitin receptor, CERK1, demonstrated that the substitution of a portion of the alphaEF helix in CERK1 with the amino acid sequence, YAQ, from the corresponding region of NFR1 enables L. japonicus nfr1 mutants to establish symbiosis with Mesorhizobium loti. Together with these results, we will discuss the molecular evolution of LysM receptor kinase.
