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Free Content From defense to symbiosis: limited alterations in the kinase domain of LysM receptor-like kinases are crucial for evolution of legume–Rhizobium symbiosis

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Summary

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 symbiosis genes independently of NF receptors. Analyses using chimeric genes consisting of the LysM receptor domain of a Lotus japonicus NF receptor, NFR1, and the kinase domain of an Arabidopsis chitin receptor, CERK1, demonstrated that substitution of a portion of the αEF helix in CERK1 with the amino acid sequence YAQ from the corresponding region of NFR1 enables L.┬ájaponicus nfr1 mutants to establish symbiosis with Mesorhizobium loti. We also showed that the kinase domains of two Lotus japonicus LysM receptor kinases, Lys6 and Lys7, which also possess the YAQ sequence, suppress the symbiotic defect of nfr1. These results strongly suggest that, in addition to adaptation of extracellular LysM domains to NFs, limited alterations in the kinase domain of chitin receptors have played a crucial role in shifting the intracellular signaling to symbiosis from defense responses, thus constituting one of the key genetic events in the evolution of root nodule symbiosis in legume plants.
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Keywords: CERK1; LysM receptor kinase; NFR1; defense; molecular evolution; symbiosis

Document Type: Research Article

Affiliations: 1: Department of Life Sciences, Faculty of Agriculture, Meiji University, Kawasaki 214-8571, Japan 2: Division of Plant Sciences, National Institute of Agrobiological Sciences, Tsukuba 305-8602, Japan 3: Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto 611-0011, Japan 4: Department of Applied Biological Sciences, Nihon University, Fujisawa 252-8501, Japan

Publication date: January 1, 2011

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