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Title: R gene-controlled host specificity in the legume-rhizobia symbiosis

item YANG, SHENGMING - University Of Kentucky
item TANG, FANG - University Of Kentucky
item GAO, MUQIANG - University Of Kentucky
item Krishnan, Hari
item ZHU, HONGYAN - University Of Kentucky

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2010
Publication Date: 10/26/2010
Citation: Yang, S., Tang, F., Gao, M., Krishnan, H.B., Zhu, H. 2010. R gene-controlled host specificity in the legume-rhizobia symbiosis. Proceedings of the National Academy of Sciences. 107(43):18735-18740.

Interpretive Summary: Rhizobia are soil-dwelling bacteria that form nodules on the roots of legume plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by legumes for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. The legume-rhizobia association is highly specific, such that each rhizobial strain establishes a symbiosis with only a limited set of host plants and vice versa. Bradyrhizobium japonicum USDA122 and Sinorhizobium fredii USDA257 can form nodules on the roots of soybean plants in a cultivar-specific manner. Despite recent advances in our understanding of the nodulation pathway, the host control of nodulation specificity remains poorly understood. In this study we have cloned and examined the role of Rj2 and Rfg1 genes in regulating soybean cultivar specificity. Information obtained from this study will help to better understand the factors that limit the formation of nitrogen-fixing nodules on North American soybean cultivars. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase the soybean yields with minimal use of nitrogen fertilizers.

Technical Abstract: Leguminous plants can enter into root nodule symbioses with nitrogen-fixing soil bacteria known as rhizobia. An intriguing but still poorly understood property of the symbiosis is its host specificity, which is controlled at multiple levels involving both rhizobial and host genes. Here we report the positional cloning of two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively. We demonstrate that Rj2 and Rfg1 are allelic genes that encode a member of the TIR-NBS-LRR class of plant resistance (R) proteins. Our finding reveals a common recognition mechanism underlying symbiotic and pathogenic host-bacteria interactions and suggests that establishment of a root nodule symbiosis require the evasion of plant immune responses triggered by rhizobial effectors.