Defense versus Symbiosis: Host Genetic Control of Nodulation Specificity in Soybean

This is a seed grant research proposal from Kentucky which is eligible for USDA EPSCoR funding. Legumes represent the second most important family of crop plants. One of the defining features of legumes is their ability to form a root symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia. A significant property of the symbiosis is its host specificity, which is controlled at multiple levels involving both rhizobial and host genes. Understanding of the host genetic control mechanisms involved in determination of host specificity will enable breeding for enhanced nitrogen fixation in crop legumes and facilitate the study of the molecular ecology of nitrogen-fixing symbiosis. A number of single dominant genes (e.g, Rj2, Rj4 and Rfgl) that restrict nodulation with specific rhizobial strains have been identified in soybean since 1960s. These dominant genes restrict nodulation through recognition of incompatible rhizobial strains, resembling those 'gene-for-gene' interactions between plants and pathogens. The observation that the Rj2 allele is located within a disease resistance (R) gene cluster on the soybean linkage group .J has led to the speculation that the dominant nodulation restriction genes may encode plant R proteins that play an important role in limiting the host range ofrhizobia. The goal of this project is to map and clone the soybean genes Rj2 and Rfgl, taking advantage of the availability of genetic and genomic tools in soybean. Cloning and characterization of these genes will not only provide novel insights into the evolution of host specificity in legume nodulation but also contribute to our understanding of the similarities and contrasts between pathogenic and symbiotic plant-microbe interactions. This project addresses the first priority described in the RFA which includes mapping and positional cloning of economically important genes in Fabaceae.