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Defense versus Symbiosis: Host Genetic Control of Nodulation Specificity in Soybean
Department of Plant and Soil Sciences
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 Rfg1) 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 of rhizobia.
The goal of this project is to map and clone the soybean genes Rj2 and Rfg1, 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.
2010 Project Description
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. It is widely believed that the host specificity is determined by specific recognition of bacterially derived Nod factors by the cognate host receptor(s). We have cloned two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively. We show that Rj2 and Rfg1 are allelic genes encoding a member of the Toll-interleukin receptor/nucleotide-binding site/leucine-rich repeat (TIR-NBS-LRR) class of plant resistance (R) proteins.
The involvement of host R genes in the control of genotype-specific infection and nodulation reveals a common recognition mechanism underlying symbiotic and pathogenic host-bacteria interactions and suggests the existence of their cognate avirulence genes derived from rhizobia. This study suggests that establishment of a root nodule symbiosis requires the evasion of plant immune responses triggered by rhizobial effectors.
Yang S, Tang F, Gao M, Krishnan HB, Zhu H (2010) R gene-controlled host specificity in the legume-rhizobia symbiosis. Proc Natl Acad Sci USA. 107(43):18735-40.