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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.
2011 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. The goal of this project was to clone the two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively. We have successfully cloned the two genes. We showed 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.
Our discovery is consistent with recent reports describing rhizobial T3SS and its secreted effectors that play an important role in modulation of host range, and suggests that establishment of a root nodule symbiosis requires the evasion of plant immune responses triggered by rhizobial effectors.
This finding may also offer novel strategies to enhance symbiotic nitrogen fixation in crop legumes. For example, the nodulation-restrictive R genes may be manipulated so that a host can deterministically interact with rhizobial inoculants with high nitrogen-fixing efficiency and exclude those indigenous strains that are highly competitive but with very low nitrogen-fixing efficiency.
We are also in the process of cloning another dominant soybean gene, called Rj4, that restricts nodulation with B. elkanii USDA61. Intriguingly Rj4 is not an R gene, and we believe that the result from this research will have significant impact in the area of plant-microbe interactions.