Elucidating Genetic and Molecular Mechanisms Underlying Symbiotic Specificity in the Medicago-Sinorhizobium Mutualism

Overview: Legumes can make their own nitrogen fertilizer by forming symbiosis with nitrogen-fixing soil bacteria called rhizobia. This interaction culminates in the formation of specialized root organs called nodules, within which the bacteria acquire the ability to convert atmospheric nitrogen into ammonia. One important characteristic of the symbiosis is its high level of specificity. Such specificity can take place at various stages of the interaction, ranging from initial bacterial infection and nodulation (nodulation specificity) to late nodule development associated with nitrogen fixation efficiency (nitrogen fixation specificity). The overall goal of this proposal is to elucidate the genetic mechanisms underlying symbiotic specificity in the Medicago-Sinorhizobium mutualism. The specific objectives of this proposal include: (1) Further characterization of the NFS1 and NFS2 genes and their roles in negative regulation of symbiotic development with Sinorhizobium meliloti Rm41; (2) Cloning and characterization of a suppressor of NFS1; and (3) Characterization of the NS1 locus in M. truncatula that restricts nodulation by Rm41. Intellectual Merit: Symbiotic specificity has long generated intense interest in the scientific community. It has been documented that domesticated crop species tend to have fewer compatible symbionts than their wild counterparts. In this case, broadening host range can lead to symbiotic nitrogen fixation in soils where the favorable strains are lacking. On the other hand, even though many legumes can nodulate with indigenous soil bacteria, nitrogen fixation efficiency varies tremendously between different plant-rhizobia combinations; in this situation, optimizing symbiotic partnerships is important to achieve high nitrogen- fixing efficiency. Despite recent advances in our understanding of the signaling pathways leading to root nodule development, the molecular mechanisms underlying natural variation in nodulation capacity and nitrogen fixation efficiency are still not well understood. Understanding the molecular basis of symbiotic specificity will facilitate the design of novel strategies to enhance the benefits of symbiotic nitrogen fixation to sustainable agriculture. Broader Impacts: The availability of nitrogen in the soil is a major limiting factor for agricultural production. Although the use of commercial fertilizer has contributed substantially to feeding the world, the production of nitrogen fertilizer consumes a lot of fossil fuels, which are limited resources. Moreover, excessive use of fertilizers causes serious environmental pollutions. In contrast, the legume-rhizobia symbiosis represents an efficient and sustainable nitrogen-fixing system, which uses the solar energy through plant photosynthesis. Thus, improvement of nitrogen fixation effectiveness will have significant implications on both agricultural and natural ecosystems. Education will also be a major focus of this proposal. The postdoc, graduate and undergraduate students recruited for this project will be trained in the areas of plant genomics, bioinformatics, and molecular biology of plant-microbe interactions. We will particularly encourage the participation of members of under-represented undergraduates to join our group. Moreover, we will contribute in a direct and substantive manner to high school education in Kentucky through a high school student and teacher outreach program at University of Kentucky.