RESEARCH-PGR: Leveraging the Medicago HapMap to characterize genome-by-genome interactions in nitrogen-fixing symbiosis

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Nitrogen is required for the synthesis of amino acids and proteins, essential components of the human diet. Nitrogen is also one of the most important nutrients required by plants, and it is normally applied as fertilizer in agricultural systems. However, legumes have the ability to establish a symbiotic association with specialized nitrogen-fixing soil bacteria through the formation of root nodules where the bacteria fix atmospheric nitrogen; the fixed nitrogen is used by the plant for protein synthesis. The symbionts have mechanisms that recognize molecular signals from each other, which in turn initiate processes that lead to the formation of root nodules. However, there is significant genetic variation among bacterial strains and among legumes in their abilities to interact symbiotically. The objective of this proposal is to identify bacterial and plant genes and gene variants responsible for the most efficient and effective interactions. This objective will be accomplished by identifying the bacterial strains that outcompete other strains in a large mixture in their ability to form nodules in roots of Medicago truncatula, the barrel medic. These experiments will be carried out with a large collection of barrel medic accessions representing most of the variation in the species. Thus, the results are expected to identify the most competitive strains and plant accessions. Quantitative analysis of the DNA sequences of the most competitive types is expected to reveal the identity of the bacterial and plant genes that are the most effective in the symbiotic interaction. The project will train several undergraduate students. Biological N-fixation mediated by legume-rhizobium symbiosis contributes N to both natural and agro-ecosystems, and results in legumes having N-rich leaves and seeds that make them some of the most important sources of dietary protein. For this reason, this symbiosis is a critical component of sustainable agricultural systems. The legume-rhizobia symbiosis represents an ideal system to study the coordination of function between a bacterial and a plant genome. Although both partners benefit from this relationship, not all partnerships are equally beneficial. The outcome of host-rhizobia interactions depends on the genotypes of the partners. This means that there is not a single rhizobium strain that confers the greatest benefit to all plant genotypes, and that different rhizobia strains are favored by different plant genotypes. The objective of this proposal is to identify the genomic basis that underlies G x G variation in mutualism, which could enable manipulation of the interaction to maximize biological N-fixation. The PIs have developed a select and resequence assay to identify strains with the greatest fitness in a diverse multistrain community. The PIs are proposing to refine and evaluate the robustness of the performance of this assay that will satisfy various biological and statistical challenges. The Broader Impacts of this project include the direct benefits to the agricultural sector and the environment that will result from more efficient nitrogen-fixation systems and the training of undergraduate students.