**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** SOIL MICROBIOTA HAVE SIGNIFICANT IMPACTS ON PLANT HEALTH. AMONG MEMBERS OF THE MICROBIOTA ARE RHIZOBIA, A GROUP OF BENEFICIAL BACTERIA WHICH FIX NITROGEN FOR HOST LEGUMES THAT, ALONG WITH OTHER BENEFICIAL MEMBERS OF THE MICROBIOTA, HAVE PROMISE AS AN ALTERNATIVE TO CHEMICAL FERTILIZATION TO PROMOTE MORE SUSTAINABLE AGRICULTURAL PRACTICES. YET, THE DEPLOYMENT OF RHIZOBIA IS CHALLENGED BY THE SO-CALLED RHIZOBIAL COMPETITION PROBLEM, WHERE INTRODUCED ELITE STRAINS OFTEN CANNOT GAIN ACCESS TO HOST NODULES BECAUSE THEY ARE OUTCOMPETED BY INDIGENOUS STRAINS THAT PROVIDE LESS BENEFIT TO PLANTS. THE GOAL OF THIS PROPOSAL IS TO CHARACTERIZE THE ENVIRONMENTAL AND GENOMIC MECHANISMS THAT MEDIATE COMPETITION TO COLONIZE HOST ROOTS. THE SPECIFIC OBJECTIVES ARE TO I) GENERATE A LIVING AND DIGITAL COLLECTION OF COMPETITIVE STRAINS THAT ARE ENRICHED IN HOST ROOT COMPARTMENTS AND ASSOCIATED WITH SUPERIOR PLANT PERFORMANCE, II) QUANTIFY COLONIZATION COMPETITIVENESS ON HOSTS FOR A FOCAL SET OF STRAINS UNDER REAL LIFE CONDITIONS, AND III) COUPLE GENOMIC AND HIGH THROUGHPUT GENETIC METHODS TO UNCOVER MECHANISMS OF COLONIZATION COMPETITIVENESS. WE HAVE USED EXPERIMENTAL AND GENOMIC METHODS TO AMASS A LARGE BODY OF FOUNDATIONAL WORK TO STUDY RHIZOBIA, BOTH IN NATIVE PLANTS AND IN CROPS. BY COMPLETING THE OBJECTIVES OF THIS PROPOSAL, WE WILL ADVANCE TOWARDS MEETING THE LONG-TERM GOAL OF DESIGNING STRAINS THAT ARE ELITE NOT ONLY IN SYMBIOSIS BUT IN COMPETITIVENESS FOR CROP PLANTS. THIS PROPOSAL IS DIRECTLY RELEVANT TO THE AFRI GOAL OF ADVANCING KNOWLEDGE AND DEVELOPING PRACTICES THAT PROMOTE SUSTAINABLE PLANT PRODUCTION SYSTEMS.