WATER INSECURITY IS A MAJOR THREAT TO SUSTAINED AGRICULTURAL PRODUCTIVITY. DROUGHTS ARE INCREASING IN FREQUENCY, DURATION AND INTENSITY, AND IN MANY REGIONS, THEY EXACERBATE DECLINING WATER RESERVES. DROUGHTS DRAMATICALLY REDUCE CROP YIELDS, WITH PARTICULARLY NEGATIVE IMPACTS ON LEGUMES DUE TO THE SENSITIVITY OF SYMBIOTIC NITROGEN FIXATION TO WATER DEFICITS. SOME PLANT-MICROBE PARTNERSHIPS IMPROVE STRESS TOLERANCE IN PLANTS, AND THE EXISTENCE OF THESE PARTNERSHIPS INDICATES THAT PLANTS MAY FAVOR, OR EVEN RECRUIT, SOIL MICROBES THAT ENHANCE STRESS TOLERANCE. MICROBES THEREFORE PROVIDE A COMPLEMENTARY STRATEGY TO CROP GERMPLASM IMPROVEMENT FOR ENHANCING THE RESILIENCE OF OUR CROPS TO DROUGHT. USING MICROBES FOR CROP PROTECTION, HOWEVER, REQUIRES KNOWLEDGE OF HOW CROPS PROMOTE THE DEVELOPMENT OF MICROBIAL COMMUNITIES AND HOW THEY DERIVE BENEFIT FROM THOSE ORGANISMS. PREVIOUS STUDIES HAVE FOUND THAT ROOT MICROBIAL COMMUNITIES ON MANY PLANT SPECIES SHOW SIMILAR CHANGES IN RESPONSE TO DROUGHT. THIS PROJECT WILL EXPLOIT THESE CONSERVED COMMUNITY RESPONSES TO IDENTIFY THE MECHANISMS DRIVING THESE RESPONSES. IN PARTICULAR, THE PROJECT WILL EVALUATE THREE POTENTIAL MECHANISMS BY WHICH DROUGHT-INDUCED CHANGES IN THE CHEMISTRY AND PHYSIOLOGY OF SOYBEAN ROOTS MAY DRIVE THE ASSEMBLY PATTERNS OF ROOT-ASSOCIATED MICROBIOTA. THESE MECHANISMS INCLUDE DROUGHT-INDUCED ACCUMULATION OF NUTRIENTS OR TOXIC REACTIVE OXYGEN SPECIES THAT DIFFERENTIALLY BENEFIT OR HARM DISTINCT MICROBIAL GROUPS, AND SOYBEAN-DRIVEN CHANGES IN THE PRODUCTION OF ANTIMICROBIAL COMPOUNDS BY SPECIFIC MICROBIAL GROUPS. THE PROJECT WILL USE MICROBIAL GENE EXPRESSION AND ROOT METABOLITE PROFILES, AS WELL AS FUNCTIONAL TESTS WITH MICROBIAL ISOLATES, TO EVALUATE THESE MECHANISMS. THE PROJECT WILL ALSO IDENTIFY LINKS BETWEEN STRESS-RELATED SOYBEAN TRAITS AND MICROBIAL COMMUNITY TRAITS AS A FIRST STEP TOWARD UNDERSTANDING THE COMMUNITY MEMBERS, COMBINATION OF MEMBERS, AND MICROBIAL FUNCTIONS THAT MAY BE DRIVING FITNESS BENEFITS IN DROUGHT-STRESSED SOYBEAN. THESE LINKAGES WILL BE IDENTIFIED BY GROWING SOYBEANS UNDER DROUGHT STRESS IN THE PRESENCE OF DIVERSE MICROBIAL COMMUNITIES, WITH AND WITHOUT DILUTION TO REDUCE COMMUNITY COMPLEXITY, AND COLLECTING MEASUREMENTS THAT PROFILE DIVERSE ABOVEGROUND AND BELOWGROUND SOYBEAN FITNESS TRAITS CONCURRENT WITH MICROBIAL COMMUNITY TRAITS. COLLECTIVELY, THE PROJECT RESULTS WILL PROVIDE A CRITICAL FOUNDATION FOR MANIPULATING, IDENTIFYING AND TESTING FOR DESIRED MICROBIAL FUNCTIONS THAT INFLUENCE CROP FITNESS DURING DROUGHT. IN-DEPTH KNOWLEDGE OF THESE PLANT-MICROBIAL ASSOCIATIONS IS CURRENTLY AN UNDER-EXPLOITED AVENUE FOR ENHANCING AGRICULTURAL PRODUCTIVITY.