LEGUMES, SUCH AS SOYBEAN, COWPEA OR MEDICAGO, ARE AMONG THE MOST IMPORTANT CROP SPECIES WORLDWIDE, AND ARE GROWN ON APPROXIMATELY 180 MILLION HA, OR 15% OF THE ARABLE LAND ON EARTH.THESE PLANTACCOUNT FOR 27% OF THE WORLD&ACUTE;S PRIMARY CROP PRODUCTION, AND 33% OF THE DIETARY NITROGEN NEEDS OF HUMANS. AGRICULTURAL CROP PRODUCTIVITY RELIES ON A CONTINUOUS SUPPLY OFNITROGEN (N) AND PHOSPHATE (P) FERTILIZERS, BECAUSE THESE NUTRIENTS MOST OFTEN LIMIT PLANT GROWTH, UNFORTUNATELY THE EXTENSIVE USE OF FERTILIZERS IN THE PAST HAS LED TO SUBSTANTIAL ENVIRONMENTAL DEGRADATION, INCLUDING THE REDUCTION IN WATER QUALITY AND EUTROPHICATION OF MARINE ECOSYSTEMS (ALGAE BLOOMS), THE DEVELOPMENT OF PHOTOCHEMICAL SMOG, AND INCREASING CONCENTRATIONS OF GREENHOUSE GASES. AN INCREASE IN THE NUTRIENT EFFICIENCY OF LEGUMES THEREFORE REPRESENTS AN URGENT RESEARCH PRIORITY TO ENSURE COST-EFFECTIVE AND ENVIRONMENTALLY SUSTAINABLE CROP PRODUCTION IN THE FUTURE.LEGUMES ARE ABLE TO FORM TRIPARTITE INTERACTIONS WITH NITROGEN (N) FIXING RHIZOBIA BACTERIA AND ARBUSCULAR MYCORRHIZAL (AM) FUNGI. BOTH SYMBIONTS CONTRIBUTE TO THE N NUTRITION OF THEIR HOST, AND AM FUNGI ALSO TRANSFER OTHER NUTRIENTS AND IMPROVE THE RESISTANCE OF THE HOST AGAINST OTHER ABIOTIC AND BIOTIC STRESSES. BOTH SYMBIOSES ARE COSTLY FOR THE HOST PLANT, AND IN EXCHANGE FOR THEIR BENEFICIAL EFFECT, THE PLANT TRANSFERS SIGNIFICANT AMOUNTS OF CARBOHYDRATES TO THESE ROOT SYMBIONTS. IN ORDER TO BE BENEFICIAL FOR THE PLANT, THE BENEFITSWEIGH THE CARBON COSTS OF THE SYMBIOSIS FOR THE HOST, BECAUSE SOURCE-TO-SINK CARBON TRANSPORT IS ONE OF THE MAJOR DETERMINANTS FOR PLANT GROWTH. IN RECENT YEARS, SIGNIFICANT PROGRESS HAS BEEN MADE TO BETTER UNDERSTAND THE CARBON TO NUTRIENT EXCHANGE IN THE AM SYMBIOSIS OR IN THE SYMBIOSIS WITH N-FIXING BACTERIA. IN CONTRAST, TRIPARTITE INTERACTIONS ARE NOT WELL STUDIED, AND IT IS NOT KNOWN HOW HOST PLANTS DISTRIBUTE THEIR CARBON RESOURCES AMONG SYMBIONTS, AND HOW THESE RESOURCE EXCHANGE PROCESSES ARE REGULATED. A BETTER UNDERSTANDING OF THESE PROCESSES IS IMPORTANT, BECAUSETHE CARBON COSTS OF BOTH SYMBIOSES CAN LEAD TO NEUTRAL OR NEGATIVE GROWTH IMPACTS UNDER CERTAIN NUTRIENT SUPPLY CONDITIONS;BOTH SYMBIOSES HAVE THE POTENTIAL TO SIGNIFICANTLY IMPROVE YIELD AND ENVIRONMENTAL SUSTAINABILITY OF CROP PRODUCTION; ANDKEY FOR AN APPLICATION OF BOTH SYMBIOSES AS EFFICIENT BIOFERTILIZERS IN SUSTAINABLE AGRICULTURE IS A BETTER UNDERSTANDING OF HOW COST TO BENEFIT RATIOS ARE CONTROLLED IN THESE IMPORTANT BENEFICIAL PLANT MICROBE INTERACTIONS.THE PROJECT WILL PROVIDE INSIGHTS INTO THESE UNDER-RESEARCHED PLANT SYMBIOSES AND WILL ALLOW US TO BETTER PREDICT AND TO MAXIMIZE SYMBIOTIC BENEFITS IN AGRICULTURAL APPLICATIONS. THE PROJECT WILL ALSO CONTRIBUTE TO THE TRAINING OF A NEW GENERATION OF SCIENTISTS, AND WILL TRAIN ONE POST-DOCTORAL SCIENTIST, ONE GRADUATE STUDENT, AND SEVERAL UNDERGRADUATE STUDENTS IN INNOVATIVE AND STATE OF THE ART SCIENTIFIC TECHNIQUES.