PLANTS HARBOR A DIVERSE ARRAY OF MICROSCOPIC BACTERIA, FUNGI, VIRUSES, ETC. THE INTERACTIONS BETWEEN PLANTS AND THESE MICROBIAL ORGANISMS CAN DETERMINE MANY IMPORTANT ASPECTS ABOUT PLANT BIOLOGY SUCH AS WHERE PLANTS CAN PERSIST AND HOW WELL THEY GROW. AMONG THIS MICROBIAL CONSORTIUM, ARBUSCULAR MYCORRHIZAL (AM) FUNGI PLAY A CENTRAL ROLE IN NOT ONLY PLANT BIOLOGY GENERALLY BUT ALSO IN SUSTAINABLE AGRICULTURE INITIATIVES. AM FUNGI ASSOCIATE WITH 80-90% OF ALL LAND PLANTS AND ARE IMPORTANT IN ALLEVIATING LOW NUTRIENT STRESS. THE RELATIONSHIP BETWEEN PLANTS AND AM FUNGI IS OFTEN CONSIDERED MUTUALLY BENEFICIAL, BUT HOW MUCH THE RELATIONSHIP BENEFITS PLANTS AND EVEN WHETHER IT IS HARMFUL VARIES DEPENDS ON THE ENVIRONMENTAL CONTEXT IN WHICH THE INTERACTION OCCURS. HOWEVER, WHILE WE UNDERSTAND THAT PLANT-AM ASSOCIATIONS AND THEIR OUTCOMES FOR PLANT HEALTH AND PERFORMANCE ARE DEPENDENT ON ENVIRONMENTAL CONTEXT, WE KNOW SURPRISINGLY LITTLE ABOUT THE MECHANISMS THAT UNDERPIN THIS INTERACTION'S CONTEXT DEPENDENCY.THE GOAL OF THIS PROPOSAL IS TO BEGIN FILLING THIS GAP IN OUR KNOWLEDGE OF HOW PLANTS AND AM FUNGI INTEGRATE INFORMATION ABOUT THEIR ENVIRONMENT TO DETERMINE WHETHER THE RELATIONSHIP WILL BE MUTUALLY BENEFICIAL OR HARMFUL TO THE PLANT. BY IDENTIFYING THE UNDERLYING MECHANISMS (E.G., SPECIFIC GENES) WE COULD POTENTIALLY IMPROVE OUR ABILITY TO PREDICT WHICH PLANT GENOTYPES WOULD BENEFIT FROM MYCORRHIZAL INOCULA IN SPECIFIC ENVIRONMENTS. THIS WOULD SATISFY A MAJOR OBJECTIVE OF SUSTAINABLE AGRICULTURE BY REMOVING THE NEED FOR ENVIRONMENTALLY HARMFUL SYNTHETIC FERTILIZERS AND UTILIZING NATURAL ALTERNATIVES WITH LIMITED ENVIRONMENTAL CONSEQUENCES.I WILL FIRST IDENTIFY SPECIFIC GENE FAMILIES (I.E., RELATED GENES WITH POTENTIALLY SIMILAR FUNCTIONS) IMPORTANT TO PLANT-AM ASSOCIATIONS BY ANALYZING GENOMES OF 42 AGRONOMICALLY IMPORTANT PLANT SPECIES LIKE CORN AND TOMATO. THEN, I WILL DETERMINE IF THE SIZE OF A GENE FAMILY IS AN IMPORTANT ASPECT OF REGULATING CONTEXT-DEPENDENCY BY DETERMINING IF LARGER GENE FAMILIES HAVE HIGHER FREQUENCIES OF CONTEXT-DEPENDENCY. GENE FAMILY SIZE IS LIKELY TO BE AN IMPORTANT COMPONENT OF CONTEXT-DEPENDENCY BECAUSE INSTEAD OF ONLY HAVING AN OPTION OF TURNING A GENE ON AND OFF, MORE FAMILY MEMBERS ALLOWS PLANTS TO SPECIFY WHERE, WHEN, AND HOW MUCH OF AN IMPORTANT FUNCTION IS PRESENT. TO TEST THESE PREDICTIONS, I WILL THEN GROW PLANTS WITH MUTATIONS IN GENES OF INTEREST FROM THE PREVIOUS ANALYSES IN A GREENHOUSE WITH/WITHOUT AM FUNGI UNDER FOUR DIFFERENT IMPORTANT ENVIRONMENTAL CONDITIONS: NO STRESS, LOW PHOSPHORUS STRESS, LOW POTASSIUM STRESS, AND HIGH SALINITY STRESS.BY BUILDING A MECHANISTIC FRAMEWORK OF CONTEXT-DEPENDENCY IN PLANT-AM INTERACTIONS, WE CAN IMPROVE OUR ABILITY TO SELECT FOR PLANT GENOTYPES THAT WOULD MOST BENEFIT FROM NATURALLY-OCCURRING AM FUNGI IN SPECIFIC ENVIRONMENTS. ULTIMATELY, THIS MAY LEAD TO LESS RELIANCE ON SYNTHETIC FERTILIZERS WHICH CAN LEAD TO TOXIC ALGAL BLOOMS AND POISONING OF DRINKING WATER THROUGH RUN-OFF.