ENGINEERING SYNTHETIC SYMBIOSES BETWEEN PLANTS AND BACTERIA TO DELIVER NITROGEN TO CROPS

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Nitrogen is an essential element of biological molecules and life on earth. Lack of usablenitrogen limits growth of microbes, plants, and animals. Lack of nitrogen in agriculturalsoils limits plant production of food, feed, fiber and fuel. Nature solved the nitrogen limitationproblem via evolution of biological nitrogen fixation in diazotrophic bacteria that reduceatmospheric N2 to NH3, which is readily assimilated into biological molecules. Biologicalnitrogen fixation is catalyzed by a complex metalloenzyme called nitrogenase whose oxygen-sensitivitymay explain its restricted distribution amongst prokaryotes. Some plants, including mostlegumes and a few non-legumes form intimate, nitrogen-fixing symbioses with diazotrophs thatprovide the plants with ammonia. As a consequence, legumes have been an integral part ofsustainable agricultural systems for thousands of years. Unfortunately, many important foodspecies, including the grasses maize/corn, rice, and wheat cannot establish effective nitrogen-fixingsymbioses with diazotrophs, which means that they are dependent on nitrogenous fertilizersfor high yield. Large-scale use of industrially-produced N-fertilizer has doubled the influxof N into the terrestrial biogeochemical N-cycle, with serious negative consequences for humanhealth and the natural environment. Therefore, the long-term sustainability of massive N-fertilizerinputs in agriculture has come into question.A team of six investigators has come together to solve the dual nitrogen problems of N-fertilizerover-use in developed countries and soil N-paucity in developing countries by developing effectiveendophytic and associative nitrogen-fixing symbioses in a model and a crop plant species.The team brings together expertise in bacterial and plant genetics, genomics, biochemistry,molecular and cell biology, physiology and synthetic biology with a deep knowledge of biologicalnitrogen fixation. The overarching goal of the project proposed here is to develop effectiveN2-fixing symbioses between the model C4-grass, Setaria viridis, or the related crop species,Zea mays, and the endophytic bacterium, Rhizobium sp. IRBG74, or the associative bacterium,Pseudomonas fluorescenes Pf5. Successful deployment of biological nitrogen fixation in modelor crop grass species will pave the way for a second Green Revolution that will increase cropyields for resource-poor farmers and decrease the use and environmental-impact of industrialN-fertilizers by wealthier farmers.