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

氮是地球上生物分子和生命的基本元素。缺乏可用的氮素限制了微生物、植物和动物的生长。农业土壤中氮的缺乏限制了植物的食物、饲料、纤维和燃料的生产。自然界通过重氮菌生物固氮的进化解决了氮素限制的问题，重氮菌将大气中的氮气还原为NH3，后者很容易被生物分子同化。生物固氮是由一种名为固氮酶的复杂金属酶催化的，它对氧的敏感性可能解释了它在原核生物中的有限分布。一些植物，包括大多数豆类和一些非豆类植物，与重氮菌形成亲密的固氮共生体，为植物提供氨。因此，几千年来，豆类一直是可持续农业系统不可或缺的一部分。不幸的是，许多重要的粮食作物，包括玉米/玉米、水稻和小麦等，不能与重氮菌建立有效的固氮共生体，这意味着它们依赖氮肥来获得高产。大量使用工业生产的氮肥使流入陆地生物地球化学氮循环的氮素翻了一番，对人类健康和自然环境造成了严重的负面后果。因此，农业中大量氮肥投入的长期可持续性受到了质疑。一个由6名研究人员组成的团队联合起来，通过在一个模式植物和一个作物物种中开发有效的内生和联合固氮共生体，解决发达国家氮肥过度使用和发展中国家土壤缺氮的双重氮素问题。该团队汇集了细菌和植物遗传学、基因组学、生物化学、分子和细胞生物学、生理学和合成生物学的专业知识，并对生物固氮有深入的了解。该项目的总体目标是在模式C4草种狗尾草或相关作物品种玉米与内生细菌根瘤菌之间建立有效的固氮共生。IRBG74，或联合细菌荧光假单胞菌Pf5。在Modelor作物草种中成功部署生物固氮将为第二次绿色革命铺平道路，这将为资源贫乏的农民增加农田，减少富裕农民使用工业氮肥并减少对环境的影响。

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.