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。可利用氮的缺乏限制了微生物、植物和动物的生长。农业土壤缺氮限制了植物生产食物、饲料、纤维和燃料。大自然通过重氮营养细菌的生物固氮进化解决了氮的限制问题，它将大气中的N2还原为NH3， NH3很容易被生物分子吸收。生物固氮是由一种称为氮酶的复杂金属酶催化的，其对氧的敏感性可以解释其在原核生物中的有限分布。一些植物，包括大多数豆科植物和少数非豆科植物，与重氮营养体形成亲密的固氮共生关系，为植物提供氨。因此，数千年来，豆类一直是可持续农业系统的一个组成部分。不幸的是，许多重要的食物物种，包括禾草、玉米、水稻和小麦，不能与重氮营养体建立有效的固氮共生关系，这意味着它们依赖氮肥来获得高产。大规模使用工业生产的氮肥使进入陆地生物地球化学氮循环的氮增加了一倍，对人类健康和自然环境产生了严重的负面影响。因此，大量氮肥投入农业的长期可持续性受到质疑。一个由六名研究人员组成的研究小组通过在一种模式植物和一种作物物种中培养有效的内生固氮共生和联合固氮共生，共同解决了发达国家氮肥过度使用和发展中国家土壤氮缺乏的双重氮问题。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方面的专业知识，并对生物固氮有深入的了解。本项目提出的总体目标是在模式c4草（Setaria viridis）或相关作物品种（Zea mays）与内生细菌（Rhizobium sp. IRBG74）或相关细菌（Pseudomonas fluorescenes Pf5）之间建立有效的固定ven2的共生关系。生物固氮在模型或作物牧草物种中的成功应用将为第二次绿色革命铺平道路，这将提高资源贫乏农民的作物产量，减少富裕农民对工业氮肥的使用和对环境的影响。

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.