REU: Chemotaxis in Soil Rhizosphere and Symbiotic Infection

趋化性是细菌通过检测重要的营养物质、毒物和其他物质，通过游泳向源头或远离源头移动的过程。涉及趋化作用的分子、细胞和遗传机制已经在一些细菌中得到了广泛的研究，如大肠杆菌。然而，几乎没有人做过任何工作来确定趋化性如何有助于细菌物种在自然环境中的生存和竞争成功。趋化性是一种非常复杂的行为反应。它既需要60多个基因的协调和功能，也需要花费细胞相当大一部分的能量资源。因此，趋化性及其调控似乎是许多细菌生活方式和竞争成功的主要决定因素。拟议的研究将探索趋化性在一种特定的土壤细菌--慢生根瘤菌--的生态成功中所起的作用。大豆芽孢杆菌是一种重要的大豆共生菌。它刺激根瘤的形成，在根瘤中，细菌将空气中的氮转化为氨，以共生交换植物通过光合作用固定的碳。根瘤的形成取决于细菌在没有宿主的情况下在土壤中生存的能力，它检测、到达和定植宿主根表面的能力，以及它到达表面少数几个易受感染的根细胞的能力。趋化性对每一个步骤的贡献将被调查。以前的研究表明，趋化性有缺陷的细菌突变体在结节形成方面的效率明显较低。相反，趋化性增强的突变体是更有效的结瘤菌。结节形成所需的几个基因的突变特别地取消了一些趋化反应。在进一步的研究中，将分离出趋化性增强、改变或功能障碍的日本双胞菌突变体，并与亲本菌株混合进行鉴定和比较，以确定哪些方面的趋化性对土壤存活、根部定植和共生感染起重要作用。长期目标是了解趋化性如何有助于成功地处理复杂、动态环境的需求，并利用这一知识来增强植物相关细菌的共生性能。

Chemotaxis is the process by which bacteria detect important nutrients, poisons and other substances and move toward or away from their source by swimming. The molecular, cellular and genetic mechanisms involved in chemotaxis have been studied extensively in a few bacteria such as E. coli. However, there has been virtually no work done to determine how chemotaxis contributes to the survival and competitive success of bacterial species in natural environments. Chemotaxis is a very sophisticated behavioral response. It requires both the co- ordination and functioning of over 60 genes and the expenditure of a significant fraction of the cell's energy resources. So it seems likely that chemotaxis and its regulation are major determinants of lifestyle and competitive success for many bacteria. The proposed studies will explore the role of chemotaxis in the ecological success of a particular soil bacterium, Bradyrhizobium japonicum. B. japonicum is an agronomically important symbiont of soybean. It stimulates the formation of nodules in which the bacterium converts nitrogen from the air into ammonia for the plant in symbiotic exchange for carbon fixed by the plant through photosynthesis. Formation of nodules depends on the ability of the bacterium to survive in the soil in the absence of its host, its ability to detect, reach and colonize the root surface of its host, and its ability to get to those few root cells on the surface that are developmentally susceptible to infection. The contribution of chemotaxis to each of these steps will be investigated. Previous studies have shown that bacterial mutants which have defective chemotaxis are significantly less efficient in nodule formation. Conversely, mutants with enhanced chemotaxis are more efficient nodulators. Mutations in several of the genes required for nodule formation specifically abolish some chemotactic responses. In further studies, mutants of B. japonicum with enhanced, altered or disfunctional chemotaxis will be isolated, characterized and compared in mixtures with the parental strain to determine which aspects of chemotaxis contribute importantly to soil survival, root colonization and symbiotic infection. Long range goals are to understand how chemotaxis contributes to success in dealing with the demands of complex, dynamic environments and to use this knowledge to enhance the symbiotic performance of plant- associated bacteria.