REU: Chemotaxis in Soil Rhizosphere and Symbiotic Infection

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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.