The goal of this project is to better understand attachment and infection thread formation of bacteria during the nitrogen-fixing symbiosis that occurs with legumes. The nitrogen-fixing symbiosis between bacteria of the Rhizobiaceae and legume plants involves a constant dialogue between partners that results in bacterial attachment to plant roots, the formation of infection threads through which the bacteria enter the root, and the development of the nodule on plant roots, in which the bacteria convert atmospheric nitrogen into ammonia that is readily assimilated by the plant. Nod factor, a bacterial signal molecule, is critical for infection thread formation and nodule development. However, some legumes differ from this paradigm. They form nitrogen-fixing associations in the absence of nodules and with bacteria other than rhizobia. Infection thread formation and invasion does not require Nod factor suggesting that other bacterial factors are important for infection thread formation and that fewer steps are required for establishment of the symbiosis.
Intellectual Merit: Most bacterial-eukaryotic cell interactions involve host receptor-protein-pili interactions followed by endocytosis. This type of protein-protein interaction could mediate the non-nodulating legume symbiosis. This project will use biochemical, genetic and genomic approaches to assess the role of bacterial pili and to look for putative plant pili-binding proteins in both nodulating and non-nodulating legume symbioses. To elucidate the role of pili in attachment and infection thread formation, candidate genes involved in pili formation will be identified in rhizobial and non-rhizobial species and mutants generated to assess how the symbiosis is affected. Pili-binding proteins also will be identified in model legumes and gene disruption techniques will be employed to determine their requirement for establishment of the association.
Broader impacts: The results of this study will provide a more concrete analysis of the earliest stages of the nitrogen-fixing symbiosis in a non-nodulating legume, and will be prepared for publication in peer-reviewed journals as well as being of topic of discussion in Dr. Hirsch's undergraduate course: Chemical Communication between Plants and Other Organisms. A postdoctoral associate will be directly involved in this research as well as a Ph.D. student. In addition, a Masters degree student will be helping the postdoctoral associate; both are female Asian-Americans.
