9808197 Cook The Rhizobium-legume symbiosis constitutes one of the major sources of reduced nitrogen in the biosphere. This relationship culminates in symbiotic nitrogen fixation within a specialized organ, termed a "nodule", that develops on the plant root. Several lines of evidence indicate that the plant controls symbiotic activity by regulating the number of nodules that form on the plant root. Recently, the PI's laboratory has identified two plant genes that are required for appropriate control of nodule number. Mutations in these genes result in an approximately 10-fold increase in the number of nodules on the plant root. Genetic analyses indicate that these mutants, named "supernumerary" (sun) and "sickle" (skl), represent separate, unlinked genes that may act independently to control nodule number. Research under this grant will further examine the nature of these mutants with the goal of understanding how the host plant regulates symbiotic development. The specific objectives of this research include (1) developing a frame work genetic pathway for nodulation involving sun, skl and other nodulation mutants, (2) examining the probable role of sun and skl in plant hormone perception, and (3) to map and clone the sun and skl genes.
