9402659 deBruijn Natural bacteria on and near roots contribute to plant growth, but little is known about how particular bacterial species form communities in the rhizosphere. Therefore, it is important to clarify the molecular basis of plant-microbe signaling and define its role in bacterial growth, survival, and colonization of roots. Recent results show that small quantities of certain plant compounds regulate transcription of genes required for infection by Rhizobium and Agrobacterium bacteria. It is postulated that other plant-induced genes in these bacteria control earlier stages of root colonization and that homologous genes are present in other rhizosphere microbes. This project will use Rhizobium and Agrobacterium as models to find novel plant-regulated genes that are important for understanding broader issues of rhizosphere biology. By moving beyond the well-studied nodulation and virulence genes in these organisms, it is hoped to build a basis for future research on rhizosphere ecology. This project specifically will 1) define chemical structures of plant signals that induce genes in soil bacteria, 2) isolate novel bacterial genes responding to the compounds, 3) deduce signal transduction pathways, and 4) assess the importance of the molecules as regulators of microbial colonization in the soil. These goals will be achieved by identifying natural signals from alfalfa (Medicago sativa L.) which induce transcription of random bioluminescence (lux) gene fusions in R. meliloti and A. tumefaciens. The tools and experience for this project are available in the complementary programs of the three co-principal investigators. D. A. Phillips has studied Rhizobium-legume interactions for many years and uses modern methods to isolate and identify compounds involved in that symbiosis. Drs. Kado and de Bruijn are experienced microbial geneticists with major research programs on Agrobacterium and Rhizobium. This project will define in molecular terms how al falfa controls root colonization by two model soil microbes, and it will develop information and tools required for testing detailed hypotheses on physiological processes that structure microbial communities in the rhizosphere. This is a collaborative project with Drs. Phillips and Kado at the University of California, Davis. ***
