This Presidential Young Investigator award is to support research on understanding the cellular and molecular mechanisms that regulate plant growth and development. The specific model to be employed involves the specific interaction between the nitroten-fixing bacterium, Rhizobium, and root nodules of legumes such as alfalfa. Specifically the PI will examine the mechanisms by which Rhizobium invades the root nodules of the plant without triggering the host defense mechanism, the genes involved in nodule development, the structural proteins of cell walls involved in nodule development and how the genetic program for nodule development is programmed. This research has fundamental importance to our understanding of cellular and molecular mechanisms of plant growth.