Gravity is one of the primary environmental signals determining the orientation of root and shoot growth in many plant species. The gravitropic response involves perception of the gravity signal, transmission of the signal, and execution of a response program that re- orients growth. Thus gravitropism constitutes a plant signal transduction pathway. This pathway is known to depend on transport and reception of the plant hormone auxin. The proposed research is a continuation of our effort to identify and characterize genes involved in gravitropic signal transduction in Arabidopsis thaliana. Here we will focus on further analysis of T-DNA-tagged mutants. We will continue the physiological and molecular genetic characterization of our previously isolated mutants to establish the functional roles of the genes affected by the mutations. Molecular cloning and characterization of wild-type alleles will further define the roles of these genes. Wild-type clones will be re-introduced into Arabidopsis by Agrobacterium-mediated stable transformation to test for complementation of mutant phenotypes. Genetic and molecular approaches will be used to elucidate interactions of these genes with each other and with previously identified genes involved in gravitropism. Stable and transient transformation experiments will be used to demonstrate the biological effects of altering the normal expression patterns of these genes. This work will provide new insights into the molecular mechanisms of gravitropism and of auxin-mediated signal transduction in plants.