Molecular and genetic analysis of IBA homeostasis and action in Arabidopsis

Auxin is an important plant phytohormone that influences virtually all aspects of plant growth and development. Despite the ubiquitous involvement of auxin in plant processes, gaps remain in our knowledge of auxin homeostasis and signal transduction. There are several endogenous forms of auxin including, indole-3-acetic acid (IAA), and indole-3-butyric acid (IBA) (Ludwig-Muller and Epstein, 1993). IBA appears to undergo a two-carbon elimination reaction similar to the one used in fatty acid beta-oxidation in peroxisomes to yield IAA (Zolman et al., 2000). To understand the roles of IBA in plants, I have used forward and reverse genetics to isolate IBA-r&barbelow;esponse ( ibr) mutants that display enhanced root elongation on inhibitory concentrations of IBA and yet maintain a wild-type phenotype on IAA. I have categorized these mutants into three distinct classes based on their root and hypocotyl phenotypes under various hormone, sugar, and light conditions. One project involves a mutant, B423, with an exogenous sucrose-dependence for germination, and an inability to produce lateral roots in response to IBA and without hormone. The phenotypes of this mutant suggest a peroxisomal deficiency including defective beta-oxidation of lipid stores and an inability to convert IBA to IAA, necessary for lateral root production. Interestingly, this ibr mutant maps to a region lacking genes known to function in peroxisomal processes. This mutant may define a novel component that has yet to be identified in other organisms. A second project involves two mutants, B705 and B839, which are sucrose independent for germination and produce wild-type numbers of lateral roots in response to IBA. We used positional cloning to determine that one mutant is defective in ACX1 and the other is defective in ACX3, two of the six fatty acyl-CoA oxidase (ACX) genes in Arabidopsis. Characterization of T-DNA insertion mutants defective in the other ACX genes revealed reduced IBA responses in a third mutant, acx4. Activity assays demonstrated that mutants defective in ACX1, ACX3, and ACX4 have reduced fatty acyl-CoA activity on specific substrates. Moreover, double mutant analysis shows that ACX2 and ACX5 also contribute to IBA response