We are using molecular genetic approaches to elucidate functions of indole-3-butyric acid (IBA) in Arabidopsis.  Although IBA is a naturally occurring form of the plant growth hormone auxin and is used commercially to promote rooting in many species, the molecular mechanisms by which it acts are only beginning to be understood.

We have isolated IBA-response (ibr) mutants; some of these mutants have b-oxidation and peroxisome biogenesis defects.  Our analysis suggests that IBA is converted into indole-3-acetic acid (IAA) using reactions analogous to those of fatty acid catabolism (b-oxidation), a largely peroxisomal process in plants.  Thus these mutants are contributing to our understanding of plant peroxisomes and the genes necessary for their biogenesis.

To understand auxin action, the functional significance of the endogenous auxins must be determined.  Identifying genes involved in converting IBA to IAA is a prerequisite to understanding the regulation and importance of this conversion.  This knowledge is essential to determine the contributions of IBA relative to other inputs to the active auxin pool, including de novo synthesis and conjugate hydrolysis.  In addition, elucidating the molecular mechanisms of IBA action in a genetically tractable plant may provide insights for agricultural IBA uses.  For example, identifying and characterizing the specific isozymes that convert IBA to IAA may facilitate their modification in difficult-to-root cultivars where IBA application is normally ineffective.

pxa1 is an IBA response mutant; it elongates its primary root on inhibitory concentrations of IBA, but is resistant to the stimulatory effects of IBA on lateral root formation. pxa1 is defective in a peroxisomal transporter (Zolman et al., 2001).