Genetic analysis of indole-3-butyric acid response mutants in Arabidopsis thaliana

Auxins are a family of hormones that influence numerous aspects of plant growth and development, including apical dominance, vascular development, tropic responses, inhibition of root elongation, and initiation of lateral roots. Indole-3-butyric acid (IBA) is a naturally-occurring auxin that efficiently induces rooting and is widely used in commercial and agricultural settings. Interestingly, one mechanism of IBA action is via its conversion to the more abundant auxin indole-3-acetic acid (IAA) in a pathway similar to fatty acid beta-oxidation. This reaction should occur in peroxisomes, which are small organelles that contain enzymes required for fatty acid beta-oxidation, the glyoxylate cycle, and branched-chain amino acid catabolism. More than 20 peroxin (PEX) proteins are required for peroxisomal biogenesis and maintenance. To better understand the in vivo role of this naturally-occurring auxin, we have identified a collection of Arabidopsis thaliana mutants that are resistant to the inhibitory effects of IBA on root elongation, but that remain sensitive to IAA. These mutants have defects in various IBA-mediated responses, which allowed us to group them into five phenotypic classes. Defects in seedling development in the absence of exogenous sucrose suggest that some of these mutants have defects in the beta-oxidation of seed storage lipids, implying that the conversion of IBA to IAA also is disrupted. Other mutants appear to have normal peroxisomal function; these may be defective in IBA transport, signaling, or responses. We have used positional information to clone the genes defective in eight distinct loci. Three mutants, pex5, pex6, and pxa1, have defects in proteins expected to act in peroxisomal biogenesis or import. In addition, we have isolated mutants with defects in the medium-chain acyl-CoA oxidase ACX3 and in two peroxisomal acyl-CoA dehydrogenases with unknown functions. Lastly, we have identified a mutant defective in CHY1, which encodes an acyl-CoA hydrolase that may act in peroxisomal valine catabolism. These results indicate that in Arabidopsis, IBA acts, at least in part, via its conversion to IAA. Moreover, IBA resistance is a powerful tool to identify genes acting in peroxisomal beta-oxidation, providing an unbiased approach for studying peroxisomal function in plants