Identification and characterization of Arabidopsis indole-3-butyric acid response mutants defective in novel peroxisomal enzymes

Genetic evidence suggests that indole-3-butyric acid (IBA) is converted to the active auxin indole-3-acetic acid (IAA) by removal of two side-chain methlene units in a process similar to fatty acid beta-oxidation. Previous studies implicate peroxisomes as the site of IBA metabolism, although the enzymes that act in this process are still being identifed. Here, we describe two IBA-response mutants, ibr1 and ibr10. Like the previously described ibr3 mutant, which disrupts a putative peroxisomal acyl-CoA oxidase/dehydrogenase, ibr1 and ibr10 display normal IAA reponses and defective IBA reponses. These defects include reduced root elongation inhibition, decreased lateral root initiation, and reduced IBA-responsive gene expression. Howerer, peroxisomal energy-generating patways necessary during early seediing development are unaffected in the mutants. Positional cloning of the genes reponsible for the mutant defects reveals that IBR1 encodes a member of the short-chain dehydrogenase/reductase family and that IBR10 resembles enoly-CoA hydratases/isomerases. Both enzymes contain C-terminal peroxisomal-targeting signals, consisent with IBA metabolism occuring in peroxisomes. We present a model in which IBR3, IBR10, and IBR1 may act sequentially in peroxisomal IBA beta-oxidation to IAA.