Originally published as Genetics Published Articles Ahead of Print on August 24, 2008.

Genetics, Vol. 180, 237-251, September 2008, Copyright © 2008
doi:10.1534/genetics.108.090399

Identification and Characterization of Arabidopsis Indole-3-Butyric Acid Response Mutants Defective in Novel Peroxisomal Enzymes

Bethany K. Zolman*,1, Naxhiely Martinez{dagger}, Arthur Millius{dagger},2, A. Raquel Adham{dagger} and Bonnie Bartel{dagger}

* Department of Biology, University of Missouri, St. Louis, Missouri 63121 and {dagger} Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005

1 Corresponding author: Department of Biology, University of Missouri, R223 Research Bldg., 1 University Blvd., St. Louis, MO 63121-4400.
E-mail: zolmanb{at}umsl.edu

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 methylene units in a process similar to fatty acid β-oxidation. Previous studies implicate peroxisomes as the site of IBA metabolism, although the enzymes that act in this process are still being identified. 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 responses and defective IBA responses. These defects include reduced root elongation inhibition, decreased lateral root initiation, and reduced IBA-responsive gene expression. However, peroxisomal energy-generating pathways necessary during early seedling development are unaffected in the mutants. Positional cloning of the genes responsible for the mutant defects reveals that IBR1 encodes a member of the short-chain dehydrogenase/reductase family and that IBR10 resembles enoyl-CoA hydratases/isomerases. Both enzymes contain C-terminal peroxisomal-targeting signals, consistent with IBA metabolism occurring in peroxisomes. We present a model in which IBR3, IBR10, and IBR1 may act sequentially in peroxisomal IBA β-oxidation to IAA.