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

Abstract

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 boxidation. 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 peroxisomaltargeting signals, consistent with IBA metabolism occurring in peroxisomes. We present a model in which IBR3, IBR10, and IBR1 may act sequentially in peroxisomal IBA b-oxidation to IAA.

Conversion of Endogenous Indole-3-Butyric Acid to Indole-3-Acetic Acid Drives Cell Expansion in Arabidopsis Seedlings

“…Mutation of genes encoding the apparent b-oxidation enzymes INDOLE-3-BUTYRIC ACID RESPONSE1 (IBR1), IBR3, or IBR10 results in IBA resistance, but does not alter IAA response or confer a dependence on exogenous carbon sources for growth following germination (Zolman et al, 2000(Zolman et al, , 2007(Zolman et al, , 2008, consistent with the possibility that these enzymes function in IBA b-oxidation but not fatty acid b-oxidation.…”

“…Similarly, mutants defective in the peroxisomal ABC transporter PXA1, which may transport substrates into the peroxisome for b-oxidation (Zolman et al, 2001b), and mutants defective in several peroxisome-targeted enzymes, including the acyl-CoA oxidase double mutant acx1-2 acx2-1 (Adham et al, 2005), the branchedchain amino acid catabolism mutant chy1-10 (Zolman et al, 2001a), and the IBA response triple mutant ibr1-2 ibr3-1 ibr10-1 (Zolman et al, 2008), are IBA resistant and IAA sensitive ( Fig. 1A; Zolman et al, 2001aZolman et al, , 2001bZolman et al, , 2008Adham et al, 2005).…”

“…Unlike the simple one-step release of free IAA from amino acid conjugates, release of IAA from IBA is suggested to require a multistep process (Zolman et al, 2007(Zolman et al, , 2008. Conversion of IBA to IAA has been demonstrated in a variety of plants (Fawcett et al, 1960; for review, see Epstein and Ludwig-Mü ller, 1993) and may involve b-oxidation of the four-carbon carboxyl side chain of IBA to the two-carbon side chain of IAA (Fawcett et al, 1960;Zolman et al, 2000Zolman et al, , 2007.…”

“…The IAA-amino acid hydrolases are predicted to be endoplasmic reticulum localized (Bartel and Fink, 1995;Davies et al, 1999) and enzymes required for IBA responses, including IBR1, IBR3, and IBR10, are peroxisomal (Zolman et al, 2007(Zolman et al, , 2008. Moreover, many peroxisome biogenesis mutants, such as peroxin5 (pex5) and pex7, are resistant to exogenous IBA, but remain IAA sensitive (Zolman et al, 2000;Woodward and Bartel, 2005b).…”

Comparative effects of plant growth regulators on leaf and stem explants of Labisia pumila var. alata

“…Consequently, additional energy would be used up and might eventually lead to insufficient energy needed for cell growth and development. Zolman et al (2008) demonstrated that energy is needed in converting NAA to IAA, hence, displaying reduced responses of NAA in root elongation.…”