Abstract
Genetic evidence in Arabidopsis (Arabidopsis thaliana) suggests that the auxin precursor indole-3-butyric acid (IBA) is converted into active indole-3-acetic acid (IAA) by peroxisomal beta-oxidation; however, direct evidence that Arabidopsis converts IBA to IAA is lacking, and the role of IBA-derived IAA is not well understood. In this work, we directly demonstrated that Arabidopsis seedlings convert IBA to IAA. Moreover, we found that several IBA-resistant, IAA-sensitive mutants were deficient in IBA-to-IAA conversion, including the indole-3-butyric acid response1 (ibr1) ibr3 ibr10 triple mutant, which is defective in three enzymes likely to be directly involved in peroxisomal IBA beta-oxidation. In addition to IBA-to-IAA conversion defects, the ibr1 ibr3 ibr10 triple mutant displayed shorter root hairs and smaller cotyledons than wild type; these cell expansion defects are suggestive of low IAA levels in certain tissues. Consistent with this possibility, we could rescue the ibr1 ibr3 ibr10 short-root-hair phenotype with exogenous auxin. A triple mutant defective in hydrolysis of IAA-amino acid conjugates, a second class of IAA precursor, displayed reduced hypocotyl elongation but normal cotyledon size and only slightly reduced root hair lengths. Our data suggest that IBA beta-oxidation and IAA-amino acid conjugate hydrolysis provide auxin for partially distinct developmental processes and that IBA-derived IAA plays a major role in driving root hair and cotyledon cell expansion during seedling development.
Highlights
The auxin indole-3-acetic acid (IAA) controls both cell division and cell expansion and thereby orchestrates many developmental events and environmental responses
We found that several indole-3-butyric acid (IBA)-resistant, IAA-sensitive mutants were deficient in IBA-to-IAA conversion, including the ibr1 ibr3 ibr10 triple mutant, which is defective in three enzymes likely to be directly involved in peroxisomal IBA β-oxidation
Because genetic evidence suggests that IBA-to-IAA conversion is impaired in mutants defective in peroxisome function or certain peroxisome-targeted enzymes, we examined the mutant biochemical defects in a feeding assay with labeled IBA
Summary
The auxin indole-3-acetic acid (IAA) controls both cell division and cell expansion and thereby orchestrates many developmental events and environmental responses. Normal plant morphogenesis and environmental responses require modulation of auxin levels by controlling biosynthesis, regulating transport, and managing storage forms (reviewed in Woodward and Bartel, 2005a). In some storage forms, the carboxyl group of IAA is conjugated to amino acids or peptides or to sugars, and free IAA can be released by hydrolases when needed (Bartel et al, 2001; Woodward and Bartel, 2005a). A second potential auxin storage form is the side chain-lengthened compound indole-3-butyric acid (IBA), which can be synthesized from IAA (Epstein and Ludwig-Müller, 1993) and is suggested to be shortened into IAA by peroxisomal β-oxidation (Bartel et al, 2001; Woodward and Bartel, 2005a). Genetic evidence suggests that the auxin activity of both IAA-amino acid conjugates and IBA requires free IAA to be released from these precursors (Bartel and Fink, 1995; Zolman et al, 2000). Mutation of Arabidopsis thaliana genes encoding IAA-amino acid hydrolases, including ILR1, IAR3, and ILL2, reduces plant sensitivity to the applied IAA-amino acid conjugates that are substrates of these enzymes, including IAA-Leu, IAA-Phe, and IAA-Ala (Bartel and Fink, 1995; Davies et al, 1999; LeClere et al, 2002; Rampey et al, 2004), which are present in Arabidopsis (Tam et al, 2000; Kowalczyk and Sandberg, 2001; Kai et al, 2007)
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