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Levels of auxin, which regulates both cell division and cell elongation in plant development, are controlled by synthesis, inactivation, transport, and the use of storage forms. However, the specific contributions of various inputs to the active auxin pool are not well understood. One auxin precursor is indole-3-butyric acid (IBA), which undergoes peroxisomal β-oxidation to release free indole-3-acetic acid (IAA). We identified ENOYL-COA HYDRATASE2 (ECH2) as an enzyme required for IBA response. Combining the ech2 mutant with previously identified iba response mutants resulted in enhanced IBA resistance, diverse auxin-related developmental defects, decreased auxin-responsive reporter activity in both untreated and auxin-treated seedlings, and decreased free IAA levels. The decreased auxin levels and responsiveness, along with the associated developmental defects, uncover previously unappreciated roles for IBA-derived IAA during seedling development, establish IBA as an important auxin precursor, and suggest that IBA-to-IAA conversion contributes to the positive feedback that maintains root auxin levels.
INTRODUCTION
Auxin is a key phytohormone that directs both cell division and cell elongation, thus regulating critical aspects of plant growth and development (reviewed in Perrot-Rechenmann, 2010). The active auxin indole-3-acetic acid (IAA) is a potent growth regulator, and its levels are modulated through synthesis, regulated transport, and storage forms (reviewed in Woodward and Bartel, 2005b). However, the relative importance of various pathways that contribute to the active auxin pool is not well understood.
The side chain of the auxin precursor indole-3-butyric acid (IBA) is two carbons longer than the IAA side chain and is shortened to IAA in numerous plants (Fawcett et al., 1960; reviewed in Epstein and Ludwig-Müller, 1993) in a peroxisome-dependent manner (Strader et al., 2010). Genetic screens in Arabidopsis thaliana have revealed that the auxin activity of IBA requires conversion to IAA through a multistep process similar to fatty acid β-oxidation, which removes two-carbon units from fatty acyl-CoA molecules. Proteins required for full IBA responsiveness include the ATP binding cassette (ABC) transporter PEROXISOMAL ABC TRANSPORTER1 (PXA1), which may transport IBA into the peroxisome for β-oxidation (Zolman et al., 2001), and proteins required for peroxisome biogenesis, such as PEROXIN4 (PEX4) (Zolman et al., 2005), PEX5 (Zolman et al., 2000), PEX6 (Zolman and Bartel, 2004), and PEX7 (Woodward and Bartel, 2005a; Ramón and Bartel, 2010). In addition, several peroxisomal enzymes are required for...