IBR3, a novel peroxisomal acyl-CoA dehydrogenase-like protein required for indole-3-butyric acid response

Indole-3-butyric acid (IBA) is an endogenous auxin that acts in  Arabidopsis  primarily via its conversion to the principal auxin indole-3-acetic acid (IAA). Genetic and biochemical evidence indicates that this conversion is similar to peroxisomal fatty acid β-oxidation, but the specific enzymes catalyzing IBA β-oxidation have not been identified. We identified an  IBA-response  mutant ( ibr3 ) with decreased responses to the inhibitory effects of IBA on root elongation or the stimulatory effects of IBA on lateral root formation. However,  ibr3  mutants respond normally to other forms of auxin, including IAA. The mutant seedlings germinate and develop normally, even in the absence of sucrose, suggesting that fatty acid β-oxidation is unaffected. Additionally, double mutants between  ibr3  and  acx3 , which is defective in an acyl-CoA oxidase acting in fatty acid β-oxidation, have enhanced IBA resistance, consistent with a distinct role for IBR3. Positional cloning revealed that  IBR3  encodes a putative acyl-CoA dehydrogenase with a consensus peroxisomal targeting signal. Based on the singular defect of this mutant in responding to IBA, we propose that IBR3 may act directly in the oxidation of IBA to IAA.