METABOLISM OF INDOLE-3-ACETIC-ACID IN ARABIDOPSIS

The metabolism of indole-3-acetic acid (IAA) was investigated in 14-d- old Arabidopsis plants grown in liquid culture. After ruling out metab olites formed as an effect of nonsterile conditions, high-level feedin g, and spontaneous interconversions, a simple metabolic pattern emerge d. Oxindole-3-acetic acid (OxIAA), OxIAA conjugated to a hexose moiety via the carboxyl group, and the conjugates indole-3-acetyl aspartic a cid (IAAsp) and indole-3-acetyl glutamate (IAGlu) were identified by m ass spectrometry as primary products of IAA fed to the plants. Refeedi ng experiments demonstrated that none of these conjugates could be hyd rolyzed back to IAA to any measurable extent at this developmental sta ge. IAAsp was further oxidized, especially when high levels of IAA wer e fed into the system, yielding OxIAAsp and OH-IAAsp. This contrasted with the metabolic fate of IAGlu, since that conjugate was not further metabolized. At IAA concentrations below 0.5 mu M, most of the suppli ed IAA was metabolized via the OxIAA pathway, whereas only a minor por tion was conjugated. However, increasing the IAA concentrations to 5 m u M drastically altered the metabolic pattern, with marked induction o f conjugation to IAAsp and IAGlu. This investigation used concentratio ns for feeding experiments that were near endogenous levels, showing t hat the metabolic pathways controlling the IAA pool size in Arabidopsi s are limited and, therefore, make good targets for mutant screens pro vided that precautions are taken to avoid inducing artificial metaboli sm.