Tryptophan-dependent indole-3-acetic acid biosynthesis by 'IAA-synthase' proceeds via indole-3-acetamide

Plants are suggested to produce their major growth promoting phytohormone, indole-3-acetic acid (IAA), via multiple redundantly operating pathways. Although great effort has been made and plenty of possible routes have been proposed based on experimental evidence, a complete pathway for IAA production has yet to be demonstrated. In this study, an in-vitro approach was taken to examine the conversion Of L-tryptophan (L-trp) to IAA by gas chromatography-mass spectrometry (GC-MS). Especially the influence of putative reaction intermediates on the enzymatic conversion of L-trp to IAA was analyzed. Among the substances tested only indole-3-acetamide (IAM) showed a pronounced effect on the L-trp conversion. We additionally report that IAM is synthesized from L-trp and that it is further converted to IAA by the utilized cell free Arabidopsis extract. Together, our results underscore the functionality of an IAM-dependent auxin biosynthesis pathway in Arabidopsis thaliana. (C) 2009 Elsevier Ltd. All rights reserved.