Comparison of indole derivatives as potential intermediates of auxin biosynthesis in Arabidopsis

Biosynthesis of indole-3-acetic acid (IAA) is crucial for the regulation of plant growth and morphological changes. However, the pathways of IAA biosynthesis have not been completely understood. Although indole derivatives have been proposed to be intermediates of the pathway, it has not been thoroughly elucidated whether the proposed intermediates are indeed converted into an active auxin, IAA, or intermediate metabolites other than IAA. In the present work, we examined 11 indole derivatives whether they are indeed IAA intermediates. The indole derivatives were examined by measuring their effects on recovery from auxin-deficiency conditions caused by L-aminooxyphenylpropionic acid (AOPP), which we previously identified as an IAA biosynthesis inhibitor. We estimated auxin activity of each indole derivative by analyzing auxin-inducible marker gene expression and root morphology in Arabidopsis. We found that indole-3-pyruvic acid most effectively recovered seedlings from auxin deficiency among the indole derivatives examined, followed by N-hydroxyl tryptamine and indole-3-acetonitrile, with effective recovery from the auxin deficiency caused by AOPP. We also found that tryptophol recovered the root growth defects. Among the indole derivatives examined, only indole-3-acrylic acid did not show effective recovery from auxin deficiency. These results provide information on which compounds are reliable intermediates of IAA biosynthesis pathways in Arabidopsis.