(IAA) The principal *auxin
of most plants. The other indole compounds that also occur in plants probably owe their auxin-like activity to conversion to IAA at, or near to, the site of action. Thus, indole pyruvic acid, indole acetaldehyde, and other compounds may act as IAA reserves. The route of IAA synthesis is from the amino acid tryptophan. Studies in vivo have shown that IAA is transported in greater quantity and inactivated or destroyed more quickly than synthetic auxins. This suggests that an important aspect in the control of auxin activity is the relative rates of synthesis, transport, and inactivation in different tissues. IAA is decomposed by light and by the enzyme IAA oxidase. It may also combine with other compounds to form an inert complex. The rapid oxidation of IAA by the plant's enzymes has limited its commercial use as a weedkiller, and synthetic auxins, e.g. 2,4-D, 2,4,5-T, and MCPA, are normally used.