The indole alkaloids brucine, yohimbine, and hypaphorine are indole-3-acetic acid-specific competitors which do not alter auxin transport

The indole alkaloids brucine and yohimbine, just like hypaphorine, counteract indole‐3‐acetic acid (IAA) activity in seedling roots, root hairs and shoots, but do not appear to alter auxin transport in roots or in cultured cells. In roots, the interactions between IAA and these three alkaloids appear competitive and specific since these molecules interact with IAA but with neither 1‐naphthaleneacetic acid (NAA) or 2,4‐dichlorophenoxyacetic acid (2,4‐D), two synthetic auxins. The data reported further support the hypothesis that hypaphorine brucine and yohimbine compete with IAA on some auxin‐binding proteins likely to be auxin receptors and that 2,4‐D and NAA are not always perceived by the same receptor as IAA or the same component of that receptor. At certain steps of plant development and in certain cells, endogenous indole alkaloids could be involved in IAA activity regulation together with other well‐described mechanisms such as conjugation or degradation. Hypaphorine with other active indole alkaloids remaining to be identified, might be regarded as a new class of IAA antagonists.