Auxin Activity of Some Indole Derivatives.

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Abstract
Since most studies on the chemical structure and biological activity of auxins have been made with aryl and aryloxy acids there is little information on the indole derivatives, although their relationship to the natural auxin indole-acetic acid (IAA) makes them of special interest. The activities of 20 indole compounds and 4 others were studied in 3 bio-assays: straight growth of sections of Avena coleoptile and Pisum stem, and curvature of slit Pisum stem. Indole itself promotes growth but there was no evidence for its conversion to IAA in vivo. Skatole inhibits growth, and in Pisum stems the inhibition is not competitive with IAA. Four hydroxy-acids, indolelactic, indoleglycolic, and 5-and7-hydroxy-IAA all have very low activity, agreeing with the low activity of hydroxy-aromatic acids. Six derivatives of oxindole and 3 of imidazole are inactive, the latter confirming the rule that 5-membered rings do not confer activity. The inactivity of indolemethylene malonic acid agrees with the inactivity of other dicarboxylic acids. Indole-3-isobutyric acid has real auxin activity in all tests. Indole-4-acetic acid has 1-2% of the activity of IAA, which accords poorly with 2 theories of the structural requirements for auxin activity. The 7-aza derivative of IAA has high activity, close to that of IAA, in all 3 assays. It is concluded that (1) auxin antagonists should be defined by the specific bioassay in which they function, (2) the empirical rules relating structure of aromatic compounds to auxin activity also hold in the indole series, but (3) the requirements for activity, though evidently stringent, are not well described by any of the recently proposed theories.
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