The metabolism of 3-indolylalkanecarboxylic acids, and their amides, nitriles and methyl esters in plant tissues

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Abstract
Plant growth-regulating activity in homologous series of $\omega $-(3-indolyl)alkanecarboxylic acids, their methyl esters, amides and nitriles has been assessed in the wheat-cylinder, pea-curvature and pea-segment tests. In wheat tissue all homologues were active except 3-indolecarboxylic acid and its corresponding ester, amide and nitrile. These compounds, together with the propio-, butyro-, valero- and capro-nitriles, were inactive in the pea tests. The breakdown of all the compounds when metabolized in wheat coleoptile and pea-stem tissues was investigated using paper chromatography, identification of metabolites being made by chromogenic tests and bio-assay. Strong evidence was obtained that $\beta $-(3-indolyl)propionic acid is itself a plant growth hormone and not a hormone precursor, and that $\beta $-oxidation of this compound within plant tissue to 3-indolecarboxylic acid, is hindered. The formation of either 3-indolylacetic or $\beta $-(3-indolyl)propionic acids from alternate homologues in the acid series is fully consistent with degradation of the side-chain by $\beta $-oxidation. All esters and amides except 3-indoleamide were readily converted within the test tissues to the corresponding acids and their activity is explicable on this basis. With the nitriles in wheat tissue, the results indicated that the terminal-CN is hydrolyzed to-COOH with all homologues except 3-indolenitrile. In pea tissue, however, only 3-indolyl-acetonitrile was hydrolyzed in this way. Evidence for $\alpha $-oxidation was obtained in wheat with all homologues except 3-indolenitrile. In pea tissue, however, only 3-indolylacetonitrile and $\zeta $-(3-indolyl)heptanonitrile were so degraded. The mechanisms underlying oxidative breakdown of the side-chain of these indole homologues are discussed. The results are shown to provide further evidence for $\beta $-oxidation of acids and $\alpha $-oxidation of nitriles within plant tissues and, in addition, it is suggested that with longer chain lengths, nitriles may also be subject to $\omega $-oxidation.