The metabolism of certain acids, amides and nitriles within plant tissues

The plant growth-regulating activities of the first six members of the homologous series $\omega $-$(2\colon 4$-$\text{dichlorophenoxy})$ alkanecarboxylic acids and the corresponding amides and nitriles have been assessed in the wheat cylinder, pea curvature, pea segment and tomato-leaf epinasty tests. The breakdown of these compounds when metabolized under standard conditions in wheat coleoptile and pea-stem tissues has also been investigated using paper chromatography together with chromogenic and bio-assay procedures. In addition, colorimetric analysis was used. The alternation in activity shown by the homologous series of acids and the production of 2:4-dichlorophenoxyacetic acid and of 2:4-dichlorophenol from alternate members indicates that $\beta $-$\text{oxidation}$ of the side-chain of these acids occurs within wheat and pea tissues. The behaviour of the amides was similar to that of the acids and explicable in terms of hydrolysis to corresponding acid within the tissue followed by $\beta $-$\text{oxidation}$. Only the first member of the nitrile series was active in the pea and tomato tests, though in the wheat cylinder test all showed some activity, that of the propionic derivative, however, being only slight. With the exception of this compound, the nitriles yielded 2:4-dichlorophenoxyacetic acid and all gave some 2:4-dichlorophenol when metabolized in wheat tissue. These results can be interpreted in terms of modification of the $-\text{CH}_{2}\text{CN}$ grouping by two mechanisms, viz. hydrolysis to corresponding acid, and a type of breakdown which we refer to as $\alpha $-$\text{oxidation}$ which leads to the production of the lower carboxylic acid. The acids produced by either route are then subject to $\beta $-$\text{oxidation}$. A study of the metabolism of 3-indolylacetonitrile provided further evidence that $\alpha $-$\text{oxidation}$ of substituted nitriles can occur in plant tissue. In this instance, the end-product of the reaction was proved to be 3-indolecarboxylic acid and the occurrence of 3-indolealdehyde as an intermediate compound was established. The mechanism of $\alpha $-$\text{oxidation}$ of nitriles is discussed and it is suggested that this may be a general reaction occurring in both plant and animal biochemistry.