FURTHER EVIDENCE FOR A CHEMICAL REACTION BETWEEN PLANT GROWTH-REGULATORS AND A PLANT SUBSTRATE

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Abstract
Increased elongation of Avena coleoptile sections brought about by 2,6-dichlorobenzoic acid is accompanied by the release of Cl ion. This observation substantiates the interpretation of the reaction of the growth regulator as a displacement of the electronegative atom by a nucleophilic substrate. The reaction of the growth regulator through the carboxyl group and the ortho position requires a basic group and a nucleophilic group on the substrate which are separated by a min. distance corresponding to that between the carboxyl group and the ortho position of benzoic acid. The cysteinyl unit of a protein appears to be the most likely such substrate in the plant cell. Cyclic reaction products between cysteine and the benzoic and phenoxyacetic acids are posssible on chemical grounds. The different activities of D and L optical isomers and cis and trans isomers, and the inhibitory effects of unsaturated lactones, arsenite, iodoacetate, p-chloromercuribenzoate, and cysteine may be explained by the ortho reaction hypothesis for plant growth-regulators.