Inhibitory Oxidation Products of Indole-3-acetic Acid. Mechanism of Action and Route of Detoxification.

The use of Escherichia coli as a model organism for studies on the biochemistry of the action of indole-3-acetic acid has revealed that two products of the photooxidation of this plant hormone, 3.hydroxymethyloxindole and 3-methyleneoxindole, are potent inhibitors of the growth of both microorganisms and higher plants (I). The latter compound was first identified by Hinman, Bauman, and Lang (2) as a product of the action of horseradish peroxidase on indole-3-acetic acid. It was subsequently shown to be identical with one of the products (1) of the riboflavin-catalyzed photooxidation of indole-a-acetic acid discovered by Galston (3). The identification of the other inhibitory substance produced in the photooxidation as 3-hydroxymethyloxindole is still tentative as it rests, in part, on indirect evidence. Inhibition of the growth of E. coli by both compounds is transient. Examination of cultures in which growth had resumed revealed that the onset of recovery coincides with the conversion of both inhibitors to 3-methyloxindole, a nontoxic substance (1). The experiments described in this report were undertaken in order to determine the enzymatic basis for the inhibitory effects of these oxindoles and to elucidate the reaction or reactions responsible for their detoxification.