The Role of 3-Methyleneoxindole in Auxin Action

A product of the enzymatic oxidation of indole-3-acetic acid, 3-methyleneoxindole, is at least 10-fold more effective than indole-3-acetic acid as a plant auxin as estimated by stimulation of elongation of excised pea and mung bean stem segments and by the stimulation of protein synthesis in the subapical regions of decapitated pea seedlings. An obligatory role for 3-methyleneoxindole in such auxin-stimulated responses is suggested by the following additional observations. 1. Chlorogenic acid, which blocks the enzymatic oxidation of indole-3-acetic acid to 3-methyleneoxindole in peas, antagonizes the stimulatory effect of indole-3-acetic acid on elongation and protein synthesis but does not interfere with the same stimulatory effects of 3-methyleneoxindole, the product of the blocked reaction. 2. 3-Methyleneoxindole reacts directly with sulfhydryl compounds; as expected, its growth-promoting activity is blocked by reduced, but not by oxidized, glutathione. Reduced glutathione also blocks the action of indole-3-acetic acid, which does not react directly with sulfhydryl compounds. 3. A synthetic auxin, 1-naphthaleneacetic acid, cannot be oxidized to 3-methyleneoxindole; however, it has another relationship to 3-methyleneoxindole metabolism as an inhibitor of the enzymatic reduction of 3-methyleneoxindole to 3-methyloxindole, an inert substance. Small amounts of 1-naphthaleneacetic acid, inactive alone, cause a considerable reduction in the amount of 3-methyleneoxindole required for optimal stimulation of growth. It is therefore suggested that 1-naphthaleneacetic acid might act by maintaining endogenous supplies of 3-methyleneoxindole at elevated intracellular levels through inhibition of 3-methyleneoxindole reductase. It is proposed that the stimulatory effects of 3-methyleneoxindole might result from its previously demonstrated ability to render a growth rate-limiting regulatory protein insensitive to its negative effector in a bacterial model system. The necessary specificity of action, both in terms of comparison to ordinary sulfhydryl reagents which do not have auxin activity and from the point of view of interaction with its cellular targets, may reside in its oxindolic structure and in the fact that it is a specific substrate for inactivating enzymes of plants, 3-methyleneoxindole reductases.