Stimulation of the oxidative decarboxylation of indole-3-acetic acid in citrus tissues by ethylene

Ethylene has been shown to stimulate the degradation of indole-3-acetic aci d (IAA) in citrus leaf tissues via the oxidative decarboxylation pathway, r esulting in the accumulation of indole-3-carboxylic acid (ICA). Preliminary data indicated that ethylene stimulates only the first step of this pathwa y, i.e. the decarboxylation of IAA which leads to the formation of indole-3 -methanol. The effect of ethylene seems to be a specific one since 2,5-norb ornadiene, an ethylene action inhibitor, significantly inhibited the stimul ation of IAA decarboxylation by ethylene. It has long been suggested that p eroxidase or a specific form of the peroxidase complex ('IAA oxidase') cata lyse this step. However, we did not observe a clear effect of ethylene on t he peroxidase system. An alternative possibility, that the stimulatory effe ct of ethylene on IAA catabolism results from increased formation of hydrog en peroxide (H2O2), a co-factor for peroxidase activity, was verified by di rect measurements of H2O2 in the tissues or by assaying the activity of glu thathione reductase, which has been shown to be induced by oxygen species. This possibility is further supported by the observations showing that IAA decarboxylation in control tissues was enhanced to the level detected in et hylene-treated tissues by application of H2O2.