Regulation of ethylene biosynthesis in higher plants by carbon dioxide

Elevated CO2 levels used with or without reduced O-2 levels regulate many biochemical and physiological processes in higher plants, among them ethylene biosynthesis. The mode of action of elevated CO2 in the regulation of ethylene biosynthesis is still a subject of much debate. Various hypotheses have been put forward to explain its model(s) of action and most of them have pointed out that CO2 regulates ethylene biosynthesis, at least in part, by counteracting ethylene action. This is thought to be mainly through the regulation of 1-aminocyclopropane-1-carboxylate (ACC) synthase, presumably the rate-limiting enzyme in the ethylene biosynthetic pathway, and in some instances ACC oxidase. The present review brings together recent developments on the biochemical, physiological and molecular bases for the regulation by CO2 of ethylene biosynthesis in higher plants. The mode of activation of ACC oxidase by CO2 is also discussed.