Inhibition of auxin-induced ethylene production in cucumber fruit discs by carbon dioxide

The induction of ethylene biosynthesis by indole-3-acetic acid (IAA) and its regulation by CO₂ was examined in cucumber (Cucumis sativus L., var. Sharp-1) fruit discs. Treatment of discs with IAA significantly stimulated ethylene production and the extent depended on IAA concentration upto 1 mM. The increase in IAA-induced ethylene production was accompanied by increase in the activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase (also known as ethylene-forming enzyme) and a massive accumulation of ACC and 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC). The induction of ethylene production by IAA was inhibited by treatment with CO₂. The extent of inhibition was dependent upon the concentration of CO₂. Carbon dioxide inhibited IAA-induced ethylene biosynthesis by suppressing ACC synthase and ACC oxidase activities. Additionally, CO₂ inhibited ACC conjugation into MACC. The results indicate that inhibition of ethylene production by CO₂ in cucumber fruit discs following IAA treatment is due to inhibition of both ACC synthesis and ACC oxidation.