Characterization of the regulation of ethylene biosynthesis in tomato fruit by carbon dioxide and diazocyclopentadiene

The regulation of ethylene biosynthesis by CO₂ and diazocyclopentadiene (DACP), both inhibitors of ethylene action, was investigated in tomato (Lycopersicon esculentum Mill. cv. 'Momotaro') fruit held at 25 °C. When the tomato fruit at the pink stage of ripeness were treated with 20% CO₂ (+ 20% O₂ + 60% N₂) or DACP, ethylene production by the fruit was rapidly decreased. The inhibition of ethylene production resulted primarily, if not solely, from the suppression of the activities of both 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. The inhibition of ACC synthase activity subsequently led to low levels of ACC. CO₂ treatment further inhibited ACC conjugation into 1-(malonylamino) cyclopropane-1-carboxylic acid (MACC). By contrast, DACP-treated fruit maintained slightly higher levels of MACC relative to the control fruit. When the fruit were transferred from the CO₂-enriched atmosphere to air, ethylene production, ACC and MACC contents and the activities of ACC synthase and ACC oxidase increased gradually to the control level after 24 h, while these values, except for MACC content, remained low in DACP-treated fruit throughout the experimental period. These results indicate that CO₂ and DACP regulate ethylene production in tomato fruit by inhibiting ACC synthase and ACC oxidase activities and further support the hypothesis that the autocatalytic signal associated with ethylene action during fruit ripening stimulates the activities of both enzymes.