INDUCTION OF ETHYLENE BIOSYNTHESIS AND POLYAMINE ACCUMULATION IN CUCUMBER FRUIT IN RESPONSE TO CARBON-DIOXIDE STRESS

Carbon dioxide stress-induced ethylene biosynthesis, respiration and polyamine accumulation in cucumber fruit (Cucumis sativus L, cv. Sharp-1) held at 25 degrees C was investigated. Control fruit produced little ethylene and the respiration rate decreased with increase in incubation time while polyamine levels decreased. Elevated CO2 induced ethylene production, respiration and polyamine accumulation. Putrescine and spermidine levels increased in response to CO2 treatment, whereas spermine levels were not significantly affected. No cadaverine was detected in all treatments. The increase in ethylene production paralleled increases in 1-aminocyclopropane-1-carboxylic acid (ACC) and the activities of both ACC synthase and in vitro ACC oxidase. Infiltration of the fruit with aminooxyacetic acid, a potent inhibitor of the conversion of S-adenosylmethionine (AdoMet) to ACC completely blocked CO2 stress-induced ethylene production. Similarly, cycloheximide, an inhibitor of nucleocytoplasmic protein synthesis effectively blocked CO2 stress induction of polyamine accumulation, ethylene production, ACC formation and the development of ACC synthase. Withdrawal of CO2 gas caused cessation of increases in ethylene production, respiration, ACC, putrescine and the activities of ACC synthase and ACC oxidase, but caused increase in spermidine and spermine levels. These data indicate that CO2 induces de novo synthesis of ACC synthase thereby causing accumulation of ACC and increase in ethylene production and suggest that the conversion of AdoMet to ACC is the rate-limiting step in CO2 stress-induced ethylene biosynthesis. The induction, however, requires continuous presence of the stimulus. The results also suggest that protein synthesis might be required for the CO2 stress induction of polyamine biosynthesis. The results further suggest that in cucumber fruit under CO2 stress, at least, the ethylene and polyamine biosynthetic pathways are not competitive.