Ethylene: A regulator of stress‐induced ACC synthase activity in nonclimacteric fruit

Abstract

In spite of precautions, harvesting, packing and shimpping of grapefruit (Citrus paradisi) can often result in injury to the peel, which in turn leads to an increased evolution of the ripening hormone ethylene by the fruit. The present study is an attempt to determine how the stress‐induced ethylene pathway is regulated using the 1‐aminocyclopropane‐1‐carboxylate (ACC) synthase (EC 4.4.1.14) action inhibitor aminoethoxyvinylglycine (AVG). Incubation of mature grapefruit flavedo discs in the presence of AVG created a super‐induction of ACC synthase (ACS) activity. Maximum enzyme activity was achieved following an incubation period of between 24 and 36 h. From AVG‐treated flavedo discs, cDNA encoding ACS was isolated, employing reverse transcriptase‐polymerase chain reaction (RT‐PCR), and the accumulation of ACS transcripts, which peaked after 36 h incubation, was found to complement the recorded enzyme activity. Saturation of ACS activity was achieved with 100 μM AVG. Incubation of flavedo with AVG significantly decreased both the conversion of S‐adenosyl‐l‐methionine (AdoMet) to ACC and the evolution of ethylene from the treated tissue, and in vitro incubation of extracted ACS enzyme with AVG led to repression of the action of the enzyme. Treatment of flavedo discs with ACC and ethylene inhibited the AVG‐induced ACS activity. Northern blot analysis showed that ethylene sumppressed the accumulation of ACS mRNA. It is suggested that in nonclimacteric fruit, the synthesis of ethylene can be regulated through a process of autoinhibition working at the level of gene expression.