Ethylene regulation of fruit ripening: Molecular aspects

Progress in ethylene regulating fruit ripening concerning its perception and signal transduction and expression of ACC synthase and ACC oxidase genes is reviewed. ACC synthase and ACC oxidase have been characterized and their genes cloned from various fruit tissues. Both ACC synthase and ACC oxidase are encoded by multigene families, and their activities are associated with fruit ripening. In climacteric fruit, the transition to autocatalytic ethylene production appears to be due to a series of events in which ACC sythase and ACC oxidase genes have been expressed developmentally. Differential expression of ACC synthase and ACC oxidase gene family members is probably involved in such a transition that ultimately controls the onset of fruit ripening. In comparison to ACC synthase and ACC oxidase, less is known about ethylene perception and signal transduction because of the difficulties in isolating and purifying ethylene receptors or ethylene-binding proteins using biochemical methods. However, the identification of the Nr tomato ripening mutant as an ethylene receptor, the applications of new potent anti-ethylene compounds and the generation of transgenic fruits with reduced ethylene production have provided evidence that ethylene receptors regulate a defined set of genes which are expressed during fruit ripening. The properties and functions of ethylene receptors, such as ETR1, are being elucidated. Application of molecular genetics, in combination with biochemical approaches, will enable us to better understand the individual steps leading from ethylene perception and signal transduction and expression of ACC synthase and ACC oxidase gene family member to the physiological responses.