The Regulatory Role of 1-aminocyclopropane-1-carboxylate Synthase Genes in Apple Fruit Shelf-life

The postharvest economic quality of mature apple (Malus x domestica Borkh.) is determined by the fruit shelf-life, which is associated with the fruit softening rate at room temperature. Ethylene is the factor that has been most studied in relation to the regulation of fruit softening. Softening rate and ethylene production differ widely among apple cultivars. 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is one of the key regulatory enzymes involved in the synthesis of ethylene. In apple, at least four members of the ACS gene family have been identified, two of which, Md4CS1 and MdACS3, have been studied extensively due to their specific expression in fruit tissue. Recent characterization of these genes has provided insight into the molecular basis of ACS gene involvement in fruit ripening, and a role of ACS genes in the regulation of system-1 and system-2 ethylene biosynthesis has been proposed. This paper reviews advances in our understanding of the relationship between ACS gene expression and the fruit shelf-life of apple, and also discusses other factors that influence fruit ripening. In addition, we prospectively overview promising research avenues that could lead to further clarification of the regulatory events involved in ripening.