Characterization of cultivar differences in alcohol acyltransferase and 1-aminocyclopropane-1-carboxylate synthase gene expression and volatile ester emission during apple fruit maturation and ripening

Alcohol acyltransferase (AAT) catalyzes the last step of volatile ester biosynthesis, and ethylene purportedly regulates AAT gene expression. In this study, expression patterns of apple (Malus × domestica Borkh.) AAT genes and ethylene biosynthesis genes of 1-aminocyclopropane-1-carboxylate synthase (ACS) were investigated in cultivars with relatively high (‘Golden Delicious’) or low (‘Granny Smith’) volatile ester production. All four AAT genes expressed stronger in ‘Golden Delicious’ than in ‘Granny Smith’. MdAAT1 and MdAAT2 are the predominant genes expressed in fruit tissues. The expression levels of MdAAT1 and MdAAT2 were increasing as ripening progressed and were consistent with the total amount of esters detected between two cultivars. The transcript levels of MdAAT3 and MdAAT4 decreased at or after the onset of ripening. The expression of MdACS1 was significantly increased at the onset of ripening in both cultivars, while the expression of MdACS3 was detected throughout the harvest period in ‘Golden Delicious’. Postharvest methylcyclopropene (1-MCP) exposure had little impact on expression of MdAAT1 and MdACS3 genes, but substantially suppressed the transcript level for MdACS1 in both cultivars, and MdAAT2 in ‘Golden Delicious’. The results indicated that (1) differential expression of AAT genes may contribute to phenotypic variation of volatile ester biosynthesis, and (2) the expression of MdACS3 may play a role on induction of AAT genes expression in early fruit development, because it was expressed prior to ACS1. (3) The climacteric expression of MdACS1 greatly enhanced the expression levels of MdAAT1 and MdAAT2 genes and the emission of aromatic volatile esters. (4) Postharvest 1-MCP treatment showed selected inhibition on gene expression of specific AAT and ACS family members.