Apple aroma: alcohol acyltransferase, a rate limiting step for ester biosynthesis, is regulated by ethylene

The role of ethylene in aroma biosynthesis of apple fruits was investigated using transgenic 'Greensleeves' apple trees suppressed for ACC-oxidase or ACC-synthase enzyme activity, and an ethylene action inhibitor (1-methycyclopropene, 1-MCP). In the transgenic lines and 1-MCP treated fruit, reductions higher than 90% in ethylene biosynthesis and respiration rates were observed in apples held at 20 °C for 14 days. We observed a major reduction in ester production in the ethylene-suppressed lines and in the 1-MCP treated fruit, with only slight differences in the levels of alcohol and aldehyde volatiles under these conditions. The activity of alcohol acyl-CoA transferase (AAT), a key enzyme in ester biosynthesis, showed an ethylene dependent pattern of regulation. Additionally, gene expression levels of specifically an AAT clone were highly regulated by ethylene. In contrast, activity and expression levels of alcohol dehydrogenase (ADH) were not affected by changes in the levels of endogenous ethylene. These results suggest that ethylene is involved in ester biosynthesis in apple via regulation of AAT.