Emitted and endogenous volatiles in ‘Tsugaru’ apple: The mechanism of ester and (E,E)-α-farnesene accumulation

Volatile compound production was studied in terms of biosynthetic gene expression in apple (‘Tsugaru’). To this end, we first analysed the endogenous and emitted volatiles in the skin of ripened apple fruit. GC–MS and GC analyses suggested that the boiling point of the endogenous compounds in apple skin is an important determinant of the composition and amount of emitted volatiles. Since esters and (E,E)-α-farnesene are the major endogenous volatiles in apple, key biosynthetic genes were isolated from apple skin and their expression patterns were analysed with ethylene biosynthetic genes. During fruit development, the onset of alcohol acyltransferase (pMdAAT) expression in the skin, which is responsible for ester biosynthesis, coincided with the accumulation of 1-aminocyclopropane-1-carboxylate synthase (MdACSs, MdACS5B) mRNA. Thereafter, pMdAAT expression remained at a high level, even when no MdACS transcripts were observed. On the other hand, the accumulation of (E,E)-α-farnesene synthase (pMdAFS1) transcripts in the skin was associated with the expression of MdACSs and 1-aminocyclopropane-1-carboxylate oxidase (MdACO1). After harvest, the inhibition of pMdAAT expression in 1-methylcyclopropene (1-MCP)-treated apple skin was incomplete. In contrast, the expression of pMdAFS1 was repressed by 1-MCP treatment concomitant with considerable inhibition of ethylene production. These results suggest that pMdAFS1 expression is controlled by ethylene, whereas pMdAAT expression is developmentally regulated in the skin of ‘Tsugaru’.