The 'fruity' attributes of ripe apples (Malus × domestica) arise from our 
perception of a combination of volatile ester compounds. Phenotypic variability 
in ester production was investigated using a segregating population from a 
'Royal Gala' (RG; high ester production) × 'Granny Smith' (GS; low ester 
production) cross, as well as in transgenic RG plants in which expression of the 
alcohol acyl transferase 1 (AAT1) gene was reduced. In the RG × GS population, 
46 quantitative trait loci (QTLs) for the production of esters and alcohols were 
identified on 15 linkage groups (LGs). The major QTL for 35 individual compounds 
was positioned on LG2 and co-located with AAT1. Multiple AAT1 gene variants were 
identified in RG and GS, but only two (AAT1-RGa and AAT1-GSa) were functional. 
AAT1-RGa and AAT1-GSa were both highly expressed in the cortex and skin of ripe 
fruit, but AAT1 protein was observed mainly in the skin. Transgenic RG 
specifically reduced in AAT1 expression showed reduced levels of most key esters 
in ripe fruit. Differences in the ripe fruit aroma could be perceived by sensory 
analysis. The transgenic lines also showed altered ratios of biosynthetic 
precursor alcohols and aldehydes, and expression of a number of ester 
biosynthetic genes increased, presumably in response to the increased substrate 
pool. These results indicate that the AAT1 locus is critical for the 
biosynthesis of esters contributing to a 'ripe apple' flavour.