Amy Iezzoni, Project Director, Michigan State University

Aroma is a complex trait that contributes to the quality of rosaceous fruit and floral products. Its perception by humans is largely determined by the relative abundance of a combination of volatile ester compounds that increase during the ripening process. Apple cultivars like ‘Royal Gala’ are characterized by a “fruity” aroma compared to cultivars like ‘Granny Smith’. In progeny from a cross between these two cultivars, a cluster of large-effect loci for 31 ester compounds was identified at the top of apple linkage group (chromosome) 2 (Souleyre et al., 2014). For one of these 31 compounds, the effect of the “fruity” allele for this complex locus increased the ester production as much as 53-fold. The gene underlying the variation at this locus was determined to encode for alcohol acyl transferase 1 (AAT1), an enzyme that catalyzes the synthesis of a broad range of esters found in ‘Royal Gala’ fruit that contribute to its “ripe apple” aroma. Two functional variants of AAT1 were identified. The variant from ‘Royal Gala’ was associated with a significant qualitative and quantitative contrast in ester profile and production compared to ‘Granny Smith’.

Apple breeders already have access to DNA tests to select for other key components of apple fruit quality, including apple texture (Md-ACS1 = Jewel 1); crispness, juiciness, acidity (Ma locus = Jewel 6); firmness (Md-PG1 = Jewel 9); and fructose concentration (LG1Fru). With genetic knowledge of which apple seedlings will most likely have desirable ester content and profile (along with the other components of apple fruit quality), breeders can plan crosses to maximize the probability of obtaining selections with excellent fruit quality. Therefore, because knowledge of this genetic region will lead to more efficient breeding of apple cultivars, it is chosen as one of RosBREED’s “Jewels in the Genome.”

Reference

Souleyre E.J., D. Chagné, X. Chen, S. Tomes, R.M. Turner, M.Y. Wang, R. Maddumage, M.B. Hunt, R.A. Winz, C. Wiedow, C. Hamiaux, S.E. Gardiner, D.D. Rowan and R.G. Atkinson. 2014. The AAT1 locus is critical for the biosynthesis of esters contributing to ‘ripe apple’ flavor in ‘Royal Gala’ and ‘Granny Smith’ apples. Plant J. 78: 903-915.