Identification of QTL for volatile organic compounds in apple (Malus x domestica)

Plant metabolites including volatile organic compounds (VOCs) play a major role in apple flavor notes. Breeding for improved flavor is a major objective in many breeding programs although there is no systematic selection for specific VOCs. Knowledge of the genetic systems controlling these compounds will facilitate breeding for enhanced concentration and combinations of these compounds in apple. This study profiled individual VOCs in ~200 individuals including commercial cultivars and their progenies from the Washington State University’s (WSU) apple breeding program, maintained at the WSU Sunrise orchard, Wenatchee. Fruit was collected, juiced and frozen during the harvest season of 2012, then assayed for VOCs via GC-MS. The accessions were genotyped using ~1000 polymorphic SNPs of a total of 9K SNPs on the Infinium® II array developed in the RosBREED (www.rosbreed.org) project. The integration of phenotypic (VOCs) and genotypic data (SNPs) to identify quantitative trait loci (QTL) linked to these traits was done using FlexQTL™ software. In all accessions, pentyl acetate showed the least concentration (0.1-102.1 ng/ml) while hexanal had the highest concentration (55.8-3390.1 ng/ml). QTL were identified for precursors such as 1-butanol on LG 2 and LG 16, and for 2-hexenal on LG 4 (2 QTL) and LG 7.Methyl-butanol, 1-pentanol and hexanal each had one QTL on LG 13, LG 10 and LG 13, respectively. Among the esters assayed, QTL were identified for butyl-acetate on LG 5 and LG 8, on LG 4 for hexyl-acetate, on LG 2, LG 4, LG 10 and LG 16 for methylbutylacetate and on LG 5 for pentylacetate. The implications of these results are discussed in the context of breeding for enhanced flavors in apple.