Functional allelic diversity of the apple alcohol acyl-transferase gene MdAAT1 associated with fruit ester volatile contents in apple cultivars

Flavour is an important key factor of apple (Malus × domestica Borkh.) fruit quality, and its improvement is an important but complex breeding goal. Acetate esters are quantitatively the most important volatile compounds in apple fruit, and only a few of them dominate the typical aroma of a cultivar. Alcohol acyl-transferase (AAT) is a key enzyme involved in the last step of ester biosynthesis. The aim of this study was to target single nucleotide polymorphisms (SNPs) in an AAT candidate gene genetically associated with ester quantitative trait loci (QTL), to enable functional marker development for marker-assisted apple breeding programs. The AAT gene inventory of apple was characterized by in-silico mining of the assembled Golden Delicious genome, and 17 putative AAT genes in total were defined. MdAAT1 located on chromosome 2 was selected as the main candidate gene associated with QTL for different acetate esters, and its allelic diversity was assessed by direct amplicon sequencing in a collection of 102 apple cultivars characterized for ester volatile profiles. Sequencing a 468 bp nucleotide sequence of the MdAAT1 coding region resulted in the detection of four SNPs. In total, 18 different SNP haplotypes/heterozygous patterns were generated from the four SNPs identified within the apple collection. Association analyses resulted in highly significant associations of both individual SNPs and distinct haplotypes with the content of four acetate esters, including hexyl acetate, butyl acetate and 2-methyl-butyl acetate. About a third (31) of the 102 apple cultivars possessed the specific MdAAT1 haplotype H1 (C-A-C-A) and were characterized by strongly decreased ester concentrations. The contrasting H8 haplotype (T-G-T-G) was found in 28 varieties but was associated with normal to elevated ester concentrations. The observed association suggests a putative causal functional relationship between MdAAT1 and production of key apple esters.