Identification of characteristic aromas and mining of key genes associated with esters biosynthesis in apple

Aroma is one of the most important quality attributes of apple (Malus domestica) and it greatly affects consumer acceptance. Volatile esters are major aromas contributing to the organoleptic quality of apple fruit. However, the molecular mechanisms underlying the regulation of volatile ester biosynthesis in apple remain elusive. In this study, gas chromatography-mass spectrometry (GC-MS) was used to identify the volatile compounds in peel and flesh of ‘Honeycrisp’ and ‘Qinguan’ apples. Although the constitution of volatiles varied widely between the peel and flesh, the volatile profiles of the peel and flesh of ripening ‘Honeycrisp’ and ‘Qinguan’ fruit were dominated by volatile esters. Based on the results obtained from GC-MS, aroma extract dilution analysis-gas chromatography-olfactometry, and omission tests, hexyl acetate as the most abundant ester in peel was suggested to be one of the characteristic aroma compounds of ‘Qinguan’ apple, contributing to its strong fruity aroma note, while hexyl 2-methyl butyrate is responsible for the strong fruity note of ‘Honeycrisp’ apple. Moreover, this study investigated the transcriptomes of ‘Qinguan’ apple fruit during development and/or postharvest storage. WGCNA results suggested that 19 genes belonging to the ester biosynthesis pathways and 11 hub transcription factor genes potentially participated in the biosynthesis and regulation of esters. To figure out key regulators of ester biosynthesis, correlation network analysis, dual-luciferase assays, and yeast one-hybrid assay were conducted and suggested that MdMYB94 trans-activated the MdAAT2 promoter and participated in the regulation of ester biosynthesis. This study provides a framework for understanding ester biosynthesis and regulation in apple.