## 3 Discussion
Combined with the measurement results of volatile profiles, the upregulated expression pattern of _MdLOX_ genes in the Jonagold apple may play a key role in the volatile compounds accumulation. Products of the LOX reaction can be converted to aldehydes by the hydroperoxide lyase (HPL) enzyme and aldehydes are subsequently reduced to the corresponding alcohol by the alcohol dehydrogenase (ADH) [12]. The alcohols resulting from ADH enzymatic activity are natural substrates for the alcohol acyltransferase (AAT), which transfers an acyl group through an oxygen-dependent reaction from acyl-CoA to the OH group of an alcoholforming an ester [39]. In our study, compared to the Granny Smith apple, the _MdAAT_ gene exhibited significantly higher expression levels in the Jonagold apple, accounting for the high ester production. Amino acids are the second most important source of volatile compounds in the aroma of apples. A previous study has reported that the branched amino acids, especially isoleucine, can produce the branched-chain esters in apples [34]. In our dataset, the key gene _MdBCAT_, which catalyzes the last step in the synthesis of branched amino acids, was higher expressed in the Jonagold apple than that in the Granny Smith apple, and might provide more precursors for the synthesis of branched-chain aldehydes, alcohols, acids and esters. Sesquiterpenes, which synthesized via the mevalonate pathway, are the main aroma volatiles from the isoprenoid family in the apple fruits [9]. In our study, there was only one terpenoid compound \(\alpha\)-farnesene (sesquiterpene) detected in Granny Smith and Jonagold apples. \(\alpha\)-Farnesene synthase (AFS) catalyzed the final step in sesquiterpene biosynthesis in apples [40]. The _MdAFS_ as well as the _MdHMGS_, _MdHMGR_ and _MdFPPS_ genes in this pathway were all upregulated in Jonagold apples compared with the Granny Smith apple, might be responsible for high production of \(\alpha\)-farnesene. In addition, phenylropenes, such as eugenol, estragole and isoestragole, also contribute to apple aroma and these compounds are derived from the phenylpropanoid pathway [17; 30]. It has been reported that estragole imparts a spicy and aniseed note to 'Ellison Orange' and 'Royal Gala' apples [17]. In this study, the compound estragole was detected in both two apple varieties, the key genes _MdPAL_ and _MdOMT_ exhibited high expression levels in Jonagold apples, possibly accelerating the reaction of enzymes in estragole biosynthesis and resulting in the accumulation of estragole. Transcriptional regulation mediated by transcription factors (TFs) is a pivotal control hierarchy for various plant biological processes, including the synthesis of volatile compounds [41; 42]. A previous study has reported that FaMYB9 is positively involved in volatile biosynthesis in strawberries, transient silencing of FaMYB9 delayed the fruit development, resulting in a significant decrease in the contents of C6 volatiles [43]. Moreover, in banana, MabZIP4 and MabZIP5 were capable of binding directly to _BanAAT_ promoter, suggesting the function of MabZIPs in controlling aroma production [42]. Additionally, the transcription factor PpNAC1 was proved to transcriptionally activate the _PpAAT_ gene and formation of volatile esters in peaches [44]. AP2/ERF transcription factors have been shown to be associated with the production of volatile terpen in sweet orange fruits [45]. In order to clarify potential transcription factors that may be involved in regulating volatile biosynthesis, expression patterns of TFs such as NAC, MYB, bZIP and ERF were analyzed. In the Jonagold apple, the MdNAC1, MdMYB5, MdERF8 and MdbZIP44 TFs had higher expression levels than in the Granny Smith apple. Combined with the aroma profiles of these two apple varieties, we speculate that these TFs can bind to the promoters of some key genes and may play important roles in regulating the synthesis of volatile compounds.