###### Abstract  
Aroma is a key quality attribute of apples, making major contributions to commercial value and consumer choice. However, the mechanism underlying molecular regulation of aroma formation genes and transcription factors remains poorly understood in apples. Here, we investigated the aroma volatile profiles of two apple varieties with distinctive flavors using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS). A total of 35 volatile compounds were identified in Granny Smith and Jonagold apples. Aldehydes were the most abundant volatiles contributing to the aroma in Granny Smith apple while esters were the dominant volatile compounds in Jonagold apple. In order to know more about the expression levels of aroma-related genes involved in the metabolic pathways, transcriptome sequencing of these two different apple varieties was conducted utilizing the Illumina platform. In total, 94 differentially expressed genes (DEGs) were found in the fatty acid metabolism, amino acid metabolism, the mevalonate pathway and phenylpropanol pathway. Furthermore, compared to the Granny Smith apple, the expression of multiple genes and transcription factors were upregulated in the Jonagold apple, which might play important roles in the synthesis of aroma volatile compounds. Our study contributes toward better understanding on the molecular mechanism of aroma synthesis in apples and provides a valuable reference for metabolic engineering and flavor improvement in the future. a 2022 23 2939 2022  
Volatile compounds in apples are mainly synthesized from fatty acids, amino acids and carbohydrates [9; 12]. Fatty acids are major precursors of aroma volatile compounds in apples [5]. Lipoxygenase (LOX) and \(\beta\)-oxidation are the two key pathways involved in fatty acid metabolism for the formation of straight-chain aldehydes, alcohols and esters, whereas branched-chain aldehydes, alcohols and esters are mainly derived from amino acids isoleucine (Ile), leucine (Leu) and valine (Val) [13; 14]. These amino acids are branched compounds of aliphatic nature and are synthesized in chloroplasts [15]. In addition, terpenoids come directly from carbohydrate metabolism and are synthesized from mevalonate (MVA) and methylerythnitol phosphate (MEP) pathway, with \(\alpha\)-farnesene as the most prominent terpenoid compound in apples [16]. Phenylropenes such as estragole and eugenol, derived from the phenylpropanol pathway, also have the influences on the apple flavor and aroma [17]. The composition and concentration of volatile compounds in apples are specific to the varieties [18; 19]. 'Granny Smith' and 'Jonagold' are two apple varieties with distinctive flavors and aroma profiles as well as characteristic skin colors. 'Granny Smith' as a late maturing, green-skinned apple variety, is believed to have descended from French crab apples grown in Australia [20]. The volatile profile of Granny Smith apples is characterized by the producing low levels of ethyl and propyl esters and free alcohols such as 2-methylbutanol [21]. 'Jonagold', a cross between 'Jonathan' by 'Golden Delicious', has gained popularity in North America, Northern Europe and China for its high yield and dessert quality [22]. The volatile profile of Jonagold exhibits an abundance of hexyl acetate, hexyl butyrate, butyl hexanoate and \(\alpha\)-farnesene [18]. Hence, Jonagold is considered one of the most aromatic apples while the Granny Smith is regarded to be one of the least aromatic apples [23; 24]. These two varieties represent two phenotypic extremes in volatile compound production and emission by apples. Although previous studies have measured the aroma profiles in Jonagold, Granny Smith and some other apple varieties [18; 22; 24], little is known about the transcriptional regulation mechanisms of volatile compounds and the expression patterns of aroma-related genes in apples.