## 2 Results and Discussion
### Transcript Level of Key LOX-HPL Pathway Genes during Development  
C6 volatile compounds have been confirmed to be produced via the LOX-HPL pathway in grape berries during the berry development period, and it has been reported that lipoxygenase (LOX), hydroperoxide lyase (HPL), alcohol dehydrogenase (ADH), and alcohol acetyl transferase (AAT) are the key enzymes that regulate the biosynthesis of C6 volatiles [18, 38, 39, 40]. In order to elucidate the effect of the LOX-HPL pathway key genes on the accumulation of C6 volatiles in these table grape cultivars, we further investigated the evolution of the transcript level of the genes in these cultivars under the 2014 vintage. Lipoxygenases (LOXs) have been reported to take charge of yielding PUFAhydroperoxides via catalysis of the oxygenation of polyunsaturated fatty acids (PUFAs) [38]. _VvLOXA_ and _VvLOXO_ have been considered representative of putative 13-LOXs [38]. In the present study, an increase in the expression of the _VvLOXA_ was observed in these cultivars after veraison, and then its expression exhibited a decrease when the grape berries reached the harvest stage (Figure 3). Additionally, the expression level of the _VvLOXA_ appeared to be higher in the \"Xiangfei\" and \"Tamina\" grape cultivars. It should be noted that _VvLOXA_ exhibited much higher expression in these grape cultivars than _VvLOXO_. A similar observation was also reported in wine-making grape cultivars [27]. Therefore, _VvLOXA_ was speculated to be the primary 13-LOX in these cultivars. The transcript level of _VvLOXO_ was higher in the \"Tamina\" and \"Xiangfei\" cultivars. However, its expression pattern was totally different. For example, an increase and then a decrease in the expression of the _VvLOXO_ was found in the \"Tamina\" cultivar during the maturation period, whereas the \"Xiangfei\" cultivar resulted in a decrease and then an increase in the _VvLOXO_ expression. Additionally, the \"Zaomeiguixiang\", \"Muscat of Alexandria\", and \"Christmas Rose\" cultivars displayed a low expression level of these LOXs during the whole berry development period (Figure 3), which resulted in a low concentration of the C6 aldehydes in these grape cultivars (Figure 2). Hydroperoxide lyases (HPLs) are the key enzymes that can cleave PUFAhydroperoxides into aldehydes and oxocads [39]. The _VvHPL1_ gene was expressed at a higher level in the \"Xiangfei\" and \"Zaomeiguixiang\" cultivars during development. However, its expression pattern in these cultivars appeared to be significantly different (Figure 3). For example, the expression of _VvHPL1_ remainedstable in the \"Xiangfei\" cultivar after veraison, and a further increase in the gene expression was observed when the grapes approached the harvest. However, this gene was rapidly expressed in the \"Zaomeiguixiang\" cultivar from the beginning of the veraison stage, and then a decrease in its expression level was found at the late stages of development. A similar evolution of this gene was reported in the Cabernet Sauvignon cultivar during berry development [39]. Regarding the other cultivars, the _VvHPL1_ gene was expressed at a constant level, followed by a decrease at the harvest. The evolution of this gene expression in these cultivars was consistent with the evolution of the C6 aldehydes during berry development. Alcohol dehydrogenases (ADHs) can convert alcohols into aldehydes and two key genes (_VvADH1_ and _VvADH2_) have been confirmed to biosynthesize the ADHs in grape berries [40]. It has been reported that _VvADH2_ played a primary role in facilitating berry ripening through enhancing its expression level at the late stage of berry development, whereas berry maturation resulted in a  
Figure 3: Transcript levels of key genes in LOX/HPL pathways in different table grape cultivars during berry development in 2014. GenBank accession numbers are as follows: _VvLOXA_ (FJ858255), _VvLOXO_ (FJ858257), _VvHDL1_ (HM627632), _VvADH1_ (AF194173), _VvADH2_ (AF194174) and _VvAT_ (AAW22989).