# Monoterpenoids Evolution and MEP Pathway Gene Expression Profiles in Seven Table Grape Varieties
## 2 Results and Discussion
### Transcript Level of MEP Pathway Key Genes during Development  
Biochemical studies showed that the monotereponoids were mainly produced by the MEP pathway during berry development and it had been documented that DXS and DXR were found as the key enzymes that could regulate the biosynthesis of monotereponids in some table grapes [3; 21; 23]. In order to explain their roles on the accumulation of monotereponids in the seven table varieties used in this study, we analyzed their transcript levels except 'Moldova' and 'Italia' in 2014 season. It was reported that a single nucleotide polymorphism (SNP) mutation in _VvDXS1_ gene could lead to the significant difference of monotereponids content in a specific grape variety [19]. In this study, increased expression of the _VvDXS1_ during berry ripening was observed in the varieties of 'Muscat of Alexandria', 'Tamina', and 'Xiangfei'. It was worthy to note that the expression of _VvDXS1_ in 'Xiangfei' increased dramatically and reached at the highest level at maturity, consistent with some previous studies on table grapes, such as [PERSON] et al. (2021) in 'Muscat Hamburg' [26] and [PERSON] et al. (2020) in 'Shine Muscat' [27], while its expression in 'Christmas Rose' remained at a low level in the whole process. These results were consistent with monotereponids accumulation found in Section 2.2 (Figure 3). It was worthwhile to notice that _VvDXS1_ showed a decreased expressional trend when approaching maturity, which was not well matched with monotereponids accumulation in 'Zaomeiguixiang' (Figure 3). _VvDXS3_ had low transcript levels in 'Christmas Rose' and 'Alexandria' varieties, while having higher expression in 'Tamina', 'Xiangfei', and 'Zaomeiguixiang'. [PERSON] et al. (2020) and [PERSON] et al. [28] have found that _VvDXS3_ was gradually up-regulated during berry development by qRT-PCR in 'Sunshine Rose' and 'Gewurztraminer', which were consistent with our findings in 'Zaomeiguixiang'. The expression pattern of _VvDXS3_ in the other four varieties was different from 'Zaomeiguixiang', with downregulation in 'Christmas Rose', no significant change in 'Alexander', a decrease and then an increase and then an increase with a decrease again at maturity in 'Xiangfei'. Figure 2: Principle component analysis of monotereponoids composition in different table grape varieties during berry development in 2013 (**A**) and 2014 (**B**). A represented ‘Muscat of Alexandria’; C represented ‘Christmas Rose’; I represented ‘Italia’; M represented ‘Moldova’; T represented ‘Tamina’; X represented ‘Xiangfei’; Z represented ‘Zmeiguixiang’. 1 represented E-L35; 2 represented E-L36; 3 represented E-L37; 4 represented E-L38. _VvDXR_ expression was increased during berry maturation in 'Christmas Rose', 'Muscat of Alexandria', and 'Xiangfei' from the verasion, which is consistent with previous findings in 'Shine Muscat' [23; 27], 'Moscato Bianco' [21], and 'Gewurztraminer' [28]. Its expression was significantly higher in 'Xiangfei' than in the other two varieties, which is consistent with the pattern of monoterreponids accumulation among these varieties found previously. The expression pattern in 'Tamina' decreased followed by an increase with the highest expression at maturity in line with the trend found in 'Kyoo' and '87-1' two table grapes [24]. The unique expression trend of _VvDXR_ in 'Zmeiguixiang', rising at the beginning of veraison and gradually decreasing with berry maturity, was not consistent with the reported, but similar to its corresponding compounds accumulation pattern. The expression pattern of _VvHDR_ with progressively higher expression during berry ripening has been reported in 'Shine Muscat' [27], 'Muscat Hamburg' [29], 'Moscato Bianco' [21], 'Muscat Blanc a Petits Grains' [22], and 'Gewurztraminer' [28]. _VvHDR_ gene expression was upregulated in 'Kyoo' during berry development, while '87-1' had lower transcript levels slightly upregulated near maturity [24].