# Monoterpenoids Evolution and MEP Pathway Gene Expression Profiles in Seven Table Grape Varieties
## 1 Introduction  
The grape is a widely cropped variety with 77.8 million tons of global output, of which the percentage of table grapes is 36%, according to the OIV (Organisation Internationale de la Vigne et du Vin, 2022). China is one of the most important table grape markets in the globe and table grapes account for about 80% of China's total grape production [1]. Consumers in China like the table grapes with red to pink color and sweetness mouthfeel. Berries with specific flavor are becoming more and more popular in the market in recent years [2]. Aroma is the important sensory parameter for grapes. There were kinds of aroma compounds found in grape berries including terpen, norisoprenoids, 2-methoxy-3-methylpyrazine, aldehydes, alcohols, and esters [3, 4]. Some monoterpenoids could desorb a flora flavor, also called Muscat, which was easily felt by the human olfactory organ [5, 6]. In grape berries, the monoterpenoids mainly exist in pericarp [7]. Their critents was determined by the genotype [8], and affected by development stage [9], environment and management [10, 11, 12]. There are two forms of monoterpenoids, free form and bound form. The former can contribute directly to the flavor, while the latter, which is a class of potential aroma compound, can be converted to free form by hydrolysis [13, 14, 15]. However, most previous studies focused on wine grapes but not on table grapes. Biologically, the biosynthesis of geranyl diphosphate (GPP), the precursor terpenoids in plants, is mainly through two pathways, methyl-erythnitrol-4-phosphate pathway (DXP/MEP) in the plastid, and mexolonate pathway (MVA) in the cytoplasm [16]. [PERSON] generally believed that the GPPs for monoterpenoids biosynthesis in grape berries mainly come from the former pathway. The MEP pathway was found start from pyruvate and G3P (3-phosphate glyceraldehyde), by DXS (1-deoxy-D-xylulose-5-phosphate synthase), the first enzyme in the MEP pathway, and they are changed in to DXP (1-deoxy-D-xylulose-5-phosphate), then converted into GPP by six enzymatic reactions. Then, the GPP was synthesized in to monoterpenoids (C10) by terpene synthases (TPSs), mainly in plastid [17; 18]. Previous studies showed that SNPs of _VvDXS1_, in particularly SNP at 1822 which led to K284N, could explain the difference of monoterpenoids contents among grape varieties to some extent [19; 20]. In recent years, researchers have also found that the accumulation of monoterpenoids was consistent with the expression pattern of some genes in the MEP pathway during berry ripening. The related studies were also mainly focused on wine grapes. [PERSON] et al. found that the monoterpenoids increased from veraison in 'Moscato Bianco' berries and it was consistent with the expression of _VvHDR_[21]. [PERSON] et al. had similar findings in 'Muscat Blanc a Petits Grains' [22]. [PERSON] et al. found a positively correlated relationship between _VvDXS1_ expression and monoterpenoids accumulation among 'Moscato Bianco' [19]. To our best knowledge, few studies were focused on table grapes. [PERSON] et al. [23] found that in 'Shine Muscat', terpenoids gradually increased during fruit development while a relatively decline in abundance of total terpenoids was observed during maturity. _VvDXS1_, _VvDXS3_, _VvDXR_, and _VvHDR_ were found to be positively correlated with terpenoid accumulation. [PERSON] et al. [24] found that the monoterpenoids concentrations increased during berry ripening in two table grapes ('Kyoho' and '87-1') two table grapes. Correlation analysis revealed a positive correlation between the expression of _VvDXS_ and _VvDXR_ with monoterpenoids content, while a negative correlation was found between the gene expression of _VvHDR_ with monoterpenoids content. From above, these results indicated that although most of the monoterpenoids mainly accumulated during berry ripening in kinds of varieties, the expressional patterns of related genes in MEP pathway were found not the same.