## 1 Introduction  
Grape berry aroma plays an essential role in consumers' choice of fruits and promotes their desire for re-consumption [1, 2]; it is also a crucial criterion in grape breeding. Generally, the interaction among multiple metabolites contributes to the formation of aroma. Many studies have found that the volatile compounds in lipids and amino acids contribute to the enhancement of aroma in fruits [3]. Esters, as key odorants, have been found to influence the aroma quality of fruits such as apples and bananas [4, 5], and polyunsaturated fatty acids have a considerable influence on the formation of esters in fruits. Ester formation in yellow-fleshed peaches has been related to the activity of alcohol acyltransferase (AAT) and gene expression [6].  
Grape crops are a commercially significant fruit crop with a long history of cultivation, and their produce can be categorized as table grapes, raisins, juice, or wine [7]. Grape quality is determined by size, color, and aroma, among which aroma has received more attention in recent years. As a non-respiratory and non-climacter fruit, grape berries have three development stages: the early stage of berry development, a short growth stagnation stage, and a maturation stage with rapid growth. The ripening of grape berries is accompanied by many physiological changes; for example, phenols (such as tannins) and aroma substances (such as pyrazines) accumulate in the early stage, while many aroma substances and their precursors accumulate the maturation stage. The volatile aroma in grape berries usually exists in the form of a \"free\" or \"bound\" glycoside [8]. A large number of volatile aroma compounds in grape berries are found in the \"bound\" form and have no aroma; however, upon hydrolysis of the glycoside, the compounds can be volatilized. The volatile aroma in grape berries consists of hexacarbons, esters, terpenoids, alcohols, ketones, and aldehydes [9]. These compounds interact with each other and determine the aroma of grape berries. As direct precursors of fruit aromatic substances, amino acids can produce large amounts of aromatic substances, such as alcohols, aldehydes, esters, ketones, volatile terpenes, and carbonyls; these are the key components of grape aroma and can be used to classify neutral, strawberry, and muscat grape resources [10; 11; 12; 13; 14; 15].  
With the development of sequencing technology, combined RNA-Seq and GC/MS have been widely utilized to explore metabolites and related genes to reveal the coloration and quality formation in fruit crops [16; 17; 18]. Although aroma is an important characteristic of grapes, the molecular regulation mechanism is still unclear. In this study, three grape cultivars were used: \"Adenauer Rose\", which originated in France, belongs to _V. vinifera_ L. with a strong muscat fragrance, and is a diploid cultivar. \"Kyoho\" belongs to _V. vinifera_\(\times\)_V. labrusca_ L. and has a strong strawberry aroma. \"Mei Xiangbao\" is a new tetraploid variety which possesses muscular and strawberry fragrances; and is crossbred from \"Adenauer Rose\" (female parent) and \"Kyoho\" (male parent). In this study, berries of these three grape cultivars in two developmental stages were explored using transcriptome and metabolome analyses, and then an integrative analysis of the transcriptome and metabolome was performed. A number of metabolites and genes associated with the formation of grape aroma were identified and quantified. The results will further our understanding of the molecular mechanism of the regulatory network governing aroma formation in grape berries and also provide us with a valuable reference for grape aroma breeding research.