Characterization of aroma biosynthetic pathways in vitis vinifera

Background and Aims: Grape-derived flavour compounds and some flavour precursors modified during fermentation, wine evolution and ageing, are fundamental in determining the organoleptic parameters used to define wine quality as the aromatic quality, persistency and complexity are wine characteristics that may influence consumer choice. Although hundreds of secondary metabolites that potentially contribute to wine flavour have been detected in grapes, the knowledge of the biosynthesis of many of these compounds is not well understood and only a few genes involved in flavour synthesis pathways have been discovered and characterised. The flavour and aroma of certain Vitis vinifera grape varieties is dominated by volatile terpenes and small volatile aldehydes. Typical monoterpenol components of Muscat cultivars, a group of Vitis vinifera aromatic varieties, are linalool, geraniol, nerol, citronellol, and alpha-terpineol. The aim of this research is the discovery and characterisation of genes involved in terpenoid biosynthesis of cv. Moscato Bianco, one of the most important Italian Muscat varieties. Methods and Results: Samples of flowers and berries at different developmental stages, from fruit set to ripening, were analysed for their aroma content and the expression of both previously characterised genes and some that are predicted to be involved in grape flavour pathways. The aroma compounds were extracted by SPE and SPME procedures and analysed by GS-MS, while the expression of characterised grape aroma genes and some candidate genes were analysed by realtime RT-PCR. Among the genes analysed, some showed a peak of expression at berry set, while others were expressed at veraison or during post-veraison. Some candidate genes showed a peak of expression in flowers: three putative geranyl or geranylgeranyl diphosphate synthases, alpha-terpineol synthase, valencene and germacrene D synthase, and three other putative terpene synthases. Candidate genes at other stages were: HMG-CoA reductase (fruit set); carotenoid cleavage dioxygenase 1 (veraison); a putative geranyl or geranylgeranyl diphosphate synthase, and three putative terpene synthases (ripening). Conclusions: The analysis of correlations between aroma accumulation and gene expression has allowed the identification of candidate genes potentially involved in biosynthesis of grape aroma compounds and these will be functionally characterised. Significance of the Study: The discovery and characterisation of genes that encode the enzymes for synthesis of flavour compounds and analysis of their activity during berry development would support decisions on management of genotype, environment and viticultural practices for improving grape flavour and aroma potential.