Biosynthetic Pathways of Phenolics in Grapes

From dihydrokaempferol, dihydroquercetin and dihydromyricetin -by action of enzyme flavonol synthetase (FLS) - kaempferol, quercetin and myricetin respectively are formed. As flavonols are present in the glycoside form, analogous to anthocyanins, it is supposed that the beginning of their synthesis is concomitant with the flavonol-3-O-glycosyltransferase enzyme formation, at the veraison beginning. 

By reduction of dihydroquercetin and dihydromyricetin, operated by the enzyme dihydroflavonol-4-reductase (DFR) (in vine the reduction of dihydrokaempferol does not occur), 2,3-trans leucocyanidin and leucodelphinidin form. Successive synthesis of (+)-catechin and (+)-gallocatechin (2,3-trans isomers) is mediated by the enzyme leucoan-thocyanidin reductase (LAR). On the other hand, the synthesis of (—)-epicatechin and (—)-epigallocatechin (2,3-cis isomers) proceeds via oxidation of 2,3-trans leucoanthocyanins into cyanidin and delphinidin (accompanied with chirality loss) mediated by the enzyme leucoan-thocyanidin dioxygenase (LDOX), which is followed by the reduction catalyzed by the enzyme anthocyanidin reductase (ANR) with reacquisition of chrality (Fujita et al., 2005). 

as supposed, epicatechin synthesis starts from cyanidin, then the production of proanthocyanidins in the skins and seeds does not occur 

From veraison, a decrease of HCTA in pulp and skin is observed (Di Stefano, unpublished data) probably due to how they are used in the acylated anythocynins synthesis. 

Production of flavonols begins before the veraison in the first period of veraison quercetin is the main flavonol present (the dihydroquercetin biosynthesis prevails) and production of myricetin is slower but, in varieties where biosynthesis of dihydromyricetin prevails, myricetin can be the main flavonol (Borsa, personal communication Squadrito etal., 2006). 

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