Proanthocyanidins, or condensed tannins, are crucial polyphenolic compounds for grape and wine quality. Recently, significant advances were achieved in understanding the biosynthesis of their main subunits, (+)-catechin and (-)-epicatechin, produced by leucoanthocyanidin reductase (LAR) and anthocyanidin reductase (ANR), respectively. Expression and enzyme activity studies have been published but no data were previously available on their regulation by hormonal status. As abscisic acid (ABA) is involved in controlling anthocyanin biosynthesis, we examined the effects of the ABA signal on the proanthocyanidin pathway. The aim of this research was to determine whether ABA application on green grapes affected tannin content and composition, LAR and ANR activity, and the expression patterns of the genes encoding them (VvANR, VvLAR1, VvLAR2) and the associated transcription regulators (VvMYB5a, VvMYBPA1) in grape skin during ripening. Our results show that ABA affects tannin content and is involved in the tannin biosynthesis pathway in grape skin by decreasing LAR and ANR activity and repressing the expression of related genes a few days after application. This treatment reduced the tannin content of green grapes without modifying their composition but had a positive impact on tannin biosynthesis during veraison, as previously demonstrated for anthocyanins, suggesting that ANR and LAR were coregulated by ABA.
