Abiotic stresses differentially affect the expression of O-methyltransferase genes related to methoxypyrazine biosynthesis in seeded and parthenocarpic fruits of Vitis vinifera (L.).

MPs (3-alkyl-2-methoxypyrazines) are grape-derived aroma compounds that are associated with detrimental herbaceous flavours in some wines. It is well known that several viticultural and environmental parameters can modulate MP concentrations in grapes, although comprehensive molecular studies have not been conducted in this field. Although the biosynthesis pathway of MPs has not been fully elucidated, four Vitis vinifera O-methyltransferase genes (VvOMT1-4) have been related to be involved in MP biosynthesis. We assessed whether different abiotic stresses induction have an impact on MP levels in grapes and wines from seeded and parthenocarpic fruits. Our results show that the timing of VvOMT3 expression is associated with the period of MPs accumulation in seeded fruits during both abiotic stresses, whereas no association was found in parthenocarpic fruits. These results are discussed in the context of how different viticultural practices can modulate VvOMT gene expression, which has a direct impact on MPs levels in wines.

Biosynthesis of methoxypyrazines: elucidating the structural/functional relationship of two Vitis viniferaO-methyltransferases capable of catalyzing the putative final step of the biosynthesis of 3-alkyl-2-methoxypyrazine.

3-Alkyl-2-methoxypyrazines (MPs) are an important food constituent and have been associated with detrimental herbaceous flavors in red wines by consumers and the wine industry. The Vitis vinifera genes O-methyltransferase 1 and 2 (VvOMT1 and VvOMT2) have been isolated in the grapevine cultivar Carmenere. These genes encode S-adenosyl-l-methionine (SAM)-dependent O-methyltransferases, which have the ability to methylate 3-alkyl-2-hydroxypyrazines (HPs)-the putative final step in MPs production. Atomic studies were performed in order to explain the differences in these VvOMT activities through their structural/functional relationship in MPs biosynthesis. Differences in enthalpy energy observed between the proteins may be due to changes of equivalent residues in the active sites of VvOMT1 (F319, L322) and VvOMT2 (L319, V322). However, docking simulations and QM/MM analyses described how residues H272 and M182 could explain the main functional differentiation observed between VvOMT1 and VvOMT2 through steric impediment, which limits the formation of the transition state in enzymes encoded by VvOMT2. Therefore, this finding could explain the decreasing catalytic efficiency observed for VvOMT2.