Impact of Vine Water Status on 3-Alkyl-2-methoxypyrazine Content and Function Verification of VvOMT2/VvOMT3 Genes Associated with 3-Alkyl-2-methoxypyrazine Accumulation in “Marselan” Grape Berries (Vitis vinifera L.)

3-Alkyl-2-methoxypyrazines (MPs) could be considered as off-flavor for red wine if the concentration exceeds a certain threshold. It is unknown whether the vine water status has an influence on MP metabolism in grape berries and, therefore, in wines. This study aimed to evaluate the effect of vine water status on MP content and on the expression level of VvOMTs; moreover, the exact functions of VvOMT2/3 were investigated. In this study, the grapevines were subjected over two years (2020 and 2021) to different levels of water constraints and the treatments were (i) light water constraint (LW); (ii) moderate water constraint (MW); and (iii) severe water constraint (SW) in comparison with well-irrigated vines used as control (CK). The results showed that six MPs, including ETMP, MEMP, MOMP, SBMP, IPMP, and IBMP, were negatively and significantly affected by water constraints. Meanwhile, the levels of VvOMT1, VvOMT2, VvOMT3, and VvOMT4 were 0.17-fold, 0.13-fold, 0.35-fold, and 0.75-fold, respectively, at 40 DAA or 60 DAA under MW treatment relative to CK in 2020. In 2021, the trend was similar to that in 2020. When VvOMT2 and VvOMT3 genes were transiently overexpressed in grape berries and callus, both their expression level and protein level were induced; in addition, IPMP, SBMP, and IBMP contents were significantly increased. Moreover, heterologous expression of VvOMT3 in tomato led to IPMP, SBMP, and IBMP accumulation, whereas VvOMT2 could only promote the accumulation of IPMP. Altogether, moderate water constraint not only improved the quality of "Marselan" grape berries but also reduced the MP content per berry. This study showed for the first time, according to our knowledge, the effect of vine water constraint on the metabolism of MPs by way of allowing the reduction of the precursors of this aromatic compound, which could be perceived as an off-flavor impacting negatively wine aromatic profiles with notes of asparagus and green pepper.