VvSnRK1-VvSS3 regulates sugar accumulation during grape berry ripening in response to abscisic acid

Sugar is the primary product of photosynthesis and is mainly transported in plants as sucrose. Sucrose synthase 3 (VvSS3) and SNF1-related protein kinase 1 (VvSnRK1) are both critical enzymes in the metabolism of sugar in plants but are not clear about the relationship between VvSS3 and VvSnRK1 in the regulation of sugar metabolism. In this study, the spatiotemporal expression of VvSS3 and VvSnRK1 was measured during three stages of grape berry development from hard nucleolus phase to maturation phase. Our results indicated that VvSS3 and VvSnRK1 isoforms overexpressed at the beginning veraison and sugar accumulation occurred at ripening stage. We also provided transgenic calli of overexpressed VvSS3 using Agrobacterium GV3101-mediated transformation and found VvSS3 is localized in cell membrane. Furthermore, up-regulation of VvSnRK1s occurred with overexpression of VvSS3 but abscisic acid (ABA) exogenous down-regulated VvSnRK1s expression. VvSS3-overexpressed transgenic calli also increased the accumulation of soluble sugar and anthocyanin content by increasing the expression of genes related to anthocyanin and ABA synthesis. Furthermore, the interaction of VvSS3 with VvSnRK1β was confirmed by yeast two-hybrid (Y2H) and Bimolecular fluorescence complementation (BiFC). These results suggest that the VvSS3 gene has an important role in sugar accumulation during fruit ripening in grapevines.