Effects of two apple tonoplast sugar transporters, MdTST1 and MdTST2, on the accumulation of sugar

The types and proportions of soluble sugars in fleshy fruits are directly related to the sweetness and flavor of the fruit. Transporters located in the vacuole membrane, especially the tonoplast sugar transporter (TST) family that mediate sugar influx into vacuoles, are the main regulators of sugar accumulation in the fruit. In this study, we identified six TST orthologs in apple and named them MdTST1–6. MdTST1 and MdTST2 were the most highly expressed genes during apple fruit development, while their encoded proteins were localized on the vacuole membrane. Overexpression of MdTST1 in Arabidopsis leaves and tomato fruits increased glucose, fructose and sucrose accumulation, especially the glucose content. MdTST2 overexpression lines exhibited massive increases in fructose and sucrose contents as compared to wild-type lines. Overexpression of MdTST2 in the tmt1–2::tDNA double mutant of Arabidopsis complemented its growth retardation phenotype and significantly increased the concentrations of hexoses and sucrose, restoring the same levels as the wild-type lines or even higher. The results revealed that MdTST1 and MdTST2 were functionally divergent, with MdTST1 more related to glucose accumulation and MdTST2 more related to fructose and sucrose accumulation. The results also indicated that MdTST1 and MdTST2 play important roles in controlling sugar contents. These findings lay a foundation for revealing the molecular mechanism of sugar accumulation in the vacuoles of apple fruit, and also provide a theoretical basis to improve sugar accumulation and fruit quality via biotechnology method.