Functional analysis of MdSUT2.1, a plasma membrane sucrose transporter from apple

Sugar content is a determinant of apple (Malus×domestica Borkh.) sweetness. However, the molecular mechanism underlying sucrose accumulation in apple fruit remains elusive. Herein, the role of the sucrose transporter MdSUT2.1 in the regulation of sucrose accumulation in apple was reported. The MdSUT2.1 gene encodes a protein with 612 amino acid residues that could be localized at the plasma membrane when expressed in tobacco leaf protoplasts. MdSUT2.1 was highly expressed in fruit and was positively correlated with sucrose accumulation during apple fruit development. Moreover, the sucrose transport activity of MdSUT2.1 was validated by complementary growth assays in a yeast mutant. MdSUT2.1 overexpression in apple and tomato resulted in significant increases in sucrose, fructose, and glucose contents compared to WT. Further analysis revealed that the expression levels of sugar metabolism- and transport-related genes SUSYs, NINVs, FRKs, HXKs, and TSTs increased in apple and tomato with MdSUT2.1 overexpression compared to WT. Finally, unlike the tonoplast sugar transporters MdTST1 and MdTST2, the promoter of MdSUT2.1 was not inducible by exogenous sugars. These findings provide valuable insights into the molecular mechanism underlying sugar accumulation in apple.