pDR196

Sugar content is the determining factor in apple sweetness. However, the molecular mechanisms underlying sucrose accumulation in apple fruits remain unclear. Here, researchers report the role of the sucrose transporter MdSUT2.1 in regulating sucrose accumulation in apples. MdSUT2.1 is highly expressed in fruits and is positively correlated with sucrose accumulation during apple fruit development. Furthermore, complementation growth assays of yeast mutants verified the sucrose transport activity of MdSUT2.1. Overexpression of MdSUT2.1 in apples and tomatoes resulted in significant increases in sucrose, fructose, and glucose contents compared with wild type (WT). Further analysis showed that compared with WT, the expression levels of sugar metabolism and transport-related genes SUSYs, NINVs, FRKs, HXKs and TSTs were increased in apples and tomatoes overexpressing MdSUT2.1. Finally, unlike the tonoplast sugar transporters MdTST1 and MdTST2, the promoter of MdSUT2.1 is not induced by exogenous sugars. These findings provide valuable insights into the molecular mechanisms underlying sugar accumulation in apples.

The SUSY7/ura3 yeast strain inserted into the potato sucrose synthase (SUSY) gene cannot grow in media with sucrose as the sole sugar source due to the lack of extracellular invertase and sucrose transporter. To study whether MdSUT2.1 has sucrose transport activity, the CDS of MdSUT2.1 and AtSUT1 were cloned into vector pDR196. pDR196-MdSUT2.1 was transformed into SUSY7/ura3 cells to generate SUSY7/ura3-MdSUT2.1 cells. Negative control SUSY7/ura3-pDR196 cells (SUSY7/ura3 cells transformed with empty vector pDR196), positive control SUSY7/ura3-AtSUT1 cells (SUSY7/ura3 cells transformed with pDR196-AtSUT1) and SUSY7/ura3-MdSUT2.1 cells could grow on an SD/–Ura/Glucose medium. However, only SUSY7/ura3-AtSUT1 cells and SUSY7/ura3-MdSUT2.1 cells were able to grow in SD/–Ura/sucrose medium, indicating that MdSUT2.1 can transport sucrose in yeast. Furthermore, SUSY7/ura3-MdSUT2.1 cells grew slowly in SD/–Ura/sucrose medium compared with SUSY7/ura3-AtSUT1 cells (Figure 1).

Figure 1. Heterologous expression of MdSUT2.1 in SUSY7/ura3 yeast. SUSY7/ura3 carrying different genes was grown on SD/–Ura/glucose (left) and SD/–Ura/sucrose (right) media. Empty vector (pDR196) and pDR196-AtSUT1 were used as negative and positive controls, respectively. (Zhang B, et al., 2023)