Overexpression of the Vacuolar Sugar Carrier AtSWEET16 Modifies Germination, Growth, and Stress Tolerance in Arabidopsis

Here, we report that SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET16) from Arabidopsis (Arabidopsis thaliana) is a vacuole-located carrier, transporting glucose (Glc), fructose (Fru), and sucrose (Suc) after heterologous expression in Xenopus laevis oocytes. The SWEET16 gene, similar to the homologs gene SWEET17, is mainly expressed in vascular parenchyma cells. Application of Glc, Fru, or Suc, as well as cold, osmotic stress, or low nitrogen, provoke the down-regulation of SWEET16 messenger RNA accumulation. SWEET16 overexpressors (35S(Pro):SWEET16) showed a number of peculiarities related to differences in sugar accumulation, such as less Glc, Fru, and Suc at the end of the night. Under cold stress, 35S(Pro):SWEET16 plants are unable to accumulate Fru, while under nitrogen starvation, both Glc and Fru, but not Suc, were less abundant. These changes of individual sugars indicate that the consequences of an increased SWEET16 activity are dependent upon the type of external stimulus. Remarkably, 35S(Pro):SWEET16 lines showed improved germination and increased freezing tolerance. The latter observation, in combination with the modified sugar levels, points to a superior function of Glc and Suc for frost tolerance. 35S(Pro):SWEET16 plants exhibited increased growth efficiency when cultivated on soil and showed improved nitrogen use efficiency when nitrate was sufficiently available, while under conditions of limiting nitrogen, wild-type biomasses were higher than those of 35S(Pro):SWEET16 plants. Our results identify SWEET16 as a vacuolar sugar facilitator, demonstrate the substantial impact of SWEET16 overexpression on various critical plant traits, and imply that SWEET16 activity must be tightly regulated to allow optimal Arabidopsis development under nonfavorable conditions.