Overexpression of the vacuolar sugar carrier AtSWEET16 modifies germination, growth and stress tolerance in Arabidopsis thaliana
Plant Physiol. 2013; 163(3):1338-52
Klemens PAW, Patzke K, Deitmer JW, Spinner L, Le Hir R, Bellini C, Bedu M, Chardon F, Krapp A, Neuhaus E

Abstract
Here we report that SWEET16 from Arabidopsis thaliana is a vacuolar-located carrier, transporting glucose, fructose and sucrose after heterologous expression in Xenopus oocytes. The SWEET16 gene is, similar to the homologues gene SWEET17, mainly expressed in vascular parenchyma cells. Application of glucose, fructose or sucrose, as well as cold-, osmotic stress or low nitrogen provoke down regulation of SWEET16 mRNA accumulation. SWEET16 overexpressors (35SPro:SWEET16) showed a number of peculiarities related to differences in sugar accumulation as e.g. less glucose, fructose and sucrose at the end of the night. Under cold stress 35SPro:SWEET16 plants are unable to accumulate fructose, while under nitrogen starvation both, glucose and fructose, but not sucrose were less abundant. These changes of individual sugars indicate that consequences of an increased SWEET16 activity are dependent upon the type of external stimulus. Remarkably, 35SPro:SWEET16-lines showed improved germination and increased freezing tolerance. Latter observation points, in combination with the modified sugar levels, to a superior function of glucose and sucrose for frost tolerance. 35SPro:SWEET16 plants exhibited increased growth efficiency when cultivated on soil and showed improved nitrogen-use efficiency when nitrate is sufficiently available, while under conditions of limiting nitrogen, wild-type biomasses were higher than those of 35SPro: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 non-favourable conditions.

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