Abstract Regulation of intracellular sugar homeostasis is maintained by regulation of activities of sugar import and export proteins residing at the tonoplast. We show here that the EARLY RESPONSE TO DEHYDRATION6-LIKE4 protein, being the closest homolog to the proton/glucose symporter ERDL6, resides in the vacuolar membrane. Gene expression and subcellular fractionation studies indicated that ERDL4 was involved in fructose allocation across the tonoplast. Overexpression of ERDL4 increased total sugar levels in leaves, due to a concomitantly induced stimulation of TST2 expression, coding for the major vacuolar sugar loader. This conclusion is supported by the finding that tst1-2 knockout lines overexpressing ERDL4 lack increased cellular sugar levels. ERDL4 activity contributing to the coordination of cellular sugar homeostasis is also indicated by two further observations. Firstly, ERDL4 and TST genes exhibit an opposite regulation during a diurnal rhythm, and secondly, the ERDL4 gene is markedly expressed during cold acclimation representing a situation in which TST activity needs to be upregulated. Moreover, ERDL4 -overexpressing plants show larger size of rosettes and roots, a delayed flowering time and increased total seed yield. Consistently, erdl4 knock-out plants show impaired cold acclimation and freezing tolerance along with reduced plant biomass. In summary, we show that modification of cytosolic fructose levels influences plant organ development and stress tolerance. One sentence summary The activity of the vacuolar sugar porter ERDL4 is important for balanced cytosolic monosaccharide homeostasis and influences plant growth and cold response in Arabidopsis The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors is: Benjamin Pommerrenig ( pommerre@bio.uni-kl.de ).

The vacuolar sugar transporterEarly Response to Dehydration 6-Like4regulates fructose signaling and plant growth