The novel chloroplast glucose transporter pGlcT2 affects adaptation to extended light periods

Intracellular sugar compartmentation is critically involved in plant development and acclimation to challenging environmental conditions. Sugar transport proteins are present in the plasma mem­brane and in membranes of organelles such as vacuoles, the Golgi apparatus and plastids. Here we report on a novel transporter of the Monosaccharide Transporter Family (MSF), that is the closest phylogenetic homolog to the chloroplast-localized glucose transporter pGlcT and that we therefore term plastidic glucose transporter 2 (pGlcT2). Its gene complemented glucose uptake deficiency of an Escherichia coli ptsG/manXYZ mutant strain and biochemical characterization revealed that this transporter specifically facilitates glucose transport, whereas other sugars do not serve as substrates. pGlcT2-GFP fusions located to the chloroplast envelope, and promoter activity analyses indicated that pGlcT2 is mainly produced in seedlings and in the rosette center of mature Arabidopsis plants. In the absence of additional sugars, pGlcT2-overexpression lines exhibited impaired germi­nation. In conjunction with molecular and metabolic data we propose that pGlcT2 acts as a glucose importer that can limit cytosolic glucose availability in developing pGlcT2-overexpressing seedlings. Overexpression, as well as deletion of pGlcT2 resulted in impaired growth efficiency under long day and especially under continuous light conditions, suggesting that pGlcT2 contributes to a release of glucose derived from starch mobilization late in the light phase. These data thus indicate that the facilitator pGlcT2 changes its direction of glucose transport during plant development and suggest that the activity of pGlcT2 must be controlled spatially and temporarily, in order to prevent developmental defects during adaptation to periods of extended light. Comparative gene expression analysis of Arabidopsis thaliana lines overexpressing or lacking the pGLCT2 (At1g05030) gene under permanent light conditions