Subcellular plant carbohydrate metabolism under elevated temperature

In many plant species, exposure to a changing environmental temperature regime induces an acclimation response that ultimately increases thermotolerance. Under elevated temperatures, membrane systems undergo remodeling to counteract destabilizing thermodynamic effects. Elevated temperature also affects photosynthesis and carbohydrate metabolism due to altered protein functions, enzyme activities, and transport across membrane systems. Here, a combination of electrolyte leakage assays and chlorophyll fluorescence measurements was applied to quantify heat tolerance before and after heat acclimation in Arabidopsis thaliana under different temperature regimes. Subcellular carbohydrate concentrations were determined through nonaqueous fractionation and 3D reconstruction of mesophyll cells and subcellular compartments using serial block-face scanning electron microscopy. Across temperature regimes between 32 and 38 °C, 7 d of heat acclimation at 34 °C most efficiently increased tissue heat tolerance. Under such conditions, cytosolic sucrose concentrations were stabilized by a shift in sucrose cleavage rates into the vacuolar compartment, while invertase-driven cytosolic sucrose cleavage was efficiently quenched by fructose and glucose acting as competitive and noncompetitive inhibitors, respectively. Finally, this study provides strong evidence for a sucrose concentration gradient from the cytosol to the vacuole, which might directly affect the physiological role and direction of sugar transport across cellular membrane systems.