Transport of sugars across the plasma membrane of beetroot protoplasts

Protoplasts isolated from beetroot tissue took up glucose preferentially whereas sucrose was transported more slowly. The (14)C-label from [(14)C]glucose and [(14)C]sucrose taken up by the cells could be detected rapidly in phosphate esters and, after feeding of [(14)C]glucose was found also in sucrose. The temperature-dependent uptake process (activation energy EA about 50 kJ · mol(-1)) seems to be carrier mediated as indicated by its substrate saturation and, for glucose, by competition experiments which revealed positions C1, C5 and C6 of the D-glucose molecule as important for effective uptake. The apparent Km(20° C) for glucose (3-O-methylglucose) was about 1 mM whereas for sucrose a significantly lower apparent affinity was determined (Km about 10 mM). When higher concentrations of glucose (5 mM) or sucrose (20 mM) were administered, the uptake process followed first-order kinetics. Carrier-mediated transport was inhibited by N,N'-dicyclohexylcarbodiimide, Na-orthovanadate, p-chloromercuribenzenesulfonic acid, and by uncouplers and ionophores. The uptake system exhibited a distinct pH optimum at pH 5.0. The results indicate that generation of a proton gradient is a prerequisite for sugar uptake across the plasma membrane. Protoplasts from the bundle regions in the hypocotyl take up glucose at higher rates than those derived from bundle-free regions. The results favour the idea that apoplastic transport of assimilates en route of unloading might be restricted to distinct areas within the storage organ (i.e. the bundle region) whereas distribution in the storage parenchyma is symplastic.