Sugar Transport across the Plasmalemma and the Tonoplast of Barley Mesophyll Protoplasts. Evidence for Different Transport Systems

Uptake of 14C-labelled glucose into protoplasts and vacuoles isolated from the protoplasts has been compared. Below 2 mol m -3 external concentrations, transfer of glucose across the plasmalemma was faster than transfer across the tonoplast. At high concentrations transfer rates were comparable. Only transfer across the plasmalemma was inhibited by uncouplers. From a comparison of the uptake of glucose, fructose, and sucrose into isolated vacuoles it is concluded that the tonoplast contains at least two different transport systems for sugars. Within a concentration range of up to 100 mol m -3, sucrose uptake exhibited saturation kinetics, whereas rates of hexose uptake increased with concentration, first non-linearly and then linearly. For sucrose, the apparent Km of uptake was between 20 and 30 mol m -3. The Km of the saturatable phase of glucose uptake was 3 to 5 mol m -3. The dominance of the linear phase of fructose uptake precluded affinity calculations. Sucrose uptake was not or only slightly decreased by glucose and fructose, whereas fructose uptake was competitively inhibited by glucose and sucrose. Fructose and sucrose inhibited uptake of glucose, but were much less effective in decreasing glucose uptake than sucrose and glucose were in inhibiting fructose uptake. Thus, the kinetic data suggest that fructose has a low affinity to both the sucrose and the glucose transport system. Whereas sucrose transfer was inhibited by pCMBS, glucose transfer was insensitive to this SH-blocker. ATP did not stimulate sugar uptake. No evidence has been obtained for the presence in barley mesophyll tonoplasts of a uridine diphosphoglucose-dependent group translocator system, which has been reported to be responsible for vacuolar sucrose synthesis from external uridine diphosphoglucose in sugar cane and red beet vacuoles (Thorn and Maretzki, 1985; Thorn et al., 1986). Hexose transfer across the tonoplast appeared to be independent of the transfer of glycosylated phenols which is stimulated by ATP. On the basis of kinetic data on sucrose and glucose uptake, we propose a model of sugar transport across the tonoplast which can explain accumulation and mobilization of fructans in fructan-synthesizing plants.