Sucrose uptake and compartmentation in sugar beet taproot tissue

Active sucrose uptake by discs of mature sugar beet (Beta vulgaris L. cv GW-D2 and USH-20) root tissue shows a biphasic dependence on external sucrose. At concentrations up to 20 millimolar sucrose, the active uptake mechanism appears to approach saturation, with an apparent K(m) of 3.6 millimolar. At higher external sucrose concentrations, a linear dependence becomes obvious indicating the probable presence of a nonsaturable, metabolically dependent uptake component. Active transport was not observed at external sucrose concentrations that caused tissue plasmolysis. Passive sucrose uptake in unplasmolyzed tissue showed a linear dependence on external sucrose concentration. The mitochondrial and/or suspected vacuolar ATPase inhibitors oligomycin, diethylstilbestrol, and N,N-dicyclohexylcarbodiimide strongly inhibited active sucrose uptake, whereas the putative plasmalemma-specific ATPase inhibitor orthovanadate was without effect.Sucrose efflux patterns from root discs indicated three distinct sucrose compartments having efflux kinetics consistent with those for cell wall, cytoplasm, and vacuole with the vacuole being the slowest releasing compartment. The sucrose contents and volumes of the compartments indicated that sucrose uptake into the vacuole was against a concentration gradient. Combined sucrose uptake/efflux analyses indicated that sucrose uptake into the vacuole is primarily an active transport process while transport into the cytoplasm is apparently passive, at least at external sucrose concentrations above 20 millimolar. We discuss the possibility that active sucrose uptake into the vacuoles of sugar beet storage cells is rate limited by passive sucrose transport to the active uptake site.