Energetics of Sucrose Transport into Protoplasts from Developing Soybean Cotyledons

The accumulation of tetraphenylphosphonium (TPP@), 5,5'-dimethyloxazolidine-2,4-dione (DMO), and a micro pH electrode were used to measure membrane potential, intracellular and extracellular pH, respec- tively, upon the addition of exogenous sucrose to soybean cotyledon protoplasts. Addition of sucrose caused a specific and transient (a) depolarization of the membrane potential (measured by TPPF accumu- lation), (b) acidification of the intracellular pH (measured by DMO accumulation), and (c) alkalization of the external medium (measured by a micro pH electrode). The time course for all these changes was similar (i.e. 5 to 10 minutes). Based on the rate of sucrose uptake and alkalization of the external medium, a stoichiometry of 1.02 to 1.10 for proton to sucrose was estimated. These data strongly support a proton/sucrose cotransporting mechanism in soybean cotyledon cells. Kinetic analyses of sugar uptake into protoplasts isolated from a rapidly growing soybean cotyledon revealed that at low external sugar concentrations, sucrose is preferentially transported over glucose (14, 22). This difference in sugar uptake is mainly due to the presence of a saturable uptake component for sucrose but not for glucose The responses of sucrose uptake into protoplasts (14, 22) and excised intact soybean cotyledons