Energetics of Sucrose Transport into Protoplasts from Developing Soybean Cotyledons

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Abstract
The accumulation of tetraphenylphosphonium (TPP+), 5,5''-dimethyl-oxazolidine-2,4-dione (DMO), and a micro pH electrode were used to measure membrane potential, and intracellular and extracellular pH, respectively, upon the addition of exogenous sucrose to soybean cotyledon protoplasts. Addition of sucrose caused a specific and transient depolarization of the membrane potential (measured by TPP+ accumulation), acidification of the intracellular pH (measured by DMO accumulation), and alkalization of the external medium (measured by a micro pH electrode). The time course for all these changes are similar (i.e., 5-10 min). Based on the rate of sucrose uptake and alkalization of the external medium, a stoichiometry of 1.02-1.10 for proton to sucrose was estimated. These data strongly support a proton/sucrose cotransporting mechanism in soybean cotyledon cells.