Direct Evidence for a Sugar Transport Mechanism in Isolated Vacuoles

Abstract

Sugar transport has been directly observed in isolated higher plant vacuoles for the first time. The latter were released from protoplasts isolated from the mesophyll of Pisum sativum L.

Uptake of l-glucose by the vacuoles was very slight in comparison with that of the d-glucose analog 3-O-methyl glucose (MeG), indicating, first, that a highly selective sugar uptake mechanism is seated in the tonoplast; and, second, that the mechanism was functioning in the isolated vacuoles.

MeG uptake was markedly sensitive to the pH of the medium, falling as the external pH rose. Addition of MgATP to buffered medium strongly promoted MeG uptake by vacuoles, but not by the protoplasts from which they were released. Treatment with the proton ionophore SF₆₈₄₇ drastically reduced uptake by the vacuoles, but had a lesser effect on uptake by the protoplasts. The inhibitory effect of SF₆₈₄₇ on uptake by the vacuoles was countered to a substantial degree by the addition of MgATP.

The influence of pH, the stimulatory effect of ATP, and the ATP-reversible inhibition by SF₆₈₄₇ all strengthen the conclusion that the observed sugar uptake reflected membrane function and was not due to a diffusional inward leak through damaged membranes.

The results are discussed in the light of currently held concepts regarding the driving force for sugar transport.