Osmotically-Induced Surface Area Expansion in Oat Protoplasts Depends on the Transmembrane Potential

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Abstract
The transmembrane potential of mesophyll cell protoplasts from Avena sativa was altered in two different ways: first by raising the external KCI concentration with and without the addition of valinomycin and, secondly, by using the proton carrier CCCP and varying the external pH. The percentage of unlysed protoplasts was determined after decreasing the osmotic pressure from 1.2 to 0.7 MPa in order to characterize their ability to enlarge their surface area. Lysis strongly increased when the KCI concentration was raised from 0 to 90 mol m−3. This effect did not depend on the presence of valinomycin. Using CCCP, lysis increased similarly if the external pH was lowered from 7.2 to 5.5. In the presence of CCCP the influence of the external KCI concentration disappeared completely. This led to the conclusion that depolarization inhibited surface area expansion. Since the expansion is thought to be based on exocytotic-like membrane incorporation, these results suggest that exocytosis is controlled, at least in part, by the transmembrane potential.