Fusion and fission of plasma-membrane material accommodates for osmotically induced changes in the surface area of guard-cell protoplasts

Stomatal movement requires large and repetitive changes in cell volume and consequently changes in surface area. The patch-clamp technique was used to monitor changes in plasma-membrane surface area of individual guard-cell protoplasts (GCPs) by measuring membrane capacitance (C-m), a parameter proportional to the surface area. The membrane capacitance increased under hypoosmotic conditions and decreased after hypertonic treatment. As the specific capacitance remained constant, this demonstrates that osmotically induced changes in surface area are associated with incorporation and removal of membrane material. Osmotically induced fusion and fission of plasma-membrane material was not affected by removal of extracellular Ca2+ Dialysing protoplasts with very low (<2 nM) or high (1 mu M) Ca2+ had no effect on changes in C-m under hypo- and hyperosmotic conditions. However, the rate of change in surface area was dependent on the size of the difference in osmotic potential applied. The larger the osmotic difference and thus changes in membrane tension caused by water influx or efflux, the faster the change in C-m. The results therefore demonstrate that osmotically induced fusion and fission of plasma-membrane material in GCPs are Ca2+-independent and modulated by membrane tension.