Membrane trafficking and osmotically induced volume changes in guard cells

Guard cells rapidly adjust their plasma membrane surface area while responding to osmotically induced volume changes. Previous studies have shown that this process is associated with membrane internalization and remobilization. To investigate how guard cells maintain membrane integrity during rapid volume changes, the effects of two membrane trafficking inhibitors on the response of intact guard cells of Vicia faba to osmotic treatments were studied. Using confocal microscopy and epidermal peels, the relationship between the area of a medial paradermal guard-cell section and guard-cell volume was determined. This allowed estimates of guard-cell volume to be made from single paradermal confocal images, and therefore allowed rapid determination of volume as cells responded to osmotic treatments. Volume changes in control cells showed exponential kinetics, and it was possible to calculate an apparent value for guard-cell hydraulic conductivity from these kinetics. Wortmannin and cytochalasin D inhibited the rate of volume loss following a 0-1.5 MPa osmotic treatment. Cytochalasin D also inhibited volume increases following a change from 1.5 MPa to 0 MPa, but wortmannin had no effect. Previous studies showing that treatment with arabinanase inhibits changes in guard-cell volume in response to osmotic treatments were confirmed. However, pressure volume curves show that the effects of arabinanase and the cytochalasin D were not due to changes in cell wall elasticity. It is suggested that arabinanase, cytochalasin D, and wortmannin cause reductions in the hydraulic conductivity of the plasma membrane, possibly via gating of aquaporins. A possible role for aquaporins in co-ordinating volume changes with membrane trafficking is discussed.