ELECTRICAL-POLARIZATION OF TOBACCO CELLS BY CA2+ ION CHANNELS

It has been suggested that neighbouring cells in higher plants co-ordinate their direction of growth by sensing the electrical polarities of their neighbours. The present work sets out to examine the role played by calcium in the response of their individual cells to externally-applied fields. The transcellular currents of cultured tobacco cells were investigated with a vibrating probe before and after the application of an artificial electric current with a density of 250-mu-A cm-2, giving a potential difference of approximately 3 mV across the cell. When calcium was omitted from the experimental medium. the externally-applied current had little effect on either the direction or magnitude of the cells' own transcellular currents. When 0.1 mM calcium was present. the external current repolarized the cells so that their own currents tended to flow in the same direction as the current applied. This was due to a large localized increase in inward current in the region nearest the positive electrode. with the outward current being more evenly spread. Adding cobalt ions (a Ca2+-channel blocker) in the presence of external calcium had little immediate effect on the transcellular currents themselves. but they lost their ability to change in response to the artificially applied current. This suggests that the cells may have detected the applied current by enhanced calcium ingress through calcium channels in the most hyperpolarized region of the membrane. An hypothesis is presented which proposes that asymmetric calcium entry results in the electrical polarization of cells by a mechanism involving both the opening of calcium-gated ion channels and the lateral movement of electrophoretically mobile channels.