CABLE CORRECTION OF MEMBRANE CURRENTS RECORDED FROM ROOT HAIRS OF ARABIDOPSIS-THALIANA L

Membrane currents were recorded under Voltage clamp from root hairs of Arabidopsis thaliana L. using the two-electrode method. Concurrent measurements of membrane voltage distal to the point of current injection were also carried out to assess the extent of current dissipation along the root hair axis. Estimates of the characteristic cable length, R, showed this parameter to be a function both of membrane voltage and of substrate concentration for transport. The mean value for lambda at 0 mV was 103 +/- 20 mu m (n = 17), but ranged by as much as 6-fold in any one cell for membrane voltages from -300 to +40 mV and was affected by 0.25 to 3-fold at any one voltage on raising [K+](0) from 0.1 to 10 mol m(-3). Current dissipation along the length of the cells lead to serious distortions of the current-voltage (I-V) characteristic, including consistent underestimates of membrane current as well as a general linearization of the I-V curve and a masking of conductance changes in the presence of transported substrates. In some experiments, microelectrodes were also placed in neighbouring epidermal cells to record the extent of intercellular coupling. Even with current-passing microelectrodes placed at the base of root hairs, coupling was less than or equal to 5% (voltage deflection of the epidermal cell less than or equal to 5% that recorded at the site of current injection), indicating an appreciable resistance to current passage between cells. These results demonstrate the feasibility of using root hairs as a 'single-cell model' in electrophysiological analyses of transport across the higher-plant plasma membrane; they also confirmed the need to correct for the cable properties of these cells on a cell-by-cell basis.