POTASSIUM-ION CHANNELS IN THE PLASMALEMMA

The potassium ion is an indispensible cytosolic component of living cells and a key osmolyte of plant cells, crossing the plasmalemma to drive physiological processes like cell growth and motor cell activity. K+ transport across the plasmalemma may be passive through channels, driven by the electrochemical gradient, K+ equilibrium potential (EK) – membrane potential (Vm), or secondary active by coupling through a carrier to the inward driving force of H+ or Na+. Known K+ channels are permeable to monovalent cations, a permeability order being K+ > Rb+ > NH4+ > Na+≥ Li+ > Cs+. The macroscopic K+ currents across a cell or protoplast surface commonly show rectification, i.e. a Vm‐dependent conductance which in turn, may be controlled by the cytosolic activity of Ca2+, of K+, of H+, or by the K+ driving force. Analysis by the patch clamp technique reveals that plant K+ channels are similar to animal channels in their single channel conductance (4 to 100 pS), but different in that a given channel population slowly activates and may not inactivate at all. Single‐channel kinetics reveal a broad range of open times (ms to s) and closed times (up to 100 s). Further progress in elucidating plant K+ channels will critically depend on molecular cloning, and the availability of channel‐specific (phyto)toxins. Copyright © 1990, Wiley Blackwell. All rights reserved