Cation channels in the plasma membrane of rye roots

Voltage-dependent cation channels in the plasma membrane of rye (Secale cereale L.) root cells have been studied electrically both in vivo, by patch-clamping protoplasts of epidermal cells, and in vitro, following the reconstitution of plasma-membrane vesicles into planar lipid bilayers (PLB). The patch-clamp technique has identified a time-dependent inward-rectified K+ current and two time-dependent outward-rectified currents, one of which is relatively selective for K+ whilst the other is non-selective between cations. In addition, a time-independent K+-current has been observed in patch-clamp recordings. This current is believed to be carried by voltage-independent channels, which are permeable to monovalent cations and whose biophysical properties correspond to those of the channel most frequently observed in PLB. Two further cation channels are frequently observed in PLB. These channels are permeable to both monovalent and divalent cations and are activated by plasmamembrane depolarization. They may have a role in intracellular signalling since they would mediate Ca2+ influx in response to plasma-membrane depolarization in vivo. In this paper the biophysical properties and physiological roles of each cation channel are discussed and compared with cation channels in the plasma membrane of root cells from contrasting cell types and plant species.