A patch clamp study of Na+transport in maize roots

The mechanisms mediating Na+ transport in higher plant roots were investigated by applying the patch clamp technique to protoplasts isolated from the cortex and stele of maize roots. In the cortex, permeation of Na+ through a time-dependent K+-selective inward rectifier was negligible. Instead, Na+ influx into maize roots probably occurs via an instantaneously-activating current. This current was partially inhibited by extracellular Ca2+, but was insensitive to extracellular TEA+, Cs+ and TTX. In outside-out patches, a plasma membrane ion channel was found which mediated an inward Na+ current which, at least in part, underlies the whole-cell instantaneously-activating current. The unitary conductance of this channel was 15 pS in 102:121 mM Na+ (outside:cytosol). Channel gating was voltage-independent and distinct from that observed for the inwardly rectifying K+-selective channel in the same cell type. Increasing extracellular Ca2+ from 0.1 to 1 mM reduced the open probability and unitary conductance of this channel. In 102 mM Na+:123 mM K+ (outside:cytosol) a PNa:PK of 2.1 was calculated. It is suggested that the plasma membrane Na+-permeable channel identified in the cortex of maize roots represents a pathway for low affinity Na+ uptake by intact maize roots. In the stele, permeation of Na+ through outwardly rectifying K+ channels was found to be negligible and the channels are thus unlikely to be involved in the transport of Na+ from the root symplasm.