A patch clamp study of Na+ transport in maize roots

The mechanisms mediating Na(+) transpdrt 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 Ca(2+), 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(+) (outsidexytosol). Channel gating was voltage-independent and distinct from that observed for the inwardly rectifying K(+)-selective channel in the same cell type. Increasing extracellular Ca(2+) 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 P(Na):P(K) 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.