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 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.