Enhancement of Na+ uptake currents, time-dependent inward-rectifying K+ channel currents, and K+ channel transcripts by K+ starvation in wheat root cells

Excessive low-affinity Na+ uptake is toxic to the growth of glycophytic plants. Recently, several reports have suggested that the interaction between K+ and Na+ uptake might represent a key factor in determining the Na+ tolerance of plants. We investigated the effects of K+ starvation on Na+ and K+ uptake mechanisms in the plasma membrane of wheat (Triticum aestivum L.) root cortex cells using the patch-clamp technique. Unexpectedly, K+ starvation of wheat seedlings was found to enhance the magnitude and frequency of occurrence of time-dependent inward-rectifying K+ channel currents (I-K(in)+). We examined whether the transcription of a wheat root K-in(+) channel gene is induced by K+ starvation. A cDNA coding for a wheat root K+ channel homolog, TaAKT1 (accession no. AF207745), was isolated. TaAKT1 mRNA levels were up-regulated in roots in response to withdrawal of K+ from the growth medium. Furthermore, K+ starvation caused an enhancement of instantaneous Na+ currents (I-Na(+)). Electrophysiological analyses suggested that I-K(in)+ and I-Na(+) are not mediated by the same transport protein based on: (a) different activation curves, (b) different time dependencies, (c) different sensitivities to external Ca2+, and (d) different cation selectivities. These data implicate a role for I-Na(+) in Na+ uptake and stress during K+ starvation, and indicate that K-in(+) channels may contribute to K+-starvation-induced K+ uptake in wheat roots.