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 pla nts. Recently, several reports have suggested that the interaction between K+ and Na+ uptake might represent a key factor in determining the Na+ toler ance of plants. We investigated the effects of K+ starvation on Na+ and Kuptake mechanisms in the plasma membrane of wheat (Triticum aestivum L.) ro ot cortex cells using the patch-clamp technique. Unexpectedly, K+ starvatio n of wheat seedlings was found to enhance the magnitude and frequency of oc currence of time-dependent inward-rectifying K+ channel currents (I-K(in)+) . We examined whether the transcription of a wheat root K-in(+) channel gen e is induced by K+ starvation. A cDNA coding for a wheat root K+ channel ho molog, TaAKT1 (accession no. AF207745), was isolated. TaAKT1 mRNA levels we re up-regulated in roots in response to withdrawal of K+ from the growth me dium. 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) di fferent activation curves, (b) different time dependencies, (c) different s ensitivities to external Ca2+, and (d) different cation selectivities. Thes e data implicate a role for I-Na(+) in Na+ uptake and stress during K+ star vation, and indicate that K-in(+) channels may contribute to K+-starvation- induced K+ uptake in wheat roots.