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Plant Physiol, April 2000, Vol. 122, pp. 1387-1398

Enhancement of Na+ Uptake Currents, Time-Dependent Inward-Rectifying K+ Channel Currents, and K+ Channel Transcripts by K+ Starvation in Wheat Root Cells1

Peter H. Buschmann, Rama Vaidyanathan, Walter Gassmann,2 and Julian I. Schroeder*

Department of Biology and Center for Molecular Genetics, University of California at San Diego, La Jolla, California 92093-0116

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 (IK+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 (INa+). Electrophysiological analyses suggested that IK+in and INa+ 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 INa+ 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.

1 This work was supported by the U.S. Department of Agriculture (grant no. 98-353-04-6684 to J.I.S.), in part by the Department of Energy (grant no. 94-ER-20148 to J.I.S.), and by a postdoctoral fellowship of the Deutsche Forschungsgemeinschaft (P.H.B.).

2 Present address: Department of Plant Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720-3102.

* Corresponding author; e-mail julian{at}biomail.ucsd.edu; fax 858-534-7108.

© 2000 American Society of Plant Physiologists


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