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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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