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Rapid Up-Regulation of HKT1, a High-Affinity Potassium Transporter Gene, in Roots of Barley and Wheat following Withdrawal of Potassium1

Tie-Bang Wang2, Walter Gassmann3, Francisco Rubio4, Julian I. Schroeder, and Anthony D.M. Glass*

Department of Botany, University of British Columbia, Vancouver, Canada V6T 1Z4 (T.-B.W., A.D.M.G.); and Department of Biology and Center for Molecular Genetics, University of California-San Diego, La Jolla, California 92093-0116 (W.G., F.R., J.I.S.)

High-affinity K+ uptake in plant roots is rapidly up-regulated when K+ is withheld and down-regulated when K+ is resupplied. These processes make important contributions to plant K+ homeostasis. A cDNA coding for a high-affinity K+ transporter, HKT1, was earlier cloned from wheat (Triticum aestivum L.) roots and functionally characterized. We demonstrate here that in both barley (Hordeum vulgare L.) and wheat roots, a rapid and large up-regulation of HKT1 mRNA levels resulted when K+ was withdrawn from growth media. This effect was specific for K+; withholding N caused a modest reduction of HKT1 mRNA levels. Up-regulation of HKT1 transcript levels in barley roots occurred within 4 h of removing K+, which corresponds to the documented increase of high-affinity K+ uptake in roots following removal of K+. Increased expression of HKT1 mRNA was evident before a decline in total root K+ concentration could be detected. Resupply of 1 mM K+ was sufficient to strongly reduce HKT1 transcript levels. In wheat root cortical cells, both membrane depolarizations in response to 100 µM K+, Cs+, and Rb+, and high-affinity K+ uptake were enhanced by K+ deprivation. Thus, in both plant systems the observed physiological changes associated with manipulating external K+ supply were correlated with levels of HKT1 mRNA expression. Implications of these findings for K+ sensing and regulation of the HKT1 mRNA levels in plant roots are discussed.

1   This research was supported by a Natural Sciences and Engineering Research Council of Canada grant to A.D.M.G. and by a U.S. Department of Agriculture grant to J.I.S.
2   Present address: Department of Botany, University of Toronto, Ontario, Canada M5S 3B2.
3   Present address: Department of Plant Biology, Koshland Hall, University of California, Berkeley, CA 94720-3102.
4   Present address: Escuela Tecnica Superior de Ingenieros Agronomos. Universidad Politecnica de Madrid, 28040 Madrid, Spain.
*   Corresponding author; e-mail aglass{at}unixg.ubc.ca; fax 1-604-822-6089.

Plant Physiol. (1998) 118: 651-659
Copyright Clearance Center:   0032-0889/98/118//09
© 1998 American Society of Plant Physiologists




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