title: The Physiological Relevance of Na+-Coupled K+-Transport.
abstract: Plant roots utilize at least two distinct pathways with high and low affinities to accumulate K+. The system for high-affinity K+ uptake, which takes place against the electrochemical K+ gradient, requires direct energization. Energization of K+ uptake via Na+ coupling has been observed in algae and was recently proposed as a mechanism for K+ uptake in wheat (Triticum aestivum L.). To investigate whether Na+ coupling has general physiological relevance in energizing K+ transport, we screened a number of species, including Arabidopsis thaliana L. Heynh. ecotype Columbia, wheat, and barley (Hordeum vulgare L.), for the presence of Na+-coupled K+ uptake. Rb+-flux analysis and electrophysiological K+-transport assays were performed in the presence and absence of Na+ and provided evidence for a coupling between K+ and Na+ transport in several aquatic species. However, all investigated terrestrial species were able to sustain growth and K+ uptake in the absence of Na+. Furthermore, the addition of Na+ was either without effect or inhibited K+ absorption. The latter characteristic was independent of growth conditions with respect to Na+ status and pH. Our results suggest that in terrestrial species Na+-coupled K+ transport has no or limited physiological relevance, whereas in certain aquatic angiosperms and algae this type of secondary transport energization plays a significant role.
authors.affiliation: Department of Biology, University of York, P.O. Box 373, York YO1 5YW, United Kingdom (F.J.M.M., D.S.).
authors.forename: FJM.
authors.initials: F
authors.lastname: Maathuis
authors.forename: D.
authors.initials: D
authors.lastname: Verlin
authors.forename: F. A.
authors.initials: FA
authors.lastname: Smith
authors.forename: D.
authors.initials: D
authors.lastname: Sanders
authors.forename: J. A.
authors.initials: JA
authors.lastname: Fernandez
authors.forename: N. A.
authors.initials: NA
authors.lastname: Walker