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WHOLE PLANT AND ECOPHYSIOLOGY

Low-Affinity Na⁺ Uptake in the Halophyte Suaeda maritima^{1,[C],[W],[OA]}

School of Pastoral Agriculture Science and Technology, Key Laboratory of Grassland Agro-ecosystem, Ministry of Agriculture, Lanzhou University, Lanzhou 730000, People's Republic of China (S.-M.W., J.-L.Z.); and Department of Biology and Environmental Science, School of Life Sciences, University of Sussex, Falmer, Brighton, Sussex BN1 9QG, United Kingdom (T.J.F.)

Na⁺ uptake by plant roots has largely been explored using species that accumulate little Na⁺ into their shoots. By way of contrast, the halophyte Suaeda maritima accumulates, without injury, concentrations of the order of 400 mM NaCl in its leaves. Here we report that cAMP and Ca²⁺ (blockers of nonselective cation channels) and Li⁺ (a competitive inhibitor of Na⁺ uptake) did not have any significant effect on the uptake of Na⁺ by the halophyte S. maritima when plants were in 25 or 150 mM NaCl (150 mM NaCl is near optimal for growth). However, the inhibitors of K⁺ channels, TEA⁺ (10 mM), Cs⁺ (3 mM), and Ba²⁺ (5 mM), significantly reduced the net uptake of Na⁺ from 150 mM NaCl over 48 h, by 54%, 24%, and 29%, respectively. TEA⁺ (10 mM), Cs⁺ (3 mM), and Ba²⁺ (1 mM) also significantly reduced ²²Na⁺ influx (measured over 2 min in 150 mM external NaCl) by 47%, 30%, and 31%, respectively. In contrast to the situation in 150 mM NaCl, neither TEA⁺ (1–10 mM) nor Cs⁺ (0.5–10 mM) significantly reduced net Na⁺ uptake or ²²Na⁺ influx in 25 mM NaCl. Ba²⁺ (at 5 mM) did significantly decrease net Na⁺ uptake (by 47%) and ²²Na⁺ influx (by 36% with 1 mM Ba²⁺) in 25 mM NaCl. K⁺ (10 or 50 mM) had no effect on ²²Na⁺ influx at concentrations below 75 mM NaCl, but the influx of ²²Na⁺ was inhibited by 50 mM K⁺ when the external concentration of NaCl was above 75 mM. The data suggest that neither nonselective cation channels nor a low-affinity cation transporter are major pathways for Na⁺ entry into root cells. We propose that two distinct low-affinity Na⁺ uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA⁺ or Cs⁺, but sensitive to Ba²⁺ and mediates Na⁺ uptake under low salinities (25 mM NaCl); pathway 2 is sensitive to TEA⁺, Cs⁺, and Ba²⁺ and mediates Na⁺ uptake under higher external salt concentrations (150 mM NaCl). Pathway 1 might be mediated by a high-affinity K transporter-type transporter and pathway 2 by an AKT1-type channel.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Timothy J. Flowers (t.j.flowers{at}sussex.ac.uk).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.104315

^* Corresponding author; e-mail t.j.flowers{at}sussex.ac.uk.

Received June 19, 2007; accepted August 23, 2007; published August 31, 2007.

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