Journal of Plant Physiology 171 (2014) 723–731 Contents lists available at ScienceDirect Journal of Plant Physiology journal h om epage: www.elsevier.com/locate/jplph Physiology The twins K + and Na + in plants  Bego ˜ na Benito a,1 , Rosario Haro a,1 , Anna Amtmann b , Tracey Ann Cuin c , Ingo Dreyer a,∗ a Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid, Madrid, Spain b Institute of Molecular, Cellular and Systems Biology (MCSB), College of Medical, Veterinary and Life Sciences (MVLS), University of Glasgow, Glasgow, UK c Biochimie et Physiologie Moléculaire des Plantes, Institut de Biologie Intégrative des Plantes, UMR 5004 CNRS/UMR 0386 INRA/Montpellier SupAgro/Université Montpellier 2, Montpellier, France a r t i c l e i n f o Article history: Received 26 July 2013 Received in revised form 1 October 2013 Accepted 2 October 2013 Available online 3 March 2014 Keywords: HKT Potassium Sodium Transport mechanism a b s t r a c t In the earth’s crust and in seawater, K + and Na + are by far the most available monovalent inorganic cations. Physico-chemically, K + and Na + are very similar, but K + is widely used by plants whereas Na + can easily reach toxic levels. Indeed, salinity is one of the major and growing threats to agricultural production. In this article, we outline the fundamental bases for the differences between Na + and K + . We present the foundation of transporter selectivity and summarize findings on transporters of the HKT type, which are reported to transport Na + and/or Na + and K + , and may play a central role in Na + utilization and detoxification in plants. Based on the structural differences in the hydration shells of K + and Na + , and by comparison with sodium channels, we present an ad hoc mechanistic model that can account for ion permeation through HKTs. © 2013 Elsevier GmbH. All rights reserved. Introduction Potassium with ∼2.1% (position 8) and sodium with ∼2.4% (posi- tion 6) are among the top ten elements present in the continental crust (position 1: oxygen 47.2%, 2: silicon 28.8%, 3: aluminum 8.0%, 4: iron 4.3%, 5: calcium 3.9%, 7: magnesium 2.2%, 9: tita- nium 0.4%, and 10: carbon 0.2%; Wedepohl, 1995). They are both far more abundant than the other alkali metals rubidium, lithium, and cesium (all <0.01%). Also in seawater, the monovalent cations Na + and K + are amongst the major dissolved ions. There, however, Na + dominates together with Cl - (both ∼500 mM) over Mg 2+ (∼50 mM), SO 4 2- (∼28 mM), Ca 2+ (∼10 mM) and K + (∼10 mM; values are for average seawater of salinity 35 2 ; Riley and Tongudai, 1967). Under these conditions, primitive cells might have found that accumula- tion of K + along with exclusion of the more abundant Na + provided Abbreviations: HAK, h igh-a ffinity K + uptake transporter (name of a transporter family); HKT , h igh-affinity K + t ransporter (name of a transporter family); KT, K + t ransporter (name of a transporter family); Ktr, K + tr ansporter (name of a transporter family); Kup, K + up take t ransporter (name of a transporter family); P, pore-forming region; TM, transmembrane -helix; Trk, tr ansport of K + (name of a transporter family).  This article is part of a Special Issue entitled “Potassium effect in plants”. ∗ Corresponding author. Tel.: +34 913364588. E-mail address: ingo.dreyer@upm.es (I. Dreyer). 1 These authors contributed equally to this work and are listed alphabetically. 2 The Practical Salinity Scale defines salinity in terms of the conductivity ratio of a sample to that of a solution of 32.4356 g of KCl at 15 ◦ C in a 1 kg solution. A sample of seawater at 15 ◦ C with conductivity equal to this KCl solution has a salinity of exactly 35 practical salinity units (psu). an efficient way to energize the plasma membrane. That the inor- ganic composition of the cytoplasm appears to be highly conserved for all eukaryotic organisms indicates that already at the outset of evolution in seawater, living cells developed a preference among the inorganic monovalent cations for K + over Na + for essential functions such as, for example, maintaining electro-neutrality and osmotic equilibrium. K + therefore became an indispensable neces- sity for living cells; a dependency also inherited by Embryophyta, which need to survive in oligotrophic environments where K + is present at much lower concentrations than in seawater (see Zörb et al., 2014, for more details on the bioavailability of K + in soils). This feature remained also conserved in higher plant species that returned to the sea after their predecessors spent a period of time on land as the sea grass Posidonia oceanica, for instance (Carpaneto et al., 1997, 1999, 2004). In plants, potassium plays a vital role in a wide range of both biophysical and biochemical processes. It exists as a monovalent cation and does not participate in covalent binding; it functions to maintain charge balance. The preservation of cell turgor pres- sure is very sensitive to a limited K + supply. Indeed, due to its high mobility, K + is usually the principle cation that contributes to vacuole and cell expansion (for K + transporters in vacuoles see Hamamoto and Uozumi, 2014; Pottosin and Dobrovinskaya, 2014). Nonetheless, over a longer time scale it can be replaced by Na + (Jeschke and Wolf, 1988) and/or organic solutes (Talbott and Zeiger, 1996), explaining the observed highly variable (10–200 mM) vac- uolar K + levels. In contrast, cytoplasmic levels are relatively stable, near 100 mM (Leigh and Wyn Jones, 1984). It is suggested that as total tissue K + concentration declines, the cytoplasm maintains a 0176-1617/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.jplph.2013.10.014