Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download molecular mechanisms and regulation of k+ transport in higher plants
Document related concepts
Cytokinesis wikipedia , lookup
Signal transduction wikipedia , lookup
Membrane potential wikipedia , lookup
Cyclic nucleotide–gated ion channel wikipedia , lookup
Endomembrane system wikipedia , lookup
Magnesium transporter wikipedia , lookup
List of types of proteins wikipedia , lookup
Transcript
3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB 10.1146/annurev.plant.54.031902.134831 Annu. Rev. Plant Biol. 2003. 54:575–603 doi: 10.1146/annurev.plant.54.031902.134831 c 2003 by Annual Reviews. All rights reserved Copyright ° MOLECULAR MECHANISMS AND REGULATION OF K+ TRANSPORT IN HIGHER PLANTS Anne-Aliénor Véry and Hervé Sentenac Biochimie et Physiologie Moléculaires des Plantes, UMR 5004 CNRS/ENSA-M/INRA/UM2, Place Viala, 34060 Montpellier Cedex 2, France; email: [email protected], [email protected] Key Words plant K+ nutrition, K+ transporter, K+ channel, Shaker, potassium homeostasis ■ Abstract Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light. CONTENTS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOLECULAR IDENTITY OF K+ TRANSPORT SYSTEMS . . . . . . . . . . . . . . . . . . Identification of Multigene Families: Chronology and Cloning Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shaker Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KCO Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . KUP/HAK/KT Transporters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HKT Transporters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOLECULAR BASES OF K+ TRANSPORT REGULATION . . . . . . . . . . . . . . . . . Modulation of Transcript Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterotetramerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identification of Regulatory Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regulation by Voltage, pH, Ca2+ , and Cyclic Nucleotides . . . . . . . . . . . . . . . . . . . . 1040-2519/03/0601-0575$14.00 576 576 576 577 582 583 584 585 586 586 587 588 590 575 3 Apr 2003 12:44 576 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC NEW LIGHT ON EARLY PHYSIOLOGICAL QUESTIONS . . . . . . . . . . . . . . . . . . K+ Uptake: More Complex than a Dual Mechanism . . . . . . . . . . . . . . . . . . . . . . . . Energization of High-Affinity K+ Uptake: Still Unresolved Questions . . . . . . . . . . Evidence for Passive K+ Release into the Xylem Sap . . . . . . . . . . . . . . . . . . . . . . . A Milestone in the Analysis of Phloem K+ Transport . . . . . . . . . . . . . . . . . . . . . . . Evidence for Involvement of K+ Transport in Control of Cell Growth . . . . . . . . . . CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591 591 592 593 593 594 594 595 INTRODUCTION Potassium (K+) can compose up to 10% of the total plant dry weight. As it is compatible with protein structure at high concentration, K+ is the most abundant cation in the cytosol where it plays a role in basic functions, such as osmoregulation, electrical neutralization of anionic groups, and control of cell membrane polarization (21, 90). The importance of these functions, along with the relatively high permeability of the plant cell membrane to K+, which renders the experimental analyses easier, probably explains why the transport of K+ has been studied more extensively than that of all the other nutrient ions, giving rise to heuristic models for the analysis of ion transport in plants. For instance, in the 1950s, pioneering work on K+ uptake in roots led to the idea that the Michaelis-Menten formalism could be applied to membrane transport analysis and, eventually, to the dual mechanism paradigm: Both high- and low-affinity transport systems are involved in absorption of most solutes in plant cells (42). In the 1970s, the demonstration that the general organization of solute transport across the plant plasma membrane conformed to the chemiosmotic scheme was based largely on electrophysiological analyses of root K+ transport (17). In the early 1990s, the first mineral ion transport systems identified in plants were two K+ channels from Arabidopsis. There has been considerable progress since then in analysis of K+ transport in plants at the molecular level, again providing a model for plant ion transport biology. This review summarizes this new information and revisits earlier physiological questions in light of recent molecular findings. MOLECULAR IDENTITY OF K+ TRANSPORT SYSTEMS Identification of Multigene Families: Chronology and Cloning Strategies The first molecular breakthrough in the analysis of membrane K+ transport in plants came in 1992 with the identification of two Arabidopsis K+ channels, AKT1 (133) and KAT1 (4), related to animal K+ channels of the Shaker family. Both AKT1 and KAT1 were cloned by functional complementation of yeast mutant strains defective for K+ uptake. DNA-based strategies and systematic sequencing programs soon 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 577 revealed a large family of K+ channels related to these two channels (Table 1), constituting the so-called plant Shaker family. In 1994, the yeast functional complementation strategy allowed another breakthrough to be made with the first cloning of a plant K+ transporter1, HKT1 from wheat (126), identifying the so-called plant HKT family (115) (Table 2). In 1997, another family of plant K+ transporters was identified simultaneously in three different cloning strategies, PCR (124), in silico analyses (73, 112), and functional complementation of yeast (47). Members of this family (Table 2) were named either HAK# (124), KT# (112), or KUP# (47, 73), and the corresponding family was named HAK (115) or KUP/HAK/KT (98). During the same period, in silico searches for new plant counterparts of animal K+ channels identified the so-called KCO channels (24, 25). At the functional level, direct evidence has been obtained by using various heterologous expression systems that members of these families are indeed endowed with K+ transport activity (see below). Other putative K+ transporters and cation transporters that might play a role in K+ transport have been identified (75, 82, 98, 125, 131), but the information available about most of these systems is still poor and essentially came from analyses of sequence and phylogenetic relationships (98). Shaker Channels Plant Shaker channels share similarities, both at the sequence and structure levels, with animal voltage-gated K+ channels forming the so-called Shaker family (70). Animal Shaker channels are made up of four subunits arranged around a central pore. The hydrophobic core of each subunit consists of six transmembrane segments (TMS), the fourth one with repetition of basic residues acting as a voltage sensor (Figure 1). A highly conserved membranar loop, located between the fifth and the sixth TMS and called the P (pore) domain, forms part of the selectivity filter of the ion-conducting pore. This loop contains a TxxTxGYGD motif, the hallmark of K+-selective channels (70). The plant Shaker cytosolic C-terminal region harbors regulatory domains comprising a putative cyclic nucleotide-binding site and, at the extreme C terminus, the so-called KHA region (rich in hydrophobic and acidic residues), which might be involved in subunit tetramerization (26) and/or channel clustering in the membrane (40). An ankyrin domain, hypothesized to be a site of interaction with regulatory proteins (133), is present in most channels (e.g., in six of the nine Arabidopsis Shakers) between the putative cyclic nucleotide–binding site and the KHA region. Most plant Shakers identified so far have been successfully expressed and characterized in heterologous systems (Xenopus oocytes, insect and mammalian cell lines, or yeast) (Table1). Like their animal counterparts, they form K+-selective channels strongly regulated by voltage. They can be regrouped into three functional subfamilies according to the voltage range within which they are active and 1 Nonchannel-like transport systems are called transporters in this review. At At At At At At At Dc Le Mc Mc Mc Ss AKT1 SPIK AKT6 AKT2 AtKC1 SKOR GORK KDC1 LKT1 KMT1 MKT1 MKT2 SPICK1 AKT2 AKT2 AtKC1 AKT1 KAT1 AKT1 — — IR IR — — OR OR Silent? — — Act ac pHe Act ac pHe — — — Act al pH Act ac pHe — Act al pH — Act ac pHe r cortex, epidermis & hair, hydathode, mesophyll, gc? Pollen f Phloem, l epidermis, gc, mesophyll, sepal r epidermis, hair, cortex & endodermis, trichome, hydathode, l epidermis, stipule r pericycle & xylem parenchyma, pollen gc, stem, f, r hair & epidermis r hair r hair, r, l l, seed capsule r epidermis, cortex & stele, f l, f, seed capsule Pulvinus gc, l phloem, f — — — — — PM PM? — PM PM — — PM PM? PM Salt stress: (−) Light & internal clock — — Salt stress: transient (+) Salt stress: (−) (& less protein) — — Light, photosynthesis & ABA: (+) salt stress: (−); low K+& BA: ( = ) Interaction with AKT1 ABA: (−); salt stress: (+) in leaf; BA & 2,4-D: (−) in root; K+ deprivation: ( = ) ABA, BA, 2,4-D, K+starvation: (−) in root; salt stress: ( = ) in root — Low K+, salt stress & ABA: ( = ) BA & 2,4-D: (−) in root — — Regulationf — Leaf movement? — — — — Stomatal movement? Xylem sap K+ loading Tube development — Phloem K+ loading/unloading? Root K+ absorption? Stomatal movement? Root K+ absorption Stomatal movement? Role 134 100 34 59 134 134 52, 79, 101, 108 2, 68 10, 31, 51, 60, 81, 108, 133, 136 101 80 29, 80, 96, 108 108, 113 4, 62, 66, 77, 104, 127, 136, 147 109 References AR184-PP54-23.sgm SKOR SKOR AtKC1 IR — WIR IR IR gc Mb.e AR184-PP54-23.tex AKT1 AKT1 AKT2 AKT1 KAT1 Act ac pH Organ/Tissued ¥ At IR pH sensit.d VÉRY KAT1 Typec AR KAT2 At SHAKER family KAT1 Groupb 578 Sp.a 12:44 Name TABLE 1 Cloned K+ channels in plants: functional properties, localization, and regulation 3 Apr 2003 LaTeX2e(2002/01/18) SENTENAC P1: GJB Vv Zm Zm At At Ss SIRK ZMK1 ZMK2 KCO family KCO1 KCO6 SPOCK1 KCO1 — — — Act al pHe Act ac pH Act ac pHe — — Act ac pH Act ac pHe — Act al pHe? Mesophyll, gc, l, r vasculature, r apex, r pericycle, sepal, pollen Mesophyll, gc Pulvinus gc (l, s, petiole), berry Coleoptile (vascular & non vascular), mesocotyl, r Coleoptile (vascular), mesocotyl, l, r Pulvinus Pulvinus l gc, f l abaxial & s gc r Phloem — — Tono — — — — — PM? — — — — Light: transient (+) — Auxin: ( = ) in coleoptile Light & internal clock Light & internal clock Mild drought stress: (−) — Ext K+ deprivation: (+) (−) in cotyledon during seed dvpt; fructose: (+); (+) day versus night; (+) in sinks versus sources (−) in berry after véraison Auxin in coleoptile: transient (+) — — — — Coleoptile elongation & bending upon gravistimulation? — Leaf movement? Leaf movement? — — — Phloem K+ loading/unloading? 129 100 23, 25, 129 107 111 107 100 100 64, 76, 102 162, 163 16 1 pH sensit.: pH sensitivity; Act ac(al) pH(e): Activation by acidic(alkaline) (external) pH; gc: guard cell; l: leaf; f: flower; r: root; s: stem. Transcriptional regulation unless otherwise mentioned; increased (+), decreased (−) or unaltered ( = ) transcript level upon indicated conditions; dvpt: development Mb.: membranar localization; PM: plasma membrane; Tono: tonoplast. LaTeX2e(2002/01/18) ABA: abscisic acid; BA: benzyladenine; 2,4-D: 2,4 dichlorophenoxyacetic acid. f e The five groups of Shakers are defined by the primary branches in the phylogenetic tree (111); Name of group: first identified group member from Arabidopsis. Type: determined by functional characterization; IR: inward rectifier; WIR: weak inward rectifier; OR: outward rectifier. d c b AR184-PP54-23.sgm — — OR WIR IR IR — — IR IR — WIR? AR184-PP54-23.tex AKT2 KAT1 AKT1 AKT2 SKOR KAT1 AKT1 AKT1 AKT2 AR a Sp.: Species; At: Arabidopsis thaliana; Dc: Daucus carota; Le: Lycopersicon esculentum; Mc: Mesembryanthemum crystallinum; Ss: Samanea saman; St: Solanum tuberosum; Ta: Triticum aestivum; Vf: Vicia faba; Vv: Vitis vinifera; Zm: Zea mays. Ss Ss St St Ta Vf 12:44 SPICK2 SPORK1 KST1 SKT1 TaAKT1 VfK1 3 Apr 2003 P1: GJB K+ TRANSPORT IN PLANTS 579 Na+ K+, Na+ K+, Na+ — Na+ K+, Na+ K+, Na+ HKT family AtHKT1 EcHKT1 EcHKT2 HvHKT1 OsHKT1 OsHKT2 TaHKT1 r l, s, r l, s, r r r r r cortex mainly Ext K+ & Na+: ( = ) — — Ext K+ depletion: (+) High ext K+ & Na+: (−) High ext K+ & Na+: (−) Ext K+ depletion: (+) — Low K+: slight (−) in root Low K+: (+) in root — K+ deprivation: (+) in shoot — — Developmentally regulated Ext K+ depletion: (+) — Low ext K+ & salt stress: (+) Salt stress: slight (+) Salt stress: slight (+) Low ext K+: slight (+) Ext K+ depletion: (+) K+ deprivation: (+) Ext K+, Na+, pH, Ca2+: ( = ) Root Na+ uptake? — — — — — — Root K+ uptake? Cell expansion? — Root hair elongation — — — Fiber elongation? — — — — — — — — — Role 120, 142 44, 87 44, 87 152 61 61 117, 126, 152 47, 73 41, 73, 112 73 73, 114 118 49 49 116 118, 124 132 135 135 135 135 8 8 8 References c f: flower; l: leaf; s: stem; r: root, sh: shoot; Regulation: increased (+), decreased (−) or unaltered ( = ) transcript level upon indicated treatment. Perm.: permeability; H+(Na+)-K+: H+(Na+)-K+ symport; HA (LA): high (low) affinity; pH sensit.: pH sensitivity; Act ac pHe: activation by acidic external pH. LaTeX2e(2002/01/18) b At: Arabidopsis thaliana; Cn: Cymodocea nodosa; Gh: Gossypium hirsutum; Hv: Hordeum vulgare; Mc: Mesembryanthemum crystallinum; Os: Oryza sativa; Ec: Eucalyptus calmaldulensis; Ta: Triticum aestivum. Na+-K+? Na+-K+ None — — — — — None f, l, s, r f, l, s, r f, l, s, r f, l, s, r, silique r (mainly), sh l l Cotton fiber r r l, s, r s mainly r mainly l, s, r r mainly r & sh r &sh Regulationc ¥ AR184-PP54-23.sgm LA LA Na+ LA Na+ — — HA K+?, LA Na+ HA K+, LA Na+ — — — — — Act ac pHe — — — Act ac pHe — — — — — Act ac pHe? Act ac pHe? Organ/Tissuec AR184-PP54-23.tex Na+-K+? Na+-K+? — HA K+, LA Na+? LA — HA HA K+, LA Na+? LA LA? — HA K+, LA Na+? LA LA? LA? — LA? HA K+, LA Na+? LA LA pH sensit.b VÉRY — — — — — H+-K+? — — — H+-K+? — — — — — — — Affinityb AR a K+, Na+? K+, Na+? — K+ K+, Na+? K+ K+ ? — K+, Na+? K+, Na+? K+ K+ — K+ K+, Na+? K+ K+ KUP/HAK/KT family AtKUP1 AtKUP2 AtKUP3 AtKUP4 AtHAK5 CnHAK1 CnHAK2 GhKT1 HvHAK1 HvHAK2 McHAK1 McHAK2 McHAK3 McHAK4 OsHAK1 OsHAK7 OsHAK10 Typeb 580 Perm.b 12:44 Namea TABLE 2 Cloned K+ transporters in plants: functional properties, localization, and regulation 3 Apr 2003 P1: GJB SENTENAC 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 581 Figure 1 Topology of K+ transport systems identified in plants. The models proposed for the three families of (putative) K+ channels identified in plants, Shaker, KCO-2P, and KCO-1P, are derived from those predicted for their animal and microbial counterparts, the Shaker (70), KCNK (106), and Kir (33) families, respectively. Plant KUP/HAK/KT transporters are related to the bacterial KUP or fungal HAK K+ transporters (115). Their proposed topology is so far based on hydrophobicity profiles only. The proposed topology of plant HKT transporters is that recently suggested by Durell et al. (39) for the whole (plant)HKT/(fungal)TRK/(bacterial)KtrB superfamily. Abbreviations: ext/cyt, extracellular/cytoplasmic side; mb, membrane; ++, positively charged amino acids in the voltage sensor; P, CNBD, Anky, KHA, EF, respectively, pore, putative cyclic nucleotide–binding, ankyrin, KHA and EF hand domains; (W)IR/OR, (weakly) inwardly/outwardly rectifying. thus their rectification properties: inward, weakly inward, and outward2 (Table 1). Comparison of their functional properties (in heterologous expression systems) with channel activity recorded in planta suggests that they are active at the plasma membrane and mediate most K+-selective voltage-gated currents that dominate the membrane K+ conductance at hyperpolarized and depolarized membrane potentials, at millimolar K+ concentrations, in numerous cell types (148). 2 Inward rectifiers are activated by negative-going membrane potentials (membrane hyperpolarization), from a threshold generally more negative than the K+ equilibrium potential (EK) except at submillimolar external K+ concentration and are therefore mainly involved in K+ uptake. Weak inward rectifiers also are activated by membrane hyperpolarization, but they never display null open probability within the physiological membrane potential range and therefore are potentially able to mediate both K+ uptake and release. Outward rectifiers activate at membrane potentials more positive than EK (membrane depolarization) and thus are specialized in K+ release. 17 Apr 2003 14:28 582 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC Biochemical and reverse genetics analyses directly support this hypothesis in Arabidopsis. The Arabidopsis Shaker family, which comprises nine members, is to date the best-characterized family of plant transport systems. The available information (Table 1) (Figure 2) suggests that these channels are likely to mediate long-term wholesale transport involved in plant K+ nutrition and/or regulation of the cell’s K+ status and osmotic potential. Multidisciplinary approaches associating reverse genetics analyses, expression studies, and electrophysiological characterizations have highlighted the role of four Shakers, AKT1, SKOR, KAT1, and SPIK, in such functions (Figure 2). In the root, AKT1 plays a role in K+ uptake from the soil solution (60) and SKOR in K+ release into the xylem sap (52). KAT1 takes part in guard cell K+ uptake but is not essential for stomatal opening (136), probably because of inward rectifier redundancy in guard cells (109). SPIK is involved in K+ uptake in pollen and is required for optimal pollen tube development and pollen competitive ability (101). The roles of the five other Shakers are less well understood. Current data support the hypothesis that AKT2 is involved in longdistance K+ transport via the phloem sap (29, 80, 96). AKT2 has also been shown to be an important contributor, along with AKT1, to the mesophyll K+ permeability (31). Like KAT1, KAT2 is likely to play a role in K+ influx into guard cells during stomatal opening (109) and GORK to mediate K+ release from these cells during stomatal closure (2). GORK is also expressed in root hairs where it could play a role in osmoregulation (68). AtKC1 is expressed in root periphery cells (68, 108) where it would be an integral component of functional K+ uptake channels (113). Only localization data have been obtained for the remaining Arabidopsis Shaker channel, AKT6 (80), revealing expression in flowers. KCO Channels KCO channels display a hydrophobic core composed of either four TMS and two P domains (KCO-2P family) or two TMS and one P domain (KCO-1P family) (Figure 1). They do not possess any TMS that might be expected to behave as voltage sensors. Their pore domains bear a high K+-permeability hallmark motif. Most of them possess putative Ca2+-binding sites (one or two EF hands) in their cytosolic C-terminal region (24, 100). K+ channels with a hydrophobic core sharing structural homologies with KCO channels exist in the animal field. Functionally, 2TMS-1P animal K+ channels are inward rectifiers (33, 70). Most 4TMS-2P animal channels have been described as leak-like channels (i.e., their open probability is weakly sensitive to voltage), with some of them gated by membrane stretch (106). In Arabidopsis, the KCO-2P family has five members and the KCO-1P family has a single member (24). To date, only KCO1, which is the first KCO gene identified in plants and belonging to the KCO-2P family, has been characterized. KCO1 has been successfully expressed in insect cells where it encodes a K+-selective outwardly rectifying channel activated by cytosolic Ca2+ (25). In addition to this 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 583 sensitivity to Ca2+, KCO1 can be functionally distinguished from outwardly rectifying Shaker channels by faster and nonsigmoidal kinetics of current activation and a higher single-channel conductance. KCO1 is expressed throughout the plant (23) (Table1). At the subcellular level, it has been localized at the tonoplast (23, 129). The effect of KCO1 disruption on mesophyll vacuolar K+ currents was examined (129). The available data suggest that the outwardly rectifying fast-activating Fast Vacuolar (FV) current, operating at low cytosolic Ca2+, was not affected by the mutation. Conversely, the density of the outwardly rectifying slowly activating Slow Vacuolar (SV) current, activated by high cytosolic Ca2+, seemed to be reduced in vacuoles from the knock-out line, suggesting that KCO1 might contribute to the SV current. KUP/HAK/KT Transporters Plants possess a family of genes encoding polypeptides homologous to K+ transporters that were first identified in Escherichia coli [named KUP, for K+ uptake (128)] and the soil yeast Schwanniomyces occidentalis [named HAK1, for highaffinity K+ (9)]. The bacterial KUP system was reported to be a constitutive lowaffinity K+ uptake system with a preponderant role at low pH, likely to operate as an H+-K+ symport (141, 158). In S. occidentalis or Neurospora crassa, HAK1 mediates high-affinity K+ uptake, probably acting as an H+-K+ symport, and is the major K+ transport system at work in conditions of K+ starvation (115). The plant homologues, called KUP, HAK, or KT (for K+ transporter), form a large family, with 13 members in Arabidopsis (98) and at least 17 members in rice (8). Little is known about the structure of these transporters. Hydrophobicity profiles suggest that they might possess 12 TMS and a long cytosolic loop between the second and third TMS (Figure 1) (8, 73, 112, 118). No region involved in ion conduction has yet been identified. Four groups of plant KUP/HAK/KT transporters can be distinguished on a phylogenetic tree (8, 118). Transporters from two groups, I and II, have been characterized at the functional level (Table 2). This was performed by expression in yeast or E. coli mutants lacking major endogenous K+ transport systems (115). Flux and growth rescue experiments suggested that some of these systems are devoted to high-affinity K+ transport, whereas others, all from group II (so far), play a preponderant role in the millimolar K+ range (Table 2). The former ones are probably active transport systems. They discriminate poorly between K+, Rb+, and Cs+ (8, 118) but are a lot less permeable to Na+ and NH+ 4 (8, 47, 124). Whether they are energized by H+ is unknown. The low-affinity transport systems are highly permeable to both K+ and Rb+, with some possibly also permeable to Na+ and Cs+ or blocked by these ions, as suggested by competitive influx experiments (73, 112, 132). Inhibition by alkaline pH was sometimes reported (49, 132), a feature consistent with the hypothesis that the corresponding systems might be endowed with H+-K+ symport activity. Recent studies have shown that these systems can mediate K+ influx and efflux (8, 49). 3 Apr 2003 12:44 584 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC Expression of both high-affinity and low-affinity KUP/HAK/KT transporters was found in various plant organs/tissues (Table 2), but their subcellular localization is mostly unknown. A low-affinity rice transporter present both in root and shoots was suggested, by GFP-tagged transient expression in onion epidermal cells, to be localized at the tonoplast (8). Activity for other transporters is expected at the plasma membrane, allowing a role in, for instance, high-affinity K+ uptake by the root. HKT Transporters Plant HKT transporters are related to the fungal Trk transporters and prokaryote KtrB and TrkH K+ transporter subunits (38, 115). Both fungal and prokaryote K+ transport systems of this family are believed to work as K+ cotransporters, where the coupling ion is H+ (12, 86) or Na+ (140), or as K+:K+ cotransport systems (57, 58), depending on the transporter and possibly also on the ionic conditions (115). In fungi, Trk transporters are the major systems involved in K+ uptake at micromolar to submillimolar K+ concentrations [at least at neutral and basic pH (115)]. HKT transporters are likely to be present in all plant species but probably not as large families (only one member in Arabidopsis thaliana). Sequence analyses have led to the hypothesis that these transporters have evolved from bacterial 2TMS K+ channels and display a core structure with eight TMS and four P-forming domains [four repeats of 1TMS-1P-1TMS (Figure 1)], with the four P loops lining a central P (38, 39). A recent investigation of AtHKT1 topology using a combination of engineered epitopes and glycosylation sites and a reporter alkaline phosphatase gene approach in E. coli did indeed provide direct support to a model with eight TMS, and N- and C-terminal cytosolic regions (72). Heterologous expression of plant HKT transporters in yeast and Xenopus oocytes revealed that these systems allow both influx and efflux of ions. They display high permeability to Na+ and, depending on the transporter and the ionic conditions, variable permeability to K+ (Table 2). For instance, TaHKT1 seems to work as a high-affinity Na+-K+ symporter in the presence of low K+ and Na+ concentrations, and as a low-affinity Na+-Na+ (co)transporter when the Na+/K+ concentration ratio in the external solution is high (50, 117). On the other hand, AtHKT1 does not seem to be significantly permeable to K+, even at low Na+ concentration (97, 142). Analyses of the molecular determinants of the Na+/K+ permeability of these transporters have identified a glycine residue highly conserved in all P domains of the K+-permeable HKT/Trk/KtrB transporters but replaced by a serine in the first P domain of AtHKT1 and OsHKT1, two transporters that are very weakly permeable to K+ (39, 61). The corresponding S to G mutation in AtHKT1 and OsHKT1 and the reverse mutation in TaHKT1 allowed the determinant role of this glycine residue to be confirmed in the K+ permeability (97). In TaHKT1, several other mutations in the P domains (39) were shown to decrease the permeability to Na+ (32, 117, 119). Deletions in the highly charged region of the cytosolic 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 585 second TMS– third TMS linker (39) also strongly decreased the permeability to Na+ (88). All HKT transporters so far identified are expressed in roots (Table 2). A broader expression pattern was reported for two members identified in Eucalyptus (44). The role of this family of transporters in plants is still unclear. Na+-coupled K+ uptake has not been evidenced in root cortex (92, 151). On the other hand, analysis of null mutations of AtHKT1, isolated as suppressor of the sos3-1 NaCl-hypersensitivity phenotype, suggested a role for the encoded system in root Na+ transport (120). Other Candidates CNGCs Plants possess a family of channels [20 members in A. thaliana (75, 98)] sharing structural homologies with the animal cyclic nucleotide–gated channels (CNGCs) that were first identified in sensory cells. CNGCs are related to the Shaker family (core structure has with six TMS and one P) but without the high K+selectivity hallmark motif in their P domain (46). Like plant Shaker channels, they harbor a putative cyclic nucleotide–binding domain in their cytoplasmic C-terminal region. Also, they often display a calmodulin-binding site. In plant CNGCs, the calmodulin-binding site (present in all channels) is located just downstream or even within the C-terminal region of the putative cyclic nucleotide–binding domain (75, 98). Very little is known about plant CNGCs relative to their ion selectivity, localization, and function (30, 98, 148). They are expected to be, like their animal counterparts, permeable to monovalent cations and/or Ca2+, regulated by cyclic nucleotides and calmodulin, and involved in cell signaling (30, 148). GLUTAMATE RECEPTORS A family of polypeptides related to animal ionotropic glutamate receptors has been found in plants (82) [20 members in Arabidopsis (78)]. All these polypeptides are likely to share the same structure, characterized by a membranar core encompassing one TMS, one P, and two TMS, and extracellular ligand-binding sites. Although the P domains of plant and animal receptors are quite distant, plant glutamate receptors might, like their animal counterparts, form cation channels permeable to K+, Na+, and/or Ca2+ (82, 105). All plant glutamate receptors are expressed in root, with some of them root specific (20). Their role in plants is unknown. LCT1 LCT1 is a wheat polypeptide shown by heterologous expression in yeast to mediate low-affinity transport of a wide range of cations [all monovalents, Ca2+, Cd2+, not Zn2+ (3, 22, 125)]. The hydrophobicity profile suggests a protein structure comprising eight to ten TMS. LCT1 has no counterpart in A. thaliana and shares no sequence homology with any gene described to date. It is expressed in both roots and leaves (125). Nonselective cation conductances have been described in vivo in wheat roots. The hypothesis of a role for LCT1 in this activity would, however, be highly speculative because poorly selective cation conductances have also been described in many species, including Arabidopsis (30). 3 Apr 2003 12:44 586 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC A family of cation proton antiporters (CPA), comprising six putative K+/H+ antiporters, has been identified in Arabidopsis (98). The latter systems (called KEA for K+ efflux antiporter) show substantial sequence similarities (up to 35% identity) with bacterial Kef (K+ efflux) antiporters regulated by glutathione (103, 157). Their tissular and subcellular localizations are unknown. Other members of the plant CPA family might be poorly specific and also transport K+ (in addition to other ions). For instance, the AtNHX1 vacuolar Na+/H+ exchanger (5) was recently shown to transport Na+ and K+ with equal affinity in reconstituted liposomes (146). Plant K+/H+ exchange activity is expected at least at the tonoplast, as a mechanism of K+ loading into the vacuole. AtNHX1 might be essentially involved in osmoregulation and Na+ detoxification from the cytoplasm, but possibly also in cytosolic pH regulation (5, 146). Finally, it has been suggested that K+/H+ exchangers might be at work at the plasma membrane, contributing to active K+ secretion in the xylem sap, for example (74). CPA FAMILY CCC FAMILY A few putative members of the cation chloride cotransporter family (CCC) have been found in plants (e.g., GenBank accession numbers NP 174333, T01896, BAB20646). In animal cells, the CCC family comprises K+-Cl−, Na+Cl−, and Na+-K+−2Cl− cotransporters (48, 54, 67). Members of this family have important roles in cellular ionic and osmotic homeostasis in animal cells. MOLECULAR BASES OF K+ TRANSPORT REGULATION As more K+ channels and transporters are identified, K+ transport in plants appears to be much more complex than originally thought in the late 1980s. Most types of K+ transport systems are encoded by large gene families, and systems from the same family are expressed differentially in various tissues. It is tempting to speculate that this diversity plays a central role in K+ transport regulation, allowing the cell to control the nature and level of K+ conductances in relation to the physiological context. Early (electro)physiological approaches have shown that various functions at the cell or whole-plant levels (e.g., osmoregulation and cell growth, guard cell movements and control of gas exchanges, or adaptation to K+ shortage or salt stress) involve regulation of K+ fluxes (21, 74). The identification of gene families encoding K+ transport systems has opened the way for molecular analysis of such regulations. Modulation of Transcript Levels Large variations in transcript levels have been found for both K+ channels and transporters in the course of plant development and in response to environmental changes. Current information is summarized in Tables 1 and 2. Studies on K+ transporters have mainly concerned transcriptional regulation by K+ availability and NaCl stress (Table 2). Increased transcript levels have been observed for some transporters of the KUP/HAK/KT and HKT families in response to K+ shortage 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 587 and are regarded as circumstantial evidence for a role of these systems in highaffinity K+ uptake (8, 73, 124, 152). Downregulation of HKT transcript levels in rice roots (61) and upregulation of HAK transcripts in common ice plant (135) upon salt stress could be adaptations limiting Na+ accumulation (61) and favoring K+-selective uptake (135). Four Arabidopsis Shaker K+ channel genes, AKT1, SKOR, AKT2, and AtKC1, have been systematically examined for transcriptional sensitivity to K+ starvation, salt stress, and hormones (108). Adaptation to K+ shortage and salt stress did not seem to involve dramatic reprogramming of the expression of these channels, except for a strong increase in AtKC1 expression in leaf peripheral tissues upon salt stress. On the other hand, treatments with abscisic acid, cytokinins, or auxin have been shown to strongly affect K+ channel expression (107, 108), providing the first molecular clues regarding hormonal control of K+ transport (145). Expression analyses of Shaker channels localized in phloem, indicating regulation by light and sugars (1, 29), have advanced molecular analysis of the coupling between phloem K+ transport and sugar production and allocation (150). Several Shakers and one KCO-2P have been shown to display light and circadian control in Samanea pulvinar motor cells, with large changes in transcript accumulation levels, suggesting that transcriptional regulation of K+ conductances plays a crucial role in osmoregulation in the pulvinar tissues (100). In summary, information to date indicates that transcriptional regulation of K+ channels and transporters is indeed a major determinant of K+ transport regulation and provides stimulating working hypotheses regarding physiological questions. Heterotetramerization The transmembrane core of plant K+ channels is very likely made up of several subunits, four in Shakers (26, 143) and in KCO-1P, and two in KCO-2P as shown in animal counterparts (84, 93, 156). This is likely to give rise to the formation of heteromultimeric structures. This hypothesis is supported by two-hybrid experiments in yeast, coexpressions in Xenopus oocytes and biochemical analyses showing that some pairs of plant Shaker polypeptides, but probably not every pair (143), are able to interact in a heterologous context (7, 36, 40, 109, 162). Formation of heteromultimeric structures could generate diversity in K+ conductances and provide further mechanisms for regulation, depending on spatial segregation of individual K+ channel subunit pools and kinetics of expression, as demonstrated for animal K+ channels (69). In the animal Shaker superfamily, heterotetramerization is possible only within subfamilies. A domain located in the channel N-terminal region is involved in subunit recognition (155). In plant Shaker channels, molecular determinants of subunit compatibility have not yet been identified. It is, however, tempting to speculate that they might be localized in the channel’s cytoplasmic C-terminal region. Indeed, biochemical studies indicate that the C-terminal region of the Arabidopsis Shaker channel AKT1 is able to self-tetramerize (26). Three domains of interactions have been identified within this region using the yeast two-hybrid 3 Apr 2003 12:44 588 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC system: the KHA domain at the extreme C terminus, which cross-interacted with the region just downstream of the hydrophobic core, and the putative cyclic nucleotide– binding domain, which cross-interacted with itself (26). Modulatory core channel subunits, unable to form functional homotetramers but able to interact with other subunits, forming heterotetramers with novel properties, have been identified in the animal Shaker superfamily (122). Such silent modulatory subunits might also exist in plants. The Arabidopsis AtKC1 Shaker was recently suggested to be one such subunit (113). AtKC1 (among others) is expressed in root peripheral tissues, along with the inward Shaker AKT1 (Table 1). Patch-clamp studies on root-hair protoplasts combined with reverse-genetics approaches revealed that AtKC1 alone (in mutant plants disrupted in the AKT1 gene) did not form inward channels, whereas expression of AKT1 alone (in mutant plants disrupted in the AtKC1 gene) gave rise to such channels (113). These channels, however, had functional features (activation threshold, ionic sensitivity, and sensitivity to external pH and Ca2+) different from those of the inward K+ channels observed in the presence of both AtKC1 and AKT1 (in wild-type plants), suggesting that AtKC1 formed with AKT1 functional heterotetrameric channels (113). Thus, different regulation of AKT1 and AtKC1 expression in roots could be a way to control root K+ uptake. It is also worth noting that the hypothesis of AtKC1 acting in all contexts as a silent Shaker subunit is weakened by the observation that some cells strongly expressing AtKC1 (e.g., trichomes) (108) apparently do not express any other Shaker gene (Figure 2) (Table 1). Identification of Regulatory Proteins Current knowledge in this field concerns K+ channels. Electrophysiological analyses in planta or in heterologous systems and reverse-genetics approaches have shown that various proteins, e.g., kinases, phosphatases, 14-3-3 proteins, syntaxins, farnesyl transferase, or G proteins, are involved in the regulation of K+ channel activity (6, 13, 28, 130). To date, the interacting networks involved in these regulations are still poorly understood. Direct searches for physically interacting partners have succeeded in two cases, with the identification of beta subunits of Shaker channels and with that of the AtPP2CA phosphatase shown to interact with the Arabidopsis AKT2 Shaker channel. In animal cells, oxydoreductases have been identified as partners of Shaker channels and called beta subunits (56, 154). They bind to the N-terminal cytosolic region of the Shaker polypeptides. Their physiological role is not well understood. However, some of these proteins have been shown to modulate channel functional properties. The reported effects mainly concern the current kinetics (induction or acceleration of fast inactivation) or the current level (changes in the number of functional channels at the cell membrane) (154). Homologues of animal beta subunits have been found in plants, e.g., 40–50% identity between rat Kvβ2-1 and Arabidopsis KAB1 (137), rice KOB1 (45), or potato KB1 (GenBank BETA SUBUNITS 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 589 accession number CAA04451.1). In vitro protein-protein interaction studies have confirmed that these plant counterparts of animal beta subunits are indeed able to physically interact with some plant Shaker channels (45, 138, 139). KAB1 was found to be expressed in various organs [root, flower, leaf, including guard cells (138)]. Immunochemical studies have revealed an association with the plasma membrane but also with endomembranes (139), suggesting that interaction with KAB1 might not be restricted to Shaker channels (all shown so far to be active at the plasma membrane). Coexpression of KAB1 with the Arabidopsis KAT1 Shaker in Xenopus oocytes resulted in increased current levels with no change in gating properties (160). The Arabidopsis phosphatase AtPP2CA was identified as an interacting partner of the Shaker channel AKT2 by yeast two-hybrid screens and in vitro binding assays (19, 149). The interaction was shown to involve the cytoplasmic C-terminal region of the channel and the C-terminal (catalytic) domain of the phosphatase. Expression of AtPP2CA was detected in all tissues where the AKT2 channel is expressed and found to be upregulated by abscisic acid (19). Coexpression of AtPP2CA with AKT2 in heterologous expression systems both quantitatively and qualitatively modified AKT2 activity: The level of current was strongly decreased and the level of rectification increased (19). The AKT2 channel has unique functional properties because it is the only weak inward rectifier characterized to date in Arabidopsis (see below). It has been suggested that AtPP2CA-mediated conversion of AKT2 from a leak-like channel into an inward rectifier plays a role in the regulation of the membrane potential (19). AtPP2CA Studies to identify kinases directly responsible for K+ channel phosphorylation have focused on the Arabidopsis KAT1 Shaker. Biochemical analyses have revealed that the phosphorylation status of this channel is sensitive to guard cell kinase activities (85, 99). Coexpression of a soybean Ca2+-dependent kinase with KAT1 in Xenopus oocytes shifted the channel activation threshold negatively and decreased the level of current (11). Direct interaction between the two proteins has not, however, been demonstrated. KINASES 14-3-3 PROTEINS Circumstantial evidence suggests that 14-3-3 proteins are regulatory partners of plant K+ channels, as shown for the human HERG K+ channel, for example (71). Overexpression of plant 14-3-3 proteins in tobacco strongly enhances the mesophyll K+ outward conductance (121). Addition of plant 14-3-3 proteins to the cell cytoplasm in patch-clamp experiments had the same effect (15). Neither the gating parameters nor the single-channel conductance were affected, suggesting that 14-3-3 proteins acted on the number of channels available for activation. Similar experiments on barley mesophyll vacuoles led to a transient inhibition of cationic SV currents and activation of cationic FV ones (28, 144). 3 Apr 2003 12:44 590 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC Regulation by Voltage, pH, Ca2+, and Cyclic Nucleotides Consistent with the chemiosmotic model of membrane energization, K+ transport in plants is dependent on the voltage and pH transmembrane gradients (74). In addition to this general control, voltage and pH can directly modulate the activity of K+ transport systems. For instance, these two effectors play crucial roles in regulation of guard cell inward and outward K+ rectifiers (13, 14). Information on molecular determinants of the sensitivity to voltage and pH of some Shaker channels is now available. On the other hand, the various (electro)physiological analyses evidencing sensitivity of K+ transport to Ca2+ (13, 42, 161) still have very few counterparts at the molecular level (25, 44, 96). Further knowledge in this field is eagerly awaited because of the importance of related questions in plant physiology. Note too that the role of the putative cyclic nucleotide–binding site identified in channels of the Shaker and CNGC families (see above) (Figure 1) has not been elucidated. The Arabidopsis Shakers KAT1 and AKT1 and one member of the CNGC family are sensitive to cyclic nucleotides (changes in voltage sensitivity for the Shakers, induction of channel activity for the CNGC) when expressed in heterologous systems (51, 62, 83), but whether this involves direct binding of the second messenger to the channel or indirect control (for instance, via activation of cyclic nucleotide–dependent protein kinases) has not been determined. VOLTAGE The plant Shaker channels characterized to date are regulated by voltage. Voltage gating has also been reported for the single member of the KCO family (KCO1) that has been characterized (25). On the other hand, current information suggests that activity of transporters of the HKT and KUP/HAK/KT families is not strongly sensitive to voltage (8, 44, 49, 50, 142). Molecular bases of voltage gating in Shakers have been extensively studied in animal outward rectifiers, highlighting the role of the positively charged fourth TMS (S4) in voltage sensing. In plants, the first structure-function relationship studies in this field were directed at analyzing the gating mechanism of the Arabidopsis inward rectifier KAT1. Plant inward and animal outward Shaker channels were shown to use S4 in a similar way for voltage sensing (95, 159). S4 movements in response to voltage changes occur in the same direction in inward and outward rectifiers, toward the cytoplasm upon membrane hyperpolarization, for example. However, an S4 movement in a given direction results in opposite effects on channel open probability in inward and outward rectifiers (18, 159). Molecular determinants of the direction of the rectification have not yet been identified. Gating analyses of the Arabidopsis weak inward rectifier AKT2 have suggested a voltage-sensing mechanism similar to that of strongly rectifying Shakers. The weak inward rectification would result from the fact that AKT2 channels with two distinct gating modes, mode 1 or mode 2, would be present and active simultaneously in the membrane (37): Mode 1 channels would behave as typical inward rectifiers, whereas mode 2 channels would behave as open leak channels, owing to a large shift of their voltage sensitivity positive to the physiological voltage range. A switch from mode 1 to mode 2 behavior would involve changes in phosphorylation 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 591 status (19). Chimera constructs with P exchange between AKT2 and the inward rectifier KST1 from potato suggest that the P domain is involved in the determination of inward versus weak inward rectification (64). Structural determinants of voltage sensitivity in KCO1, which lacks a Shaker-like voltage sensor, have not yet been examined. All cloned Shaker channels so far characterized are strongly regulated by pH (Table 1). The inward rectifiers are activated by external acidification. Two kinds of mechanisms have been shown to be involved in this regulation: either an increase in macroscopic current level upon acidification without change in gating properties [as observed in the guard cell channel KAT1 (109, 147)] or a positive shift of channel activation threshold without change in conductance [as shown in the guard cell channels KAT2, KST1, and SIRK, and in the pollen channel SPIK (63, 101, 109, 111)]. Activation upon cytosolic acidification has also been reported for KAT1 and KST1 (62, 64). The outward root stellar rectifier SKOR is conversely inhibited by external and cytosolic acidification. A decrease in current level without change in gating properties or in single-channel conductance was observed, suggesting that pH modulated the number of SKOR channels available for activation (79). The two weak inward rectifiers characterized [AKT2 and ZMK2 (80, 96, 107)] also are inhibited by external and cytosolic acidification. However, contrary to what was observed for SKOR, AKT2 single-channel conductance was sensitive to external pH (96), indicating a different mechanism of pH regulation. Molecular studies to identify determinants of pH sensitivity in plant Shaker channels have mainly focused on histidine residues. Mutagenesis experiments on each of the two extracellular histidines of KST1 (one in the third-TMS-fourthTMS linker and one in the P domain; the latter histidine is highly conserved in plant Shakers) strongly altered the channel’s pH sensitivity, suggesting that both histidines are involved in external pH sensing (63). Similar experiments on KAT1, which possesses a single extracellular histidine—the conserved one in the P— did not lead to any modification of the channel’s pH sensitivity, indicating that the mechanism of pH sensing is different in the two inward channels (65), as suggested by analysis of pH effect on their current/voltage (I/V) relationships (see above). Swapping the P domain between KST1 and the weak inward rectifier AKT2 showed the importance of this domain in the external (but not internal) pH sensitivity of AKT2 (64). This hypothesis has recently received further support from site-directed mutagenesis experiments targeting the AKT2 P domain (53). pH NEW LIGHT ON EARLY PHYSIOLOGICAL QUESTIONS K+ Uptake: More Complex than a Dual Mechanism In the early 1950s, Epstein and coworkers proposed applying the Michaelis-Menten formalism to the process of ion absorption. In a study of (86Rb+)K+ uptake in barley roots, they observed biphasic uptake kinetics with respect to K+ external concentration and postulated that two types of K+ transport systems were at work at the 3 Apr 2003 12:44 592 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC root cell plasma membrane (42, 43): high-affinity transport systems (mechanism 1) saturating with a Km of 10–40 µM, highly selective for K+ over Na+, and low-affinity transport systems (mechanism 2) saturating with a Km in the 10 mM range, showing weak selectivity for K+ over other alkali cations. A similar dual mechanism was thereafter described in different tissues of many plant species (42, 74). Consistent with the dual mechanism model, activities of both high-affinity K+ transport systems with characteristics of Epstein’s mechanism 1 and channels with features fitting to mechanism 2 were subsequently demonstrated in root cortical cells (90, 91). However, the situation now appears to be much more complex. Indeed, numerous K+ transport systems are likely to be involved in root K+ uptake (Tables 1, 2) (Figure 2). In Arabidopsis, for instance, K+-selective channels [encoded by the AKT1 and AtKC1 Shaker genes (60, 113)], nonselective cation channels [not yet identified at the molecular level (91)], and probably the highaffinity K+ transporter KUP4 (114) are at work at the plasma membrane of root periphery cells. In addition, four other root KUP/HAK transporters (Table 2) might contribute to root K+ uptake. Furthermore, it is now clear that channels can play a role in both high- and low-affinity K+ uptake. This has been demonstrated for the AKT1 channel in roots (60) and suggested for SPIK in pollen (101). Finally, characterization of K+ transporters in heterologous systems indicated that they do not all have a preponderant role within the same K+ concentration range (Table 2): Some KUP/HAK transporters display high-affinity K+ transport activity, whereas others have been described as mediating low-affinity K+ transport [e.g., the barley root HAK2 transporter, (132)]. In other words, it is likely that various transporters and channels contribute to K+ uptake in plant cells and that the two types of transport systems are less different, regarding the affinity of transport, than initially thought. Energization of High-Affinity K+ Uptake: Still Unresolved Questions Discovery of high-affinity K+ uptake in roots (Epstein’s mechanism 1) raised questions about the underlying energization process. Pioneering electrophysiological analyses in the early 1970s revealed that K+ transport occurred against the K+ electrochemical gradient, in other words, that the transport was active, when the external concentration of this ion was below a threshold in the 0.1–0.5 mM range (74, 90). The identity and energetic coupling of the systems responsible for this active transport have been highly debated subsequently. The current consensus, based on electrophysiological analyses, is that active K+ uptake is mediated by H+-K+ symporters (90, 123). Activity of one such symporter has been evidenced at the plasma membrane of Arabidopsis root cortical cells (89). Reverse-genetics studies have revealed that channels [e.g., AKT1 in root cortical cells, and probably SPIK in pollen (60, 101)] can contribute to K+ uptake at K+ concentrations in the 10–100 µM range. This suggests that the concentration 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 593 threshold below which only active transport systems are able to drive K+ uptake can be much lower than initially predicted, at least in some physiological contexts. There has been little progress from molecular studies in identifying active K+ transport systems and elucidating their mode of energization. A number of highaffinity K+ transport systems have been identified (Table 2), but whether and how they “energize” active K+ transport is for the most part unknown. Information on this subject has been obtained for only one member of the HKT family, the wheat root transporter TaHKT1, which uses the Na+ electrochemical gradient when heterologously expressed (50, 117). This finding is puzzling in that Na+-coupled K+ transport has not been observed in vivo in higher plants (123, 151). Furthermore, electrophysiological analyses in wheat roots would suggest an inhibitory effect of external Na+ on high-affinity K+ uptake (92). TaHKT1 is thus unlikely to play an important role in K+ uptake from the soil. Circumstantial evidence (high selectivity for K+ against Na+ and inhibition by NH+ 4 ) suggests that root active transport systems are members of the KUP/HAK/KT family (123). Evidence for Passive K+ Release into the Xylem Sap Studies of ion release to the xylem are often subject to difficulties in interpreting the experimental data (27, 74, 110) because reliable measurements of membrane potential remain scarce and estimations of apoplastic ion activities are uncertain (55). Thus, although outward K+ channels were identified at the plasma membrane of xylem parenchyma cells, their involvement in xylem sap K+ loading was still a matter of debate because there was no clear evidence for sufficiently depolarized resting membrane potential (27). Furthermore, few electrophysiological and pharmacological data supported the hypothesis that K+ secretion into the xylem occurred against the K+ electrochemical gradient and thus was mediated by active transport systems (74, 110). The molecular identification of the Arabidopsis outward Shaker SKOR enabled, by using a reverse genetics approach, investigation of this question to go forward (52). Evidence that passive secretion of K+ into the xylem sap through outward K+ channels can actually occur, at least in some physiological contexts, was provided. In these experiments, SKOR activity contributed to ∼50% of K+ translocation toward the shoots (52). A Milestone in the Analysis of Phloem K+ Transport Phloem K+ transport has been suggested as playing a major role in the integration of K+ transport at the whole-plant level. Briefly, the amount of K+ retranslocated from the shoots to the roots via the phloem, dependent on shoot growth rate and K+ availability, would control the amount of K+ secreted into the root xylem and, ultimately, tune root K+ absorption to the shoot demand for growth (35, 94, 153). Furthermore, phloem K+ loading in sources and unloading in sinks, by taking part in the creation of the osmotic potential gradient along the phloem tubes, would play a role in controlling phloem sap flow rate, and hence in the regulation of photoassimilate delivery to the sinks. In spite of their physiological importance, 3 Apr 2003 12:44 594 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC the transport mechanisms mediating phloem K+ loading/unloading are still poorly understood. The recent identification of a Shaker channel expressed in phloem tissues [AKT2 in Arabidopsis, VfK1 in Vicia faba (1, 80, 96)] should enable this analysis to progress. Because this channel is expressed in both source and sink phloem tissues and is potentially able to mediate both K+ influx and efflux owing to its weak rectification property, it has been suggested as being involved in phloem sap K+ loading in sources and unloading in sinks (80, 96). The sensitivity of its expression to light, photoassimilates, and hormones (1, 29, 108) might play a role in controlling phloem K+ transport in relation to sugar production and translocation. Evidence for Involvement of K+ Transport in Control of Cell Growth Owing to the major role of K+ in building plant cell turgor, K+ transport systems were thought to be major actors in controlling cell growth. Molecular analyses have recently provided direct support to this hypothesis, mainly in two tip-growing model cells, root hair, and pollen. Disruption of the high-affinity K+ transporter gene AtKUP4 [widely expressed throughout the plant (Table 2)] decreased the overall root high-affinity K+ absorption by ∼40% and completely prevented root hair elongation. Surprisingly, increasing the external concentration of K+ up to 50 mM did not restore root hair elongation (114). Furthermore, disruption of the inward Shaker AKT1 gene expressed in root hairs and known to significantly contribute to both high- and low-affinity K+ uptake in root has not been reported to affect root-hair development (68, 113). Thus, AtKUP4 would be involved in a specific K+ transport process, essential for root-hair elongation (114). Another study concerned the Arabidopsis pollen inward Shaker gene SPIK (101). Disruption of this gene resulted in a strong decrease in the rate of pollen tube growth in a wide range of external K+ concentrations (5 µM to 1 mM). Because electrophysiological analyses indicated that SPIK was a main component of the inward K+ conductance of the pollen plasma membrane, the impairment of tube development in the mutant was ascribed to a deficit in K+ uptake. A few other studies have provided more circumstantial evidence for a role of K+ transport in cell-growth control, by showing that growth peaks are accompanied by increased accumulation of transcripts encoding K+ uptake systems in different cell types (107, 116). CONCLUSION Considerable progress in the analysis of K+ transport in plants has been made over the past few years. Several putative families of K+ channels and transporters have been identified, revealing a much more complex situation than initially thought. However, DNA-based strategies coupled to functional analyses have allowed the role of some K+ transport systems in the plant to be determined. Information in this domain mainly relates to the plasma membrane and K+ channels from the Shaker family. An exciting backdrop of information already exists regarding 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 595 several functions at the cell or whole-plant levels, such as K+ influx or efflux into or from guard cells during stomatal movements and K+ uptake in the elongating pollen tube, or root K+ uptake and secretion into the xylem sap. Localization and functional characterization of the many still-orphan K+ transport systems will undoubtedly result in further progress in our understanding of K+ transport in plants. ACKNOWLEDGMENTS We are grateful to Michèle Rambier for valuable help in compiling the bibliography and to Jean-Baptiste Thibaud and Sabine Zimmermann for critical comments on the manuscript. The Annual Review of Plant Biology is online at http://plant.annualreviews.org LITERATURE CITED 1. Ache P, Becker D, Deeken R, Dreyer I, Weber H, et al. 2001. VFK1, a Vicia faba K+ channel involved in phloem unloading. Plant J. 27:571–80 2. Ache P, Becker D, Ivashikina N, Dietrich P, Roelfsema MR, Hedrich R. 2000. GORK, a delayed outward rectifier expressed in guard cells of Arabidopsis thaliana, is a K+-selective, K+-sensing ion channel. FEBS Lett. 486:93–98 3. Amtmann A, Fischer M, Marsh EL, Stefanovic A, Sanders D, Schachtman DP. 2001. The wheat cDNA LCT1 generates hypersensitivity to sodium in a salt-sensitive yeast strain. Plant Physiol. 126:1061–71 4. Anderson JA, Huprikar SS, Kochian LV, Lucas WJ, Gaber RF. 1992. Functional expression of a probable Arabidopsis thaliana potassium channel in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. USA 89:3736–40 5. Apse MP, Aharon GS, Snedden WA, Blumwald E. 1999. Salt tolerance conferred by overexpression of a vacuolar Na+/H+ antiport in Arabidopsis. Science 285:1256–58 6. Assmann SM. 2002. Heterotrimeric and 7. 8. 9. 10. 11. unconventional GTP binding proteins in plant cell signaling. Plant Cell 14:S355– 73 Baizabal-Aguirre VM, Clemens S, Uozumi N, Schroeder JI. 1999. Suppression of inward-rectifying K+ channels KAT1 and AKT2 by dominant negative point mutations in the KAT1 alphasubunit. J. Membr. Biol. 167:119–25 Bañuelos MA, Garciadeblas B, Cubero B, Rodrı́guez-Navarro A. 2002. Inventory and functional characterization of the HAK potassium transporters of rice. Plant Physiol. 130:784–94 Bañuelos MA, Klein RD, AlexanderBowman SJ, Rodrı́guez-Navarro A. 1995. A potassium transporter of the yeast Schwanniomyces occidentalis homologous to the Kup system of Escherichia coli has a high concentrative capacity. EMBO J. 14:3021–27 Basset M, Conejero G, Lepetit M, Foucroy P, Sentenac H. 1995. Organization and expression of the gene coding for the potassium transport system AKT1 of Arabidopsis thaliana. Plant Mol. Biol. 29:947–58 Berkowitz GA, Zhang X, Mercier R, Leng 3 Apr 2003 12:44 596 12. 13. 14. 15. 16. 17. 18. 19. 20. AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC Q, Lawton M. 2000. Co-expression of calcium-dependent protein kinase with the inward rectified guard cell K+ channel KAT1 alters current parameters in Xenopus laevis oocytes. Plant Cell Physiol. 41:785–90 Bihler H, Gaber RF, Slayman CL, Bertl A. 1999. The presumed potassium carrier Trk2p in Saccharomyces cerevisiae determines an H+-dependent, K+-independent current. FEBS Lett. 447:115–20 Blatt MR. 2000. Cellular signaling and volume control in stomatal movements in plants. Annu. Rev. Cell Dev. Biol. 16:221– 41 Blatt MR, Grabov A. 1997. Signalling gates in abscisic acid-mediated control of guard cell ion channels. Physiol. Plant. 100:481–90 Booij PP, Roberts MR, Vogelzang SA, Kraayenhof R, de Boer AH. 1999. 14-3-3 proteins double the number of outwardrectifying K+ channels available for activation in tomato cells. Plant J. 20:673– 83 Buschmann PH, Vaidyanathan R, Gassmann W, Schroeder JI. 2000. Enhancement of Na+ uptake currents, timedependent inward-rectifying K+ channel currents, and K+ channel transcripts by K+ starvation in wheat root cells. Plant Physiol. 122:1387–97 Cheeseman JM, Hanson JB. 1979. Energy-linked potassium influx as related to cell potential in corn roots. Plant Physiol. 64:842–45 Chen J, Piper DR, Sanguinetti MC. 2002. Voltage sensing and activation gating of HCN pacemaker channels. Trends Cardiovasc. Med. 12:42–45 Chérel I, Michard E, Platet N, Mouline K, Alcon C, et al. 2002. Physical and functional interaction of the Arabidopsis K+ channel AKT2 and phosphatase AtPP2CA. Plant Cell 14:1133–46 Chiu JC, Brenner ED, DeSalle R, Nitabach MN, Holmes TC, Coruzzi GM. 2002. Phylogenetic and expression anal- 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. ysis of the glutamate-receptor-like gene family in Arabidopsis thaliana. Mol. Biol. Evol. 19:1066–82 Clarkson DT, Hanson JB. 1980. The mineral nutrition of higher plants. Annu. Rev. Plant Physiol. 31:239–98 Clemens S, Antosiewicz DM, Ward JM, Schachtman DP, Schroeder JI. 1998. The plant cDNA LCT1 mediates the uptake of calcium and cadmium in yeast. Proc. Natl. Acad. Sci. USA 95:12043–48 Czempinski K, Frachisse J-M, Maurel C, Barbier-Brygoo H, Mueller-Roeber B. 2002. Vacuolar membrane localization of the Arabidopsis ‘two-pore’ K+ channel KCO1. Plant J. 29:809–20 Czempinski K, Gaedeke N, Zimmermann S, Müller-Röber B. 1999. Molecular mechanisms and regulation of plant ion channels. J. Exp. Bot. 50:955–66 Czempinski K, Zimmermann S, Ehrhardt T, Müller-Röber B. 1997. New structure and function in plant K+ channels: KCO1, an outward rectifier with a steep Ca2+ dependency. EMBO J. 16:2565–75 Daram P, Urbach S, Gaymard F, Sentenac H, Chérel I. 1997. Tetramerization of the AKT1 plant potassium channel involves its C-terminal cytoplasmic domain. EMBO J. 16:3455–63 de Boer AH. 1999. Potassium translocation into root xylem. Plant Biol. 1:36–45 de Boer AH. 2002. Plant 14-3-3 proteins assist ion channels and pumps. Biochem. Soc. Trans. 30:416–21 Deeken R, Sanders C, Ache P, Hedrich R. 2000. Developmental and light-dependent regulation of a phloem-localised K+ channel of Arabidopsis thaliana. Plant J. 23:285–90 Demidchik V, Davenport RJ, Tester M. 2002. Nonselective cation channels in plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. 53:67–107 Dennison KL, Robertson WR, Lewis BD, Hirsch RE, Sussman MR, Spalding EP. 2001. Functions of AKT1 and AKT2 potassium channels determined by 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. studies of single and double mutants of Arabidopsis. Plant Physiol. 127:1012–19 Diatloff E, Kumar R, Schachtman DP. 1998. Site directed mutagenesis reduces the Na+ affinity of HKT1, an Na+ energized high affinity K+ transporter. FEBS Lett. 432:31–36 Doupnik CA, Davidson N, Lester HA. 1995. The inward rectifier potassium channel family. Curr. Opin. Neurobiol. 5: 268–77 Downey P, Szabò I, Ivashikina N, Negro A, Guzzo F, et al. 2000. KDC1, a novel carrot root hair K+ channel. Cloning, characterization, and expression in mammalian cells. J. Biol. Chem. 275:39420–26 Drew MC, Saker LR. 1984. Uptake and long-distance transport of phosphate, potassium and chloride in relation to internal ion concentrations in barley: evidence for a non-allosteric regulation. Planta 160:500–7 Dreyer I, Antunes S, Hoshi T, MüllerRöber B, Palme K, et al. 1997. Plant K+ channel alpha-subunits assemble indiscriminately. Biophys. J. 72:2143–50 Dreyer I, Michard E, Lacombe B, Thibaud J-B. 2001. A plant Shaker-like K+ channel switches between two distinct gating modes resulting in either inwardrectifying or “leak” current. FEBS Lett. 505:233–39 Durell SR, Guy HR. 1999. Structural models of the KtrB, TrkH, and Trk1,2 symporters based on the structure of the KcsA K+ channel. Biophys. J. 77:789– 807 Durell SR, Hao Y, Nakamura T, Bakker EP, Guy HR. 1999. Evolutionary relationship between K+ channels and symporters. Biophys. J. 77:775–88 Ehrhardt T, Zimmermann S, MüllerRöber B. 1997. Association of plant K+in channels is mediated by conserved C termini and does not affect subunit assembly. FEBS Lett. 409:166–70 Elumalai RP, Nagpal P, Reed JW. 2002. A mutation in the Arabidopsis KT2/KUP2 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 597 potassium transporter gene affects shoot cell expansion. Plant Cell 14:119–31 Epstein E. 1973. Mechanisms of ion transport through plant cell membranes. Int. Rev. Cytol. 34:123–68 Epstein E, Rains DW, Elzam OE. 1963. Resolution of dual mechanisms of potassium absorption by barley roots. Proc. Natl. Acad. Sci. USA 49:684–92 Fairbairn DJ, Liu W, Schachtman DP, Gomez-Gallego S, Day SR, Teasdale RD. 2000. Characterisation of two distinct HKT1-like potassium transporters from Eucalyptus camaldulensis. Plant Mol. Biol. 43:515–25 Fang Z, Kamasani U, Berkowitz GA. 1998. Molecular cloning and expression characterization of a rice K+ channel beta subunit. Plant Mol. Biol. 37:597–606 Finn JT, Grunwald ME, Yau KW. 1996. Cyclic nucleotide-gated ion channels: an extended family with diverse functions. Annu. Rev. Physiol. 58:395–426 Fu H-H, Luan S. 1998. AtKuP1: a dualaffinity K+ transporter from Arabidopsis. Plant Cell 10:63–73 Gamba G, Saltzberg SN, Lombardi M, Miyanoshita A, Lytton J, et al. 1993. Primary structure and functional expression of a cDNA encoding the thiazidesensitive, electroneutral sodium-chloride cotransporter. Proc. Natl. Acad. Sci. USA 90:2749–53 Garciadeblas B, Benito B, Rodrı́guezNavarro A. 2002. Molecular cloning and functional expression in bacteria of the potassium transporters CnHAK1 and CnHAK2 of the seagrass Cymodocea nodosa. Plant Mol. Biol. 50:623–33 Gassman W, Rubio F, Schroeder JI. 1996. Alkali cation selectivity of the wheat root high-affinity potassium transporter HKT1. Plant J. 10:869–82 Gaymard F, Cerutti M, Horeau C, Lemaillet G, Urbach S, et al. 1996. The baculovirus/insect cell system as an alternative to Xenopus oocytes. First characterization of the AKT1 K+ 3 Apr 2003 12:44 598 52. 53. 54. 55. 56. 57. 58. 59. 60. AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC channel from Arabidopsis thaliana. J. Biol. Chem. 271:22863–70 Gaymard F, Pilot G, Lacombe B, Bouchez D, Bruneau D, et al. 1998. Identification and disruption of a plant shaker-like outward channel involved in K+ release into the xylem sap. Cell 94:647–55 Geiger D, Becker D, Lacombe B, Hedrich R. 2002. Outer pore residues control the H+ and K+ sensitivity of the Arabidopsis potassium channel AKT3. Plant Cell 14:1859–68 Gillen CM, Brill S, Payne JA, Forbush B. 1996. Molecular cloning and functional expression of the K-Cl cotransporter from rabbit, rat, and human. A new member of the cation-chloride cotransporter family. J. Biol. Chem. 271:16237–44 Grignon C, Sentenac H. 1991. pH and ionic conditions in the apoplast. Annu. Rev. Plant Physiol. Plant Mol. Biol. 42: 103–28 Gulbis JM, Mann S, MacKinnon R. 1999. Structure of a voltage-dependent K+ channel beta subunit. Cell 97:943–52 Haro R, Rodrı́guez-Navarro A. 2002. Molecular analysis of the mechanism of potassium uptake through the TRK1 transporter of Saccharomyces cerevisiae. Biochim. Biophys. Acta 1564:114–22 Haro R, Sainz L, Rubio F, Rodrı́guezNavarro A. 1999. Cloning of two genes encoding potassium transporters in Neurospora crassa and expression of the corresponding cDNAs in Saccharomyces cerevisiae. Mol. Microbiol. 31:511–20 Hartje S, Zimmermann S, Klonus D, Mueller-Roeber B. 2000. Functional characterisation of LKT1, a K+ uptake channel from tomato root hairs, and comparison with the closely related potato inwardly rectifying K+ channel SKT1 after expression in Xenopus oocytes. Planta 210:723– 31 Hirsch RE, Lewis BD, Spalding EP, Sussman MR. 1998. A role for the AKT1 potassium channel in plant nutrition. Science 280:918–21 61. Horie T, Yoshida K, Nakayama H, Yamada K, Oiki S, Shinmyo A. 2001. Two types of HKT transporters with different properties of Na+ and K+ transport in Oryza sativa. Plant J. 27:129–38 62. Hoshi T. 1995. Regulation of voltage dependence of the KAT1 channel by intracellular factors. J. Gen. Physiol. 105:309– 28 63. Hoth S, Dreyer I, Dietrich P, Becker D, Müller-Röber B, Hedrich R. 1997. Molecular basis of plant-specific acid activation of K+ uptake channels. Proc. Natl. Acad. Sci. USA 94:4806–10 64. Hoth S, Geiger D, Becker D, Hedrich R. 2001. The pore of plant K+ channels is involved in voltage and pH sensing: domain-swapping between different K+ channel alpha-subunits. Plant Cell 13: 943–52 65. Hoth S, Hedrich R. 1999. Distinct molecular bases for pH sensitivity of the guard cell K+ channels KST1 and KAT1. J. Biol. Chem. 274:11599–603 66. Ichida AM, Pei Z-M, Baizabal-Aguirre VM, Turner KJ, Schroeder JI. 1997. Expression of a Cs+-resistant guard cell K+ channel confers Cs+-resistant, lightinduced stomatal opening in transgenic Arabidopsis. Plant Cell 9:1843–57 67. Isenring P, Forbush B. 1997. Ion and bumetanide binding by the Na-K-Cl cotransporter. Importance of transmembrane domains. J. Biol. Chem. 272: 24556–62 68. Ivashikina N, Becker D, Ache P, Meyerhoff O, Felle HH, Hedrich R. 2001. K+ channel profile and electrical properties of Arabidopsis root hairs. FEBS Lett. 508:463–69 69. Jan LY, Jan YN. 1990. How might the diversity of potassium channels be generated? Trends Neurosci. 13:415–19 70. Jan LY, Jan YN. 1997. Cloned potassium channels from eukaryotes and prokaryotes. Annu. Rev. Neurosci. 20:91–123 71. Kagan A, Melman YF, Krumerman A, McDonald TV. 2002. 14-3-3 amplifies and 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 72. 73. 74. 75. 76. 77. 78. 79. 80. prolongs adrenergic stimulation of HERG K+ channel activity. EMBO J. 21:1889–98 Kato Y, Sakaguchi M, Mori Y, Saito K, Nakamura T, et al. 2001. Evidence in support of a four transmembrane-poretransmembrane topology model for the Arabidopsis thaliana Na+/K+ translocating AtHKT1 protein, a member of the superfamily of K+ transporters. Proc. Natl. Acad. Sci. USA 98:6488–93 Kim EJ, Kwak JM, Uozumi N, Schroeder JI. 1998. AtKUP1: an Arabidopsis gene encoding high-affinity potassium transport activity. Plant Cell 10:51–62 Kochian LV, Lucas WJ. 1988. Potassium transport in roots. Adv. Bot. Res. 15:93– 178 Köhler C, Merkle T, Neuhaus G. 1999. Characterisation of a novel gene family of putative cyclic nucleotide- and calmodulin-regulated ion channels in Arabidopsis haliana. Plant J. 18:97–104 Kopka J, Provart NJ, Müller-Röber B. 1997. Potato guard cells respond to drying soil by a complex change in the expression of genes related to carbon metabolism and turgor regulation. Plant J. 11:871–82 Kwak JM, Murata Y, Baizabal-Aguirre VM, Merrill J, Wang M, et al. 2001. Dominant negative guard cell K+ channel mutants reduce inward-rectifying K+ currents and light-induced stomatal opening in Arabidopsis. Plant Physiol. 127:473– 85 Lacombe B, Becker D, Hedrich R, DeSalle R, Hollmann M, et al. 2001. The identity of plant glutamate receptors. Science 292:1486–87 Lacombe B, Pilot G, Gaymard F, Sentenac H, Thibaud J-B. 2000. pH control of the plant outwardly-rectifying potassium channel SKOR. FEBS Lett. 466:351–54 Lacombe B, Pilot G, Michard E, Gaymard F, Sentenac H, Thibaud J-B. 2000. A Shaker-like K+ channel with weak rectification is expressed in both source and sink phloem tissues of Arabidopsis. Plant Cell 12:837–51 599 81. Lagarde D, Basset M, Lepetit M, Conejero G, Gaymard F, et al. 1996. Tissuespecific expression of Arabidopsis AKT1 gene is consistent with a role in K+ nutrition. Plant J. 9:195–203 82. Lam HM, Chiu J, Hsieh MH, Meisel L, Oliveira IC, et al. 1998. Glutamatereceptor genes in plants. Nature 396:125– 26 83. Leng Q, Mercier RW, Yao W, Berkowitz GA. 1999. Cloning and first functional characterization of a plant cyclic nucleotide-gated cation channel. Plant Physiol. 121:753–61 84. Lesage F, Reyes R, Fink M, Duprat F, Guillemare E, Lazdunski M. 1996. Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge. EMBO J. 15: 6400–7 85. Li J, Lee Y-RJ, Assmann SM. 1998. Guard cells possess a calcium-dependent protein kinase that phosphorylates the KAT1 potassium channel. Plant Physiol. 116:785–95 86. Lichtenberg-Fraté H, Reid JD, Heyer M, Höfer M. 1996. The SpTRK gene encodes a potassium-specific transport protein TKHp in Schizosaccharomyces pombe. J. Membr. Biol. 152:169–81 87. Liu W, Fairbairn DJ, Reid RJ, Schachtman DP. 2001. Characterization of two HKT1 homologues from Eucalyptus camaldulensis that display intrinsic osmosensing capability. Plant Physiol. 127:283– 94 88. Liu W, Schachtman DP, Zhang W. 2000. Partial deletion of a loop region in the high affinity K+ transporter HKT1 changes ionic permeability leading to increased salt tolerance. J. Biol. Chem. 275:27924– 32 89. Maathuis FJM, Sanders D. 1994. Mechanism of high-affinity potassium uptake in roots of Arabidopsis thaliana. Proc. Natl. Acad. Sci. USA 91:9272–76 90. Maathuis FJM, Sanders D. 1996. Mechanisms of potassium absorption by higher plant roots. Physiol. Plant. 96:158–68 3 Apr 2003 12:44 600 AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC 91. Maathuis FJM, Sanders D. 2001. Sodium uptake in Arabidopsis roots is regulated by cyclic nucleotides. Plant Physiol. 127:1617–25 92. Maathuis FJM, Velin D, Smith FA, Sanders D, Fernández JA, Walker NA. 1996. The physiological relevance of Na+-coupled K+ transport. Plant Physiol. 112:1609–16 93. MacKinnon R. 1991. Determination of the subunit stoichiometry of a voltageactivated potassium channel. Nature 350:232–35 94. Marschner H, Kirkby EA, Cakmak I. 1996. Effect of mineral nutritional status on shoot-root partitioning of photoassimilates and cycling of mineral nutrients. J. Exp. Bot. 47:1255–63 95. Marten I, Hoshi T. 1998. The N-terminus of the K channel KAT1 controls its voltage-dependent gating by altering the membrane electric field. Biophys. J. 74:2953–62 96. Marten I, Hoth S, Deeken R, Ache P, Ketchum KA, et al. 1999. AKT3, a phloem-localized K+ channel, is blocked by protons. Proc. Natl. Acad. Sci. USA 96:7581–86 97. Mäser P, Hosoo Y, Goshima S, Horie T, Eckelman B, et al. 2002. Glycine residues in potassium channel-like selectivity filters determine potassium selectivity in four-loop-per-subunit HKT transporters from plants. Proc. Natl. Acad. Sci. USA 99:6428–33 98. Mäser P, Thomine S, Schroeder JI, Ward JM, Hirschi K, et al. 2001. Phylogenetic relationships within cation transporter families of Arabidopsis. Plant Physiol. 126:1646–67 99. Mori IC, Uozumi N, Muto S. 2000. Phosphorylation of the inward-rectifying potassium channel KAT1 by ABR kinase in Vicia guard cells. Plant Cell Physiol. 41:850–56 100. Moshelion M, Becker D, Czempinski K, Mueller-Roeber B, Attali B, et al. 2002. Diurnal and circadian regulation of puta- 101. 102. 103. 104. 105. 106. 107. 108. 109. tive potassium channels in a leaf moving organ. Plant Physiol. 128:634–42 Mouline K, Véry A-A, Gaymard F, Boucherez J, Pilot G, et al. 2002. Pollen tube development and competitive ability are impaired by disruption of a Shaker K+ channel in Arabidopsis. Genes Dev. 16:339–50 Müller-Röber B, Ellenberg J, Provart N, Willmitzer L, Busch H, et al. 1995. Cloning and electrophysiological analysis of KST1, an inward rectifying K+ channel expressed in potato guard cells. EMBO J. 14:2409–16 Munro AW, Ritchie GY, Lamb AJ, Douglas RM, Booth IR. 1991. The cloning and DNA sequence of the gene for the glutathione-regulated potassiumefflux system KefC of Escherichia coli. Mol. Microbiol. 5:607–16 Nakamura RL, McKendree WL Jr, Hirsch RE, Sedbrook JC, Gaber RF, Sussman MR. 1995. Expression of an Arabidopsis potassium channel gene in guard cells. Plant Physiol. 109:371–74 Nakanishi N, Shneider NA, Axel R. 1990. A family of glutamate receptor genes: evidence for the formation of heteromultimeric receptors with distinct channel properties. Neuron 5:569–81 Patel AJ, Honoré E. 2001. Properties and modulation of mammalian 2P domain K+ channels. Trends Neurosci. 24:339–46 Philippar K, Fuchs I, Lüthen H, Hoth S, Bauer CS, et al. 1999. Auxin-induced K+ channel expression represents an essential step in coleoptile growth and gravitropism. Proc. Natl. Acad. Sci. USA 96:12186–91 Pilot G, Gaymard F, Mouline K, Chérel I, Sentenac H. 2003. Regulated expression of Arabidopsis Shaker K+ channel genes involved in K+ uptake and distribution in the plant. Plant Mol. Biol. 51:773–787 Pilot G, Lacombe B, Gaymard F, Chérel I, Boucherez J, et al. 2001. Guard cell inward K+ channel activity in Arabidopsis involves expression of the twin channel 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. subunits KAT1 and KAT2. J. Biol. Chem. 276:3215–21 Pitman MG. 1977. Ion transport into the xylem. Annu. Rev. Plant Physiol. 28:71– 88 Pratelli R, Lacombe B, Torregrosa L, Gaymard F, Romieu C, et al. 2002. A grapevine gene encoding a guard cell K+ channel displays developmental regulation in the grapevine berry. Plant Physiol. 128:564–77 Quintero FJ, Blatt MR. 1997. A new family of K+ transporters from Arabidopsis that are conserved across phyla. FEBS Lett. 415:206–11 Reintanz B, Szyroki A, Ivashikina N, Ache P, Godde M, et al. 2002. AtKC1, a silent Arabidopsis potassium channel alpha-subunit modulates root hair K+ influx. Proc. Natl. Acad. Sci. USA 99:4079– 84 Rigas S, Debrosses G, Haralampidis K, Vicente-Agullo F, Feldmann K, et al. 2001. Trh1 encodes a potassium transporter required for tip growth in Arabidopsis root hairs. Plant Cell 13:139–51 Rodrı́guez-Navarro A. 2000. Potassium transport in fungi and plants. Biochim. Biophys. Acta 1469:1–30 Ruan Y-L, Llewellyn DJ, Furbank RT. 2001. The control of single-celled cotton fiber elongation by developmentally reversible gating of plasmodesmata and coordinated expression of sucrose and K+ transporters and expansin. Plant Cell 13:47–60 Rubio F, Gassmann W, Schroeder JI. 1995. Sodium-driven potassium uptake by the plant potassium transporter HKT1 and mutations conferring salt tolerance. Science 270:1660–63 Rubio F, Santa-Marı́a GE, Rodrı́guezNavarro A. 2000. Cloning of Arabidopsis and barley cDNAs encoding HAK potassium transporters in root and shoot cells. Physiol. Plant. 109:34–43 Rubio F, Schwarz M, Gassmann W, Schroeder JI. 1999. Genetic selection of 120. 121. 122. 123. 124. 125. 126. 127. 128. 601 mutations in the high affinity K+ transporter HKT1 that define functions of a loop site for reduced Na+ permeability and increased Na+ tolerance. J. Biol. Chem. 274:6839–47 Rus A, Yokoi S, Sharkhuu A, Reddy M, Lee BH, et al. 2001. AtHKT1 is a salt tolerance determinant that controls Na+ entry into plant roots. Proc. Natl. Acad. Sci. USA 98:14150–55 Saalbach G, Schwerdel M, Natura G, Buschmann P, Christov V, Dahse I. 1997. Over-expression of plant 14-3-3 proteins in tobacco: enhancement of the plasmalemma K+ conductance of mesophyll cells. FEBS Lett. 413:294–98 Salinas M, Duprat F, Heurteaux C, Hugnot J-P, Lazdunski M. 1997. New modulatory alpha subunits for mammalian Shab K+ channels. J. Biol. Chem. 272:24371–79 Santa-Marı́a GE, Danna CH, Czibener C. 2000. High-affinity potassium transport in barley roots. Ammonium-sensitive and -insensitive pathways. Plant Physiol. 123:297–306 Santa-Marı́a GE, Rubio F, Dubcovsky J, Rodrı́guez-Navarro A. 1997. The HAK1 gene of barley is a member of a large gene family and encodes a high-affinity potassium transporter. Plant Cell 9:2281–89 Schachtman DP, Kumar R, Schroeder JI, Marsh EL. 1997. Molecular and functional characterization of a novel lowaffinity cation transporter (LCT1) in higher plants. Proc. Natl. Acad. Sci. USA 94:11079–84 Schachtman DP, Schroeder JI. 1994. Structure and transport mechanism of a high-affinity potassium uptake transporter from higher plants. Nature 370:655–58 Schachtman DP, Schroeder JI, Lucas WJ, Anderson JA, Gaber RF. 1992. Expression of an inward-rectifying potassium channel by the Arabidopsis KAT1 cDNA. Science 258:1654–58 Schleyer M, Bakker EP. 1993. Nucleotide sequence and 30 -end deletion studies indicate that the K+-uptake protein Kup from 3 Apr 2003 12:44 602 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. AR VÉRY AR184-PP54-23.tex ¥ AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB SENTENAC Escherichia coli is composed of a hydrophobic core linked to a large and partially essential hydrophilic C terminus. J. Bacteriol. 175:6925–31 Schönknecht G, Spoormaker P, Steinmeyer R, Brüggemann L, Ache P, et al. 2002. KCO1 is a component of the slowvacuolar (SV) ion channel. FEBS Lett. 511:28–32 Schroeder JI, Allen GJ, Hugouvieux V, Kwak JM, Waner D. 2001. Guard cell signal transduction. Annu. Rev. Plant Physiol. Plant Mol. Biol. 52:627–58 Schuurink RC, Shartzer SF, Fath A, Jones RL. 1998. Characterization of a calmodulin-binding transporter from the plasma membrane of barley aleurone. Proc. Natl. Acad. Sci. USA 95:1944–49 Senn ME, Rubio F, Bañuelos MA, Rodrı́guez-Navarro A. 2001. Comparative functional features of plant potassium HvHAK1 and HvHAK2 transporters. J. Biol. Chem. 276:44563–69 Sentenac H, Bonneaud N, Minet M, Lacroute F, Salmon J-M, et al. 1992. Cloning and expression in yeast of a plant potassium ion transport system. Science 256:663–65 Su H, Golldack D, Katsuhara M, Zhao C, Bohnert HJ. 2001. Expression and stressdependent induction of potassium channel transcripts in the common ice plant. Plant Physiol. 125:604–14 Su H, Golldack D, Zhao C, Bohnert HJ. 2002. The expression of HAK-type K+ transporters is regulated in response to salinity stress in common ice plant. Plant Physiol. 129:1482–93 Szyroki A, Ivashikina N, Dietrich P, Roelfsema MR, Ache P, et al. 2001. KAT1 is not essential for stomatal opening. Proc. Natl. Acad. Sci. USA 98:2917–21 Tang H, Vasconcelos AC, Berkowitz GA. 1995. Evidence that plant K+ channel proteins have two different types of subunits. Plant Physiol. 109:327–30 Tang H, Vasconcelos AC, Berkowitz GA. 1996. Physical association of KAB1 with 139. 140. 141. 142. 143. 144. 145. 146. 147. plant K+ channel alpha subunits. Plant Cell 8:1545–53 Tang H, Vasconcelos AC, Ma J, Berkowitz GA. 1998. In vivo expression pattern of a plant K+ channel ß subunit protein. Plant Sci. 134:117–28 Tholema N, Bakker EP, Suzuki A, Nakamura T. 1999. Change to alanine of one out of four selectivity filter glycines in KtrB causes a two orders of magnitude decrease in the affinities for both K+ and Na+ of the Na+ dependent K+ uptake system KtrAB from Vibrio alginolyticus. FEBS Lett. 450:217–20 Trchounian A, Kobayashi H. 1999. Kup is the major K+ uptake system in Escherichia coli upon hyper-osmotic stress at a low pH. FEBS Lett. 447:144–48 Uozumi N, Kim EJ, Rubio F, Yamaguchi T, Muto S, et al. 2000. The Arabidopsis HKT1 gene homolog mediates inward Na+ currents in Xenopus laevis oocytes and Na+ uptake in Saccharomyces cerevisiae. Plant Physiol. 122:1249–59 Urbach S, Chérel I, Sentenac H, Gaymard F. 2000. Biochemical characterization of the Arabidopsis K+ channels KAT1 and AKT1 expressed or co-expressed in insect cells. Plant J. 23:527–38 Van den Wijngaard PW, Bunney TD, Roobeek I, Schönknecht G, de Boer AH. 2001. Slow vacuolar channels from barley mesophyll cells are regulated by 14-3-3 proteins. FEBS Lett. 488:100–4 Van Steveninck RFM. 1976. Effect of hormones and related substances on ion transport. In Encyclopedia of Plant Physiology, ed. U Lüttge, MG Pitman, 2A:307– 42. Berlin: Springer-Verlag Venema K, Quintero FJ, Pardo JM, Donaire JP. 2002. The Arabidopsis Na+/H+ exchanger AtNHX1 catalyzes low affinity Na+ and K+ transport in reconstituted liposomes. J. Biol. Chem. 277:2413– 18 Véry A-A, Gaymard F, Bosseux C, Sentenac H, Thibaud J-B. 1995. Expression of a cloned plant K+ channel in Xenopus 3 Apr 2003 12:44 AR AR184-PP54-23.tex AR184-PP54-23.sgm LaTeX2e(2002/01/18) P1: GJB K+ TRANSPORT IN PLANTS 148. 149. 150. 151. 152. 153. 154. 155. 156. oocytes: analysis of macroscopic currents. Plant J. 7:321–32 Véry A-A, Sentenac H. 2002. Cation channels in the Arabidopsis plasma membrane. Trends Plant Sci. 7:168–75 Vranová E, Tähtiharju S, Sriprang R, Willekens H, Heino P, et al. 2001. The AKT3 potassium channel protein interacts with the AtPP2CA protein phosphatase 2C. J. Exp. Bot. 52:181–82 Vreugdenhil D. 1985. Source-to-sink gradient of potassium in the phloem. Planta 211:105–11 Walker NA, Sanders D, Maathuis FJ. 1996. High-affinity potassium uptake in plants. Science 273:977–79 Wang T-B, Gassmann W, Rubio F, Schroeder JI, Glass ADM. 1998. Rapid up-regulation of HKT1, a high-affinity potassium transporter gene, in roots of barley and wheat following withdrawal of potassium. Plant Physiol. 118:651– 59 White PJ. 1997. The regulation of K+ influx into roots of rye (Secale cereale L.) seedlings by negative feedback via the K+ flux from shoot to root in the phloem. J. Exp. Bot. 48:2063–73 Xu J, Li M. 1998. Auxiliary subunits of Shaker-type potassium channels. Trends Cardiovasc. Med. 8:229–34 Xu J, Yu W, Jan YN, Jan LY, Li M. 1995. Assembly of voltage-gated potassium channels. Conserved hydrophilic motifs determine subfamily-specific interactions between the alpha-subunits. J. Biol. Chem. 270:24761–68 Yang J, Jan YN, Jan LY. 1995. Deter- 157. 158. 159. 160. 161. 162. 163. 603 mination of the subunit stoichiometry of an inwardly rectifying potassium channel. Neuron 15:1441–47 Yao W, Hadjeb N, Berkowitz GA. 1997. Molecular cloning and characterization of the first plant K(Na)/proton antiporter. Plant Physiol. 114S:200 Zakharyan E, Trchounian A. 2001. K+ influx by Kup in Escherichia coli is accompanied by a decrease in H+ efflux. FEMS Microbiol. Lett. 204:61–64 Zei PC, Aldrich RW. 1998. Voltagedependent gating of single wild-type and S4 mutant KAT1 inward rectifier potassium channels. J. Gen. Physiol. 112:679– 713 Zhang X, Ma J, Berkowitz GA. 1999. Evaluation of functional interaction between K+ channel alpha- and betasubunits and putative inactivation gating by co-expression in Xenopus laevis oocytes. Plant Physiol. 121:995–1002 Zimmermann S, Ehrhardt T, Plesch G, Müller-Röber B. 1999. Ion channels in plant signaling. Cell Mol. Life Sci. 55: 183–203 Zimmermann S, Hartje S, Ehrhardt T, Plesch G, Mueller-Roeber B. 2001. The K+ channel SKT1 is co-expressed with KST1 in potato guard cells—both channels can co-assemble via their conserved KT domains. Plant J. 28:517–27 Zimmermann S, Talke I, Ehrhardt T, Nast G, Müller-Röber B. 1998. Characterization of SKT1, an inwardly rectifying potassium channel from potato, by heterologous expression in insect cells. Plant Physiol. 116:879–90 17 Apr 2003 17:4 AR AR184-23-COLOR.tex AR184-23-COLOR.SGM LaTeX2e(2002/01/18) P1: GDL Figure 2 Expression and function of K+ transport systems in Arabidopsis. In silico analyses have identified several families of genes likely to encode K+ transport systems in the Arabidopsis genome. So far, localization data have been obtained for 16 genes, indicated in the figure. Detailed information mainly concerns K+ channels of the Shaker family. Indicated in bold are the five Shaker channels for which a function in planta has been determined based on complementary approaches, including localization at the tissue/cell level, functional characterization of K+ transport activity in heterologous systems, and reverse genetics analyses. In the root, the inward channel AKT1 plays a role in K+ uptake from the soil (60) and the outward channel SKOR in K+ secretion into the xylem sap toward the shoots (52). In guard cells, KAT1 mediates part of the influx of K+ during stomatal opening (77, 136) and GORK K+ efflux during stomatal closure (2). In pollen, SPIK mediates K+ influx into the growing pollen tube, allowing tube development (101).
