GORK, a delayed outward rectifier expressed in guard cells of Arabidopsis thaliana, is a K(+)-selective, K(+)-sensing ion channel

Stomatal action directly feeds back on leaf turgor: new insights into the regulation of the plant water status from non-invasive pressure probe measurements

Uptake of CO(2) by the leaf is associated with loss of water. Control of stomatal aperture by volume changes of guard cell pairs optimizes the efficiency of water use. Under water stress, the protein kinase OPEN STOMATA 1 (OST1) activates the guard-cell anion release channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1), and thereby triggers stomatal closure. Plants with mutated OST1 and SLAC1 are defective in guard-cell turgor regulation. To study the effect of stomatal movement on leaf turgor using intact leaves of Arabidopsis, we used a new pressure probe to monitor transpiration and turgor pressure simultaneously and non-invasively. This probe permits routine easy access to parameters related to water status and stomatal conductance under physiological conditions using the model plant Arabidopsis thaliana. Long-term leaf turgor pressure recordings over several weeks showed a drop in turgor during the day and recovery at night. Thus pressure changes directly correlated with the degree of plant transpiration. Leaf turgor of wild-type plants responded to CO(2), light, humidity, ozone and abscisic acid (ABA) in a guard cell-specific manner. Pressure probe measurements of mutants lacking OST1 and SLAC1 function indicated impairment in stomatal responses to light and humidity. In contrast to wild-type plants, leaves from well-watered ost1 plants exposed to a dry atmosphere wilted after light-induced stomatal opening. Experiments with open stomata mutants indicated that the hydraulic conductance of leaf stomata is higher than that of the root-shoot continuum. Thus leaf turgor appears to rely to a large extent on the anion channel activity of autonomously regulated stomatal guard cells.

Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Reactive oxygen species (ROS) are central to plant stress response, signalling, development and a multitude of other processes. In this study, the plasma-membrane hydroxyl radical (HR)-activated K(+) channel responsible for K(+) efflux from root cells during stress accompanied by ROS generation is characterised. The channel showed 16-pS unitary conductance and was sensitive to Ca(2+), tetraethylammonium, Ba(2+), Cs(+) and free-radical scavengers. The channel was not found in the gork1-1 mutant, which lacks a major plasma-membrane outwardly rectifying K(+) channel. In intact Arabidopsis roots, both HRs and stress induced a dramatic K(+) efflux that was much smaller in gork1-1 plants. Tests with electron paramagnetic resonance spectroscopy showed that NaCl can stimulate HR generation in roots and this might lead to K(+)-channel activation. In animals, activation of K(+)-efflux channels by HRs can trigger programmed cell death (PCD). PCD symptoms in Arabidopsis roots developed much more slowly in gork1-1 and wild-type plants treated with K(+)-channel blockers or HR scavengers. Therefore, similar to animal counterparts, plant HR-activated K(+) channels are also involved in PCD. Overall, this study provides new insight into the regulation of plant cation transport by ROS and demonstrates possible physiological properties of plant HR-activated K(+) channels.

Guard cell anion channel SLAC1 is regulated by CDPK protein kinases with distinct Ca2+ affinities

In response to drought stress, the phytohormone abscisic acid (ABA) induces stomatal closure. Thereby the stress hormone activates guard cell anion channels in a calcium-dependent, as well as -independent, manner. Open stomata 1 protein kinase (OST1) and ABI1 protein phosphatase (ABA insensitive 1) represent key components of calcium-independent ABA signaling. Recently, the guard cell anion channel SLAC1 was identified. When expressed heterologously SLAC1 remained electrically silent. Upon coexpression with Ca(2+)-independent OST1, however, SLAC1 anion channels appear activated in an ABI1-dependent manner. Mutants lacking distinct calcium-dependent protein kinases (CPKs) appeared impaired in ABA stimulation of guard cell ion channels, too. To study SLAC1 activation via the calcium-dependent ABA pathway, we studied the SLAC1 response to CPKs in the Xenopus laevis oocyte system. Split YFP-based protein-protein interaction assays, using SLAC1 as the bait, identified guard cell expressed CPK21 and 23 as major interacting partners. Upon coexpression of SLAC1 with CPK21 and 23, anion currents document SLAC1 stimulation by these guard cell protein kinases. Ca(2+)-sensitive activation of SLAC1, however, could be assigned to the CPK21 pathway only because CPK23 turned out to be rather Ca(2+)-insensitive. In line with activation by OST1, CPK activation of the guard cell anion channel was suppressed by ABI1. Thus the CPK and OST1 branch of ABA signal transduction in guard cells seem to converge on the level of SLAC1 under the control of the ABI1/ABA-receptor complex.

Potassium-dependent wood formation in poplar: seasonal aspects and environmental limitations

Potassium availability and acquisition are pivotal for the generation of biomass and thus wood formation in growing poplar trees. Here, we focus on the role of potassium (K(+)) in wood production, transitions between dormancy and active growth, and limiting environmental conditions. Molecular mechanisms, such as expression and activity of K(+) transporters and channels controlling seasonal changes in wood formation, are discussed.

Proteomic profiling of tandem affinity purified 14-3-3 protein complexes in Arabidopsis thaliana

In eukaryotes, 14-3-3 dimers regulate hundreds of functionally diverse proteins (clients), typically in phosphorylation-dependent interactions. To uncover new clients, 14-3-3 omega (At1g78300) from Arabidopsis was engineered with a "tandem affinity purification" tag and expressed in transgenic plants. Purified complexes were analyzed by tandem MS. Results indicate that 14-3-3 omega can dimerize with at least 10 of the 12 14-3-3 isoforms expressed in Arabidopsis. The identification here of 121 putative clients provides support for in vivo 14-3-3 interactions with a diverse array of proteins, including those involved in: (i) Ion transport, such as a K(+) channel (GORK), a Cl(-) channel (CLCg), Ca(2+) channels belonging to the glutamate receptor family (1.2, 2.1, 2.9, 3.4, 3.7); (ii) hormone signaling, such as ACC synthase (isoforms ACS-6, -7 and -8 involved in ethylene synthesis) and the brassinolide receptors BRI1 and BAK1; (iii) transcription, such as 7 WRKY family transcription factors; (iv) metabolism, such as phosphoenol pyruvate carboxylase; and (v) lipid signaling, such as phospholipase D (beta and gamma). More than 80% (101) of these putative clients represent previously unidentified 14-3-3 interactors. These results raise the number of putative 14-3-3 clients identified in plants to over 300.

What makes a gate? The ins and outs of Kv-like K+ channels in plants

Gating of K(+) and other ion channels is 'hard-wired' within the channel protein. So it remains a puzzle how closely related channels in plants can show an unusually diverse range of biophysical properties. Gating of these channels lies at the heart of K(+) mineral nutrition, signalling, abiotic and biotic stress responses in plants. Thus, our knowledge of the molecular mechanics underpinning K(+) channel gating will be important for rational engineering of related traits in agricultural crops. Several key studies have added significantly to our understanding of channel gating in plants and have challenged current thinking about analogous processes found in animal K(+) channels. Such studies highlight how much of K(+) channel gating remains to be explored in plants.

Potassium nutrition, sodium toxicity, and calcium signaling: connections through the CBL-CIPK network

Plant roots take up numerous minerals from the soil. Some minerals (e.g., K(+)) are essential nutrients and others (e.g., Na(+)) are toxic for plant growth and development. In addition to the absolute level, the balance among the minerals is critical for their physiological functions. For instance, [K(+)]/[Na(+)] ratio and homeostasis often determine plant growth rate. Either low-K or high-Na in the soil represents a stress condition that severely affects plant life and agricultural production. Earlier observations indicated that higher soil Ca2(+) improve plants growth under low-K or high-Na condition, implying functional interaction among the three cations. Recent studies have begun to delineate the signaling mechanisms underlying such interactions. Either low-K(+) or high-Na(+) can trigger cellular Ca2(+) changes that lead to activation of complex signaling networks. One such network consists of Ca2(+) sensor proteins (e.g., CBLs) interacting with their target kinases (CIPKs). The CBL-CIPK signaling modules interact with and regulate the activity of a number of transporting proteins involved in the uptake and translocation of K(+) and Na(+), maintaining the "balance" of these cations in plants under stress conditions.

K+ transport in plants: physiology and molecular biology

Potassium (K(+)) is an essential nutrient and the most abundant cation in plant cells. Plants have a wide variety of transport systems for K(+) acquisition, catalyzing K(+) uptake across a wide spectrum of external concentrations, and mediating K(+) movement within the plant as well as its efflux into the environment. K(+) transport responds to variations in external K(+) supply, to the presence of other ions in the root environment, and to a range of plant stresses, via Ca(2+) signaling cascades and regulatory proteins. This review will summarize the molecular identities of known K(+) transporters, and examine how this information supports physiological investigations of K(+) transport and studies of plant stress responses in a changing environment.

Distinct roles of the last transmembrane domain in controlling Arabidopsis K+ channel activity

The family of voltage-gated potassium channels in plants presumably evolved from a common ancestor and includes both inward-rectifying (K(in)) channels that allow plant cells to accumulate K(+) and outward-rectifying (K(out)) channels that mediate K(+) efflux. Despite their close structural similarities, the activity of K(in) channels is largely independent of K(+) and depends only on the transmembrane voltage, whereas that of K(out) channels responds to the membrane voltage and the prevailing extracellular K(+) concentration. Gating of potassium channels is achieved by structural rearrangements within the last transmembrane domain (S6). Here we investigated the functional equivalence of the S6 helices of the K(in) channel KAT1 and the K(out) channel SKOR by domain-swapping and site-directed mutagenesis. Channel mutants and chimeras were analyzed after expression in Xenopus oocytes. We identified two discrete regions that influence gating differently in both channels, demonstrating a lack of functional complementarity between KAT1 and SKOR. Our findings are supported by molecular models of KAT1 and SKOR in the open and closed states. The role of the S6 segment in gating evolved differently during specialization of the two channel subclasses, posing an obstacle for the transfer of the K(+)-sensor from K(out) to K(in) channels.

Plant ion channels: gene families, physiology, and functional genomics analyses

Distinct potassium, anion, and calcium channels in the plasma membrane and vacuolar membrane of plant cells have been identified and characterized by patch clamping. Primarily owing to advances in Arabidopsis genetics and genomics, and yeast functional complementation, many of the corresponding genes have been identified. Recent advances in our understanding of ion channel genes that mediate signal transduction and ion transport are discussed here. Some plant ion channels, for example, ALMT and SLAC anion channel subunits, are unique. The majority of plant ion channel families exhibit homology to animal genes; such families include both hyperpolarization- and depolarization-activated Shaker-type potassium channels, CLC chloride transporters/channels, cyclic nucleotide-gated channels, and ionotropic glutamate receptor homologs. These plant ion channels offer unique opportunities to analyze the structural mechanisms and functions of ion channels. Here we review gene families of selected plant ion channel classes and discuss unique structure-function aspects and their physiological roles in plant cell signaling and transport.

Outward-rectifying K+ channel activities regulate cell elongation and cell division of tobacco BY-2 cells

Potassium ions (K+) are required for plant growth and development, including cell division and cell elongation/expansion, which are mediated by the K+ transport system. In this study, we investigated the role of K+ in cell division using tobacco BY-2 protoplast cultures. Gene expression analysis revealed induction of the Shaker-like outward K+ channel gene, NTORK1, under cell-division conditions, whereas the inward K+ channel genes NKT1 and NtKC1 were induced under both cell-elongation and cell-division conditions. Repression of NTORK1 gene expression by expression of its antisense construct repressed cell division but accelerated cell elongation even under conditions promoting cell division. A decrease in the K+ content of cells and cellular osmotic pressure in dividing cells suggested that an increase in cell osmotic pressure by K+ uptake is not required for cell division. In contrast, K+ depletion, which reduced cell-division activity, decreased cytoplasmic pH as monitored using a fluorescent pH indicator, SNARF-1. Application of K+ or the cytoplasmic alkalizing reagent (NH(4))(2)SO(4) increased cytoplasmic pH and suppressed the reduction in cell-division activity. These results suggest that the K+ taken up into cells is used to regulate cytoplasmic pH during cell division.

Effects of top excision on the potassium accumulation and expression of potassium channel genes in tobacco

The effects of the removal of the shoot apex of tobacco on the relative transcript levels of potassium channel genes, determined by real-time PCR, and on the relationship between the expression of genes encoding potassium channels and potassium concentration, were studied. The results from the study indicated that comparatively more assimilates of photosynthesis were allocated to the apex in control plants than in both decapitated and IAA-treated decapitated plants. By contrast, dry matter in the upper leaves, roots, and stems in both decapitated and IAA-treated plants was significantly increased relative to control plants. The potassium level in whole plants decreased post-decapitation compared with control plants, and so did the potassium concentration in middle and upper leaves, stem, and roots. Expression of NKT1, NtKC1, NTORK1, and NKT2 was inhibited by decapitation in tobacco leaves with a gradual reduction after decapitation, but was induced in roots. The relative expression of NKT1, NTORK1, and NKT2 in tobacco leaves was higher than that in roots, whereas the expression of NtKC1 was higher in roots. The levels of inhibition and induction of NKT1, NtKC1, NTORK1, and NKT2 in leaves and roots, respectively, associated with decapitation were reduced by the application of IAA on the cut surface of the decapitated stem. Further results showed that the level of endogenous auxin IAA in decapitated plants, which dropped in leaves and increased in roots by 140.7% at 14 d compared with the control plant, might be attributed to the change in the expression of potassium channel genes. The results suggest that there is a reciprocal relationship among endogenous auxin IAA, expression of potassium channel genes and potassium accumulation. They further imply that the endogenous IAA probably plays a role in regulating the expression of potassium channel genes, and that variations in expression of these genes affected the accumulation and distribution of potassium in tobacco.

Targeting of vacuolar membrane localized members of the TPK channel family

Four members of the tandem-pore potassium channel family of Arabidopsis thaliana (TPK1, 2, 3, and 5) reside in the vacuolar membrane, whereas TPK4 is a plasma membrane K(+)-channel. By constructing chimeras between TPK1 and TPK4, we attempted to identify channel domains involved in the trafficking process and found that the TPK1 cytoplasmic C-terminal domain (CT) is critical for the ER- as well as Golgi-sorting steps. Following site-directed mutagenesis, we identified a diacidic motif (DLE) required for ER-export of TPK1. However, this diacidic motif in the C-terminus is not conserved among other members of the TPK family, and TPK3 sorting is independent of its CT. Moreover, the 14-3-3 binding site of TPK1, essential for channel activation, is not involved in channel sorting.

Heteromerization of Arabidopsis Kv channel alpha-subunits: Data and prospects

Potassium translocation in plants is accomplished by a large variety of transport systems. Most of the available molecular information on these proteins concerns voltage-gated potassium channels (Kv channels). The Arabidopsis genome comprises nine genes encoding alpha-subunits of Kv channels. Based on knowledge of their animal homologues, and on biochemical investigations, it is broadly admitted that four such polypeptides must assemble to yield a functional Kv channel. The intrinsic functional properties of Kv channel alpha-subunits have been described by expressing them in suitable heterologous contexts where homo-tetrameric channels could be characterized. However, due to the high similarity of both the polypeptidic sequence and the structural scheme of Kv channel alpha-subunits, formation of heteromeric Kv channels by at least two types of alpha-subunits is conceivable. Several examples of such heteromeric plant Kv channels have been studied in heterologous expression systems and evidence that heteromerization actually occurs in planta has now been published. It is therefore challenging to uncover the physiological role of this heteromerization. Fine tuning of Kv channels by heteromerisation could be relevant not only to potassium transport but also to electrical signaling within the plant.

Membrane transporters for nitrogen, phosphate and potassium uptake in plants

Nitrogen, phosphorous and potassium are essential nutrients for plant growth and development. However, their contents in soils are limited so that crop production needs to invest a lot for fertilizer supply. To explore the genetic potentialities of crops (or plants) for their nutrient utilization efficiency has been an important research task for many years. In fact, a number of evidences have revealed that plants, during their evolution, have developed many morphological, physiological, biochemical and molecular adaptation mechanisms for acquiring nitrate, phosphate and potassium under stress conditions. Recent discoveries of many transporters and channels for nitrate, phosphate and potassium uptake have opened up opportunities to study the molecular regulatory mechanisms for acquisition of these nutrients. This review aims to briefly discuss the genes and gene families for these transporters and channels. In addition, the functions and regulation of some important transporters and channels are particularly emphasized.

Role and influence of mycorrhizal fungi on radiocesium accumulation by plants

This review summarizes current knowledge on the contribution of mycorrhizal fungi to radiocesium immobilization and plant accumulation. These root symbionts develop extended hyphae in soils and readily contribute to the soil-to-plant transfer of some nutrients. Available data show that ecto-mycorrhizal (ECM) fungi can accumulate high concentration of radiocesium in their extraradical phase while radiocesium uptake and accumulation by arbuscular mycorrhizal (AM) fungi is limited. Yet, both ECM and AM fungi can transport radiocesium to their host plants, but this transport is low. In addition, mycorrhizal fungi could thus either store radiocesium in their intraradical phase or limit its root-to-shoot translocation. The review discusses the impact of soil characteristics, and fungal and plant transporters on radiocesium uptake and accumulation in plants, as well as the potential role of mycorrhizal fungi in phytoremediation strategies.

Single mutations convert an outward K+ channel into an inward K+ channel

Shaker-type K(+) channels in plants display distinct voltage-sensing properties despite sharing sequence and structural similarity. For example, an Arabidopsis K(+) channel (SKOR) and a tomato K(+) channel (LKT1) share high amino acid sequence similarity and identical domain structures; however, SKOR conducts outward K(+) current and is activated by positive membrane potentials (depolarization), whereas LKT1 conducts inward current and is activated by negative membrane potentials (hyperpolarization). The structural basis for the "opposite" voltage-sensing properties of SKOR and LKT1 remains unknown. Using a screening procedure combined with random mutagenesis, we identified in the SKOR channel single amino acid mutations that converted an outward-conducting channel into an inward-conducting channel. Further domain-swapping and random mutagenesis produced similar results, suggesting functional interactions between several regions of SKOR protein that lead to specific voltage-sensing properties. Dramatic changes in rectifying properties can be caused by single amino acid mutations, providing evidence that the inward and outward channels in the Shaker family from plants may derive from the same ancestor.

Plant cells must pass a K+ threshold to re-enter the cell cycle

Potassium is an inevitable component of plant life, and potassium channels play a pivotal role in plant growth and development. The role of potassium and of K(+) channels in plant cell division and cell-cycle progression, however, has not been determined so far. K(+) channel blocker studies with synchronized tobacco BY-2 cells revealed that K(+) uptake is required for proper cell-cycle progression during the transition from G(1) to S phase. Electrophysiological studies (patch-clamp and voltage-clamp techniques) showed a cell-cycle dependency of K(+) channel activities and reduced driving force for K(+) uptake in dividing cells. Among the four Shaker-like K(+) channel genes expressed in BY-2 cells, NKT1 represents an inwardly rectifying K(+) channel that mediates K(+) uptake. NKT1 is transcriptionally induced during G(1) phase, while transcripts of the outward-rectifier NTORK1 dominate S phase. Elongating BY-2 cells appeared hyperpolarized (-101 +/- 11 mV), and had elevated osmotic pressure and approximately twice the turgor pressure when compared with depolarized (-64 +/- 8 mV) dividing cells. This indicates that cells have to gain a threshold K(+) level to re-enter the cell cycle. Based on these findings, turgor regulation through modulation of K(+) channel density in plant cell division and cell-cycle progression is discussed.

Roles of ion channels and transporters in guard cell signal transduction

Stomatal complexes consist of pairs of guard cells and the pore they enclose. Reversible changes in guard cell volume alter the aperture of the pore and provide the major regulatory mechanism for control of gas exchange between the plant and the environment. Stomatal movement is facilitated by the activity of ion channels and ion transporters found in the plasma membrane and vacuolar membrane of guard cells. Progress in recent years has elucidated the molecular identities of many guard cell transport proteins, and described their modulation by various cellular signal transduction components during stomatal opening and closure prompted by environmental and endogenous stimuli.

In planta AKT2 subunits constitute a pH- and Ca2+-sensitive inward rectifying K+ channel

Heterologous expression of plant genes in yeast and animal cells represents a common approach to study plant ion channels. When expressed in Xenopus oocytes and COS cells the Arabidopsis Shaker-like K+ channel, AKT2 forms a weakly voltage-dependent channel, blocked by Ca2+ and protons. Channels with these characteristics, however, were not found in AKT2-expressing Arabidopsis cell types. To understand this phenomenon, we employed Agrobacterium-mediated transient transformation to functionally characterise Arabidopsis thaliana channels in Nicotiana benthamiana mesophyll cells. In this expression system we used AtTPK4 as a control for voltage-independent A. thaliana channels. Agrobacteria harbouring GFP-tagged constructs with the coding sequences of AKT2 and AtTPK4 were infiltrated into intact tobacco leaves. With quantitative RT-PCR analyses channel transcripts of AKT2 and AtTPK4 were determined in transformed leaves. These results were confirmed by Western blots with V5 epitope-tagged AKT2 and AtTPK4 proteins, showing that the channel protein was indeed synthesised. For functional analysis of these channels, mesophyll protoplasts were isolated from infiltrated leaf sections. Patch-clamp studies revealed that AKT2 channels in mesophyll protoplasts retained Ca2+ and pH sensitivity, characteristics of the heterologously expressed protein, but displayed pronounced differences in voltage-dependence and kinetics. AKT2-transformed mesophyll cells, displayed inward-rectifying, rather than voltage-independent K+ channels, initially characterised in AKT2-expressing animal cells. In contrast, AtTPK4 showed the same electrophysiological characteristics both, in oocytes and plant cells. Our data suggest that heterologous systems do not always possess all regulatory components for functional expression of plant channels. Therefore, transient expression of plant proteins in planta provides an additional research tool for rapid biophysical analysis of plant ion channels.

K+ channel activity in plants: genes, regulations and functions

Potassium (K(+)) is the most abundant cation in the cytosol, and plant growth requires that large amounts of K(+) are transported from the soil to the growing organs. K(+) uptake and fluxes within the plant are mediated by several families of transporters and channels. Here, we describe the different families of K(+)-selective channels that have been identified in plants, the so-called Shaker, TPK and Kir-like channels, and what is known so far on their regulations and physiological functions in the plant.

30-year progress of membrane transport in plants

In the past 30 years enormous progress was made in plant membrane biology and transport physiology, a fact reflected in the appearance of textbooks. The first book dedicated to 'Membrane Transport in Plants' was published on the occasion of the 'International Workshop on Membrane Transport in Plants' held at the Nuclear Research Center, Jülich, Germany [Zimmermann and Dainty (eds) 1974] and was followed in 1976 by a related volume 'Transport in plants II' in the 'Encyclopedia of plant physiology' [Lüttge and Pitman (eds) 1976]. A broad spectrum of topics including thermodynamics of transport processes, water relations, primary reactions of photosynthesis, as well as more conventional aspects of membrane transport was presented. The aim of the editors of the first book was to bring advanced thermodynamical concepts to the attention of biologists and to show physical chemists and biophysicist what the more complex biological systems were like. To bundle known data on membrane transport in plants and relevant fields for mutual understanding, interdisciplinary research and clarification of problems were considered highly important for further progress in this scientific area of plant physiology. The present review will critically evaluate the progress in research in membrane transport in plants that was achieved during the past. How did 'Membrane Transport in Plants' progress within the 30 years between the publication of the first book about this topic (Zimmermann and Dainty 1974), a recent one with the same title (Blatt 2004), and today?

Molecular template for a voltage sensor in a novel K+ channel. I. Identification and functional characterization of KvLm, a voltage-gated K+ channel from Listeria monocytogenes

The fundamental principles underlying voltage sensing, a hallmark feature of electrically excitable cells, are still enigmatic and the subject of intense scrutiny and controversy. Here we show that a novel prokaryotic voltage-gated K⁺ (Kv) channel from Listeria monocytogenes (KvLm) embodies a rudimentary, yet robust, sensor sufficient to endow it with voltage-dependent features comparable to those of eukaryotic Kv channels. The most conspicuous feature of the KvLm sequence is the nature of the sensor components: the motif is recognizable; it appears, however, to contain only three out of eight charged residues known to be conserved in eukaryotic Kv channels and accepted to be deterministic for folding and sensing. Despite the atypical sensor sequence, flux assays of KvLm reconstituted in liposomes disclosed a channel pore that is highly selective for K⁺ and is blocked by conventional Kv channel blockers. Single-channel currents recorded in symmetric K⁺ solutions from patches of enlarged Escherichia coli (spheroplasts) expressing KvLm showed that channel open probability sharply increases with depolarization, a hallmark feature of Kv channels. The identification of a voltage sensor module in KvLm with a voltage dependence comparable to that of other eukaryotic Kv channels yet encoded by a sequence that departs significantly from the consensus sequence of a eukaryotic voltage sensor establishes a molecular blueprint of a minimal sequence for a voltage sensor.

Properties of shaker-type potassium channels in higher plants

Potassium (K(+)), the most abundant cation in biological organisms, plays a crucial role in the survival and development of plant cells, modulation of basic mechanisms such as enzyme activity, electrical membrane potentials, plant turgor and cellular homeostasis. Due to the absence of a Na(+)/K(+) exchanger, which widely exists in animal cells, K(+) channels and some type of K(+) transporters function as K(+) uptake systems in plants. Plant voltage-dependent K(+) channels, which display striking topological and functional similarities with the voltage-dependent six-transmembrane segment animal Shaker-type K(+) channels, have been found to play an important role in the plasma membrane of a variety of tissues and organs in higher plants. Outward-rectifying, inward-rectifying and weakly-rectifying K(+) channels have been identified and play a crucial role in K(+) homeostasis in plant cells. To adapt to the environmental conditions, plants must take advantage of the large variety of Shaker-type K(+) channels naturally present in the plant kingdom. This review summarizes the extensive data on the structure, function, membrane topogenesis, heteromerization, expression, localization, physiological roles and modulation of Shaker-type K(+) channels from various plant species. The accumulated results also help in understanding the similarities and differences in the properties of Shaker-type K(+) channels in plants in comparison to those of Shaker channels in animals and bacteria.

Intracellular K+ sensing of SKOR, a Shaker-type K+ channel from Arabidopsis

Most K+ channels in plants are structurally classified into the Shaker family named after the shaker K+ channel in Drosophila. Plant K+ channels function in many physiological processes including osmotic regulation and K+ nutrition. An outwardly rectifying K+ channel, SKOR, mediates the delivery of K+ from stelar cells to the xylem in the roots, a critical step in the long-distance distribution of K+ from roots to the upper parts of the plant. Here we report that SKOR channel activity is strictly dependent on intracellular K+ concentrations. Activation by K+ did not affect the kinetics of voltage dependence in SKOR, indicating that a voltage-independent gating mechanism underlies the K+ sensing process. Further analysis showed that the C-terminal non-transmembrane region of the SKOR protein was required for this sensing process. The intracellular K+ sensing mechanism couples SKOR activity to K+ nutrition status in the 'source cells', thereby establishing a supply-based unloading system for the regulation of K+ distribution.

External K+ modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanism

Plant outward-rectifying K+ channels mediate K+ efflux from guard cells during stomatal closure and from root cells into the xylem for root-shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K+ concentration, although the ions carrying the current are derived from inside the cell. This K+ dependence confers a sensitivity to the extracellular K+ concentration ([K+]) that ensures that the channels mediate K+ efflux only, regardless of the [K+] prevailing outside. We investigated the mechanism of K+-dependent gating of the K+ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K+ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an 'S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K+ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K+-sensory process in plants.

Interactive domains between pore loops of the yeast K+ channel TOK1 associate with extracellular K+ sensitivity

Gating of the outward-rectifying K+ channel TOK1 of Saccharomyces cerevisiae is controlled by membrane voltage and extracellular K+ concentration. Previous studies identified two kinetically distinct effects of K+, and site-mutagenic analysis associated these K+-dependencies with domains of the extracellular turrets of the channel protein. We have mapped the TOK1 pore domains to extant K+ channel crystal structures to target additional residues contributing to TOK1 gating. Leu270, located in the first pore domain of TOK1, was found to be critical for gating and its K+ sensitivity. Analysis of amino acid substitutions indicated that spatial position of the polypeptide backbone is a primary factor determining gating sensitivity to K+. The strongest effects, with L270Y, L270F and L270W, led to more than a 30-fold decrease in apparent K+ affinity and an inversion in the apparent K+-dependence of voltage-dependent gating compared with the wild-type current. A partial rescue of wild-type gating was obtained on substitution in the second pore domain with the double mutant L270D/A428Y. These, and additional results, demarcate extracellular domains that are associated with the K+-sensitivity of TOK1 and they offer primary evidence for a synergy in gating between the two pore domains of TOK1, demonstrating an unexpected degree of long-distance interaction across the mouth of the K+ channel.

Plant responses to potassium deficiencies: a role for potassium transport proteins

The availability of potassium to the plant is highly variable, due to complex soil dynamics, which are strongly influenced by root-soil interactions. A low plant potassium status triggers expression of high affinity K+ transporters, up-regulates some K+ channels, and activates signalling cascades, some of which are similar to those involved in wounding and other stress responses. The molecules that signal low K+ status in plants include reactive oxygen species and phytohormones, such as auxin, ethylene and jasmonic acid. Apart from up-regulation of transport proteins and adjustment of metabolic processes, potassium deprivation triggers developmental responses in roots. All these acclimation strategies enable plants to survive and compete for nutrients in a dynamic environment with a variable availability of potassium.

A procedure for localisation and electrophysiological characterisation of ion channels heterologously expressed in a plant context

BACKGROUND

In silico analyses based on sequence similarities with animal channels have identified a large number of plant genes likely to encode ion channels. The attempts made to characterise such putative plant channels at the functional level have most often relied on electrophysiological analyses in classical expression systems, such as Xenopus oocytes or mammalian cells. In a number of cases, these expression systems have failed so far to provide functional data and one can speculate that using a plant expression system instead of an animal one might provide a more efficient way towards functional characterisation of plant channels, and a more realistic context to investigate regulation of plant channels.

RESULTS

With the aim of developing a plant expression system readily amenable to electrophysiological analyses, we optimised experimental conditions for preparation and transformation of tobacco mesophyll protoplasts and engineered expression plasmids, that were designed to allow subcellular localisation and functional characterisation of ion channels eventually in presence of their putative (possibly over-expressed) regulatory partners. Two inward K+ channels from the Shaker family were functionally expressed in this system: not only the compliant KAT1 but also the recalcitrant AKT1 channel, which remains electrically silent when expressed in Xenopus oocytes or in mammalian cells.

CONCLUSION

The level of endogenous currents in control protoplasts seems compatible with the use of the described experimental procedures for the characterisation of plant ion channels, by studying for instance their subcellular localisation, functional properties, structure-function relationships, interacting partners and regulation, very likely in a more realistic context than the classically used animal systems.

Inward rectification of the AKT2 channel abolished by voltage-dependent phosphorylation

The Arabidopsis K(+) channel AKT2 possesses the remarkable property that its voltage threshold for activation can be either within the physiological range (gating mode 1), or shifted towards considerably more positive voltages (gating mode 2). Gating mode 1 AKT2 channels behave as delayed K(+)-selective inward rectifiers; while gating mode 2 AKT2 channels are K(+)-selective 'open leaks' in the physiological range of membrane potential. In the present study we have investigated modulation of AKT2 current by effectors of phosphatases/kinases in COS cells and Xenopus oocytes. These experiments show that (i) dephosphorylation can result in AKT2 channel silencing; and (ii) phosphorylation by protein kinase A (PKA) favors both recruitment of silenced AKT2 channels and transition from gating mode 1 to gating mode 2. Interestingly, phosphorylation of AKT2 by PKA in COS cells and Xenopus oocytes is favored by hyperpolarization. Two PKA phosphorylation sites (S210 and S329) were pinpointed in the region of the pore inner mouth. The role of these phosphorylation sites in the switch between the two gating modes was assessed by electrophysiological characterization of mutant channels. The molecular aspects of AKT2 regulation by phosphorylation, and the possible physiological meaning of such regulation in the plant context, are discussed.

Polar-localised poplar K+ channel capable of controlling electrical properties of wood-forming cells

In previous studies, we have shown that annual expression profiles of cambial and wood tissue with respect to the Shaker K+ channel PTORK correlate with cambial activity. To follow PTORK-gene activity on the cellular level, we isolated the respective promoter regions and generated transgenic Arabidopsis plants expressing the GUS gene under the control of the PTORK promoter. Cross-sections of petioles showed PTORK-driven signals predominantly in the xylem parenchyma surrounding the vessels and in the phloem. Antibodies raised against a unique N-terminal region of PTORK in histo-immunochemical analyses recognised this K+-release channel in growth-active poplar plants only. PTORK labelling was found in differentiating xylem cells (young fibres) and mature xylem (vessel-associated cells of the ray parenchyma). Patch-clamp measurements on fibre cell protoplasts, derived from young poplar twigs, identified outward-rectifying K+ channels as the major K+ conductance of this cell type, which resembled the biophysical properties of PTORK when expressed in Xenopus oocytes.

Differential expression of K+ channels between guard cells and subsidiary cells within the maize stomatal complex

Grass stomata are characterized by dumbbell-shaped guard cells forming a complex with a pair of specialized epidermal cells, the subsidiary cells. Stomatal movement is accomplished by a reversible exchange of potassium and chloride between these two cell types. To gain insight into the molecular machinery involved in K+ transport within the stomatal complex of Zea mays, we determined the spatial and temporal expression pattern of potassium channels in the maize leaf. KZM2 and ZORK were isolated and identified as new members of the plant Shaker K+ channel family. Northern blot analysis identified fully developed leaves as the predominant site of KZM2 expression. Following enzymatic digestion and separation of leaf tissue into epidermal, mesophyll, and vascular fractions, KZM2 and ZORK transcripts were localized in the epidermis. Using a collection of individually isolated guard cell or subsidiary cell protoplasts, ZORK transcripts were found in both cell types while KZM2 was exclusively expressed in the guard cell population. The previously identified K+ channel genes ZMK1 and KZM1 were expressed in subsidiary cells and guard cells, respectively, whereas ZMK2 transcripts were not detected. These data indicate that the interaction between subsidiary cells and guard cells is based on overlapping as well as differential expression of K+ channels in the two cell types of the maize stomatal complex.

Nucleotides and Mg2+ ions differentially regulate K+ channels and non-selective cation channels present in cells forming the stomatal complex

Voltage-dependent inward-rectifying (K(in)) and outward-rectifying (K(out)) K(+) channels are capable of mediating K(+) fluxes across the plasma membrane. Previous studies on guard cells or heterologously expressed K(+) channels provided evidence for the requirement of ATP to maintain K(+) channel activity. Here, the nucleotide and Mg(2+) dependencies of time-dependent K(in) and K(out) channels from maize subsidiary cells were examined, showing that MgATP as well as MgADP function as channel activators. In addition to K(out) channels, these studies revealed the presence of another outward-rectifying channel type (MgC) in the plasma membrane that however gates in a nucleotide-independent manner. MgC represents a new channel type distinguished from K(out) channels by fast activation kinetics, inhibition by elevated intracellular Mg(2+) concentration, permeability for K(+) as well as for Na(+) and insensitivity towards TEA(+). Similar observations made for guard cells from Zea mays and Vicia faba suggest a conserved regulation of channel-mediated K(+) and Na(+) transport in both cell types and species.

In the light of stomatal opening: new insights into 'the Watergate'

Stomata can be regarded as hydraulically driven valves in the leaf surface, which open to allow CO2 uptake and close to prevent excessive loss of water. Movement of these 'Watergates' is regulated by environmental conditions, such as light, CO2 and humidity. Guard cells can sense environmental conditions and function as motor cells within the stomatal complex. Stomatal movement results from the transport of K+ salts across the guard cell membranes. In this review, we discuss the biophysical principles and mechanisms of stomatal movement and relate these to ion transport at the plasma membrane and vacuolar membrane. Studies with isolated guard cells, combined with recordings on single guard cells in intact plants, revealed that light stimulates stomatal opening via blue light-specific and photosynthetic-active radiation-dependent pathways. In addition, guard cells sense changes in air humidity and the water status of distant tissues via the stress hormone abscisic acid (ABA). Guard cells thus provide an excellent system to study cross-talk, as multiple signaling pathways induce both short- and long-term responses in these sensory cells.

Plant K(in) and K(out) channels: approaching the trait of opposite rectification by analyzing more than 250 KAT1-SKOR chimeras

Members of the Shaker-like plant K(+) channel family share a common structure, but are highly diverse in their function: they behave as either hyperpolarization-activated inward-rectifying (K(in)) channels, or leak-like (K(weak)) channels, or depolarization-activated outward-rectifying (K(out)) channels. Here we created 256 chimeras between the K(in) channel KAT1 and the K(out) channel SKOR. The chimeras were screened in a potassium-uptake deficient yeast strain to identify those, which mediate potassium inward currents, i.e., which are functionally equivalent to KAT1. This strategy allowed us to identify three chimeras which differ from KAT1 in three parts of the polypeptide: the cytosolic N-terminus, the cytosolic C-terminus, and the putative voltage-sensor S4. Additionally, mutations in the K(out) channel SKOR were generated in order to localize molecular entities underlying its depolarization activation. The triple mutant SKOR-D312N-M313L-I314G, carrying amino-acid changes in the S6 segment, was identified as a channel which did not display any rectification in the tested voltage-range.

AKT2/3 subunits render guard cell K+ channels Ca2+ sensitive

Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the K+ inward rectifier. To confirm the akt2/3-1 phenotype, two independent knockout mutants, akt2-1 and akt2::En-1 were tested, demonstrating that the loss of AKT2/3 indeed affects the Ca2+ dependence of guard cell inward rectifier. In contrast to AKT2 knockout plants, AKT1, AtKC1, and KAT1 loss-of-function mutants retained Ca2+ block of the guard cell inward rectifier. When expressed in HEK293 cells, AKT2 channel displayed a pronounced susceptibility toward extracellular Ca2+, while the dominant guard cell K+ channel KAT2 was Ca2+ insensitive. Thus, we conclude that the AKT2/3 subunit constitutes the Ca2+ sensitivity of the guard cell K+ uptake channel.

Guard cell metabolism and CO2 sensing

In this review we concentrate on guard cell metabolism and CO2 sensing. Although a matter of some controversy, it is generally accepted that the Calvin cycle plays a minor role in stomatal movements. Recent data emphasise the importance of guard cell starch degradation and of carbon import from the guard cell apoplast in promoting and maintaining stomatal opening. Chloroplast maltose and glucose transporters appear to be crucial to the export of carbon from both guard and mesophyll cells. The way guard cells sense CO2 remains an unresolved question. However, a better understanding of the cellular events downstream from CO2 sensing is emerging. We now recognise that there are common as well as unique steps in abscisic acid (ABA) and CO2 signalling pathways. For example, while ABA and CO2 both trigger increases in cytoplasmic free calcium, unlike ABA, CO2 does not promote a cytoplasmic pH change. Future advances in this area are likely to result from the increased use of techniques and resources, such as, reverse genetics, novel mutants, confocal imaging, and microarray analyses of the guard cell transcriptome.

Assembly of plant Shaker-like K(out) channels requires two distinct sites of the channel alpha-subunit

SKOR and GORK are outward-rectifying plant potassium channels from Arabidopsis thaliana. They belong to the Shaker superfamily of voltage-dependent K(+) channels. Channels of this class are composed of four alpha-subunits and subunit assembly is a prerequisite for channel function. In this study the assembly mechanism of SKOR was investigated using the yeast two-hybrid system and functional assays in Xenopus oocytes and in yeast. We demonstrate that SKOR and GORK physically interact and assemble into heteromeric K(out) channels. Deletion mutants and chimeric proteins generated from SKOR and the K(in) channel alpha-subunit KAT1 revealed that the cytoplasmic C-terminus of SKOR determines channel assembly. Two domains that are crucial for channel assembly were identified: i), a proximal interacting region comprising a putative cyclic nucleotide-binding domain together with 33 amino acids just upstream of this domain, and ii), a distal interacting region showing some resemblance to the K(T) domain of KAT1. Both regions contributed differently to channel assembly. Whereas the proximal interacting region was found to be active on its own, the distal interacting region required an intact proximal interacting region to be active. K(out) alpha-subunits did not assemble with K(in) alpha-subunits because of the absence of interaction between their assembly sites.

Abscisic acid and 14-3-3 proteins control K channel activity in barley embryonic root

Germination of seeds proceeds in general in two phases, an initial imbibition phase and a subsequent growth phase. In grasses like barley, the latter phase is evident as the emergence of the embryonic root (radicle). The hormone abscisic acid (ABA) inhibits germination because it prevents the embryo from entering and completing the growth phase. Genetic and physiological studies have identified many steps in the ABA signal transduction cascade, but how it prevents radicle elongation is still not clear. For elongation growth to proceed, uptake of osmotically active substances (mainly K(+)) is essential. Therefore, we have addressed the question of how the activity of K(+) permeable ion channels in the plasma membrane of radicle cells is regulated under conditions of slow (+ABA) and rapid germination (+fusicoccin). We found that ABA arrests radicle growth, inhibits net K(+) uptake and reduces the activity of K(+) (in) channels as measured with the patch-clamp technique. In contrast, fusicoccin (FC), a well-known stimulator of germination, stimulates radicle growth, net K(+) uptake and reduces the activity of K(+) (out) channels. Both types of channels are under the control of 14-3-3 proteins, known as integral components of signal transduction pathways and instrumental in FC action. Intriguingly, 14-3-3 affected both channels in an opposite fashion: whereas K(+) (in) channel activity was fully dependent upon 14-3-3 proteins, K(+) (out) channel activity was reduced by 14-3-3 proteins by 60%. Together with previous data showing that 14-3-3 proteins control the activity of the plasma membrane H(+)-ATPase, this makes 14-3-3 a prime candidate for molecular master regulator of the cellular osmo-pump. Regulation of the osmo-pump activity by ABA and FC is an important mechanism in controlling the growth of the embryonic root during seed germination.

AtTPK4, an Arabidopsis tandem-pore K+ channel, poised to control the pollen membrane voltage in a pH- and Ca2+-dependent manner

The Arabidopsis tandem-pore K(+) (TPK) channels displaying four transmembrane domains and two pore regions share structural homologies with their animal counterparts of the KCNK family. In contrast to the Shaker-like Arabidopsis channels (six transmembrane domains/one pore region), the functional properties and the biological role of plant TPK channels have not been elucidated yet. Here, we show that AtTPK4 (KCO4) localizes to the plasma membrane and is predominantly expressed in pollen. AtTPK4 (KCO4) resembles the electrical properties of a voltage-independent K(+) channel after expression in Xenopus oocytes and yeast. Hyperpolarizing as well as depolarizing membrane voltages elicited instantaneous K(+) currents, which were blocked by extracellular calcium and cytoplasmic protons. Functional complementation assays using a K(+) transport-deficient yeast confirmed the biophysical and pharmacological properties of the AtTPK4 channel. The features of AtTPK4 point toward a role in potassium homeostasis and membrane voltage control of the growing pollen tube. Thus, AtTPK4 represents a member of plant tandem-pore-K(+) channels, resembling the characteristics of its animal counterparts as well as plant-specific features with respect to modulation of channel activity by acidosis and calcium.

Potassium and carrot embryogenesis: are K+ channels necessary for development?

The expression pattern of the KDC1 gene, coding for an inwardly-rectifying K(+) channel of Daucus carota , is described in several embryo stages and seedling tissues. Relative quantitative RT-PCR experiments indicated that, during (somatic) embryonic development, the KDC1 transcript appears as early as the globular stage and that the transcript level remains constant throughout the successive heart and torpedo stages. Thereafter, the KDC1 transcript is preferentially expressed in plant roots, but is also present in other tissues, and in particular, in the shoot apical meristem. In situ hybridisation experiments showed that in embryos KDC1 mRNA is detectable preferentially in protoderm cells with a stage dependent expression pattern. At later times, the hybridisation signal is particularly evident in root hairs, root epidermis and endodermis, but is also observed in single cell layers corresponding to L1 of the shoot apical meristem and leaf primordia. Promoter studies with the beta -glucuronidase reporter gene confirm preferential expression of KDC1 in embryo protoderm cells and in plant root epidermis and root hairs. Western blot analysis of embryonic proteins and immunolocalisation experiments on somatic embryos sections revealed the presence of KDC1 during embryo development. Consistent with these observations, patch-clamp experiments performed on protoplasts isolated from embryos at the torpedo stage demonstrated the presence of functional inward rectifying K(+) channels. This is the first report on the expression of a plant ion channel during embryo development.

Cytoplasmic alkalization precedes reactive oxygen species production during methyl jasmonate- and abscisic acid-induced stomatal closure

Signaling events during abscisic acid (ABA) or methyl jasmonate (MJ)-induced stomatal closure were examined in Arabidopsis wild type, ABA-insensitive (ost1-2), and MJ-insensitive mutants (jar1-1) in order to examine a crosstalk between ABA and MJ signal transduction. Some of the experiments were performed on epidermal strips of Pisum sativum. Stomata of jar1-1 mutant plants are insensitive to MJ but are able to close in response to ABA. However, their sensitivity to ABA is less than that of wild-type plants. Reciprocally, the stomata of ost1-2 are insensitive to ABA but are able to close in response to MJ to a lesser extent compared to wild-type plants. Both MJ and ABA promote H(2)O(2) production in wild-type guard cells, while exogenous application of diphenylene iodonium (DPI) chloride, an inhibitor of NAD(P)H oxidases, results in the suppression of ABA- and MJ-induced stomatal closure. ABA elevates H(2)O(2) production in wild-type and jar1-1 guard cells but not in ost1-2, whereas MJ induces H(2)O(2) production in both wild-type and ost1-2 guard cells, but not in jar1-1. MJ-induced stomatal closing is suppressed in the NAD(P)H oxidase double mutant atrbohD/F and in the outward potassium channel mutant gork1. Furthermore, MJ induces alkalization in guard cell cytosol, and MJ-induced stomatal closing is inhibited by butyrate. Analyses of the kinetics of cytosolic pH changes and reactive oxygen species (ROS) production show that the alkalization of cytoplasm precedes ROS production during the stomatal response to both ABA and MJ. Our results further indicate that JAR1, as OST1, functions upstream of ROS produced by NAD(P)H oxidases and that the cytoplasmic alkalization precedes ROS production during MJ or ABA signal transduction in guard cells.

The poplar K+ channel KPT1 is associated with K+ uptake during stomatal opening and bud development

To gain insights into the performance of poplar guard cells, we have measured stomatal conductance and aperture, guard cell K+ content and K+-channel activity of the guard cell plasma membrane in intact poplar leaves. In contrast to Arabidopsis, broad bean and tobacco grown under same conditions, poplar stomata operated just in the dynamic range - any change in conductance altered the rate of photosynthesis. In response to light, CO2 and abscisic acid (ABA), the stomatal opening velocity was two to five times faster than that measured for Arabidopsis thaliana, Nicotiana tabacum and Vicia faba. When stomata opened, the K+ content of guard cells increased almost twofold, indicating that the very fast stomatal opening in this species is mediated via potassium uptake. Following impalement of single guard cells embedded in their natural environment of intact leaves with triple-barrelled microelectrodes, time-dependent inward and outward-rectifying K+-channel-mediated currents of large amplitude were recorded. To analyse the molecular nature of genes encoding guard cell K+-uptake channels, we cloned K+-transporter Populustremula (KPT)1 and functionally expressed this potassium channel in a K+-uptake-deficient Escherichia coli mutant. In addition to guard cells, this K+-transporter gene was expressed in buds, where the KPT1 gene activity strongly correlated with bud break. Thus, KPT1 represents one of only few poplar genes associated with bud flush.

Regulation of K+ channel activities in plants: from physiological to molecular aspects

Plant voltage-gated channels belonging to the Shaker family participate in sustained K+ transport processes at the cell and whole plant levels, such as K+ uptake from the soil solution, long-distance K+ transport in the xylem and phloem, and K+ fluxes in guard cells during stomatal movements. The attention here is focused on the regulation of these transport systems by protein-protein interactions. Clues to the identity of the regulatory mechanisms have been provided by electrophysiological approaches in planta or in heterologous systems, and through analogies with their animal counterparts. It has been shown that, like their animal homologues, plant voltage-gated channels can assemble as homo- or heterotetramers associating polypeptides encoded by different Shaker genes, and that they can bind auxiliary subunits homologous to those identified in mammals. Furthermore, several regulatory processes (involving, for example, protein kinases and phosphatases, G proteins, 14-3-3s, or syntaxins) might be common to plant and animal Shakers. However, the molecular identification of plant channel partners is still at its beginning. This paper reviews current knowledge on plant K+ channel regulation at the physiological and molecular levels, in the light of the corresponding knowledge in animal cells, and discusses perspectives for the deciphering of regulatory networks in the future.

Regulation of the ABA-sensitive Arabidopsis potassium channel gene GORK in response to water stress

The phytohormone abscisic acid (ABA) regulates many stress-related processes in plants. In this context ABA mediates the responsiveness of plants to environmental stresses such as drought, cold or salt. In response to water stress, ABA induces stomatal closure by activating Ca2+, K+ and anion channels in guard cells. To understand the signalling pathways that regulate these turgor control elements, we studied the transcriptional control of the K+ release channel gene GORK that is expressed in guard cells, roots and vascular tissue. GORK transcription was up-regulated upon onset of drought, salt stress and cold. The wilting hormone ABA that integrates responses to these stimuli induced GORK expression in seedlings in a time- and concentration-dependent manner and this induction was dependent on extracellular Ca2+. ABA-responsive expression of GORK was impaired in the ABA-insensitive mutants abi1-1 and abi2-1, indicating that these protein phosphatases are regulators of GORK expression. Application of ABA to suspension-cultured cells for 2 min followed by a 4 h chase was sufficient to manifest transcriptional activation of the K+ channel gene. As predicted for a process involved in drought adaptation, only 12-24 h after the release of the stress hormone, GORK mRNA slowly decreased. In contrast to other tissues, GORK expression as well as K+(out) channel activity in guard cells is ABA insensitive, allowing the plant to adjust stomatal movement and water status control separately.

Isolation of AtSUC2 promoter-GFP-marked companion cells for patch-clamp studies and expression profiling

K+ channels control K+ homeostasis and the membrane potential in the sieve element/companion cell complexes. K+ channels from Arabidopsis phloem cells expressing green fluorescent protein (GFP) under the control of the AtSUC2 promoter were analysed using the patch-clamp technique and quantitative RT-PCR. Single green fluorescent protoplasts were selected after being isolated enzymatically from vascular strands of rosette leaves. Companion cell protoplasts, which could be recognized by their nucleus, vacuole and chloroplasts, and by their expression of the phloem-specific marker genes SUC2 and AHA3, formed the basis for a cell-specific cDNA library and expressed sequence tag (EST) collection. Although we used primers for all members of the Shaker K+ channel family, we identified only AKT2, KAT1 and KCO6 transcripts. In addition, we also detected transcripts for AtPP2CA, a protein phosphatase, that interacts with AKT2/3. In line with the presence of the K+ channel transcripts, patch-clamp experiments identified distinct K+ channel types. Time-dependent inward rectifying K+ currents were activated upon hyperpolarization and were characterized by a pronounced Ca2+-sensitivity and inhibition by protons. Whole-cell inward currents were carried by single K+-selective channels with a unitary conductance of approximately 4 pS. Outward rectifying K+ channels (approximately 19 pS), with sigmoidal activation kinetics, were elicited upon depolarization. These two dominant phloem K+ channel types provide a versatile mechanism to mediate K+ fluxes required for phloem action and potassium cycling.

Tumour development in Arabidopsis thaliana involves the Shaker-like K+ channels AKT1 and AKT2/3

After completion of the Arabidopsis genome-sequencing programme, crown galls induced by Agrobacterium tumefaciens may become a model system to study plant tumour development. The molecular mechanisms of nutrient supply to support tumour growth and development are still unknown. In this study, we have identified a unique profile of Shaker-like potassium channels in agrobacteria-induced Arabidopsis tumours. Comparing the gene expression pattern of rapidly growing tumours with that of non-infected tissues, we found the suppression of shoot in favour of root-specific K+ channels. Among these, the upregulation of AKT1 and AtKC1 and the suppression of AKT2/3 and GORK were most pronounced. As a consequence, K+ uptake and accumulation were elevated in the tumour (163 mm) compared to control tissues (92 mm). Patch clamp studies on tumour protoplasts identified a population expressing the electrical properties of the AKT1 K+ channel. Furthermore, plants lacking a functional AKT1 or the AKT2/3 phloem K+ channel gene did not support tumour growth. This indicates that the delivery of potassium by AKT1 and the direction of assimilates, triggered by AKT2/3, are essential for tumour growth.

The K+ channel KZM1 mediates potassium uptake into the phloem and guard cells of the C4 grass Zea mays

In search of K(+) channel genes expressed in the leaf of the C(4) plant Zea mays, we isolated the cDNA of KZM1 (for K(+) channel Zea mays 1). KZM1 showed highest similarity to the Arabidopsis K(+) channels KAT1 and KAT2, which are localized in guard cells and phloem. When expressed in Xenopus oocytes, KZM1 exhibited the characteristic features of an inward-rectifying, potassium-selective channel. In contrast to KAT1- and KAT2-type K(+) channels, however, KZM1 currents were insensitive to external pH changes. Northern blot analyses identified the leaf, nodes, and silks as sites of KZM1 expression. Following the separation of maize leaves into epidermal, mesophyll, and vascular fractions, quantitative real-time reverse transcriptase-PCR allowed us to localize KZM1 transcripts predominantly in vascular strands and the epidermis. Cell tissue separation and KZM1 localization were followed with marker genes such as the bundle sheath-specific ribulose-1,5-bisphosphate carboxylase, the phloem K(+) channel ZMK2, and the putative sucrose transporter ZmSUT1. When expressed in Xenopus oocytes, ZmSUT1 mediated proton-coupled sucrose symport. Coexpression of ZmSUT1 with the phloem K(+) channels KZM1 and ZMK2 revealed that ZMK2 is able to stabilize the membrane potential during phloem loading/unloading processes and KZM1 to mediate K(+) uptake. During leaf development, sink-source transitions, and diurnal changes, KZM1 is constitutively expressed, pointing to a housekeeping function of this channel in K(+) homeostasis of the maize leaf. Therefore, the voltage-dependent K(+)-uptake channel KZM1 seems to mediate K(+) retrieval and K(+) loading into the phloem as well as K(+)-dependent stomatal opening.