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

The Arabidopsis outward K+ channel GORK is involved in regulation of stomatal movements and plant transpiration

Microscopic pores present in the epidermis of plant aerial organs, called stomata, allow gas exchanges between the inner photosynthetic tissue and the atmosphere. Regulation of stomatal aperture, preventing excess transpirational vapor loss, relies on turgor changes of two highly differentiated epidermal cells surrounding the pore, the guard cells. Increased guard cell turgor due to increased solute accumulation results in stomatal opening, whereas decreased guard cell turgor due to decreased solute accumulation results in stomatal closing. Here we provide direct evidence, based on reverse genetics approaches, that the Arabidopsis GORK Shaker gene encodes the major voltage-gated outwardly rectifying K(+) channel of the guard cell membrane. Expression of GORK dominant negative mutant polypeptides in transgenic Arabidopsis was found to strongly reduce outwardly rectifying K(+) channel activity in the guard cell membrane, and disruption of the GORK gene (T-DNA insertion knockout mutant) fully suppressed this activity. Bioassays on epidermal peels revealed that disruption of GORK activity resulted in impaired stomatal closure in response to darkness or the stress hormone abscisic acid [corrected]. Transpiration measurements on excised rosettes and intact plants (grown in hydroponic conditions or submitted to water stress) revealed that absence of GORK activity resulted in increased water consumption. The whole set of data indicates that GORK is likely to play a crucial role in adaptation to drought in fluctuating environments.

Molecular dissection of the contribution of negatively and positively charged residues in S2, S3, and S4 to the final membrane topology of the voltage sensor in the K+ channel, KAT1

Voltage-dependent ion channels control changes in ion permeability in response to membrane potential changes. The voltage sensor in channel proteins consists of the highly positively charged segment, S4, and the negatively charged segments, S2 and S3. The process involved in the integration of the protein into the membrane remains to be elucidated. In this study, we used in vitro translation and translocation experiments to evaluate interactions between residues in the voltage sensor of a hyperpolarization-activated potassium channel, KAT1, and their effect on the final topology in the endoplasmic reticulum (ER) membrane. A D95V mutation in S2 showed less S3-S4 integration into the membrane, whereas a D105V mutation allowed S4 to be released into the ER lumen. These results indicate that Asp(95) assists in the membrane insertion of S3-S4 and that Asp(105) helps in preventing S4 from being releasing into the ER lumen. The charge reversal mutation, R171D, in S4 rescued the D105R mutation and prevented S4 release into the ER lumen. A series of constructs containing different C-terminal truncations of S4 showed that Arg(174) was required for correct integration of S3 and S4 into the membrane. Interactions between Asp(105) and Arg(171) and between negative residues in S2 or S3 and Arg(174) may be formed transiently during membrane integration. These data clarify the role of charged residues in S2, S3, and S4 and identify posttranslational electrostatic interactions between charged residues that are required to achieve the correct voltage sensor topology in the ER membrane.

Structure and function of potassium channels in plants: some inferences about the molecular origin of inward rectification in KAT1 channels (Review)

Potassium channels in plants play a variety of important physiological roles including K(+) uptake into roots, stomatal and leaf movements, and release of K(+) into the xylem. This review summarizes current knowledge about a class of plant genes whose products are K(+) channel-forming proteins. Potassium channels of this class belong to a superfamily characterized by six membrane-spanning domains (S1-6), a positively charged S4 domain and a region between the S5 and S6 segments that forms the channel selectivity filter. These channels are voltage dependent, which means the membrane potential modifies the probability of opening (P(o)). However, despite these channels sharing the same topology as the outward-rectifying K(+) channels, which are activated by membrane depolarization, some plant K(+) channels such as KAT1/2 and KST1 open with hyperpolarizing voltages. In outward-rectifying K(+) channels, the change in P(o) is achieved through a voltage sensor formed by the S4 segment that detects the voltage transferring its energy to the gate that controls pore opening. This coupling is achieved by an outward displacement of the charges contained in S4. In KAT1, most of the results indicate that S4 is the voltage sensor. However, how the movement of S4 leads to opening remains unanswered. On the basis of recent data, we propose here that in plant-inward rectifiers an inward movement of S4 leads to channel opening and that the difference between it and outward-rectifying channels resides in the mechanism that couples gating charge displacement with pore opening.

Molecular mechanisms and regulation of K+ transport in higher plants

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.

Poplar potassium transporters capable of controlling K+ homeostasis and K+-dependent xylogenesis

The cambial K+ content of poplar increases during the growth period in a K+ supply dependent manner. Upon K+ starvation or application of tetraethylammoniumchloride (TEA+), a K+ channel blocker, the average vessel lumen and expansion zone area were significantly reduced. In search for the molecular basis of potassium-dependent xylogenesis in poplar, K+ transporters homologous to those of known function in Arabidopis phloem- and xylem-physiology were isolated from a poplar wood EST library. The expression profile of three distinct K+ channel types and one K+ transporter, Populus tremula K+ uptake transporter 1 (PtKUP1), was analysed by quantitative RT-PCR. Thereby, we found P. tremula outward rectifying K+ channel (PTORK) and P. tremula K+ channel 2 (PTK2) correlated with the seasonal wood production. K+ transporter P. tremula 1 (KPT1) was predominantly found in guard cells. Following the heterologous expression in Xenopus oocytes the biophysical properties of the different channels were determined. PTORK, upon membrane de-polarization mediates potassium release. PTK2 is almost voltage independent, carrying inward K+ flux at hyperpolarized potential and K+ release upon de-polarization. PtKUP1 was expressed in a K+ uptake-deficient Escherichia coli strain, where this K+ transporter rescued K+-dependent growth. In order to link the different K+ transporters to the cambial activity and wood production, we compared the expression profiles to seasonal changes in the K+ content of the bark as well as xylem vessel diameter. Thereby, we found PTORK and PTK2 transcripts to follow the annual K+ variations in poplar branches. PtKUP1 was expressed at a low level throughout the year, suggesting a housekeeping function. From these data, we conclude that K+ channels are involved in the regulation of K+-dependent wood production.

Genes for calcium-permeable channels in the plasma membrane of plant root cells

In plant cells, Ca(2+) is required for both structural and biophysical roles. In addition, changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) orchestrate responses to developmental and environmental signals. In many instances, [Ca(2+)](cyt) is increased by Ca(2+) influx across the plasma membrane through ion channels. Although the electrophysiological and biochemical characteristics of Ca(2+)-permeable channels in the plasma membrane of plant cells are well known, genes encoding putative Ca(2+)-permeable channels have only recently been identified. By comparing the tissue expression patterns and electrophysiology of Ca(2+)-permeable channels in the plasma membrane of root cells with those of genes encoding candidate plasma membrane Ca(2+) channels, the genetic counterparts of specific Ca(2+)-permeable channels can be deduced. Sequence homologies and the physiology of transgenic antisense plants suggest that the Arabidopsis AtTPC1 gene encodes a depolarisation-activated Ca(2+) channel. Members of the annexin gene family are likely to encode hyperpolarisation-activated Ca(2+) channels, based on their corresponding occurrence in secretory or elongating root cells, their inhibition by La(3+) and nifedipine, and their increased activity as [Ca(2+)](cyt) is raised. Based on their electrophysiology and tissue expression patterns, AtSKOR encodes a depolarisation-activated outward-rectifying (Ca(2+)-permeable) K(+) channel (KORC) in stelar cells and AtGORK is likely to encode a KORC in the plasma membrane of other Arabidopsis root cells. Two candidate gene families, of cyclic-nucleotide gated channels (CNGC) and ionotropic glutamate receptor (GLR) homologues, are proposed as the genetic correlates of voltage-independent cation (VIC) channels.

Identification of K(+) channels in the plasma membrane of maize subsidiary cells

The stomatal complex of Zea mays consists of two guard cells with the pore in between them and two flanking subsidiary cells. Both guard cells and subsidiary cells are important elements for stoma physiology because a well-coordinated transmembrane shuttle transport of potassium and chloride ions occurs between these cells during stomatal movement. To shed light upon the corresponding transport systems from subsidiary cells, subsidiary cell protoplasts were enzymatically isolated and in turn, analyzed with the patch-clamp technique. Thereby, two K(+)-selective channel types were identified in the plasma membrane of subsidiary cells. With regard to their voltage-dependent gating behavior, they may act as hyperpolarization-dependent K(+) uptake and depolarization-activated K(+) release channels during stomatal movement. Interestingly, the K(+) channels from subsidiary cells and guard cells similarly responded to membrane voltage as well as to changes in the K(+) gradient. Further, the inward- and outward-rectifying K(+) current amplitude decreased upon a rise in the intracellular free Ca(2+) level from 2 nM to the micro M-range. The results indicate that the plasma membrane of subsidiary cells and guard cells has to be inversely polarized in order to achieve the anti-parallel direction of K(+) fluxes between these cell types during stomatal movement.

Cation channels in the Arabidopsis plasma membrane

In vivo analyses have identified different functional types of ion channels in various plant tissues and cells. The Arabidopsis genome contains approximately 70 genes for ion channels, of which 57 might be cation-selective channels (K(+), Ca(2+) or poorly discriminating channels). Here, we describe the different families of (putative) cation channels: the Shakers, the two-P-domain and Kir K(+) channels (encoded by the KCO genes), the cyclic-nucleotide-gated channels, the glutamate receptors, and the Ca(2+) channel TPC1. We also compare molecular data with the data obtained in planta, which should lead to a better understanding of the identity of these channels and provide clues about their roles in plant nutrition and cell signalling.

AtKC1, a silent Arabidopsis potassium channel alpha -subunit modulates root hair K+ influx

Ion channels in roots allow the plant to gain access to nutrients. The composition of the individual ion channels and the functional contribution of different alpha-subunits is largely unknown. Focusing on K(+)-selective ion channels, we have characterized AtKC1, a new alpha-subunit from the Arabidopsis shaker-like ion channel family. Promoter-beta-glucuronidase (GUS) studies identified AtKC1 expression predominantly in root hairs and root endodermis. Specific antibodies recognized AtKC1 at the plasma membrane. To analyze further the abundance and the functional contribution of the different K(+) channels alpha-subunits in root cells, we performed real-time reverse transcription-PCR and patch-clamp experiments on isolated root hair protoplasts. Studying all shaker-like ion channel alpha-subunits, we only found the K(+) inward rectifier AtKC1 and AKT1 and the K(+) outward rectifier GORK to be expressed in this cell type. Akt1 knockout plants essentially lacked inward rectifying K(+) currents. In contrast, inward rectifying K(+) currents were present in AtKC1 knockout plants, but fundamentally altered with respect to gating and cation sensitivity. This indicates that the AtKC1 alpha-subunit represents an integral component of functional root hair K(+) uptake channels.

A grapevine gene encoding a guard cell K(+) channel displays developmental regulation in the grapevine berry

SIRK is a K(+) channel identified in grapevine (Vitis vinifera), belonging to the so-called Shaker family. The highest sequence similarities it shares with the members of this family are found with channels of the KAT type, although SIRK displays a small ankyrin domain. This atypical feature provides a key to understand the evolution of the plant Shaker family. Expression in Xenopus laevis oocytes indicated that SIRK is an inwardly rectifying channel displaying functional properties very similar to those of KAT2. The activity of SIRK promoter region fused to the GUS reporter gene was analyzed in both grapevine and Arabidopsis. Like other KAT-like channels, SIRK is expressed in guard cells. In Arabidopsis, the construct is also expressed in xylem parenchyma. Semiquantitative reverse transcriptase-polymerase chain reaction experiments indicated that SIRK transcript was present at low levels in the berry, during the first stages of berry growth. After veraison, the period of berry development that corresponds to the inception of ripening and that is associated with large biochemical and structural modifications, such as evolution of stomata in nonfunctional lenticels and degeneration of xylem vasculature, the transcript was no longer detected. The whole set of data suggests that in the berries SIRK is expressed in guard cells and, possibly, in xylem tissues. The encoded channel polypeptide could therefore play a role in the regulation of transpiration and water fluxes in grapevine fruits.

Pollen tube development and competitive ability are impaired by disruption of a Shaker K(+) channel in Arabidopsis

Sexual reproduction in plants requires elongation of the pollen tube through the transmitting tissues toward the ovary. Tube growth rate is a major determinant of pollen competitive ability. We report that a K(+) channel of the Shaker family in Arabidopsis, SPIK, plays an important role in pollen tube development. SPIK was found to be specifically expressed in pollen. When SPIK was heterologously expressed in COS cells, its product formed hyperpolarization-activated K(+) channels. Disruption (T-DNA insertion) of the SPIK coding sequence strongly affected inwardly rectifying K(+)-channel activity in the pollen-grain plasma membrane. Measurements of membrane potential in growing pollen tubes yielded data compatible with a contribution of SPIK to K(+) influx. In vitro pollen germination assays were performed, revealing that the disruption results in impaired pollen tube growth. Analysis of the transmission rate of the disrupted allele in the progeny of heterozygous plants revealed a decrease in pollen competitive ability, the probability of fertilization by mutant pollen being approximately 1.6 times lower than that by wild-type pollen. The whole set of data supports the hypothesis that functional expression of SPIK plays a role in K(+) uptake in the growing pollen tube, and thereby in tube development and pollen competitive ability.

KCO1 is a component of the slow-vacuolar (SV) ion channel

The Arabidopsis double pore K+ channel KCO1 was fused to green fluorescent protein and expressed in tobacco protoplasts. Microscopic analysis revealed a bright green fluorescence at the vacuolar membrane. RT-PCR experiments showed that KCO1 is expressed in the mesophyll. Vacuoles from Arabidopsis wild-type and kco1 knockout plants were isolated for patch-clamp analyses. Currents mediated by slow-activating vacuolar (SV) channels of mesophyll cell vacuoles were significantly smaller in kco1 plants compared to the wild-type. This shows that KCO1 is involved in the formation of SV channels.

Nonselective cation channels in plants

Nonselective cation channels are a diverse group of ion channels characterized by their low discrimination between many essential and toxic cations. They are ubiquitous in plant tissues and are active in the plasma membrane, tonoplast, and other endomembranes. Members of this group are likely to function in low-affinity nutrient uptake, in distribution of cations within and between cells, and as plant Ca2+ channels. They are gated by diverse mechanisms, which can include voltage, cyclic nucleotides, glutamate, reactive oxygen species, and stretch. These channels dominate tonoplast cation transport, and the selectivity and gating mechanisms of tonoplast nonselective cation channels are comprehensively reviewed here. This review presents the first classification of plant nonselective cation channels and the first full description of nonselective cation channel candidate sequences in the Arabidopsis genome.

K(+) channel profile and electrical properties of Arabidopsis root hairs

Ion channels and solute transporters in the plasma membrane of root hairs are proposed to control nutrient uptake, osmoregulation and polar growth. Here we analyzed the molecular components of potassium transport in Arabidopsis root hairs by combining K(+)-selective electrodes, reverse transcription-PCR, and patch-clamp measurements. The two inward rectifiers AKT1 and ATKC1 as well as the outward rectifier GORK dominated the root hair K(+) channel pool. Root hairs of AKT1 and ATKC1 loss-of-function plants completely lack the K(+) uptake channel or exhibited altered properties, respectively. Upon oligochitin-elicitor treatment of root hairs, transient changes in K(+) fluxes and membrane polarization were recorded in wild-type plants, while akt1-1 root hairs showed a reduced amplitude and pronounced delay in the potassium re-uptake process. This indicates that AKT1 and ATKC1 represent essential alpha-subunits of the inward rectifier. Green fluorescent protein (GFP) fluorescence following ballistic bombardment with GORK promoter-GFP constructs as well as analysis of promoter-GUS lines identified this K(+) outward rectifier as a novel ion channel expressed in root hairs. Based on the expression profile and the electrical properties of the root hair plasma membrane we conclude that AKT1-, ATKC- and GORK-mediated potassium transport is essential for osmoregulation and repolarization of the membrane potential in response to elicitors.

From milliseconds to millions of years: guard cells and environmental responses

During the past year, significant advances have been made in our understanding of stomatal development and its response to climate change, and in our knowledge of how guard cell Ca(2+) oscillations encode environmental signals. Recent studies on (de)phosphorylation mechanisms have provided new information on how guard cells respond to abscisic acid and blue light.

The role of ion channels in light-dependent stomatal opening

Stomatal opening represents a major determinant of plant productivity and stress management. Because plants lose water essentially through open stomata, volume control of the pore-forming guard cells represents a key step in the regulation of plant water status. These sensory cells are able to integrate various signals such as light, auxin, abscisic acid, and CO(2). Following signal perception, changes in membrane potential and activity of ion transporters finally lead to the accumulation of potassium salts and turgor pressure formation. This review analyses recent progress in molecular aspects of ion channel regulation and suggests how these developments impact on our understanding of light- and auxin-dependent stomatal action.

A plant Shaker-like K+ channel switches between two distinct gating modes resulting in either inward-rectifying or "leak" current

Among the Shaker-like plant potassium channels, AKT2 is remarkable because it mediates both instantaneous "leak-like" and time-dependent hyperpolarisation-activated currents. This unique gating behaviour has been analysed in Xenopus oocytes and in COS and Chinese hamster ovary cells. Whole-cell and single-channel data show that (i) AKT2 channels display two distinct gating modes, (ii) the gating of a given AKT2 channel can change from one mode to the other and (iii) this conversion is under the control of post-translational factor(s). This behaviour is strongly reminiscent of that of the KCNK2 channel, recently reported to be controlled by its phosphorylation state.

VFK1, a Vicia faba K(+) channel involved in phloem unloading

In search of a K(+) channel involved in phloem transport we screened a Vicia faba cotyledon cDNA library taking advantage of a set of degenerated primers, flanking regions conserved among K(+) uptake channels. We cloned VFK1 (for Vicia faba K(+) channel 1) characterised by a structure known from the Shaker family of plant K(+) channels. When co-expressed with a KAT1 mutant in Xenopus oocytes, heteromers revealed the biophysical properties of a K(+) selective, proton-blocked channel. Northern blot analyses showed high levels of expression in cotyledons, flowers, stem and leaves. Using in situ PCR techniques we could localise the K(+) channel mRNA in the phloem. In the stem VFK1 expression levels were higher in the lower internodes. There channel transcripts increased in the light and thus under conditions of increased photosynthate allocation. VFK1 transcripts are elevated in sink leaves, and rise in source leaves during the experimental transition into sinks. Fructose- rather than sucrose- or glucose-feeding via the petiole induced VFK1 gene activity. We therefore monitored the fructose sensitivity of the sieve tube potential through cut aphid stylets. In response to an 1 h fructose treatment the sieve tube potential shift increased from 19 mV to 53 mV per 10-fold change in K(+) concentration. Under these conditions K(+) channels dominated the electrical properties of the plasma membrane. Based on the phloem localisation and expression patterns of VFK1 we conclude that this K(+) channel is involved in sugar unloading and K(+) retrieval.

Escherichia coli as an expression system for K(+) transport systems from plants

The value of the Escherichia coli expression system has long been established because of its effectiveness in characterizing the structure and function of exogenously expressed proteins. When eukaryotic membrane proteins are functionally expressed in E. coli, this organism can serve as an alternative to eukaryotic host cells. A few examples have been reported of functional expression of animal and plant membrane proteins in E. coli. This mini-review describes the following findings: 1) homologous K(+) transporters exist in prokaryotic cells and in eukaryotic cells; 2) plant K(+) transporters can functionally complement mutant K(+) transporter genes in E. coli; and 3) membrane structures of plant K(+) transporters can be elucidated in an E. coli system. These experimental findings suggest the possibility of utilizing the E. coli bacterium as an expression system for other eukaryotic membrane transport proteins.

Phylogenetic relationships within cation transporter families of Arabidopsis

Uptake and translocation of cationic nutrients play essential roles in physiological processes including plant growth, nutrition, signal transduction, and development. Approximately 5% of the Arabidopsis genome appears to encode membrane transport proteins. These proteins are classified in 46 unique families containing approximately 880 members. In addition, several hundred putative transporters have not yet been assigned to families. In this paper, we have analyzed the phylogenetic relationships of over 150 cation transport proteins. This analysis has focused on cation transporter gene families for which initial characterizations have been achieved for individual members, including potassium transporters and channels, sodium transporters, calcium antiporters, cyclic nucleotide-gated channels, cation diffusion facilitator proteins, natural resistance-associated macrophage proteins (NRAMP), and Zn-regulated transporter Fe-regulated transporter-like proteins. Phylogenetic trees of each family define the evolutionary relationships of the members to each other. These families contain numerous members, indicating diverse functions in vivo. Closely related isoforms and separate subfamilies exist within many of these gene families, indicating possible redundancies and specialized functions. To facilitate their further study, the PlantsT database (http://plantst.sdsc.edu) has been created that includes alignments of the analyzed cation transporters and their chromosomal locations.

GUARD CELL SIGNAL TRANSDUCTION

Guard cells surround stomatal pores in the epidermis of plant leaves and stems. Stomatal pore opening is essential for CO2 influx into leaves for photosynthetic carbon fixation. In exchange, plants lose over 95% of their water via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate hormonal stimuli, light signals, water status, CO2, temperature, and other environmental conditions to modulate stomatal apertures for regulation of gas exchange and plant survival under diverse conditions. Stomatal guard cells have become a highly developed model system for characterizing early signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. In this review we focus on recent advances in understanding signal transduction mechanisms in guard cells.

Single guard cell recordings in intact plants: light-induced hyperpolarization of the plasma membrane

Guard cells are electrically isolated from other plant cells and therefore offer the unique possibility to conduct current- and voltage-clamp recordings on single cells in an intact plant. Guard cells in their natural environment were impaled with double-barreled electrodes and found to exhibit three physiological states. A minority of cells were classified as far-depolarized cells. These cells exhibited positive membrane potentials and were dominated by the activity of voltage-dependent anion channels. All other cells displayed both outward and inward rectifying K+-channel activity. These cells were either depolarized or hyperpolarized, with average membrane potentials of -41 mV (SD 16) and -112 mV (SD 19), respectively. Depolarized guard cells extrude K+ through outward rectifying channels, while K+ is taken up via inward rectifying channels in hyperpolarized cells. Upon a light/dark transition, guard cells that were hyperpolarized in the light switched to the depolarized state. The depolarization was accompanied by a 35 pA decrease in pump current and an increase in the conductance of inward rectifying channels. Both an increase in pump current and a decrease in the conductance of the inward rectifier were triggered by blue light, while red light was ineffective. From these studies we conclude that light modulates plasma membrane transport through large membrane potential changes, reversing the K+-efflux via outward rectifying channels to a K+-influx via inward rectifying channels.

KAT1 is not essential for stomatal opening

It is generally accepted that K(+) uptake into guard cells via inward-rectifying K(+) channels is required for stomatal opening. To test whether the guard cell K(+) channel KAT1 is essential for stomatal opening, a knockout mutant, KAT1En-1, was isolated from an En-1 mutagenized Arabidopsis thaliana population. Stomatal action and K(+) uptake, however, were not impaired in KAT1-deficient plants. Reverse transcription-PCR experiments with isolated guard cell protoplasts showed that in addition to KAT1, the K(+) channels AKT1, AKT2/3, AtKC1, and KAT2 were expressed in this cell type. In impalement measurements, intact guard cells exhibited inward-rectifying K(+) currents across the plasma membrane of both wild-type and KAT1En-1 plants. This study demonstrates that multiple K(+) channel transcripts exist in guard cells and that KAT1 is not essential for stomatal action.