Properties of Single K and Cl Channels in Asclepias tuberosa Protoplasts

The essential role of anionic transport in plant cells: the pollen tube as a case study

Plasma membrane anion transporters play fundamental roles in plant cell biology, especially in stomatal closure and nutrition. Notwithstanding, a lot is still unknown about the specific function of these transporters, their specific localization, or molecular nature. Here the fundamental roles of anionic transport in plant cells are reviewed. Special attention will be paid to them in the control of pollen tube growth. Pollen tubes are extreme examples of cellular polarity as they grow exclusively in their apical extremity. Their unique cell biology has been extensively exploited for fundamental understanding of cellular growth and morphogenesis. Non-invasive methods have demonstrated that tube growth is governed by different ion fluxes, with different properties and distribution. Not much is known about the nature of the membrane transporters responsible for anionic transport and their regulation in the pollen tube. Recent data indicate the importance of chloride (Cl(-)) transfer across the plasma membrane for pollen germination and pollen tube growth. A general overview is presented of the well-known accumulated data in terms of biophysical and functional characterization, transcriptomics, and genomic description of pollen ionic transport, and the various controversies around the role of anionic fluxes during pollen tube germination, growth, and development. It is concluded that, like all other plant cells so far analysed, pollen tubes depend on anion fluxes for a number of fundamental homeostatic properties.

pH regulation in an acidophilic green alga - a quantitative analysis

Short-term cytosolic pH regulation has three components: H(+) binding by buffering groups; H(+) transport out of the cytosol; and H(+) transport into the vacuole. To understand the large differences plants show in their tolerance to acidic environments, these three components were quantified in the acidophilic unicellular green alga Eremosphaera viridis. Intracellular pH was recorded using ion-selective microelectrodes, whereas constant doses of weak acid were applied over different time intervals. A mathematical model was developed that describes the recorded cytosolic pH changes. Recordings of cytosolic K(+) and Na(+) activities, and application of anion channel inhibitors, revealed which ion fluxes electrically compensate H(+) transport. The cytosolic buffer capacity was beta = 30 mM. Acidification resulted in a substantial and constant H(+) efflux that was probably driven by the plasmalemma H(+)-ATPase, and a proportional pH regulation caused by H(+) pumped into the vacuole. Under severe cytosolic acidification (> or = 1 pH) more than 50% of the ATP synthesized was used for H(+) pumping. While H(+) influx into the vacuole was compensated by cation release, H(+) efflux out of the cell was compensated by anion efflux. The data presented here give a complete and quantitative picture of the ion fluxes during acid loading in an acidophilic green plant cell.

Plasma membrane anion channels in higher plants and their putative functions in roots

Recent years have seen considerable progress in identifying anion channel activities in higher plant cells. This review outlines the functional properties of plasma membrane anion channels in plant cells and discusses their likely roles in root function. Plant anion channels can be grouped according to their voltage dependence and kinetics: (1) depolarization-activated anion channels which mediate either anion efflux (R and S types) or anion influx (outwardly rectifying type); (2) hyperpolarization-activated anion channels which mediate anion efflux, and (3) anion channels activated by light or membrane stretch. These types of anion channel are apparent in root cells where they may function in anion homeostasis, membrane stabilization, osmoregulation, boron tolerance and regulation of passive salt loading into the xylem vessels. In addition, roots possess anion channels exhibiting unique properties which are consistent with them having specialized functions in root physiology. Most notable are the organic anion selective channels, which are regulated by extracellular Al3+ or the phosphate status of the plant. Finally, although the molecular identities of plant anion channels remain elusive, the diverse electrophysiological properties of plant anion channels suggest that large and diverse multigene families probably encode these channels.

The delivery of salts to the xylem. Three types of anion conductance in the plasmalemma of the xylem parenchyma of roots of barley

To explore possible pathways for anions to enter the xylem in the root during the transport of salts to the shoot, we used the patch-clamp method on protoplasts prepared from the xylem parenchyma of barley (Hordeum vulgare L.) plants. K(+) currents were suppressed by tetraethylammonium or N-methylglucamine in the solutions in the pipette and the bath, and the permeating anions were Cl(-) or NO(3)(-). We recorded the activities of three distinct anion conductances: (a) an inwardly rectifying anion channel (X-IRAC), characterized by activation at hyperpolarization and open times of up to several seconds; (b) a quickly activating anion conductance (X-QUAC), important for anion efflux at voltages between -50 mV and the equilibrium potential of the prevailing anion; and (c) a slowly activating anion conductance (X-SLAC), activating above -100 mV. Both X-IRAC and X-QUAC were permeable for Cl(-) and NO(3)(-); X-QUAC was also permeable for malate. The occurrence of X-IRAC became more frequent with an increase in cytoplasmic Ca(2+), while the occurrence of X-QUAC decreased. Anion currents through X-SLAC, and particularly through X-QUAC, were estimated to be large enough to account for reported rates of xylem loading, which is in accordance with the notion that xylem loading is a passive process.

Evaluation of functional interaction between K(+) channel alpha- and beta-subunits and putative inactivation gating by Co-expression in Xenopus laevis oocytes

Animal K(+) channel alpha- (pore-forming) subunits form native proteins by association with beta-subunits, which are thought to affect channel function by modifying electrophysiological parameters of currents (often by inducing fast inactivation) or by stabilizing the protein complex. We evaluated the functional association of KAT1, a plant K(+) channel alpha-subunit, and KAB1 (a putative homolog of animal K(+) channel beta-subunits) by co-expression in Xenopus laevis oocytes. Oocytes expressing KAT1 displayed inward-rectifying, non-inactivating K(+) currents that were similar in magnitude to those reported in prior studies. K(+) currents recorded from oocytes expressing both KAT1 and KAB1 had similar gating kinetics. However, co-expression resulted in greater total current, consistent with the possibility that KAB1 is a beta-subunit that stabilizes and therefore enhances surface expression of K(+) channel protein complexes formed by alpha-subunits such as KAT1. K(+) channel protein complexes formed by alpha-subunits such as KAT1 that undergo (voltage-dependent) inactivation do so by means of a "ball and chain" mechanism; the ball portion of the protein complex (which can be formed by the N terminus of either an alpha- or beta-subunit) occludes the channel pore. KAT1 was co-expressed in oocytes with an animal K(+) channel alpha-subunit (hKv1.4) known to contain the N-terminal ball and chain. Inward currents through heteromeric hKv1. 4:KAT1 channels did undergo typical voltage-dependent inactivation. These results suggest that inward currents through K(+) channel proteins formed at least in part by KAT1 polypeptides are capable of inactivation, but the structural component facilitating inactivation is not present when channel complexes are formed by either KAT1 or KAB1 in the absence of additional subunits.

Calcium-activated, voltage-dependent, non-selective cation currents in endosperm plasma membrane from higher plants

Single-channel and whole-cell patch-clamp techniques were used to characterize the electrophysiological behaviour of plasma membranes from freshly isolated, non-enzyme-treated endosperm protoplasts. A non-selective monovalent cation channel with a single-channel conductance of 22 pS in solutions with physiological potassium concentrations was observed in inside-out patches. The channel passes outward current at depolarized potentials and is highly selective for cations over anions, but discriminates poorly between lithium, sodium, potassium, rubidium and caesium ions. Specific potassium channel blockers were ineffective. The channel kinetics were apparently complex, with burst-like openings and rapid closures within a single burst. Single-channel openings were more frequent both for depolarizing pulses and maintained positive potentials. Channel activity was also increased by elevated cytoplasmic concentrations of either calcium or barium. Subsequent exposure of patches to low calcium, EGTA-buffered solutions resulted in large decreases in activity. Under whole-cell current clamp, small negative resting potentials were observed. A slowly developing outward current evoked by depolarizing pulses was seen in whole-cell recordings.

ATP-Dependent Regulation of an Anion Channel at the Plasma Membrane of Protoplasts from Epidermal Cells of Arabidopsis Hypocotyls

Although Arabidopsis is the object of many genetic and molecular biology investigations, relatively few studies deal with regulation of its transmembrane ion exchanges. To clarify the role of ion transport in plant development, organ-and tissue-specific ion channels must be studied. We identified a voltage-dependent anion channel in epidermal cells of Arabidopsis hypocotyls, thus providing a new example of the occurrence of voltage-dependent anion channels in a specific plant cell type distinct from the stomatal guard cell. The Arabidopsis hypocotyl anion channel is able to function under two modes characterized by different voltage dependences and different kinetic behaviors. This switch between a fast and a slow mode is controlled by ATP. In the presence of intracellular ATP (fast mode), the channels are closed at resting potentials, and whole-cell currents activate upon depolarization. After activation, the anion current deactivates rapidly and more and more completely at potentials negative to the peak. In the absence of ATP, the current switches from this fast mode to a mode characterized by a slow and incomplete deactivation at resting potentials. In addition, the whole-cell currents can be correlated with the activity of single channels. In the outside-out configuration, the presence of ATP modulates the mean lifetimes of the open and closed states of the channel at hyperpolarized potentials, thus controlling its open probability. The fact that ATP-dependent voltage regulation was observed in both whole-cell and outside-out configurations suggests that a single type of anion channel can switch between two modes with distinct functional properties.

Evidence that plant K+ channel proteins have two different types of subunits

Plant K+ channel proteins have been previously characterized as tetramers of membrane-spanning alpha subunit polypeptides. Recent studies have identified a 39-kD, hydrophilic polypeptide that is a structural component of purified animal K+ channel proteins. We have cloned and sequenced an Arabidopsis thaliana cDNA encoding a 38.4-kD polypeptide that has a sequence homologous to the animal K+ channel beta subunit. Southern and northern analyses indicate the presence of a gene encoding this cDNA in the Arabidopsis genome and that its transcription product is present in Arabidopsis cells. To our knowledge, this is the first report to document the presence of K+ channel beta subunits in plants.

Ion channels in the plasma membrane of protoplasts from the halophytic angiosperm Zostera muelleri

Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halphytic angiosperm Zostera muelleri. In wholecell preparations, both outward and inward rectifying currents that activate in a time- and voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K+. The channels responsible for the outward currents have a unit conductance of approximately 70 pS and are five times more permeable to K+ than to Na+. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies with a conductance of 6 pS. The reversal potentials of these channels indicate a significant permeability to K+. In addition, the plasma membrane contains a much larger K+ channel with a conductance of 300 pS. Single channel recordings also indicate the existence of two Cl- channels, with conductances of 20 and 80 pS with distinct substates. The membrane potential difference of perfused protoplasts showed rapid action potentials of up to 50 mV from the resting level. The frequency of these action potentials increased as the external osmolarity was decreased. The action potentials disappeared with the addition of Gd3+, an effect that is reversible upon washout.

Characterization of ion channels in the plasma membrane of epidermal cells of expanding pea (Pisum sativum arg) leaves

Ion channels in isolated patches of the plasma membrane of pea (Pisum sativum arg) epidermal cells were studied with the patch-clamp technique. One anion and one cation channel were dominantly present in most trials. The anion channel conducts nitrate, halides and malate, with a conductance in symmetrical 100 mM Cl- of 300 pS and can be blocked by SITS when applied to the cytoplasmic side of the membrane. The cation channel poorly discriminates between potassium, sodium and lithium, is not blocked by either TEA or Ba2+, and has a conductance of 35 pS in symmetrical 100 mM K+. The open probability of the cation channel increases with increase of the Ca2+ concentration on the cytoplasmic side of the membrane from 0.1 to 1 microM. The possible role of these two channels in the physiology of epidermal cells is discussed.

Plasmalemmal, voltage-dependent ionic currents from excitable pulvinar motor cells of Mimosa pudica

Plasmalemmal ionic currents from excitable motor cells of the primary pulvinus of Mimosa pudica were investigated by patch-clamp techniques. In almost all of the enzymatically isolated protoplasts, a delayed rectifier potassium current was activated by depolarization, while no currents were detected upon hyperpolarization. This sustained outward current was reversibly blocked by Ba and TEA and serves to repolarize the membrane potential. Outward single channel currents that very likely underly the macroscopic outward potassium current had an elementary conductance of approximately 20 pS. In addition, in a few protoplasts held at hyperpolarized potentials, depolarization-activated transient inward currents were observed, and under current clamp, action potential-like responses were triggered by depolarizing current injections or by mechanical perturbations. The activation characteristics of both inward currents and spikes showed striking similarities compared to those of action potentials in situ.

Novel ion channels in the protists, Mougeotia and Saprolegnia, using sub-gigaseals

Protoplasts of the filamentous alga, Mougeotia, and the filamentous fungal oomycete, Saprolegnia ferax, exhibit two K+ ion channels (2-6 pA) using the patch-clamp technique when the seals are less than 1 G omega (about 100 M omega). The membrane potential of the protoplasts was near 0 mV as measured intracellularly with double-barreled micropipettes; thus, inward K+ flux is due solely to concentration differences. Although conductances are in the range expected for K+ channels, the activity at 0 mV is not seen in other organisms under gigaseal conditions. This paper draws attention to the usefulness of this subsidiary patch-clamp technique and the novel characteristics of ion channels in Mougeotia and Saprolegnia.

Localized Patch Clamping of Plasma Membrane of a Polarized Plant Cell : Laser Microsurgery of the Fucus spiralis Rhizoid Cell Wall

We used an ultraviolet laser to rupture a small region of cell wall of a polarized Fucus spiralis rhizoid cell and gained localized access to the plasma membrane at the growing apex. Careful control of cell turgor enabled a small portion of plasma membrane-bound cytoplasm to be exposed. Gigaohm seals allowing single-channel recordings were obtained with a high success rate using this method with conventional patch clamp techniques.

Two types of anion channel currents in guard cells with distinct voltage regulation

Transpirational water loss by plants is reduced by closing of stomatal pores in the leaf epidermis. Anion channels in the plasma membrane of guard cells may provide a key molecular mechanism for control of stomatal closing in leaves. However, central questions regarding the regulation, diversity, and function of anion channels in guard cells and other higher plant cells remain unanswered. We show here that two highly distinct types of depolarization-activated anion currents operate in the plasma membrane of Vicia faba guard cells. One described type of anion channel was activated rapidly within 50 ms by depolarization, inactivated during prolonged stimulation, and deactivated rapidly at hyperpolarized potentials (R-type anion current). The other depolarization-activated anion current showed extremely slow voltage-dependent activation and deactivation (S-type anion current) and lacked inactivation. The distinct voltage and time dependencies of R-type and S-type anion channels suggest that they may play a role during depolarization-associated signal transduction in higher plant cells and that these anion channels may contribute to different processes in the regulation of stomatal movements. In particular, the slow and sustained nature of S-type anion channel activation revealed here leads us to hypothesize that S-type anion channels may provide a central molecular mechanism for control of stomatal closing, which is accompanied by long-term anion efflux and depolarization.

Potassium channels from the plasma membrane of rye roots characterized following incorporation into planar lipid bilayers

Plasma membrane was purified from roots of rye (Secale cereale L. cv. Rheidol) by aqueous-polymer two-phase partitioning and incorporated into planar bilayers of 1-palmitoyl-2-oleoyl phosphatidylethanolamine by stirring with an osmotic gradient. Since plasmamembrane vesicles were predominantly oriented with their cytoplasmic face internal, when fused to the bilayer the cytoplasmic side of channels faced the trans chamber. In asymmetrical (cis:trans) 280∶100 mM KCl, five distinct K(+)-selective channels were detected with mean chord-conductances (between +30 and -30 mV; volyages cis with respect to trans) of 500 pS, 194 pS, 49 pS, 21 pS and 10 pS. The frequencies of incorporation of these K(+) channels into the bilayer were 48, 21, 50, 10 and 9%, in the order given (data from 159 bilayers). Only the 49 pS channel was characterized further in this paper, but the remarkable diversity of K(+) channels found in this preparation is noteworthy and is the subject of further study. In symmetrical KCl solutions, the 49 pS channel exhibited non-ohmic unitary-current/voltage relationships. The chord-conductance (between +30 and-30 mV) of the channel in symmetrical 100 mM KCl was 39 pS. The unitary current was greater at positive voltages than at corresponding negative voltages and showed considerable rectification with increasing positive and negative voltages. This would represent 'inward rectification' in vivo. Gating of the channel was not voltage-dependent and the channel was open for approx. 80% of the time. Presumably this is not the case in vivo, but we are at present uncertain of the in vivo controls of channel gating. The distribution of channel-open times could be approximated by the sum of two negative exponential functions, yielding two open-state time constants (τo, the apparent mean lifetime of the channel-open state) of 1.0 ms and 5.7 s. The distribution of channel-closed times was best approximated by the sum of three negative exponential functions, yielding time constants (τc, the apparent mean lifetime of the channel-closed state) of 1.1 ms, 51 ms and 11 s. This indicates at least a five-state kinetic model for the activity of the channel. The selectivity of the 49 pS channel, determined from both reversal potentials under biionic conditions (100 mM KCl∶100 mM cation chloride) and from conductance measurements in symmetrical 100 mM cation chloride, was Rb(+)≥ K(+) > Cs(+) > Na(+) > Li(+) > tetraethylammonium (TEA(+)). The 49 pS channel was reversibly inhibited by quinine (1 mM) but TEA(+) (10 mM), Ba(2+) (3 mM), Ca(2+) (1 mM), 4-aminopyridine (1 mM) and charybdotoxin (3 μM) were without effect when applied to the extracellular (cis) surface.

Patch clamping protoplasts from vascular plants : method for the quick isolation of protoplasts having a high success rate of gigaseal formation

A method is described for the isolation of protoplasts (Pisum sativum, Phaseolus vulgaris, Avena sativa, Arabidopsis thaliana) in preparation for ion flux studies using patch clamp electrophysiology. Protoplasts that have been exposed to hydrolytic, cell wall degrading, enzymes for as little as 5 minutes form gigaseals (seal resistance higher than 10 giga Ohm) with the patch pipette with success rates greater than 40%. Sealing of these protoplasts is fast, averaging less than 2 minutes. This method yields high rates of gigaseal formation in a variety of tissues from both monocots and dicots and will enhance data collection in ion flux studies of plasma membranes of vascular plants.

Inward Rectifying K Channels in the Plasma Membrane of Arabidopsis thaliana

Ion channels in the plasma membrane of protoplasts isolated from cultured cells of Arabidopsis thaliana were studied by means of the patch-clamp technique applied in the whole-cell configuration. In some protoplasts, depolarizing pulses and, in other protoplasts, hyperpolarizing pulses elicited time-dependent currents; both kinds of current were only rarely observed in the same protoplast. The hyperpolarization-activated inward rectifying currents, the focus of this paper, appeared to be due to the relatively slow opening of channels (activation time constant = 150 to 300 milliseconds), which closed at positive potentials. The reversal potential of this current, measured in the presence of different ion concentrations (symmetrical or asymmetrical K(+) and Cl(-) or gluconate), was always close to the electrochemical equilibrium potential of K(+). The currents were inhibited by 10 millimolar tetraethylammonium, a K(+) channel blocker. These data show that the hyperpolarization-activated currents flow through K(+) channels, which can provide a pathway for the passive diffusion of K(+) down its electrochemical gradient.

The k/na selectivity of a cation channel in the plasma membrane of root cells does not differ in salt-tolerant and salt-sensitive wheat species

The characteristics of cation outward rectifier channels were studied in protoplasts from wheat root (Triticum aestivum L. and Triticum turgidum L.) cells using the patch clamp technique. The cation outward rectifier channels were voltage-dependent with a single channel conductance of 32 +/- 1 picosiemens in 100 millimolar KCl. Whole-cell currents were dominated by the activity of the cation outward rectifiers. The time- and voltage-dependence of these currents was accounted for by the summed behavior of individual channels recorded from outside-out detached patches. The K(+)/Na(+) permeability ratio of these channels was measured in a salt-sensitive and salt-tolerant genotype of wheat that differ in rates of Na(+) accumulation, using a voltage ramp protocol on protoplasts in the whole-cell configuration. Permeability ratios were calculated from shifts in reversal potentials following ion substitutions. There were no significant differences in the K(+)/Na(+) permeability ratios of these channels in root cells from either of the two genotypes tested. The permeability ratio for K(+)/Cl(-) was greater than 50:1. The K(+)/Na(+) permeability ratio averaged 30:1, which is two to four times more selective than the same type of channel in guard cells and suspension culture cells. Lowering the Ca(2+) concentration in the bath solution to 0.1 millimolar in the presence of 100 millimolar Na(+) had no significant effect on the K(+)/Na(+) permeability ratios of the channel. It seems unlikely that the mechanism of salt tolerance in wheat is based on differences in the K(+)/Na(+) selectivity of these channels.

Ion channels in the plasma membrane of Amaranthus protoplasts: one cation and one anion channel dominate the conductance

This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons of Amaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+ greater than Na+ much greater than Cl- (1:0.16:0.03): whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of subconductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (Vm) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2 mM) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO3- greater than Cl- greater than K+ greater than Aspartate (2.04:1:0.18 to 0.09:0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clamp Vm at a preferred value in the face of events which would otherwise perturb Vm.

Whole-cell and single-channel currents across the plasmalemma of corn shoot suspension cells

Whole-cell sealed-on pipettes have been used to measure electrical properties of the plasmalemma surrounding protoplasts isolated from Black Mexican sweet corn shoot cells from suspension culture. In these protoplasts the membrane resting potential (Vm) was found to be -59 +/- 23 mV (n = 23) in 1 mM Ko+. The mean Vm became more negative as [K+]o decreased, but was more positive than the K+ equilibrium potential. There was no evidence of electrogenic pump activity. We describe four features of the current-voltage characteristic of the plasmalemma of these protoplasts which show voltage-gated channel activity. Depolarization of the whole-cell membrane from the resting potential activates time- and voltage-dependent outward current through K(+)-selective channels. A local minimum in the outward current-voltage curve near Vm = 150 mV suggests that these currents are mediated by two populations of K(+)-selective channels. The absence of this minimum in the presence of verapamil suggests that the activation of one channel population depends on the influx of Ca2+ into the cytoplasm. We identify unitary currents from two K(+)-selective channel populations (40 and 125 pS) which open when the membrane is depolarized; it is possible that these mediate the outward whole-cell current. Hyperpolarization of the membrane from the resting potential produces time- and voltage-dependent inward whole-cell current. Current activation is fast and follows an exponential time course. The current saturates and in some cases decreases at membrane potentials more negative than -175 mV. This current is conducted by poorly selective K+ channels, where PCl/PK = 0.43 +/- 0.15. We describe a low conductance (20 pS) channel population of unknown selectivity which opens when the membrane is hyperpolarized. It is possible that these channels mediate inward whole-cell current. When the membrane is hyperpolarized to potentials more negative than -250 mV large, irregular inward current is activated. A third type of inward whole-cell current is briefly described. This activates slowly and with a U-shaped current-voltage curve over the range of membrane potentials -90 less than Vm less than 0 mV.

Substrate regulation of single potassium and chloride ion channels in Arabidopsis plasma membrane

Patch clamp measurements of excised inside-out patches of Arabidopsis thaliana plasma membrane reveal at least two ion channels which conduct either potassium or chloride. The conductance of the potassium channel ranged from 5 to 70 picosiemens depending on KCl concentration. The conductance increased linearly with increasing cytoplasmic-side [KCl]; the extent of this dependence declined as extracytoplasmic-side [KCl] was increased. This indicates that substrate regulation of the potassium channel is a consequence of the molecular architecture of the channel: in particular, multi-ion binding sites within the channel pore. The chloride channel conductance (ranging from 5-40 picosiemens) was independent of cytoplasmic-side [KCl] until a threshold concentration of about 300 millimolar was reached. Such behavior is expected only if the channel is allosterically regulated by cytoplasmic-side K(+) and/or Cl(-). The median open times of either channel (about 200 milliseconds for the potassium channel and 20 milliseconds for the chloride channel) were unaffected by substrate concentrations.

Pharmacology of the Ca2(+)-dependent K+ channel in corn protoplasts

We investigated the sensitivity of the Ca2(+)-dependent K+ current, IK(Ca), present in corn protoplasts, to different K+ channel blockers. IK(Ca) was inhibited by external Cs+ (10 mM), Ba2+ (10 mM), and quinine (0.5 mM): reagents which block many types of outward-rectifying K+ channels. In contrast 4-aminopyridine (5 mM), an inhibitor of delayed rectifier or inactivating K+ currents, had no effect. Neither of the peptide toxins, apamin or charybdotoxin, specific for Ca2(+)-dependent K+ channels in animal cells, inhibited currents when used in the nanomolar concentration range. However, higher levels of charybdotoxin (10 microM) caused marked reduction of IK(Ca).

Potassium ion channels in the plasmalemma

The potassium ion is an indispensible cytosolic component of living cells and a key osmolyte of plant cells, crossing the plasmalemma to drive physiological processes like cell growth and motor cell activity. K(+) transport across the plasmalemma may be passive through channels, driven by the electrochemical gradient, K(+) equilibrium potential (E(K) ) - membrane potential (V(m) ), or secondary active by coupling through a carrier to the inward driving force of H(+) or Na(+) . Known K(+) channels are permeable to monovalent cations, a permeability order being K(+) > Rb(+) > NH(4) (+) > Na(+) ≥ Li(+) > Cs(+) . The macroscopic K(+) currents across a cell or protoplast surface commonly show rectification, i.e. a V(m) -dependent conductance which in turn, may be controlled by the cytosolic activity of Ca(2+) , of K(+) , of H(+) , or by the K(+) driving force. Analysis by the patch clamp technique reveals that plant K(+) channels are similar to animal channels in their single channel conductance (4 to 100 pS), but different in that a given channel population slowly activates and may not inactivate at all. Single-channel kinetics reveal a broad range of open times (ms to s) and closed times (up to 100 s). Further progress in elucidating plant K(+) channels will critically depend on molecular cloning, and the availability of channel-specific (phyto)toxins.

Tansley Review No. 21 Plant ion channels: whole-cell and single channel studies

Ion channels are proteins which catalyse rapid, passive, electrogenic uniport of ions through pores spanning an otherwise poorly permeable lipid bilayer. Among other processes, fluxes through ion channels are responsible for action potentials - large, transient changes in membrane potential which have been known of in plants for over 100 years. Much disparate information on ion channels in plant cells has accumulated over the past few years. In an attempt to synthesize these data, the properties of at least 18 different ion channels are collated in this review. Channels are initially classified according to ion selectivity (Ca²⁺ , Cl^- , K⁺ and H⁺ ); then gating characteristics (i.e. control of opening and closing), unitary conductance and pharmacology are used to distinguish further different sub-types of channels. To provide a background for this overview, the fundamental properties which define ion channels in animal cells, namely conduction, selectivity and gating, are described. Appropriate techniques for the study of ion channels are also assessed. The review concludes with a discussion on the role of ion channels in plant cells, although any comment on functions beyond turgor regulation and general statements about signalling remains largely speculative. The study of ion channels in plant cells is still at an early stage and it is hoped that this review will provide a framework upon which further work in both algae and vascular plants can be based. CONTENTS Summary 305 I. Introduction: plant electrophysiology 306 II. A general description of ion channels 306 III. Ion channels in plants 310 IV. Ca²⁺ channels 313 V. Cl^- channels 315 VI. K⁺ channels in the plasma membrane 318 VII. K⁺ channels in the tonoplast 322 VIII. Channels in thylakoids 324 IX. H⁺ channels 324 X. Functions of channels 325 XI. Conclusions 328 Acknowledgements 328 References 329.

Red light regulates calcium-activated potassium channels in mougeotia plasma membrane

The alga Mougeotia has a large central chloroplast whose positioning is regulated by photoactivation of phytochrome, possibly via modulation of cytosolic calcium (Serlin B, Roux SJ [1984] Proc Natl Acad Sci USA 81: 6368-6372). We used the patch clamp technique to examine the effects of red and far-red light on ion channel activity in the plasma membrane of Mougeotia protoplasts to determine if ion channels play a role in chloroplast movement. Patch clamping in the cell-attached mode reveals two channels of about 2 and 4 picoamperes amplitude at 0 millivolt (inside pipette) and estimated conductances of 30 and 65 picosiemens. They are activated by red light irradiation after a lag period of about 2 to 5 minutes. Far-red light, when applied immediately after red light irradiation, reverses this activation; otherwise it has no effect. This result implicates phytochrome. The addition of the calcium ionophore, A23187, also activates ion channel activity after a lag of a few minutes. The channels are not specific for calcium since they are present when calcium is removed from the external and pipette media. They are inhibited by quaternary ammonium ions. Thus, we believe they are calcium-activated potassium channels. Their possible role in chloroplast positioning is discussed.

Involvement of ion channels and active transport in osmoregulation and signaling of higher plant cells

The transport of inorganic and organic ions across the plasma membrane and organelle membranes of higher plants by ion channels, electrogenic pumps and co-transporters is essential to vital processes such as osmoregulation, growth, development, signal transduction and the storage of solutes. Recent studies have led to the identification of specialized transport proteins in the plasma membrane and vacuolar membrane of higher plant cells. Here we have integrated the functional aspects of these membrane proteins into a model which proposes a novel basis for ion transport processes involved in the regulation of gas exchange in leaves.

Characterization of potassium-dependent currents in protoplasts of corn suspension cells

Protoplasts obtained from corn (Zea mays) suspension cells were studied using the whole cell patch-clamp technique. One time-independent current, as well as two time-dependent currents were identified. All three currents were reduced by tetraethylammonium (9 millimolar), a K(+) channel blocker. The time-independent current had a nearly linear current-voltage relationship and its reversal potential, defined as the voltage at which there is zero current, was highly dependent on the extracellular potassium concentration. One of the two time-dependent currents was activated, with rapid kinetics, by membrane hyperpolarization to potentials more negative than -100 millivolts. The second time-dependent current was activated with a sigmoidal time course by membrane depolarization to potentials more positive than -60 millivolts. It exhibited no inactivation and was carried primarily by potassium ions. These characteristics suggest that this latter current is caused by the voltage-dependent opening of delayed-rectifier K(+) channels. These three currents, which are not generated by the plasmalemma H(+)-ATPase, are likely to assist in the regulation of the cellular K(+) fluxes and membrane potential.

Quantitative analysis of outward rectifying K+ channel currents in guard cell protoplasts from Vicia faba

A quantitative analysis of the time and voltage dependence of outward-rectifying K+ currents (IK+, out) in guard cells from Vicia faba is described using the whole-cell patch-clamp technique. After step depolarizations from -75 mV to potentials positive to -40 mV, time-dependent outward currents were produced, which have recently been identified as K+ channel currents. This K+ current was characterized according to its time dependence and its steady-state activation. IK+, out could be described in terms of a Hodgkin-Huxley type conductance. Activation of the current in time was sigmoid and was well fitted by raising the activation variable to the second power. Deactivating tail currents were single exponentials, which suggests that only one conductance underlies this slow outward K+ current. Rates of channel closing were strongly dependent on the membrane potential, while rates of channel opening showed only limited voltage dependence leading to a highly asymmetric voltage dependence for channel closing and opening. The presented analysis provides a quantitative basis for the understanding of IK+, out channel gating and IK+, out channel functions in plant cells.

Channel-mediated K(+) flux in barley aleurone protoplasts

Gibberellic acid (GA3) stimulates K(+) efflux from the barley (Hordeum vulgare L. cv. Himalaya) aleurone. We investigated the mechanism of K(+) flux across the plasma membrane of aleurone protoplasts using patch-clamp techniques. Potassium-ion currents, measured over the entire surface of the protoplast plasma membrane, were induced when the electrochemical gradient for K(+) was inward (into the cytoplasm). The magnitude and voltage-dependence of this inward current were the same in protoplasts treated with GA3 and in control protoplasts (no GA3). Inward currents activated by negative shifts in the membrane potential (EM) from the Nernst potential for K(+) (EK) showed membrane conductance to be a function of the electrochemical gradient (i.e. EM-EK). Single-channel influx currents of K(+) were recorded in small patches of the plasma membrane. These channels had a single-channel conductance of 5-10 pS with 100 mM K(+) on the inside and 10 mM K(+) on the outside of the plasma membrane. Single-channel currents, like whole-cell currents, were the same in protoplasts treated with GA3 and control protoplasts. Voltage-gated efflux currents were found only in protoplasts tha thad been incubated without GA3. We conclude that K(+) influx in the aleurone is mediated by channels and these membrane proteins are not greatly effected by GA3.

Potassium Channels in Motor Cells of Samanea saman: A Patch-Clamp Study

Leaflet movements in Samanea saman are driven by the shrinking and swelling of cells in opposing (extensor and flexor) regions of the motor organ (pulvinus). Changes in cell volume, in turn, depend upon large changes in motor cell content of K(+), Cl(-) and other ions. We performed patch-clamp experiments on extensor and flexor protoplasts, to determine whether their plasma membranes contain channels capable of carrying the large K(+) currents that flow during leaflet movement. Recordings in the "whole-cell" mode reveal depolarization-activated K(+) currents in extensor and flexor cells that increase slowly (t((1/2)) = ca. 2 seconds) and remain active for minutes. Recordings from excised patches reveal a single channel conductance of ca. 20 picosiemens in both cell types. The magnitude of the K(+) currents is adequate to account quantitatively for K(+) loss, previously measured in vivo during cell shrinkage. The K(+) channel blockers tetraethylammonium (5 millimolar) or quinine (1 millimolar) blocked channel opening and decreased light- and dark-promoted movements of excised leaflets. These results provide evidence for the role of potassium channels in leaflet movement.