Rapid effect of light on the K+ channel in the plasmalemma ofNitella

Mitochondria provide the main source of cytosolic ATP for activation of outward-rectifying K+ channels in mesophyll protoplast of chlorophyll-deficient mutant rice (OsCHLH) seedlings

The role of mitochondria in providing intracellular ATP that controls the activity of plasma membrane outward-rectifying K+ channels was evaluated. The OsCHLH rice mutant, which lacks chlorophyll in the thylakoids, was isolated by T-DNA gene trapping (Jung, K.-H., Hur, J., Ryu, C.-H., Choi, Y., Chung, Y.-Y., Miyao, A., Hirochika, H., and An, G. (2003) Plant Cell Physiol. 44, 463-472). The OsCHLH mutant is unable to fix CO2 and exhibits reduced growth. Wild type and mutant plants exhibit similar rates of respiratory O2 uptake in the dark, whereas the rate of photosynthetic O2 evolution by the mutant was negligible during illumination. During dark respiration the wild type and mutant exhibited similar levels of cytoplasmic ATP. In the mutant oligomycin treatment (an inhibitor of mitochondrial F1F0-ATPase) drastically reduced ATP production. The fact that this was reversed by the addition of glucose suggested that the mutant produced ATP exclusively from mitochondria but not from chloroplasts. In whole cell patch clamp experiments, the activity of outward-rectifying K+ channels of rice mesophyll cells showed ATP-dependent currents, which were 1.5-fold greater in wild type than in mutant cells. Channels in both wild type and mutant cells were deactivated by the removal of cytosolic ATP, whereas in the presence of ATP the channels remained active. We conclude that mesophyll cells in the OsCHLH rice mutant derive ATP from mitochondrial respiration, and that this is critical for the normal function of plasma membrane outward-rectifying K+ channels.

Low-pH-mediated elevations in cytosolic calcium are inhibited by aluminium: a potential mechanism for aluminium toxicity

Aluminium, the most abundant metal in the earth's crust, is highly toxic to most plant species. One of the prevailing dogmas is that aluminium exerts this effect by disrupting cellular calcium homeostasis. However, recent research gives strongly conflicting results: aluminium was shown to provoke either an increase or a decrease in cytosolic free calcium concentration ([Ca2+]c). To solve this question, we have adopted a novel approach: [Ca2+]c measurements in intact plant roots as opposed to isolated cells, and the correlative measurements of intracellular and external pH. The results obtained show that plant roots respond to low external pH by a sustained elevation in [Ca2+]c. In the presence of aluminium, this pH-mediated elevation in [Ca2+]c does not occur, therefore any potential calcium-mediated protection against low pH is likely to be irreversibly inhibited. The severity of the inhibitory effect of aluminium on [Ca2+]c depends on the concentration of external calcium, thus perhaps explaining why the effects of aluminium toxicity are ameliorated in calcium-rich soils. It seems possible that a primary toxic effect of aluminium might be to impair calcium-mediated plant defence responses against low pH.

Repetitive Ca2+ spikes in a unicellular green alga

Cytosolic Ca2+ activity ([Ca2+]cy) and membrane potential were measured simultaneously in the unicellular green alga Eremosphaera viridis. Steady state [Ca2+]cy was about 160 nM. A 'light-off' stimulus induced a transient elevation of [Ca2+]cy ([Ca2+]cy spike) in parallel with a transient hyperpolarization of the plasma membrane. Caffeine and Sr2+, known to release Ca2+ from intracellular stores in animal cells, induced repetitive [Ca2+]cy spikes in Eremosphaera which were always accompanied by parallel repetitive transient hyperpolarizations. These transient hyperpolarizations could be used as an indicator for [Ca2+]cy spikes. Repetitive [Ca2+]cy spikes in Eremosphaera were similar to repetitive [Ca2+]cy spikes in excitable animal cells. The mechanisms underlying these [Ca2+]cy oscillations seem to be comparable in animal and plant cells.

Calcium -regulated channels and their bearing on physiological activities in Characean cells

Calcium is involved in the regulation of cytoplasmic streaming, membrane excitation, turgor regulation and salt tolerance in the giant internodal cells of the Characeae. To analyse the mechanism of Ca 2+ action, model systems were used, namely, tonoplast-free and plasma m em brane-perm eabilized cells. In the former, the plasma membrane remained intact and its activity could be investigated by m anipulating the cytoplasmic and external media, whereas in the latter, the tonoplast remained intact and its activity could be studied by altering the bathing solution. Studies using these model systems have established the presence of voltage-dependent Ca 2+ -channels in the plasma membrane and Ca 2+ - dependent ion channels in both the plasma membrane and the tonoplast. To further analyse Ca 2+ action on the basis of single channel activities, patch-clam p techniques were applied to plasmolysed protoplasts and isolated cytoplasmic drops. Channel activities were measured using both cell-attached and excised membrane patch modes. A fresh-water member of the Characeae, Nitellopsis , becomes salt-tolerant if millimolar amounts of Ca 2+ are present in the external medium. Under these conditions, excised patches of the plasma membrane exhibit K + -channel activity with unitary conductances of 25-50 pS and a permeability ratio ( P Na / P K ) of 0.28. These K + channels were closed by external Ca 2+ when ATP was present on the cytoplasmic side of the membrane. ATP could be replaced with AMP, which suggests that ATP acted neither as an energy source nor as a substrate for protein phosphorylation, but rather as an effector. In the tonoplast, K + channels having a unitary conductance of 75 pS and a P Na / P K ratio of 0.2 were not activated by Ca 2+ when it was present on the cytoplasmic side of the excised patches, but these channels were activated by Ca 2+ injected into the cytoplasmic drop, which suggested the involvement of an unknown cytoplasmic factor(s) that mediates the Ca 2+ signal. The brackish water Characeae, Lamprothamnium , can regulate elevated turgor induced by hypotonic treatment only when millimolar amounts of Ca 2+ are present in the external medium. In this situation, elevated turgor may first activate Ca 2+ channels and the increased level of Ca 2+ in the cytoplasm may then activate K + and Cl - channels in both the plasma membrane and the tonoplast. In the cytoplasmic-drop-attached mode, single K + channel current-voltage measurements established that the K + channel exhibited a unitary conductance of 50 pS for negative shifts of the voltage, while under positive shifts in the voltage 100 pS channel conductance was observed. The channel with a P Na / P K of 0.02 is highly selective for K + against Na + and this channel is directly activated by Ca 2+ added to the cytoplasmic side of the excised patch. These results suggest that, in Nitellopsis and Lamprothamnium , Ca 2+ regulation of channels in both the plasma membrane and the tonoplast may form the molecular basis for Ca 2+ -regulated physiological functions such as salt tolerance and turgor regulation in characean cells. The mode of Ca 2+ regulation is discussed in light of current findings.

An apparatus for studying rapid electrophysiological responses to light demonstrated on Arabidopsis leaves

An apparatus for making high-resolution measurements of electrophysiological changes induced by light in plant cells was constructed. Its main components were a xenon arc lamp, an electronic shutter, a liquid light-guide, a computer equipped with an analog-to-digital converter and a computer program that controlled the shutter and data acquisition. The apparatus was used to examine transient changes in membrane potential (Vm) that occur upon illumination in Arabidopsis leaves. Light-on induced a transient hyperpolarization of 4 mV after a lag time of 0.53 s. It was followed by a much larger transient depolarization that peaked 31 s after light-on. The Vm returned to near its original value after approximately 3 min. The early changes in Vm have been proposed to result from effects of photosynthetically produced ATP on the activities of H(+)-ATPases and K+ channels at the plasma membrane. The kinetics of the initial hyperpolarization were found to be reasonably consistent with such a mechanism. It is expected that the apparatus described here will be useful in future investigations of this and other electrophysiological responses to light.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Patch-clamp studies on the anomalous mole fraction effect of the K+ channel in cytoplasmic droplets of Nitella: an attempt to distinguish between a multi-ion single-file pore and an enzyme kinetic model with lazy state

Patch-clamp studies have been employed in order to check whether the assumption of a multi-ion single-file pore is necessary for the explanation of the anomalous mole fraction effect or whether this effect can also be explained by a single-barrier enzyme kinetic model. Experiments in the cell-attached configuration were done on the tonoplast membrane of cytoplasmic droplets of Nitella in solutions containing 150 mol m-3 of K+ plus Tl+ with seven different K+/Tl+ ratios. At first sight, the results seem to support the multi-ion single-file pore, because apparent open channel conductivity displays the anomalous mole fraction effect, whereas open-probability has not been found to be dependent on the K+/Tl+ ratio. Changes in open probability would be expected for a single-barrier enzyme kinetic model with a lazy state. On the other hand, the lazy-state model is more successful in explaining the measured I-V curves. The entire slope of the apparent open channel current-voltage curves rotates with changing K+/Tl+ ratios in the whole voltage range between -100 and +80 mV. Numerical calculations on the basis of multi-ion single-file pores could create the anomalous mole fraction effect only in a limited voltage range with intersecting I-V curves. The apparent absence of an effect on open probability which is postulated by the lazy-state model can be explained if switching into and out of the lazy state is faster than can be resolved by the temporal resolution of 1 msec.

Linear analysis applied to the comparative study of the I-D-P phase of chlorophyll fluorescence as induced by actinic PS-II light, PS-I light and changes in CO2-concentration

The investigation of the kinetics of chlorophyll-fluorescence under continuous background light enables the application of linearizing conditions. This approach, which provides a quantitative evaluation by means of curve-fitting routines, is applied to the investigation of the linear kinetics of the I-D-P phase. Using changes in PS II-light, PS I-light and in CO2-concentration as input signals showed that a pool at the acceptor side of PS I, in addition to the plastoquinone pool, plays an essential role in the generation of the dip. The occurrence of the dip is related to the sign of the faster one of the two components related to the I-D and the D-P phase. This sign can be inverted by the ratio of PS I and PS II light. However, model calculations show that the change of this sign does not allow a decision which one of the two components is related to which one of the two pools. The dependence of the sign of the faster component on light conditions can generate different types of I-D-P transitions, namely nearly monophasic increases, sigmoid responses or dips. As these phenomena are already created by the linear responses, non-linear effects or additional loops between PS II and PS I are not required for the explanation of the basic features.

Injected inositol 1,4,5-trisphosphate activates Ca2(+)-sensitive K+ channels in the plasmalemma of Eremosphaera viridis

InsP3, and established mediator of intracellular Ca2+ signals in animal cells, is microinjected into the cytoplasm of Eremosphaera viridis. InsP3, but not Ins, InsP1, InsP2 or F2,6-P2 induce a transient opening of Ca2(+)-dependent K+ channels in the plasmalemma of this alga. This effect is indicated by a transient polarization (TP) with a simultaneous increase of membrane conductance. The TP is inhibited by TMB8 (2 mM), an intracellular Ca2+ antagonist or by BAPTA (20 mM), microinjected together with InsP3. The results suggest that InsP3 initiates an increase in the cytoplasmic Ca2+ activity and an activation of Ca2(+)-dependent membrane currents, hence, opening of K+ channels.

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.