Separation of tonoplast and plasma membrane potential and resistance in cells of oat coleoptiles

Functional Evaluation of Proteins in Watery and Gel Saliva of Aphids

Gel and watery saliva are regarded as key players in aphid-pIant interactions. The salivary composition seems to be influenced by the variable environment encountered by the stylet tip. Milieu sensing has been postulated to provide information needed for proper stylet navigation and for the required switches between gel and watery saliva secretion during stylet progress. Both the chemical and physical factors involved in sensing of the stylet's environment are discussed. To investigate the salivary proteome, proteins were collected from dissected gland extracts or artificial diets in a range of studies. We discuss the advantages and disadvantages of either collection method. Several proteins were identified by functional assays or by use of proteomic tools, while most of their functions still remain unknown. These studies disclosed the presence of at least two proteins carrying numerous sulfhydryl groups that may act as the structural backbone of the salivary sheath. Furthermore, cell-wall degrading proteins such a pectinases, pectin methylesterases, polygalacturonases, and cellulases as well as diverse Ca²⁺-binding proteins (e.g., regucalcin, ARMET proteins) were detected. Suppression of the plant defense may be a common goal of salivary proteins. Salivary proteases are likely involved in the breakdown of sieve-element proteins to invalidate plant defense or to increase the availability of organic N compounds. Salivary polyphenoloxidases, peroxidases and oxidoreductases were suggested to detoxify, e.g., plant phenols. During the last years, an increasing number of salivary proteins have been categorized under the term 'effector'. Effectors may act in the suppression (C002 or MIF cytokine) or the induction (e.g., Mp10 or Mp 42) of plant defense, respectively. A remarkable component of watery saliva seems the protein GroEL that originates from Buchnera aphidicola, the obligate symbiont of aphids and probably reflects an excretory product that induces plant defense responses. Furthermore, chitin fragments in the saliva may trigger defense reactions (e.g., callose deposition). The functions of identified proteins and protein classes are discussed with regard to physical and chemical characteristics of apoplasmic and symplasmic plant compartments.

The use of microelectrodes to investigate compartmentation and the transport of metabolized inorganic ions in plants

Microelectrode measurements can be used to investigate both the intracellular pools of ions and membrane transport processes of single living cells. Microelectrodes can report these processes in the surface layers of root and leaf cells of intact plants. By careful manipulation of the plant, a minimum of disruption is produced and therefore the information obtained from these measurements most probably represents the 'in vivo' situation. Microelectrodes can be used to assay for the activity of particular transport systems in the plasma membrane of cells. Compartmental concentrations of inorganic metabolite ions have been measured by several different methods and the results obtained for the cytosol are compared. Ion-selective microelectrodes have been used to measure the activities of ions in the apoplast, cytosol and vacuole of single cells. New sensors for these microelectrodes are being produced which offer lower detection limits and the opportunity to measure other previously unmeasured ions. Measurements can be used to determine the intracellular steady-state activities or report the response of cells to environmental changes.

Effects of light and inhibitors of ATP-synthesis on the chloride carrier of the alga Valonia utricularis: is the carrier a chloride pump?

The effect of metabolic inhibitors, such as cyanide, antimycin A and azide was studied on the chloride transport system of the giant marine alga Valonia utricularis by using the charge pulse relaxation method. Two clearly defined voltage relaxations were resolved. The addition of 10-30 microM cyanide to the artificial sea water (ASW) bathing the algal cells increased the time constants of the slow voltage relaxation, tau 2, significantly when the algal cells were kept in the dark. The cyanide-effect reached a plateau value at 100-300 microM and was fully reversible when cyanide was removed from the ASW. Analysis of the charge pulse data in terms of the Läuger-model demonstrated that the translocation rates of the free, kS, and the charged carrier, kAS, decreased. The decrease of kS was more pronounced than that of kAS. 10 microM antimycin A and 3 mM azide had similar effects on the rate constants when the light was switched off. Upon illumination the cyanide- and antimycin A-, but not the azide-mediated effects disappeared. At concentrations higher than 1 mM cyanide caused a further, dramatic decrease of kS and kAS, while the surface concentration of the carrier molecules, N0, was not affected. This cyanide-effect was also reversible, but not light-dependent. Measurements of the ATP level showed that 3 mM cyanide reduced the ATP level by about 70% both under light and dark conditions. In the presence of 30 microM cyanide (or 10 microM antimycin A) the ATP level decreased by about 50%, but only in the dark. These results suggest two different effects of cyanide on the Cl(-)-carrier system: in the micromolar concentration range cyanide (and antimycin A) reduced predominantly the translocation of the free carrier by inhibition of ATP synthesis by oxidative phosphorylation, whereas in the millimolar concentration range cyanide apparently inhibits the translocation rates of both the free and charged carriers by its binding to the carrier. The results provide some evidence that the chloride transport of V. utricularis could be coupled to metabolic energy but it is an open question whether it is a pump or not.

Vacuolar proton-pumping pyrophosphatase in Beta vulgaris shows vectorial activation by potassium

Previous work with membrane vesicles has demonstrated an absolute dependence on K+ for proton translocation by the inorganic pyrophosphatase (H(+)-PPase: EC 3.6.1.1) from the vacuolar membrane (tonoplast) of higher plants. Using intact vacuoles from sugar beet (Beta vulgaris) storage tissue, we have monitored PP1-dependent currents by patch clamp in 'whole vacuole' mode. Serial K+ substitutions were made at both tonoplast faces. The results show that K+ activation occurs only at the cytosolic face.

Biphasic voltage relaxation pattern observed in cells of Eremosphaera viridis after injection of charge-pulses of short duration: detection of tip clogging of intracellular microelectrodes by charge-pulse technique

Charge pulse experiments performed on the peat-bog alga Eremosphaera viridis revealed an unusual voltage relaxation behaviour. Injection of charge pulses of 1 microseconds duration resulted in an immediate charging of the membranes (time constant of the order of 40 ns). Nevertheless, the potential-measuring microelectrode recorded an exponential increase in membrane voltage with a time constant of about 1.3 ms. The maximum voltage value was recorded after about 3 ms, followed by an exponential decay with a time constant of about 9.6 ms. This biphasic time course was independent of the amplitude of the injected charge and of the location of the impaled microelectrodes in the vacuole. Centrifuged cells in which the chloroplasts and the other organelles were pelleted in one part of the cells showed the same electrical response. Electrical breakdown of the cell membranes resulted in the disappearance of the biphasic voltage response. In this case only the decaying relaxation process could be recorded with a time constant of 3 ms. After resealing of the membranes the original biphasic relaxation response was restored. Increasing concentrations of KCl in the bathing medium reduced both time constants almost correspondingly. The experimental findings were evaluated with an electrical equivalent circuit. Theoretical analysis with reference to the experimental data suggested that the delayed voltage response of the potential-recording electrode resulted from a membrane seal across the tip of this electrode. The resistance of this seal was calculated to be about 400 M omega. The specific resistances and capacitances of tonoplast and plasmalemma membranes were calculated from the decaying part of the biphasic relaxation curves. The average values were found to be 2.58 omega.m2 and 5 mF.m-2. The investigations reported here suggest that charge pulse experiments can be generally used for the detection of membrane and cytoplasmic material clogging of the tip of intracellular microelectrodes, a problem with which most electrophysiologists are faced when interpreting data obtained from impaled microelectrodes.

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.

Electrical properties of soybean plasma membrane measured in heterotrophic suspension callus

Previous work on heterotrophic suspension-cultured cells has failed to detect the electrogenic processes normally associated with the plasma membranes of non-animal cells. This study reports measurements on heterotrophic cells from soybean (Glycine max L.) suspension cultures, which are shown to be amenable to impalement with microelectrodes. The plasma membrane clearly exhibits fundamental characteristics which are common to many other plant cell types: (i) a resting membrane potential significantly more negative than-100mV (measured value:121±4mV); (ii) obvious electrogenic activity, as evidenced by the marked depolarization of the membrane (87±6mV) by cyanide, and by the fact the membrane potential was frequently more negative than the equilibrium potential for K(+); (iii) a finite permeability to K(+) ions; (iv) electrophoretic transport of glucose. The development of a recording medium consisting primarily of 1:5 diluted growth medium was critical for successful impalement of these cells. It is proposed that the novel identification of electrogenic processes in heterotrophic suspension-cultured cells results from the deployment of electrodes with relatively dilute filling solutions, thus avoiding substantial changes in intracellular ion concentrations.The overwhelming majority of cells in soybean suspension cultures exist in small clusters, and the possibility of intercellular coupling potentially precludes assessment of membrane specific resistance and current density. Furthermore, as with most higher-plant cells, the vacuole occupies a large fraction of the intracellular volume. However, a model in which the measuring electrode is cytosolically located and the cells are electrically well-coupled is the only one which satisfactorily generates values for membrane specific resistance in a manner which is not strongly dependent on the number of cells in the cluster: other models in which the electrode tip is located in the vacuole and-or the impaled cell is electrically isolated from the others do not seem to apply. The measured values of membrane specific resistance are in the range 5.4 to 8.4 ω·m(2), which is in excellent agreement with comparable measurements on other plant and fungal cells. The results are discussed with respect to mechanisms of transmembrane signalling in soybean, as well as to general electrophysiological studies on higher-plant cells in suspension culture and in tissues.

Mobile charges in the cell membranes ofHalicystis parvula

Charge-pulse experiments were performed on cells of the giant marine algaHalicystis parvula. At normal pH (8.2), the voltage decay following a charge-pulse of 500 ns duration fed to the vacuole could be described by summing two exponential relaxations. The amplitudes and time constants of these relaxations were widely separated. The parameters of the two relaxation processes were found to be pH-dependent. Reduction of the external pH value from pH 8.2 to 5 resulted in a complete change of the two relaxation processes within a few minutes. Only one relaxation process could be observed at pH 5, within the time resolution of our instrumentation. The experimental data could not be explained by a two-membrane model with reasonable values for the specific capacitances of tonoplast and plasmalemma. The results of the charge-pulse relaxations were found to be consistent with the assumption that both membranes have very similar electrical properties and that both contain mobile charges with a total surface concentration of about 30 nmol·m(-2) and a translocation-rate constant of about 500·s(-1). The mobile charges became neutralized at pH 5 hhich led to a decrease of the apparent specific capacitance of the algal cells. They are presumably either part of a transport system for cations or connected with the chloride pump ofHalicystis parvula.

The integration of whole-root and cellular hydraulic conductivities in cereal roots

The hydraulic conductivities of excised whole root systems of wheat (Triticum aestivum L. cv. Atou) and of single excised roots of wheat and maize (Zea mays L. cv. Passat) were measured using an osmotically induced back-flow technique. Ninety minutes after excision the values for single excised roots ranged from 1.6·10(-8) to 5.5·10(-8) m·s(-1)·MPa(-1) in wheat and from 0.9·10(-8) to 4.8·10(-8) m·s(-1)·MPa(-1) in maize. The main source of variation was a decrease in the value as root length increased. The hydraulic conductivities of whole root systems, but not of single excised roots, were smaller 15 h after excision. This was not caused by occlusion of the xylem at the cut end of the coleoptile. The hydraulic conductivities of epidermal, cortical and endodermal cells were measured using a pressure probe. Epidermal and cortical cells of both wheat and maize roots gave mean values of 1.2·10(-7) m·s(-1)·MPa(-1) but in endodermal cells (measured only in wheat) the mean value was 0.5·10(-7) m·s(-1)·MPa(-1). The cellular hydraulic conductivities were used to calculate the root hydraulic conductivities expected if water flow across the root was via transcellular (vacuole-to-vacuole), apoplasmic or symplasmic pathways. The results indicate that, in freshly excised roots, the bulk of water flow is unlikely to be via the transcellular pathway. This is in contrast to our previous conclusion (H. Jones, A.D. Tomos, R.A. Leigh and R.G. Wyn Jones 1983, Planta 158, 230-236) which was based on results obtained with whole root systems of wheat measured 14-15 h after excision and which probably gave artefactually low values for root hydraulic conductivity. It is now concluded that, near the root tip, water flow could be through a symplasmic pathway in which the only substantial resistances to water flow are provided by the outer epidermal and the inner endodermal plasma membranes. Further from the tip, the measured hydraulic conductivities of the roots are consistent with flow either through the symplasmic or apoplasmic pathways.

Some characteristics of anion transport at the tonoplast of oat roots, determined from the effects of anions on pyrophosphatedependent proton transport

The effects of anions on inorganicpyrophosphate-dependent H(+)-transport in isolated tonoplast vesicles from oat (Avena sativa L.) roots were determined. Both fluorescent and radioactive probes were used to measure formation of pH gradients and membrane potential in the vesicles. Pyrophosphate hydrolysis by the H(+)-translocating pyrophosphatase was unaffected by anions. Nonetheless, some anions (Cl(-), Br(-) and NO3-) stimulated H(+)-transport while others (malate, [Formula: see text] and iminodiacetate) did not. These differential effects were abolished when the membrane potential was clamped at zero mV using potassium and valinomycin. Stimulation of H(+)-transport by Cl(-) showed saturation kinetics whereas that by NO3- consisted of both a saturable component and a linear phase. For Cl(-) and NO3-, the saturable phase had a K m of about 2 mol·m(-3). The anions that stimulated H(+)-transport also dissipated the membrane potential (Δψ.) generated by the pyrophosphatase. It is suggested that the stimulatory anions cross the tonoplast in response to the positive Δψ generated by the pyrophosphatase, causing dissipation of Δψ and stimulation of ΔpH, as expected by the chemiosmotic hypothesis. The work is discussed in relation to recent studies of the effects of anions on ATP-dependent H(+)-transport at the tonoplast, and its relevance to anion accumulation in the vacuole in vivo is considered.

Electrical properties of the vertically growing root tip of Lepidium sativum L

Electrical transmembrane potential differences and resistances in different tissues of intact root tips of Lepidium sativum L. were investigated in a humid atmosphere by conventional glass-microelectrode techniques with the reference electrode at the surface (apoplast) of the root. The resting potential (inside negative) in cells of the root cap rose from-80 mV in external cell layers (secretion cells) to approx.-140 mV in central cells (statocytes). Measurements of the electric input resistance within the apoplast of the root tip (calyptra, meristem and elongation zone) yielded a preference for longitudinal contact (resistance per length of tissue approx. 3.4 GOhm m(-1)) compared with transversal contact (approx. 14 GOhm m(-1)). Similarly, the symplastic coupling expressed as the characteristic length (L) where a signal is reduced to 1/c compared with the origin yielded L y =390 μm in the longitudinal (y) direction and L x =140 μm in the transversal (x) direction. Cable analytical treatment of the symplastic input resistances (approx. 10 MOhm) resulted in low membrane resistances in the y-direction at the ends of cells compared with the membrane resistances in the x-direction (approx. 0.2 Ohm m(2)) of the lateral membranes in the approximately cylindrical cells. This anisotropy is discussed in terms of model calculations. The resistivity of the symplast was calculated to be about 2.5 Ohm m. The input current-voltage relationship displayed a slight curvature with increasing slope for the more negative membrane potential typical of membranes with electrogenic pumps. Even after massive electrical stimulation in the range from-50 to-150mV carried out to trace current-voltage curves, electrical excitations (action potentials) were not detected in the cells investigated.

Role of the ATPase of sugar-cane vacuoles in energization of the tonoplast

Vacuoles of sugar-cane suspension cells contained a tonoplast-bound ATPase which was exclusively located on the cytoplasmic side of the vacuole. Vanadate and diethylstilbestrol had little effect on the vacuolar ATPase. ATP was the optimum substrate for the tonoplast ATPase, but there was also evidence for tonoplast-bound GDP-hydrolyzing and GTP-hydrolyzing enzymes which can interfere with the ATPase assay. Other phosphate anhydrides and esters were not hydrolyzed. The addition of MgATP polarized the tonoplast from about 0 mV to an interior-positive value of about +20 mV; MgADP and MgGTP had much less effect; MgGDP and ATP (in the absence of magnesium) had no effect on the membrane potential. The polarization of the tonoplast was insensitive to valinomycin, nigericin, and inhibitors of plasmalemma ATPase, but was strongly reduced by the uncoupler carbonylcyanide m-chlorophenylhydrazone. These data are interpreted as evidence for the action of tonoplast-bound ATPase as a pump which translocates protons into the vacuoles. The activity of the ATPase was highly specific for MgATP2-; the other important ionic states of ATP:ATP4-, HATP3-, MgHATP-, and Mg2ATP neither stimulated nor inhibited. The same was true for Mg2+. Since the protons were not brought to the catalytic site by protonation of the substrate, the tonoplast-ATPase may pick up the proton for translocation from the cytoplasm. The saturation kinetics for MgATP2- hydrolysis were biphasic, the higher affinity ATPase with Km value of 0.7 mM seems to be the physiologically relevant activity.

Rapid response of the plasma-membrane potential in oat coleoptiles to auxin and other weak acids

We have compared the effects of the auxin, indole-3-acetic acid (IAA) with that of other weak acids on the plasma-membrane potential of oat (Avena sativa L.) coleoptile cells. Cells treated with 1 μM IAA at pH 6 depolarize 20-25 mV in 10-12 min, but they then repolarize, until by 20-25 min their potentials are about 25 mV more negative than the initial value. Similar concentrations of benzoic and butyric acids cause the initial depolarization, but not the subsequent hyperpolarization. The hyperpolarization is therefore specific to IAA. All the weak acids, including IAA, evoke a rapid hyperpolarization when their concentrations are raised to 10 mM. This result indicates that at high concentrations, the uptake of undissociated weak acids activates electrogenic proton pumping, most likely by lowering cytoplasmic pH. In contrast, the hyperpolarization observed with concentrations of IAA four orders of magnitude lower appears to be a specific hormonal effect. This specific, auxin-induced hyperpolarization occurs at the same time as the initiation of net proton secretion and supports the hypothesis that auxin initiates extension growth by increasing proton pumping.