Role of "active" potassium transport in the regulation of cytoplasmic pH by nonanimal cells

The vacuolar K+/H+ exchangers and calmodulin-like CML18 constitute a pH-sensing module that regulates K+ status in Arabidopsis

Shifts in cytosolic pH have been recognized as key signaling events and mounting evidence supports the interdependence between H+ and Ca2+ signaling in eukaryotic cells. Among the cellular pH-stats, K+/H+ exchange at various membranes is paramount in plant cells. Vacuolar K+/H+ exchangers of the NHX (Na+,K+/H+ exchanger) family control luminal pH and, together with K+ and H+ transporters at the plasma membrane, have been suggested to also regulate cytoplasmic pH. We show the regulation of vacuolar K+/H+ exchange by cytoplasmic pH and the calmodulin-like protein CML18 in Arabidopsis. The crystal structure and physicochemical properties of CML18 indicate that this protein senses pH shifts. Interaction of CML18 with tonoplast exchangers NHX1 and NHX2 was favored at acidic pH, a physiological condition elicited by K+ starvation in Arabidopsis roots, whereas excess K+ produced cytoplasmic alkalinization and CML18 dissociation. These results imply that the pH-responsive NHX-CML18 module is an essential component of the cellular K+- and pH-stats.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

An insight into the mechanisms of homeostasis in extremophiles

The homeostasis of extremophiles is one that is a diamond hidden in the rough. The way extremophiles adapt to their extreme environments gives a clue into the true extent of what is possible when it comes to life. The discovery of new extremophiles is ever-expanding and an explosion of knowledge surrounding their successful existence in extreme environments is obviously perceived in scientific literature. The present review paper aims to provide a comprehensive view on the different mechanisms governing the extreme adaptations of extremophiles, along with insights and discussions on what the limits of life can possibly be. The membrane adaptations that are vital for survival are discussed in detail. It was found that there are many alterations in the genetic makeup of such extremophiles when compared to their mesophilic counterparts. Apart from the several proteins involved, the significance of chaperones, efflux systems, DNA repair proteins and a host of other enzymes that adapt to maintain functionality, are enlisted, and explained. A deeper understanding of the underlying mechanisms could have a plethora of applications in the industry. There are cases when certain microbes can withstand extreme doses of antibiotics. Such microbes accumulate numerous genetic elements (or plasmids) that possess genes for multiple drug resistance (MDR). A deeper understanding of such mechanisms helps in the development of potential approaches and therapeutic schemes for treating pathogen-mediated outbreaks. An in-depth analysis of the parameters - radiation, pressure, temperature, pH value and metal resistance - are discussed in this review, and the key to survival in these precarious niches is described.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

SAUR proteins and PP2C.D phosphatases regulate H+-ATPases and K+ channels to control stomatal movements

Activation of plasma membrane (PM) H+-ATPase activity is crucial in guard cells to promote light-stimulated stomatal opening, and in growing organs to promote cell expansion. In growing organs, SMALL AUXIN UP RNA (SAUR) proteins inhibit the PP2C.D2, PP2C.D5, and PP2C.D6 (PP2C.D2/5/6) phosphatases, thereby preventing dephosphorylation of the penultimate phosphothreonine of PM H+-ATPases and trapping them in the activated state to promote cell expansion. To elucidate whether SAUR-PP2C.D regulatory modules also affect reversible cell expansion, we examined stomatal apertures and conductances of Arabidopsis thaliana plants with altered SAUR or PP2C.D activity. Here, we report that the pp2c.d2/5/6 triple knockout mutant plants and plant lines overexpressing SAUR fusion proteins exhibit enhanced stomatal apertures and conductances. Reciprocally, saur56 saur60 double mutants, lacking two SAUR genes normally expressed in guard cells, displayed reduced apertures and conductances, as did plants overexpressing PP2C.D5. Although altered PM H+-ATPase activity contributes to these stomatal phenotypes, voltage clamp analysis showed significant changes also in K+ channel gating in lines with altered SAUR and PP2C.D function. Together, our findings demonstrate that SAUR and PP2C.D proteins act antagonistically to facilitate stomatal movements through a concerted targeting of both ATP-dependent H+ pumping and channel-mediated K+ transport.

Regulation and activity of CaTrk1, CaAcu1 and CaHak1, the three plasma membrane potassium transporters in Candida albicans

Wild-type cells of Candida albicans, the most common human fungal pathogen, are able to grow at very low micromolar concentrations of potassium in the external milieu. One of the reasons behind that behaviour is the existence of three different types of K⁺ transporters in their plasma membrane: Trk1, Acu1 and Hak1. This work shows that the transporters are very differently regulated at the transcriptional level upon exposure to saline stress, pH alterations or K⁺ starvation. We propose that different transporters take the lead in the diverse environmental conditions, Trk1 being the "house-keeping" one, and Acu1/Hak1 dominating upon K⁺ limiting conditions. Heterologous expression of the genes coding for the three transporters in a Saccharomyces cerevisiae strain lacking its endogenous potassium transporters showed that all of them mediated cation transport but with very different efficiencies. Moreover, expression of the transporters in S. cerevisiae also affected other physiological characteristics such as sodium and lithium tolerance, membrane potential or intracellular pH, being, in general, CaTrk1 the most effective in keeping these parameters close to the usual wild-type physiological levels.

Communication between the Plasma Membrane and Tonoplast Is an Emergent Property of Ion Transport

Serial membranes that operate through a common compartment, the cytosol for transport at the plasma membrane and tonoplast, are intrinsically connected and communicate through this pool of solutes.

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

A Thermodynamic Model of Monovalent Cation Homeostasis in the Yeast Saccharomyces cerevisiae

Cationic and heavy metal toxicity is involved in a substantial number of diseases in mammals and crop plants. Therefore, the understanding of tightly regulated transporter activities, as well as conceiving the interplay of regulatory mechanisms, is of substantial interest. A generalized thermodynamic description is developed for the complex interplay of the plasma membrane ion transporters, membrane potential and the consumption of energy for maintaining and restoring specific intracellular cation concentrations. This concept is applied to the homeostasis of cation concentrations in the yeast cells of S. cerevisiae. The thermodynamic approach allows to model passive ion fluxes driven by the electrochemical potential differences, but also primary or secondary active transport processes driven by the inter- play of different ions (symport, antiport) or by ATP consumption (ATPases). The model-confronted with experimental data-reproduces the experimentally observed potassium and proton fluxes induced by the external stimuli KCl and glucose. The estimated phenomenological constants combine kinetic parameters and transport coefficients. These are in good agreement with the biological understanding of the transporters thus providing a better understanding of the control exerted by the coupled fluxes. The model predicts the flux of additional ion species, like e.g. chloride, as a potential candidate for counterbalancing positive charges. Furthermore, the effect of a second KCl stimulus is simulated, predicting a reduced cellular response for cells that were first exposed to a high KCl stimulus compared to cells pretreated with a mild KCl stimulus. By describing the generalized forces that are responsible for a given flow, the model provides information and suggestions for new experiments. Furthermore, it can be extended to other systems such as e.g. Candida albicans, or selected plant cells.

Wide pH range tolerance in extremophiles: towards understanding an important phenomenon for future biotechnology

Microorganisms that inhabit the extreme pH environments are classified as acidophiles and alkaliphiles. A number of studies emerged from extreme high (hot springs, hydrothermal vents) as well as low temperature (arctic and antarctic regions, sea water, ice shelf, marine sediments, cold deserts, glaciers, temperate forests, and plantations) environments have highlighted the occurrence of microorganisms (thermophiles/psychrophiles) with the ability to tolerate wide pH range, from acidic to alkaline (1.5-14.0 in some cases), under laboratory conditions. However, the sampling source (soil/sediment) of these microorganisms showed the pH to be neutral or slightly acidic/alkaline. The aim of the present review is to discuss the phenomenon of wide pH range tolerance possessed by these microorganisms as a hidden character in perspective of their habitats, possible mechanisms, phylogeny, ecological and biotechnological relevance, and future perspectives. It is believed that the genome is a probable reservoir of the hidden variations. The extremophiles have the ability to adapt against the environmental change that is probably through the expression/regulation of the specific genes that were already present in the genome. The phenomenon is likely to have broad implications in biotechnology, including both environmental (such as bioremediation, biodegradation, and biocontrol), and industrial applications (as a source of novel extremozymes and many other useful bioactive compounds with wide pH range tolerance).

Quantitative description of ion transport via plasma membrane of yeast and small cells

Modeling of ion transport via plasma membrane needs identification and quantitative understanding of the involved processes. Brief characterization of main ion transport systems of a yeast cell (Pma1, Ena1, TOK1, Nha1, Trk1, Trk2, non-selective cation conductance) and determining the exact number of molecules of each transporter per a typical cell allow us to predict the corresponding ion flows. In this review a comparison of ion transport in small yeast cell and several animal cell types is provided. The importance of cell volume to surface ratio is emphasized. The role of cell wall and lipid rafts is discussed in respect to required increase in spatial and temporary resolution of measurements. Conclusions are formulated to describe specific features of ion transport in a yeast cell. Potential directions of future research are outlined based on the assumptions.

Exploring emergent properties in cellular homeostasis using OnGuard to model K+ and other ion transport in guard cells

It is widely recognized that the nature and characteristics of transport across eukaryotic membranes are so complex as to defy intuitive understanding. In these circumstances, quantitative mathematical modeling is an essential tool, both to integrate detailed knowledge of individual transporters and to extract the properties emergent from their interactions. As the first, fully integrated and quantitative modeling environment for the study of ion transport dynamics in a plant cell, OnGuard offers a unique tool for exploring homeostatic properties emerging from the interactions of ion transport, both at the plasma membrane and tonoplast in the guard cell. OnGuard has already yielded detail sufficient to guide phenotypic and mutational studies, and it represents a key step toward 'reverse engineering' of stomatal guard cell physiology, based on rational design and testing in simulation, to improve water use efficiency and carbon assimilation. Its construction from the HoTSig libraries enables translation of the software to other cell types, including growing root hairs and pollen. The problems inherent to transport are nonetheless challenging, and are compounded for those unfamiliar with conceptual 'mindset' of the modeler. Here we set out guidelines for the use of OnGuard and outline a standardized approach that will enable users to advance quickly to its application both in the classroom and laboratory. We also highlight the uncanny and emergent property of OnGuard models to reproduce the 'communication' evident between the plasma membrane and tonoplast of the guard cell.

Systems analysis of guard cell membrane transport for enhanced stomatal dynamics and water use efficiency

Stomatal transpiration is at the center of a crisis in water availability and crop production that is expected to unfold over the next 20 to 30 years. Global water usage has increased 6-fold in the past 100 years, twice as fast as the human population, and is expected to double again before 2030, driven mainly by irrigation and agriculture. Guard cell membrane transport is integral to controlling stomatal aperture and offers important targets for genetic manipulation to improve crop performance. However, its complexity presents a formidable barrier to exploring such possibilities. With few exceptions, mutations that increase water use efficiency commonly have been found to do so with substantial costs to the rate of carbon assimilation, reflecting the trade-off in CO₂ availability with suppressed stomatal transpiration. One approach yet to be explored in detail relies on quantitative systems analysis of the guard cell. Our deep knowledge of transport and homeostasis in these cells gives real substance to the prospect for reverse engineering of stomatal responses, using in silico design in directing genetic manipulation for improved water use and crop yields. Here we address this problem with a focus on stomatal kinetics, taking advantage of the OnGuard software and models of the stomatal guard cell recently developed for exploring stomatal physiology. Our analysis suggests that manipulations of single transporter populations are likely to have unforeseen consequences. Channel gating, especially of the dominant K⁺ channels, appears the most favorable target for experimental manipulation.

Coordination of K+ transporters in neurospora: TRK1 is scarce and constitutive, while HAK1 is abundant and highly regulated

Fungi, plants, and bacteria accumulate potassium via two distinct molecular machines not directly coupled to ATP hydrolysis. The first, designated TRK, HKT, or KTR, has eight transmembrane helices and is folded like known potassium channels, while the second, designated HAK, KT, or KUP, has 12 transmembrane helices and resembles MFS class proteins. One of each type functions in the model organism Neurospora crassa, where both are readily accessible for biochemical, genetic, and electrophysiological characterization. We have now determined the operating balance between Trk1p and Hak1p under several important conditions, including potassium limitation and carbon starvation. Growth measurements, epitope tagging, and quantitative Western blotting have shown the gene HAK1 to be much more highly regulated than is TRK1. This conclusion follows from three experimental results: (i) Trk1p is expressed constitutively but at low levels, and it is barely sensitive to extracellular [K(+)] and/or the coexpression of HAK1; (ii) Hak1p is abundant but is markedly depressed by elevated extracellular concentrations of K(+) and by coexpression of TRK1; and (iii) Carbon starvation slowly enhances Hak1p expression and depresses Trk1p expression, yielding steady-state Hak1p:Trk1p ratios of ∼500:1, viz., 10- to 50-fold larger than that in K(+)- and carbon-replete cells. Additionally, it appears that both potassium transporters can adjust kinetically to sustained low-K(+) stress by means of progressively increasing transporter affinity for extracellular K(+). The underlying observations are (iv) that K(+) influx via Trk1p remains nearly constant at ∼9 mM/h when extracellular K(+) is progressively depleted below 0.05 mM and (v) that K(+) influx via Hak1p remains at ∼3 mM/h when extracellular K(+) is depleted below 0.1 mM.

OnGuard, a computational platform for quantitative kinetic modeling of guard cell physiology

Stomatal guard cells play a key role in gas exchange for photosynthesis while minimizing transpirational water loss from plants by opening and closing the stomatal pore. Foliar gas exchange has long been incorporated into mathematical models, several of which are robust enough to recapitulate transpirational characteristics at the whole-plant and community levels. Few models of stomata have been developed from the bottom up, however, and none are sufficiently generalized to be widely applicable in predicting stomatal behavior at a cellular level. We describe here the construction of computational models for the guard cell, building on the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. The OnGuard software was constructed with the HoTSig library to incorporate explicitly all of the fundamental properties for transporters at the plasma membrane and tonoplast, the salient features of osmolite metabolism, and the major controls of cytosolic-free Ca²⁺ concentration and pH. The library engenders a structured approach to tier and interrelate computational elements, and the OnGuard software allows ready access to parameters and equations 'on the fly' while enabling the network of components within each model to interact computationally. We show that an OnGuard model readily achieves stability in a set of physiologically sensible baseline or Reference States; we also show the robustness of these Reference States in adjusting to changes in environmental parameters and the activities of major groups of transporters both at the tonoplast and plasma membrane. The following article addresses the predictive power of the OnGuard model to generate unexpected and counterintuitive outputs.

Systems dynamic modeling of the stomatal guard cell predicts emergent behaviors in transport, signaling, and volume control

The dynamics of stomatal movements and their consequences for photosynthesis and transpirational water loss have long been incorporated into mathematical models, but none have been developed from the bottom up that are widely applicable in predicting stomatal behavior at a cellular level. We previously established a systems dynamic model incorporating explicitly the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. Here we describe the behavior of the model in response to experimentally documented changes in primary pump activities and malate (Mal) synthesis imposed over a diurnal cycle. We show that the model successfully recapitulates the cyclic variations in H⁺, K⁺, Cl⁻, and Mal concentrations in the cytosol and vacuole known for guard cells. It also yields a number of unexpected and counterintuitive outputs. Among these, we report a diurnal elevation in cytosolic-free Ca²⁺ concentration and an exchange of vacuolar Cl⁻ with Mal, both of which find substantiation in the literature but had previously been suggested to require additional and complex levels of regulation. These findings highlight the true predictive power of the OnGuard model in providing a framework for systems analysis of stomatal guard cells, and they demonstrate the utility of the OnGuard software and HoTSig library in exploring fundamental problems in cellular physiology and homeostasis.

Potassium starvation in yeast: mechanisms of homeostasis revealed by mathematical modeling

The intrinsic ability of cells to adapt to a wide range of environmental conditions is a fundamental process required for survival. Potassium is the most abundant cation in living cells and is required for essential cellular processes, including the regulation of cell volume, pH and protein synthesis. Yeast cells can grow from low micromolar to molar potassium concentrations and utilize sophisticated control mechanisms to keep the internal potassium concentration in a viable range. We developed a mathematical model for Saccharomyces cerevisiae to explore the complex interplay between biophysical forces and molecular regulation facilitating potassium homeostasis. By using a novel inference method (“the reverse tracking algorithm”) we predicted and then verified experimentally that the main regulators under conditions of potassium starvation are proton fluxes responding to changes of potassium concentrations. In contrast to the prevailing view, we show that regulation of the main potassium transport systems (Trk1,2 and Nha1) in the plasma membrane is not sufficient to achieve homeostasis.

Without potassium, all living cells will die; it has to be present in sufficient amounts for the proper function of most cell types. Disturbances in potassium levels in animal cells result in potentially fatal conditions and it is also an essential nutrient for plants and fungi. Cells have developed effective mechanisms for surviving under adverse environmental conditions of low external potassium. The question is how. Using the eukaryotic model organism, baker's yeast (Saccharomyces cerevisiae), we modeled how potassium homeostasis takes place. This is because, through mathematical modeling and experimentation, we found that the electro-chemical forces regulating potassium concentrations are coupled to proton fluxes, which respond to external conditions in order to maintain a viable potassium level within the cells. Our results challenge the current understanding of potassium homeostasis in baker's yeast, and could potentially be extended to other microorganisms, including non-conventional yeasts such as the pathogenic Candida albicans, and plant cells. In the future, the fundamental bases for this descriptive and predictive model might contribute to the development of new treatments for fungal infections, or developments in crop sciences.

Protocol: optimised electrophyiological analysis of intact guard cells from Arabidopsis

Genetic resources available for Arabidopsis thaliana make this species particularly attractive as a model for molecular genetic studies of guard cell homeostasis, transport and signalling, but this facility is not matched by accessible tools for quantitative analysis of transport in the intact cell. We have developed a reliable set of procedures for voltage clamp analysis of guard cells from Arabidopsis leaves. These procedures greatly simplify electrophysiological recordings, extending the duration of measurements and scope for analysis of the predominant K+ and anion channels of intact stomatal guard cells to that achieved previously in work with Vicia and tobacco guard cells.

A bicistronic, Ubiquitin-10 promoter-based vector cassette for transient transformation and functional analysis of membrane transport demonstrates the utility of quantitative voltage clamp studies on intact Arabidopsis root epidermis

To date the use of fluorescent reporter constructs in analysing membrane transport has been limited primarily to cell lines expressing stably either the tagged transporter protein(s) or markers to identify lineages of interest. Strategies for transient expression have yet to be exploited in transport analysis, despite their wide application in cellular imaging studies. Here we describe a Gateway-compatible, bicistronic vector, incorporating the constitutive Ubiqutin-10 gene promoter of Arabidopsis that gives prolonged expression after transient transformation and enables fluorescence marking of cells without a fusion construct. We show that Arabidopsis root epidermal cells are readily transformed by co-cultivation with Agrobacterium and are tractable for quantitative electrophysiological analysis. As a proof of principle, we transiently transformed Arabidopsis with the bicistronic vector carrying GFP as the fluorescent marker and, separately, the integral plasma membrane protein SYP121 essential for the inward K+ channel current. We demonstrate that transient expression of SYP121 in syp121 mutant plants is sufficient to rescue the K+ current in vivo. The combination of transient expression and use of the bicistronic vector promises significant advantages for studies of membrane transport and nutrient acquisition in roots.

Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.

HKT1 mediates sodium uniport in roots. Pitfalls in the expression of HKT1 in yeast

The function of HKT1 in roots is controversial. We tackled this controversy by studying Na+ uptake in barley (Hordeum vulgare) roots, cloning the HvHKT1 gene, and expressing the HvHKT1 cDNA in yeast (Saccharomyces cerevisiae) cells. High-affinity Na+ uptake was not detected in plants growing at high K+ but appeared soon after exposing the plants to a K(+)-free medium. It was a uniport, insensitive to external K+ at the beginning of K+ starvation and inhibitable by K+ several hours later. The expression of HvHKT1 in yeast was Na+ (or K+) uniport, Na(+)-K+ symport, or a mix of both, depending on the construct from which the transporter was expressed. The Na+ uniport function was insensitive to external K+ and mimicked the Na+ uptake carried out by the roots at the beginning of K+ starvation. The K+ uniport function only took place in yeast cells that were completely K+ starved and disappeared when internal K+ increased, which makes it unlikely that HvHKT1 mediates K+ uptake in roots. Mutation of the first in-frame AUG codon of HvHKT1 to CUC changed the uniport function into symport. The expression of the symport from either mutants or constructs keeping the first in-frame AUG took place only in K(+)-starved cells, while the uniport was expressed in all conditions. We discuss here that the symport occurs only in heterologous expression. It is most likely related to the K+ inhibitable Na+ uptake process of roots that heterologous systems fail to reproduce.

Epitope Tagging of the Yeast K+ Carrier Trk2p Demonstrates Folding That Is Consistent with a Channel-like Structure

TRK family proteins, which mediate the concentrative uptake of potassium by plant cells, fungi, and bacteria, resemble primitive potassium channels in sequence and have recently been proposed actually to fold like potassium channels in a 4-MPM motif (Durell, S. R., and Guy, H. R. (1999) Biophys. J. 77, 789 - 807), instead of like conventional substrate porters in the 12-TM motif (Gaber, R. F., Styles, C. A., and Fink, G. R. (1988) Mol. Cell. Biol. 8, 2848-2859). The known fungal members of this family possess a very long hydrophilic loop, positioned intracellularly in the K(+)-channel model and extracellularly in the substrate porter model. This and two shorter hydrophilic segments have been tested as topological markers for the true folding pattern of TRK proteins using Saccharomyces cerevisiae Trk2p. Hemagglutinin epitope tags were inserted into all three segments, and the enhanced green fluorescent protein (EGFP) was fused to the C terminus of Trk2p. The gene constructs were expressed from a high copy plasmid, and sidedness of the tags was determined by native fluorescence (EGFP), indirect immunofluorescence, and immunoelectron microscopy. Both the long-loop tag and the C-terminal EGFP fusion allowed abundant protein to reach the plasma membrane and support normal yeast growth. In all determinations, the long-loop tag was localized to the inner surface of the yeast cell plasma membrane, thus strongly supporting the channel-like folding model. Additional observations showed (i). membrane-associated Trk2p to lie in proteolipid rafts; (ii). significant tagged protein, expressed from the plasmid, to be sequestered in cytoplasmic vesicular-tubular clusters; and (iii). suppression of such clusters by yeast growth in 5-10% glycerol. This chaperone-like effect may assist other membrane proteins (overexpressed or heterologously expressed) to function within the yeast plasma membrane.

Role of the plasma membrane H+-ATPase in auxin-induced elongation growth: historical and new aspects

This article will cover historical and recent aspects of reactions and mechanisms involved in the auxin-induced signalling cascade that terminates in the dramatic elongation growth of cells and plant organs. Massive evidence has accumulated that the final target of auxin action is the plasma membrane H(+)-ATPase, which excretes H(+) ions into the cell wall compartment and, in an antiport, takes up K(+) ions through an inwardly rectifying K(+) channel. The auxin-enhanced H(+) pumping lowers the cell wall pH, activates pH-sensitive enzymes and proteins within the wall, and initiates cell-wall loosening and extension growth. These processes, induced by auxin or by the "super-auxin" fusicoccin, can be blocked instantly and specifically by a voltage inhibition of the H(+)-ATPase due to removal of K(+) ions or the addition of K(+)-channel blockers. Vice versa, H(+) pumping and growth are immediately switched on by addition of K(+) ions. Furthermore, the treatment of segments either with auxin or with fusicoccin (which activates the H(+)-ATPase irreversibly) or with acid buffers (from outside) causes an identical transformation and degradation pattern of cell wall constituents during cell-wall loosening and growth. These and other results described below are in agreement with the acid-growth theory of elongation growth. However, objections to this theory are also discussed.

Cell physiological aspects of the plasma membrane electrogenic H+ pump

This article deals with cell physiological aspects of the plasma membrane electrogenic proton (H+) pump and emphasizes the contribution of the giant algal cells of the Characeae in elucidating the mechanism of the pump. First, a history of the development of intracellular perfusion techniques in characean internodal cells is described, including preparation of tonoplast-free cells. Then, an outline of the hypothesis of the electrogenic H+ pump proposed by Kitasato is introduced, who prophesied the existence of an electric potential generated by an active H+ efflux. Subsequently, a history of finding ATP as the direct energy source of the electrogenic ion pump is presented. Quantitative agreement between the pump current and the ATP-dependent H+ efflux supports the notion that the ion carried by the electrogenic ion pump is H+. The role of the H+ pump in regulation of the cytosolic pH is discussed. Mechanisms of light-induced potential change through photosynthesis-controlled activation of the H+ pump are discussed in terms of changes in the levels of adenine nucleotides and in modulation of the Km value for the ATP of H+-ATPase. Recent progress in the molecular mechanism of the blue-light-induced activation of the H+-ATPase in guard cells is presented. However, there are cases where H+-ATPase activity is inhibited by blue light, indicating the flexibility of the control mechanisms of H+-ATPase activity. Finally, modulation of H+-pumping or H+-ATPase activities in response to environmental factors, such as anoxia, membrane excitation, osmotic and salt stresses, nutrient deficiencies and aluminum toxicity are described. Discussions are presented on the regulation of the electrogenic H+ pump.

Intracellular pH homeostasis in the filamentous fungus Aspergillus niger

Intracellular pH homeostasis in the filamentous fungus Aspergillus niger was measured in real time by 31P NMR during perfusion in the NMR tube of fungal biomass immobilized in Ca2+-alginate beads. The fungus maintained constant cytoplasmic pH (pH(cyt)) and vacuolar pH (pH(vac)) values of 7.6 and 6.2, respectively, when the extracellular pH (pH(ex)) was varied between 1.5 and 7.0 in the presence of citrate. Intracellular metabolism did not collapse until a Delta pH over the cytoplasmic membrane of 6.6-6.7 was reached (pH(ex) 0.7-0.8). Maintenance of these large pH differences was possible without increased respiration compared to pH(ex) 5.8. Perfusion in the presence of various hexoses and pentoses (pH(ex) 5.8) revealed that the magnitude of Delta pH values over the cytoplasmic and vacuolar membrane could be linked to the carbon catabolite repressing properties of the carbon source. Also, larger Delta pH values coincided with a higher degree of respiration and increased accumulation of polyphosphate. Addition of protonophore (carbonyl cyanide m-chlorophenylhydrazone, CCCP) to the perfusion buffer led to decreased ATP levels, increased respiration and a partial (1 microm CCCP), transient (2 microm CCCP) or permanent (10 microm CCCP) collapse of the vacuolar membrane Delta pH. Nonlethal levels of the metabolic inhibitor azide (N3-, 0.1 mm) caused a transient decrease in pH(cyt) that was closely paralleled by a transient vacuolar acidification. Vacuolar H+ influx in response to cytoplasmic acidification, also observed during extreme medium acidification, indicates a role in pH homeostasis for this organelle. Finally, 31P NMR spectra of citric acid producing A. niger mycelium showed that despite a combination of low pH(ex) (1.8) and a high acid-secreting capacity, pH(cyt) and pH(vac) values were still well maintained (pH 7.5 and 6.4, respectively).

The role of cytosolic potassium and pH in the growth of barley roots

In an earlier paper we showed that in fully developed barley (Hordeum vulgare L.) root epidermal cells a decrease in cytosolic K+ was associated with an acidification of the cytosol (D.J. Walker, R. A. Leigh, A.J. Miller [1996] Proc Natl Acad Sci USA 93: 10510-10514). To show that these changes in cytosolic ion concentrations contributed to the decreased growth of K+-starved roots, we first measured whether similar changes occurred in cells of the growing zone. Triple-barreled ion-selective microelectrodes were used to measure cytosolic K+ activity and pH in cells 0.5 to 1.0 mm from the root tip. In plants growing from 7 to 21 d after germination under K+-replete conditions, the mean values did not change significantly, with values ranging from 80 to 84 mM for K+ and 7.3 to 7.4 for pH. However, in K+-starved plants (external [K+], 2 &mgr;M), the mean cytosolic K+ activity and pH had declined to 44 mM and 7.0, respectively, after 14 d. For whole roots, sap osmolality was always lower in K+-starved than in K+-replete plants, whereas elongation rate and dry matter accumulation were significantly decreased after 14 and 16 d of K+ starvation. The rate of protein synthesis in root tips did not change for K+-replete plants but declined significantly with age in K+-starved plants. Butyrate treatment decreased cytosolic pH and diminished the rate of protein synthesis in K+-replete roots. Procaine treatment of K+-starved roots gave an alkalinization of the cytosol and increased protein synthesis rate. These results show that changes in both cytosolic pH and K+ can be significant factors in inhibiting protein synthesis and root growth during K+ deficiency.

Potassium homeostasis in vacuolate plant cells

Plant cells contain two major pools of K+, one in the vacuole and one in the cytosol. The behavior of K+ concentrations in these pools is fundamental to understanding the way this nutrient affects plant growth. Triple-barreled microelectrodes have been used to obtain the first fully quantitative measurements of the changes in K+ activity (aK) in the vacuole and cytosol of barley (Hordeum vulgare L.) root cells grown in different K+ concentrations. The electrodes incorporate a pH-selective barrel allowing each measurement to be assigned to either the cytosol or vacuole. The measurements revealed that vacuolar aK declined linearly with decreases in tissue K+ concentration, whereas cytosolic aK initially remained constant in both epidermal and cortical cells but then declined at different rates in each cell type. An unexpected finding was that cytoplasmic pH declined in parallel with cytosolic aK, but acidification of the cytosol with butyrate did not reveal any short-term link between these two parameters. These measurements show the very different responses of the vacuolar and cytosolic K+ pools to changes in K+ availability and also show that cytosolic K+ homeostasis differs quantitatively in different cell types. The data have been used in thermodynamic calculations to predict the need for, and likely mechanisms of, active K+ transport into the vacuole and cytosol. The direction of active K+ transport at the vacuolar membrane changes with tissue K+ status.

Integration and regulation of hyphal tip growth

Hyphal tip growth is an exquisitely controlled process that forms developmentally regulated, species-specific, even-diameter tubes at rates of up to about 50 μm/min. The traditional view is that this process results from the balance between the expansive force of turgor pressure and the controlled extensibility of the apical cell wall. While these elements are involved, the model places regulation into either the global domain (turgor pressure) or the extracellular environment (the cell wall), neither of which seem well suited to the level of control evinced. Recent evidence suggests that F-actin-rich elements of the cytoskeleton are important in tip morphogenesis. Our current models propose that tip expansion is regulated (restrained under normal turgor pressure and protruded under low turgor) by a peripheral network of F-actin that is attached to the plasmalemma and the cell wall by integrin-containing linkages, thus placing control in the cytoplasm where it is accessible to normal intracellular regulatory systems. The F-actin system also functions in cytoplasmic and organelle motility; control of plasmalemma-located, stretch-activated, Ca2+-transporting, ion channel distribution; vectoral vesicle transport; and exocytosis. Regulation of the system may involve Ca2+, the concentration of which is influenced by the tip-high gradient of the stretch-activated channels, thus suggesting a possible feedback regulation mechanism. Key words: tip growth, fungi, stretch-activated channels, F-actin, Ca2+, hyphae.

NO3- transport across the plasma membrane of Arabidopsis thaliana root hairs: kinetic control by pH and membrane voltage

High-affinity nitrate transport was examined in intact root hair cells of Arabidopsis thaliana using electrophysiological recordings to characterise the response of the plasma membrane to NO3- challenge and to quantify transport activity. The NO3(-)-associated membrane current was determined using a three-electrode voltage clamp to bring membrane voltage under experimental control and to compensate for current dissipation along the longitudinal cell axis. Nitrate transport was evident in the roots of seedlings grown in the absence of a nitrogen source, but only 4-6 days postgermination. In 6-day-old seedlings, additions of 5-100 microM NO3- to the bathing medium resulted in membrane depolarizations of 8-43 mV, and membrane voltage (Vm) recovered on washing NO3- from the bath. Voltage clamp measurements carried out immediately before and following NO3- additions showed that the NO3(-)-evoked depolarizations were the consequence of an inward-directed current that appeared across the entire range of accessible voltages (-300 to +50 mV). Both membrane depolarizations and NO3(-)-evoked currents recorded at the free-running voltage displayed quasi-Michaelian kinetics, with apparent values for Km of 23 +/- 6 and 44 +/- 11 microM, respectively and, for the current, a maximum of 5.1 +/- 0.9 muA cm-2. The NO3- current showed a pronounced voltage sensitivity within the normal physiological range between -250 and -100 mV, as could be demonstrated under voltage clamp, and increasing the bathing pH from 6.1 to 7.4-8.0 reduced the current and the associated membrane depolarizations 3- to 8-fold. Analyses showed a well-defined interaction between the kinetic variables of membrane voltage, pHo and [NO3-]o. At a constant pHo of 6.1, depolarization from -250 to -150 mV resulted in an approximate 3-fold reduction in the maximum current but a 10% rise in the apparent affinity for NO3-. By contrast, the same depolarization effected an approximate 20% fall in the Km for transport as a function in [H+]o. These, and additional characteristics of the transport current implicate a carrier cycle in which NO3- binding is kinetically isolated from the rate-limiting step of membrane charge transit, and they indicate a charge-coupling stoichiometry of 2(H+) per NO3- anion transported across the membrane. The results concur with previous studies showing a high-affinity NO3- transport system in Arabidopsis that is inducible following a period of nitrogen-limiting growth, but they underline the importance of voltage as a kinetic factor controlling NO3- transport at the plant plasma membrane.

Mechanism of high-affinity potassium uptake in roots of Arabidopsis thaliana

Potassium is a major nutrient in higher plants, where it plays a role in turgor regulation, charge balance, leaf movement, and protein synthesis. Terrestrial plants are able to sustain growth at micromolar external K+ concentrations, at which K+ uptake across the plasma membrane of root cells must be energized despite the presence of a highly negative membrane potential. However, the mechanism of energization has long remained obscure. Therefore, whole-cell mode patch clamping has been applied to root protoplasts from Arabidopsis thaliana to characterize membrane currents resulting from the application of micromolar K+. Analysis of whole cell current/voltage relationships in the presence and absence of micromolar K+ enabled direct testing of K+ transport for possible energization by cytoplasmic ATP and the respective trans-membrane gradients of Na+, Ca2+, and H+. Subtracted current/voltage relations for K(+)-dependent membrane currents are independent of ATP and reverse at potentials that imply H(+)-coupled K+ transport with a ratio of 1 H+:K+. Furthermore, the reversal potential of the K+ current shifts negative as external H+ activity is decreased. K(+)-dependent currents saturate in the micromolar concentration range with an apparent Km of 30 microM, a value in close agreement with previously reported Km values for high-affinity K+ uptake. We conclude that our results are consistent with the view that high-affinity K+ uptake in higher plants is mediated by a H+:K+ symport mechanism, competent in driving K+ accumulation to equilibrium ratios in excess of 10(6)-fold.

Changes in cytosolic pH and calcium of guard cells precede stomatal movements

Stomatal opening is induced by indoleacetic acid (IAA), cytokinins, and fusicoccin (FC), whereas stomatal closure is induced by abscisic acid (ABA). To test the effect of these growth regulators on guard cell cytosolic Ca2+ ([Ca2+]cyt) and pH (pHcyt), epidermal strips were taken from the lower side of leaves of the orchid Paphiopedilum tonsum and were loaded with acetomethoxy-esterified forms of the Ca2+ indicator fluo-3 or the pH indicator 2',7'-bis(2-carboxyethyl)-5(6)carboxyfluorescein. Basal [Ca2+]cyt ranged from 0.05 to 0.3 M and was 0.22 +/- 0.015 (n = 21). Increases in both [Ca2+]cyt and pHcyt were observed in guard cells after application of 10-100 M ABA to open stomata, and these preceded stomatal closure. The increase in [Ca2+]cyt ranged from 1.5- to 3-fold and was seen in 7 of 10 experiments. Guard cell alkalinization began within 2 min of ABA treatment and continued for the next 8 min. The increase ranged from 0.04 to 0.3 pH unit and was seen in 13 of 14 experiments. Guard cell [Ca2+]cyt increased, whereas pHcyt decreased after treatment of closed stomata with IAA, kinetin, or FC. In response to 50-100 M IAA, [Ca2+]cyt increased 1.5- to 2-fold in all cases, and pHcyt decreased 0.2-0.4 unit within 5 min in 7 experiments. Within 12 min, 10-100 M kinetin caused [Ca2+]cyt to increase in 28 of 34 experiments (1.3- to 2.5-fold) and pHcyt fell 0.1-0.4 unit in 15 of 17 treatments. The response to 10-50 M FC was similar in both time and magnitude. These results show that stomatal opening is accompanied by an increase in [Ca2+]cyt and cytosolic acidification in the guard cells, whereas stomatal closure is preceded by an increase in [Ca2+]cyt and cytosolic alkalinization in the guard cells. The order of these events is still uncertain, but changes in pHcyt are correlated with stomatal movement, and these changes may be an important factor in the regulation of guard cell movement.

Chemoattractant-induced membrane hyperpolarization in Dictyostelium discoideum. A possible role for cyclic GMP

Membrane potentials of the cellular slime mold Dictyostelium discoideum were monitored after chemotactic stimulation by measuring the distribution of the lipophilic cation tetraphenylphosphonium. Stimulation with the chemoattractant cAMP induces a transient membrane hyperpolarization which reaches its most negative value between 1-3 min after stimulation. This hyperpolarization is consistent with the opening of potassium channels. Measurements in streamer F mutant cells reveal that cGMP likely plays a role in the regulation of the cAMP-induced hyperpolarization.

Voltage dependence of the Chara proton pump revealed by current-voltage measurement during rapid metabolic blockade with cyanide

It is generally agreed that solute transport across the Chara plasma membrane is energized by a proton electrochemical gradient maintained by an H(+)-extruding ATPase. Nonetheless, as deduced from steady-state current-voltage (I-V) measurements, the kinetic and thermodynamic constraints on H(+)-ATPase function remain in dispute. Uncertainties necessarily surround long-term effects of the relatively nonspecific antagonists used in the past; but a second, and potentially more serious problem has sprung from the custom of subtracting, across the voltage spectrum, currents recorded following pump inhibition from currents measured in the control. This practice must fail to yield the true I-V profile for the pump when treatments alter the thermodynamic pressure on transport. We have reviewed these issues, using rapid metabolic blockade with cyanide and fitting the resultant whole-cell I-V and difference-current-voltage (dI-V) relations to a reaction kinetic model for the pump and parallel, ensemble leak. Measurements were carried out after blocking excitation with LaCl3, so that steady-state currents could be recorded under voltage clamp between -400 and +100 mV. Exposures to 1 mM NaCN (CN) and 0.4 mM salicylhydroxamic acid (SHAM) depolarized (positive-going) Chara membrane potentials by 44-112 mV with a mean half time of 5.4 +/- 0.8 sec (n = 13). ATP contents, which were followed in parallel experiments, decayed coincidently with a mean half time of 5.3 +/- 0.9 sec [( ATP]t = 0, 0.74 +/- 0.3 mM; [ATP]t = infinity, 0.23 +/- 0.02 mM). Current-voltage response to metabolic blockade was described quantitatively in context of these changes in ATP content and the consequent reduction in pump turnover rate accompanied by variable declines in ensemble leak conductance. Analyses of dI-V curves (+/- CN + SHAM) as well as of families of I-V curves taken at times during CN + SHAM exposures indicated a stoichiometry for the pump of one charge (H+) transported per ATP hydrolyzed and an equilibrium potential near -420 mV at neutral external pH; under these conditions, the pump accounted for approximately 60-75% of the total membrane conductance near Vm. Complementary results were obtained also in fitting previously published I-V data gathered over the external pH range 4.5-7.5. Kinetic features deduced for the pump were dominated by a slow step preceding H+ unloading outside, and by recycling and loading steps on the inside which were in rapid equilibrium.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of fusicoccin action: kinetic modification and inactivation of K(+) channels in guard cells

Fusicoccin commonly is thought to promote secondary solute transport via an increase in electrical driving force which follows the enhancement of primary, "electrogenic" H(+) extrusion by the plant plasma membrane H(+)-ATPase. However, previous electrical studies ofVicia faba L. guard cells in FC (Blatt, 1988, Planta174, 187) demonstrated, in addition to a limited rise in pump current, appreciable declines in membrane conductance near and positive to the free-running membrane potential (V m). Much of the current at these potentials could have been carried by outward-rectifying K(+) channels which were progressively inactivated in FC. We have examined this possibility in electrical studies, using whole-cell currents measured under voltage clamp to quantitate steadystate and kinetic characteristics of the K(+) channels both before and during exposure to FC; channels block in tetraethylammonium chloride was exploited to assess changes in background 'leak' currents. The cells showed little evidence of primary pump activity, a fact which further simplified analyses. Under these conditions, outward-directed K(+) channel current contributed to charge balance maintainingV m, and adding 10 μM FC on average depolarized (positive-going)V m. Steady-state current-voltage relations revealed changes both in K(+) channel and in leak currents underlying the voltage response. Changes in the leak were variable, but on average the leak equilibrium potential was shifted (+)19 mV and leak conductance declined by 21% over 30-40 min in FC. Potassium currents were inactivated irreversibly and with halftimes (current maxima) of 6.2-10.7 min. Inactivation was voltage-dependent, so that the activation ("gating") potential for the current was shifted, positive-going, with time in FC. Channel gating kinetics, inferred from the macroscopic currents, were also affected; current rise at positive potentials accelerated 4.5-fold and more, but in a manner apparently independent of voltage and extracellular potassium concentration. Current decay at negative potentials was quickened, also. These results identify the outward-rectifying K(+) channels as one site of action for FC at a higher plant cell membrane; they complete the link introduced in the preceding paper between K(+) channel current, K(+)((86)Rb(+)) flux and irreversible cation uptake in the toxin. The data also offer some insights toward a kinetic description of channel gating. Finally, they provide a vehicle for interpreting FC-induced changes in K(+) and net H(+) flux, and in membrane potential without the necessity for postulating gross changes in H(+) pumping.

Mechanisms of fusicoccin action: evidence for concerted modulations of secondary K(+) transport in a higher plant cell

Fusicoccin (FC) has long been known to promote K(+) uptake in higher plant cells, including stomatal guard cells, yet the precise mechanism behind this enhancement remains uncertain. Membrane hyperpolarization, thought to arise from primary H(+) pumping stimulated in FC, could help drive K(+) uptake, but the extent to which FC stimulates influx and uptake frequently exceeds any reasonable estimates from Constant Field Theory based on changes in the free-running membrane potential (V m) alone; furthermore, unidirectional flux analyses have shown that in the toxin K(+) ((86)Rb(+)) exchange plummets to 10% of the control (G.M. Clint and E.A.C. MacRobbie 1984, J. Exp. Bot.35 180-192). Thus, the activities of specific pathways for K(+) movement across the membrane could be modified in FC. We have explored a role for K(+) channels in mediating these fluxes in guard cells ofVicia faba L. The correspondence between FC-induced changes in chemical ((86)Rb(+)) flux and in electrical current under voltage clamp was followed, using the K(+) channel blocker tetraethylammonium chloride (TEA) to probe tracer and charge movement through K(+)-selective channels. Parallel flux and electrical measurements were carried out when cells showed little evidence of primary pump activity, thus simplifying analyses. Under these conditions, outward-directed K(+) channel current contributed appreciably to charge balance maintainingV m, and adding 10 mM TEA to block the current depolarized (positive-going)V m; TEA also reduced(86)Rb(+) efflux by 68-80%. Following treatments with 10 μM FC, both K(+) channel current and(86)Rb(+) efflux decayed, irreversbly and without apparent lag, to 10%-15% of the controls and with equivalent half-times (approx. 4 min). Fusicoccin also enhanced(86)Rb(+) influx by 13.9-fold, but the influx proved largely insensitive to TEA. Overall, FC promotednet cation uptake in 0.1 mM K(+) (Rb(+)), despite membrane potentials which were 30-60 mVpositive of the K(+) equilibrium potential. These results tentatively link (chemical) cation efflux to charge movement through the K(+) channels. They offer evidence of an energy-coupled mechanism for K(+) uptake in guard cells. Finally, the data reaffirm early suspicions that FC alters profoundly the K(+) transport capacity of the cells, independent of any changes in membrane potential.

Transcellular ion currents and extension of Neurospora crassa hyphae

Hyphae of Neurospora crassa, like many other tip-growing organisms, drive endogenous electric currents through themselves such that positive charges flow into the apical region and exit from the trunk. In order to identify the ions that carry the current, the complete growth medium was replaced by media lacking various constituents. Omission of K+ or of phosphate diminished the zone of inward current, effectively shifting the current pattern towards the apex. Omission of glucose markedly reduced both inward and outward currents; addition of sodium azide virtually abolished the flow of electric current. Growing hyphae also generate a longitudinal pH gradient: the medium surrounding the apex is slightly more alkaline than the bulk phase, while medium adjacent to the trunk turns acid. The results suggest that Neurospora hyphae generate a proton current; protons are expelled distally by the H+-ATPase and return into the apical region by a number of pathways, including the symport of protons with phosphate and potassium ions. Calcium influx may also contribute to the electric current that enters the apical region. There seems to be no simple obligatory linkage between the intensity of the transcellular electric current and the rate of hyphal extension. Calcium ions, however, are required in micromolar concentrations for extensions and morphogenesis of hyphal tips.

Mechanisms of fusicoccin action: A dominant role for secondary transport in a higher-plant cell

Fusicoccin (FC) is commonly thought to promote "electrogenic" H(+) extrusion through its action on the H(+)-ATPase of the plant plasma membrane. Nonetheless, essential support from rigorous electrophysiological analysis has remained largely absent. The present investigation surveys the effects of FC on the charge transport properties at the membrane of a higher-plant cell - stomatal guard cells of Vicia faba L. - for which the electrical geometry is defined, and from which the voltage-dependent kinetic characteristic for the pump has been identified. Current-voltage (I-V) relations of the guard cells were determined before and during treatments with FC, and during brief exposures to NaCN plus salicylhydroxamic acid. Responses of the pump and of the ensemble of secondary transport processes were identified in the whole-membrane conductance-voltage relations and in the difference-current-voltage (dI-V) characteristic for the pump. In 0.1 mM K(+), exposure to 10 μM FC shifted guard-cell potentials negative by 29-61 mV. Current-and conductance-voltage profiles indicated limited changes in the pump I-V characteristic, an observation which was confirmed through explicit kinetic analysis of pump dI-V relations. However, the voltage response was accompanied by a 1.5-to 2.6-fold fall in membrane conductance. These results challenge conventional views of fusicoccin action by ascribing the electrical responses to reduced current passage through secondary transport pathways as well as to enhanced electrogenic ion pumping.

Transcellular proton current in Achlya bisexualis hyphae: relationship to polarized growth

Growing hyphae of Achlya bisexualis drive an electric current through themselves, such that positive charge flows into the apical region (the anterior 300 micron) and exits distally along the hyphal trunk. They also generate a gradient of extracellular pH, such that the medium surrounding the apex is slightly alkaline whereas that along the hyphal trunk is acid. To explore the genesis of these gradients and their relationship to polarized extension, we examined the effects of changes in the composition of the growth medium. The transcellular electric current was most pronounced in medium rich in amino acids. In leaner medium, containing limited amounts of amino acids or none at all, the current was attenuated or absent. We interpret the results to mean that inward current represents H+/amino acid symport, mediated by porters that are preferentially localized in the apical region. Apical alkalinity may be due to ammonia production. Outward current, and perhaps also the generation of metabolic acid, reflects the distribution of the H+-ATPase, which is excluded from the apex but is abundant along the hyphal trunk. Thanks to the spatial segregation of transport functions, protons characteristically flow into the apical region. However, since hyphae grow apically and at the same rate despite wide variations in current pattern, the flow of electric charge through the hyphae cannot be required to polarize extension or to localize the tip.

Potassium-proton symport in Neurospora: kinetic control by pH and membrane potential

Active transport of potassium in K+-starved Neurospora was previously shown to resemble closely potassium uptake in yeast, Chlorella, and higher plants, for which K+ pumps or K+/H+-ATPases had been proposed. For Neurospora, however, potassium-proton cotransport was demonstrated to operate, with a coupling ratio of 1 H+ to 1 K+ taken inward so that K+, but not H+, moves against its electrochemical gradient (Rodriguez-Navarro et al., J. Gen. Physiol. 87:649-674). In the present experiments, the current-voltage (I-V) characteristic of K+-H+ cotransport in spherical cells of Neurospora has been studied with a voltage-clamp technique, using difference-current methods to dissect it from other ion-transport processes in the Neurospora plasma membrane. Addition of 5-200 microM K+ to the bathing medium causes 10-150 mV depolarization of the unclamped membrane, and yields a sigmoid I-V curve with a steep slope (maximal conductance of 10-30 microS/cm2) for voltages of -300 to -100 mV, i.e., in the normal physiologic range. Outside that range the apparent I-V curve of the K+-H+ symport saturates for both hyperpolarization and depolarization. It fails to cross the voltage axis at its predicted reversal potential, however, an effect which can be attributed to failure of the I-V difference method under reversing conditions. In the absence of voltage clamping, inhibitors-such as cyanide or vanadate-which block the primary proton pump in Neurospora also promptly inhibit K+ transport and K+-H+ currents. But when voltage clamping is used to offset the depolarizing effects of pump blockade, the inhibitors have no immediate effect on K+-H+ currents. Thus, the inhibition of K+ transport usually observed with these agents reflects the kinetic effect of membrane depolarization rather than any direct chemical action or the cotransport system itself. Detailed study of the effects of [K+]o and pHo on the I-V curve for K+-H+ symport has revealed that increasing membrane potential systematically decreases the apparent affinity of the transporter for K+, but increases affinity for protons (Km range: for [K+]o, 15-45 microM; for [H+]o, 10-35 nM). This behavior is consistent with two distinct reaction-kinetic models, in which (i) a neutral carrier binds K+ first and H+ last in the forward direction of transport, or (ii) a negatively charged carrier (-2) binds H+ first and K+ last.

Electrical characteristics of stomatal guard cells: The ionic basis of the membrane potential and the consequence of potassium chlorides leakage from microelectrodes

The membrane electrical characteristics of stomatal guard cells in epidermal strips from Vicia faba L. and Commelina communis L. were explored using conventional electrophysiological methods, but with double-barrelled microelectrodes containing dilute electrolyte solutions. When electrodes were filled with the customary 1-3 M KCl solutions, membrane potentials and resistances were low, typically decaying over 2-5 min to near-30 mV and <0.2 kω·cm(2) in cells bathed in 0.1 mM KCl and 1 mM Ca(2+), pH 7.4. By contrast, cells impaled with electrodes containing 50 or 200 mM K(+)-acetate gave values of-182±7 mV and 16±2 kω·cm(2) (input resistances 0.8-3.1 Gω, n=54). Potentials as high as (-) 282 mV (inside negative) were recorded, and impalement were held for up to 2 h without appreciable decline in either membrane parameter. Comparison of results obtained with several electrolytes indicated that Cl(-) leakage from the microelectrode was primarily responsible for the decline in potential and resistance recorded with the molar KCl electrolytes. Guard cells loaded with salt from the electrodes also acquired marked potential and conductance responses to external Ca(2+), which are tentatively ascribed to a K(+) conductance (channel) at the guard cell plasma membrane.Measurements using dilute K(+)-acetate-filled electrodes revealed, in the guard cells, electrical properties common to plant and fungal cell membranes. The cells showed a high selectivity for K(+) over Na(+) (permeability ratio PNa/PK=0.006) and a near-Nernstian potential response to external pH over the range 4.5-7.4 (apparent PH/PK=500-600). Little response to external Ca(2+) was observed, and the cells were virtually insensitive to CO2. These results are discussed in the context of primary, charge-carrying transport at the guard cell plasma membrane, and with reference to possible mechanisms for K(+) transport during stomatal movements. They discount previous notions of Ca(2+)-and CO2-mediated transport control. It is argued, also, that passive (diffusional) mechanisms are unlikely to contribute to K(+) uptake during stomatal opening, despite membrane potentials which, under certain, well-defined conditions, lie negative of the potassium equilibrium potential likely prevailing.