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  • 1
    Electronic Resource
    Electronic Resource
    Anion Channels in Plants (1992)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Plant Physiology and Plant Molecular Biology 43 (1992), S. 351-373 
    Details
    ISSN: 1040-2519
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.pp.43.060192.002031
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  • 2
    Electronic Resource
    Electronic Resource
    Guard cell pressure/aperture characteristics measured with the pressure probe (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 18 (1995), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Pressure within guard cells in strips of intact epidermis of Tradescantia virginiana was controlled with a pressure probe apparatus after the guard cells had been filled with silicone oil. Pressure was increased and decreased incrementally between 0.0 and 4.1 MPa to cause inflation and deflation of the guard cells. At steady-state guard cell pressures, the width of the stomatal pore was recorded and plotted against pressure. The pressure required for near-maximum aperture was 4.1 MPa. Aperture as a function of pressure was sigmoidal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1995.tb00583.x
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  • 3
    Electronic Resource
    Electronic Resource
    Plant aquaporins: multifunctional water and solute channels with expanding roles (2002)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 25 (2002), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.0016-8025.2001.00791.x
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  • 4
    Electronic Resource
    Electronic Resource
    Calcium-dependent K current in plasma membranes of dermal cells of developing bean cotyledons (2004)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 27 (2004), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: In developing seeds of bean (Phaseolus vulgaris L.), phloem-imported assimilates (largely sucrose and potassium) are released from coats to seed apoplasm and subsequently retrieved by the dermal cell complexes of cotyledons. To investigate the mechanisms of K+ uptake by the cotyledons, protoplasts of dermal cell complexes were isolated and whole-cell currents across their plasma membranes were measured with the patch-clamp technique. A weakly rectified cation current displaying a voltage-dependent blockade by external Ca2+ and acidic pH, dominated the conductance of the protoplasts. The P haseolus v ulgaris Cotyledon Dermal-cell pH and Calcium-dependent Cation Conductance (Pv-CD-pHCaCC) was highly selective for K+ over Ca2+ and Cl–. For K+ current through Pv-CD-pHCaCC a sigmoid shaped current–voltage (I–V) curve was observed with negative conductance at voltages between −200 and −140 mV. This negative K+ conductance was Ca2+ dependent. With other univalent cations (Na+, Rb+, NH4+) the currents were smaller and were not Ca2+ dependent. Reversal potentials remained constant when external K+ was substituted with these cations, suggesting that Pv-CD-pHCaCC channels were non-selective. The Pv-CD-pHCaCC would provide a pathway for K+ and other univalent cation influx into developing cotyledons. These cation influxes could be co-ordinated with sucrose influx via pH and Ca2+dependence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.2004.01152.x
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  • 5
    Electronic Resource
    Electronic Resource
    The effect of different growing conditions on water relations parameters of leaf epidermal cells of Tradescantia virginiana L. (1984)
    Springer
    Oecologia 62 (1984), S. 110-117 
    Details
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Tradescantia virginiana L. plants were cultivated under contrasting conditions of temperature, humidity, light quality and intensity, and nutrient status in order to investigate the effect of growth conditions on the water relations parameters of the leaf epidermal cells. Turgor pressure (P), volumetric elastic modulus (ɛ), half-time of water potential equilibration (T 1/2), hydraulic conductivity (L p ) were measured with the miniaturized pressure probe in single cells of the upper and lower epidermis of leaves. Turgor differed (range: 0.1 bar to 7.2 bar) between treatments with lowest values under warm and humid conditions and additional supply of fertilizer, and highest values under conditions of low air humidity and low nutrient supply. The volumetric elastic modulus changed by 2 orders of magnitude (range: 3.0 bar to 350 bar, 158 cells), but ɛ was only affected by the treatments, in as much as it was dependent on turgor. The turgor dependence of ɛ, measured on intact leaves of T. virginiana, was similar to that for cells of the isolated (peeled) lower epidermis, where ɛ as a function of turgor was linear over the whole range of turgors. This result has implications for the discussion of pressure/volume curves as measured by the pressure bomb where changes in “bulk leaf ɛ” are frequently discussed as “adaptations” to certain treatments. The measurements of the hydraulic conductivity indicate that this parameter varies between treatments (range of means: 2.4×10-6 cm s-1 bar-1 to 13.4×10-6 cm s-1 bar-1). There was a negative correlation for L p in cells of intact leaves as a function of turgor which was altered by the growing conditions. However, a correlation with turgor could not be found for cells from isolated epidermis or cells from a uniform population of plants. The large variation in L p from cell to cell observed in the present and in previous studies was accounted for in a study of 100 cells from a uniform population of plants by the propagation of measurement errors in calculating L p . The results suggest that in T. virginiana cellular water relations are changed mainly by the turgor dependence of ɛ.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00377383
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  • 6
    Electronic Resource
    Electronic Resource
    A channel that allows inwardly directed fluxes of anions in protoplasts derived from wheat roots (1994)
    Springer
    Planta 192 (1994), S. 295-305 
    Details
    ISSN: 1432-2048
    Keywords: Chloride channel ; Nitrate ; Potassium channel ; Salinity ; Triticum roots
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract An anion channel that only allows outward current flow (anion influx) has been identified in protoplasts derived from wheat (Triticum aestivum L., Triticum turgidum L.) roots. The anion outward rectifier (anion OR) measured by patch-clamp of whole cells activated very quickly, usually reaching a steady-state level in less than 100 ms and was easily distinguished from the cation outward rectifier (cation OR) which activated more slowly during membrane depolarisation. The anion OR is permeable to NO 3 − and Cl−, moderately permeable to I−, and relatively impermeable to H2PO4/− and ClO4/−. An anomalous mole-fraction effect between ClO4/ − and Cl− was observed on the outward current, indicating that the channel is a multi-ion pore. The anion OR is gated by both voltage and external anion concentration such that it activates near to the equilibrium potential for the permeant anion. It activated at more negative membrane potentials when NO 3 − was substituted for Cl− in the external medium, indicating that the channel may function to allow NO 3 − influx under luxuriant external NO 3 − concentrations. For most experiments, K+ and Cl− were the main cation and anion in solution, and under these conditions it appeared likely that the anion OR functioned in membrane-potential regulation by facilitating a Cl− influx at membrane potentials more positive than the chloride reversal potential (ECl). If ECl was more negative than the K+ reversal potential (EK) then the anion OR dominated but both the anion and cation ORs occurred together when the membrane potential difference (Vm) was positive of both ECl and EK. The cation OR was inhibited by increasing external Cl− concentrations, but the anion OR was not affected by external K+ or Na+ concentration. The anion-transport inhibitors, zinc and phenylglyoxal were ineffective in blocking the anion OR. 4,4′-Di-isothiocyanostilbene-2, 2′-disulfonic acid (DIDS) irreversibly blocked about 34% of the current when applied extracellularly at a concentration of 25 μM, and about 69% at a concentration of 200 μM. However, DIDS (200 μM) also occasionally acted as an irreversible blocker of the cation OR. Perchlorate blocked irreversibly 75% of the current at an external concentration of 10 mM and did not block the cation OR. Whole-cell currents also indicated that the anion OR was insensitive to external pH (pH=5–7) and calcium concentration ([Ca2+]=0.1–10 mM). Increasing intracellular calcium concentration significantly increased the occurrence of the fast outward current in whole cells (P 〈 0.005, X2 test). With approximately 10 nM calcium inside the cell the anion outward current was observed in 64% (n = 45) of cells and with 50 nM calcium inside the cell the anion current was observed in 88% (n = 69) of cells. Single-anion OR channels observed in outside-out patches had a conductance in 300 mM KCl (external) of about 4 pS. When voltage pulses were applied to outside-out patches the average currents were similar to those observed in whole cells. The significance of the anion OR as a likely route for Cl uptake in high salinities is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00198563
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  • 7
    Electronic Resource
    Electronic Resource
    Inward membrane current inChara inflata: II. Effects of pH, Cl−-channel blockers and NH 4 + , and significance for the hyperpolarized state (1986)
    Springer
    The journal of membrane biology 89 (1986), S. 153-161 
    Details
    ISSN: 1432-1424
    Keywords: Cl− channels ; Chara inflata ; pH ; channel blockers ; NH 4 +
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The Cl− component of the voltage- and time-dependent inward current activated by hyperpolarizing the membrane ofChara inflata increases exponentially as the external pH, pH o , is lowered from 7 with the membrane potential difference (PD) kept constant. Lanthanum and anthracene-9-carboxylic acid (A-9-C, a Cl− channel blocker) both blocked the Cl− component and removed the pH o sensitivity of the inward current. Lanthanum, however, also decreased the K+ conductance. The hyperpolarized membrane is depolarized by A-9-C in a manner similar to that caused by the removal of external Cl−. Low external concentrations of NH 4 + stimulated the Cl− component of the inward current probably as a result of a change in cytoplasmic pH rather than as a result of a change in cytoplasmic [Cl−], since the effect was observed in Cl−-free solutions. The results show that the membrane PD, at hyperpolarized levels, is most likely determined by two factors: the proton extrusion pump, provided it has a reversal PD more negative than about −300 mV, and a voltage-dependent Cl− leak.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869711
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  • 8
    Electronic Resource
    Electronic Resource
    Inward membrane current inChara inflata: I. A voltage- and time-dependent Cl− component (1986)
    Springer
    The journal of membrane biology 89 (1986), S. 139-152 
    Details
    ISSN: 1432-1424
    Keywords: Cl− channels ; ion channels ; Chara inflata ; membranes ; punchthrough
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary An inward current which increases in magnitude over a period of seconds is activated when the membrane ofChara inflata (a green alga) in a K+-conductive state is hyperpolarized by a voltage clamp. The peak current and the half-time of activation are exponentially dependent on membrane potential difference. It was found by using an external Cl− electrode that the component exponentially dependent on potential was due to an efflux of Cl−. The measured current-voltage curves and the kinetics of deactivation of the current showed that other time-dependent components contributed to the net inward current. The “punchthrough” theory of Coster (Biophys. J. 5:669–686, 1965) does not adequately explain the inward current since a “punchthrough potential” could not be obtained, and the inward current was distinctly time dependent. The voltage and time dependence of the inward current strongly suggests that the Cl− efflux activated by hyperpolarization is through voltage-gated channels which open more frequently as the membrane is hyperpolarized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869710
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  • 9
    Electronic Resource
    Electronic Resource
    Ion channels in the plasma membrane ofAmaranthus protoplasts: One cation and one anion channel dominate the conductance (1991)
    Springer
    The journal of membrane biology 121 (1991), S. 223-236 
    Details
    ISSN: 1432-1424
    Keywords: plant ion channels ; patch clamp ; plasma membrane ; anion channel ; cation channel ; multistate channel
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons ofAmaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+〉Na+〉Cl− (1∶0.16∶0.03); whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of sub-conductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (V m ) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2mm) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO 3 − 〉Cl−〉K+〉Aspartate (2.04∶1∶0.18 to 0.09∶0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clampV m at a preferred value in the face of events which would otherwise perturbV m .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01951556
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  • 10
    Electronic Resource
    Electronic Resource
    Pump and K+ inward rectifiers in the plasmalemma of wheat root protoplasts (1994)
    Springer
    The journal of membrane biology 139 (1994), S. 103-116 
    Details
    ISSN: 1432-1424
    Keywords: Pump ; Inward rectifiers ; Wheat ; K+ channels ; Plasma membrane
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract An electrogenic pump, a slowly activating K+ inward rectifier and an intermittent, “spiky,” K+ inward rectifier, have been identified in the plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high external concentrations of K+ of 100 m m, the pump can maintain the membrane potential difference (PD) down to −180 mV, more negative than the electrochemical equilibrium potentials of the various ions in the system. The slowly activating K+ inward rectifier, apparent in about 23% of protoplasts, allows inward current flow when the membrane PD becomes more negative than the electrochemical equilibrium potential for K+ by about 50 mV. The current usually consists of two exponentially rising components, the time constant of one about 10 times greater than the other. The longer time constant is voltage dependent, while the smaller time constant shows little voltage dependence. The rectifier deactivates, on return of the PD to less negative levels, with a single exponential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, allows intermittent current, of considerable magnitude, through the plasmalemma at PDs usually more negative than about −140 mV. Patch clamp experiments on detached outside-out patches show that a possibly multi-state K+ channel, with maximum conductance greater than 400 pS, may constitute this rectifier. The paper also considers the role of the pump and the K+ inward rectifiers in physiological processes in the cell.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00232429
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