D. Cornel

Plasma Membrane Depolarization Induced by Abscisic Acid in Arabidopsis Suspension Cells Involves Reduction of Proton Pumping in Addition to Anion Channel Activation, Which Are Both Ca2+ Dependent

In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H+-ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H+ pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H+-ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca2+]cyt increase induced by ABA since increase in [Ca2+]cyt enhanced anion channels and impaired H+-ATPases.

Plasmalemma Abscisic Acid Perception Leads to RAB18 Expression via Phospholipase D Activation in Arabidopsis Suspension Cells

Abscisic acid (ABA) plays a key role in the control of stomatal aperture by regulating ion channel activities and water exchanges across the plasma membrane of guard cells. Changes in cytoplasmic calcium content and activation of anion and outward-rectifying K+ channels are among the earliest cellular responses to ABA in guard cells. In Arabidopsis suspension cells, we have demonstrated that outer plasmalemma perception of ABA triggered similar early events. Furthermore, a Ca2+influx and the activation of anion channels are part of the ABA-signaling pathway leading to the specific expression ofRAB18. Here, we determine whether phospholipases are involved in ABA-induced RAB18 expression. Phospholipase C is not implicated in this ABA pathway. Using a transphosphatidylation reaction, we show that ABA plasmalemma perception results in a transient stimulation of phospholipase D (PLD) activity, which is necessary for RAB18 expression. Further experiments showed that PLD activation was unlikely to be regulated by heterotrimeric G proteins. We also observed that ABA-dependent stimulation of PLD was necessary for the activation of plasma anion current. However, when ABA activation of plasma anion channels was inhibited, the ABA-dependent activation of PLD was unchanged. Thus, we conclude that in Arabidopsis suspension cells, ABA stimulation of PLD acts upstream from anion channels in the transduction pathway leading to RAB18 expression.

Electrogenic active proton pump in Hevea brasiliensis laticiferous cells: Its role in activating sucrose/H+ and glucose/H+ symports at the plasma membrane

Abstract The transplasmalemmal electrical gradient recorded in laticiferous cells at steady state was −113 ± 21 mV. Sucrose and glucose depolarize the plasmalemma of laticiferous cells by about 15 to 25 mV. Our results show that with depolarization due to sucrose (1 mM) or glucose (1 mM) a slight alkalinization (0.1 to 0.2 pH units) can be detected on the outer surface of the cell. Fructose and 3-O-methyl-glucose have no such effect. The extent of depolarization due to the addition of sugars is lower than the electrogenic component of the membrane potential produced by the functioning of the H+-excretion pump (vanadate sensitive-ATPase). Furthermore, in the presence of vanadate or DNP, with glucose or sucrose no shift in pH value was observed. The effect of phlorizin has been tested on the shift of the membrane potential due to sugar uptake across the plasmalemma: neither sucrose nor glucose demonstrate any further depolarization and alkalinization in the presence of phlorizin. Stimulation of the H+-pump by ethylene hyperpolarizes cells by approximately −40 mV and increases the extent of the depolarization induced by sugar transport. These results suggest an active transport of the sugars from the apoplasm towards the cytosol. Evidence for the existence of H+ cotransport with sucrose and/or glucose at the plasmalemma is discussed hereafter.

Energetics of OH− or H+ dependent nitrate uptake by Catharanthus roseus cells: Electrophysiological effects

Abstract The electrical potential across the plasmalemma and the tonoplast were recorded, in Catharanthus roseus cells, by pushing a glass microelectrode through a cell with the tip consecutively in the cell wall, the cytoplasm and the vacuole. The electrical potential difference between the cytoplasm and the external medium (ECO) was about −71 mV and the mean potential difference at the tonoplast (EVC) about +22 mV. In culture conditions, during the first two days following the transfer of the cells into a fresh medium, nitrate uptake by the cells was marked by a hyperpolarization of the plasmalemma (about −18 mV) and a simultaneous alkalinization of the external medium (1.3 pH units). Similar data were also obtained in short experiments (less than 10 min) with cells bathed in nitrate solutions (10 mM NaNO3). Evidence for the existence of nitrate cotransport with H+ or OH− as counterions at the plasmalemma is discussed.

Sugar absorption and potassium channels in protoplasts of Hevea brasiliensis laticiferous vessels

Abstract Laticiferous vessels of Hevea brasiliensis were isolated from stem segments by enzymatic digestion of the cell wall. The laticiferous vessels keep their anastomosed network shape as in the intact tissue. After cell-wall hydrolysis, the isolated laticiferous protoplasts have a negative polarization (negative inside) and functional voltage sensitive inward K + channels. The transplasmalemmal electric gradient is lower than that for laticiferous vessels in place inside the tissue in the same medium (about −35 mV instead of −115 mV). In the absence of cell wall, and therefore of cell-wall invertase, we observe that sucrose induces depolarization as in intact vessels. Whereas fructose and 3- O -methylglucose do not produce any depolarization on intact laticiferous vessels, a depolarization of laticiferous vessel protoplasts is observed upon addition of these sugars under the conditions used for glucose and sucrose. With sucrose, the depolarizing effect was blocked by phlorizin as in intact vessels. Furthermore, the link between transport of different sugars and the function of the proton excretion pump endorses the hypothesis that electrogenic symports are present on the plasmalemma of the laticiferous vessels. Two specific symports, one to transport hexoses and the other to transport sucrose, could be involved.

Nitrate Uptake in Catharanthus Roseus Cells: Electrophysiological Effects

Data on electrophysiological parameters of nitrate uptake are rather rare in plant cells. The distinction between the electrical effect of NO3 − at the plasmalemma and at the tonoplast turns out to be too complex for a heterogeneously polarized system, especially because the localization of the microelectrode tip in the cytosol or vacuole is often unknown. Transcellular potential changes connected to an external alkalinization upon addition of nitrate have often been considered as an indication of an unbalanced stoichiometry of a nitrate cotransport mechanism located at the plasma membrane. The wide-spread neglect of the positive trans-tonoplast potential in studies of nitrate effects on membrane potential can lead to errors in interpreting hyperpolarization phenomena (Rona et al. 1980 a,b; Barbier-Brygoo et al. 1985; Chedhomme and Rona 1988). For cell hyperpolarization, it has been suggested that NO3 −/OH− antiport functions as the main transport mechanism for nitrate across the plasmalemma (Thibaud and Grignon 1981; Monestiez et al. 1987), in the case of cell depolarization upon addition of nitrate a NO3 −/2H+ symport (Ullrich and Novacky 1981; Ullrich 1987) has been proposed. Nevertheless, it has been reported in the literature that the electropositive gradient across the tonoplast is also affected by nitrate uptake into the vacuole, causing a dissipation of the passive trans-tonoplast potential (Poole and Blumwald 1987; Leigh and Pope 1987) and a decrease of the positive electrogenic component in connection with the partial inhibition of the tonoplast ATPase (Bennett and Spanswick 1984; Sze 1984; Jochem et al. 1984; Griffith et al. 1986; Chedhomme and Rona 1986).

Some Characteristics of Tonoplast NO3 − Transport Processes on Acer Pseudoplatanus L. Cells

Data on the NO3 − accumulation mechanisms at the tonoplast has remained widely unknown. NO3 − uptake induced a depolarization or an hyperpolarization of cells (Ullrich and Novacky, 1981; Thibaud and Grignon, 1981). This can be classically attributed to an electrogenic 0H−/NO3 − antiport or H+/NO3 − symport at the plasmalemma (Rona and Cornel, 1984; Thibaud and Grignon, 1981). But the existence of an NO3inhibited and Cl−-stimulated Mg-ATPase on tonoplast is now well established (Sze, 1985). Furthermore, NO3 − vacuolar net accumulation could give rise to depolarization of the tonoplastic membrane. Nitrate induced hyperpolarization in cells could result partly from the depolarization of the tonoplast. The aim of these experiments was to investigate some of the NO3 − vacuolar uptake characteristics and those of related electrical events.