Electrical Evidence for Turgor Inhibition of Proton Extrusion in Sugar Beet Taproot

Abstract

Sections of sugar beet (Beta vulgaris L.) taproot were incubated in various concentrations of mannitol. At 0.4, 0.6, and 0.8 molar, the membrane electrical potential difference (E.) averaged about -130 millivolts; at 0.2 molar, about -90 millivolts; and at 0 molar, between -60 and -80 millivolts. Additions of 10 millivolts acetate to the incubation solutions (all at pH 5) enhanced the membrane polarity to about -200 millivolts. We conclude from these and previous findings that high turgor inhibits proton extrusion in the sugar beet, but that proton extrusion can be activated in fully turgid tissue by acidification of the cytoplasm. A possible function of this turgor effect may be the control of turgor itself.sliced and pushed into Lucite tissue holders.Tissue-containing holders were incubated in a solution (referred to as IX here and in previous publications [3,7]) composed of 1 mM NaCl, 1 mm Ca(NO3)2, 1 mm KH2PO4, and 0.25 mM MgSO4. This solution was variously supplemented with mannitol, acetic acid, and NaCl as described in the individual experiments. The pH of I X was 5.0, and solutions supplemented with acetic acid were adjusted back to pH 5.0 with NaOH. After incubation times of one to several hours the tissue-containing holders were inserted into Lucite chambers fitted with inlet and outlet ports for the perfusion solutions. These chambers were used in an assembly for implanting glass microelectrodes used for the measurement of Em (3, 7).Incubation of plant tissues in solutions of mannitol, or other osmotica, sometimes stimulates acidification of the bathing media (4,5,12,17) and enhances the rate of solute uptake (2,5,12,14,17). Because many solutes are cotransported with protons across plant cell membranes (3, 7, 1 1), the observed effects of the osmotica upon plant tissues can be explained by the hypothesis that high turgor inhibits the proton extrusion pump located in the plasmalemma. The action of this pump contributes to the membrane Em' and the A[H+] across the membrane, and thereby develops the proton motive force (= Em-59ApH, units in mV, T = 250C) that drives cotransport (11,15 Figure 1. Contrary to our experience with oat coleoptiles, stable, longlasting measurements in I X with 0 M mannitol were difficult to obtain. These measurements, whether stable, unstable, long lasting, or brief were typically of a low negative value. We incorporated into Figure 1 all measurements more negative than -60 mV of at least 15-s duration. The -60 mV cutoff was chosen because negative readings up to -40 mV could be obtained by pushing the electrode into broken cell fragments. At 0.2 and 0.4 M mannitol stable measurements were easier to obtain, but at 0.6 and 0.8 M mannitol, implantations became difficult again as the tissue became flaccid. (Osmotic concentration of the cell solution ranged from 0.6 to 0.75 M [17]). Attempts to measure Em during transitions from one mannitol concentration to another failed, presumably because of the movement of the tissue during changes in turgor.Taking the measurement...

Pressure Regulation of the Electrical Properties of Growing Arabidopsis thaliana L. Root Hairs

“…In this case, the results confirm previous reports (Kinraide and Wyse, 1986;Li and Delrot, 1987). In hypo-osmotic treatments, the depolarization and increased conductance are consistent with increased ion efflux from the cell, with C1-I a likely candidate.…”

“…Although there are reports of osmoticum-induced changes in the membrane potential of higher plant cells (Kinraide and Wyse, 1986;Li and Delrot, 1987), it is possible that the electrical properties of root hairs-membrane potential, conductance, input resistance, and cell-to-cell coupling-were unaffected by changes in turgor pressure because the root hairs simply lack any turgor regulation. Therefore, the effect of indirect modulation of turgor by extracellular osmotica was examined.…”

“…In higher plants, it is known that hyperosmotic treatment causes hyperpolarization of the membrane potential (Kinraide and Wyse, 1986;Li and Delrot, 1987) and extracellular acidification (Reinhold et al, 1984;Curti et al, 1993), which has been attributed to modulation of the proton-pumping activity, resulting in short-term osmoregulation to maintain a constant turgor. Turgor is the "driving force" in cellular expansion.…”

Osmotic Dependence of the Transmembrane Potential Difference of Broadbean Mesocarp Cells

“…Although this possibility has received support from proton fluxes measurements (3,9,19,25), little information is still available on the osmotic dependence of the PD, particularly in higher plant cells (10,15). Ion fluxes, which may act as compensation charge for H+ fluxes, may be also affected by osmotic conditions, and it is interesting to know whether and how the changes in proton-pumping activity are translated in terms of PD.…”

Carbohydrate transport in discs of storage parenchyma of celery petioles

“…However, generally observed effects of low turgor seem to be (i) stimulation of F,,,^^ of the sugar carrier (at constant K^ and (ii) acidification of uptake medium by the tissue. The combination of these two effects has led to the working-hypothesis that the lowering of turgor stimulates the plasmalemma H^-ATPase providing enhanced proton motive force to drive the sugar/H^-cotransporter more quickly (Reinhold, Seiden & Volokita, 1984;Kinraide & Wyse, 1986;Wyse, Zamski & Tomos, 1986;Daie, 19876).…”