Turgor regulation in osmotically stressed Arabidopsis epidermal root cells. Direct support for the role of inorganic ion uptake as revealed by concurrent flux and cell turgor measurements

Abstract

Hyperosmotic stress is known to significantly enhance net uptake of inorganic ions into plant cells. Direct evidence for cell turgor recovery via such a mechanism, however, is still lacking. In the present study, we performed concurrent measurements of net ion fluxes (with the noninvasive microelectrode ion flux estimation technique) and cell turgor changes (with the pressure-probe technique) to provide direct evidence that inorganic ion uptake regulates turgor in osmotically stressed Arabidopsis epidermal root cells. Immediately after onset of hyperosmotic stress (100/100 mM mannitol/sorbitol treatment), the cell turgor dropped from 0.65 to about 0.25 MPa. Turgor recovery started within 2 to 10 min after the treatment and was accompanied by a significant (30-80 nmol m-2 s-1) increase in uptake of K+, Cl-, and Na+ by root cells. In most cells, almost complete (>90% of initial values) recovery of the cell turgor was observed within 40 to 50 min after stress onset. In another set of experiments, we combined the voltage-clamp and the microelectrode ion flux estimation techniques to show that this process is, in part, mediated by voltage-gated K+ transporters at the cell plasma membrane. The possible physiological significance of these findings is discussed.

Figure 1

Example of turgor measurements from an Arabidopsis root epidermal cell. The dotted baseline is the pressure required to offset capillary action to bring the APW/oil interface to the tip. This is the “zero” pressure used in measurements of cell turgor pressure. The micropipette was impaled into the cell, and the position of the meniscus was adjusted to the tip by applying additional pressure to the piston. To make sure that the pipette tip did not become plugged, the meniscus was periodically (every 1.5–2 min) brought out of the cell to clear the tip and then immediately returned to the tip. Due to restricted flow through the small aperture of the micropipette, the pressures recorded during meniscus movement are overshoots and do not reflect the pressure in the cell. For this reason, cell turgor pressure was measured after the rapid transient peak, when the meniscus was adjusted to the tip, but before the slower movement of the meniscus back from the tip because of expansion of the teflon tubing. Hyperosmotic treatment (100/100 mm mannitol/sorbitol) was given at time zero. Cell turgor recovery started approximately 5 min after stress onset. At the end of experiment, the probe was taken out of the cell, and the meniscus position was adjusted in APW (indicated by an arrow) to assure the baseline pressure had not change during the experiment.

Figure 2

Kinetics of osmotically induced changes in cell turgor (A) and net fluxes of K⁺ (B), Cl⁻ (C), and Na⁺ (D) in Arabidopsis root cells. Data are mean ± se (n = 7 individual plants for data presented in A and n = 5 for ion flux data shown in B–D). Hyperosmotic treatment was given at time zero. Almost complete (>90% of initial value) turgor recovery was observed within 40 min after stress onset.

Figure 3

Osmotically induced changes in plasma MP in Arabidopsis root cells. A, Example of a long-term record from one typical cell. B, Average data for seven plants (error bars are se). Immediate and prolonged manniol-induced hyperpolarization was observed in all cells.

Figure 4

Control of net K⁺ fluxes by voltage clamping of Arabidopsis root hair cell. A typical root hair impaled with a double-barreled microelectrode for voltage-clamp and current measurements and a K⁺-selective MIFE microelectrode for K⁺ flux measurements is shown in Figure 7B. A, Voltage-clamping protocol for one typical experiment. A bipolar staircase voltage clamp was given, above (−300 mV) and below (−20 mV) the resting potential difference (RPD). Current (1; scale, 20 nA/division) and voltage (2; 50 mV/division) traces are shown. The dotted line indicates the initial level of MP (−175 mV). B, Net K⁺ fluxes (inward positive) measured in voltage-clamp experiments. The cell clamping at potentials more negative than RPD caused a significant increase in uptake of K⁺, indicating a direct control of applied voltage over K⁺ transporters at the plasma membrane (Babourina et al., 2001). Error bars are se (n = 8–12).

Figure 5

A model illustrating pathways of fast turgor adjustment in Arabidopsis root cells. Hyperosmotic shock, sensed via an osmosensor, activates the H⁺-ATPase. The hyperpolarization (Lew, 1996) increases net K⁺ uptake thorough an inward K⁺ channel and concomitantly decreases K⁺ efflux through an outward K⁺ channel. Both the hyperpolarized potential and extracellular acidification increase uptake of Cl⁻ through a H⁺/Cl⁻ symporter.

Figure 6

Oscillations in net K⁺ flux (A) and external pH (B) measured, 20 min after onset of hyperosmotic stress. Two typical examples from two individual plants are shown. Such oscillations are expected to arise from the presence of feedback loops in mechanism of cell osmotic adjustment as suggested by our model (Fig. 5).

Figure 7

Microphotgraphic examples of pressure probe (A) and ion-flux/voltage-clamp measurements (B). The micropipette tip is indicated by an arrow in A. It is located in the vacuole. In B, the double-barreled microelectrode is impaled into the cytoplasm (Lew, 2000). The ion fluxes from the root hair were measured parallel to the root surface. Bar = 20 μm.