Is Modulation of the Rate of Proton Pumping a Key Event in Osmoregulation?

Abstract

ABSTRAC1The net uptake of 3-0-methylglucose into leaf segments obtained from Senecio mikanioides Otto, and net proton efflux from the segments, were both promoted when the osmotic potential of the medium was decreased by addition of mannitol, sorbitol, or polyethylene glycol (optimal osmolarity, 0.3 Osmolar for mannitol and sorbitol). The effect was not due to promotion of 'aging', since the antibiotic cerulenin suppressed aging without reducing the size of the mannitol stimulation; further, mannitol did not accelerate aging. Neither was the effect ascribable to diminished efflux (i.e. reduced 'leak' because: first, visualization of the unidirectional supr fluxes by double labeling indicated that the effect of added osmoticum was to promote influx rather than to reduce effiux; second, compartment analysis did not suggest any effect of mannitol on the rate constants for efflux from either the slowly equilibrating or more rapidly equilibrating compartment. The effect was not specific to poly-ols since it was also obtained with betaine and choline chloride. Since methyl glucose is not taken up into the phloem it could not be ascribed to a turgor effect on phloem loading. We conclude that the effect may reflect osmoregulation. As the sugar flux is probably driven by protonmotive force, it is likely that the effects on proton flux and on supr flux are related. We suggest that the plasmalemma-sited proton pump is sensitive to the hydrostatic pressure gradient across the plasmalemma-cell wall complex, and functions both as detector and as effector in osmoregulation.We have earlier (8, 9) brought evidence which suggests that plasmalemma transport of sugars into pea mesophyll protoplasts in the light (though possibly not in the dark) is driven by pmf2 as is believed to be the case in many other plant systems (11). Apart from its sensitivity to light, the sugar transport system of leaf tissue also shows sensitivity to turgor. Because experiments in which external osmotic pressure is manipulated in the hypotonic range are not practicable with isolated protoplasts, we have carried out these experiments with leaf segments of Senecio mikanioides from which the lower epidermis had been removed. We have observed that a change towards more negative external water potential brings about an increased rate ofproton extrusion (also reported by 5, 7) as well as an increase in sugar influx. The observed effect of turgor on sugar transport may thus well be mediated via the proton extrusion pump and the pmf. In a brief preliminary communication (12), we proposed that these responses to low turgor reflected osmotic regulation, and suggested that the proton pump might play a key role in the mechanism 'Supported by of osmotic adaptation. However, a number of alternative explanations for the observed effects suggested themselves, which we thought necessary to investigate. We have presented the results of our investigation in detail. MATERIALS AND METHODSLeaves which had almost reached their maximum size were collected from plants of Senecio...

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

“…This pump may also be the detector of turgor-a possibility given current hypotheses that turgor detection resides in the plasma membrane (1,18). The possibility that the proton extrusion pump may be both detector and effector in osmoregulation has already been proposed by Reinhold et al (12).…”

“…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.…”

Enhanced Net K+ Uptake Capacity of NaCl-Adapted Cells

“…A substantial component of increased H+ pumping was vanadate-sensitive and it has been inferred that the plasma membrane H+-ATPase has a central function in the response to alterations in the osmotic or ionic environments (2,17,18,26). Intracellular sugar uptake has been linked to this enhanced H+ pumping capacity (17).…”

“…Proton extrusion across the plasma membrane is stimulated by a reduction in the external water potential mediated by any number of osmotic solutes (17,18,26,29). A substantial component of increased H+ pumping was vanadate-sensitive and it has been inferred that the plasma membrane H+-ATPase has a central function in the response to alterations in the osmotic or ionic environments (2,17,18,26).…”

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

“…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.…”