Osmoregulation or Turgor Regulation in <i>Chara</i>?

A new method, the turgor clamp, was developed to test the effects of turgor on cell enlargement. The method used a pressure probe to remove or inject cell solution and change the turgor without altering the external environment of the cell walls. After the injections, the cells were permanently at the new turgor and required no further manipulation. Internode cells of Chara corallina grew rapidly with the pressure probe in place when growth was monitored with a position transducer. Growth-induced water potentials were negligible and turgor effects could be studied simply. As turgor was decreased, there was a threshold below which no growth occurred, and only reversible elastic/viscoelastic changes could be seen. Above the threshold, growth was superimposed on the elastic/viscoelastic effects. The rate of growth did not depend on turgor. Instead, the rate was highly dependent on energy metabolism as shown by inhibitors that rapidly abolished growth without changing the turgor. However, turgors could be driven above the maximum normally attainable by the cell, and these caused growth to respond as though plastic deformation of the walls was beginning, but the deformation caused wounding. Growth was inhibited when turgor was changed with osmotica but not inhibited when similar changes were made with the turgor clamp. It was concluded that osmotica caused side effects that could be mistaken for turgor effects. The presence of a turgor threshold indicates that turgor was required for growth. However, because turgor did not control the rate, it appears incorrect to consider the rate to be determined by a turgor-dependent plastic deformation of wall polymers. Instead, above the turgor threshold, the rapid response to energy inhibitors suggests a control by metabolic reactions causing synthesis and/or extension of wall polymers.It is well accepted that fp,2 is involved in cell enlargement. The evidence is based mostly on varying the i,p with osmotica or soil water deficits and observing changes in growth rates. These treatments also alter the solute and/or pressure environment of the cell walls (2,11,40) (9,10,20,25,31,35,42), but the tensions may not have completely simulated ip, which is multidimensional (31,40).After the elegant investigations of isolated cell walls by Preston and coworkers (33,35), it was proposed that 4,p causes an irreversible deformation of the wall to a larger size by a time-dependent plastic yielding resembling a steady creep (27,28,33). In some studies, creep appeared proportional to the 6p above a minimum termed the yield threshold (8,35). Lockhart (27,28) formalized these concepts in an equation of the form:where G is the relative growth rate (in .m3m s-or s-'), /, is the turgor (MPa), Y is the yield threshold turgor (MPa), and M is the wall extensibility (m3. m-3 . s-'. MPa-1 or s-. MPa-1).Equation 1 has been extensively used (2, 40) but often without distinguishing growth from elastic changes. In a central study in Nitella (18)

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“…C corallina was an ideal species for this work because it shows no activity for 4p regulation, and osmoregulation is slow (1 …”

Section: Discussionmentioning

confidence: 99%

Enlargement in Chara Studied with a Turgor Clamp

Zhu

Boyer²

1992

Plant Physiol.

106

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“…The related freshwater alga Chara corallina also fails to maintain constant turgor during osmotic stress (23). Modulation of the strength of the cell wall dominates growth-rate control among higher plants (24)(25)(26) and in the giant sporangiophore of the fungus Phycomyces (27).…”

Section: Discussionmentioning

confidence: 99%

Extension growth of the water mold Achlya: interplay of turgor and wall strength.

Money

Harold

1992

Proc. Natl. Acad. Sci. U.S.A.

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When hyphae of the water mold Achlya were subjected to osmotic stress, imposed with polyethylene glycol (PEG)-300 or sucrose, turgor pressure fell in proportion to the increase in external osmotic pressure. There was no evidence of turgor regulation, even over a period of days, yet the extension rate was unaffected until turgor was reduced to less than a third of the normal level of 0.6-0.8 MPa (6-8 bars). Measurements of the pressure at which the hyphae burst indicate that they respond to osmotic stress by softening their apical cell walls, sustaining extension growth despite reduced turgor pressure.The effect of osmolytes excluded by the wall was very different; superfusion ofgrowing hyphae with PEG-6000 or dextran-6000 reduced turgor and stopped extension but did not induce wall softenig. Furthermore, the hyphae did not resume growth during an hour or more of continuous exposure to these substances. Although the two classes of osmolytes have the same effect on turgor, they may induce different strains within the cell wall; this might then affect the capacity of the organism to detect the drop in turgor or to soften its cell wall. The interplay between turgor and wall strength supports the proposition that turgor supplies the driving force for extension and that production of the standard hyphal form requires a balance between hydrostatic pressure and a resistive cell wall.Most plant and microbial cells are highly pressurized structures whose walls withstand the tension exerted by up to 2.0 MPa (20 bars) of hydrostatic pressure (1-3). This pressure, or turgor, is generated by water influx due to the difference in osmotic pressure between the protoplasm and the external milieu. Turgor provides skeletal support and, so it is thought, the driving force for expansion. Biophysical models suggest that turgor exerts an isotropic force upon the inner surface of the cell wall, and localized compliance results in the generation of cellular form (4-7).Many organisms, both prokaryotes and eukaryotes, respond to osmotic stress by the accumulation of compatible solutes (1, 3, 8-10), either by synthesis or by uptake from the external medium. The resumption of growth parallels solute accumulation; although few experiments have measured changes in total solute concentration, it seems likely that turgor is rebuilt under these conditions. These studies indicate that turgor is necessary for the growth of walled cells but do not identify its role in the manifold processes that underlie surface expansion.The lower eukaryote Achlya is a convenient subject for investigations on the role of turgor in apical growth; it produces relatively large aseptate hyphae that lend themselves to direct measurement of turgor with a micropipetbased pressure probe (11). We report here that hyphae of Achlya lack a mechanism for turgor regulation but instead sustain apical growth with reduced turgor by softening their cell walls. Our observations reinforce the view that turgor supplies the mechanical force for extension and provide some insight...

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“…Spanswick (Cornell University, Ithaca, NY), was cultured in FW medium in the laboratory as previously described (Bisson and Bartholomew, 1984). Chara longifolia (previously identified as C. buckellii) was collected from Waldsea Lake, a saline lake in Saskatchewan, Canada, and cultured in either FW or SW medium.…”

Section: Materials a N D Methodsmentioning

confidence: 99%

Plasma Membrane Na+ Transport in a Salt-Tolerant Charophyte (Isotopic Fluxes, Electrophysiology, and Thermodynamics in Plants Adapted to Saltwater and Freshwater)

Kiegle

Bisson

1996

Plant Physiology

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In salt-tolerant Chara longifolia, enhanced Na+ efflux plays an important role in maintaining low cytoplasmic Na+. When it is cultured in fresh water (FW), C. longifolia has a higher Na+ efflux than the obligate FW Chara corallina, although pH dependence and inhibitor profiles are similar for both species (1. Whittington and M.A. Bisson 119941 j Exp Bot 45: 657-665). When it is cultured in saltwater, C. longifolia has a Na+ efflux of 264 f 14 nmol m-'s-' at pH 7, 13 times higher than FW-adapted cultures and 31 times higher than C. corallina. As in FW-adapted plants, efflux is highest at pH 5, but pH dependence is less steep and more linear in cells adapted to saltwater. In plants of both species from FW cultures, Na+ efflux is inhibited by Li+ at pH 5 but not at pH 7 or 9, whereas in the salt-adapted C. longifolia, Li+ inhibits Na+ efflux at pH 7 and 9 but not at pH 5. Amiloride inhibits Na+ efflux in salt-adapted cells but not in FW cells. We conclude that a new type of Na+ efflux system is induced in salt-adapted plants, although both systems have characteristics suggestive of a Na+/H+ antiport. In all cases, a 1 :1 Na+/H+ antiport would have sufficient energy to maintain the cytoplasmic Na+ activities measured at pH 5 and 7 but not at pH 9, which suggests that another efflux system must be operating at p H 9.

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Osmoregulation or Turgor Regulation in Chara?

Cited by 64 publications

References 10 publications

Enlargement in Chara Studied with a Turgor Clamp

Enlargement in Chara Studied with a Turgor Clamp

Extension growth of the water mold Achlya: interplay of turgor and wall strength.

Plasma Membrane Na+ Transport in a Salt-Tolerant Charophyte (Isotopic Fluxes, Electrophysiology, and Thermodynamics in Plants Adapted to Saltwater and Freshwater)

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