Journal of Experimental Botany, Vol. 55, No. 396, pp. 449-461, February 1, 2004
© 2004 Oxford University Press
Regulation of Growth, Development and Whole Organism Physiology |
A cohesion/tension mechanism explains the gating of water channels (aquaporins) in Chara internodes by high concentration
Received 3 April 2003; Accepted 17 October 2003
Department of Plant Ecology, Bayreuth University, D-95440 Bayreuth, Germany
* To whom correspondence should be addressed. Fax: +49 921 55 2564. E-mail: ernst.steudle{at}uni-bayreuth.de
Isolated internodes of Chara corallina have been used to study the gating of aquaporins (water channels) in the presence of high concentrations of osmotic solutes of different size (molecular weight). Osmolytes were acetone and three glycol ethers: ethylene glycol monomethyl ether (EGMME), diethylene glycol monomethyl ether (DEGMME), and triethylene glycol monoethyl ether (TEGMEE). The ‘osmotic efficiency’ of osmolytes was quite different. Their reflection coefficients ranged between 0.15 (acetone), 0.59 (EGMME), 0.78 (DEGMME), and 0.80 (TEGMEE). Bulk water permeability (Lp) and diffusive permeabilities (Ps) of heavy water (HDO), hydrogen peroxide (H2O2), acetone, and glycol ethers (EGMME, DEGMME, and TEGMEE) were measured using a cell pressure probe. Cells were treated with different concentrations of osmotic solutes of up to 800 mM (
2.0 MPa of osmotic pressure). Inhibition of aquaporin activity increased with both increasing concentration and size of solutes (reflection coefficients). As cell Lp decreased, Ps increased, indicating that water and solutes used different passages across the plasma membrane. Similar to earlier findings of an osmotic gating of ion channels, a cohesion/tension model of the gating of water channels in Chara internodes by high concentration is proposed. According to the model, tensions (negative pressures) within water channels affected the open/closed state by changing the free energy between states and favoured a distorted/collapsed rather than the open state. They should have differed depending on the concentration and size of solutes that are more or less excluded from aquaporins. The bigger the solute, the lower was the concentration required to induce a reversible closure of aquaporins, as predicted by the model.
Key words: Aquaporins, Chara, cohesion/tension, gating, hydraulic conductivity, reflection coefficient, water channels.
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