WATER CHANNELS IN PLANTS - DO BASIC CONCEPTS OF WATER TRANSPORT CHANGE

A review and re-examination of literature data shows that highly selec tive water channels (aquaporins) have marked effects on the overall tr ansport properties of the plasma membrane of plant cells. The applicat ion of the channel blocker HgCl2 (50 mu M) or of high external concent rations of permeating solutes reduced the water permeability (hydrauli c conductivity, Lp) of Chara internodes down to 25% of the control, In treated cells, reflection coefficients (sigma(s)) of permeating low m olecular weight organic test solutes (alcohols, amides, ketones) were markedly reduced as well, but solute permeability (permeability coeffi cient, P-s) remained constant. A similar relation between Lp and sigma (s) was found with untreated cells of isolated epidermis of Tradescant ia virginiana. The results can not be interpreted in terms of conventi onal membrane models (pore or solubility membrane); for instance, the classical frictional pore model (Dainty and Ginzburg, 1963) fails to e xplain low sigma(s) of rapidly permeating solutes. The results fit int o a model which treats the membrane as a composite structure with prot einaceous arrays (containing water channels) in parallel with lipid ar rays ('composite transport model'), Test solutes predominantly pass ac ross the lipid array which was not affected by treatments. Water, howe ver, largely uses the water channel array which was affected, When usi ng heavy water (HDO) as an osmotic solute, the transport pattern chang ed as predicted by the model, As indicated by low channel reflection c oefficients of test solutes, water channels did not completely exclude small uncharged molecules and do show some permeability for the test solutes used. Low sigma(s) values of water channels are interpreted by a single-file mechanism of water and solute flow. Absolute values of transport coefficients (Lp, P-s, sigma(s)) represent mixed values to w hich the different membrane arrays contribute according to concepts av ailable from irreversible thermodynamics. The patchy structure of the cell membrane results in a circulation flow of water in the membrane, The fact that water channels can be triggered by factors such as heavy metals and high concentration suggests that water transport can be re gulated by opening or closing water channels. The results have consequ ences for our basic understanding of osmosis and water transport in pl ants.