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Gating of water channels (aquaporins) in plants: effects of osmotic and oxidative stresses and the role of unstirred layers
(2006)
- There is accumulating evidence that water channels (aquaporins; AQPs) play an important role in plant water relations at the levels of cells, tissues, organs, and whole plants. AQPs facilitate the rapid, passive exchange of water across cell membranes. Most of the water permeability of plasma membranes is due to AQP activity, i.e. the open/closed state of AQPs can be ‘gated’ in order to regulate water movement. Many internal (metabolic) factors or external (environmental) stresses have been found to cause a gating of AQPs, but the precise gating mechanisms are not well understood. Using pressure probe techniques, water and solute flows across cell membranes (internodes of giant green alga Chara and of cortical cells of corn root) and across an entire organ (roots of young corn seedlings) were studied. Based on detailed information on the function of AQPs, two new gating mechanisms of AQPs have been proposed. One is the ‘cohesion/tension (C/T) mechanism’ for the osmotic gating of AQPs, which is used to interpret the effect of high concentration on cell hydraulic conductivity (Lp) during osmotic stress. The other one is the ‘oxidative gating mechanism’ of AQPs in the presence of hydroxyl radicals (*OH) or of hydrogen peroxide (H2O2). Evidence for an osmotic gating of AQPs was provided from measurements of effects of high concentration on cell Lp in Chara internodes. The osmotic dehydration is thought to be caused by the fact that the exclusion of solutes from AQPs creates tensions (negative pressures) within the water channel pores, when the osmolyte is present on both sides of the membrane. This should have affected the open/closed state by changing the free energy between states favouring a reversible distorted/collapsed state rather than the open. As expected from the C/T model, effects increased with increasing both the concentration and size of osmolytes. Pore volumes of AQPs in the plasma membrane of Chara internodes were estimated from exponential ‘dehydration curves’. The analysis of osmotic responses showed that there were narrow pores with a volume of 2.3 ± 0.2 nm3 and bigger ones with a volume of between 5.5 ± 0.8 and 6.1 ± 0.8 nm3. Alternatively, pore volumes were estimated from ratios between osmotic ( ) and diffusional (Pd) water flow as derived from measurements of hydraulic and isotopic water flows, which represent the number of water molecules (N) in a single-file pore transporting water. Values of N ranged between 35 and 60, which referred to volumes of 0.51 and 0.88 nm3/pore. This value was substantially smaller than that obtained during osmotic dehydration, but bigger than values from literature. The difference may be due to an underestimation caused by unstirred layers (USLs), which would affect Pd rather than Pf (Lp). Based on analytical solutions from diffusion kinetics and measured experimental results, even for the most rapidly permeating solute heavy water (HDO), USLs should have resulted in an underestimation of Pd by less than 30 %. Evidence for an ‘oxidative gating’ of AQPs was derived from experiments with Chara internodes, which was then tested for higher plant tissue cells as well (root cortical cells of corn). In the presence of hydroxyl radicals (*OH) as produced during the Fenton reaction (Fe2+ + H2O2 = Fe3+ + OH- + *OH), cell Lp of Chara internodes was dramatically but reversibly reduced. In experiments with young corn roots, in the presence of hydrogen peroxide (H2O2), half times of water flows increased at the level of both entire roots and individual cortical cells by factors of 3 and 9, respectively, i.e., hydraulic conductivity decreased by the same factors. Results indicated that there might be a common interaction between the redox state (oxidative stress) and water relations (water stress) in plants. For solutes rapidly permeating through cell membranes, closure of water channel resulted in anomalous osmosis (negative reflection coefficients), i.e., in the striking situation that a cell did not shrink but swelled in a hypertonic medium. For the first time, anomalous osmosis has been demonstrated in roots as well, i.e., for an entire organ and in the presence of a rather complicated osmotic barrier. The phenomenon has been interpreted in terms of the composite transport structure of both the cell membrane and the root. In the presence of a rapidly permeating solute like acetone, channel closure resulted in a situation that the solute moved faster than the water, and the reflection coefficient (ss) reversed its sign.
