Characteristics of Hg- and Zn-sensitive Water Channels in the Plasma Membrane ofCharaCells*

Foliar spraying of zinc oxide nanoparticles improves water transport and nitrogen metabolism in tomato (Solanum lycopersicum L.) seedlings mitigating the negative impacts of cadmium

The use of bio-nanotechnology in agriculture-such as the biological applications of metal oxide nanoparticles (NPs)-greatly improves crop yield and quality under different abiotic stress factors including soil metal contamination. Here, we explore the effectiveness of zinc oxide (ZnO)-NPs (0, 50 mg/L) foliar spraying to ameliorate the detrimental effects of cadmium (Cd) on the water transport and nitrogen metabolism in tomato (Solanum lycopersicum Mill. cv. Chibli F1) plants grown on a Cd-supplied (CdCl2; 0, 10, 40 μM) Hoagland nutrient solution. The results depicted that the individually studied factors (ZnO-NPs and Cd) had a significant impact on all the physiological parameters analyzed. Independently to the Cd concentration, ZnO-NPs-sprayed plants showed significantly higher dry weight (DW) in both leaves and roots compared to the non-sprayed ones, which was in consonance with higher and lower levels of Zn2+ and Cd2+ ions, respectively, in these organs. Interestingly, ZnO-NPs spraying improved water status in all Cd-treated plants as evidenced by the increase in root hydraulic conductance (L0), apoplastic water pathway percentage, and leaf and root relative water content (RWC), compared to the non-sprayed plants. This improved water balance was associated with a significant accumulation of osmoprotectant osmolytes, such as proline and soluble sugars in the plant organs, reducing electrolyte leakage (EL), and osmotic potential (ψπ). Also, ZnO-NPs spraying significantly improved NO3- and NH4+ assimilation in the leaf and root tissues of all Cd-treated plants, leading to a reduction in NH4+ toxicity. Our findings point out new insights into how ZnO-NPs affect water transport and nitrogen metabolism in Cd-stressed plants and support their use to improve crop resilience against Cd-contaminated soils.

The differential expressions of aquaporins underline the diverse strategies of cucumber and tomato against salinity and zinc stress

Salinity and excess zinc are two main problems that have limited agriculture in recent years. Aquaporins are crucial in regulating the passage of water and solutes through cells and may be essential for mitigating abiotic stresses. In the present study, the adaptive response to moderate salinity (60 mM NaCl) and excess Zn (1 mM ZnSO4 ) were compared alone and in combination in Cucumis sativus L. and Solanum lycopersicum L. Water relations, gas exchange and the differential expression of all aquaporins were analysed. The results showed that cucumber plants under salinity maintained the internal movement of water through osmotic adjustment and the overexpression of specific PIPs aquaporins, following a "conservation strategy". As tomato has a high tolerance to salinity, the physiological parameters and the expression of most aquaporins remained unchanged. ZnSO4 was shown to be stressful for both plant species. While cucumber upregulated 7 aquaporin isoforms, the expression of aquaporins increased in a generalized manner in tomato. Despite the differences, water relations and transpiration were adjusted in both plants, allowing the RWC in the shoot to be maintained. The aquaporin regulation in cucumber plants facing NaCl+ZnSO4 stress was similar in the two treatments containing NaCl, evidencing the predominance of salt in stress. However, in tomato, the induced expression of specific isoforms to deal with the combined stress differed from independent stresses. The results clarify the key role of aquaporin regulation in facing abiotic stresses and their possible use as markers of tolerance to salinity and heavy metals in plants.

Calcium control of the hydraulic resistance in cells of Chara corallina

The hydraulic resistance (the reciprocal of the hydraulic conductivity Lp) Lp^-1 was measured in cells of Chara corallina by the method of transcellular osmosis. Treatment of cells with 100 mM KCl decreased Lp^-1 significantly. Subsequent treatment of the cells with 70 mM CaCl₂ recovered the decreased Lp^-1 to the original value. To know whether K⁺ or Ca²⁺/Mg²⁺ acts on the cell wall and/or the membrane, the hydraulic resistances of the cell wall (Lp_w^-1) and that of the membrane (Lp_m^-1) were determined in one and the same cell. For this, a pair of cells (twin cells) were made from an internodal cell, one used for measurement of Lp^-1 and the other used for the measurement of Lp_w^-1. From Lp^-1 and Lp_w^-1, Lp_m^-1 was calculated. Both Lp^-1 and Lp_w^-1 were decreased by K⁺, while Lp_m^-1 was not affected by K⁺. The same result was obtained with 5 mM EGTA. Lp_w^-1 was decreased more than it was by KCl but Lp_m^-1 remained constant after EGTA treatment. The recovery of the K⁺-decreased Lp^-1 with Ca²⁺ can be explained exclusively by the recovery of Lp_w^-1 with Ca²⁺. The Ca²⁺ recovery of Lp_w^-1 was observed in the intact cell wall but not in the cell wall tube isolated from an internodal cell. The different response to Ca²⁺ between the intact cell wall and the isolated cell wall was discussed in relation to the tension in the cell wall which may be an important factor for the ionic regulation of hydraulic conductivity.

Age dependence of the hydraulic resistances of the plasma membrane and the tonoplast (vacuolar membrane) in cells of Chara corallina

Hydraulic resistances (reciprocals of hydraulic conductivities) of the cell (Lp^-1), the cell wall (Lp_w^-1), the membrane (Lp_m^-1), the plasma membrane (Lp_pm^-1), and the tonoplast (Lp_tp^-1) were determined in individual internodal cells of Chara corallina and their dependence on the cell age was studied. The thickness of the cell wall (d) was adopted as an index of the cell age, since the cell wall of spring-grown young cells (sg-cells) was found to be significantly thinner than that of winter-spent old cells (ws-cells). Both Lp_w^-1 and Lp_m^-1 were found to increase with cell age. Since Lp_m^-1 is the sum of Lp_pm^-1 and Lp_tp^-1, their dependence on the wall thickness was studied. It was found that both Lp_pm^-1 and Lp_tp^-1 increase with cell age using d as a proxy and that the former is distinctly higher than the latter. The ratio Lp_pm^-1/Lp_tp^-1 amounts to 30 for 5 μm of d, indicating that the tonoplast is a negligible barrier to osmotic water flow. The ratio decreases with the increase in d and amounts to 5.0 for 11 μm of d, showing that the tonoplast ages faster than the plasma membrane. The physiological meaning of the age dependence of hydraulic resistance of the tonoplast was discussed in terms of the role of the vacuole in the osmoregulation of the cytoplasm.

Targeting aquaporins to alleviate hazardous metal(loid)s imposed stress in plants

Uptake of hazardous metal(loid)s adversely affects plants and imposes a threat to the entire food chain. Here, the role of aquaporins (AQPs) providing tolerance against hazardous metal(loid)s in plants is discussed to provide a perspective on the present understanding, knowledge gaps, and opportunities. Plants adopt complex molecular and physiological mechanisms for better tolerance, adaptability, and survival under metal(loid)s stress. Water conservation in plants is one such primary strategies regulated by AQPs, a family of channel-forming proteins facilitating the transport of water and many other solutes. The strategy is more evident with reports suggesting differential expression of AQPs adopted by plants to cope with the heavy metal stress. In this regard, numerous studies showing enhanced tolerance against hazardous elements in plants due to AQPs activity are discussed. Consequently, present understanding of various aspects of AQPs, such as tertiary-structure, transport activity, solute-specificity, differential expression, gating mechanism, and subcellular localization, are reviewed. Similarly, various tools and techniques are discussed in detail aiming at efficient utilization of resources and knowledge to combat metal(loid)s stress. The scope of AQP transgenesis focusing on heavy metal stresses is also highlighted. The information provided here will be helpful to design efficient strategies for the development of metal(loid)s stress-tolerant crops.

Differential Aquaporin Response to Distinct Effects of Two Zn Concentrations after Foliar Application in Pak Choi (Brassica rapa L.) Plants

Zinc (Zn) is considered an essential element with beneficial effects on plant cells; however, as a heavy metal, it may induce adverse effects on plants if its concentration exceeds a threshold. In this work, the effects of short-term and prolonged application of low (25 µM) and high (500 µM) Zn concentrations on pak choi (Brassica rapa L.) plants were evaluated. For this, two experiments were conducted. In the first, the effects of short-term (15 h) and partial foliar application were evaluated, and in the second a long-term (15 day) foliar application was applied. The results indicate that at short-term, Zn may induce a rapid hydraulic signal from the sprayed leaves to the roots, leading to changes in root hydraulic conductance but without effects on the whole-leaf gas exchange parameters. Root accumulation of Zn may prevent leaf damage. The role of different root and leaf aquaporin isoforms in the mediation of this signal is discussed, since significant variations in PIP1 and PIP2 gene expression were observed. In the second experiment, low Zn concentration had a beneficial effect on plant growth and specific aquaporin isoforms were differentially regulated at the transcriptional level in the roots. By contrast, the high Zn concentration had a detrimental effect on growth, with reductions in the root hydraulic conductance, leaf photosynthesis rate and Ca2+ uptake in the roots. The abundance of the PIP1 isoforms was significantly increased during this response. Therefore, a 25 µM Zn dose resulted in a positive effect in pak choi growth through an increased root hydraulic conductance.

The use of biovesicles to improve the efficiency of Zn foliar fertilization

Foliar fertilization is becoming very popular since it is highly efficient, minimizes contamination, and is target oriented. However, the low mobility of several nutrients and low penetration through the leaf cuticle should be addressed, to establish an efficient leaf applied fertilizer. During the last few years, nanotechnologies - such as nanocarriers - have been studied with regard to improving the penetration and delivery of compounds. This work describes the use of membrane vesicles obtained from Brassica oleracea L. as nanobiocarriers of Zn and the evaluation of their potential as a foliar fertilizer, also in Brassica. The results show a high Zn encapsulation efficiency and high delivery into protoplasts. Also, the foliar fertilization experiments demonstrated a very effective system of Zn nanofertilization.

Surface pH changes suggest a role for H+/OH- channels in salinity response of Chara australis

To understand salt stress, the full impact of salinity on plant cell physiology has to be resolved. Electrical measurements suggest that salinity inhibits the proton pump and opens putative H+/OH- channels all over the cell surface of salt sensitive Chara australis (Beilby and Al Khazaaly 2009; Al Khazaaly and Beilby 2012). The channels open transiently at first, causing a characteristic noise in membrane potential difference (PD), and after longer exposure remain open with a typical current-voltage (I/V) profile, both abolished by the addition of 1 mM ZnCl2, the main known blocker of animal H+ channels. The cells were imaged with confocal microscopy, using fluorescein isothiocyanate (FITC) coupled to dextran 70 to illuminate the pH changes outside the cell wall in artificial fresh water (AFW) and in saline medium. In the early saline exposure, we observed alkaline patches (bright fluorescent spots) appearing transiently in random spatial distribution. After longer exposure, some of the spots became fixed in space. Saline also abolished or diminished the pH banding pattern observed in the untreated control cells. ZnCl2 suppressed the alkaline spot formation in saline and the pH banding pattern in AFW. The osmotic component of the saline stress did not produce transient bright spots or affect banding. The displacement of H+ from the cell wall charges, the H+/OH- channel conductance/density, and self-organization are discussed. No homologies to animal H+ channels were found. Salinity activation of the H+/OH- channels might contribute to saline response in roots of land plants and leaves of aquatic angiosperms.

Dynamic regulation of the root hydraulic conductivity of barley plants in response to salinity/osmotic stress

Salinity stress significantly reduces the root hydraulic conductivity (Lpr) of several plant species including barley (Hordeum vulgare). Here we characterized changes in the Lpr of barley plants in response to salinity/osmotic stress in detail using a pressure chamber. Salt-tolerant and intermediate barley cultivars, K305 and Haruna-nijyo, but not a salt-sensitive cultivar, I743, exhibited characteristic time-dependent Lpr changes induced by 100 mM NaCl. An identical response was evoked by isotonic sorbitol, indicating that this phenomenon was triggered by osmotic imbalances. Further examination of this mechanism using barley cv. Haruna-nijyo plants in combination with the use of various inhibitors suggested that various cellular processes such as protein phosphorylation/dephosphorylation and membrane internalization appear to be involved. Interestingly, the three above-mentioned barley cultivars did not exhibit a remarkable difference in root cell sap osmolality under hypertonic conditions, in contrast to the case of Lpr. The possible biological significance of the regulation of Lpr in barley plants upon salinity/osmotic stress is discussed.

The cadmium-tolerant pea (Pisum sativum L.) mutant SGECdt is more sensitive to mercury: assessing plant water relations

Heavy metals have multiple effects on plant growth and physiology, including perturbation of plant water status. These effects were assessed by exposing the unique Cd-tolerant and Cd-accumulating pea (Pisum sativum L.) mutant SGECd(t) and its wild-type (WT) line SGE to either cadmium (1, 4 μM CdCl2) or mercury (0.5, 1, 2 μM HgCl2) in hydroponic culture for 12 days. When exposed to Cd, SGECd(t) accumulated more Cd in roots, xylem sap, and shoot, and had considerably more biomass than WT plants. WT plants lost circa 0.2 MPa turgor when grown in 4 μM CdCl2, despite massive decreases in whole-plant transpiration rate and stomatal conductance. In contrast, root Hg accumulation was similar in both genotypes, but WT plants accumulated more Hg in leaves and had a higher stomatal conductance, and root and shoot biomass compared with SGECd(t). Shoot excision resulted in greater root-pressure induced xylem exudation of SGECd(t) in the absence of Cd or Hg and following Cd exposure, whereas the opposite response or no genotypic differences occurred following Hg exposure. Exposing plants that had not been treated with metal to 50 μM CdCl2 for 1h increased root xylem exudation of WT, whereas 50 μM HgCl2 inhibited and eliminated genotypic differences in root xylem exudation, suggesting differences between WT and SGECd(t) plants in aquaporin function. Thus, root water transport might be involved in mechanisms of increased tolerance and accumulation of Cd in the SGECd(t) mutant. However, the lack of cross-tolerance to Cd and Hg stress in the mutant indicates metal-specific mechanisms related to plant adaptation.

Polarity of water transport across epidermal cell membranes in Tradescantia virginiana

Using the automated cell pressure probe, small and highly reproducible hydrostatic pressure clamp (PC) and pressure relaxation (PR) tests (typically, applied step change in pressure = 0.02 MPa and overall change in volume = 30 pL, respectively) were applied to individual Tradescantia virginiana epidermal cells to determine both exosmotic and endosmotic hydraulic conductivity (L(p)(OUT) and L(p)(IN), respectively). Within-cell reproducibility of measured hydraulic parameters depended on the method used, with the PR method giving a lower average coefficient of variation (15.2%, 5.8%, and 19.0% for half-time, cell volume [V(o)], and hydraulic conductivity [L(p)], respectively) than the PC method (25.4%, 22.0%, and 24.2%, respectively). V(o) as determined from PC and PR tests was 1.1 to 2.7 nL and in the range of optically estimated V(o) values of 1.5 to 4.9 nL. For the same cell, V(o) and L(p) estimates were significantly lower (about 15% and 30%, respectively) when determined by PC compared with PR. Both methods, however, showed significantly higher L(p)(OUT) than L(p)(IN) (L(p)(OUT)/L(p)(IN) ≅ 1.20). Because these results were obtained using small and reversible hydrostatically driven flows in the same cell, the 20% outward biased polarity of water transport is most likely not due to artifacts associated with unstirred layers or to direct effects of externally applied osmotica on the membrane, as has been suggested in previous studies. The rapid reversibility of applied flow direction, particularly for the PR method, and the lack of a clear increase in L(p)(OUT)/L(p)(IN) over a wide range of L(p) values suggest that the observed polarity is an intrinsic biophysical property of the intact membrane/protein complex.

Zinc ions block H⁺/OH⁻ channels in Chara australis

Chara australis cells exposed to media of pH 10 and above exhibit high conductance, arising from the opening of H⁺/OH⁻ channels in the plasma membrane. This high conductance can be totally inhibited by 1.0 mm ZnCl₂ and restored by 0.5 mm 2-mercaptoethanol (ME). Important for carbon fixation, H⁺/OH⁻ channels play a key role in cell pH banding. Banding was also shown to be abolished by 1.0 mm ZnCl₂ and restored in some cells by ME. The proton pump is also involved in banding, but was little affected by ZnCl₂ over the periods needed for the inhibition of H⁺/OH⁻ channels. Previously, we postulated that H⁺/OH⁻ channels open transiently at the onset of saline stress in salt-sensitive C. australis, causing membrane potential difference (PD) noise; and remain open in latter stages of saline stress, contributing to cell deterioration. ZnCl₂ totally inhibited the saline noise and the upwardly concave I/V characteristics associated with the putative H⁺/OH⁻ currents. Again, ME reversed both these effects. We discuss the mode of action of zinc ions and ME with reference to animal voltage-gated H⁺ channels and water channels.

Gating of aquaporins by heavy metals in Allium cepa L. epidermal cells

Changes in the water permeability, aquaporin (AQP) activity, of leaf cells were investigated in response to different heavy metals (Zn(2+), Pb(2+), Cd(2+), Hg(2+)). The cell pressure probe experiments were performed on onion epidermal cells as a model system. Heavy metal solutions at different concentrations (0.05 μM-2 mM) were used in our experiments. We showed that the investigated metal ions can be arranged in order of decreasing toxicity (expressed as a decrease in water permeability) as follows: Hg>Cd>Pb>Zn. Our results showed that β-mercaptoethanol treatment (10 mM solution) partially reverses the effect of AQP gating. The magnitude of this reverse differed depending on the metal and its concentration. The time course studies of the process showed that the gating of AQPs occurred within the first 10 min after the application of a metal. We also showed that after 20-40 min from the onset of metal treatment, the water flow through AQPs stabilized and remained constant. We observed that irrespective of the metal applied, the effect of AQP gating can be recorded within the first 10 min after the administration of metal ions. More generally, our results indicate that the toxic effects of investigated metal ions on the cellular level may involve AQP gating.

Impact of cadmium and zinc on growth and water status of Zygophyllum fabago in two contrasting metallicolous populations from SE Spain: comparison at whole plant and tissue level

Cadmium and zinc accumulation and toxicity were assessed in whole plants and callus culture of two Zygophyllum fabago populations originating from two metallicolous sites in Murcia (southeast Spain), La Peña and Mazarrón, the first containing 2.8-times more Cd and five-times more Zn than the second. Seedlings from both ecotypes were exposed for 3 weeks to 1 or 10 μm Cd, and to 10 or 100 μm Zn in nutrient solution in a controlled environment. Calli from both ecotypes were exposed to 0.01, 0.1 or 1 mm Cd, and to 0.1, 1 or 5 mm Zn. Plants from both populations exhibited similar tolerance to Zn, while tolerance to Cd appeared more important in plants from La Peña than those from Mazarrón. Only minor differences were recorded in final Cd accumulation, with higher Cd retention in roots and stems of plants from La Peña. In both populations, transient decreases in the rate of Zn intake and translocation from root to shoot were recorded. This reduction in ion uptake was not more efficient for the population from the most contaminated area compared to the less contaminated area. Similar concentrations of Cd were found in cotyledon-derived calli from the two populations, but absorbed Cd induced conspicuous water stress in calli issues from Mazarrón but not in those from La Peña. It is concluded that, beside comparable levels of heavy metal concentration in tissues, the physiological strategy of tolerance may differ according to the metal and according to the considered population.

Comparison of two ecotypes of the metal hyperaccumulator Thlaspi caerulescens (J. & C. PRESL) at the transcriptional level

This paper investigates differences in gene expression among the two Thlaspi caerulescens ecotypes La Calamine (LC) and Lellingen (LE) that have been shown to differ in metal tolerance and metal uptake. LC originates from a metalliferous soil and tolerates higher metal concentrations than LE which originates from a non-metalliferous soil. The two ecotypes were treated with different levels of zinc in solution culture, and differences in gene expression were assessed through application of a cDNA microarray consisting of 1,700 root and 2,700 shoot cDNAs. Hybridisation of root and shoot cDNA from the two ecotypes revealed a total of 257 differentially expressed genes. The regulation of selected genes was verified by quantitative reverse transcriptase polymerase chain reaction. Comparison of the expression profiles of the two ecotypes suggests that LC has a higher capacity to cope with reactive oxygen species and to avoid the formation of peroxynitrite. Furthermore, increased transcripts for the genes encoding for water channel proteins could explain the higher Zn tolerance of LC compared to LE. The higher Zn tolerance of LC was reflected by a lower expression of the genes involved in disease and defence mechanisms. The results of this study provide a valuable set of data that may help to improve our understanding of the mechanisms employed by plants to tolerate toxic concentrations of metal in the soil.

Deactivation of aquaporins decreases internal conductance to CO2 diffusion in tobacco leaves grown under long-term drought

We compared the diffusion conductance to CO₂ from the intercellular air space to the chloroplasts (internal conductance (g _i)) between tobacco leaves acclimated to long-term drought (drought-acclimated (DA)) and those grown under sufficient irrigation (well-watered (WW)), and analysed the changes in g _i in relation to the leaf anatomical characteristics and a possible CO₂ transporter, aquaporin. The g _i, which was estimated by combined analyses of CO₂ gas exchange with chlorophyll fluorescence, in the DA plants was approximately half of that in the WW plants. The mesophyll and chloroplast surface areas exposing the intercellular air space, which potentially affect g _i, were not significantly different between the WW and DA plants. The amounts of plasma membrane aquaporins (PIP), immunochemically determined using radish PIP antibodies, were unrelated to g _i. After treatment with HgCl₂, an aquaporin inhibitor, the water permeability of the leaf tissues (measured as the weight loss of fully-turgid leaf disks without the abaxial epidermis in 1 m sorbitol) in WW plants decreased with an increase in HgCl₂ concentration. The g _i in the WW plants decreased to similar levels to the DA plants when the detached leaflets were fed with 0.5 mm HgCl₂. In contrast, both water permeability and g _i were insensitive to HgCl₂ treatments in DA plants. These results suggest that deactivation of aquaporins is responsible for the significant reduction in g _i observed in plants growing under long-term drought.

Measurement and simulation of water and methanol transport in algal cells

BACKGROUND

Experimental data and a complementary biophysical model are presented to describe the dynamic response of a unicellular microalga to osmotic processes encountered during cryopreservation.

METHOD OF APPROACH

Chlorococcum texanum (C. texanum) were mounted on a cryoperfusion microscope stage and exposed sequentially to various solutions of sucrose and methanol. Transient volumetric excursions were determined by capturing images of cells in real time and utilizing image analysis software to calculate cell volumes. A biophysical model was applied to the data via inverse analysis in order to determine the plasma membrane permeability to water and to methanol. The data were also used to determine the elastic modulus of the cell wall and its effect on cell volume. A three-parameter (hydraulic conductivity (Lp), solute permeability; (omega), and reflection coefficient, (sigma)) membrane transport model was fit to data obtained during methanol perfusion to obtain constitutive property values. These results were compared with the property values obtained for a two coefficient (Lp and omega) model.

RESULTS

The three-parameter model gave a value for sigma not consistent with practical physical interpretation. Thus, the two-coefficient model is the preferred approach for describing simultaneous water and methanol transport. The rate of both water and methanol transport were strongly dependent on temperature over the measured temperature range (25 degrees C to -5 degrees C) and cells were appreciably more permeable to methanol than to water at all measured temperatures.

CONCLUSION

These results may explain in part why methanol is an effective cryoprotective agent for microalgae.

A cohesion/tension mechanism explains the gating of water channels (aquaporins) in Chara internodes by high concentration

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 ( approximately 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.

Aquaporin functionality in relation to H+-ATPase activity in root cells of Capsicum annuum grown under salinity

As water and nutrient uptake should be related in the response of plants to salinity, the aim of this paper is to establish whether or not aquaporin functionality is related to H+-ATPase activity in root cells of pepper (Capsicum annuum L.) plants. Thus, H+-ATPase activity was measured in plasma membrane vesicles isolated from roots and aquaporin functionality was measured using a cell pressure probe in intact roots. Salinity was applied as 60 mM NaCl or 60 mM KCl, to determine which ion (Na+, K+ or Cl-) is producing the effects. We also investigated whether the effects of both salts were ameliorated by Ca2+. Similar results were obtained for cell hydraulic conductivity, Lpc, and H+-ATPase activity, large reductions in the presence at NaCl or KCl and an ameliorative effect of Ca2+. However, fusicoccin (an activator of H+-ATPase) did not alter osmotic water permeability of protoplasts isolated from roots. Addition of Hg2+ inhibited both ATPase and aquaporins, but ATPase also contains Hg-binding sites. Therefore, the results indicate that H+-ATPase and aquaporin activities may not be related in pepper plants.

Fluoride inhibits root water transport and affects leaf expansion and gas exchange in aspen (Populus tremuloides) seedlings

The effects of sodium fluoride (0.3, 5 and 10 mM NaF) on root hydraulic conductivity, and gas exchange processes were examined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. A long-term exposure of roots to NaF significantly decreased root hydraulic conductivity (Lp) and stomatal conductance ( gs). Root absorbed NaF significantly affected electrolyte leakage in leaf tissues and substantially restricted leaf expansion. NaF did not significantly affect leaf chlorophyll contents but decreased net photosynthesis (Pn). A short-term exposure of excised roots to 5 mM NaF and KF significantly decreased root water flow (Qv) with a concomitant decline in root respiration and reduced gs when applied through intact roots or excised stems. The same molar concentration of NaCl also decreased Qv and gs in intact seedlings, but to a lesser extent than NaF or KF, and did not significantly affect root respiration. The results suggest that fluoride metabolically inhibited Qv or Lp, probably by affecting water channel activity. We suggest that the metabolic inhibition of Lp by root-absorbed fluoride affected gas exchange and leaf expansion in aspen seedlings.

Molecular physiology of aquaporins in plants

In plants, membrane channels of the major intrinsic protein (MIP) super-family exhibit a high diversity with, for instance, 35 homologues in the model species Arabidopsis thaliana. As has been found in other organisms, plant MIPs function as membrane channels permeable to water (aquaporins) and in some cases to small nonelectrolytes. The aim of the present article is to integrate into plant physiology what has been recently learned about the molecular and functional properties of aquaporins in plants. Exhaustive compilation of data in the literature shows that the numerous aquaporin isoforms of plants have specific expression patterns throughout plant development and in response to environmental stimuli. The diversity of aquaporin homologues in plants can also be explained in part by their presence in multiple subcellular compartments. In recent years, there have been numerous reports that describe the activity of water channels in purified membrane vesicles, in isolated organelles or protoplasts, and in intact plant cells or even tissues. Altogether, these data suggest that the transport of water and solutes across plant membranes concerns many facets of plant physiology. Because of the high degree of compartmentation of plant cells, aquaporins may play a critical role in cell osmoregulation. Water uptake in roots represents a typical process in which to investigate the role of aquaporins in transcellular water transport, and the mechanisms and regulations involved are discussed.

Plant aquaporins: multifunctional water and solute channels with expanding roles

There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.

Effects of HgCl(2) on CO(2) dependence of leaf photosynthesis: evidence indicating involvement of aquaporins in CO(2) diffusion across the plasma membrane

Experiments were conducted to examine whether mercury-sensitive aquaporins facilitate photosynthetic CO(2) diffusion across the plasma membrane of leaf mesophyll cells. Discs without abaxial epidermes from Vicia faba leaflets were treated with HgCl(2), an inhibitor of aquaporins. Hydraulic conductivity of the plasma membrane of these discs, measured as the weight loss of the discs in the 1 M sorbitol solution, was inhibited by sub-mM concentrations of HgCl(2) by 70 to 80%. Photosynthetic CO(2) fixation was also inhibited by the HgCl(2) treatment in a similar concentration range. When 0.3 mM HgCl(2) solution was fed to the V. faba leaflets with intact epidermes via the transpiration stream, the rate of photosynthesis on leaf area basis (A) measured at photosynthetically active photon flux density of 700 micromol m(-2) s(-1) and at leaf temperature of 25 degrees C, decreased by about 20 to 30% at any CO(2) concentration in the intercellular spaces (C(i)). However, when CO(2) concentration in the chloroplast stroma (C(c)) was calculated from fluorescence and gas exchange data and A was plotted against C(c), A at low C(c) concentrations did not differ before and after the treatment. The conductance for CO(2) diffusion from the intercellular spaces to the chloroplast stroma (g(i)) decreased to 40 and 30% of the control value, when the leaflets were fed with 0.3 mM and 1.2 mM HgCl(2), respectively. Similar results were obtained with leaves of Phaseolus vulgaris. Although effects of HgCl(2) were not specific, the present results showed that HgCl(2) consistently lowered g(i). It is, thus, probable that the photosynthetic CO(2) uptake across the plasma membrane of the mesophyll cells is facilitated by mercury-sensitive aquaporins.

Low aquaporin content and low osmotic water permeability of the plasma and vacuolar membranes of a CAM plant Graptopetalum paraguayense: comparison with radish

Aquaporin facilitates the osmotic water transport across biomembranes and is involved in the transcellular and intracellular water flow in plants. We immunochemically quantified the aquaporin level in leaf plasma membranes (PM) and tonoplast of Graptopetalum paraguayense, a Crassulacean acid metabolism (CAM) plant. The aquaporin content in the Graptopetalum tonoplast was approximately 1% of that of radish. The content was calculated to be about 3 microg mg(-1) of tonoplast protein. The level of PM aquaporin in Graptopetalum was determined to be less than 20% of that of radish, in which an aquaporin was a major protein of the PM. The PM aquaporin was detected in the mesophyll tissue of Graptopetalum leaf by tissue print immunoblotting. The osmotic water permeability of PM and tonoplast vesicles prepared from both plants was determined with a stopped-flow spectrophotometer. The water permeability of PM was lower than that of the tonoplast in both plants. The Graptopetalum PM vesicles hardly showed water permeability, although the tonoplast showed a relatively high permeability. The water permeability changed depending on the assay temperature and was also partially inhibited by a sulfhydryl reagent. Furthermore, measurement of the rate of swelling and shrinking in different mannitol concentrations revealed that the protoplasts of Graptopetalum showed low water permeability. These results suggest that the low content of aquaporins in PM and tonoplast is one of the causes of the low water permeability of GRAPTOPETALUM: The relationship between the water-storage function of succulent leaves of CAM plants and the low aquaporin level is also discussed.

THE COHESION-TENSION MECHANISM AND THE ACQUISITION OF WATER BY PLANT ROOTS

The physical basis and evidence in support of the cohesion-tension theory of the ascent of sap in plants are reviewed. The focus is on the recent discussion of challenges to the cohesion-tension mechanism based on measurements with the pressure probe. Limitations of pressure probes to measure tensions (negative pressures) in intact transpiring plants are critically assessed. The possible role of the cohesion-tension mechanism during the acquisition of water and solutes by plant roots is discussed.

Role of Ca2+ in membrane excitation and cell motility in Characean cells as a model system

The Characeae internodal cell is excitable and generates an action potential. The depolarizing phase of the action potential primarily reflects the activation of Cl- channels, which takes place in a Ca2+-dependent manner. Namely, an increase in the Ca2+ influx takes place at the very beginning of the action potential and heightens the Ca2+ concentration in the cytoplasm ([Ca2+]c). The high [Ca2+]c activates Cl- channels at the plasmalemma, resulting in a large depolarization. The high [Ca2+]c also acts as a signal to induce a tonoplast action potential and the instantaneous cessation of cytoplasmic streaming; the tonoplast action potential also is caused by Ca2+-induced activation of Cl- channels at the tonoplast, and the cessation is a result of inhibition of the actin-myosin interaction by Ca2+. When the cytoplasm of the Characeae cell, especially in Nitella flexilis, is hydrated rapidly, [Ca2+], also increases through Ca2+ release from an intracellular store(s). The release may be triggered by the stretching of endomembranes caused by osmotic swelling of the Ca2+ store lumens. Although the origin of Ca2+ is different from that in the case of an action potential, high [Ca2+]c not only induces membrane depolarization through activation of the Cl- channel in a Ca2+-dependent manner but also inhibits cytoplasmic streaming in Characeae.

Onion root water transport sensitive to water channel and K+ channel inhibitors

Transroot osmotic water flux (Jos) and radial hydraulic conductivity (Lpr) in onion roots were greatly increased by three means; infiltration of roots by pressurization, repetition of osmosis and chilling at 5 degrees C. Jos was strongly reduced by the water channel inhibitor HgCl2 (91%) and the K+ channel inhibitor nonyltriethylammonium (C9, 75%), which actually made the membrane potential of root cells less sensitive to K+. C9 decreased the rate of turgor reduction induced by sorbitol solution to the same extent as HgCl2. Thus, C9 is assumed to decrease the hydraulic conductivity (Lp) of the plasma membrane by blocking water channels, although possible inhibition of the plasmodesmata of the root symplast by C9 cannot be excluded. Onion roots transported water from the tip to the base in the absence of the osmotic gradient. This non-osmotic water flux (Jnos) was equivalent to Jos induced by 0.029 M sorbitol. Jnos increased when Jos was increased by repetition of osmosis and decreased when Jos was decreased by either HgCl2 or by C9. The correlation between Jnos and Jos suggests that non-osmotic water transport occurs via the same pathways as those for osmotic water transport.

Transport and metabolic degradation of hydrogen peroxide in Chara corallina: model calculations and measurements with the pressure probe suggest transport of H(2)O(2) across water channels

A mathematical model is presented that describes permeation of hydrogen peroxide across a cell membrane and the implications of solute decomposition by catalase inside the cell. The model was checked and analysed by means of a numerical calculation that raised predictions for measured osmotic pressure relaxation curves. Predictions were tested with isolated internodal cells of CHARA: corallina, a model system for investigating interactions between water and solute transport in plant cells. Series of biphasic osmotic pressure relaxation curves with different concentrations of H(2)O(2) of up to 350 mol m(-3) are presented. A detailed description of determination of permeability (P(s)) and reflection coefficients (sigma(s)) for H(2)O(2) is given in the presence of the chemical reaction in the cell. Mean values were P(s)=(3.6+/-1.0) 10(-6) m s(-1) and sigma(s)=(0.33+/-0.12) (+/-SD, N=6 cells). Besides transport properties, coefficients for the catalase reaction following a Michaelis-Menten type of kinetics were determined. Mean values of the Michaelis constant (k(M)) and the maximum rate of decompositon (v(max)) were k(M)=(85+/-55) mol m(-3) and v(max)=(49+/-40) nmol (s cell)(-1), respectively. The absolute values of P:(s) and sigma(s) of H(2)O(2) indicated that hydrogen peroxide, a molecule with chemical properties close to that of water, uses water channels (aquaporins) to cross the cell membrane rapidly. When water channels were inhibited with the blocker mercuric chloride (HgCl(2)), the permeabilities of both water and H(2)O(2) were substantially reduced. In fact, for the latter, it was not measurable. It is suggested that some of the water channels in CHARA: (and, perhaps, in other species) serve as 'peroxoporins' rather than as 'aquaporins'.

Inhibition of water channels by HgCl2 in intact wheat root cells

To assess the extent of water flow through channels in the membranes of intact higher plant cells, the effects of HgCl2 on hydraulic conductivity (LP) of wheat (Triticum aestivum L.) root cells were investigated using a pressure probe. The LP of root cells was reduced by 75% in the presence of 100 &mgr;M HgCl2. The K+-channel blocker tetraethylammonium had no effect on the LP at concentrations that normally block K+ channels. HgCl2 rapidly depolarized the membrane potential (Vm) of the root cells. The dose-response relationship of inhibition of LP and depolarization of Vm were not significantly different, with half-maximal inhibition occurring at 4. 6 and 7.8 &mgr;M, respectively. The inhibition of LP and the depolarization of Vm caused by HgCl2 were partially reversed by beta-mercaptoethanol. The inhibition of LP by HgCl2 was similar in magnitude to that caused by hypoxia, and the addition of HgCl2 to hypoxia-treated cells did not result in further inhibition. We compared the LP of intact cells with that predicted from a model of cortical cells incorporating water flow across both the plasma membrane and the tonoplast using measured values of water permeability from isolated membranes, and found that HgCl2 has other effects in addition to the direct inhibition of water channels.

Water stress inhibits hydraulic conductance and leaf growth in rice seedlings but not the transport of water via mercury-sensitive water channels in the root

The mechanisms by which moderate water stress (adding polyethylene glycol 6000 to the root medium) induces a sustained inhibition of growth in emerging first leaves of intact rice (Oryza sativa) seedlings was investigated under growth-chamber conditions. Early (24 h) inhibition of leaf growth was not related to changes in root size or in osmotic potential gradients and cell wall-yielding characteristics in the leaf-expansion zone of stressed seedlings. However, reductions in root-to-leaf hydraulic conductance (L) were measured in two rice cultivars after 4 or 24 h at various levels of water stress, and these reductions correlated well with the inhibition of leaf growth. We assayed L by a psychrometric method and, in intact seedlings, by a novel osmotic-jump method. The addition of 0.5 mM HgCl2 to the root medium to inhibit water transport through Hg-sensitive water channels in the roots did not inhibit leaf growth in unstressed seedlings. However, both leaf growth and L were additionally reduced (by 49% and 43%, respectively) within minutes of adding HgCl2 to roots of water-stressed seedlings. Water stress therefore appeared to increase the transport of water via Hg-sensitive water channels. Other mechanisms were apparently involved in inhibiting overall L and leaf growth.

Osmotic water permeability of isolated protoplasts. Modifications during development

A transference chamber was developed to measure the osmotic water permeability coefficient (Pos) in protoplasts 40 to 120 μm in diameter. The protoplast was held by a micropipette and submitted to a steep osmotic gradient created in the transference chamber. Pos was derived from the changes in protoplast dimensions, as measured using a light microscope. Permeabilities were in the range 1 to 1000 μm s-1 for the various types of protoplasts tested. The precision for Pos was </=40%, and within this limit, no asymmetry in the water fluxes was observed. Measurements on protoplasts isolated from 2- to 5-d-old roots revealed a dramatic increase in Pos during root development. A shift in Pos from 10 to 500 μm s-1 occurred within less than 48 h. This phenomenon was found in maize (Zea mays), wheat (Triticum aestivum), and rape (Brassica napus) roots. These results show that early developmental processes modify water-transport properties of the plasma membrane, and that the transference chamber is adapted to the study of water-transport mechanisms in native membranes.

Significance of plasmalemma aquaporins for water-transport in Arabidopsis thaliana

The plant plasma membrane intrinsic protein, PIP1b, facilitates water transport. These features were characterized in Xenopus oocytes and it has asked whether aquaporins are relevant for water transport in plants. In order to elucidate this uncertainty Arabidopsis thaliana was transformed with an anti-sense construct targeted to the PIP1b gene. Molecular analysis revealed that the anti-sense lines have reduced steady-state levels of PIP1b and the highly homologous PIP1a mRNA. The cell membrane water permeability was analyzed by swelling of protoplasts, which had been transferred into hypotonic conditions. The results indicate that the reduced expression of the specific aquaporins decreases the cellular osmotic water permeability coefficient approximately three times. The morphology and development of the anti-sense lines resembles that of control plants, with the exception of the root system, which is five times as abundant as that of control plants. Xylem pressure measurement suggests that the increase of root mass compensates the reduced cellular water permeability in order to ensure a sufficient water supply to the plant. The results obtained by this study, therefore, clearly demonstrate that aquaporins are important for plant water transport.