Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

New evidence about the relationship between water channel activity and calcium in salinity-stressed pepper plants

This study, of how Ca2+ availability (intracellular, extracellular or linked to the membrane) influences the functionality of aquaporins of pepper (Capsicum annuum L.) plants grown under salinity stress, was carried out in plants treated with NaCl (50 mM), CaCl2 (10 mM), and CaCl2 (10 mM) + NaCl (50 mM). For this, water transport through the plasma membrane of isolated protoplasts, and the involvement of aquaporins and calcium (extracellular, intracellular and linked to the membrane) has been determined. After these treatments, it could be seen that the calcium concentration was reduced in the apoplast, in the cells and on the plasma membrane of roots of pepper plants grown under saline conditions; these concentrations were increased or restored when extra calcium was added to the nutrient solution. Protoplasts extracted from plants grown under Ca2+ starvation showed no aquaporin functionality. However, for the protoplasts to which calcium was added, an increase of aquaporin functionality of the plasma membrane was observed [osmotic water permeability (Pf) inhibition after Hg addition]. Interestingly, when verapamil (a Ca2+ channel blocker) was added, no functionality was observed, even when Ca2+ was added with verapamil. Therefore, calcium seems to be involved in plasma membrane aquaporin regulation via a chain of processes within the cell but not by alteration of the stability of the plasma membrane.     

Different blocking effects of HgCl2 and NaCl on aquaporins of pepper plants

In this study we have compared the short-term effects of both NaCl and HgCl₂ on aquaporins of Capsicum annuum L. plants, in order to determine whether or not they are similar. Stomatal conductance, turgor, root hydraulic conductance and water status were measured after 0.5, 2, 4 and 6 h of NaCl (60 mmol/L) or HgCl₂ (50 μmol/L) treatment. When 60 mmol/L NaCl was added to the nutrient solution, a large decrease in stomatal conductance was observed after 2 h. However, when HgCl₂ (50 μmol/L) was added, the decrease occurred after 4 h. The number of open stomata closed was always lower in plants treated with HgCl₂ than in plants treated with NaCl. The water content of the Hg²⁺-treated plants was decreased, compared with controls and NaCl-treated. The root hydraulic conductance decreased after HgCl₂ and NaCl treatment plants. Turgor of leaf epidermal cells was greatly reduced in plants treated with HgCl₂, but remained constant in the NaCl treatment, compared with control plants. The fact that the stomatal conductance was reduced more rapidly after NaCl addition, followed by the stomatal closure, and that both water content and turgor did not differ from the control suggests that in NaCl-treated plants there must be a signal moving from root to shoot. Therefore, the control of plant homeostasis through a combined regulation of root and stomatal exchanges may be dependent on aquaporin regulation.     

Birth of water channel proteins-the aquaporins

If we compare aquaporin (as a proteic pathway for water permeation across biological membranes) with a child we can say that he had a very long gestation period. His possible existence was predicted for a long time (Overton in 1985, Stein and Danielli in 1956), some of his features (transport of water and its reversible inhibition) were assigned by Macey and Farmer in 1970, however this child was first detected by Benga and coworkers in 1986. We clearly demonstrated for the first time the presence and location of a water channel at the human RBC membrane among the polypeptides migrating in the region having 35-60 kDa on the electrophoretogram of RBC membranes, labeled with 203Hg-PCMBS in the conditions of specific inhibition of water diffusion; I suggested that a minor membrane protein that binds PCMBS is involved in water transport and also indicated the way in which the specific protein could be further characterized: by purification and reconstitution in liposomes. Our landmark papers in 1986 can be compared with the first detection of a child "in utero" by ultrasonography, since we discovered one of the essential components of the "aquaporin child" (a molecular weight of 35-60 kDa for the glycosylated component); we have also indicated the way to recognize him after birth (among other children of his group!): placing the isolated children in a certain environment and asking them to perform the same task (one should read: reconstitution studies in liposomes and measurement of water permeability), like aligning athletes for a running test. This was the only certain way to know that the child is really the fastest runner and not just one that is helping (by various means) another child to be fastest runner. A "new child" was observed in 1988 by Agre and coworkers, who identified a novel integral membrane protein in human RBCs having a non-glycosylated component of 28 kDa and a glycosylated component migrating as a diffuse band of 35-60 kDa; they suggested that the new protein (nick-named CHIP28 in 1991) may play a role in linkage of the membrane skeleton to the lipid bilayer. In 1992 Agre and coworkers suggested that CHIP28 is a functional unit of membrane water channels; by reconstitution in liposomes it was demonstrated that CHIP28 is a water channel itself rather than a water channel regulator. In other words the child we first detected was recognized as having the predicted qualities only in 1992. In 1993 CHIP28 was renamed aquaporin 1. Looking in retrospect, asking the crucial question, when was the first water channel protein, aquaporin 1, discovered, a fair and clear cut answer would be: the first water channel protein, now called aquaporin 1, was identified or "seen" in situ in the human RBC membrane by Benga and coworkers in 1986. It was again "seen" when it was by chance purified by Agre and coworkers in 1988 and was again identified when its main feature, the water transport property was found by Agre and coworkers in 1992. If a comparison with the discovery of The New World of America is made, the first man who has "seen" a part, very small indeed, of The New Land was Columbus; later, others, including Amerigo Vespucci (from whom the name derived), have better "seen" a larger part of the new Continent and in the subsequent years many explorers discovered the complexity of the Americas!     

The structural basis of water permeation and proton exclusion in aquaporins (Review)

Aquaporins (AQPs) represent a ubiquitous class of integral membrane proteins that play critical roles in cellular osmoregulations in microbes, plants and mammals. AQPs primarily function as water-conducting channels, whereas members of a sub-class of AQPs, termed aquaglyceroporins, are permeable to small neutral solutes such as glycerol. While AQPs facilitate transmembrane permeation of water and/or small neutral solutes, they preclude the conduction of protons. Consequently, openings of AQP channels allow rapid water diffusion down an osmotic gradient without dissipating electrochemical potentials. Molecular structures of AQPs portray unique features that define the two central functions of AQP channels: effective water permeation and strict proton exclusion. This review describes AQP structures known to date and discusses the mechanisms underlying water permeation, proton exclusion and water permeability regulation.     

The interactive effects of temperature and osmotic potential on the growth of marine isolates of <Emphasis Type="Italic">Fusarium solani</Emphasis>

The mycelial growth of 18 Fusarium solani strains isolated from sea beds of the south-eastern coast of Spain was tested on potato-dextrose-agar adjusted to different osmotic potentials with either KCl or NaCl (−1.50 to −144.54 bars) in 10 °C intervals ranging from 15 to 35 °C. Fungal growth was determined by measuring colony diameter after 4 days of incubation. Mycelial growth was maximal at 25 °C. The quantity and frequency pattern of mycelial growth of F. solani differ significantly at 15 and 25 °C, with maximal growth occurring at the highest water potential tested (−1.50 bars); and at 35 °C, with a maximal mycelial growth at −13.79 bars. The effect of water potential was independent of salt composition. The general growth pattern of F. solani showed declining growth at potentials below −41.79 bars. Fungal growth at 35 °C was always higher than that grow at 15 °C, of all the water potentials tested. Significant differences observed in the response of mycelia to water potential and temperature as main and interactive effects. The viability of cultures was increasingly inhibited as the water potential dropped, but some growth was still observed at −99.56 bars. These findings could indicate that marine strains of F. solani have a physiological mechanism that permits survival in environments with low water potential. The observed differences in viability and the magnitude of growth could indicate that the biological factors governing potential and actual growth are affected by osmotic potential in different ways.     

Different cation stresses affect specifically osmotic root hydraulic conductance, involving aquaporins, ATPase and xylem loading of ions in Capsicum annuum, L. plants

In order to study the effect of nutrient stress on water uptake in pepper plants (Capsicum annuum L.), the excess or deficiency of the main cations involved in plant nutrition (K(+), Mg(2+), Ca(2+)) and two different degrees of salinity were related to the activity of plasma membrane H(+)-ATPase, the pH of the xylem sap, nutrient flux into the xylem (J(s)) and to a number of parameters related to water relations, such as root hydraulic conductance (L(0)), stomatal conductance (g(s)) and aquaporin activity. Excess of K(+), Ca(+) and NaCl produced a toxic effect on L(0) while Mg(2+) starvation produced a positive effect, which was in agreement with aquaporin functionality, but not with ATPase activity. The xylem pH was altered only by Ca treatments. The results obtained with each treatment could suggest that detection of the quality of the nutrient supply being received by roots can be related to aquaporins functionality, but also that each cation stress triggers specific responses that have to be assessed individually.     

Osmotic versus toxic effects of NaCl on pepper plants

Water relations, mineral composition, growth and root morphology were studied in pepper plants (Capsicum annuum L. cv California Wonder). Two NaCl concentrations (30 and 60 mM) and two nutrient solutions in which the concentrations of macronutrients were increased were used to assess the ionic and osmotic effects of NaCl in these plants. The hydraulic conductivity (L_o), stomatal conductance (g_s), percentage of open stomata and pressure potential (Ψ_p) decreased with all treatments, in a similar way for 30 mM NaCl and for its iso-osmotic solution of macronutrients, however, the decrease was higher for 60 mM NaCl than for its iso-osmotic solution. Ion analyses also revealed that nutrient concentrations were altered greatly at 60 mM NaCl. Also, changes in morphology, such as increases in cortex cell size and in intercellular spaces, were detected. Therefore, at low salinity, the effect of NaCl was mainly osmotic, however, under higher salinity also the toxicity of Na⁺ and Cl⁻ participate.     

Cu1+, but not Cu2+ is capable of inhibition of AQP4 permeability in an in vitro CHO cell based model

Aquaporin 4 (AQP4) is an important water channel in the central nervous system which is implicated in several neurological disorders. Due to its significance, the identification of molecules which are able to modulate its activity is quite important for potential therapeutic applications. Here we used a novel screening method involving CHO cell lines which stably express AQP4 to test for potential molecules of interest. Using this method we identified a metal ion, Cu¹⁺, which is able to inhibit AQP4 activity in a cell model, an interaction which has not been previously described. This inhibition was effective at concentrations greater than 500 nM in the CHO cell model, and was confirmed in a proteoliposome based model. Furthermore, the binding sites for Cu¹⁺ inhibition of AQP4 are identified as cysteine 178 and cysteine 253 on the intracellular domain of the protein via the synthesis of AQP4 containing point mutations to remove these cysteines. These results suggest that Cu¹⁺ is able to access intracellular binding sites and inhibit AQP4 in a cell based model.     

Osmotic water permeability of plasma and vacuolar membranes in protoplasts I. High osmotic water permeability in radish ( Raphanus sativus ) root cells as measured by a new method

Intra- and transcellular water movements in plants are regulated by the water permeability of the plasma membrane (PM) and vacuolar membrane (VM) in plant cells. In the present study, we investigated the osmotic water permeability of both PM (P _f1) and VM (P _f2), as well as the bulk osmotic water permeability of a protoplast (P _f(bulk)) isolated from radish (Raphanus sativus) roots. The values of P _f(bulk) and P _f2 were determined from the swelling/shrinking rate of protoplasts and isolated vacuoles under hypo- or hypertonic conditions. In order to minimize the effect of unstirred layer, we monitored dropping or rising protoplasts (vacuoles) in sorbitol solutions as they swelled or shrunk. P _f1 was calculated from P _f(bulk) and P _f2 by using the ‘three-compartment model’, which describes the theoretical relationship between P _f1, P _f2 and P _f(bulk) (Kuwagata and Murai-Hatano in J Plant Res, 2007). The time-dependent changes in the volume of protoplasts and isolated vacuoles fitted well to the theoretical curves, and solute permeation of PM and VM was able to be neglected for measuring the osmotic water permeability. High osmotic water permeability of more than 500 μm s⁻¹, indicating high activity of aquaporins (water channels), was observed in both PM and VM in radish root cells. This method has the advantage that P _f1 and P _f2 can be measured accurately in individual higher plant cells. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. It includes four appendices, four tables and two figures. Mari Murai-Hatano and Tsuneo Kuwagata contributed equally to the paper. An erratum to this article is available at .     

Insights into structural mechanisms of gating induced regulation of aquaporins

Aquaporin family comprises of transmembrane channels that are specialized in conducting water and certain small, uncharged molecules across cell membranes. Essential roles of aquaporins in various physiological and pathophysiological conditions have attracted great scientific interest. Pioneering structural studies on aquaporins have almost solved the basic question of mechanism of selective water transport through these channels. Another important structural aspect of aquaporins which seeks attention is that how the flow of water through the channel is regulated by the mechanism of gating. Aquaporins are also regulated at the protein level, i.e. by trafficking which includes changes in their expression levels in the membrane. Availability of high resolution structures along with numerous molecular dynamics simulation studies have helped to gain an understanding of the structural mechanisms by which water flux through aquaporins is controlled. This review will summarize the highlights regarding structural features of aquaporins, mechanisms governing water permeation, proton exclusion and substrate specificity, and describe the structural insights into the mechanisms of aquaporin gating whereby water conduction is regulated by post translational modifications, such as phosphorylation.     

Osmotic water permeability of plasma and vacuolar membranes in protoplasts II. Theoretical basis

Water permeability of the plasma membrane (PM) and the vacuolar membrane (VM) is important for intracellular and transcellular water movement in plants, because mature plant cells have large central vacuoles. We have developed a new method for measuring the osmotic water permeability of the PM and VM (P _f1 and P _f2, respectively) in individual plant cells. Here, the theoretical basis and procedure of the method are discussed. Protoplasts isolated from higher plant tissues are used to measure P _f1 and P _f2. Because of the semi-permeability (selective permeability) of cellular membranes, protoplasts swell or shrink under hypotonic or hypertonic conditions. A theoretical three-compartment model is presented for simulating time-dependent volume changes in the vacuolar and cytoplasmic spaces in a protoplast during osmotic excursions. The model describes the theoretical relationships between P _f1, P _f2 and the bulk osmotic water permeability of protoplasts (P _f(bulk)). The procedure for measuring the osmotic water permeability is: (1) P _f(bulk) is calculated from the time when half of the total change in protoplast volume is completed, by assuming that the protoplast has a single barrier to water movement across it (two-compartment model); (2) P _f2 of vacuoles isolated from protoplasts is obtained in the same manner; and (3) P _f1 is determined from P _f(bulk) and P _f2 according to the three-compartment model. The theoretical relationship between P _fl (m s⁻¹) and L _Pl (hydraulic conductivity, l=1, 2) (m s⁻¹ Pa⁻¹) is also discussed. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorised users. Tsuneo Kuwagata and Mari Murai-Hatano contributed equally to the paper.     

Osmotic water permeability of plasma and vacuolar membranes in protoplasts I. High osmotic water permeability in radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis>) root cells as measured by a new method

Intra- and transcellular water movements in plants are regulated by the water permeability of the plasma membrane (PM) and vacuolar membrane (VM) in plant cells. In the present study, we investigated the osmotic water permeability of both PM (P ( f1)) and VM (P ( f2)), as well as the bulk osmotic water permeability of a protoplast (P ( f(bulk))) isolated from radish (Raphanus sativus) roots. The values of P ( f(bulk)) and P ( f2) were determined from the swelling/shrinking rate of protoplasts and isolated vacuoles under hypo- or hypertonic conditions. In order to minimize the effect of unstirred layer, we monitored dropping or rising protoplasts (vacuoles) in sorbitol solutions as they swelled or shrunk. P ( f1) was calculated from P ( f(bulk)) and P ( f2) by using the 'three-compartment model', which describes the theoretical relationship between P ( f1), P ( f2) and P ( f(bulk)) (Kuwagata and Murai-Hatano in J Plant Res, 2007). The time-dependent changes in the volume of protoplasts and isolated vacuoles fitted well to the theoretical curves, and solute permeation of PM and VM was able to be neglected for measuring the osmotic water permeability. High osmotic water permeability of more than 500 mum s(-1), indicating high activity of aquaporins (water channels), was observed in both PM and VM in radish root cells. This method has the advantage that P ( f1) and P ( f2) can be measured accurately in individual higher plant cells.     

Myocardial water handling and the role of aquaporins

Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.     

Elevated CO2 alleviates negative effects of salinity on broccoli (Brassica oleracea L. var Italica) plants by modulating water balance through aquaporins abundance

In the global change scenario, increased CO₂ may favour water use efficiency (WUE) by plants. By contrast, in arid and semiarid areas, salinity may reduce water uptake from soils. However, an elevated WUE does not ensure a reduced water uptake and upon salinity this fact may constitute an advantage for plant tolerance. In this work, we aimed to determine the combined effects of enhanced [CO₂] and salinity on the plant water status, in relation to the regulation of PIP aquaporins, in the root and leaf tissues of broccoli plants (Brassica oleracea L. var Italica), under these two environmental factors. Thus, different salinity concentrations (0, 60 and 90 mM NaCl) were applied under ambient (380 ppm) and elevated (800 ppm) [CO₂]. Under non-salinised conditions, stomatal conductance (G_s) and transpiration rate (E) decreased with rising [CO₂] whereas water potential (Ψ_ω) was maintained stable, which caused a reduction in the root hydraulic conductance (L₀). In addition, PIP1 and PIP2 abundance in the roots was decreased compared to ambient [CO₂]. Under salinity, the greater stomatal closure observed at elevated [CO₂] – compared to that at ambient [CO₂] – caused a greater reduction in G_s and E and allowed plants to maintain their water balance. In addition, a lower decrease in L₀ under salt stress was observed at elevated [CO₂], when comparing with the decrease at ambient [CO₂]. Modifications in PIP1 and PIP2 abundance or their functionality in the roots is discussed. In fact, an improved water status of the broccoli plants treated with 90 mM NaCl and elevated [CO₂], evidenced by a higher Ψ_ω, was observed together with higher photosynthetic rate and water use efficiency. These factors conferred on the salinised broccoli plants greater leaf area and biomass at elevated [CO₂], in comparison with ambient [CO₂]. We can conclude that, under elevated [CO₂] and salt stress, the water flow is influenced by the tight control of the aquaporins in the roots and leaves of broccoli plants and that increased PIP1 and PIP2 abundance in these organs provides a mechanism of tolerance that maintains the plant water status.     

Short-term succession dynamics of macrobenthos in a salinity-stressed estuary

It is postulated that early succession is influenced by environmental conditions directly after a disturbance event and the temporal scale of succeeding disturbances. For example, Rincon Bayou, Texas, U.S.A. is an estuary stressed by large fluctuations in salinity ranging from oligohaline to hypersaline conditions; leading to the question: are benthic succession dynamics interrupted or affected by the temporal scale of salinity fluctuations? To answer this question, succession dynamics were investigated by manipulation of 1) initial environmental conditions, and 2) time between severe disturbances. The influence of environmental conditions and time course of succession were investigated by comparing two experiments. A synoptic-deployment experiment where samples were deployed at the same environmental conditions and a synoptic-retrieval experiment where samples were collected from deployments under different environmental conditions. Both experiments were conducted at varying time courses of 2-, 4- and 8-week durations between severe disturbances. Severe disturbances were simulated with defaunated sediment. Ambient conditions were simultaneously sampled as a reference. Environmental conditions directly affected succession. In the ambient study, succession occurred in conjunction with changes in environmental conditions. In the synoptic-deployment and synoptic-retrieval studies, environmental conditions at the time of retrieval were more important in determining community structure than deployment duration or initial environmental conditions at time of deployment. Initial environmental conditions were not the primary mechanism in regulation of succession because succession is likely interrupted by frequent salinity disturbances. The salinity-stressed estuary of Rincon Bayou appears to be in a constant state of early and intermediate succession because of frequent salinity-related disturbances.     

Brassinolide may control aquaporin activities in Arabidopsis thaliana

 It is usually assumed that aquaporins present in the cellular membranes could be an important route in the control of water flux in plants, but evidence for this hypothesis is scarce. In this paper, we report measurements of the osmotic permeability (P _os ) of protoplasts isolated from hypocotyls of wild-type and mutant Arabidopsis thaliana (L.) Heynh. Mutants were affected in their growth and exhibited different sensitivities to the phytohormone, brassinolide. For the two mutants studied (cpd: constitutive photomorphogenesis and dwarfism; bri1: brassinosteroid insensitive), hypocotyl length was correlated to P _os for the protoplasts. Under experimental conditions where hypocotyl growth had ceased, restoration of root, hypocotyl and petiole growth by brassinolide was correlated with an increase in P _os of the hypocotyl protoplasts. We consider that the increase in P _os of the hypocotyl cells was needed because these cells were part of the transcellular water pathway of the plant. This is the first time, to our knowledge, that brassinolide has been shown to be involved in the modification of the water-transport properties of cell membranes. Our results also emphasize the importance of aquaporins and the transcellular pathway in water transport under normal growth conditions. Received: 15 January 2000 / Accepted: 18 May 2000     

Aquaporins: Another piece in the osmotic puzzle

Osmolarity not only plays a key role in cellular homeostasis but also challenges cell survival. The molecular understanding of osmosis has not yet been completely achieved, and the discovery of aquaporins as molecular entities involved in water transport has caused osmosis to again become a focus of research. The main questions that need to be answered are the mechanism underlying the osmotic permeability coefficients and the extent to which aquaporins change our understanding of osmosis. Here, attempts to answer these questions are discussed. Critical aspects of the state of the state of knowledge on osmosis, a topic that has been studied since 19th century, are reviewed and integrated with the available information provided by in vivo, in vitro and in silico approaches.     

A molecular modeling approach defines a new group of Nodulin 26-like aquaporins in plants

The three-dimensional models built for the Nod26-like aquaporins all exhibit the typical α-helical fold of other aquaporins containing the two ar/R and NPA constriction filters along the central water channel. Besides these structural homologies, they readily differ with respect to the amino acid residues forming the ar/R selective filter. According to these discrepancies in both the hydrophilicity and pore size of the ar/R filter, Nod26-like aquaporins can be distributed in three subgroups corresponding to NIP-1, NIP-II and a third subgroup of Nod26-like aquaporins exhibiting a highly hydrophilic and widely open filter. However, all Nod26-like aquaporins display a bipartite distribution of electrostatic charges along the water channel with an electropositive extracellular vestibular portion followed by an electronegative cytosolic vestibular portion. The specific transport of water, non-ionic solutes (glycerol, urea, ammoniac), ions and gas (NH₃) across the Nod26-like obviously depends on the electrostatic and conformational properties of their central water channel.     

Distribution and roles of aquaporins in salivary glands

Salivary glands are involved in secretion of saliva, which is known to participate in the protection and hydratation of mucosal structures within the oral cavity, oropharynx and oesophagus, the initiation of digestion, some antimicrobial defence, and the protection from chemical and mechanical stress. Saliva secretion is a watery fluid containing electrolytes and a mixture of proteins and can be stimulated by muscarinic and adrenergic agonists. Since water movement is involved in saliva secretion, the expression, localization and function of aquaporins (AQPs) have been studied in salivary glands. This review will focus on the expression, localization and functional roles of the AQPs identified in salivary glands. The presence of AQP1, AQP5 and AQP8 has been generally accepted by many, while the presence of AQP3, AQP4, AQP6 and AQP7 still remains controversial. Functionally, AQP5 seems to be the only AQP thus far to be clearly playing a major role in the salivary secretion process. Modifications in AQPs expression and/or distribution have been reported in xerostomic conditions.