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First published online November 17, 2006; 10.1104/pp.106.090092 Plant Physiology 143:122-133 (2007) © 2007 American Society of Plant Biologists
Putative Role of Aquaporins in Variable Hydraulic Conductance of Leaves in Response to Light1Unité Mixte de Recherche 547, Physiologie Intégrée de l'Arbre Fruitier et Forestier, Institut National de la Recherche Agronomique, 63039 Clermont-Ferrand, France (H.C., T.S.B.); Unité Mixte de Recherche 547, Physiologie Intégrée de l'Arbre Fruitier et Forestier, Université Blaise Pascal, 63170 Aubière, France (J.-S.V., N.B., S.H., A.G., S.S.); Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 (M.T.T.); and United States Forest Service, South Burlington, Vermont 05403 (M.T.T.)
Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (Kleaf) to irradiance. The effects of light and temperature on Kleaf are described. Under dark conditions, Kleaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, Kleaf values of light-exposed leaves responded to temperature and 0.1 mM cycloheximide treatments. Furthermore, Kleaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that Kleaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low Kleaf in the dark was associated with down-regulation, whereas high Kleaf in the light was associated with up-regulation of JrPIP2. Light responses of Kleaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indicating the importance of de novo protein biosynthesis in this process. Our results indicate that walnut leaves can rapidly change their hydraulic conductance and suggest that these changes can be explained by regulation of plasma membrane aquaporins. Model simulation suggests that variable leaf hydraulic conductance in walnut might enhance leaf gas exchanges while buffering leaf water status in response to ambient light fluctuations.
Terrestrial plants face the contradictory demand of maximizing carbon dioxide uptake for photosynthesis while minimizing water loss to the atmosphere. Plants dynamically and finely adjust their hydraulic efficiency by offering a series of resistances in different organs along the soil-plant-atmosphere continuum (Tyree and Zimmermann, 2002  ). For instance, the leaf seems to contribute 50% of the hydraulic resistance of the aerial part of plants, which represents about 30% of the whole plant resistance (Nardini and Tyree, 1999; Tsuda and Tyree, 2000; Sack et al., 2003), but this share would, of course, vary if leaf resistance is itself variable (Sack and Holbrook, 2006). Evidence linking hydraulic conductance to plasma membrane aquaporins (plasma intrinsic proteins [PIPs]), which facilitate water transport through cell membranes, has been well documented for roots (Javot and Maurel, 2002; Tyerman et al., 2002). Conversely, the full significance of PIPs in leaf conductance to water is widely lacking.
Water movement through a leaf can follow an apoplastic (through the cell walls) or a cell-to-cell route. The latter includes symplastic (through plasmodesmata) and transcellular (across cell membrane) routes (Canny 1988
In the recent literature, many physiological and molecular data dealing with the importance of PIPs in the regulation of root conductance to water have been published (Javot and Maurel, 2002
In contrast to living tissues, hydraulic conductance of xylem conduits (dead cells) is entirely governed by physical processes. For instance, hydraulic conductance is determined by the dimensions of the conduits and the structure of their walls according to the Poiseuille-Hagen law (Sperry et al., 2005
The objective of this study was to investigate the relative importance of plasma membrane aquaporins in leaf hydraulic conductance by combining physiological and molecular approaches. This study was focused on leaves of the walnut tree, where leaf hydraulic conductance has been shown to increase in response to light (Sack et al., 2002
Light and Temperature Effects on Leaf Hydraulic Conductance
To understand the light and temperature effects on leaf hydraulic conductance, the experimental setup in Figure 1 was used (see "Materials and Methods"). The results of a typical experiment are shown in Figure 2
for a control leaf at 25°C (statistics for these treatments are shown in Fig. 8). In the dark condition, walnut leaves exhibited a small and constant conductance value (Kleaf < 3 mmol s–1 m–2 MPa–1). When leaves were exposed to light, their hydraulic conductance showed a dramatic 400% increase. Kleaf increase was exponential to a maximal value reached after approximately 1 h with a half-time T
Response curves obtained at different temperatures were qualitatively similar, but differed quantitatively (Fig. 3 ). Kleaf values in the dark increased slightly with temperatures below 35°C (Fig. 3A). When Kleaf was corrected for changes in water viscosity with temperature (Fig. 3B), the dependence of Kleaf in full darkness with temperature was nearly absent. A significant increase remained, however, between 25°C and 35°C. Kleaf values in full light increased considerably and linearly with temperature (Fig. 3B). This response was not attributable to a change in water viscosity (Fig. 3B). Finally, T  values strongly decreased with temperature both for the dark-to-light and light-to-dark phases (Fig. 3C). At 5°C, the light response was absent; hence, it was not possible to compute T.
Stomatal Movements Length and width of stomatal pores were constant whatever the treatments (Fig. 4, A and B), with the exception of control illuminated leaves that tended to exhibit more open stomata (Fig. 4, D and E ). Hydraulic conductance on a surface area basis of stomatal pores was neither qualitatively nor quantitatively related to Kleaf values measured in this study (Fig. 4C). Indeed, the discrepancy between the two parameters was huge, stomatal Kleaf values being 5 to 7 orders of magnitude higher.
Light Response of Kleaf and Aquaporin Transcripts To obtain consistent comparison between the pattern of Kleaf during the light phase and that of JrPIP2,1 and JrPIP2,2 abundance, real-time PCR was performed over a time course of 120 min (Fig. 5 ). Five time points were selected: 0, 15, 30, 60, and 120 min, which were representative of Kleaf dynamics. As was shown in Figure 5, the time course of Kleaf was well correlated with the time course of JrPIP2 abundance. Indeed, JrPIP2 transcripts were increased during the first 15 min after the light came on and continued to increase with increasing Kleaf through the time course. They exhibited strong abundance at the end of the time course, when Kleaf reached its maximal value.
To gain further insight into the link between Kleaf dynamics and the relative abundance of JrPIP2 transcripts, these latter were investigated together under light and dark conditions (Fig. 6 ). Leaves under the dark condition (dark) exhibited low Kleaf values, which was associated with relatively low transcript abundance of both JrPIP2,1 and JrPIP2,2 (Fig. 6). On the contrary, leaves exposed for 90-min light treatment (light) displayed a higher value of Kleaf and a strong increase in transcript level of JrPIP2,1 and JrPIP2,2 (>27-fold). Interestingly, this situation was almost completely reversed when the light was turned off again (back to dark). In this condition, Kleaf value dropped to its initial value and abundance of JrPIP2 decreased significantly as well. In summary, Kleaf values and transcript abundance of JrPIP2,1 and JrPIP2,2 appeared to be positively correlated.
CHX Effects on Kleaf and Aquaporin Transcripts
Typical results with leaves perfused under positive pressure with CHX during dark and light phases are shown in Figure 7
(statistics for these treatments are shown in Fig. 8A
). A 100 µM CHX perfusion during the dark phase had no effect on Kleaf (top section). When such leaves were illuminated, their Kleaf value was not significantly different from that in the dark phase. When CHX was supplied during the light phase, Kleaf values rapidly returned (T
Because light-induced stimulation of Kleaf was markedly inhibited by a 100 µM CHX perfusion (Fig. 7) and Kleaf was found to be linked to JrPIP2 abundance (Figs. 5 and 6), we wanted to determine whether this inhibition can be explained by potential modification in JrPIP2 abundance. When CHX solution was perfused in the dark period, leaves that showed no Kleaf response to light consistently exhibited a small accumulation of JrPIP2,1 and JrPIP2,2 transcripts compared to the CHX-untreated leaves (control-light). A similar pattern was found when CHX was supplied during the light phase, when Kleaf was at its maximal value (CHX-light 2; Fig. 8). Indeed, within less than 1 h after CHX application, both Kleaf values and accumulation of JrPIP2,1 and JrPIP2,2 strongly decreased and reached the level under dark conditions (control-dark).
To get insight into the functional significance of leaf aquaporins on leaf-water relations, experiments were conducted on transpiring leaves and treated with 100 µM CHX under dark and light conditions as above. Results were similar to those obtained with leaves perfused under positive pressure (Fig. 9 ). Indeed, both CHX treatments significantly reduced Kleaf under light conditions (Fig. 9C). This reduction resulted from significantly more negative leaf water potential values (Fig. 9B), whereas leaf absorption remained unchanged (Fig. 9A). Leaves treated with CHX remained green and healthy looking.
This study demonstrates that leaf hydraulic conductance (Kleaf) in walnut leaves is variable and can respond to some environmental stimuli (light and temperature) or CHX treatment (an inhibitor of protein biosynthesis). Stomatal observations and direct leaf evaporation rate measurements confirmed that stomatal movements did not explain the variations in Kleaf values. In addition, the magnitude of Kleaf was shown to be closely linked to the abundance of PIP2 transcripts.
In this study, most leaf hydraulic conductance values were obtained by perfusing submerged leaves with water under high pressure. This technique saturates the intercellular spaces in the leaves with water, including the stomatal chambers. Two possible drawbacks might be associated with the technique. First, as liquid water flows through the stomata, the hydraulic conductance of the stomatal pore is measured in a series with the leaf hydraulic conductance (Tyree et al., 2005
The water channel activity of JrPIP2,1 and JrPIP2,2 was previously demonstrated by expression in Xenopus laevis oocytes (Sakr et al., 2003
Transcript abundance of these two aquaporins (JrPIP2,1 and JrPIP2,2) was substantially up-regulated by light in walnut leaves (Fig. 5), which is in accordance with the capacity of aquaporin gene expression to change in response to a variety of environmental stimuli, such as salt stress (Yamada et al., 1995
The precise mechanism by which light modulates the accumulation of JrPIP2 transcripts is unclear at this moment and several hypotheses must be considered. For instance, it is well known that the transcriptional regulation of many light-regulated genes involves specific cis-elements in their promoter region that are recognized by light-specific trans-acting factors (Green et al., 1988
Like root, leaf hydraulic conductance has been reported to respond to different environmental factors (Nardini et al., 2003
The second pathway was observed in response to light and corresponded to a high conductance value (Fig. 2). In contrast to the dark route, the conductance of the light-induced pathway was proven to be highly modulated by both temperature and CHX and closely related to strong accumulation of PIP2 aquaporin transcripts. The temperature effect on the conductance of the light pathway was not explained by changes in water viscosity (Fig. 3) and rather can reflect modifications in the properties of the lipid matrix and/or the activity of the membrane protein (Sack et al., 2004
We demonstrated that conductance of the light-induced water pathway was positively related to light-mediated up-regulation of walnut JrPIP2: (1) leaves that exhibited a high Kleaf value contained a higher abundance of JrPIP2,1 and JrPIP2,2 transcripts (Fig. 6); (2) JrPIP2 transcripts and Kleaf value share similar time courses (Fig. 5); and (3) this response to light was reversible because Kleaf value and relative abundance of JrPIP2 transcripts dropped together when the light was turned off again (Fig. 6). Although our data showed a close relation between transcript abundance and activity of aquaporins (Kleaf), we cannot exclude the notion that light might regulate plasma membrane aquaporins at multiple levels (transcriptional and posttranslational levels), as is the case for other proteins (Lillo and Appenroth, 2001
Both Kleaf and JrPIP2 transcripts decreased and returned to typical dark levels when CHX was supplied to leaves during the light period (CHX-light 2; Fig. 8B). This decrease is much faster in the presence of CHX than when the light was turned off (Fig. 6). These findings suggest that application of CHX under light conditions could act at the transcript level (by enhancing the instability of aquaporin transcript) and/or at the protein level (by decreasing the turnover of aquaporins and/or that of components essential to aquaporin activity), as already proposed in the literature (Bogre et al., 1997
Based on the conclusions of our experiments, we can propose a putative resistive model for water transport in walnut leaves under dark and light conditions (Fig. 10
). We will present the most parsimonious model for clarity. In a leaf, water first takes a vascular route up to the terminal veins. The hydraulic resistance (Rv) of this vascular pathway is probably constant for well-watered leaves (Cochard et al., 2004
The leaf hydraulic resistance (Rleaf = 1/Kleaf) is equal to:
Our result suggests that aquaporin regulation may play a major role in the control of leaf water status and hence leaf physiology. Leaf water status is determined by hydraulic conductance of the water pathway and leaf transpiration rate. When aquaporin activity was inhibited in transpiring detached leaves, a substantial drop in bulk leaf water potential was observed (Fig. 9). This suggests that conductance upstream of the mesophyll cells was altered in this experiment. This plays in favor of aquaporin expression at the entry of the symplasm, potentially in leaf vein bundle sheaths. This expression would strongly be implicated in the regulation of leaf water status.
In planta, a drop in water loss and, hence, carbon uptake, would be predicted because walnut operates at a water potential very close to the point of xylem cavitation and cell turgor loss. Under these conditions, any decrease in tree hydraulic conductance could result in stomatal closure (Cochard et al., 2002 Many different model calculations can be done based on various paradigms, but option 3 above compares the net assimilation rate of a plant that expresses aquaporins versus the same plant that does not express aquaporins. The purpose of this calculation was not to compare a plant that invests more carbon in roots and stems instead of expressing aquaporins. These computations revealed enhanced net carbon gain during daylight hours in plants that modulated root and leaf hydraulic conductance during times of high irradiance (Fig. 11 ). An added benefit of the third paradigm is that reduced carbon investment in roots will translate into reduced overall respiration. A high energetic cost associated with the increase in leaf conductance during the light phase may explain why the aquaporin-mediated water pathway is not activated when transpiration is low at night.
In conclusion, leaf hydraulic conductance seems tightly correlated to the environmental conditions with the effect of maximizing leaf gas exchange while buffering leaf water status. The ability to modulate Kleaf is consistent with the revised composite water transport model for leaves, which includes an aquaporin-mediated symplasmic and/or transcellular pathway.
Plant Material Experiments were conducted during the summers of 2004 and 2005 on a mature, 10-m-tall walnut (Juglans regia) tree growing in an orchard at the Institut National de la Recherche Agronomique site of Crouël (Clermont-Ferrand, France). Leafy shoots were randomly collected from the basal sun-exposed part of the tree and immediately recut under water. Shoots were enclosed in a black plastic bag and allowed to rehydrate in full darkness for at least 24 h. Leaves with typically five leaflets were then cut from the shoots and their hydraulic conductance determined as described below.
Leaf hydraulic conductance on a surface area basis (Kleaf, mmol s–1 m–2 MPa–1) was measured with XYL'EM apparatus (Bronkhorst). The principle was to measure the water flow (F; mmol s–1) entering the petiole of a cut leaf when exposed to positive pressure (+P; MPa) or allowed to transpire and exposed to negative leaf pressure (–P) as determined by a Scholander chamber. Upon steady state, Kleaf was computed as:
Most experiments were performed in the positive pressure mode (+P). The evaporative method (–P) was used only with leaves treated with CHX. Our XYL'EM apparatus was equipped with a pressure transducer and two flow meters (Liquiflow; Bronkhorst; 15 and 20 g h–1 ranges). The XYL'EM was interfaced with a computer to log different data automatically. In essence, the techniques were similar to the high-pressure flow meter and the evaporative/flux methods described by Tyree et al. (1995) To determine the light dependence of Kleaf, the experimental setup represented in Figure 1 was used. A cylindrical container made of transparent Plexiglas with two compartments was designed. The outer compartment was 4 cm thick and contained tap water continuously renewed to prevent overheating when lamps were on. Light was provided by two 1,000-W iodine lamps (Philips) delivering approximately 1,400 µmol m–2 s–1 at leaf level as measured by a Quantum sensor (Licor 1600 porometer). When the positive pressure technique was used, two leaves were submerged in the inner compartment filled with 9 L of tap water. The water temperature in this compartment was adjusted with a regulated bath (Ministat; Huber) and continuously aerated. Preliminary experiments revealed that temperature was homogeneous in the inner compartment (<0.5°C variation) because of the stirring provided by aeration. When lights were turned on, the temperature rose, transiently, less than 1°C above the preset value (see Fig. 2). When the evaporative technique was used, the inner compartment was of course empty and air was stirred with a small fan. An approximately 5°C air temperature increase was measured upon exposure to light. Petioles were coupled by compression fittings to Luer tubes and connected to the XYL'EM apparatus. The flow entering each leaf was measured independently and simultaneously by the two flow meters. Leaves were supplied with water from the XYL'EM 2-L captive air tank. For one leaf, it was possible to bypass this main reservoir and to supply the leaf with the solution contained in a 10-mL syringe positioned before the flow meter. The pressure in the syringe equaled the pressure in the main reservoir. A 3-way stopcock at the leaf entrance was used to purge the Luer tube and rapidly supply the leaf with the solution in the syringe. The leaf not connected to the syringe was used as a control. We first filled the reservoirs with deionized, filtered (0.1 µm) water delivered by an ultrapure water system (Milli Qplus 185; Millipore). However, we were unable to obtain steady Kleaf values because conductance was slowly decreasing to zero with time. The cause of this plugging was not identified. When deionized water was further distilled with a classical water-distilling apparatus (Schott Geräte GmbH), the problem was solved and Kleaf values were no longer decreasing with time. Experiments with deionized water were not included in this analysis. A typical experiment was as follows. The water temperature in the container was first adjusted to 25°C. Two leaves were detached from the shoot, rehydrated in full darkness, and immediately inserted in the container and connected to the XYL'EM apparatus with pressure in the reservoirs adjusted to, respectively, 0.2 MPa and 0 MPa in the +P and –P modes, respectively. The container was first entirely covered with a black plastic sheet to reduce light levels below 1 µmol m–2 s–1. Typically, after one-half hour, the flow reached a stable value and the lights were switched on. The flow value increased to a new stable value after 1 to 2 h and the lights were switched off. Whereas the control leaf was always treated as described above, the experimental leaf was exposed to a CHX solution at a predefined time. At the end of the experiment, the projected leaf area (LA; m2) was measured with a LI-3000A area meter and a LI-3050A belt conveyer (LI-COR). Water in the inner compartment was renewed between two experiments to avoid possible artifacts caused by the accumulation of CHX in the water bath.
Temperature Dependence of Leaf Kleaf Light Response
 20°C and T are water viscosity at 20°C and T. Viscosity values were from Lide (1996).
To quantify the contrasted time responses at different temperatures, we computed the half time (T
Effect of CHX on Kleaf Light Response
With the –P method, leaf water potential (=balance pressure) was estimated by the Scholander-Hammel pressure chamber. Because leaves were provided with distilled water, xylem pressure potential (as measured by the pressure chamber) should approximately equal the water potential (Tyree and Zimmermann, 2002
When leaves were infiltrated under pressure, liquid water eventually passed through stomata. Stomata may therefore represent extra resistance in series with leaf mesophyll hydraulic resistance. To quantify the putative effect of stomatal openness on total leaf hydraulic conductance, we measured the dimensions of the stomata pore with a cryogenic scanning electron microscope (Cryo-SEM, model SEM 505; Philips). The observations were performed on nine selected control or experimental leaves from the different treatments described above. Treatments were selected to obtain high or low Kleaf values under high or low light conditions. After the Kleaf value of the selected leaves was determined, they were rapidly removed from the water bath and immediately submerged in a bath of liquid nitrogen and stored at –80°C until examination. A small leaf part (approximately 5 x 5 mm) from the middle part of a leaflet was detached under liquid nitrogen and rapidly transferred, still frozen, in the Cryo-SEM chamber. Stomata were observed at about –100°C and 2,500x magnification. For each sample, we measured the length and the width of 10 to 15 pores to the nearest 0.05 µm.
Following Tyree et al. (2005)
is the water viscosity at 20°C (10–3 Pa s), L and l are the pore major and minor axes (m), e is the stomatal pore depth (15 x 10–6 m), D is stomatal density (m–2), and 1015/18 is a conversion factor between m3 s–1 Pa–1 m–2 to mmol s–1 MPa–1 m–2. Stomatal density (2.30 x 108 m–2; SE = 6.90 x 106; n = 30) was measured with a light microscope by counting the number of stomata in the field of the microscope (0.11 mm2). Only e was not measured in this study and roughly estimated to be 15 µm. According to the equation above, it can be seen that K is inversely proportional to e. Doubling e will divide K by 2, which will not change our conclusions.
A subset of control and treated leaves from the experiments described above were sampled for an analysis of aquaporin transcript abundance by real-time quantitative reverse transcription (RT)-PCR. Leaves were rapidly disconnected from the XYL'EM apparatus and immediately immersed in liquid nitrogen. Samples were stored at –80°C until analysis.
Total RNA was extracted from about 500 mg of leaves as described by Chang et al. (1993) For first-strand cDNA synthesis, 2 µg of total RNA were reverse transcribed using oligo(dT) and SuperScript III (Invitrogen) following the protocol of the supplier.
Expression analysis of two aquaporin isoforms (JrPIP2,1 and JrPIP2,2) was done by real-time RT-PCR amplification, using an iCycler iQ (Bio-Rad Laboratories) and SYBR green as a fluorescent dye. Each PCR reaction (50 µL) contained the following: reverse-transcribed cDNA (1 µL of 1:5 dilution of the first cDNA strands); PCR buffer (1x), the corresponding primer (each primer 0.4 µM), dNTP mix (each 0.25 mM); MgCl2 (2 mM), platinium Taq polymerase (1 unit; CLONTECH), and SYBR green I (1/1,000; Sigma). After a heat step at 94°C for 4 min, PCR cycling conditions were 35 cycles of denaturation (94°C, 15 s), annealing (58.5°C, 15 s), and elongation (72°C, 20 s). Because the primer pairs WC11/WC12 and WC21/WC22 were initially shown to be specific for JrPIP2,1 and JrPIP2,2, respectively (Sakr et al., 2003
Specificity of amplification was confirmed by determining the melt curves for the PCR products at the end of each run and by using a gel electrophoresis. Real-time PCR amplifications were done on at least three independent experiments and every run was carried out in triplicate. The walnut EF-1
We are grateful to Andrea Nardini, Missy Holbrook, and two anonymous referees for constructive comments on this work. The assistance of Christian Bodet, Maurice Crocombette, and Celine Leitao was much appreciated. Received September 20, 2006; accepted November 5, 2006; published November 17, 2006.
1 This work was supported by the Ministère de la Recherche et de l'Education Nationale and the Institut National de la Recherche Agronomique (grants to H.C., J.-S.V., T.S.B., N.B., S.H., A.G., and S.S.).  The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Soulaiman Sakr (soulaiman.sakr{at}univ-bpclermont.fr). www.plantphysiol.org/cgi/doi/10.1104/pp.106.090092 * Corresponding author; e-mail soulaiman.sakr{at}univ-bpclermont.fr; fax 33–4–73–40–79–16.
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