Planta DOI 10.1007/s00425-013-2014-x
Original Article
Glutathione and transpiration as key factors conditioning oxidative stress in Arabidopsis thaliana exposed to uranium Iker Aranjuelo · Fany Doustaly · Jana Cela · Rosa Porcel · Maren Müller · Ricardo Aroca · Sergi Munné‑Bosch · Jacques Bourguignon
Received: 24 September 2013 / Accepted: 12 December 2013 © Springer-Verlag Berlin Heidelberg 2014
Abstract Although oxidative stress has been previously described in plants exposed to uranium (U), some uncertainty remains about the role of glutathione and tocopherol availability in the different responsiveness of plants to photo-oxidative damage. Moreover, in most cases, little consideration is given to the role of water transport in shoot heavy metal accumulation. Here, we investigated Electronic supplementary material The online version of this article (doi:10.1007/s00425-013-2014-x) contains supplementary material, which is available to authorized users. I. Aranjuelo (*) Instituto de Agrobiotecnología, Universidad Pública de Navarra-CSIC-Gobierno de Navarra, Campus de Arrosadía, 31192 Mutilva Baja, Spain e-mail: [email protected] I. Aranjuelo · F. Doustaly · J. Bourguignon (*) CEA, iRTSV, Laboratoire Physiologie Cellulaire Végétale (PCV), CEA, CNRS (UMR 5168) UJF, INRA (USC1359), CEAGrenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France e-mail: [email protected] I. Aranjuelo · F. Doustaly · J. Bourguignon PCV, Université Grenoble Alpes, 38041 Grenoble, France I. Aranjuelo · F. Doustaly · J. Bourguignon CNRS, UMR5168, PCV, 38054 Grenoble, France I. Aranjuelo · F. Doustaly · J. Bourguignon INRA, USC1359, PCV, 38054 Grenoble, France J. Cela · M. Müller · S. Munné‑Bosch Unitat de Fisologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 643, 08028 Barcelona, Spain R. Porcel · R. Aroca Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín (CSIC), Profesor Albareda 1, 18008 Granada, Spain
the effect of uranyl nitrate exposure (50 μM) on PSII and parameters involved in water transport (leaf transpiration and aquaporin gene expression) of Arabidopsis wild type (WT) and mutant plants that are deficient in tocopherol (vte1: null α/γ-tocopherol and vte4: null α-tocopherol) and glutathione biosynthesis (high content: cad1.3 and low content: cad2.1). We show how U exposure induced photosynthetic inhibition that entailed an electron sink/source imbalance that caused PSII photoinhibition in the mutants. The WT was the only line where U did not damage PSII. The increase in energy thermal dissipation observed in all the plants exposed to U did not avoid photo-oxidative damage of mutants. The maintenance of control of glutathione and malondialdehyde contents probed to be target points for the overcoming of photoinhibition in the WT. The relationship between leaf U content and leaf transpiration confirmed the relevance of water transport in heavy metals partitioning and accumulation in leaves, with the consequent implication of susceptibility to oxidative stress. Keywords Arabidopsis · Chlorophyll fluorescence · Photosynthesis · Plant hormones · Transpiration · Tocopherol · Uranium
Introduction Heavy metals (HM) is a collective term that applies to the group of metals and metalloids with a density over 4 g cm−3. This group is made up of compounds such as uranium (U), cadmium (Cd), lead (Pb) and others. Unlike most organic pollutants, heavy metals are elements found naturally in the earth’s crust. In Europe, the amount of waste generated (excluding agricultural waste) reaches 1,300 million tonnes (of which 36 million tonnes are
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hazardous waste) (European Environment Agency 1999). As described by the European Environment Agency (EEA), in 1999 in Western Europe 1,500,000 contaminated areas were estimated and 300,000 were identified (European Environment Agency 1999). In the case of France, the European agency indicates that there are 700,000 potential cases and 895 confirmed contamination sites. However, the report also notes that most European countries are in an early stage of identification and registration of contaminated sites. The origins of the pollutants are the combustion of fossil fuels (cars, etc.), industry, slurry and waste, and raw materials (applied to agriculture) and phosphates (Battarbee et al. 1988). Uranium is a naturally occurring radionuclide and heavy metal with an average concentration in the continental crust of 1.7 ppm (Wedepohl 1995). In the soluble form, U may be taken up by plants and can therefore contaminate the food chain (Neves et al. 2012). U is a chemotoxic and a radiotoxic element: chemical toxicity is particularly significant in compounds containing natural uranium, whereas its radiological toxicity is directly related to ionizing radiation effects of enriched U (Ribera et al. 1996 and references therein). Although the effect of U exposure has been previously tested in human and animal species (Ribera et al. 1996), information on the effect of U exposure in plants is scarce (Vanhoudt et al. 2008, 2011a, b, c; Viehweger et al. 2011). Similar to descriptions for other heavy metals such as Cd (López-Millán et al. 2009) and Cr (Rodriguez et al. 2012), U has been characterised as an inducing oxidative stress agent in plants (Vanhoudt et al. 2008, 2011a, b, c; Viehweger et al. 2011). Most of these studies have been focused on the expression of genes involved in the removal of reactive oxygen species (ROS). The steadystate level of ROS in cells needs to be tightly regulated to avoid their accumulation in excess that might induce oxidative damage. Under stress conditions it has been described (Schützendübel and Polle 2002; Villiers et al. 2011) to be an increase/decrease in the capacity of the antioxidative defence system. Plants have developed three main mechanisms to diminish photooxidation. First, although under high-energy donation conditions might cause photo-oxidative damage, in some cases, the plants might prevent the production of ROS by diminishing the electron transport chain. Second, the plants might diminish photooxidation of the photosystem II light-harvesting antenna through the xanthophyll cycle-dependent thermal dissipation and changes in photosynthetic light-harvesting complex II antennae (Verhoeven et al. 1999; Janik et al. 2013). The third mechanism consists in the capacity to scavenge ROS formed by an integrated system of enzymatic (e.g. superoxide dismutase, catalase, ascorbate peroxidase; Mittler et al. 2004) and non-enzymatic antioxidants [e.g. ascorbate (AsA), glutathione (GSH) and tocopherols Asada (1999)].
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Glutathione (GSH) is the major non-protein thiol source in plants and among its many functions it serves as one of the major scavengers of peroxides (May et al. 1998). It can act as an antioxidant by scavenging radicals, resulting in the oxidation of GSH to glutathione disulphide (GSSG). GSH is related to ascorbate (AsA) through the ascorbate– glutathione cycle (AsA–GSH cycle). The ascorbate–glutathione cycle is the most important antioxidant cycle in plants (Foyer and Noctor 2011). Higher ascorbate levels in uranium-exposed plants have been suggested to increase antioxidant defences via the ascorbate–glutathione pathway (Vanhoudt et al. 2008, 2011a, b, c). Tocopherols also have been described as important antioxidants (Cela et al. 2011). Tocopherols are lipophilic antioxidants that are part of the vitamin E group and are only synthesized in plants (Munné-Bosch 2005). Stress-tolerant plants show increased tocopherol levels, whereas in stress-sensitive plants tocopherol content decreases with consequent oxidative damage (Munné-Bosch and Alegre 2002). In cooperation with other antioxidants, tocopherol plays a part in reducing ROS levels (Munné-Bosch 2005). α-Tocopherol can physically quench, and therefore deactivate 1O2 in chloroplasts. It has been estimated that before being degraded, one molecule of α-tocopherol can deactivate up to 120 1O2 molecules by resonance energy transfer (Fahrenholzt et al. 1974). In addition, α-tocopherol can chemically scavenge 1O2 and lipid peroxyl radicals. Although previous studies (MunnéBosch and Alegre 2002; Ricciarelli et al. 2001) reflect the relevance of tocopherols in oxidative stress regulation, up to the present time its role in plants exposed to HM has only been investigated under challenge with cadmium and copper (Collin et al. 2008). The implications of oxidative stress in light capture and gas exchange parameters have received little attention in studies analysing the U effect on plant performance. However, this is a matter of major concern because photosynthesis represents a major energy sink, and because keeping control of photosynthetic rates is essential for plant development. Changes in C assimilation require modifications in the partitioning of the absorbed energy between heat dissipation and photochemistry. When photosynthesis decreases and the light excitation energy is in excess, over-excitation of the photosynthetic pigments in the antenna can occur. Impairment of photosynthetic function will lead to excessive excitation energy in photosystem II (PSII), leading to the accumulation of ROS, and resulting in oxidative stress (Chaves et al. 2009; Lawlor and Tezara 2009). Research has indicated that stomatal closure is involved in the inhibition of photosynthesis in plants exposed to different HM, such as Cr (Subrahmanyam 2008; Rodriguez et al. 2012) and Cd (López-Millán et al. 2009). However, even if those studies describe a reduction in stomatal conductance of between 25 and 60 %, the mechanisms leading to
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regulation of water transport in plants exposed to HM have been poorly questioned. The role of glutathione and tocopherol in the responsiveness of plants to U exposure has not been explored. Furthermore, the consequences of oxidative stress in photosynthetic performance remain to be elucidated in U-treated plants. The aim of this study was to characterize the role of these two major antioxidants (glutathione and tocopherol) in the response of Arabidopsis thaliana plants to U exposure. Furthermore, we also investigated the mechanisms leading to the limitations in water transport described in other HM studies by following hormone and antioxidant contents together with analyses of the expression of genes encoding aquaporins. To clarify these aspects, we examined the effect of U on Arabidopsis mutants with altered abilities in accumulating glutathione (cad1.3, cad2.1) (Howden et al. 1995) and tocopherol biosynthesis (vte1) (Porfirova et al. 2002) by completing a physiological and photosystem II chlorophyll fluorescence characterization.
Materials and methods Experimental design In this study, we worked with Arabidopsis wild type (Col0, WT), together with tocopherol (vte1: null α/γ-tocopherol content and vte4: null α-tocopherol) (Porfirova et al. 2002) and glutathione (cad1.3: high content and cad2.1 low content) mutants (Howden et al. 1995). The seeds were germinated by placing one seed per seed holder (Araponics S.A, Liège, Belgium) filled with agar solution (0.65 %). Plants were grown in hydroponically controlled pots of 350 ml. For each treatment combination, four pots containing our four plants per pot were grown. Nutrient solution was replaced every 2 days. The photoperiod was 10 h, with a photosynthetic photon flux density (PPFD) of ≈110 μmol photons m−2 s−1. The day/night temperature was 22/20 °C and the relative humidity was maintained close to 65 %. The mineral solution was Hoagland’s. The pH of the medium was checked and adjusted daily to 5.3–5.6. When plants were 50 days old, half of them were selected randomly and were placed into a new solution containing uranyl nitrate (50 μM) dissolved in milliQ water solution. The plants were grown in this media over 48 h. After U exposure, roots were first rinsed with 10 mM Na2CO3, then with water, and finally samples were stored at −80 °C. The experiment was repeated three times to improve the repeatability of obtained results. Uranium quantification U was determined in shoot (about 100 mg of fresh material) samples dried for one night at 80 °C and mineralized
in 9 ml of 65 % (v/v) HNO3 (Suprapur; Merck) and 3 ml of 30 % (v/v) HCl (Suprapur, Merck) at 125 °C. After complete evaporation of the mixture, residual material was dissolved in 1 % (v/v) HNO3. The U concentration in the extract was then determined using ICP-MS (HP4500 ChemStation ICP-MS device; Yokogawa Analytical Systems) following 238 U. Glutathione and ascorbate content and determination of oxidative damage to lipids For determination of glutathione content and oxidative damage to lipids, aliquots of leaves were homogenized in sulfosalicilic acid 5 % (w/v) in an ice-cold Potter–Elvehjem homogenizer and centrifuged at 20,000g, 10 min, 0–4 °C. The supernatants were kept at −70 °C for subsequent determinations. Ascorbic acid (AsA) and dehydroascorbic acid (DHA), which were extracted with m-phosphoric acid (6 %) and 0.2 mM diethylenetriaminepentaacetic acid (DTPA), were determined using the ascorbate oxidase assay as described (Turcsányi et al. 2000). The oxidized state was calculated as AsA/(AsA + DHA) × 100. One millilitre of supernatant was neutralized by 1.5 ml 0.5 M K-phosphate buffer (pH 7.5). The standard incubation medium was a mixture of: 100 μl 0.1 M sodium phosphate buffer (pH 7.5) containing 5 mM EDTA, 50 μl 6 mM 5,5-dithiobis-(-2-nitrobenzoic acid), 25 μl 2 mM NADPH, and 25 units of glutathione reductase (GR). The reaction was initiated by the addition of 25 μl glutathione standard or extract. The change in absorbance at 412 nm was recorded for 4 min. The calibration curve was made using oxidized glutathione (GSSG) in the range of 0–100 μmol. Lipid peroxidation was estimated as the content of 2-thiobarbituric acid reactive substances (TBARS) and expressed as equivalents of malondialdehyde (MDA). The chromogen was formed by mixing 200 μl of supernatants with 1 ml of a reaction mixture containing 15 % (w/v) trichloroacetic acid (TCA), 0.375 % (w/v) 2-thiobarbituric acid (TBA), 0.1 % (w/v) butyl hydroxytoluene, and 0.25 N HCl, and by incubating the mixture at 100 °C for 30 min (Minotti and Aust 1987). After cooling to room temperature, tubes were centrifuged at 800g for 5 min and the supernatant was used for spectrophotometric reading at 532 and 600 nm. The calibration curve was made using MDA in the range of 0.1–10 μmol. A blank for all samples was prepared by replacing the sample with extraction medium, and controls for each sample were prepared by replacing TBA with 0.25 N HCl. In all cases, 0.1 % (w/v) butyl hydroxytoluene was included in the reaction mixtures to prevent artifactual formation of TBARS during the acid-heating step of the assay.
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Gas exchange and chlorophyll fluorescence determinations Fully expanded apical leaves were enclosed in a LI-COR 6400 gas exchange portable photosynthesis system (LICOR, Lincoln, Nebraska, USA). The gas exchange determinations were conducted at 25 °C with a photosynthetic photon flux density of 90 μmol m−2 s−1. Photosynthetic activity was calculated using equations developed by von Caemmerer and Farquhar (1981). Stomatal conductance (gs) was determined as described by (Harley et al. 1992). The electron transport rate (ETR) was measured as described by Harley et al. (1992). Steady-state modulated chlorophyll fluorescence was determined with a portable fluorimeter (Mini-PAM; Walz, Effeltrich, Germany). Light-adapted components of chlorophyll fluorescence were measured: Steadystate fluorescence (F), maximal fluorescence (F′m), variable fluorescence F′v equivalent to (F′m − F) and quantum yield of PS II photochemistry (ΦPSII) equivalent to (F′m − F)/F′m. Leaves were then dark adapted to obtain Fo (minimum fluorescence), Fm (maximum fluorescence), Fv variable fluorescence (equivalent to Fm − Fo) and Fv/Fm (maximum quantum yield of PSII photochemistry, equivalent to (Fm − Fo)/Fm). Leaves were dark adapted for at least 20 min, after which Fv/Fm values reach about 95 % of the pre-dawn values. F′o was estimated as Fo/ ((Fvc/Fm) + (Fo/F′m)). F′o was used to calculate photochemical quenching (qP), photochemical quenching of fluorescence (equivalent to F′m − F)/(F′m − F′o)), and F′v/F′m, the intrinsic efficiency of open PSII centres during illumination (equivalent to (F′m − F′o)/F′m). Three different leaves of each individual were measured. All data were corrected for changes in the fluorescence detector sensitivity induced by temperature variation of the Mini-Pam. Photochemical quenching was estimated as (F′m − Fs)/F′v. Non-photochemical quenching (NPQ) was calculated as (Fm − F′m) − 1. The variable F′v/F′m was calculated and was an indicator of absorbed light dissipated thermally. Pigment content Extracts for pigment analysis were prepared by grinding 100 mg fresh weight in a cold mortar with 10 ml of ethanol (95 %, v/v). The homogenate was centrifuged at 3,165g for 10 min at 4 °C. An aliquot of 1 ml from the supernatant was taken, 4 ml of 95 % ethanol were added, and the absorbance measured at 750, 665, 649 and 470 nm. Absorbance determinations were carried out with a Spectronic 2000 (Bausch and Lomb, Rochester, USA) spectrophotometer. Chlorophyll a, b, total and carotenoid (Chla, Chlb, Chla + b and Cx + c, respectively) contents were determined according to Liechenthaler (Lichtenthaler 1987). In brief, Chla = 13.36 × A665 − 5.19 × A649; Chlb = 27.43 ×
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A649 − 8.12 × A665, Chla + b = 5.24 × A664 + 22.24 × A649, Cx + c = (1,000 × A470 − 2.13 Chla − 97.46 Chlb)/198. Hormones and tocopherols Phytohormones, including ABA, JA and SA were measured by UPLC-MS/MS as described (Müller and Munné-Bosch 2011). Deuterium-labelled hormones were used to estimate recovery rates for each sample. Tocopherols were measured by HPLC as described by Cela et al. (2011). Expression of plasma membrane intrinsic proteins isoforms (PIP1.1; PIP1.2; PIP2.1; PIP2.5 and PIP2.6) RNA isolation Total RNA was isolated from Arabidopsis leaves by phenol/ chloroform extraction (Kay et al. 1987). DNase treatment of total RNA was performed according to Qiagen’s protocol (Quantitect Reverse Transcription KIT Cat#205311, Qiagen, CA). Quantitative real‑time RT‑PCR The expression of each plasma membrane intrinsic pro‑ tein (PIP) gene was studied by real-time PCR using an iCycler (Bio-Rad, Hercules, California, USA). cDNAs were obtained from 2.5 μg of total DNase-treated RNA in a 20 μl reaction containing oligo(dT)15 primer (Promega, Madison, WI), 10 mM dNTP (Invitrogen, Carlsbad, California, USA), 0.1 M DTT (Invitrogen, Carlsbad, CA, USA), 40 U of RNase inhibitor (Promega, Madison, WI), 5X first strand buffer (Invitrogen) and 200 U of Superscript II Reverse Transcriptase (Invitrogen) with the temperature recommended by the enzyme supplier. The primer sets used, including that of reference gene actin, to amplify each studied gene in the synthesized cDNAs are shown in supplemental Table 1. Each 23 μl reaction contained 3 μl of a 1:10 dilution of the cDNA, 10.5 μl of Master Mix (Bio-Rad Laboratories S.A, Madrid), 8.6 μl of deionised water and 0.45 μl of each primer pair. The PCR programme consisted of a 3-min incubation at 95 °C to activate the hot-start recombinant Taq DNA polymerase, followed by 31 cycles of 30 s at 94 °C, 30 s at the established annealing temperature and 30 s at 72 °C, where the fluorescence signal was measured, followed by 1 cycle of 1 min at 95 °C, 1 min at 70 °C and 60 cycles of 10 s at 70 °C. The specificity of the PCR amplification procedure was checked with a heat dissociation protocol (from 70 to 100 °C) after the final cycle of the PCR. Real-time PCR experiments were carried out four times with biological independent samples, with the threshold cycle (CT) determined in triplicate. The relative levels
Planta Table 1 Uranium (U) exposure (50 μM) effect on leaf net photosynthesis (An), electron transport rate (ETR) and the ETR/An ratio of Arabidopsis thaliana wild type (WT, ecotype Columbia), tocopherol
(null α/γ-tocopherol: vte1 and vte4: null in α-tocopherol) and glutathione (high content: cad1.3 and low content: cad2.1) mutants
Line
Treatment
An (μmol CO2 m−2s−1)
ETR (μmol e−2 s−1)
ETR/An (μmol CO2 μmol e−2)
WT
Control U Control U Control U Control U Control
9.63 ± 0.77 a 0.08 ± 0.24 e 8.28 ± 0.46 ab 0.95 ± 0.14 e 6.71 ± 0.07 bc 0.46 ± 0.06 e 5.65 ± 0.69 c 1.32 ± 0.78 e 3.5 ± 0.25 d
176.61 ± 8.21 a 191.33 ± 12.59 a 175.89 ± 5.11 a 155.55 ± 4.87 ab 175.42 ± 9.27 a 148.24 ± 5.28 b 181.31 ± 4.69 a 157.75 ± 10.77 ab 188,03 ± 3.10 a
18.63 ± 2.01 f 700.48 ± 166.54 a 20.80 ± 1.74 f 122.69 ± 35.76 d 26.77 ± 1.50 f 334.61 ± 68.74 bc 32.2 ± 6.25 f 251.18 ± 95.03 c 58.57 ± 3.18 e
U
0.43 ± 0.08 e
169.06 ± 5.12 ab
393.37 ± 4.45 b
Vte 1 Vte 4 Cad 1.3 Cad 2.1
Each value represents the mean of 4 replicates ± SE. Statistical analysis was made by a two-factor analysis of the variance (ANOVA). When significant differences were detected in ANOVA, LSD analysis was applied. Means that differed significantly (P
Original Article
Glutathione and transpiration as key factors conditioning oxidative stress in Arabidopsis thaliana exposed to uranium Iker Aranjuelo · Fany Doustaly · Jana Cela · Rosa Porcel · Maren Müller · Ricardo Aroca · Sergi Munné‑Bosch · Jacques Bourguignon
Received: 24 September 2013 / Accepted: 12 December 2013 © Springer-Verlag Berlin Heidelberg 2014
Abstract Although oxidative stress has been previously described in plants exposed to uranium (U), some uncertainty remains about the role of glutathione and tocopherol availability in the different responsiveness of plants to photo-oxidative damage. Moreover, in most cases, little consideration is given to the role of water transport in shoot heavy metal accumulation. Here, we investigated Electronic supplementary material The online version of this article (doi:10.1007/s00425-013-2014-x) contains supplementary material, which is available to authorized users. I. Aranjuelo (*) Instituto de Agrobiotecnología, Universidad Pública de Navarra-CSIC-Gobierno de Navarra, Campus de Arrosadía, 31192 Mutilva Baja, Spain e-mail: [email protected] I. Aranjuelo · F. Doustaly · J. Bourguignon (*) CEA, iRTSV, Laboratoire Physiologie Cellulaire Végétale (PCV), CEA, CNRS (UMR 5168) UJF, INRA (USC1359), CEAGrenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France e-mail: [email protected] I. Aranjuelo · F. Doustaly · J. Bourguignon PCV, Université Grenoble Alpes, 38041 Grenoble, France I. Aranjuelo · F. Doustaly · J. Bourguignon CNRS, UMR5168, PCV, 38054 Grenoble, France I. Aranjuelo · F. Doustaly · J. Bourguignon INRA, USC1359, PCV, 38054 Grenoble, France J. Cela · M. Müller · S. Munné‑Bosch Unitat de Fisologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 643, 08028 Barcelona, Spain R. Porcel · R. Aroca Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín (CSIC), Profesor Albareda 1, 18008 Granada, Spain
the effect of uranyl nitrate exposure (50 μM) on PSII and parameters involved in water transport (leaf transpiration and aquaporin gene expression) of Arabidopsis wild type (WT) and mutant plants that are deficient in tocopherol (vte1: null α/γ-tocopherol and vte4: null α-tocopherol) and glutathione biosynthesis (high content: cad1.3 and low content: cad2.1). We show how U exposure induced photosynthetic inhibition that entailed an electron sink/source imbalance that caused PSII photoinhibition in the mutants. The WT was the only line where U did not damage PSII. The increase in energy thermal dissipation observed in all the plants exposed to U did not avoid photo-oxidative damage of mutants. The maintenance of control of glutathione and malondialdehyde contents probed to be target points for the overcoming of photoinhibition in the WT. The relationship between leaf U content and leaf transpiration confirmed the relevance of water transport in heavy metals partitioning and accumulation in leaves, with the consequent implication of susceptibility to oxidative stress. Keywords Arabidopsis · Chlorophyll fluorescence · Photosynthesis · Plant hormones · Transpiration · Tocopherol · Uranium
Introduction Heavy metals (HM) is a collective term that applies to the group of metals and metalloids with a density over 4 g cm−3. This group is made up of compounds such as uranium (U), cadmium (Cd), lead (Pb) and others. Unlike most organic pollutants, heavy metals are elements found naturally in the earth’s crust. In Europe, the amount of waste generated (excluding agricultural waste) reaches 1,300 million tonnes (of which 36 million tonnes are
13
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hazardous waste) (European Environment Agency 1999). As described by the European Environment Agency (EEA), in 1999 in Western Europe 1,500,000 contaminated areas were estimated and 300,000 were identified (European Environment Agency 1999). In the case of France, the European agency indicates that there are 700,000 potential cases and 895 confirmed contamination sites. However, the report also notes that most European countries are in an early stage of identification and registration of contaminated sites. The origins of the pollutants are the combustion of fossil fuels (cars, etc.), industry, slurry and waste, and raw materials (applied to agriculture) and phosphates (Battarbee et al. 1988). Uranium is a naturally occurring radionuclide and heavy metal with an average concentration in the continental crust of 1.7 ppm (Wedepohl 1995). In the soluble form, U may be taken up by plants and can therefore contaminate the food chain (Neves et al. 2012). U is a chemotoxic and a radiotoxic element: chemical toxicity is particularly significant in compounds containing natural uranium, whereas its radiological toxicity is directly related to ionizing radiation effects of enriched U (Ribera et al. 1996 and references therein). Although the effect of U exposure has been previously tested in human and animal species (Ribera et al. 1996), information on the effect of U exposure in plants is scarce (Vanhoudt et al. 2008, 2011a, b, c; Viehweger et al. 2011). Similar to descriptions for other heavy metals such as Cd (López-Millán et al. 2009) and Cr (Rodriguez et al. 2012), U has been characterised as an inducing oxidative stress agent in plants (Vanhoudt et al. 2008, 2011a, b, c; Viehweger et al. 2011). Most of these studies have been focused on the expression of genes involved in the removal of reactive oxygen species (ROS). The steadystate level of ROS in cells needs to be tightly regulated to avoid their accumulation in excess that might induce oxidative damage. Under stress conditions it has been described (Schützendübel and Polle 2002; Villiers et al. 2011) to be an increase/decrease in the capacity of the antioxidative defence system. Plants have developed three main mechanisms to diminish photooxidation. First, although under high-energy donation conditions might cause photo-oxidative damage, in some cases, the plants might prevent the production of ROS by diminishing the electron transport chain. Second, the plants might diminish photooxidation of the photosystem II light-harvesting antenna through the xanthophyll cycle-dependent thermal dissipation and changes in photosynthetic light-harvesting complex II antennae (Verhoeven et al. 1999; Janik et al. 2013). The third mechanism consists in the capacity to scavenge ROS formed by an integrated system of enzymatic (e.g. superoxide dismutase, catalase, ascorbate peroxidase; Mittler et al. 2004) and non-enzymatic antioxidants [e.g. ascorbate (AsA), glutathione (GSH) and tocopherols Asada (1999)].
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Glutathione (GSH) is the major non-protein thiol source in plants and among its many functions it serves as one of the major scavengers of peroxides (May et al. 1998). It can act as an antioxidant by scavenging radicals, resulting in the oxidation of GSH to glutathione disulphide (GSSG). GSH is related to ascorbate (AsA) through the ascorbate– glutathione cycle (AsA–GSH cycle). The ascorbate–glutathione cycle is the most important antioxidant cycle in plants (Foyer and Noctor 2011). Higher ascorbate levels in uranium-exposed plants have been suggested to increase antioxidant defences via the ascorbate–glutathione pathway (Vanhoudt et al. 2008, 2011a, b, c). Tocopherols also have been described as important antioxidants (Cela et al. 2011). Tocopherols are lipophilic antioxidants that are part of the vitamin E group and are only synthesized in plants (Munné-Bosch 2005). Stress-tolerant plants show increased tocopherol levels, whereas in stress-sensitive plants tocopherol content decreases with consequent oxidative damage (Munné-Bosch and Alegre 2002). In cooperation with other antioxidants, tocopherol plays a part in reducing ROS levels (Munné-Bosch 2005). α-Tocopherol can physically quench, and therefore deactivate 1O2 in chloroplasts. It has been estimated that before being degraded, one molecule of α-tocopherol can deactivate up to 120 1O2 molecules by resonance energy transfer (Fahrenholzt et al. 1974). In addition, α-tocopherol can chemically scavenge 1O2 and lipid peroxyl radicals. Although previous studies (MunnéBosch and Alegre 2002; Ricciarelli et al. 2001) reflect the relevance of tocopherols in oxidative stress regulation, up to the present time its role in plants exposed to HM has only been investigated under challenge with cadmium and copper (Collin et al. 2008). The implications of oxidative stress in light capture and gas exchange parameters have received little attention in studies analysing the U effect on plant performance. However, this is a matter of major concern because photosynthesis represents a major energy sink, and because keeping control of photosynthetic rates is essential for plant development. Changes in C assimilation require modifications in the partitioning of the absorbed energy between heat dissipation and photochemistry. When photosynthesis decreases and the light excitation energy is in excess, over-excitation of the photosynthetic pigments in the antenna can occur. Impairment of photosynthetic function will lead to excessive excitation energy in photosystem II (PSII), leading to the accumulation of ROS, and resulting in oxidative stress (Chaves et al. 2009; Lawlor and Tezara 2009). Research has indicated that stomatal closure is involved in the inhibition of photosynthesis in plants exposed to different HM, such as Cr (Subrahmanyam 2008; Rodriguez et al. 2012) and Cd (López-Millán et al. 2009). However, even if those studies describe a reduction in stomatal conductance of between 25 and 60 %, the mechanisms leading to
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regulation of water transport in plants exposed to HM have been poorly questioned. The role of glutathione and tocopherol in the responsiveness of plants to U exposure has not been explored. Furthermore, the consequences of oxidative stress in photosynthetic performance remain to be elucidated in U-treated plants. The aim of this study was to characterize the role of these two major antioxidants (glutathione and tocopherol) in the response of Arabidopsis thaliana plants to U exposure. Furthermore, we also investigated the mechanisms leading to the limitations in water transport described in other HM studies by following hormone and antioxidant contents together with analyses of the expression of genes encoding aquaporins. To clarify these aspects, we examined the effect of U on Arabidopsis mutants with altered abilities in accumulating glutathione (cad1.3, cad2.1) (Howden et al. 1995) and tocopherol biosynthesis (vte1) (Porfirova et al. 2002) by completing a physiological and photosystem II chlorophyll fluorescence characterization.
Materials and methods Experimental design In this study, we worked with Arabidopsis wild type (Col0, WT), together with tocopherol (vte1: null α/γ-tocopherol content and vte4: null α-tocopherol) (Porfirova et al. 2002) and glutathione (cad1.3: high content and cad2.1 low content) mutants (Howden et al. 1995). The seeds were germinated by placing one seed per seed holder (Araponics S.A, Liège, Belgium) filled with agar solution (0.65 %). Plants were grown in hydroponically controlled pots of 350 ml. For each treatment combination, four pots containing our four plants per pot were grown. Nutrient solution was replaced every 2 days. The photoperiod was 10 h, with a photosynthetic photon flux density (PPFD) of ≈110 μmol photons m−2 s−1. The day/night temperature was 22/20 °C and the relative humidity was maintained close to 65 %. The mineral solution was Hoagland’s. The pH of the medium was checked and adjusted daily to 5.3–5.6. When plants were 50 days old, half of them were selected randomly and were placed into a new solution containing uranyl nitrate (50 μM) dissolved in milliQ water solution. The plants were grown in this media over 48 h. After U exposure, roots were first rinsed with 10 mM Na2CO3, then with water, and finally samples were stored at −80 °C. The experiment was repeated three times to improve the repeatability of obtained results. Uranium quantification U was determined in shoot (about 100 mg of fresh material) samples dried for one night at 80 °C and mineralized
in 9 ml of 65 % (v/v) HNO3 (Suprapur; Merck) and 3 ml of 30 % (v/v) HCl (Suprapur, Merck) at 125 °C. After complete evaporation of the mixture, residual material was dissolved in 1 % (v/v) HNO3. The U concentration in the extract was then determined using ICP-MS (HP4500 ChemStation ICP-MS device; Yokogawa Analytical Systems) following 238 U. Glutathione and ascorbate content and determination of oxidative damage to lipids For determination of glutathione content and oxidative damage to lipids, aliquots of leaves were homogenized in sulfosalicilic acid 5 % (w/v) in an ice-cold Potter–Elvehjem homogenizer and centrifuged at 20,000g, 10 min, 0–4 °C. The supernatants were kept at −70 °C for subsequent determinations. Ascorbic acid (AsA) and dehydroascorbic acid (DHA), which were extracted with m-phosphoric acid (6 %) and 0.2 mM diethylenetriaminepentaacetic acid (DTPA), were determined using the ascorbate oxidase assay as described (Turcsányi et al. 2000). The oxidized state was calculated as AsA/(AsA + DHA) × 100. One millilitre of supernatant was neutralized by 1.5 ml 0.5 M K-phosphate buffer (pH 7.5). The standard incubation medium was a mixture of: 100 μl 0.1 M sodium phosphate buffer (pH 7.5) containing 5 mM EDTA, 50 μl 6 mM 5,5-dithiobis-(-2-nitrobenzoic acid), 25 μl 2 mM NADPH, and 25 units of glutathione reductase (GR). The reaction was initiated by the addition of 25 μl glutathione standard or extract. The change in absorbance at 412 nm was recorded for 4 min. The calibration curve was made using oxidized glutathione (GSSG) in the range of 0–100 μmol. Lipid peroxidation was estimated as the content of 2-thiobarbituric acid reactive substances (TBARS) and expressed as equivalents of malondialdehyde (MDA). The chromogen was formed by mixing 200 μl of supernatants with 1 ml of a reaction mixture containing 15 % (w/v) trichloroacetic acid (TCA), 0.375 % (w/v) 2-thiobarbituric acid (TBA), 0.1 % (w/v) butyl hydroxytoluene, and 0.25 N HCl, and by incubating the mixture at 100 °C for 30 min (Minotti and Aust 1987). After cooling to room temperature, tubes were centrifuged at 800g for 5 min and the supernatant was used for spectrophotometric reading at 532 and 600 nm. The calibration curve was made using MDA in the range of 0.1–10 μmol. A blank for all samples was prepared by replacing the sample with extraction medium, and controls for each sample were prepared by replacing TBA with 0.25 N HCl. In all cases, 0.1 % (w/v) butyl hydroxytoluene was included in the reaction mixtures to prevent artifactual formation of TBARS during the acid-heating step of the assay.
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Gas exchange and chlorophyll fluorescence determinations Fully expanded apical leaves were enclosed in a LI-COR 6400 gas exchange portable photosynthesis system (LICOR, Lincoln, Nebraska, USA). The gas exchange determinations were conducted at 25 °C with a photosynthetic photon flux density of 90 μmol m−2 s−1. Photosynthetic activity was calculated using equations developed by von Caemmerer and Farquhar (1981). Stomatal conductance (gs) was determined as described by (Harley et al. 1992). The electron transport rate (ETR) was measured as described by Harley et al. (1992). Steady-state modulated chlorophyll fluorescence was determined with a portable fluorimeter (Mini-PAM; Walz, Effeltrich, Germany). Light-adapted components of chlorophyll fluorescence were measured: Steadystate fluorescence (F), maximal fluorescence (F′m), variable fluorescence F′v equivalent to (F′m − F) and quantum yield of PS II photochemistry (ΦPSII) equivalent to (F′m − F)/F′m. Leaves were then dark adapted to obtain Fo (minimum fluorescence), Fm (maximum fluorescence), Fv variable fluorescence (equivalent to Fm − Fo) and Fv/Fm (maximum quantum yield of PSII photochemistry, equivalent to (Fm − Fo)/Fm). Leaves were dark adapted for at least 20 min, after which Fv/Fm values reach about 95 % of the pre-dawn values. F′o was estimated as Fo/ ((Fvc/Fm) + (Fo/F′m)). F′o was used to calculate photochemical quenching (qP), photochemical quenching of fluorescence (equivalent to F′m − F)/(F′m − F′o)), and F′v/F′m, the intrinsic efficiency of open PSII centres during illumination (equivalent to (F′m − F′o)/F′m). Three different leaves of each individual were measured. All data were corrected for changes in the fluorescence detector sensitivity induced by temperature variation of the Mini-Pam. Photochemical quenching was estimated as (F′m − Fs)/F′v. Non-photochemical quenching (NPQ) was calculated as (Fm − F′m) − 1. The variable F′v/F′m was calculated and was an indicator of absorbed light dissipated thermally. Pigment content Extracts for pigment analysis were prepared by grinding 100 mg fresh weight in a cold mortar with 10 ml of ethanol (95 %, v/v). The homogenate was centrifuged at 3,165g for 10 min at 4 °C. An aliquot of 1 ml from the supernatant was taken, 4 ml of 95 % ethanol were added, and the absorbance measured at 750, 665, 649 and 470 nm. Absorbance determinations were carried out with a Spectronic 2000 (Bausch and Lomb, Rochester, USA) spectrophotometer. Chlorophyll a, b, total and carotenoid (Chla, Chlb, Chla + b and Cx + c, respectively) contents were determined according to Liechenthaler (Lichtenthaler 1987). In brief, Chla = 13.36 × A665 − 5.19 × A649; Chlb = 27.43 ×
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A649 − 8.12 × A665, Chla + b = 5.24 × A664 + 22.24 × A649, Cx + c = (1,000 × A470 − 2.13 Chla − 97.46 Chlb)/198. Hormones and tocopherols Phytohormones, including ABA, JA and SA were measured by UPLC-MS/MS as described (Müller and Munné-Bosch 2011). Deuterium-labelled hormones were used to estimate recovery rates for each sample. Tocopherols were measured by HPLC as described by Cela et al. (2011). Expression of plasma membrane intrinsic proteins isoforms (PIP1.1; PIP1.2; PIP2.1; PIP2.5 and PIP2.6) RNA isolation Total RNA was isolated from Arabidopsis leaves by phenol/ chloroform extraction (Kay et al. 1987). DNase treatment of total RNA was performed according to Qiagen’s protocol (Quantitect Reverse Transcription KIT Cat#205311, Qiagen, CA). Quantitative real‑time RT‑PCR The expression of each plasma membrane intrinsic pro‑ tein (PIP) gene was studied by real-time PCR using an iCycler (Bio-Rad, Hercules, California, USA). cDNAs were obtained from 2.5 μg of total DNase-treated RNA in a 20 μl reaction containing oligo(dT)15 primer (Promega, Madison, WI), 10 mM dNTP (Invitrogen, Carlsbad, California, USA), 0.1 M DTT (Invitrogen, Carlsbad, CA, USA), 40 U of RNase inhibitor (Promega, Madison, WI), 5X first strand buffer (Invitrogen) and 200 U of Superscript II Reverse Transcriptase (Invitrogen) with the temperature recommended by the enzyme supplier. The primer sets used, including that of reference gene actin, to amplify each studied gene in the synthesized cDNAs are shown in supplemental Table 1. Each 23 μl reaction contained 3 μl of a 1:10 dilution of the cDNA, 10.5 μl of Master Mix (Bio-Rad Laboratories S.A, Madrid), 8.6 μl of deionised water and 0.45 μl of each primer pair. The PCR programme consisted of a 3-min incubation at 95 °C to activate the hot-start recombinant Taq DNA polymerase, followed by 31 cycles of 30 s at 94 °C, 30 s at the established annealing temperature and 30 s at 72 °C, where the fluorescence signal was measured, followed by 1 cycle of 1 min at 95 °C, 1 min at 70 °C and 60 cycles of 10 s at 70 °C. The specificity of the PCR amplification procedure was checked with a heat dissociation protocol (from 70 to 100 °C) after the final cycle of the PCR. Real-time PCR experiments were carried out four times with biological independent samples, with the threshold cycle (CT) determined in triplicate. The relative levels
Planta Table 1 Uranium (U) exposure (50 μM) effect on leaf net photosynthesis (An), electron transport rate (ETR) and the ETR/An ratio of Arabidopsis thaliana wild type (WT, ecotype Columbia), tocopherol
(null α/γ-tocopherol: vte1 and vte4: null in α-tocopherol) and glutathione (high content: cad1.3 and low content: cad2.1) mutants
Line
Treatment
An (μmol CO2 m−2s−1)
ETR (μmol e−2 s−1)
ETR/An (μmol CO2 μmol e−2)
WT
Control U Control U Control U Control U Control
9.63 ± 0.77 a 0.08 ± 0.24 e 8.28 ± 0.46 ab 0.95 ± 0.14 e 6.71 ± 0.07 bc 0.46 ± 0.06 e 5.65 ± 0.69 c 1.32 ± 0.78 e 3.5 ± 0.25 d
176.61 ± 8.21 a 191.33 ± 12.59 a 175.89 ± 5.11 a 155.55 ± 4.87 ab 175.42 ± 9.27 a 148.24 ± 5.28 b 181.31 ± 4.69 a 157.75 ± 10.77 ab 188,03 ± 3.10 a
18.63 ± 2.01 f 700.48 ± 166.54 a 20.80 ± 1.74 f 122.69 ± 35.76 d 26.77 ± 1.50 f 334.61 ± 68.74 bc 32.2 ± 6.25 f 251.18 ± 95.03 c 58.57 ± 3.18 e
U
0.43 ± 0.08 e
169.06 ± 5.12 ab
393.37 ± 4.45 b
Vte 1 Vte 4 Cad 1.3 Cad 2.1
Each value represents the mean of 4 replicates ± SE. Statistical analysis was made by a two-factor analysis of the variance (ANOVA). When significant differences were detected in ANOVA, LSD analysis was applied. Means that differed significantly (P
