High level of reduced glutathione contributes to detoxification of lipid peroxide‐derived reactive carbonyl species in transgenic Arabidopsis overexpressing glutathione reductase under aluminum stress

Lipid peroxide‐derived reactive carbonyl species (RCS), generated downstream of reactive oxygen species (ROS), are critical damage‐inducing species in plant aluminum (Al) toxicity. In mammals, RCS are scavenged primarily by glutathione (reduced form of glutathione, GSH), but in plant Al stress, contribution of GSH to RCS detoxification has not been evaluated. In this study, Arabidopsis plants overexpressing the gene AtGR1 (accession code At3g24170), encoding glutathione reductase (GR), were generated, and their performance under Al stress was examined. These transgenic plants (GR‐OE plants) showed higher GSH levels and GSH/GSSG (oxidized form of GSH) ratio, and an improved Al tolerance as they suffered less inhibition of root growth than wild‐type under Al stress. Exogenous application of 4‐hydroxy‐2‐nonenal, an RCS responsible for Al toxicity in roots, markedly inhibited root growth in wild‐type plants. GR‐OE plants suffered significantly smaller inhibition, indicating that the enhanced GSH level increased the capacity of RCS detoxification. The generation of H₂O₂ due to Al stress in GR‐OE plants was lower by 26% than in wild‐type. Levels of various RCS, such as malondialdehyde, butyraldehyde, phenylacetaldehyde, (E)‐2‐heptenal and n‐octanal, were suppressed by more than 50%. These results indicate that high levels of GSH and GSH/GSSG ratio by GR overexpression contributed to the suppression of not only ROS, but also RCS. Thus, the maintenance of GSH level by overexpressing GR reinforces dual detoxification functions in plants and is an efficient approach to enhance Al tolerance.     

Antioxidant enzyme activities in subcellular fractions of larvae of the black swallowtail butterfly,Papilio polyxenes

The black swallowtail butterfly, Papilio polyxenes, larvae are specialized feeders of pro-oxidant rich plants of Apiaceae and Rutaceae. An important defense against toxic forms of oxygen species generated by ingestion of the pro-oxidants, are the antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT), GSH-dependent glutathione peroxidases (selenium-dependent glutathione peroxidase [GPOX] and peroxidase activity of selenium-independent glutathione-S-transferase [GT_px]), and glutathione reductase (GR). The subcellular distribution of these enzymes in black swallowtail larvae was investigated and was found to resemble the patterns described for larvae of two other lepidopteran species: the southern armyworm, Spodoptera eridania, and the cabbage looper, Trichoplusia ni. The confinement of SOD in the cytosol and mitochondria was typically eukaryotic, but the relative proportion (1:1) was markedly different from the mammalian pattern (4:1; cytosol:mitochondria). The most obvious difference between the black swallowtail and other lepidoptera as a group, and mammalian species, is in very wide intracellular distributions of CAT, GTpx, and GR in insect species. Insects possess very low levels of a GPOX-like activity which reduces both H₂O₂ and organic peroxides. Consequently, insects have elaborate activities with a wide subcellular distribution of both CAT which decomposes H₂O₂, and GTpx which decomposes organic peroxides. The reduction of peroxides is dependent on GSH, which in this process is oxidized to GSSG. GR which reduces GSSG to GSH is also of wide subcellular distribution, analogous to the distribution pattern of GTpx.     

Increased intracellular H2O2 availability preferentially drives glutathione accumulation in vacuoles and chloroplasts

One biochemical response to increased H₂O₂ availability is the accumulation of glutathione disulphide (GSSG), the disulphide form of the key redox buffer glutathione. It remains unclear how this potentially important oxidative stress response impacts on the different sub‐cellular glutathione pools. We addressed this question by using two independent in situ glutathione labelling techniques in Arabidopsis wild type (Col‐0) and the GSSG‐accumulating cat2 mutant. A comparison of in situ labelling with monochlorobimane (MCB) and in vitro labelling with monobromobimane (MBB) revealed that, whereas in situ labelling of Col‐0 leaf glutathione was complete within 2 h incubation, about 50% of leaf glutathione remained inaccessible to MCB in cat2. High‐performance liquid chromatography (HPLC) and enzymatic assays showed that this correlated tightly with the glutathione redox state, pointing to significant in vivo pools of GSSG in cat2 that were unavailable for MCB labelling. Immunogold labelling of leaf sections to estimate sub‐cellular glutathione distribution showed that the accumulated GSSG in cat2 was associated with only a minor increase in cytosolic glutathione but with a 3‐ and 10‐fold increase in plastid and vacuolar pools, respectively. The data are used to estimate compartment‐specific glutathione concentrations under optimal and oxidative stress conditions, and the implications for redox homeostasis and signalling are discussed.     

Enhanced sensitivity to oxidative stress in transgenic tobacco plants with decreased glutathione reductase activity leads to a decrease in ascorbate pool and ascorbate redox state

To investigate the possible mechanisms of glutathione reductase (GR) in protecting against oxidative stress, we obtained transgenic tobacco (Nicotiana tabacum) plants with 30–70% decreased GR activity by using a gene encoding tobacco chloroplastic GR for the RNAi construct. We investigated the responses of wild type and transgenic plants to oxidative stress induced by application of methyl viologen in vivo. Analyses of CO₂ assimilation, maximal efficiency of photosystem II photochemistry, leaf bleaching, and oxidative damage to lipids demonstrated that transgenic plants exhibited enhanced sensitivity to oxidative stress. Under oxidative stress, there was a greater decrease in reduced to oxidized glutathione ratio but a greater increase in reduced glutathione in transgenic plants than in wild type plants. In addition, transgenic plants showed a greater decrease in reduced ascorbate and reduced to oxidized ascorbate ratio than wild type plants. However, there were neither differences in the levels of NADP and NADPH and in the total foliar activities of monodehydroascorbate reductase and dehydroascorbate reductase between wild type and transgenic plant. MV treatment induced an increase in the activities of GR, ascorbate peroxidase, superoxide dismutase, and catalase. Furthermore, accumulation of H₂O₂ in chloroplasts was observed in transgenic plants but not in wild type plants. Our results suggest that capacity for regeneration of glutathione by GR plays an important role in protecting against oxidative stress by maintaining ascorbate pool and ascorbate redox state.     

Speciation dependant antioxidative response in roots and leaves of sorghum (Sorghum bicolor (L.) Moench cv CO 27) under Cr(III) and Cr(VI) stress

Growth, lipid peroxidation, H₂O₂ produciton and the response of the antioxidant enzymes and metabolites of the ascorbate glutathione pathway to oxidative stress caused by two concentrations (50 and 100 µM) of Cr(III) and Cr(VI) was studied in 15 day old seedlings of sorghum (Sorghum bicolor (L.) Moench cv CO 27) after 10 days of treatment. Cr accumulation in sorghum plants was concentration and organ dependant. There was no significant growth retardation of plants under 50 µM Cr(III) stress. 100 µM Cr(VI) was most toxic of all the treatments in terms of root and leaf growth and oxidative stress. 50 µM Cr(VI) treated roots exhibited high significant increase in superoxide dismutase (SOD), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) (p < 0.01) and significant increases in catalse (CAT), ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) (p < 0.05). A high increase in ascorbic acid (AA) level was seen in roots of 50 µM Cr(VI) treated plants in comparison with control. Levels of reduced glutathione (GSH) showed a varied and complex response in all the treatments in both plant parts. GSH/GSSG ratio was not affected by Cr(III) treatment in leaves, in contrast, roots exhibited significant reduction in the ratio. Results indicate that GSH depletion increased sensitivity to oxidative stress (Cr(VI) roots and leaves and Cr(III) 100 µM roots) and AA in tandem with APX compensated for GSH depletion by acting directly on H₂O₂ and the mechanism of defensive response in roots as well as leaves varied in its degree and effectiveness due to the concentration dependant differences observed in translocation of the element itself, reactive oxygen species (ROS) generation and enzyme inhibition based on the oxidation state supplied to the plants.     

Evidence of a significant role of glutathione reductase in the sulfur assimilation pathway

With the objective of studying the role of glutathione reductase (GR) in the accumulation of cysteine and methionine, we generated transgenic tobacco and Arabidopsis lines overexpressing the cytosolic AtGR1 and the plastidic AtGR2 genes. The transgenic plants had higher contents of cysteine and glutathione. To understand why cysteine levels increased in these plants, we also used gr1 and gr2 mutants. The results showed that the transgenic plants have higher levels of sulfite, cysteine, glutathione and methionine, which are downstream to adenosine 5′ phosphosulfate reductase (APR) activity. However, the mutants had lower levels of these metabolites, while the sulfate content increased. A feeding experiment using ³⁴SO₄^2– also showed that the levels of APR downstream metabolites increased in the transgenic lines and decreased in gr1 compared with their controls. These findings, and the results obtained from the expression levels of several genes related to the sulfur pathway, suggest that GR plays an essential role in the sulfur assimilation pathway by supporting the activity of APR, the key enzyme in this pathway. GR recycles the oxidized form of glutathione (GSSG) back to reduce glutathione (GSH), which serves as an electron donor for APR activity. The phenotypes of the transgenic plants and the mutants are not significantly altered under non‐stress and oxidative stress conditions. However, when germinating on sulfur‐deficient medium, the transgenic plants grew better, while the mutants were more sensitive than the control plants. The results give substantial evidence of the yet unreported function of GR in the sulfur assimilation pathway.     

Selenium Pretreatment Upregulates the Antioxidant Defense and Methylglyoxal Detoxification System and Confers Enhanced Tolerance to Drought Stress in Rapeseed Seedlings

In order to observe the possible regulatory role of selenium (Se) in relation to the changes in ascorbate (AsA) glutathione (GSH) levels and to the activities of antioxidant and glyoxalase pathway enzymes, rapeseed (Brassica napus) seedlings were grown in Petri dishes. A set of 10-day-old seedlings was pretreated with 25 μM Se (Sodium selenate) for 48 h. Two levels of drought stress (10% and 20% PEG) were imposed separately as well as on Se-pretreated seedlings, which were grown for another 48 h. Drought stress, at any level, caused a significant increase in GSH and glutathione disulfide (GSSG) content; however, the AsA content increased only under mild stress. The activity of ascorbate peroxidase (APX) was not affected by drought stress. The monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) activity increased only under mild stress (10% PEG). The activity of dehydroascorbate reductase (DHAR), glutathione S-transferase (GST), glutathione peroxidase (GPX), and glyoxalase I (Gly I) activity significantly increased under any level of drought stress, while catalase (CAT) and glyoxalase II (Gly II) activity decreased. A sharp increase in hydrogen peroxide (H₂O₂) and lipid peroxidation (MDA content) was induced by drought stress. On the other hand, Se-pretreated seedlings exposed to drought stress showed a rise in AsA and GSH content, maintained a high GSH/GSSG ratio, and evidenced increased activities of APX, DHAR, MDHAR, GR, GST, GPX, CAT, Gly I, and Gly II as compared with the drought-stressed plants without Se. These seedlings showed a concomitant decrease in GSSG content, H₂O₂, and the level of lipid peroxidation. The results indicate that the exogenous application of Se increased the tolerance of the plants to drought-induced oxidative damage by enhancing their antioxidant defense and methylglyoxal detoxification systems.     

Glucose-6-phosphate dehydrogenase plays a central role in modulating reduced glutathione levels in reed callus under salt stress

In the present study, we investigated the role of glucose-6-phosphate dehydrogenase (G6PDH) in regulating the levels of reduced form of glutathione (GSH) to the tolerance of calli from two reed ecotypes, Phragmites communis Trin. dune reed (DR) and swamp reed (SR), in a long-term salt stress. G6PDH activity was higher in SR callus than that of DR callus under 50–150 mM NaCl treatments. In contrast, at higher NaCl concentrations (300–600 mM), G6PDH activity was lower in SR callus. A similar profile was observed in GSH contents, glutathione reductase (GR) and glutathione peroxidase (GPX) activities in both salt-stressed calli. After G6PDH activity and expression were reduced in glycerol treatments, GSH contents and GR and GPX activity decreased strongly in both calli. Simultaneously, NaCl-induced hydrogen peroxide (H₂O₂) accumulation was also abolished. Exogenous application of H₂O₂ increased G6PDH, GR, and GPX activities and GSH contents in the control conditions and glycerol treatment. Diphenylene iodonium (DPI), a plasma membrane (PM) NADPH oxidase inhibitor, which counteracted NaCl-induced H₂O₂ accumulation, decreased these enzymes activities and GSH contents. Furthermore, exogenous application of H₂O₂ abolished the N-acetyl-l-cysteine (NAC)-induced decrease in G6PDH activity, and DPI suppressed the effect of buthionine sulfoximine (BSO) on induction of G6PDH activity. Western-blot analyses showed that G6PDH expression was stimulated by NaCl and H₂O₂, and blocked by DPI in DR callus. Taken together, G6PDH activity involved in GSH maintenance and H₂O₂ accumulation under salt stress. And H₂O₂ regulated G6PDH, GR, and GPX activities to maintain GSH levels. In the process, G6PDH plays a central role.     

Exogenous Selenium Mitigates Salt Stress in Soybean by Improving Growth,Physiology, Glutathione Homeostasis and Antioxidant Defense

The mechanism of selenium (Se)-induced salt tolerance was studied in moderately sensitive soybean (Glycine max L.) plants. To execute this view, soybean plants were imposed with salt stress (EC 6 dS m⁻¹ ) applying NaCl. In other treatments, Se (0, 25, 50 and 75 µM Na₂SeO₄) was sprayed as co-application with that level of salt stress. Plant height, stem diameter, leaf area, SPAD value decreased noticeably under salt stress. Altered proline (Pro) level, together with decreased leaf relative water content (RWC) was observed in salt-affected plants. Salt stress resulted in brutal oxidative damage and increased the content of H₂O₂, MDA level and electrolyte leakage. Exogenous Se spray alleviated oxidative damage through boosting up the antioxidant defense system by increasing the activity of antioxidant enzymes such as catalase (CAT), peroxidase (POD) and glutathione reductase (GR), as well as by improving non-enzymatic antioxidants like glutathione (GSH) and GSH/glutathione disulfide (GSSG). The upregulated antioxidant defense system, restored Pro and leaf RWC, higher SPAD value conferred better growth and development in Se-sprayed salt-affected soybean plants which altogether put forth for the progressive yield contributing parameters and finally, seed yield. Among different doses of Se, soybean plants sprayed with 50 µM Na₂SeO₄ showed better salt tolerance.     

The oxidative stress caused by NaCl in Azolla caroliniana is mitigated by nitrate

In order to assess the role of the antioxidant defense system against salt treatment, the activities of some antioxidative enzymes and levels of some nonenzymatic antioxidants were estimated in Azolla caroliniana subjected to NaCl treatment (50 mM) for 10 days in absence or presence of nitrate. In A. caroliniana, salt treatment in absence of nitrate preferentially enhanced electrolyte leakage, lipid peroxidation, and H₂O₂ content. Also, the specific activitiy of guaiacol peroxidase (POX), glutathione reductase (GR), catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD) increased. In addition, reduced glutathione level increased and consequently, glutathione/oxidized glutathione (GSH/GSSG) ratio increased. Accumulation of Na⁺ increased significantly by salinity stress which resulted in a significant decrease in K⁺ accumulation, accordingly, K⁺/Na⁺ ratio decreased. Replacement of potassium chloride by potassium nitrate in nutrient solution under salt stress (50 mM NaCl) exhibited a reduction in electrolyte leakage, lipid peroxidation, and H₂O₂ contents. Conversely, the specific activity of APX, POX, GR, CAT, and SOD increased. The content of total ascorbate decreased, in contrast, reduced and GSSG increased and the ratio of GSH/GSSG increased 2.3-fold compared to the control value. Sodium ion accumulation was minimized in the presence of nitrate, potassium ion accumulation increased and as a result, K⁺/Na⁺ ratio increased when compared with the corresponding salinized plants. The differential changes in the specific activity of antioxidant enzymes due to NaCl treatment and nitrate may be useful as markers for recognizing salt tolerance in A. caroliniana.     

Changes in glutathione redox cycle during diapause determination and termination in the bivoltine silkworm,Bombyx moil

To explore whether glutathione regulates diapause determination and termina tion in the bivoltine silkworm Bombyx mori, we monitored the changes in glutathione redox cycle in the ovary of both diapanse and nondiapauseegg producers, as well as those in dia pause eggs incubated at different temperatures. The activity ofthioredoxin reductase （TrxR） was detected in ovaries but not in eggs, while neither ovaries nor eggs showed activity of glutathione peroxidase. A lower reduced glutathione/oxidized glutathione （GSH/GSSG） ratio was observed in the ovary of diapauseegg producers, due to weaker reduction of oxidized glutathione （GSSG） to the reduced glutathione （GSH） catalyzed by glutathione reductase （GR） and TrxR. This indicates an oxidative shift in the glutathione redox cy cle during diapause determination. Compared with the 25℃treated diapause eggs, the 5℃treated diapause eggs showed lower GSH/GSSG ratio, a result of stronger oxidation of GSH catalyzed by thioredoxin peroxidase and weaker reduction of GSSG catalyzed by GR. Our study demonstrated the important regulatory role of glutathione in diapause determination and termination of the bivoltine silkworm.     

Effect of Selenium on Ascorbate�CGlutathione Metabolism During PEG-induced Water Deficit in Trifolium repens L.

To elucidate the effect of selenium (Se) on the ascorbate?Cglutathione (ASC?CGSH) cycle under drought stress, the activities of antioxidant enzymes and the levels of molecules involved in ASC?CGSH metabolism were studied in Trifolium repens seedlings subjected to polyethylene glycol (PEG)-induced water deficit alone or combined with 5???M Na₂SeO₄. Compared to the control, H₂O₂, thiobarbituric acid reactive substances (TBARS), ascorbate (ASC), dehydroascorbate (DHA), and glutathione disulfide (GSSG) contents increased, whereas a constant content of glutathione (GSH) and decreases in ASC/DHA and GSH/GSSG ratios were observed in the presence of PEG. The activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were upregulated, except for monodehydroascorbate reductase (MDHAR) activity during PEG-induced water deficit. Se application decreased the contents of H₂O₂, TBARS, DHA, and GSSG, increased the levels of GSH and ASC, and inhibited the decreases of ASC/DHA and GSH/GSSG ratios. Although it did not affect APX activity significantly, Se addition improved the activities of MDHAR, DHAR, and GR. Furthermore, GR activity showed the highest increase followed by that of DHAR and MDHAR in decreasing order. These data indicated that fluctuations in ASC?CGSH metabolism resulting from Se may have a positive effect on drought stress mitigation, and the regulation in the ASC?CGSH cycle can be attributed mainly to GR and DHAR in PEG?+?Se-treated T. repens seedlings.     

Loss‐of‐function mutations in the APX1 gene result in enhanced selenium tolerance in Arabidopsis thaliana

It is generally recognized that excess selenium (Se) has a negative effect on the growth and development of plants. Numerous studies have identified key genes involved in selenium tolerance in plants; however, our understanding of its molecular mechanisms is far from complete. In this study, we isolated an Arabidopsis selenium‐resistant mutant from the mutant XVE pool lines because of its increased root growth and fresh weight in Se stress, and cloned the gene, which encodes the cytosolic ascorbate peroxidase (APX1). Two other APX1 gene knockout allelic lines were also selenium resistant, and the APX1‐complementary COM1 restored the growth state of wild type under Se stress. In addition, these APX1 allelic lines accumulated more Se than did wild‐type plants when subjected to Se stress. Further analysis revealed that the APX1‐mediated Se tolerance was associated, at least in part, with the enhanced activities of antioxidant enzymes catalase, glutathione peroxidase and glutathione reductase. Moreover, enhanced Se resistance of the mutants was associated with glutathione (GSH), which had the higher expression level of GSH1 gene involved in GSH synthesis and consequently increased GSH content. Our results provide genetic evidence indicating that loss‐of‐function of APX1 results in tolerance to Se stress.     

Exogenous salicylic acid regulates reactive oxygen species metabolism and ascorbate–glutathione cycle in <Emphasis Type="Italic">Nitraria tangutorum</Emphasis> Bobr. under salinity stress

The effect of 0.5–1.5 mM salicylic acid (SA) on modulating reactive oxygen species metabolism and ascorbate–glutathione cycle in NaCl-stressed Nitraria tangutorum seedlings was investigated. The individual plant fresh weight (PFW) and plant dry weight (PDW) significantly increased under 100 mM NaCl while remained unchanged or decreased under 200–400 mM NaCl compared to the control. Superoxide anion (O ₂ ^·? ), hydrogen peroxide (H₂O₂), thiobarbituric acid reactive substances (TBARS), reduced ascorbate (AsA), dehydroascorbate (DHA), reduced glutathione (GSH) and oxidized glutathione (GSSG) increased whereas the ratios of AsA/DHA and GSH/GSSG decreased under varied NaCl treatments. Ascorbate peroxidase (APX) and glutathione reductase (GR) activities were enhanced while dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) activities remained unvaried under 100–400 mM NaCl stresses. In addition, exogenous SA further increased PFW, PDW and root/shoot ratio. SA effectively diminished O ₂ ^·? accumulation. H₂O₂ and TBARS decreased under 0.5 and 1.0 mM SA treatments compared to those without SA. 0.5 mM of SA increased while 1.0 and 1.5 mM SA decreased APX activities. DHAR activities were elevated by 0.5 and 1.0 mM SA but not by 1.5 mM SA. MDHAR and GR activities kept constant or significantly increased at varying SA concentrations. Under SA treatments, AsA and GSH contents further increased, DHA and GSSG levels remained unaltered, while the decreases in AsA/DHA and GSH/GSSG ratios were inhibited. The above results demonstrated that the enhanced tolerance of N. tangutorum seedlings conferred by SA could be attributed mainly to the elevated GR and DHAR activities as well as the increased AsA/DHA and GSH/GSSG ratios.     

Glutathione reductase plays an anti-apoptotic role against oxidative stress in human hepatoma cells

The cellular roles of glutathione reductase (GR) in the reactive oxygen species (ROS)-induced apoptosis were studied using the HepG2 cells transfected with GR. The overexpression of GR caused a marked enhancement in reduced and oxidized glutathione (GSH/GSSG) ratio, and significantly decreased ROS levels in the stable transfectants. Hydrogen peroxide (H₂O₂), under the optimal condition for apoptosis, significantly decreased cellular viability and total GSH content, and rather increased ROS level, apoptotic percentage and caspase-3 activity in the mock-transfected cells. However, hydrogen peroxide could not largely generate these apoptotic changes in cellular viability, ROS level, apoptotic percentage, caspase-3 activity and total GSH content in the cells overexpressing GR. Taken together, GR may play a protective role against oxidative stress.     

Mechanistic differences in the uptake of salicylic acid glucose conjugates by vacuolar membrane‐enriched vesicles isolated from Arabidopsis thaliana

Salicylic acid (SA) is a plant hormone involved in a number of physiological responses including both local and systemic resistance of plants to pathogens. In Arabidopsis, SA is glucosylated to form either SA 2‐O‐β‐d ‐glucose (SAG) or SA glucose ester (SGE). In this study, we show that SAG accumulates in the vacuole of Arabidopsis, while the majority of SGE was located outside the vacuole. The uptake of SAG by vacuolar membrane‐enriched vesicles isolated from Arabidopsis was stimulated by the addition of MgATP and was inhibited by both vanadate (ABC transporter inhibitor) and bafilomycin A₁ (vacuolar H⁺‐ATPase inhibitor), suggesting that SAG uptake involves both an ABC transporter and H⁺‐antiporter. Despite its absence in the vacuole, we observed the MgATP‐dependent uptake of SGE by Arabidopsis vacuolar membrane‐enriched vesicles. SGE uptake was not inhibited by vanadate but was inhibited by bafilomycin A₁ and gramicidin D providing evidence that uptake was dependent on an H⁺‐antiporter. The uptake of both SAG and SGE was also inhibited by quercetin and verapamil (two known inhibitors of multidrug efflux pumps) and salicin and arbutin. MgATP‐dependent SAG and SGE uptake exhibited Michaelis–Menten‐type saturation kinetics. The vacuolar enriched‐membrane vesicles had a 46‐fold greater affinity and a 10‐fold greater transport activity with SGE than with SAG. We propose that in Arabidopsis, SAG is transported into the vacuole to serve as a long‐term storage form of SA while SGE, although also transported into the vacuole, is easily hydrolyzed to release the active hormone which can then be remobilized to other cellular locations.     

Antioxidative responses to different altitudes in leaves of alpine plant Polygonum viviparum in summer

Mountain environmental stresses result in increased formation of hydrogen peroxide (H₂O₂) and accumulation of malondialdehyde (MDA) in leaves of Polygonum viviparum. The activities of several antioxidative system enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and the contents of several non-enzymatic antioxidants such as reduced form of ascorbate (ASC), dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) were investigated in leaves of P. viviparum, which were collected from three altitudes (2,200, 3,200, and 3,900 m) of Tianshan Mountain in China. The activities of these four antioxidative enzymes were accompanied by increases of H₂O₂ levels from 2,200 to 3,200 m. However, the activities of CAT and POD were decreased, whereas the activities of SOD and GR continually increased at 3,900 m. Analyses of isoforms of SOD, CAT, POD, and GR showed that the leaves of P. viviparum exposed different altitude conditions are capable of differentially altering the intensity. Additionally, two new isoforms of SOD were detected at 3900 m. A continual increase in the ASC, ASC to DHA ratio, GSH and GSH/[GSH + GSSG] ratio, and the activity of DHAR were observed in leaves of P. viviparum with the elevation of altitude. These results suggest that the higher contents of ASC, GSH as well as an increase in reduced redox state may be essential to antioxidation processes in the leaves of P. viviparum, whereas antioxidant enzymes system is a cofactor in the processes.