Response of the ascorbate-glutathione cycle to re-aeration following hypoxia in lupine roots

Ascorbate-glutathione cycle in wheat and rice seedlings under anoxia and subsequent reaeration

The most important part of the plant antioxidant system is the ascorbate-glutathione cycle (AGC), the activity of which is observed upon exposure to a range of stressors, including lack of O2, and oxidative stress occurring immediately after the restoration of oxygen access, hereafter termed reaeration or post-anoxia. The operation of the AGC (enzymes and low-molecular components) in wheat (Triticum aestivum, cv. Leningradka, non-resistant to hypoxia) and rice (Oryza sativa, cv. Liman, resistant) seedlings after 24 h anoxia and 1 h or 24 h reaeration was studied. Significant accumulation of oxidized forms of ascorbate and glutathione was revealed in the non-resistant plant (wheat) after 24 h of anoxia and reaeration, indicating the development of oxidative stress. In the resistant plant (rice), reduced forms of these antioxidants prevailed both in normoxia and under stress, which may indicate their intensive reduction. In wheat, the activities of ascorbate peroxidase and dehydroascorbate reductase in shoots, and monodehydroascorbate reductase and glutathione reductase in roots decreased under anoxia and reaeration. The activity of antioxidant enzymes was maintained in rice under lack of oxygen (ascorbate peroxidase, glutathione reductase) and increased during post-anoxia (AGC reductases). Anoxia stimulated accumulation of mRNA of the organellar ascorbate peroxidase genes OsAPX3, OsAPX5 in shoots, and OsAPX3-5 and OsAPX7 in roots. At post-anoxia, the contribution of the OsAPX1 and OsAPX2 genes encoding the cytosolic forms of the enzyme increased in the whole plant, and so did that of the OsAPX8 gene for the plastid form of the enzyme. The accumulation of mRNA of the genes OsMDAR2 and OsMDAR4 encoding peroxisomal and cytosolic monodehydroascorbate reductase as well as the OsGR2 and OsGR3 for cytosolic and organellar glutathione reductase was activated during reaeration in shoots and roots. In most cases, O2 deficiency activated the genes encoding the peroxisomal, plastid, and mitochondrial forms of the enzymes, and upon reaeration, an enhanced activity of the genes encoding the cytoplasmic forms was observed. Taken together, the inactivation of AGC enzymes was revealed in wheat seedlings during anoxia and subsequent reaeration, which disrupted the effective operation of the cycle and triggered the accumulation of oxidized forms of ascorbate and glutathione. In rice, anoxia led to the maintenance of the activity of AGC enzymes, and reaeration stimulated it, including at the level of gene expression, which ensured the effective operation of AGC.

Phytotoxic mechanism of allelochemical liquiritin on root growth of lettuce seedlings

As the main active ingredient of the traditional Chinese medicine Glycyrrhiza uralensis Fisch, liquiritin has multiple biological activities, including anti-inflammatory, antihepatotoxicity, immune regulation, anti-virus and anti-cancer. In addition, liquiritin has been recognized as an allelochemical that displays markedly inhibitory effects on the growth of target plants, G. uralensis and lettuce. However, its phytotoxic mechanism remains unknown. In the present study, the mode of action of liquiritin against root growth of lettuce seedling was researched. After treatments with liquiritin, the cell division in root tips of lettuce seedlings was partly arrested, and the cell viability and root vitality were obviously lost. At the same time, overproduction of reactive oxygen species (ROS), malondialdehyde (MDA) and proline (Pro) in lettuce seedlings were induced by liquiritin. The results indicated that the phytotoxic effects of liquiritin was probably dependent on the induction of ROS overproduction, resulting in membrane lipids peroxidation following with cell death and mitosis process disorder.

Effect of selenium on growth and antioxidant enzyme activities of wine related yeasts

The use of supplements in the diet is a common practice to address nutritional deficiencies. Selenium is an essential micronutrient with an antioxidant and anti-carcinogenic role in human and animal health. There is increasing interest in developing nutritional supplements such as yeast cells enriched with selenium. The possibility of producing beverages, namely wine, with selenium-enriched yeasts, led us to investigate the selenium tolerance of six wine related yeasts. The production of such cells may hamper selenium toxicity problems. Above certain concentrations selenium can be toxic inducing oxidative stress and yeast species can show different tolerance. This work aimed at studying selenium tolerance of a diversity of wine related yeasts, thus antioxidant response mechanisms with different concentrations of sodium selenite were evaluated. Viability assays demonstrated that the yeast Torulaspora delbrueckii showed the highest tolerance for the tested levels of 100 µg mL(-1) of sodium selenite. The evaluation of antioxidative enzyme activities showed the best performance for concentrations of 250 and 100 µg mL(-1), respectively for the yeast species Saccharomyces cerevisiae and Hanseniaspora guilliermondii. These results encourage future studies on the possibility to use pre-enriched yeast cells as selenium supplement in wine production.

Mechanism of artemisinin phytotoxicity action: induction of reactive oxygen species and cell death in lettuce seedlings

Artemisinin has been recognized as an allelochemical that inhibits growth of several plant species. However, its mode of action is not well clarified. In this study, the mechanism of artemisinin phytotoxicity on lettuce seedlings was investigated. Root and shoot elongation of lettuce seedlings were inhibited by artemisinin in a concentration-dependent manner. The compound effectively arrested cell division and caused loss of cell viability in root tips of lettuce. Overproduction of reactive oxygen species (ROS) was induced by artemisinin. Lipid peroxidation, proline overproduction and reduction of chlorophyll content in lettuce seedlings were found after treatments. These results suggested that artemisinin could induce ROS overproduction, which caused membrane lipids peroxidation and cell death, and impacted mitosis and physiological processes, resulting in growth inhibition of receptor plants.

Zinc deficiency differentially affects redox homeostasis of rice genotypes contrasting in ascorbate level

Zinc (Zn) deficiency is an important mineral disorder affecting rice production, and is associated with the formation of oxidative stress in plant tissue. In this study we investigated processes of oxidative stress formation as affected by ascorbate (AsA) in two pairs of contrasting rice genotypes: (i) two indica lines differing in field tolerance to Zn deficiency and AsA metabolism, i.e. RIL46 (tolerant) and IR74 (sensitive); (ii) the japonica wild-type Nipponbare (tolerant) and the AsA deficient TOS17 mutant line ND6172 (sensitive) having a 20-30% lower AsA level due to the knockout of an AsA biosynthetic gene (OsGME1). Plants were grown hydroponically under +Zn and -Zn conditions for 21 days and samples were investigated after 7, 14, and 21 days of treatment. Tissue Zn concentrations below 20mg kg(-1) in the -Zn treatment induced the formation of visible symptoms of Zn deficiency from day 14 in all genotypes, but especially in the sensitive IR74. Significant increases in lipid peroxidation were observed in the leaves of the sensitive genotypes IR74 and ND6172, and in the roots of IR74, but not in the tolerant genotypes. At day 21, the tolerant genotypes RIL46 and Nipponbare had significantly higher AsA levels in both shoots and roots compared to the sensitive lines. Consistently, higher levels of hydrogen peroxide formation in leaves and roots of the sensitive genotypes were detected using staining methods. Differences in foliar hydrogen peroxide formation between IR74 and RIL46 became apparent on day 7 and between ND6172 and Nipponbare on day 14. Similarly, genotypic differences in hydrogen peroxide formation in the roots were seen on day 21. In conclusion, our data demonstrate that Zn deficiency leads to a redox imbalance in roots and shoots prior to the occurrence of visible symptoms, and that the antioxidant AsA plays an important role in maintaining the redox homeostasis under Zn deficiency.

Lemna minor exposed to fluoranthene: growth, biochemical, physiological and histochemical changes

Polycyclic aromatic hydrocarbons (PAHs) represent one of the major groups of organic contaminants in the aquatic environment. Duckweed (Lemna minor L.) is a common aquatic plant widely used in phytotoxicity tests for xenobiotic substances. The goal of this study was to assess the growth and the physiological, biochemical and histochemical changes in duckweed exposed for 4 and 10 days to fluoranthene (FLT, 0.1 and 1 mgL(-1)). Nonsignificant changes in number of plants, biomass production, leaf area size, content of chlorophylls a and b and carotenoids and parameters of chlorophyll fluorescence recorded after 4 and 10 days of exposure to FLT were in contrast with considerable changes at biochemical and histochemical levels. Higher occurrence of reactive oxygen species (ROS) caused by an exposure to FLT after 10 days as compared to control (hydrogen peroxide elevated by 13% in the 0.1 mgL(-1) and by 41% in the 1 mgL(-1) FLT; superoxide anion radical by 52% and 115% respectively) reflected in an increase in the activities of antioxidant enzymes (superoxide dismutase by 3% in both treatments, catalase by 9% and 1% respectively, ascorbate peroxidase by 21% and 5% respectively, guaiacol peroxidase by 12% in the 0.1 mgL(-1) FLT). Even the content of antioxidant compounds like ascorbate (by 20% in the 1 mgL(-1) FLT) or total thiols (reduced forms by 15% in the 0.1 mgL(-1) and 8% in the 1 mgL(-1) FLT, oxidized forms by 36% in the 0.1 mgL(-1) FLT) increased. Increased amount of ROS was followed by an increase in malondialdehyde content (by 33% in the 0.1 mgL(-1) and 79% in the 1 mgL(-1) FLT). Whereas in plants treated by the 0.1 mgL(-1) FLT the contents of total proteins and phenols increased by 15% and 25%, respectively, the 1 mgL(-1) FLT caused decrease of their contents by 32% and 7%. Microscopic observations of duckweed roots also confirmed the presence of ROS and related histochemical changes at the cellular and tissue levels. The assessment of phytotoxicity of organic pollutant in duckweed based only on the evaluation of growth parameters could not fully cover the irreversible changes already running at the level of biochemical processes.

Phytotoxic action mechanism of hapalocyclamide in lettuce seedlings

Hapalocyclamide (cyclo-thiazole-L-alanine-oxazole-D-alanine-D-thiazoline-d-phenylalanine), a hexapeptide phytotoxic compound, was isolated from the terrestrial cyanobacterium Hapalosiphon sp. The phytotoxic action of the compound was investigated in lettuce (Lactuca sativa L. cv. Great Lakes no. 366) by determining its effects on several physiological processes. Hapalocyclamide effectively inhibited mitosis process in root tips, which resulted in the suppression of primary root growth of lettuce. The compound also induced overproduction of reactive oxygen species (ROS) and loss of cell viability in root cells. Moreover, hapalocyclamide-induced lipid peroxidation in both roots and shoots. Therefore, the primary action of hapalocyclamide to suppress lettuce growth might be caused by ROS overproduction, which induces major oxidative damage to membrane lipids, resulting in cell death and growth inhibition.

Ascorbic acid formation and profiling of genes expressed in its synthesis and recycling in apple leaves of different ages

Ascorbic acid (AsA), as a unique antioxidant and enzyme cofactor, has multiple roles in plants. However, there is very limited information on the mechanism of AsA accumulation and controlling in leaves. In this study, we determined AsA accumulation levels, analyzed expression patterns of the genes involved in synthesizing via l-galactose pathway and recycling as well as enzyme activities in apple (Malus domestica Borkh) leaves with different age. AsA content was found to increase with leaf development, reaching the highest level in 20-day-old leaves. This level was maintained in mature leaves until the dropping in senescent leaves. Comparing with young and senescent leaves, mature leaves had higher capability for AsA synthesis with high expression levels and activity of l-galactose dehydrogenase and l-galactono-1,4-lactone dehydrogenase. The mRNA expression of genes involved in AsA synthesis also showed highest abundance in 20-day-old leaves, though GDP-mannose-3',5'-epimerase and l-galactose-1-phosphate phosphatase expression reached the highest levels before 20 days old. These results suggest that AsA accumulation in apple leaves mainly occurs during the transition phase from young to mature leaves with high rates of synthesis and recycling, and that l-galactose-1-phosphate phosphatase could play an important role in regulating AsA biosynthesis via the l-galactose pathway.

Response mechanisms of antioxidants in bryophyte (Hypnum plumaeforme) under the stress of single or combined Pb and/or Ni

The short-term responses and mechanisms of antioxidants in moss Hypnum plumaeforme subjected to single or combined Pb and/or Ni stress has been revealed in this study, in order to clarify (1) the relationship between the stress intensity and antioxidant fluctuation, (2) the difference between single and combined stress, and (3) the possibility of biomonitoring by the application of antioxidant fluctuation under stress. The results showed that the stress induced dose dependent formation of reactive oxygen species (ROS) and subsequent lipid peroxidation. Total chlorophyll (Chl) content and superoxide dismutase (SOD) activity were initiated under lower stress but were inhibited under higher stress. Both single and combined stress decreased catalase (CAT) activity but increased peroxidase (POD) activity, indicating POD in the moss played an important role in resisting the oxidative stress induced by Pb and Ni. The accumulation of (.)O2(-) and H2O2 in H. plumaeforme was respectively related to the low activity of SOD and the decreased activity of CAT. The study indicated that Pb and Ni had synergistic effect in inducing the oxidative stress in moss H. plumaeforme, especially under the combination of high concentration of Ni (0.1, 1.0 mM) and Pb. POD and CAT activity, as well as H2O2 and MDA content, which increased or decreased regularly with a dose dependent under Pb and Ni stress, could be used as an effective indicator in moss biomonitoring, especially in the case of light pollution caused by heavy metals without the changes in the appearance of mosses.

Ability of lupine seeds to germinate and to tolerate desiccation as related to changes in free radical level and antioxidants in freshly harvested seeds

Seeds of yellow lupine (Lupinus luteus L. cv. Juno) were collected throughout their development on the mother plant to determine whether the ability to germinate and to tolerate desiccation is related to the level of free radicals and the changes in the redox state of ascorbate and glutathione as well as the activities of antioxidative enzymes. Electron paramagnetic resonance (EPR)-based analyses showed that development of lupine seed was accompanied by generation of free radicals with g(1) and g(2) values of 2.0049+/-0.0004 and 2.0029+/-0.0003, respectively. Free radical level increased significantly 25 DAF and decreased thereafter. The amount of hydrogen peroxide was high in fresh immature seeds and decreased during maturation drying. Ascorbate accumulated in lupine embryos during early seed filling stage whereas glutathione content increased during late seed filling phase. During maturation drying the redox state of both ascorbate and glutathione pools shifted towards the oxidized forms. While superoxide dismutase (SOD, EC 1.15.1.1), and ascorbate peroxidase (APX, EC 1.11.1.11) activities remained high at the early seed filling stage the activities of both dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) and that of catalase (CAT, EC 1.11.1.6) increased before seeds reached physiological maturity and decreased thereafter. The changes of isoform patterns of antioxidative enzymes were observed during seed maturation. Immature lupine seeds tested immediately after harvest acquired the ability to germinate when less than half-filled and reached high tolerance to desiccation just after physiological maturity. The physiological implications of the changes in antioxidative machinery for the acquisition of desiccation tolerance and seeds germinability are discussed.

The effect of water stress on the antioxidant content, protective enzyme activities, proline content and lipid peroxidation in wheat seedling

An investigation was conducted to study the effect of water stress on the antioxidant content, protective enzyme activities, proline content and lipid peroxidation in wheat seedlings. Drought stress increases the amount of Reactive Oxygen Species (ROS), leading to metabolic disorders. It is now known that higher levels of activity-protective mechanisms render the cells more enduring against environmental stress including drought. Two widely cultivated cultivars of wheat in Iran, Sab. and N. Sar. were grown up according to the hydroponic method. Having reached the stage of 4-5 leaves growth; the plants were kept under 4, 8 and 12 bars potential resulting from using Polyethylene Glycol 8000 (PEG 8000). Hogland solution was used as the control. Then the amount of ascorbate, glutathione, superoxide dismutase and catalase activity, proline and lipid Peroxidation was measured in cut samples of the leaves. The result indicated an increase in the amount of Ascorbate and Glutathione as the stress was intensified in the case of Sab. Moreover, the reduced form of Ascorbate (ASC) and Glutathione (GSH) were higher in Sab. at 8 and 12 bars. The amount of Proline accumulation was considerably higher in Sab. than N. Sar. SOD activity, on the other hand, diminished at 8 and 12 bar levels. CAT activity is also regarded as a limiting factor. Lipid peroxidation was also geared up as the stress was intensified. These limiting factors rendered N. Sar. cultivar more sensitive to water stress resulting from PEG8000 compared to Sab.

A comparative study of water distribution, free radical production and activation of antioxidative metabolism in germinating pea seeds

The aim of this study was to investigate whether there is a relationship between hydration of the embryo axes and cotyledons and the resumption of the oxidative metabolism in both organs of germinating seeds of pea (Pisum sativum L. cv. Piast). Nuclear magnetic resonance ((1)H-NMR) spectroscopy and imaging were used to study temporal and spatial water uptake and distribution in pea seeds. The observations revealed that water penetrates into the seed through the hilum, micropyle and embryo axes, and cotyledons hydrate to different extents. Thus, inhomogeneous water distribution may influence the resumption of oxidative metabolism. Electron paramagnetic resonance (EPR) measurements showed that seed germination was accompanied by the generation of free radicals with g(1) and g(2) values of 2.0032 and 2.0052, respectively. The values of spectroscopic splitting coefficients suggest that they are quinone radicals. The highest content of free radicals was observed in embryo axes immediately after emergence of the radicle. Glutathione content decreased during the entire germination period in both embryo axes and cotyledons. A different profile was observed for ascorbate, with significant increases in embryo axes, coinciding with radicle protrusion. Electrophoretic analysis showed that superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) were present in dry seeds and were activated later during germination, especially in embryo axes. The presence of all antioxidative enzymes as well as low molecular antioxidants in dry seeds allowed the antioxidative machinery to be active as soon as the enzymes were reactivated by seed imbibition. The observed changes in free radical levels, antioxidant contents and enzymatic activities in embryo axes and cotyledons appear to be more closely related to metabolic and developmental processes associated with preparation for germination, and do not correspond directly to the hydration of the tissues.

An ultrasensitive fluorescent assay for the in vivo quantification of superoxide radical in organisms

Superoxide radical is a very important parameter of oxidative stress involved in a variety of biological phenomena; therefore, its in vivo study is of utmost significance. However, its accurate detection is a challenge due to its short lifetime and its very low physiological concentration. All current assays are qualitative and nonspecific, and at best they are performed in vitro. The current dihydroethidine-based assay overcomes all these problems and introduces the following novelties. First, it measures the in vivo superoxide production in animals, plants, and microorganisms. Second, it is ultrasensitive and very simple in that it can measure superoxide radical as low as 1.5 pmol in biological samples as low as 5 mg. Third, the very high sensitivity of the assay renders possible, for the first time, the measurement of the actual rate of formation of superoxide radical under physiological and simulated nonphysiological conditions.