Drought acclimation confers oxidative stress tolerance by inducing co-ordinated antioxidant defense at cellular and subcellular level in leaves of wheat seedlings

Wheat ( Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods (drought acclimation). The antioxidant defense in terms of redox metabolites and enzymes in leaf cells, chloroplasts, and mitochondria was examined in relation to ROS-induced membrane damage. Drought-acclimated seedlings modulated growth by maintaining favorable turgor potential and RWC and were able to limit H₂O₂ accumulation and membrane damage as compared with non-acclimated plants during severe water stress conditions. This was due to systematic upregulation of H₂O₂-metabolizing enzymes especially ascorbate peroxidase (APX, EC 1.11.1.11) and by maintaining ascorbate–glutathione redox pool in acclimated plants. By contrast, failure in the induction of APX and ascorbate–glutathione cycle enzymes makes the chloroplast susceptible to oxidative stress in non-acclimated plants. Non-acclimated plants protected the leaf mitochondria from oxidative stress by upregulating superoxide dismutase (SOD, EC 1.15.1.1), APX, and glutathione reductase (GR, EC 1.6.4.2) activities. Rewatering led to rapid enhancement in all the antioxidant defense components in non-acclimated plants, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or downregulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced oxidative stress tolerance by well-co-ordinated induction of antioxidant defense both at the chloroplast and at the mitochondrial level.     

Drought-induced spikelet sterility is associated with an inefficient antioxidant defence in rice panicles

Water stress-induced spikelet sterility limits rice production under upland conditions. The causes of spikelet sterility under drought stress are poorly understood. In this study the role of antioxidant defence management in drought-induced spikelet sterility was investigated in two rice ( Oryza sativa ) genotypes differing in drought resistance. Drought-resistant N22 genotype showed less water stress-induced spikelet sterility when compared to the susceptible N118 genotype under upland conditions. The N22 panicles maintained higher RWC and turgor potential and lower H₂O₂ levels across the developmental stages under water stress than that of N118 panicles. Drought-induced enhancement in superoxide dismutase (SOD, EC 1.15.1.1) activity coupled with higher ascorbate (AsA), glutathione (GSH) content and enhanced ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) activities resulted in lower H₂O₂ levels in N22 panicles. In contrast, insufficient enhancement in SOD, APX and GR activities resulted in relatively higher H₂O₂ levels under water stress in N118 panicles. The N22 panicles exhibited a higher number of SOD and APX isozymes in comparison with N118 panicles that might provide better reactive oxygen species scavenging. Hence it is concluded that well-equipped antioxidant defence plays an important role in minimizing water stress-induced spikelet sterility in upland rice.     

NaCl treatment markedly enhances H2O2-scavenging system in leaves of halophyte Suaeda salsa

The C₃ halophyte Suaeda salsa L. grown under the high concentration of NaCl (200 m M ) was used to investigate the role of the hydrogen peroxide (H₂O₂)-scavenging system [catalase, ascorbate peroxidase, glutathione reductase (GR), ascorbic acid, and glutathione (GSH)] in removal of reactive oxygen species. The activity of catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and GR (EC 1.6.4.2) increased significantly after 7 days of NaCl treatment. The isoform patterns of CAT and GR were not affected, but the staining intensities were significantly increased by NaCl treatment. Activities of both the thylakoid-bound APX or GR and stromal APX (S-APX) or GR in the chloroplasts were markedly enhanced under high salinity. Fifty percent of APX in the chloroplasts is thylakoid-bound APX. S-APX and GR activity represented about 74–78 and 64–71% of the total soluble leaf APX and GR activity, respectively. Salt treatment increased the contents of ascorbic acid and GSH. By contrast, a decreased content of H₂O₂ was found in the leaves of NaCl-treated S . salsa . The level of membrane lipid peroxidation decreased slightly after NaCl treatment. The plants grew well with high rate of net photosynthesis under high salinity. These data suggest that upregulation of the H₂O₂-scavenging system in plant cells, especially in the chloroplasts, is at least one component of the tolerance adaptations of halophytes to high salinity.     

Does grafting provide tomato plants an advantage against H2O2 production under conditions of thermal shock?

Non-grafted tomato plants ( Lycopersicon esculentum L. cv. Tmknvf₂) and grafted tomato plants ( L. esculentum L. cv. Tmknvf₂ ×  L. esculentum L. cv. RX-335) were grown for 30 days at three different temperatures (10°C, 25°C and 35°C). In the leaves of these plants, the enzymatic activities of SOD, GPX, CAT, APX, DHAR and GR were analysed, as were the concentrations of total H₂O₂, ascorbate and glutathione as well as foliar DW. Regardless of whether the plant was grafted or not, our results indicate that the thermal stress occurred mainly at 35°C, with the following effects: (1) high SOD activity; (2) H₂O₂ accumulation; (3) foliar-biomass reduction; (4) low GPX, CAT, APX, DHAR and GR activities; and (5) high concentrations of ascorbate and glutathione. In addition, our data show these effects to be much weaker in grafted than in non-grafted plants, directly reflected in greater biomass production. Therefore, the use of grafted plants under excessively high temperatures may offer an advantage over non-grafted plants in terms of resistance against thermal shock.     

Response of antioxidant systems to NaCl stress in the halophyte Cakile maritima

The effects of salt stress on antioxidative activities were investigated in a coastal halophyte, Cakile maritima . Two Tunisian accessions, Jerba and Tabarka, were compared. Plants were subjected to 100, 200, or 400 m M NaCl for 20 days. Parameters of oxidative stress [malondialdehyde (MDA), electrolyte leakage (EL), and hydrogen peroxide (H₂O₂) concentration], activities of several enzymes [superoxide dismutase (SOD), catalase (CAT), peroxydase (POD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR)], and antioxidant molecules (ascorbate, ASC, and glutathione, GSH) were determined. Growth of Jerba plants was improved at 100 m M NaCl as compared to that of control. Tabarka growth was inhibited by salt at all NaCl concentrations. The relative salt tolerance of Jerba was associated with high antioxidant enzyme activities and glutathione content, together with low MDA content, EL, and H₂O₂ concentration. Lower antioxidant activities and higher MDA content, EL, and H₂O₂ concentration were found in Tabarka. As a whole, these data suggest that the capacity to limit oxidative damage is important for salt tolerance of C. maritima .     

Senescence-related changes in the antioxidant status of ginkgo and birch leaves during autumn yellowing

The antioxidant status of birch and ginkgo leaves during autumnal senescence was characterized by the activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX) and superoxide dismutase (SOD). The contents of leaf H₂O₂ and ascorbate were used as indicators of oxidative stress. Degradation of chlorophyll (chl) during natural senescence was not accompanied either by an increase of H₂O₂ or by a decrease of reduced ascorbate. A transient decrease of reduced ascorbate in ginkgo and birch leaves in early senescence was accompanied by CAT inactivation. The activity of ionically-bound PODs was stimulated in late senescence in both species, when more than 30% of chl was degraded. Induction of MnSOD in both species and new isoforms of CuZnSOD in birch in late senescence was accompanied by the disappearance of other CuZnSOD isoforms in birch and FeSOD in ginkgo. The role of antioxidative enzymes in keeping ascorbate reduced and endogenous H₂O₂ at low levels in senescent leaves of deciduous trees was discussed.     

Relative importance of Na+ and Cl– in NaCl-induced antioxidant systems in roots of rice seedlings

The present study examined the response of antioxidant systems to NaCl stress and the relative importance of Na⁺ and Cl^– in NaCl-induced antioxidant systems in roots of rice seedlings. NaCl treatment caused an increase in the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) in roots of rice seedlings, but had no effect on the activities of superoxide dismutase (SOD) and catalase (CAT). There were detectable differences in APX and GR isoenzymes between control and NaCl-treated roots. Levels of activity for SOD and CAT isoenzymes did not change in NaCl-stressed roots compared with the control roots. NaCl treatment produced an increase in H₂O₂, ascorbate (AsA), dehydro-ascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) levels. Treatment with 50 m M Na-gluconate (whose anion is not permeable to membrane) led to a similar Na⁺ level in roots to that with 100 m M NaCl. It was found that treatment with 50 m M Na-gluconate affected H₂O₂, AsA, and DHA levels, APX and GR activities, OsAPX and OsGR mRNA induction in the same way as 100 m M NaCl. These observed changes seem to be mediated by Na⁺ toxicity and not by Cl^– toxicity. On the other hand, it was found that NaCl, but not Na-gluconate and NaNO₃, caused an increase in GSH and GSSG levels, indicating that Cl^–, rather than Na⁺, is responsible for the NaCl-increased GSH and GSSG levels in roots of rice seedlings.     

Metabolism of hydrogen peroxide by the scavenging system in Chlamydomonas reinhardtii

The metabolism of hydrogen peroxide by the scavenging system was studied in Chlamydomonas grown in a selenium-lacking and a selenium-containing medium. In cells of the former, 40% of external hydrogen peroxide (H₂O₂) was scavenged by ascorbate peroxidase (AsAP; EC 1.11.1.11) and the residual H₂O₂ by catalase (EC 1.11.1.6). The enzymes involved in the ascorbate-glutathione cycle including AsAP. were localized in the chloroplast. In cells of the latter, glutathione peroxidase (GSHP; EC 1.11.1.9) functioned primarily in the removal of external H₂O₂. GSHP was located solely in the cytosol. The Chlamydomonas AsAP was relatively stable in ascorbate-depleted medium as compared with chloroplast AsAP of higher plants. No inactivation of the enzyme was found upon its incubation with hydroxyurea, an inhibitor of the chloroplast enzyme of higher plants. The enzyme showed higher specificity with pyrogallol than with ascorbate. The amino acid sequences in the N-terminal region of Chlamvdomonas AsAP showed no significant similarity to any other AsAP from higher plants and Euglena . The enzyme had a molecular mass of 34 kDa. The K_m values of the enzyme for ascorbate and H₂O₂ were 5.2±0.3 and 25±3.4 μ M , respectively. Hydrogen peroxide was generated at a rate of 6.1±0.8 μmol mg^-1 chlorophyll h^-1 in intact chloroplasts isolated from Chlamydomonas cells grown in the presence of Na-selenite, and it diffused from the organelles into the medium.     

Fine and coarse regulation of reactive oxygen species in the salt tolerant mutants of barnyard grass and their wild-type parents under salt stress

The growth of the wild-type and three salt tolerant mutants of barnyard grass ( Echinochloa crusgalli L.) under salt stress was investigated in relation to oxidative stress and activities of the antioxidant enzymes superoxide dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6), phenol peroxidase (POD: EC 1.11.1.7), glutathione reductase (GR: EC 1.8.1.7) and ascorbate peroxidase (APX: EC 1.11.1.1). The three mutants ( fows B17, B19 and B21) grew significantly better than the wild-type under salt stress (200 m M NaCl) but some salt sensitive individuals were still detectable in the populations of the mutants though in smaller numbers compared with the wild-type. The salt sensitive plants had slower growth rates, higher rates of lipid peroxidation and higher levels of reactive oxygen species (ROS) in their leaves compared with the more tolerant plants from the same genotype. These sensitivity responses were maximized when the plants were grown under high light intensity suggesting that the chloroplast could be a main source of ROS under salt stress. However, the salt sensitivity did not correlate with reduced K ⁺/Na ⁺ ratios or enhanced Na ⁺ uptake indicating that the sensitivity responses may be mainly because of accumulation of ROS rather than ion toxicity. SOD activities did not correlate to salt tolerance. Salt stress resulted in up to 10-fold increase in CAT activity in the sensitive plants but lower activities were found in the tolerant ones. In contrast, the activities of POD, APX and GR were down regulated in the sensitive plants compared with the tolerant ones. A correlation between plant growth, accumulation of ROS and differential modulation of antioxidant enzymes is discussed. We conclude that loss of activities of POD, APX and GR causes loss of fine regulation of ROS levels and hence the plants experience oxidative stress although they have high CAT activities.     

Characterization of an ascorbate peroxidase in plastids of tobacco BY-2 cells

In higher plants, ascorbate peroxidase (APX; EC 1.11.1.11), the major H₂O₂-scavenging enzyme, occurs in several distinct isoenzymes that are localized in cytosol and various cell organelles. Here, we have purified and characterized an APX from the soluble fraction of plastids of non-photosynthetic tobacco BY-2 cells. The plastidic APX was a monomer with a molecular weight of 34 000. The enzymatic properties of the plastidic APX, including the rapid inactivation by H₂O₂ in ascorbate-depleted medium, were highly comparable with those of the chloroplastic stromal APX of spinach and tea leaves. However, the other chloroplastic APX isoenzyme, the thylakoid-membrane bound APX, was not detected in the plastids of the BY-2 cells. The N-terminal amino acid sequence of the plastidic APX was completely identical with the deduced amino acid sequence of a previously identified cDNA sequence of tobacco chloroplastic APX. When a green fluorescence protein gene tagged with the chloroplast-targeting signal sequence of APX was expressed in the BY-2 cells, the fluorescence protein exclusively localized into plastids, and not into mitochondria. We conclude that plastidic APX in non-photosynthetic tissues is the same as the chloroplastic APX that occurs in leaves.     

Genetic manipulation of proline levels affects antioxidants in soybean subjected to simultaneous drought and heat stresses

It was assumed that the genetic manipulation of the proline (Pro) level would also affect the (homo)glutathione content as both compounds have a common precursor, glutamate. To test this hypothesis, the levels of Pro, reduced and oxidized (homo)glutathione [(h)GSH and (h)GSSG] and other antioxidants were compared under simultaneous drought and heat stress conditions and in a control treatment in a time course experiment on wild-type soybean ( Glycine max cv. Ibis) and on transgenic plants containing the cDNA coding for l -Δ¹-pyrroline-5-carboxylate reductase (EC 1.5.1.2), the last enzyme involved in Pro synthesis, in the sense and antisense directions. At the end of the recovery period, the highest H₂O₂ and lipid hydroperoxide concentrations were observed in the antisense transformants, which exhibited the greatest injury, while the lowest H₂O₂ content was detected in the sense transformants, which exhibited the lowest injury percentage. During stress treatment, the highest Pro and ascorbate (AA) levels were detected in the sense transformants, while the highest GSH and hGSH contents, AA/dehydroascorbate (DHA) and (h)GSH/(h)GSSG ratios and ascorbate peroxidase (APX) activity were found in the antisense transformants. The greatest APX (EC 1.11.1.11) activity was observed in the first part of the stress treatment in the antisense transformants, and the greatest glutathione reductase (EC 1.6.4.2) activity was observed in the second part of the treatment in the same genotype. The present experiments indicate that the manipulation of Pro synthesis affects not only the (h)GSH concentrations, but also the levels of other antioxidants.     

Exogenous treatment with salicylic acid leads to increased antioxidant capacity in leaves of barley plants exposed to paraquat

Our previous study suggests that salicylic acid mediates tolerance in barley plants to paraquat (Ananieva et al. 2002). To further define the role of SA in paraquat induced responses, we analysed the capacity of the antioxidative defence system by measuring the activities of several antioxidative enzymes: superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1), catalase (CAT, EC 1.11.1.6), and guaiacol peroxidase (POX, EC 1.11.1.7). Twelve-day-old barley seedlings were supplied with 500 micromol/L SA or 10 micromol/L Pq via the transpiration stream and kept in the dark for 24 h. Then they were exposed to 100 micromol m(-2) s(-1) PAR and samples were taken 6 h after the light exposure. Treatment of seedlings with 10 micromol/L Pq reduced the activity of APX and GR, did not affect the activity of POX and DHAR but caused over a 40% increase in the activity of CAT. Pre-treatment with 500 micromol/L SA for 24 h in the dark before Pq application increased the activities of the studied enzymes in both the chloroplasts (SOD activity) and the other compartments of the cell (POX, CAT activity). The effect of SA pre-treatment was highly expressed on DHAR and POX activity. The data suggest that SA antagonizes Pq effects, via elicitation of an antioxidative response in barley plants.     

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 m M NaCl) on protective enzyme activities under ambient (350 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ concentrations were investigated in two barley cultivars ( Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO₂, upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO₂ alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO₂ and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO₂ mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.     

Plant glutathione peroxidases

Oxidative stress in plants causes the induction of several enzymes, including superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2). The first two are responsible for converting superoxide to H₂O₂ and its subsequent reduction to H₂O, and the third is involved in recycling of ascorbate. Glutathione peroxidases (GPXs, EC 1.11.1.9) are a family of key enzymes involved in scavenging oxyradicals in animals. Only recently, indications for the existence of this enzyme in plants were reported. Genes with significant sequence homology to one member of the animal GPX family, namely phospholipid hydroperoxide glutathione peroxidase (PHGPX), were isolated from several plants. Cit-SAP, the protein product encoded by the citrus csa gene, which is induced by salt-stress, is so far the only plant PHGPX that has been isolated and characterized. This protein differs from the animal PHGPX in its rate of enzymatic activity and in containing a Cys instead of selenocysteine (Sec) as its presumed catalytic residue. The physiological role of Cit-SAP and its homologs in other plants is not yet known.     

Investigations on the antioxidative defence responses to NaCl stress in a mangrove, Bruguiera parviflora: Differential regulations of isoforms of some antioxidative enzymes

Two-month-old healthy seedlings of a true mangrove, Bruguiera parviflora, raised from propagules in normal nursery conditions were subjected to varying concentrations of NaCl for 45 d under hydroponic culture conditions to investigate the defence potentials of antioxidative enzymes against NaCl stress imposed oxidative stress. Changes in the activities of the antioxidative enzymes catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POX), glutathione reductase (GR) and superoxide dismutase (SOD) were assayed in leaves to monitor the temporal regulation. Among the oxidative stress triggered chemicals, the level of H₂O₂ was significantly increased while total ascorbate and total glutathione content decreased. The ratio of reduced to oxidized glutathiones, however, increased due to decreased levels of oxidized glutathione in the leaf tissue. Among the five antioxidative enzymes monitored, the APX, POX, GR and SOD specific activities were significantly enhanced at high concentration (400 mM NaCl), while the catalase activities declined, suggesting both up and downregulations of antioxidative enzymes occurred due to NaCl imposed osmotic and ionic stress. Analysis of the stress induced alterations in the isoforms of CAT, APX, POX, GR and SOD revealed differential regulations of the isoforms of these enzymes. In B. parviflora one isoform of each of Mn-SOD and Cu/Zn-SOD while three isoforms of Fe-SOD were observed by activity staining gel. Of these, only Mn-SOD and Fe-SOD2 content was preferentially elevated by NaCl treatment, whereas isoforms of Cu/Zn-SOD, Fe-SOD1 and Fe-SOD3 remained unchanged. Similarly, out of the six isoforms of POX, the POX-1,-2,-3 and -6 were enhanced due to salt stress but the levels of POX-4 and -5 remained same as in control plants suggesting preferential upregulation of selective POX isoforms. Activity staining gel revealed only one prominent band of APX and this band increased with increased salt concentration. Similarly, two isoforms of GR (GR1 and GR2) were visualized on activity staining gel and both these isoforms increased upon salt stress. In this mangrove four CAT-isoforms were identified, among which the prominent CAT-2 isoform level was maximally reduced again suggesting differential downregulation of CAT isoforms by NaCl stress. The results presented in this communication are the first report on the resolutions of isoforms APX, POX and GR out of five antioxidative enzymes studied in the leaf tissue of a true mangrove. The differential changes in the levels of the isoforms due to NaCl stress may be useful as markers for recognizing salt tolerance in mangroves. Further, detailed analysis of the isoforms of these antioxidative enzymes is required for using the various isoforms as salt stress markers. Our results indicate that the overproduction of H₂O₂ by NaCl treatment functions as a signal of salt stress and causes upregulation of APX, POX, GR and deactivations of CAT in B. parviflora. The concentrations of malondialdehyde, a product of lipid peroxidation and lipoxygenase activity remained unchanged in leaves treated with different concentrations of NaCl, which again suggests that the elevated levels of the antioxidant enzymes protect the plants against the activated oxygen species thus avoiding lipid peroxidation during salt stress.     

Antioxidative protection in a drought-resistant maize strain during leaf senescence

Leaves of 7- and 18-day-old plants of two maize strains, one resistant (LIZA) and one sensitive (LG11) to water stress, were floated in 1 m M paraquat and 1 m M H₂O₂ for 12 h in light and in darkness. The aim of this work was to analyse the effects of these substances on the activities of enzymes involved in the scavenging of active oxygen species during senescence. Three senescence parameters; chlorophyll loss, lipid peroxidation and conductivity; showed a general cell damage caused by both oxidative treatments and revealed a higher tolerance of LIZA than LG11 to paraquat and H₂O₂ both in light and in darkness. Activities of antioxidative enzymes increased by the effect of oxidative treatments in young and senescent leaves of the drought-resistant maize strain LIZA. These increases were about 3-to 6-fold in glutathione reductase. 3-to 4-fold in superoxide dismutase and 2-fold in ascorbate peroxidase activities. The possible correlation between water stress resistance. senescence and the potential of antioxidant enzymes was analysed.     

Enhanced antioxidant enzymes are associated with reduced hydrogen peroxide in barley roots under saline stress

Antioxidant enzymes are related to the resistance to various abiotic stresses including salinity. Barley is relatively tolerant to saline stress among crop plants, but little information is available on barley antioxidant enzymes under salinity stress. We investigated temporal and spatial responses of activities and isoform profiles of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), non-specific peroxidase (POX), and glutathione reductase (GR) to saline stress in barley seedlings treated with 200 mM NaCl for 0, 1, 2, 5 days, respectively. In the control plant, hydrogen peroxide content was about 2-fold higher in the root than in the shoot. Under saline stress, hydrogen peroxide content was decreased drastically by 70% at 2 d after NaCl treatment (DAT) in the root. In the leaf, however, the content was remained unchanged by 2 DAT and increased about 14 % at 5 DAT. In general, the activities of antioxidant enzymes were increased in the root and shoot under saline stress. But the increase was more significant and consistent in the root. The activities of SOD, CAT, APX, POX, and GR were increased significantly in the root within 1 DAT, and various elevated levels were maintained by 5 DAT. Among the antioxidant enzymes, CAT activity was increased the most drastically. The significant increase in the activities of SOD, CAT, APX, POX, and GR in the NaCl-stressed barley root was highly correlated with the increased expression of the constitutive isoforms as well as the induced ones. The hydrogen peroxide content in the root.     

Nitric oxide, induced by wounding, mediates redox regulation in pelargonium leaves

The subject of this study was the participation of nitric oxide (NO) in plant responses to wounding, promoted by nicking of pelargonium ( Pelargonium peltatum L.) leaves. Bio-imaging with the fluorochrome 4,5-diaminofluorescein diacetate (DAF-2DA) and electrochemical in situ measurement of NO showed early (within minutes) and transient (2 h) NO generation after wounding restricted to the site of injury. In order to clarify the functional role of NO in relation to modulation of the redox balance during wounding, a pharmacological approach was used. A positive correlation was found between NO generation and regulation of the redox state. NO caused a slight restriction of post-wounded O₂⁻ production, in contrast to the periodic and marked increase in H₂O₂ level. The observed changes were accompanied by time-dependent inhibition of catalase (CAT) and ascorbate peroxidase (APX) activity. The effect was specific to NO, since the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) reversed the inhibition of CAT and APX, as well as temporarily enhancing H₂O₂ synthesis. Finally, cooperation of NO/H₂O₂ restricted the depletion of the low-molecular weight antioxidant pool ( i.e . ascorbic acid and thiols) was positively correlated with sealing and reconstruction changes in injured pelargonium leaves ( i.e . lignin formation and callose deposition). The above results clearly suggest that NO may promote restoration of wounded tissue through stabilisation of the cell redox state and stimulation of the wound scarring processes.     

Oxidative stress induced by long-term plum pox virus infection in peach (Prunus persica)

In this study, the effect of long-term plum pox virus (PPV) infection on the response of certain antioxidant enzymes at the subcellular level was studied in peach plants ( Prunus persica (L.) Batch) (cv. GF305), which are characterized by great susceptibility to the virus. In infected plants, a decrease in the efficiency of excitation energy capture by PSII ( F _v'/ F _m') was observed, which was accompanied by a decrease in non-photochemical quenching (NPQ). p -Hydroxy-mercury benzoic acid (pHMB)-insensitive ascorbate peroxidase (APX) activity (class III peroxidase) was detected in both chloroplast and soluble fractions. In soluble fractions from inoculated peaches, a significant increase in pHMB-sensitive APX activity and a significant decrease in superoxide dismutase (SOD) activity were observed. These changes were correlated with the observations in isolated chloroplasts, where an increase in both pHMB-sensitive and pHMB-insensitive APX activities was observed, whereas significant decreases in SOD, monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) activities were produced. According to these results, as a consequence of PPV infection, an oxidative stress, indicated by an increase in lipid peroxidation and protein oxidation, was produced in peach leaves, which was monitored by the diaminobenzidine (DAB) peroxidase-coupled H₂O₂ probe. PPV infection produced an alteration in chloroplast ultrastructure, giving rise to dilated thylakoid membranes. PPV-infected peach leaves showed a decreased amount of starch in chloroplasts from palisade parenchyma, as well as an increase in the number and size of plastoglobuli, in relation to control plants. The results suggest that long-term PPV infection produces an oxidative stress, and that an antioxidative metabolism imbalance may be related to the progress of PPV infection and symptoms in peach plants.