Ultraviolet-B-induced oxidative stress and antioxidant defense system responses in ascorbate-deficient vtc1 mutants of Arabidopsis thaliana

Ultraviolet-B (UV-B) radiation has a negative impact on plant cells, and results in the generation of reactive oxygen species (ROS). In order to increase our understanding of the effects of UV-B on antioxidant processes, we investigated the response of an ascorbate-deficient Arabidopsis thaliana mutant vtc1 to short-term increased UV-B exposure. After UV-B supplementation, vtc1 mutants exhibited oxidative damage. Evidence for damage included an increase in H(2)O(2) content and the production of thiobarbituric acid reactive substances (TBARS); a decrease in chlorophyll content and chlorophyll fluorescence parameters were also reported. The vtc1 mutants had higher total glutathione than the wild type (WT) during the first day of UV-B treatment. We found reduced ratio of glutathione/total glutathione and increased ratio of dehydroascorbate/total ascorbate in the vtc1 mutants, compared to the WT plants. In addition, the enzymes responsible for ROS scavenging, including superoxide dismutase, catalase, and ascorbate peroxidase, had insufficient activity in the vtc1 mutants, compared to the WT plants. The same reduced activity in the vtc1 mutants was reported for the enzymes responsible for the regeneration of ascorbate and glutathione (including monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase). These results suggest that the ascorbate-deficient mutant vtc1 is more sensitive to supplementary UV-B treatment than WT plants and ascorbate can be considered an important antioxidant for UV-B radiation.     

Defense potentials to NaCl in a mangrove, Bruguiera parviflora: differential changes of isoforms of some antioxidative enzymes

In order to assess the role of the antioxidative defense system against salt treatment, the activities of some antioxidative enzymes and levels of antioxidants were monitored in a true mangrove, Bruguiera parviflora, subjected to varying levels of NaCl under hydroponic culture. In the leaves of B. parviflora, salt treatment preferentially enhanced the content of H2O2 as well as the activity of ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), and superoxide dismutase (SOD), whereas it induced the decrease of total ascorbate and glutathione (GSH+GSSG) content as well as catalase (CAT) activity. Analysis of isoforms of antioxidative enzymes by native PAGE and activity staining revealed that leaves of B. parviflora had one isoform each of Mn-SOD and Cu/Zn-SOD and three isoforms of Fe-SOD. Expression of Mn-SOD and Fe-SOD-2 was preferentially elevated by NaCl. Similarly, out of the six isoforms of GPX, the GPX-1, 2, 3 and 6 were enhanced by salt treatment but the levels of GPX-4 and -5 changed minimally as compared to those of a control. Activity staining gel revealed only one prominent isoform of APX and two isoforms of GR (GR-1 and GR-2), all of these isoforms increased upon salt exposure. Four CAT-isoforms were identified, among which the prominent CAT-2 isoform level was maximally reduced, suggesting differential down regulation of CAT isoforms by NaCl. The concentrations of malondialdehyde (MDA), a product of lipid peroxidation, remained unchanged in leaves of the plant treated with different concentrations of NaCl. This suggests that plants are protected against activated oxygen species by the elevated levels of certain antioxidative enzymes, thus avoiding lipid peroxidation during salt exposure. The differential changes in the levels of the isoforms due to NaCl treatment may be useful as markers for recognizing salt tolerance in mangroves.     

UV-B induced changes in antioxidant enzymes and their isoforms in cucumber (Cucumis sativus L.) cotyledons

To assess the role of antioxidant defense system on exposure to ultra-violet-B (UV-B) radiation, the activities of antioxidant enzymes superoxide dismutase (SOD), ascorbic acid peroxidase (APX), glutathione reductase (GR) and guaiacol peroxidase (GPX), as well as the level of antioxidants ascorbic acid (AA) and alpha-tocopherol were monitored in cucumber (Cucumis sativus L. var long green) cotyledons. UV-B enhanced the activity of antioxidant enzymes as well as AA content, but decreased the level of alpha-tocopherol. Significant increase was observed in the activities of SOD and GPX. Analysis of isoforms of antioxidant enzymes by native-PAGE and activity staining revealed three isoforms of GPX in unexposed dark-grown cotyledons (control), and their intensity was enhanced by UV-B exposure. In addition, four new isoforms of GPX were observed in cotyledons after UV-B exposure. Although no new isoforms were observed for the other antioxidant enzymes, the activities of their existing isoforms were enhanced by UV-B.     

Amelioration of Ozone-Induced Oxidative Damage in Wheat Plants Grown under High Carbon Dioxide (Role of Antioxidant Enzymes)

O3-induced changes in growth, oxidative damage to protein, and specific activities of certain antioxidant enzymes were investigated in wheat plants (Triticum aestivum L. cv Roblin) grown under ambient or high CO2. High CO2 enhanced shoot biomass of wheat plants, whereas O3 exposure decreased shoot biomass. The shoot biomass was relatively unaffected in plants grown under a combination of high CO2 and O3. O3 exposure under ambient CO2 decreased photosynthetic pigments, soluble proteins, and ribulose-1,5-bisphosphate carboxylase/oxygenase protein and enhanced oxidative damage to proteins, but these effects were not observed in plants exposed to O3 under high CO2. O3 exposure initially enhanced the specific activities of superoxide dismutase, peroxidase, glutathione reductase, and ascorbate peroxidase irrespective of growth in ambient or high CO2. However, the specific activities decreased in plants with prolonged exposure to O3 under ambient CO2 but not in plants exposed to O3 under high CO2. Native gels revealed preferential changes in the isoform composition of superoxide dismutase, peroxidases, and ascorbate peroxidase of plants grown under a combination of high CO2 and O3. Furthermore, growth under high CO2 and O3 led to the synthesis of one new isoform of glutathione reductase. This could explain why plants grown under a combination of high CO2 and O3 are capable of resisting O3-induced damage to growth and proteins compared to plants exposed to O3 under ambient CO2.     

Scavenging of superoxide and hydrogen peroxide in blue‐green algae (cyanobacteria)

Blue‐green algae (cyanobacteria) have evolved as the most primitive, oxygenic, plant‐type photosynthetic organisms. Within a single prokaryotic cell, they have uniquely accommodated both oxygenic photosynthesis and aerobic respiration, which are known to produce superoxide and hydrogen peroxide as inevitable byproducts. Two types of superoxide dismutase have been characterized in both N₂‐fixing and non‐N₂‐fixing cyanobacteria, namely cytosolic iron‐containing superoxide dismutase and thylakoid‐bound manganese‐containing superoxide dismutase. No qualitative differences between various cell types (vegetative cells, heterocysts) were found. In contrast to chloroplasts, most of the cyanobacterial species show catalatic activity. From two species the corresponding enzymes have been characterized as typical prokaryotic (bifunctional) catalase‐peroxidases with homologies to cytochrome c peroxidases and ascorbate peroxidases. In addition to catalatic activity, some strains exhibit ascorbate peroxidase activity, but to date there are no reports detailing purification and characterization.
Cyanobacteria were found to contain low intracellular ascorbate concentrations (30‐100 µ M ) and 2‐5 m M glutathione. Both monodehydroascorbate and glutathione reductase activities were detected in most species examined, whereas dehydroascorbate reductase activity was absent. The question as to whether a glutathione‐ascorbate cycle exists in cyanobacteria cannot be answered at present.     

Changes in the antioxidative systems in mitochondria during ripening of pepper fruits

The presence of enzymes of the ascorbate–glutathione cycle was studied in mitochondria purified from green and red pepper (Capsicum annuum L.) fruits. All four enzymes, ascorbate peroxidase (APX; EC 1.11.1.11), monodehydroascorbate reductase (MDHAR; EC 1.6.5.4), dehydroascorbate reductase (DHAR; EC 1.8.5.1) and glutathione reductase (GR; EC 1.6.4.2) were present in the isolated mitochondria of both fruit ripening stages. The activity of the reductive ascorbate–glutathione cycle enzymes (MDHAR, GR and DHAR) was higher in mitochondria isolated from green than from red fruits, while APX and the antioxidative enzyme superoxide dismutase (SOD; EC 1.15.1.1) were higher in the red fruits. The levels of ascorbate and L-galactono-γ-lactone dehydrogenase (GLDH; EC 1.3.2.3) activity were found to be similar in the mitochondria of both fruits. The higher APX and Mn-SOD specific activities in mitochondria from red fruits might play a role in avoiding the accumulation of any activated oxygen species generated in these mitochondria, and suggests an active role for these enzymes during ripening.     

Phytochelatin plays a role in UV-B tolerance in N2-fixing cyanobacterium Anabaena doliolum

To study the role of Cd-induced phytochelatins in UV-B tolerance, lipid peroxidation, antioxidative enzymes (superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase), glutathione arid phytochelatin contents were measured in buthionine sulphoximine treated and untreated cells of Anabaena doliolum. Cd-pretreatment of the cyanobacterium reduced the lipid peroxidation as well as the antioxidative enzymes in comparison to UV-B treatment alone, whereas the phytochelatin content demonstrated an increase. In contrast to this, buthionine sulphoximine-induced inhibition of phytochelatin synthase, dramatically decreased the Cd-induced co-tolerance against UV-B, hence demonstrating that phytochelatin not only protects the cyanobacterium from heavy metal but participates in UV-B tolerance as well.     

Investigation of supplemental ultraviolet-B-induced changes in antioxidative defense system and leaf proteome in radish (Raphanus sativus L. cv Truthful): an insight to plant response under high oxidative stress

Impact of supplemental UV-B (sUV-B) has been investigated on photosynthetic pigments, antioxidative enzymes, metabolites, and protein profiling of radish plants under realistic field conditions. Exposure of sUV-B leads to oxidative damage in plants. However, plants possess a number of UV-protection mechanisms including a stimulation of antioxidant defense system. It caused alteration in reactive oxygen species metabolism primarily by decreasing catalase activity vis-à-vis enhanced activities of other enzymatic (superoxide dismutase, ascorbate peroxidase, and glutathione reductase) and non-enzymatic (ascorbic acid) antioxidants. Qualitative analysis of samples also showed significant reductions in photosynthetic pigments and protein content. After sUV-B exposure, protein profile showed differences mainly at eight points—126.8, 84.8, 71.9, 61.5, 47.8, 40.6, 38.9, and 17.5 kDa, whereas protein(s) of 38.9 kDa showed increment. Results of the present investigation clearly showed the adverse effect of sUV-B on total biomass at final harvest.     

REACTIVE OXYGEN METABOLISM IN INTERTIDAL FUCUS SPP. (PHAEOPHYCEAE)

Our previous research suggests that interspecific variation in stress tolerance in intertidal Fucus spp. (Phaeophyceae) is partially mediated by differences in the production of, or ability to detoxify, reactive oxygen. Here we report on the content of antioxidants (ascorbate, glutathione, carotenoids, and tocopherols) and protective enzymes (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) involved in reactive oxygen metabolism in three species of intertidal brown algae— Fucus spiralis L., F. evanescens C. Ag., and F. distichus L.—that differ in stress tolerance and position in the intertidal zone. Contents of the major antioxidants were similar in the three species and were not correlated with stress tolerance. The least stress tolerant species, F. distichus, had the lowest activity of reactive-oxygen-scavenging enzymes, although F. spiralis, the species with the highest stress tolerance, and F. evanescens contained similar activities of antioxidant enzymes on a fresh-weight basis. However, the activities of superoxide dismutase and ascorbate peroxidase in F. evanescens are lower than those of F. spiralis when expressed on the basis of chlorophyll. These data show that the ratio between reactive oxygen protection and production might be more important than the absolute content of antioxidants and protective enzymes. It also shows the importance of localization of detoxifying mechanisms and avoidance of oxidative stress.     

The antioxidants of legume nodule mitochondria

The mitochondria of legume root nodules are critical to sustain the energy-intensive process of nitrogen fixation. They also generate reactive oxygen species at high rates and thus require the protection of antioxidant enzymes and metabolites. We show here that highly purified mitochondria from bean nodules (Phaseolus vulgaris L. cv. Contender x Rhizobium leguminosarum bv. phaseoli strain 3622) contain ascorbate peroxidase primarily in the inner membrane (with lesser amounts detected occasionally in the matrix), guaiacol peroxidases in the outer membrane and matrix, and manganese superoxide dismutase (MnSOD) and an ascorbate-regenerating system in the matrix. This regenerating system relies on homoglutathione (instead of glutathione) and pyridine nucleotides as electron donors and involves the enzymes monodehydroascorbate reductase, dehydroascorbate reductase, and homoglutathione reductase. Homoglutathione is synthesized in the cytosol and taken up by the mitochondria and bacteroids. Although bacteroids synthesize glutathione, it is not exported to the plant in significant amounts. We propose a model for the detoxification of peroxides in nodule mitochondria in which membrane-bound ascorbate peroxidase scavenges the peroxide formed by the electron transport chain using ascorbate provided by L-galactono-1,4-lactone dehydrogenase in the inner membrane. The resulting monodehydroascorbate and dehydroascorbate can be recycled in the matrix or cytosol. In the matrix, the peroxides formed by oxidative reactions and by MnSOD may be scavenged by specific isozymes of guaiacol peroxidase, ascorbate peroxidase, and catalase.     

Antioxidants and Manganese Deficiency in Needles of Norway Spruce (Picea abies L.) Trees

Chlorotic and green needles from Norway spruce (Picea abies L.) trees were sampled in the Calcareous Bavarian Alps in winter. The needles were used for analysis of the mineral and pigment contents, the levels of antioxidants (ascorbate, glutathione), and the activities of protective enzymes (superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate radical reductase, dehydroascorbate reductase, glutathione reductase). In addition, the activities of two respiratory enzymes (glucose-6-phosphate dehydrogenase, NAD-malate dehydrogenase), which might provide the NADPH necessary for functioning of the antioxidative system, were determined. We found that chlorotic needles were severely manganese deficient (3 to 6 micrograms Mn per gram dry weight as compared with up to 190 micrograms Mn per gram dry weight in green needles) but had a similar dry weight to fresh weight ratio, had a similar protein content, and showed no evidence for enhanced lipid peroxidation as compared with green needles. In chlorotic needles, the level of total ascorbate and the activities of superoxide dismutase, monodehydroascorbate radical reductase, NAD-malate dehydrogenase, and glucose-6-phosphate dehydrogenase were significantly increased, whereas the levels of ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase, and glutathione were not affected. The ratio of ascorbate to dehydroascorbate was similar in both green and chlorotic needles. These results suggest that in spruce needles monodehydroascorbate radical reductase is the key enzyme involved in maintaining ascorbate in its reduced state. The reductant necessary for this process may have been supplied at the expense of photosynthate.     

Effect of short-term salt stress on oxidative stress markers and antioxidant enzymes activity in tocopherol-deficient Arabidopsis thaliana plants

Changes of carotenoids and anthocyanins content, lipid peroxidation, and activity of antioxidant enzymes were studied in wild type and tocopherol-deficient lines vte1 and vte4 of Arabidopsis thaliana subjected to 200 mM NaCI during 24 h. The salt stress enhanced the intensity of lipid peroxidation to different extent in all three plant lines. Salt stress resulted in an increase of carotenoid content and activity of catalase, ascorbate peroxidase, guaiacol peroxidase and glutathione reductase in wild type and tocopherol-deficient vte1 mutant. However, the increase in anthocyanins concentration was observed in vte1 mutants only. In vte4 mutant, which contain gamma-tocopherol instead of alpha-tocopherol, the response to salt stress occurred via coordinative action of superoxide dismutase and enzymes of ascorbate-glutathione cycle, in particular, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and glutathione-S-transferase. It can be concluded, that salt stress was accompanied by oxidative stress in three studied lines, however different mechanisms involved in adaptation of wild type and tocopherol-deficient lines to salt stress.     

Cadmium and copper induction of oxidative stress and antioxidative response in tomato (<Emphasis Type="Italic">Solanum lycopersicon</Emphasis>) leaves

We compare cadmium and copper induced oxidative stress in tomato leaves and the antioxidative enzyme response during a time course of 96 h. Plants were subjected to 25 μM of CdCl₂ or CuSO₄ and malondialdehyde (MDA) level and activity of guaiacol peroxidase, superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase were determined. The results showed that there was an early increase in the MDA level and in the guaiacol peroxidase activity more pronounced with copper exposure during almost all the time course of the experiment. The activity of superoxide dismutase and catalase was induced very early after cadmium and copper treatment, reached a maximal value after 12 h and then declined but it remained always slightly higher than the control at the end of the experiment. Ascorbate peroxidase activity pathway was similar to superoxide dismutase or catalase with a maximal activity after 48 h of heavy metal exposure. Induction of glutathione reductase activity observed only under copper exposure is maintained during almost all the experimental time. The antioxidative activity developed by tomato leaves is more induced by copper treatment. This can be related to the ability of this metal to induce more than cadmium an accumulation of reactive oxygen species (ROS) at the cellular level. Decline in the antioxidative enzymes activity at the end of the experiment can be a consequence of cadmium- and copper-inducing a further ROS formation that might affect enzymes activity.     

Differential accumulation of antioxidant mRNAs in Arabidopsis thaliana exposed to ozone.

Antioxidant isoenzymes function to eliminate free radicals and are localized to several different subcellular compartments within the plant cell. In Arabidopsis thaliana exposed to ozone (O3), we have monitored the accumulation of mRNAs encoding both cytosolic and chloroplastic antioxidant isoenzymes. Two different O3 exposure protocols yielded similar results. Upon O3 exposure, the steady-state levels of three mRNAs encoding cytosolic antioxidant isoenzymes (ascorbate peroxidase, copper/zinc superoxide dismutase, and glutathione S-transferase) increase. The glutathione S-transferase mRNA responds very quickly to the oxidative stress (2-fold increase in 30 min) and is elevated to very high levels, especially in plants grown with a 16-h photoperiod. In contrast, O3 exposure causes a decline in the levels of two chloroplastic antioxidant mRNAs (iron superoxide dismutase and glutathione reductase) and two photosynthetic protein mRNAs (chlorophyll a/b-binding protein and ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit). We show that this decline does not include all mRNAs encoding chloroplast-targeted proteins, since O3 causes an elevation of mRNA encoding the chloroplast-localized tryptophan biosynthetic enzyme phosphoribosylanthranilate transferase. Two alternative hypotheses that could explain this differential mRNA accumulation in response to O3 are discussed.     

Enhancement of the Tolerance to Oxidative Stress in Cucumber (Cucumis sativus L.) Seedlings by UV-B Irradiation: Possible Involvement of Phenolic Compounds and Antioxidative Enzymes

Cucumis sativus L.) seedlings were irradiated or not irradiated with UV-B for several days in environment-controlled growth chambers. The first leaves irradiated with UV-B were retarded in growth but simultaneously acquired a remarkably high tolerance to oxidative stress, as induced by paraquat treatment, compared with the non-irradiated leaves. This enhanced tolerance was observed within 1d after the start of UV-B irradiation and was maintained during the 12 d period of UV-B treatment. The effects of UV-B on several antioxidative enzymes were examined, and activities of superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase, but not of glutathione reductase, were found to be enhanced. However, activation of these enzymes occurred only from 6 d after the start of irradiation. In contrast, accumulation of phenolic compounds was observed within 1d after the start of UV-B irradiation. HPLC analysis of phenolic compounds showed the distinct enhancement of a substance, which may have antioxidative properties in cucumber seedlings irradiated with UV-B. On the basis of these results, we conclude that not only antioxidative enzymes but also other factors in cucumber seedlings irradiated with UV-B, such as phenolic compounds, may participate in the enhanced tolerance to oxidative stress. Received 10 June 2000/ Accepted in revised form 1 July 2000     

Up-regulation of the leaf mitochondrial and peroxisomal antioxidative systems in response to salt-induced oxidative stress in the wild salt-tolerant tomato species Lycopersicon pennellii

The response of the antioxidative systems of leaf cell mitochondria and peroxisomes of the cultivated tomato Lycopersicon esculentum (Lem) and its wild salt-tolerant related species Lycopersicon pennellii (Lpa) to NaCl 100 mM stress was investigated. Salt-dependent oxidative stress was evident in Lem mitochondria as indicated by their raised levels of lipid peroxidation and H2O2 content whereas their reduced ascorbate and reduced glutathione contents decreased. Concomitantly, SOD activity decreased whereas APX and GPX activities remained at control level. In contrast, the mitochondria of salt-treated Lpa did not exhibit salt-induced oxidative stress. In their case salinity induced an increase in the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione-dependent peroxidase (GPX). Lpa peroxisomes exhibited increased SOD, APX, MDHAR and catalase activity and their lipid peroxidation and H2O2 levels were not affected by the salt treatment. The activities of all these enzymes remained at control level in peroxisomes of salt-treated Lem plants. The salt-induced increase in the antioxidant enzyme activities in the Lpa plants conferred cross-tolerance towards enhanced mitochondrial and peroxisomal reactive oxygen species production imposed by salicylhydroxamic acid (SHAM) and 3-amino-1,2,4-triazole (3-AT), respectively.     

植物谷胱甘肽过氧化物酶研究进展

氧化胁迫可诱导植物多种防御酶的产生, 其中包括超氧化物歧化酶(SOD, EC1.15.1.1)、抗坏血酸过氧化物酶(APX, EC1.11.1.11)、过氧化氢酶(CAT, E.C.1.11.1.6 )和谷胱甘肽过氧化物酶(GPXs,EC1.11.1.9)。它们在清除活性氧过程中起着不同的作用。GPXs是动物体内清除氧自由基的主要酶类,但它在植物中的功能报道甚少。最近几年研究表明, 植物体内也存在类似于哺乳动物的GPXs家族, 并对其功能研究已初见端倪。本文综述了有关GPXs的结构以及植物GPXs功能的研究进展。     

Salinity up-regulates the antioxidative system in root mitochondria and peroxisomes of the wild salt-tolerant tomato species Lycopersicon pennellii

The effect of salinity on the antioxidative system of root mitochondria and peroxisomes of a cultivated tomato Lycopersicon esculentum (Lem) and its wild salt-tolerant related species L. pennellii (Lpa) was studied. Salt stress induced oxidative stress in Lem mitochondria, as indicated by the increased levels of lipid peroxidation and H(2)O(2). These changes were associated with decreased activities of superoxide dismutase (SOD) and guaiacol peroxidases (POD) and contents of ascorbate (ASC) and glutathione (GSH). By contrast, in mitochondria of salt-treated Lpa plants both H(2)O(2) and lipid peroxidation levels decreased while the levels of ASC and GSH and activities of SOD, several isoforms of ascorbate peroxidase (APX), and POD increased. Similarly to mitochondria, peroxisomes isolated from roots of salt-treated Lpa plants exhibited also decreased levels of lipid peroxidation and H(2)O(2) and increased SOD, ascorbate peroxidase (APX), and catalase (CAT) activities. In spite of the fact that salt stress decreased activities of antioxidant enzymes in Lem peroxisome, oxidative stress was not evident in these organelles.     

Antioxidant response of wheat roots to drought acclimation

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. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H₂O₂ and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate–glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H₂O₂ followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate–glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.