Antioxidative Defense Potential to Salinity in the Euhalophyte Salicornia brachiata

The antioxidative defense mechanism to salinity was assessed by monitoring the activities of some antioxidative enzymes and levels of antioxidants in an obligate halophyte, Salicornia brachiata, subjected to varying levels of NaCl (0, 200, 400, and 600 mM) under hydroponic culture. In the shoots of S. brachiata, salt treatment preferentially enhanced the activities of ascorbate peroxidase (APX), guaiacol peroxidase (POX), glutathione reductase (GR), and superoxide dismutase (SOD), whereas it induced the decrease of catalase (CAT) activity. Similarly, salinity caused an increase in total glutathione content (GSH + GSSG) and a decrease in total ascorbate content. Growth of S. brachiata was optimum at 200 mM NaCl and decreased with further increase in salinity. Salinity caused an increase in Na⁺ content and a decrease in K⁺ content of shoots. Proline levels did not change at low (0-200 mM NaCl) or moderate (400 mM NaCl) salinities, whereas a significant increase in proline level was observed at high salinity (600 mM NaCl). Accumulation of Na⁺ may have a certain role in osmotic homeostasis under low and moderate salinities in S. brachiata. Parameters of oxidative stress such as malondialdehyde (MDA), a product of lipid peroxidation, and H₂O₂ concentrations decreased at low salinity (200 mM NaCl) and increased at moderate (400 mM NaCl) and high salinities (600 mM NaCl). As a whole, our results suggest that the capacity to limit ionic and oxidative damage by the elevated levels of certain antioxidative enzymes and antioxidant molecules is important for salt tolerance of S. brachiata.     

Alleviation of osmotic stress in Brassica napus,B. campestris,and B. juncea by ascorbic acid application

The roles of ascorbic acid (AsA, 1 mM) under an osmotic stress [induced by 15 % (m/v) polyethylene glycol, PEG-6000] were investigated by examining morphological and physiological attributes in Brassica species. The osmotic stress reduced the fresh and dry masses, leaf relative water content (RWC), and chlorophyll (Chl) content, whereas increased the proline (Pro), malondialdehyde (MDA), and H₂O₂ content, and lipoxygenase (LOX) activity. The ascorbate content in B. napus, B. campestris, and B. juncea decreased, increased, and remained unaltered, respectively. The dehydroascorbate (DHA) content increased only in B. napus. The AsA/DHA ratio was reduced by the osmotic stress in all the species except B. juncea. The osmotic stress increased the glutathione (GSH) content only in B. juncea, but increased the glutathione disulfide (GSSG) content and decreased the GSH/GSSG ratio in all the species. The osmotic stress increased the activities of ascorbate peroxidase (APX) (except in B. napus), glutathione reductase (GR) (except in B. napus), glutathione S-transferase (GST) (except in B. juncea), and glutathione peroxidase (GPX), and decreased the activities of catalase (CAT) and monodehydroascorbate reductase (MDHAR) (only in B. campestris). The osmotic stress decreased the glyoxalase I (Gly I) and increased glyoxalase II (Gly II) activities. The application of AsA in combination with PEG improved the fresh mass, RWC, and Chl content, whereas decreased the Pro, MDA, and H₂O₂ content in comparison with PEG alone. The AsA addition improved AsA-GSH cycle components and improved the activities of all antioxidant and glyoxalase enzymes in most of the cases. So, exogenous AsA improved physiological adaptation and alleviated oxidative damage under the osmotic stress by improving the antioxidant and glyoxalase systems. According to measured parameters, B. juncea can be recognized as more drought tolerant than B. napus and B. campestris.     

Arsenite treatment induces oxidative stress,upregulates antioxidant system,and causes phytochelatin synthesis in rice seedlings

The effects of arsenite treatment on generation of reactive oxygen species, induction of oxidative stress, response of antioxidative system, and synthesis of phytochelatins were investigated in two indica rice (Oryza sativa L.) cvs. Malviya-36 and Pant-12 grown in sand cultures for a period of 5–20 days. Arsenite (As₂O₃; 25 and 50 μM) treatment resulted in increased formation of superoxide anion (O₂^.−), elevated levels of H₂O₂ and thiobarbituric acid reactive substances, showing enhanced lipid peroxidation. An enhanced level of ascorbate (AA) and glutathione (GSH) was observed irrespective of the variation in the level of dehydroascorbate (DHA) and oxidized glutathione (GSSG) which in turn influenced redox ratios AA/DHA and GSH/GSSG. With progressive arsenite treatment, synthesis of total acid soluble thiols and phytochelatins (PC) increased in the seedlings. Among antioxidative enzymes, the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), total ascorbate peroxidase (APX, EC 1.11.1.11), chloroplastic ascorbate peroxidase, guaiacol peroxidase (EC 1.11.1.7), monodehydroascorbate reductase (EC 1.6.5.4), and glutathione reductase (EC 1.6.4.2) increased in arsenite treated seedlings, while dehyroascorbate reductase (EC 1.8.5.1) activity declined initially during 5–10 days and increased thereafter. Results suggest that arsenite treatment causes oxidative stress in rice seedlings, increases the levels of many enzymatic and non-enzymatic antioxidants, and induces synthesis of thiols and PCs, which may serve as important components in mitigating arsenite-induced oxidative damage.     

Production of reactive oxygen species and development of antioxidative systems during <Emphasis Type="Italic">in vitro</Emphasis> growth and <Emphasis Type="Italic">ex vitro</Emphasis> transfer

Ex vitro transfer is often stressful for in vitro grown plantlets. Water stress and photoinhibition, often accompanying the acclimatization of in vitro grown plantlets to ex vitro conditions, are probably the main factors promoting production of reactive oxygen species (ROS) and in consequence oxidative stress. The extent of the damaging effects of ROS depends on the effectiveness of the antioxidative systems which include low molecular mass antioxidants (ascorbate, glutathione, tocopherols, carotenoids, phenols) and antioxidative enzymes (superoxide dismutase, ascorbate peroxidase, catalase, glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase). This review is focused on ROS production and development of antioxidative system during in vitro growth and their further changes during ex vitro transfer.     

Oxidative stress responses and shock proteins in the unicellular cyanobacterium Synechococcus R2 (PCC-7942)

Oxidative stress responses were tested in the unicellular cyanobacterium Synechococcus PCC 7942 (R2). Cells were exposed to hydrogen peroxide, cumene hydroperoxide and high light intensities. Activities of ascorbate peroxidase and catalase were correlated with the extent and time-course of oxidative stresses. Ascorbate peroxidase was found to be the major enzyme involved in the removal of hydrogen peroxide under the tested oxidative stresses. Catalase activity was inhibited in cells treated with high H₂O₂ concentrations, and was not induced under photo-oxidative stress. Regeneration of ascorbate in peroxide-treated cells was found to involve mainly monodehydroascorbate reductase and to a lesser extent dehydroascorbate reductase. The induction of the antioxidative enzymes was dependent on light and was inhibited by chloramphenicol. Peroxide treatment was found to induce the synthesis of eight proteins, four of which were also induced by heat shock.Abbreviations ASC ascorbate - DHA dehydroascorbate - MDA monodehydroascorbate - GSH reduced glutathione - GSSG oxidized glutathione - ASC Per ascorbate peroxidase - DHA red. dehydroascorbate reductase - MDA red. monodehydroascorbate reductase - GSSG red. glutathione reductase - HSP heat shock proteins - PSP peroxide shock proteins - C_m chloramphenicol     

Exogenous Hydrogen Peroxide Changes Antioxidant Enzyme Activity and Protects Ultrastructure in Leaves of Two Cucumber Ecotypes Under Osmotic Stress

Cucumber (Cucumis sativus L.) varieties cv. Jinchun no. 4 (a North China ecotype) and cv. Lvfeng no. 6 (a South China ecotype) were cultivated to explore the effects of osmotic stress on the ultrastructure of chloroplasts and mitochondria, as well as to assess the possible protective effect of exogenous hydrogen peroxide (H₂O₂). Under osmotic stress induced by 10% polyethylene glycol 6000, 84.3% of the chloroplasts in Jinchun no. 4 were abnormal, whereas 88.6% were abnormal in Lvfeng no. 6. Abnormal mitochondria occurred in these two strains at rates of 78.5 and 87.1%, respectively. The stress condition disintegrated the membranes of most chloroplasts and mitochondria in the leaf cells of both cucumber ecotypes, and it also increased the malondialdehyde (MDA) content. We subjected the two cultivars to a combined treatment with H₂O₂ and osmotic stress and made the following observations: (1) Abnormal chloroplasts occurred at rates of 25.7 and 28.6%, and abnormal mitochondria were observed at rates of 22.9 and 32.8%, respectively. (2) Most of the investigated membranes were well organized in leaves of Jinchun no. 4 and Lvfeng no. 6, and the levels of endogenous H₂O₂, superoxide anion, and MDA were lower. Osmotic stress and exogenous H₂O₂ both increased the activities of antioxidative enzymes such as manganese superoxide dismutase, glutathione peroxidase, catalase, guaiacol peroxidase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase, and the antioxidants ascorbate and reduced glutathione. The combined effect of osmotic stress and exogenous H₂O₂ resulted in the highest antioxidant activities in both cucumber ecotypes. We propose that exogenous H₂O₂ increases antioxidant activity in cucumber leaves and thereby decreases lipid peroxidation to some extent, thus protecting the ultrastructure of most chloroplasts and mitochondria under osmotic stress.     

Exogenous Nitric Oxide (as Sodium Nitroprusside) Ameliorates Polyethylene Glycol-Induced Osmotic Stress in Hydroponically Grown Maize Roots

The present study was designed to examine whether exogenous sodium nitroprusside (SNP) supplementation has any ameliorating action against PEG-induced osmotic stress in Zea mays cv. FRB-73 roots. Twenty percent or 40 % polyethylene glycol (PEG6000; ?0.5 MPa and ?1.76 MPa, respectively) treatment alone or in combination with 150 and 300 μM SNP was applied to hydroponically grown maize roots for 72 h. Although only catalase (CAT) activity increased when maize roots were exposed to PEG-induced osmotic stress, induction of this antioxidant enzyme was inadequate to detoxify the extreme levels of reactive oxygen species, as evidenced by growth, water content, superoxide anion radical (O ₂ ^?? ), hydroxyl radical (OH^?) scavenging activity, and TBARS content. However, supplementation of PEG-exposed specimens with SNP significantly alleviated stress-induced damage through effective water management and enhancement of antioxidant defense markers including the enzymatic/non-enzymatic systems. Exogenously applied SNP under stress resulted in the up-regulation of glutathione peroxidase (GPX), glutathione S-transferase (GST), ascorbate peroxidase (APX), glutathione reductase (GR), total ascorbate, and glutathione contents involved in ascorbate–glutathione cycle. On the other hand, growth rate, osmotic potential, CAT, APX, GR, and GPX increased in maize roots exposed to both concentrations of SNP alone, but activities of monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase decreased. Based on the above results, an exogenous supply of both 150 and 300 μM SNP to maize roots was protective for PEG-induced toxicity. The present study provides new insights into the mechanisms of SNP (NO donor) amelioration of PEG-induced osmotic stress damages in hydroponically grown maize roots.     

Biphasic effect of copper on the ascorbate-glutathione pathway in primary leaves of Phaseolus vulgaris seedlings during the early stages of metal assimilation

Copper‐imposed oxidative stress and antioxidative defence responses were investigated in the primary leaves of Phaseolus vulgaris L. plants grown on hydroponics containing 50 μM CuSO₄. Copper mainly accumulates in roots; therefore, an increase of the copper content in the leaves was only observed 48 h after the start of the copper supply. Nevertheless, an increase of the thiobarbituric acid reactive metabolites (TBArm) content, an indication of stress, occurred immediately following copper application. Because the ascorbate‐glutathione pathway is considered as a major antioxidative defence mechanism, the evolution of the enzymes and the related metabolites involved in this pathway were studied in the primary leaves as a function of plant copper assimilation. The capacities of monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were increased before elevated amounts of copper could be detected in the leaves. The early enhancement of glutathione reductase was only temporary. After copper accumulation in the leaves, a second increase of the glutathione reductase capacity and also an increase of the ascorbate peroxidase capacity (EC 1.11.1.11) were observed. These changes in enzymatic capacity modified the level of the metabolites involved. Increase of the ascorbate pool and maintenance in its reduced form was observed immediately after the start of the treatment. In the beginning of the experiment, the glutathione disulphide/reduced glutathione ratio was higher in the treated plants as compared to the controls. However, towards the end of the experiment, the total glutathione pool, as well as the reduced glutathione content, increased, resulting in a lower ratio value for the treated plants. In conlusion, copper‐imposed oxidative stress, as well as the antioxidative defence response in the leaves, appears to be biphasic. An indirect preventive effect on the antioxidative defence system was observed during the first phase before the leaf copper content increased. A root‐to‐shoot signalling system appears to be involved. Direct oxidation by copper of reduced cell metabolites occurred during the second phase when the leaf copper content was enhanced.     

Effects of iso-osmotic NaCl and mannitol on growth, proline content, and antioxidant defense in Mammillaria gracilis Pfeiff. in vitro-grown cultures

Effects of iso-osmotic concentrations of NaCl and mannitol were studied in Mammilaria gracilis (Cactaceae) in both calli and tumors grown in vitro. In both tissues, relative growth rates were reduced under osmotic stress, which were accompanied by a decrease in both tissue water and K⁺ content. However, growth was inhibited to a lesser extent after exposure to NaCl, when accumulation of Na⁺ ions was observed. In calli, only salinity increased proline content, whereas with tumors proline accumulated after both osmotic stresses. Osmotic stresses also induced oxidative damage in both cactus tissues, although higher oxidative injury was caused by mannitol in calli and by salt in tumors. Low iso-osmotic concentrations of NaCl (75 mM) and mannitol (150 mM) increased peroxidase, ascorbate peroxidase, and esterase activities, whereas elevated catalase activity was recorded only after mannitol treatment in both tissues. High osmotic stress generally decreased enzymatic activities. However, in calli, esterase activity increased in response to high salinity, whereas ascorbate peroxidase activity was enhanced after high mannitol stress. In conclusion, both in vitro-grown cactus tissues were found to be sensitive to osmotic stress caused by either mannitol or NaCl, but accumulation of Na⁺ ions in response to salt somewhat contributed to osmotic adjustment. However, more prominent oxidative damage induced by NaCl compared to mannitol in tumor could be related to ion toxicity. The mechanisms that mediate responses to salt- and mannitol-induced osmotic stresses differed and were dependent on tissue type.     

ANTIOXIDATIVE PROTECTION OF PERIDINIUM GATUNENSE IN LAKE KINNERET: SEASONAL AND DAILY VARIATION1

A comprehensive antioxidative mechanism was found in the freshwater dinoflagellate Peridinium gatunense Lemm. during the spring bloom in Lake Kinneret. Ascorbate was present throughout the bloom period and was responsible, together with catalase, for the elimination of photosynthetically produced H₂O₂. As glutathione concentrations and ascorbate regenerative enzymes were negligible during mid-spring, ascorbate was presumably biosynthesized during the photosynthetically active period. Antioxidative activity increased overall at the end of the spring in conjunction with elevated ambient stress conditions, for example high light. Under such circumstances, ascorbate was regenerated. Ascorbate levels doubled when cells were exposed to an increase in irradiance from 60 to 600 μmol photons·m^?2·s^?1, and on addition of H₂O₂, concentrations increased a further 20-fold. Significant antioxidative activity was also noted in the dark, although this was dependent on the presence of H₂O₂. Diurnal changes in antioxidants and their regenerative enzymes were observed. The activities of mono-dehydroascorbate reductase, glutathione reductase, and ascorbate concentrations showed ultraradian periodicity and were completely in phase throughout the day/night period. Dehydroascorbate reductase activity and glutathione concentrations were also in phase but showed aperiodic variation, as did ascorbate peroxidase activity. Superoxide dismutase and catalase activities were generally out of phase during the 24-h period but did show ultraradian periodicity. Lake samples entrained under constant light revealed an inate 12-h rhythm for catalase activity, during at least 36 h.     

Exogenous salicylic acid alleviates NaCl toxicity and increases antioxidative enzyme activity in <Emphasis Type="Italic">Lycopersicon esculentum</Emphasis>

Effects of exogenous salicylic acid (SA) on plant growth, contents of Na, K, Ca and Mg, activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and catalase (CAT), and contents of ascorbate and glutathione were investigated in tomato (Lycopersicon esculentum L.) plants treated with 100 mM NaCl. NaCl treatment significantly increased H₂O₂ content and lipid peroxidation indicated by accumulation of thiobarbituric acid reactive substances (TBARS). A foliar spray of 1 mM SA significantly decreased lipid peroxidation caused by NaCl and improved the plant growth. This alleviation of NaCl toxicity by SA was related to decreases in Na contents, increases in K and Mg contents in shoots and roots, and increases in the activities of SOD, CAT, GPX and DHAR and the contents of ascorbate and glutathione.     

Ascorbate deficient semi-dwarf asfL1 mutant of Lathyrus sativus exhibits alterations in antioxidant defense

An ascorbate-deficient semi-dwarf mutant asfL-1 was detected in 250 Gy γ-ray treated grass pea (Lathyrus sativus L.) cv. BioR-231. The mutant contained only 42 % of leaf and 20 % of root ascorbate content of mother control (MC). I investigated the possible causes of ascorbate deficiency and its effect on growth and antioxidant defense in control and 150 mM NaCl-treated seedling after 60 d growth period. Ascorbate deficiency was due to significant reduction in activities of monodehydroascorbate reductase and dehydroascorbate reductase as well as increase in ascorbate oxidase, leading to considerable decrease in redox state. Despite low ascorbate pool and decrease in ascorbate peroxidase activity, shoot and root biomass production in asfL-1 mutant were similar to MC plants, even at NaCl treatment. High accumulation of glutathione (GSH) coupled with high activities of GSH reductase, catalase, GSH peroxidase and peroxidase in both tissues of the mutant permitted efficient recycling of GSH and scavenging of H₂O₂ through well integrated catalase/peroxidase system, despite high superoxide dismutase activity under NaCl treatment. The collapse of this system led to inhibition of growth in NaCl-treated mother plants. Together, the results suggested that asfL-1 plants undertook a major reshuffle in its antioxidant defense machinery, which effectively counterbalanced the negative impact of ascorbate deficiency and remained unperturbed by NaCl treatment to maintain normal growth and biomass production.     

Reactive oxygen species, ascorbate–glutathione pool, and enzymes of their metabolism in drought-sensitive and tolerant indica rice (Oryza sativa L.) seedlings subjected to progressing levels of water deficit

Water deficit for rice is a worldwide concern, and to produce drought-tolerant varieties, it is essential to elucidate molecular mechanisms associated with water deficit tolerance. In the present study, we investigated the differential responses of nonenzymatic antioxidants ascorbate (AsA), glutathione (GSH), and their redox pool as well as activity levels of enzymes of ascorbate–glutathione cycle in seedlings of drought-sensitive rice (Oryza sativa L.) cv. Malviya-36 and drought-tolerant cv. Brown Gora subjected to water deficit treatment of ?1.0 and ?2.1 MPa for 24–72 h using PEG-6000 in sand cultures. Water deficit caused increased production of reactive oxygen species such as O₂?^?, H₂O₂, and HO? in the tissues, and the level of production was higher in the sensitive than the tolerant cultivar. Water deficit caused reduction in AsA and GSH and decline in their redox ratios (AsA/DHA and GSH/GSSG) with lesser decline in tolerant than the sensitive seedlings. With progressive level of water deficit, the activities of monodehydroascorbate reductase, dehydroascorbate reductase, ascorbate peroxidase (APX), and glutathione transferase increased in the seedlings of both rice cultivars, but the increased activity levels were higher in the seedlings of drought-tolerant cv. Brown Gora compared to the sensitive cv. Malviya-36. Greater accumulation of proline was observed in stressed seedlings of tolerant than the sensitive cultivar. In-gel activity staining of APX revealed varying numbers of their isoforms and their differential expression in sensitive and tolerant seedlings under water deficit. Results suggest that an enhanced oxidative stress tolerance by a well-coordinated cellular redox state of ascorbate and glutathione in reduced forms and induction of antioxidant defense system by elevated activity levels of enzymes of ascorbate–glutathione cycle is associated with water deficit tolerance in rice.     

Ammonium-induced proline and sucrose accumulation, and their significance in antioxidative activity and osmotic adjustment

Excess of ammonia generates oxidative and osmotic stress, and results in an accumulation of compatible solutes. The aim of this study was to investigate the physiological significance of excess ammonium-induced proline and sucrose accumulation on antioxidative activity and osmotic adjustment. The detached leaves of white clover (Trifolium repense L.) were fed with 0, 10, 50, 100, and 200 mM NH₄Cl, and the contribution of proline and sucrose to osmotic adjustment and their relationship with antioxidative enzymes activity were assessed. A gradual decline of relative water content and osmotic potential (Ψ_π) with increasing NH₄Cl feeding level was accompanied by an increase in ammonia concentration. Significant accumulation of proline and sucrose was observed when NH₄Cl was fed over 100 mM compared with control (0 mM NH₄Cl). The increase in enzyme activity was significant only at 200 mM for ascorbate peroxidase (APOD) and over 100 mM NH₄Cl for guaiacol peroxidase (GPOD) and catalase (CAT). The contribution of proline and sucrose to osmotic adjustment over 100 mM, where proline and sucrose accumulation was more important, maintained at control levels or significantly decreased. The content of proline and sucrose as affected by NH₄Cl feeding level was positively related with the activity of APOD, GPOD, and CAT. These results suggest that proline and sucrose accumulation induced by the excess of ammonium has a more influential role in antioxidative activity rather than osmotic adjustment.     

Spermidine pretreatment enhances heat tolerance in rice seedlings through modulating antioxidative and glyoxalase systems

This study was undertaken to investigate the possible involvement of the antioxidant defense and glyoxalase systems in protecting rice seedlings from heat-induced damage in the presence of spermidine (Spd). Hydroponically grown 14-day-old seedlings were subjected to foliar spray with Spd (1 mM, 24 h) prior to heat stress (42 °C, 48 h) followed by subsequent recovery (27 °C, 48 h). Lipoxygenase activity, malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and proline (Pro) content increased significantly whereas fresh weight (FW) and chlorophyll (Chl) content decreased during heat stress and after recovery, indicating unrecoverable damage to rice seedlings. Heat-induced damage was also evident in decreased levels of ascorbate (AsA), glutathione (GSH), and AsA and GSH redox ratios. Superoxide dismutase (SOD) and catalase (CAT) activities increased during heat stress but declined after recovery. Activities of glutathione peroxidase (GPX), ascorbate peroxidase (APX), monodehydroascorbate reductase, dehydroascorbate reductase (DHAR) and glutathione reductase (GR) decreased during heat stress but an opposite trend for most of these enzymes was observed after recovery. Heat stress also resulted in significant increases in the activities of glyoxalase enzymes (Gly I and Gly II). In contrast, exogenous Spd protected rice seedlings from heat-induced damage as marked by lower levels of MDA, H₂O₂, and Pro content coupled with increased levels of AsA, GSH, FW, Chl, and AsA and GSH redox status. After recovery, Spd-pretreated heat-exposed seedlings displayed higher activities of SOD, CAT, GPX, GST APX, DHAR and GR as well as of Gly I and Gly II. In addition, polyamine analysis revealed that exogenously applied Spd significantly elevated the levels of free and soluble conjugated Spd. Therefore, we conclude from our results that heat exposure provoked an oxidative burden while enhancement of the antioxidative and glyoxalase systems by Spd rendered rice seedlings more tolerant to heat stress. Further, co-induction of the antioxidative and glyoxalase systems was closely associated with Spd mediated enhanced level of GSH.     

Suitability of the antioxidative system as marker of magnesium deficiencyin <Emphasis Type="Italic">Capsicum annuum</Emphasis> L. plants under controlled conditions

To explore the viability of using enzyme activities and their substrates as an alternative tool for the determination of mineral (i.e., Mg) critical values, a detailed characterization of the response of the antioxidative system of Capsicum annuum L. leaves under Mg deficiency was carried out. The response of each selected enzyme activity and substrate [i.e., superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR); ascorbate and glutathione pool; protein and chlorophyll concentration] was subjected to mathematical modelling in order to calculate Mg critical values (x_c). Our x_c values ranged from 0.70 to 0.14%, on a dry weight (DW) basis, for GR activity and total glutathione concentration, respectively. Our results suggest that, under Mg deficiency, cells enhance their antioxidative defence system by initially increasing their SOD and GR activities. Subsequently, higher GSH/GSSG ratios were observed, probably due to a greater increase in GR activity (x_c = 0.70% DW) than in total glutathione concentration (x_c = 0.14% DW). In contrast, x_c values for total ascorbate concentrations (x_c = 0.29% DW) were higher than those for DHAR activities (x_c = 0.19% DW). In an attempt to study the limitations regarding the utilization of these enzymes and substrates as markers of Mg critical values in pepper, the x_c values here obtained were compared to those based on growth parameters that have been reported in the literature. Overall, the results indicate that some enzymes and substrates, such as total ascorbate concentration, 1/protein ratio, and DHAR activity, might be suitable markers for the determination of Mg critical values in pepper plants under controlled conditions.     

Pre treatment with Bacillus subtilis mitigates drought induced photo-oxidative damages in okra by modulating antioxidant system and photochemical activity

Growth promoting potential of Bacillus subtilis (BS) in drought stressed Abelmoschus esculentus (L.) Moench (okra) was assessed by measuring the chlorophyll stability index (CSI), chlorophyll a (Chl-a) fluorescence, leaf osmotic potential and lipid peroxidation by malondialdehyde content, emission of reactive oxygen species (ROS), osmolyte content and the activity of non-enzyme and enzyme antioxidants. BS treatment significantly increased the leaf osmotic potential, osmolyte production and the activity of non-enzyme and enzyme antioxidants under drought stress. BS treatment mitigated the drought-induced reduction in Chl a fluorescence and CSI. Concomitant increase in total sugar, proline, non-enzyme antioxidants [glutathione and ascorbate] and enzyme antioxidants like superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase and dehydroascorbate reductase modulate the intracellular ROS concentration in okra to resist the stress induced oxidative damage in BS treated plants led to fast recovery and less photodamage.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12298-021-00982-8.     

Other factors than apoplastic ascorbate contribute to the differential ozone tolerance of two clones of Trifolium repens L.

Apoplastic reactive oxygen intermediates, which are formed during the exposure of a higher plant to ozone (O₃), have been proposed to be detoxified by apoplastic ascorbate (ASC). An investigation to determine whether the differential sensitivity of two white clover clones (Trifolium repens L. cv Regal) to O₃ was related with their levels of ASC, glutathione derivatives or with the total antioxidative capacity. In contrast to what might be expected, the sensitive clone of white clover (NC-S) constitutively showed a 72% higher concentration of apoplastic ASC compared to the O₃-tolerant clone (NC-R). Furthermore, NC-S also showed a higher redox status of apoplastic ASC. These results indicate that higher ASC levels in the apoplast of NC-S are not sufficient to induce a higher O₃ tolerance. The redox status, but not the absolute concentration of homoglutathione in the symplast was found to be constitutively higher in NC-R than in NC-S. It is not clear, however, whether homoglutathione is a direct cause of the differential O₃ detoxification capacity of both clones. Total antioxidative capacity measurements ruled out the contribution of other low-molecular antioxidants to the relative tolerance of NC-R. It was concluded that elevated apoplastic ASC levels can not always be sufficient to render a plant O₃ tolerant.     

Nitric oxide modulates antioxidant defense and the methylglyoxal detoxification system and reduces salinity-induced damage of wheat seedlings

The present study investigates the possible regulatory role of exogenous nitric oxide (NO) in antioxidant defense and methylglyoxal (MG) detoxification systems of wheat seedlings exposed to salt stress (150 and 300 mM NaCl, 4 days). Seedlings were pre-treated for 24 h with 1 mM sodium nitroprusside, a NO donor, and then subjected to salt stress. The ascorbate (AsA) content decreased significantly with increased salt stress. The amount of reduced glutathione (GSH) and glutathione disulfide (GSSG) and the GSH/GSSG ratio increased with an increase in the level of salt stress. The glutathione S-transferase (GST) activity increased significantly with severe salt stress (300 mM). The ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT) and glutathione peroxidase (GPX) activities did not show significant changes in response to salt stress. The glutathione reductase (GR), glyoxalase I (Gly I), and glyoxalase II (Gly II) activities decreased upon the imposition of salt stress, especially at 300 mM NaCl, with a concomitant increase in the H₂O₂ and lipid peroxidation levels. Exogenous NO pre-treatment of the seedlings had little influence on the non-enzymatic and enzymatic components compared to the seedlings of the untreated control. Further investigation revealed that NO pre-treatment had a synergistic effect; that is, the pre-treatment increased the AsA and GSH content and the GSH/GSSG ratio, as well as the activities of MDHAR, DHAR, GR, GST, GPX, Gly I, and Gly II in most of the seedlings subjected to salt stress. These results suggest that the exogenous application of NO rendered the plants more tolerant to salinity-induced oxidative damage by enhancing their antioxidant defense and MG detoxification systems.     

Nitric oxide alleviates manganese toxicity by preventing oxidative stress in excised rice leaves

In the present study, we have investigated the effects of nitric oxide (NO) on alleviating manganese (Mn)-induced oxidative stress in rice leaves. Exogenous MnCl₂ treatment to excised rice leaves for 24 and 48 h resulted in increased production of H₂O₂ and lipid peroxides, decline in the levels of antioxidants, glutathione and ascorbic acid, and increased activities of antioxidative enzymes, superoxide dismutase, guaiacol peroxidase, catalase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Treatment of rice leaves with 100 μM sodium nitroprusside (SNP), a NO donor, was effective in reducing Mn-induced increased levels of H₂O₂, lipid peroxides and increased activities of antioxidative enzymes. The levels of reduced ascorbate and glutathione were considerably recovered due to SNP treatment. The effect of SNP was reversed by the addition of NO scavenger, 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (c-PTIO) suggesting that ameliorating effect of SNP is due to release of NO. The results indicate that MnCl₂ induces oxidative stress in excised rice leaves, lowers the levels of reduced ascorbate and glutathione, and elevates activities of the key antioxidative enzymes. NO appears to provide a protection to the rice leaves against Mn-induced oxidative stress and that exogenous NO application could be advantageous in combating the deleterious effects of Mn-toxicity in rice plants.