Nitric oxide acts as an antioxidant and delays methyl jasmonate-induced senescence of rice leaves

In the present study, we evaluate the protective effect of nitric oxide (NO) against senescence of rice leaves promoted by methyl jasmonate (MJ). Senescence of rice leaves was determined by the decrease of protein content. MJ treatment resulted in (1) induction of leaf senescence, (2) increase in H₂O₂ and malondialdehyde (MDA) contents, (3) decrease in reduced form glutathione (GSH) and ascorbic acid (AsA) contents, and (4) increase in antioxidative enzyme activities (ascorbate peroxidase, glutathione reductase, peroxidase and catalase). All these MJ effects were reduced by free radical scavengers such as sodium benzoate and GSH. NO donors [N-tert-butyl-α-phenylnitrone (PBN), sodium nitroprusside, 3-morpholinosydonimine, and AsA+NaNO₂] were effective in reducing MJ-induced leaf senescence. PBN prevented MJ-induced increase in the contents of H₂O₂ and MDA, decrease in the contents of GSH and AsA, and increase in the activities of antioxidative enzymes. The protective effect of PBN on MJ-promoted senescence, MJ-increased H₂O₂ content and lipid peroxidation, MJ-decreased GSH and AsA, and MJ-increased antioxidative enzyme activities was reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide, a NO-specific scavenger, suggesting that the protective effect of PBN is attributable to NO released. Reduction of MJ-induced senescence by NO in rice leaves is most likely mediated through its ability to scavenge active oxygen species including H₂O₂     

The Effects of Exogenous Ascorbic Acid on the Mechanism of Physiological and Biochemical Responses to Nitrate Uptake in Two Rice Cultivars (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) Under Aluminum Stress

As a major antioxidant in plants, ascorbic acid (AsA) plays a very important role in the response to aluminum (Al) stress. However, the effect of AsA on the mitigation of Al toxicity and the mechanism of nitrate nitrogen (NO₃ ^?–N) uptake by plants under Al stress are unclear. In this study, a hydroponic experiment was conducted using peak 1 A rice (sterile line, Indica) with weaker resistance to Al and peak 1 superior 5 rice (F1 hybrid, Indica) with stronger resistance to Al to study the effects of exogenous AsA on the physiological and biochemical responses to NO₃ ^?–N uptake by rice roots exposed to 50 μmol L^?1 Al. Al stress induced increases in the concentrations of H₂O₂ and malondialdehyde (MDA) and in the activities of antioxidant enzymes [such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX)]. Plasma membrane (PM) H⁺-ATPase and H⁺-pump activities, endogenous AsA content and NO₃ ^?–N uptake in rice roots decreased under Al stress. After treatment with 2 mmol L^?1 exogenous AsA combined with Al, concentrations of H₂O₂ and MDA in roots notably decreased, and endogenous AsA content and activities of SOD, POD, CAT, and APX in rice roots increased significantly; furthermore, the interaction of PM H⁺-ATPase and the 14-3-3 protein was also enhanced significantly compared with that in control plants without AsA treatment, which clearly increased NO₃ ^?–N uptake. Based on all of these data, the application of AsA significantly reduced the accumulation of H₂O₂ and MDA and increased the activities of PM H⁺-ATPase and the H⁺-pump by increasing the endogenous AsA content, the antioxidant enzyme activities, and the interaction of PM H⁺-ATPase and the 14-3-3 protein in the roots of the two rice cultivars under Al stress, thereby improving the uptake of NO₃ ^?–N in rice.     

Aluminum-inhibited root growth of rice seedlings is mediated through putrescine accumulation

The effects of AlCl₃ on growth and polyamine levels of rice roots were investigated. When rice roots were treated with AlCl₃, root growth was markedly inhibited. AlCl₃ treatment resulted in a higher putrescine content and lower spermidine and spermine contents in rice roots. d-Argnine and α-methylornithine, inhibitors of putrescine biosynthesis, caused a reduced content of putrescine in rice roots under Al stress. AlCl₃ treatment also resulted in a decrease in diamine oxidase activity in rice roots. The growth of rice roots in the presence of AlCl₃ was recovered after the addition of d-arginine or α-methylornithine. The protective effect of d-arginine or α-methylornithine in counteracting AlCl₃-inhibited growth of rice roots is unlikely caused by reduction of Al uptake. Furthermore, the effect of the growth recovery in AlCl₃-treated rice roots by d-arginine or α-methylornithine was reversed by the addition of putrescine. Our results strongly suggest that putrescine accumulation is a factor causing growth inhibition of rice roots under Al tress. Evidence is also presented to show that lignification is responsible for putrescine- and AlCl₃-inhibited growth of rice roots.     

Heat shock-induced ascorbic acid accumulation in leaves increases cadmium tolerance of rice (Oryza sativa L.) seedlings

Ascorbic acid (AsA) is the most abundant antioxidant in plants and plays a role in responding to oxidative stress. It has been shown that AsA plays a role in protecting against abiotic stresses. Rice seedlings stressed with 5 μM CdCl₂ showed typical Cd toxicity (chlorosis and increase in malondialdehyde content). Rice seedlings pretreated with heat shock at 45°C (HS) or H₂O₂ under non-HS conditions resulted in the increase in ascorbic acid (AsA) content and the AsA/dehydroascorbate ratio in rice leaves. Exogenous application of AsA or L-galactonone-1, 4-lactone (GalL), a biosynthetic precursor of AsA, under non-HS conditions, which resulted in an increase in AsA content in leaves, enhanced subsequent Cd tolerance of rice seedlings. Pretreatment with imidazole, an inhibitor of NADPH oxidase, under HS conditions significantly decreased H₂O₂ and AsA contents in leaves and reduced subsequent Cd tolerance of rice seedlings. We also observed that pretreatment with lycorine, which is known to inhibit the conversion of GalL to AsA, significantly inhibited HS-induced AsA accumulation in leaves and reduced HS-induced protection against subsequent Cd stress of rice stress. It appears that HS- or H₂O₂-induced protection against subsequent Cd stress of rice seedlings is mediated through AsA. The time-course analyses of HS in rice seedlings demonstrated that the accumulation of H₂O₂ preceded the increase in AsA. Based on the data obtained in this study, it could be concluded that the early accumulation of H₂O₂ during HS signals the increase in AsA content, which in turn protects rice seedlings from oxidative damage caused by Cd.     

Exogenous sodium nitroprusside and glutathione alleviate copper toxicity by reducing copper uptake and oxidative damage in rice (Oryza sativa L.) seedlings

Nitric oxide (NO) and glutathione (GSH) regulate a variety of physiological processes and stress responses; however, their involvement in mitigating Cu toxicity in plants has not been extensively studied. This study investigated the interactive effect of exogenous sodium nitroprusside (SNP) and GSH on Cu homeostasis and Cu-induced oxidative damage in rice seedlings. Hydroponically grown 12-day-old seedlings were subjected to 100 μM CuSO₄ alone and in combination with 200 μM SNP (an NO donor) and 200 μM GSH. Cu exposure for 48 h resulted in toxicity symptoms such as stunted growth, chlorosis, and rolling in leaves. Cu toxicity was also manifested by a sharp increase in lipoxygenase (LOX) activity, lipid peroxidation (MDA), hydrogen peroxide (H₂O₂), proline (Pro) content, and rapid reductions in biomass, chlorophyll (Chl), and relative water content (RWC). Cu-caused oxidative stress was evident by overaccumulation of reactive oxygen species (ROS; superoxide (O₂ ^?–) and H₂O₂). Ascorbate (AsA) content decreased while GSH and phytochelatin (PC) content increased significantly in Cu-stressed seedlings. Exogenous SNP, GSH, or SNP?+?GSH decreased toxicity symptoms and diminished a Cu-induced increase in LOX activity, O₂ ^?–, H₂O₂, MDA, and Pro content. They also counteracted a Cu-induced increase in superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and glyoxalase I and glyoxalase II activities, which paralleled changes in ROS and MDA levels. These seedlings also showed a significant increase in catalase (CAT), glutathione peroxidase (GPX), dehydroascorbate reductase (DHAR), glutathione S-transferase (GST) activities, and AsA and PC content compared with the seedlings stressed with Cu alone. Cu analysis revealed that SNP and GSH restricted the accumulation of Cu in the roots and leaves of Cu-stressed seedlings. Our results suggest that Cu exposure provoked an oxidative burden while reduced Cu uptake and modulating the antioxidant defense and glyoxalase systems by adding SNP and GSH play an important role in alleviating Cu toxicity. Furthermore, the protective action of GSH and SNP?+?GSH was more efficient than SNP alone.     

Nanosilver-Promoted Lateral Root Development in Rice is Mediated through Hydrogen Peroxide

Nanosilver (10⁻⁹ m) refers to particles comprising 20–15,000 silver atoms, exhibiting high stability and specific surface area. At present, nanosilver has been used in agricultural cultivation and production. This study examined the effects of nanosilver on growth and development of rice root systems. Study results showed that fresh weight of rice belowground organs and root length both increased significantly by 5% and 25%, respectively, after rice radicles were treated with 2 ppm of nanosilver for three days. However, the H₂O₂ level reached its peak at 2 days from treatment, but the activities of the antioxidant enzymes CAT, APX, and GR were inhibited by 2 ppm of nanosilver treatment. The results showed that nanosilver treatment inhibited the antioxidant enzyme activity of rice roots. The treatment of rice radicles with 5 μM H₂O₂ promoted root development and the same was observed when nanosilver was used for treatment. Moreover, ascorbic acid (AsA) is a H₂O₂ scavenger and therefore rice root development was inhibited when AsA was added to rice radicles together with either treatment of nanosilver or H₂O₂. In summary, nanosilver treatment of rice radicles promoted root growth and development via the regulation of H₂O₂ and not the O₂^•− pathway.     

Cell wall peroxidase activity,hydrogen peroxide level and NaCl-inhibited root growth of rice seedlings

The changes in cell-wall peroxidase (POD) activity and H₂O₂ level in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mM progressively reduced root growth and increased ionically bound cell-wall POD activity. NaCl had no effect on covalently bound cell-wall POD activities. The reduction of root growth by NaCl is closely correlated with the increase in H₂O₂ level. Exogenous H₂O₂ was found to inhibit root growth of rice seedlings. Since ammonium and proline accumulation are associated with root growth inhibition caused by NaCl, we determined the effects of NH₄Cl or proline on root growth, cell-wall POD activity and H₂O₂level in roots. External application of NH₄Cl or proline markedly inhibited root growth, increased cell-wall POD activity and increased H₂O₂ level in roots of rice seedlings in the absence of NaCl. An increase in cell-wall POD activity and H₂O₂ level preceded inhibition of root growth caused by NaCl, NH₄Cl or proline. NaCl or proline treatment also increased NADH-POD and diamine oxidase (DAO) activities in roots of rice seedlings, suggesting that NADH-POD and DAO contribute to the H₂O₂ generation in the cell wall of NaCl- or proline-treated roots. NH₄Cl treatment increased NADH-POD activity but had no effect on DAO activity, suggesting that NADH-POD but not DAO is responsible for H₂O₂ generation in cell wall of NH₄Cl-treated roots.     

Aluminum-induced oxidative stress and changes in antioxidant defenses in the roots of rice varieties differing in Al tolerance

The effects of aluminum (Al) on root elongation, lipid peroxidation, hydrogen peroxide (H₂O₂) accumulation, antioxidant levels, antioxidant enzymatic activity, and lignin content in the roots of the Al-tolerant rice variety azucena and the Al-sensitive variety IR64 were investigated. Treatment with Al induced a greater decrease in root elongation and a greater increase in H₂O₂ and lipid peroxidation as determined by the total thiobarbituric acid-reactive substance (TBARS) level in IR64 than in azucena. Azucena had significantly higher levels of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and glutathione peroxidase GSH POD activity compared with IR64. The concentrations of reduced glutathione (GSH) and ascorbic acid, and the GSH/GSSG ratio (reduced vs. oxidized glutathione) were also higher in azucena than in IR64 in the presence of Al. The addition of 1 mg/L GSH improved root elongation in both varieties and decreased H₂O₂ production under Al stress. By contrast, treatment with buthionine sulfoximine, a specific inhibitor of GSH synthesis, decreased root elongation in azucena and stimulated H₂O₂ production in both varieties. Moreover, Al treatment significantly increased the cytoplasmic activity of peroxidase (POD) as well as the levels of POD bound ionically and covalently to cell walls in the Al-sensitive variety. The lignin content was also increased. Treatment with exogenous H₂O₂ also increased the lignin content and decreased root elongation in IR64. These results suggest that Al induces lignification in the roots of Al-sensitive rice varieties, probably through an increase in H₂O₂ accumulation.     

Cadmium toxicity and translocation in rice seedlings are reduced by hydrogen peroxide pretreatment

A hydroponic experiment was carried out to study the role of hydrogen peroxide (H₂O₂) in enhancing tolerance and reducing translocation of cadmium (Cd) in rice seedlings. Plant growth (length and biomass of shoot and root) was significantly repressed by Cd exposure. However, pretreatment with 100 μM H₂O₂ for 1d mitigated Cd stress by inducing enzyme activities for antioxidation (e.g., superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX)) and detoxification (e.g., glutathione S-transferase (GST)) as well as by elevating contents of reduced glutathione (GSH) and ascorbic acid (AsA). As a result, H₂O₂ and malondialdehyde (MDA) content decreased in plants and the seedling growth was less inhibited. On the other hand, H₂O₂ pretreatment decreased Cd concentration in shoots, thus lowered the ratio of Cd concentration in shoots and roots (S/R), indicating that H₂O₂ may affect Cd distribution in rice seedlings. The improved Cd tolerance is partly due to an enhanced antioxidative system that efficiently prevents the accumulation of H₂O₂ during Cd stress. Increased Cd sequestration in rice roots may contribute to the decline of Cd translocation.     

Methyl jasmonate alleviates arsenic-induced oxidative damage and modulates the ascorbate–glutathione cycle in oilseed rape roots

Methyl jasmonate (MJ) is an important plant growth regulator, involves in various physiological processes of plants. In the present study, role of MJ in tolerance to oilseed rape (Brassica napus L.) roots under arsenic (As) stress was investigated. The treatments were comprised of three MJ doses (0, 0.1, and 1 µM) and two levels of As (0 and 200 µM). Arsenic stress resulted in oxidative damage as evidenced by decreased root growth and enhanced reactive oxygen species and lipid peroxidation. However, plants treated with MJ decreased the H₂O₂ and O₂ ^·? contents in roots and have higher antioxidant activities. Importantly, results showed that MJ enhanced the redox states of AsA and GSH, and the related enzymes involved in the AsA–GSH cycle. Moreover, MJ also induced the secondary metabolites related enzymes (PAL and PPO) activities, under As stress. PAL and PPO expression was further increased by MJ application in the roots of B. napus under As stress. MJ also reduced the total As content compared with As alone treated plants. These findings suggest the role of MJ in mitigation of the As-induced oxidative damage by regulating AsA and GSH redox states and by reducing As uptake in both cultivars.     

Arbuscular mycorrhizas alter root system architecture of Citrus tangerine through regulating metabolism of endogenous polyamines

Cadmium (Cd) is readily taken up by the roots of rice seedlings, leading to growth reduction. H₂O₂ is a constituent of oxidative metabolism and is itself a reactive oxygen species. In this study, the participation of H₂O₂ in CdCl₂-inhibited growth of rice roots was investigated. CdCl₂ treatment increased H₂O₂ production in rice roots. CdCl₂ treatment had no effect on the activities of superoxide dismutase, ascorbate peroxidase, and glutathione reductase, but inhibited the activity of catalase (CAT) in rice roots. CdCl₂-inhibited root growth and -increased H₂O₂ content were lessened in the presence of diphenyleneiodonium chloride, an inhibitor of H₂O₂ generating NADPH oxidase. However, this stimulation of root growth in CdCl₂-treated seedlings is small (about 5%). Calcium (Ca) is important in many physiological processes in plants. Attempts were also made to determine whether the action of Ca on CdCl₂-inhibited growth of rice roots is associated with H₂O₂. CaCl₂ application reduced the production of H₂O₂, the decrease in CAT activity, and the inhibition of root growth caused by CdCl₂. The effects of CaCl₂ application could be reversed by exogenous H₂O₂. Our results indicate that the Cd causes a decline in CAT and to a lower extent a stimulation of NADPH oxidase in rice roots, with the subsequent generation of H₂O₂, an agent responsible for growth inhibition.     

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.     

Induction of Betalain Pigmentation in Hairy Roots of Red Beet under Different Radiation Sources

The effects of aluminum on lipid peroxidation and activities of antioxidative enzymes were investigated in detached rice leaves treated with 0 to 5 mM AlCl₃ at pH 4.0 in the light. AlCl₃ enhanced the content of malondialdehyde but not the content of H₂O₂. Superoxide dismutase activity was reduced by AlCl₃, while catalase and glutathione reductase activities were increased. Peroxidase and ascorbate peroxidase activities were increased only after prolonged treatment, when toxicity occurred. The results give evidence that Al treatment caused oxidative stress and in turn, it caused lipid peroxidation.     

H_2S对干旱胁迫下白三叶幼苗抗氧化防御能力的影响

以‘拉丁诺’白三叶(Trifolium repens cv.‘Ladino’)为试验材料,研究外源H2S处理对PEG6 000(聚乙二醇)模拟干旱胁迫下白三叶叶片相对含水量(RWC)、膜脂过氧化、活性氧成分、抗氧化酶、抗坏血酸-谷胱甘肽循环代谢和非酶抗氧化物质的影响,以揭示H_2S调控白三叶抗旱性的生理机制。结果显示:(1)0.2 mmol/L的外源NaHS(H_2S供体)能显著提高干旱胁迫下白三叶的叶片相对含水量,维持显著较低的电解质渗透率(EL)和丙二醛(MDA)含量。(2)与直接干旱胁迫相比,干旱胁迫下外源添加NaHS处理的白三叶叶片内超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性显著增强,抗坏血酸-谷胱甘肽循环代谢中关键酶抗坏血酸过氧化物酶(APX)、脱氢抗坏血酸还原酶(DHAR)、单脱水抗坏血酸还原酶(MDHAR)和谷胱甘肽还原酶(GR)活性及其抗氧化中间产物抗坏血酸(AsA)、谷胱甘肽(GSH)含量也显著提高。(3)叶片类黄酮、总酚和原花青素的含量在一定的胁迫时间范围内亦显著增加,并伴随着活性氧成分O_2~(-·)产生速率和H_2O_2水平降低。研究认为,外源H2S能通过促进干旱胁迫下白三叶体内的多重抗氧化防御能力来提高其幼苗的抗旱性。     

The involvement of hydrogen peroxide in abscisic acid-induced activities of ascorbate peroxidase and glutathione reductase in rice roots

We have monitored the changes in antioxidant enzyme activities and H₂O₂ concentrations in roots of rice (Oryza sativa L., cv. Taichung Native 1) seedlings treated with exogenous abscisic acid(ABA). Decrease in superoxide dismutase (SOD) and catalase (CAT) activities was observed in rice roots in the presence of ABA. However, ascorbate peroxide (APX) and glutathione reductase (GR) activities were increased after the ABA treatment. ABA treatment resulted in an increase in H₂O₂ concentrations in rice roots. Pre-treatment with dimethylthiourea, a chemical trap for H₂O₂, and diphenyleneiodonium chloride (DPI), a well known inhibitor of NADPH oxidase, inhibited ABA-induced accumulation of H₂O₂ and ABA-induced activities of APX and GR. ABA-induced accumulation of H₂O₂ was found to be prior to ABA-induced activities of APX and GR. Our results suggest that H₂O₂ is involved in ABA-induced APX and GR activities in rice roots.     

Involvement of glutathione in heat shock– and hydrogen peroxide–induced cadmium tolerance of rice (Oryza sativa L.) seedlings

The role of reduced glutathione (GSH) in heat shock (HS)- and H₂O₂-induced protection of rice (Oryza sativa L., cv. Taichung 1) seedlings from Cd stress was investigated. HS- and H₂O₂-pretreatment resulted in an increase in GSH content in leaves of rice seedlings. Addition of exogenous GSH under non-HS conditions, which resulted in an increase in GSH in leaves, enhanced subsequent Cd tolerance of rice seedlings. Pretreatment with buthionine sulfoximine (BSO), a specific inhibitor of GSH synthesis, which effectively inhibited GSH content induced by HS and H₂O₂, reduced subsequent Cd tolerance. Furthermore, the effect of BSO on HS- and H₂O₂-induced GSH accumulation and toxicity by subsequent Cd stress can be reversed by the addition of GSH. The time-course analyses of HS in rice seedlings demonstrated that the accumulation of H₂O₂ preceded the increase in GSH. Based on the data obtained in this study, it could be concluded that the early accumulation of H₂O₂ during HS signals the increase in GSH content, which in turn protects rice seedlings from oxidative damage caused by Cd.     

Manganese-excess induces oxidative stress,lowers the pool of antioxidants and elevates activities of key antioxidative enzymes in rice seedlings

Manganese (Mn) is an essential element for plant growth but in excess, specially in acidic soils, it can become phytotoxic. In order to investigate whether oxidative stress is associated with the expression of Mn toxicity during early seedling establishment of rice plants, we examined the changes in the level of reactive oxygen species (ROS), oxidative stress induced an alteration in the level of non-enzymic antioxidants and activities of antioxidative enzymes in rice seedlings grown in sand cultures containing 3 and 6 mM MnCl₂. Mn treatment inhibited growth of rice seedlings, the metal increasingly accumulated in roots and shoots and caused damage to membranes. Mn treated plants showed increased generation of superoxide anion (O₂ ^.−), elevated levels of H₂O₂ and thiobarbituric acid reactive substances (TBARS) and decline in protein thiol. The level of nonprotein thiol, however, increased due to Mn treatment. A decline in contents of reduced ascorbate (AsA) and glutathione (GSH) as well as decline in ratios of their reduced to oxidize forms was observed in Mn-treated seedlings. The activities of antioxidative enzymes superoxide dismutase (SOD) and its isoforms Mn SOD, Cu/Zn SOD, Fe SOD as well as guaiacol peroxidase (GPX) increased in the seedlings due to Mn treatment however, catalase (CAT) activity increased in 10 days old seedlings but it declined by 20 days under Mn treatment. The enzymes of Halliwell-Asada cycle, ascorbate peroxidase (APX) monodehydoascorbate reductase (MDHAR), dehyroascorbate reductase (DHAR) and glutathione reductase (GR) increased significantly in Mn treated seedlings over controls. Results suggest that in rice seedlings excess Mn induces oxidative stress, imbalances the levels of antioxidants and the antioxidative enzymes SOD, GPX, APX and GR appear to play an important role in scavenging ROS and withstanding oxidative stress induced by Mn.     

The decline in ascorbic acid content is associated with cadmium toxicity of rice seedlings

Cadmium (Cd) toxicity of rice (Oryza sativa L. cv. Taichung Native 1) seedlings was evaluated by the decrease in chlorophyll content and the increase in malondialdehyde (MDA) in the second leaves of rice seedlings. CdCl₂ (5 μM) treatment was accompanied by a decrease in the contents of ascorbic acid (AsA) and AsA + dehydroascorbate (DHA) and in the ratios of AsA/DHA in leaves. However, CdCl₂ treatment resulted in an increase in DHA content in leaves. Moreover, the decrease in AsA content was prior to the occurrence of chlorosis and associated with the increase in MDA content in the leaves of seedlings treated with Cd. Pretreatment with 0.5 mM AsA or l-galactono-1,4-lactone (GalL), the biosynthetic precursor of AsA, for 6 h resulted in an increase in the contents of AsA and reduced glutathione (GSH), the ratios of AsA/DHA and GSH/oxidized glutathione, and the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) in the leaves of rice seedlings. Quantitative RT-PCR was applied to quantify the mRNA levels for OsAPX and OsGR genes from rice leaves to examine the effect of AsA or GalL pretreatment on the expression of OsAPX and OsGR genes in rice leaves. The expression of OsAPX2, OsAPX3, OsAPX4, OsAPX5, OsAPX6, OsAPX7, and OsGR1 was increased by AsA or GalL pretreatment. Rice seedlings pretreated with AsA or GalL were observed to reduce the subsequent Cd-induced toxicity. Our results suggest that AsA content may play a role in regulating Cd toxicity of rice seedlings.     

Arginine decarboxylase ADC2 enhances salt tolerance through increasing ROS scavenging enzyme activity in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

In this study, we investigated the possible role of hemin in alleviating zinc (Zn), lead (Pb) and chromium (Cr) toxicity in rice seedlings grown hydroponically by analyzing the morphological and physiological parameters. Our results showed that exposure of rice seedlings to excess Zn, Pb or Cr could cause severe leaf chlorosis, inhibit photosynthetic activity and consequently suppress plant growth. The concentration of O₂ ^?? and H₂O₂ significantly increased and the activities of antioxidative enzymes decreased in roots of rice seedlings under metal exposure. The combined treatments (hemin?+?ZnSO₄, hemin?+?Pb(NO₃)₂ and hemin?+?K₂Cr₂O₇), on the other hand, significantly enhanced the photosynthesis- and plant growth-related parameters compared with their corresponding heavy-metal-stress alone. Combined treatments dramatically stimulated the activities of superoxide dismutase (SOD), ascorbic peroxidase (APX) and glutathione reductase (GR) as well as the concentrations of ascorbic acid (AsA) and glutathione (GSH) as compared with the metal- stress alone. The concentrations of reactive oxygen species (ROS, e.g. O₂ ^?? and H₂O₂) were significantly reduced in the metal plus hemin treatments. Hemin addition also reduced metal accumulation in rice seedlings especially in root tissues. These findings suggest that hemin-elevated levels of antioxidants, activities of antioxidative enzymes and hemin-reduced accumulation of heavy-metal could confer resistance against Zn, Pb, and Cr stress in rice seedlings, resulting in improved pigments accumulation, photosynthetic attributes and plant growth.     

Jasmonic acid regulates the ascorbate–glutathione cycle in <Emphasis Type="Italic">Malus baccata</Emphasis> Borkh. roots under low root-zone temperature

Jasmonic acid (JA), which is an important phytohormone, plays a key role in plant growth, development and stress responses. Here, Malus baccata Borkh. seedlings were used to study the mechanism by which JA alleviates the oxidative damage induced by low root-zone temperature (5 °C) through regulating the ascorbate–glutathione (AsA–GSH) cycle. The roots of M. baccata Borkh. were subjected to three treatments [5 °C, 5 °C + JA, and 5 °C + ibuprofen (IBU)] for 0, 12, 24, and 48 h. The results showed that treatment with low root-zone temperature could modulate the non-enzymatic and enzymatic components of the AsA–GSH cycle, significantly inducing the accumulation of MDA and H₂O₂. Additionally, the endogenous JA content changed dramatically, and the expression levels of the related genes [lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC)] showed different trends. In plants pretreated with JA, the endogenous JA content increased at 24 h, and the gene expression levels of LOX, AOS, and AOC were upregulated. We also found a marked increase in the activities of antioxidant enzymes [ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR)], a decrease in oxidized glutathione (GSSG) and an increased GSH/GSSG ratio, which resulted in lower MDA and H₂O₂ contents. Thus, the oxidative stress was alleviated. Plants pretreated with IBU experienced an opposite effect on the function of the AsA–GSH cycle and the gene expression in the JA synthesis route relative to those subjected to exogenous JA treatment, indicating that endogenous JA can alleviate oxidative damage by regulating the function of the AsA–GSH cycle under low root-zone temperature.