Coordinated Actions of Glyoxalase and Antioxidant Defense Systems in Conferring Abiotic Stress Tolerance in Plants

Inoculation of Brevibacterium linens RS16 in Oryza sativa genotypes enhanced salinity resistance: Impacts on photosynthetic traits and foliar volatile emissions

The emission of volatiles in response to salt stress in rice cultivars has not been studied much to date. Studies addressing the regulation of stress induced volatile emission by halotolerant plant growth promoting bacteria containing ACC (1-aminocyclopropane-1-carboxylate) deaminase are also limited. The objective of the present study was to investigate the salt alleviation potential of bacteria by regulating photosynthetic characteristics and volatile emissions in rice cultivars, and to compare the effects of the bacteria inoculation and salt responses between two rice genotypes. The interactive effects of soil salinity (0, 50, and 100 mM NaCl) and inoculation with Brevibacterium linens RS16 on ACC accumulation, ACC oxidase activity, carbon assimilation and stress volatile emissions after stress application were studied in the moderately salt resistant (FL478) and the salt-sensitive (IR29) rice (Oryza sativa L.) cultivars. It was observed that salt stress reduced foliage photosynthetic rate, but induced foliage ACC accumulation, foliage ACC oxidase activity, and the emissions of all the major classes of volatile organic compounds (VOCs) including the lipoxygenase pathway volatiles, light-weight oxygenated volatiles, long-chained saturated aldehydes, benzenoids, geranylgeranyl diphosphate pathway products, and mono- and sesquiterpenes. All these characteristics scaled up quantitatively with increasing salt stress. The effects of salt stress were more pronounced in the salt-sensitive genotype IR29 compared to the moderately salt resistant FL478 genotype. However, the bacterial inoculation significantly enhanced photosynthesis, and decreased ACC accumulation and the ACC oxidase activity, and VOC emissions both in control and salt-treated plants. Taken together, these results suggested that the ACC deaminase-containing Brevibacterium linens RS16 reduces the temporal regulation of VOC emissions and increases the plant physiological activity by reducing the availability of ethylene precursor ACC and the ACC oxidase activity under salt stress.

Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Aluminum effects on photosynthesis, reactive oxygen species and methylglyoxal detoxification in two Citrus species differing in aluminum tolerance

Citrus are mainly grown in low pH soils with high active aluminum (Al). 'Xuegan' (Citrus sinensis (L.) Osbeck) and 'Shatian pummelo' (Citrus grandis (L.) Osbeck) seedlings were fertilized for 18 weeks with nutrient solution containing either 0 mM (control) or 1 mM (Al toxicity) AlCl3·6H2O. Aluminum induced decreases of biomass, leaf photosynthesis, relative water content and total soluble protein levels, and increases of methylglyoxal levels only occurred in C. grandis roots and leaves. Besides, the Al-induced decreases of pigments and alterations of chlorophyll a fluorescence transients and fluorescence parameters were greater in C. grandis leaves than those in C. sinensis leaves. Aluminum-treated C. grandis had higher stem and leaf Al levels and similar root Al levels relative to Al-treated C. sinensis, but lower Al distribution in roots and Al uptake per plant. Aluminum toxicity decreased nitrogen, phosphorus, potassium, calcium, magnesium and sulfur uptake per plant in C. grandis and C. sinensis seedlings, with the exception of Al-treated C. sinensis seedlings exhibiting increased sulfur uptake per plant and unaltered magnesium uptake per plant. Under Al-stress, macroelement uptake per plant was higher in C. sinensis than that in C. grandis. Aluminum toxicity decreased the ratios of reduced glutathione/(reduced + oxidized glutathione) and of ascorbate/(ascorbate + dehydroascorbate) only in C. grandis roots and leaves. The activities of most antioxidant enzymes, sulfur metabolism-related enzymes and glyoxalases and the levels of S-containing compounds were higher in Al-treated C. sinensis roots and leaves than those in Al-treated C. grandis ones. Thus, C. sinensis displayed higher Al tolerance than C. grandis did. The higher Al tolerance of C. sinensis might involve: (i) more Al accumulation in roots and less transport of Al from roots to shoots; (ii) efficient maintenance of nutrient homeostasis; and (iii) efficient maintenance of redox homeostasis via detoxification systems of reactive oxygen species and methylglyoxal.

Heavy metal tolerance in contrasting ecotypes of Alyssum montanum

The response of metallicolous (M) and nonmetallicolous (NM) Alyssum montanum ecotypes to multi-metal stress was investigated under in vitro condition and compared in this study. Shoot cultures were simultaneously treated with 0.7 mM ZnSO₄, 3.0 μM Pb(NO₃)₂ and 16.4 μM CdCl₂ for 8 weeks and evaluated for their morphogenetic and ultrastructural reaction, growth tolerance as well as ability to Zn, Pb, and Cd uptake. Moreover, tissue localization and concentrations of antioxidant compounds were determined in order to elucidate the potential role of ROS-scavenging machinery in plant tolerance to metal toxicity. The results clearly demonstrated that M specimens treated with heavy metals showed less phytotoxic symptoms and low level of lipid peroxidation than reference NM one. The enhanced tolerance of M ecotype resulted from heavy metals detoxification in trichomes and intracellular leaf compartments as well as balanced ROS accumulation. The inactivation of ROS in M plants was based on peroxidase-flavonoid system, while in NM plants such relationship was not detected and amounts of antioxidant enzymes or phenolic compounds was comparable to untreated specimens or decreased significantly. Considering the procumbent growth of such hemicryptophyte which reproduce effectively in the presence of heavy metals but is characterized by low biomass production, it is proposed to exploit M ecotype of A. montanum in revegetation schemes of polluted calamine wastes to provide the prompt stabilization of areas prone to erosion.

Genomic and proteomic analysis of lignin degrading and polyhydroxyalkanoate accumulating β-proteobacterium Pandoraea sp. ISTKB

BACKGROUND

Lignin is a major component of plant biomass and is recalcitrant to degradation due to its complex and heterogeneous aromatic structure. The biomass-based research mainly focuses on polysaccharides component of biomass and lignin is discarded as waste with very limited usage. The sustainability and success of plant polysaccharide-based biorefinery can be possible if lignin is utilized in improved ways and with minimal waste generation. Discovering new microbial strains and understanding their enzyme system for lignin degradation are necessary for its conversion into fuel and chemicals. The Pandoraea sp. ISTKB was previously characterized for lignin degradation and successfully applied for pretreatment of sugarcane bagasse and polyhydroxyalkanoate (PHA) production. In this study, genomic analysis and proteomics on aromatic polymer kraft lignin and vanillic acid are performed to find the important enzymes for polymer utilization.

RESULTS

Genomic analysis of Pandoraea sp. ISTKB revealed the presence of strong lignin degradation machinery and identified various candidate genes responsible for lignin degradation and PHA production. We also applied label-free quantitative proteomic approach to identify the expression profile on monoaromatic compound vanillic acid (VA) and polyaromatic kraft lignin (KL). Genomic and proteomic analysis simultaneously discovered Dyp-type peroxidase, peroxidases, glycolate oxidase, aldehyde oxidase, GMC oxidoreductase, laccases, quinone oxidoreductase, dioxygenases, monooxygenases, glutathione-dependent etherases, dehydrogenases, reductases, and methyltransferases and various other recently reported enzyme systems such as superoxide dismutases or catalase-peroxidase for lignin degradation. A strong stress response and detoxification mechanism was discovered. The two important gene clusters for lignin degradation and three PHA polymerase spanning gene clusters were identified and all the clusters were functionally active on KL-VA.

CONCLUSIONS

The unusual aerobic '-CoA'-mediated degradation pathway of phenylacetate and benzoate (reported only in 16 and 4-5% of total sequenced bacterial genomes), peroxidase-accessory enzyme system, and fenton chemistry based are the major pathways observed for lignin degradation. Both ortho and meta ring cleavage pathways for aromatic compound degradation were observed in expression profile. Genomic and proteomic approaches provided validation to this strain's robust machinery for the metabolism of recalcitrant compounds and PHA production and provide an opportunity to target important enzymes for lignin valorization in future.

Manipulation of glyoxalase pathway confers tolerance to multiple stresses in rice

Crop plants face a multitude of diverse abiotic and biotic stresses in the farmers' fields. Although there now exists a considerable knowledge of the underlying mechanisms of response to individual stresses, the crosstalk between response pathways to various abiotic and biotic stresses remains enigmatic. Here, we investigated if the cytotoxic metabolite methylglyoxal (MG), excess of which is generated as a common consequence of many abiotic and biotic stresses, may serve as a key molecule linking responses to diverse stresses. For this, we generated transgenic rice plants overexpressing the entire two-step glyoxalase pathway for MG detoxification. Through assessment of various morphological, physiological and agronomic parameters, we found that glyoxalase-overexpression imparts tolerance towards abiotic stresses like salinity, drought and heat and also provides resistance towards damage caused by the sheath blight fungus (Rhizoctonia solani) toxin phenylacetic acid. We show that the mechanism of observed tolerance of the glyoxalase-overexpressing plants towards these diverse abiotic and biotic stresses involves improved MG detoxification and reduced oxidative damage leading to better protection of chloroplast and mitochondrial ultrastructure and maintained photosynthetic efficiency under stress conditions. Together, our findings indicate that MG may serve as a key link between abiotic and biotic stress response in plants.

Signaling molecule methylglyoxal ameliorates cadmium injury in wheat (Triticum aestivum L) by a coordinated induction of glutathione pool and glyoxalase system

Methylglyoxal (MG) now is found to be an emerging signaling molecule. It can relieve the toxicity of cadmium (Cd), however its alleviating mechanism still remains unknown. In this study, compared with the Cd-stressed seedlings without MG treatment, MG treatment could stimulate the activities of glutathione reductase (GR) and gamma-glutamylcysteine synthetase (γ-ECS) in Cd-stressed wheat seedlings, which in turn induced an increase of reduced glutathione (GSH). Adversely, the activated enzymes related to GSH biosynthesis and increased GSH were weakened by N-acetyl-L-cysteine (NAC, MG scavenger), 2,4-dihydroxy-benzylamine (DHBA) and 1,3-bischloroethyl-nitrosourea (BCNU, both are specific inhibitors of GR), buthionine sulfoximine (BSO, a specific inhibitors of GSH biosynthesis), and N-ethylmaleimide (NEM, GSH scavenger), respectively. In addition, MG increased the activities of glyoxalase I (Gly I) and glyoxalase II (Gly II) in Cd-treated seedlings, followed by declining an increase in endogenous MG as comparision to Cd-stressed seedlings alone. On the contrary, the increased glyoxalase activity and decreased endogenous MG level were reversed by NAC and specific inhibitors of Gly I (isoascorbate, IAS; squaric acid, SA). Furthermore, MG alleviated an increase in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) in Cd-treated wheat seedlings. These results indicated that MG could alleviate Cd toxicity and improve the growth of Cd-stressed wheat seedlings by a coordinated induction of glutathione pool and glyoxalase system.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

Heat and Drought Stresses in Crops and Approaches for Their Mitigation

Drought and heat are major abiotic stresses that reduce crop productivity and weaken global food security, especially given the current and growing impacts of climate change and increases in the occurrence and severity of both stress factors. Plants have developed dynamic responses at the morphological, physiological and biochemical levels allowing them to escape and/or adapt to unfavorable environmental conditions. Nevertheless, even the mildest heat and drought stress negatively affects crop yield. Further, several independent studies have shown that increased temperature and drought can reduce crop yields by as much as 50%. Response to stress is complex and involves several factors including signaling, transcription factors, hormones, and secondary metabolites. The reproductive phase of development, leading to the grain production is shown to be more sensitive to heat stress in several crops. Advances coming from biotechnology including progress in genomics and information technology may mitigate the detrimental effects of heat and drought through the use of agronomic management practices and the development of crop varieties with increased productivity under stress. This review presents recent progress in key areas relevant to plant drought and heat tolerance. Furthermore, an overview and implications of physiological, biochemical and genetic aspects in the context of heat and drought are presented. Potential strategies to improve crop productivity are discussed.

The calcium-dependent kinase OsCPK24 functions in cold stress responses in rice

Calcium-dependent protein kinases (CPKs) are serine/threonine protein kinases that function in plant stress responses. Although CPKs are recognized as key messengers in signal transduction, the specific roles of CPKs and the molecular mechanisms underlying their activity remain largely unknown. Here, we characterized the function of OsCPK24, a cytosol-localized calcium-dependent protein kinase in rice. OsCPK24 was universally and highly expressed in rice plants and was induced by cold treatment. Whereas OsCPK24 knockdown plants exhibited increased sensitivity to cold compared to wild type (WT), OsCPK24-overexpressing plants exhibited increased cold tolerance. Plants overexpressing OsCPK24 exhibited increased accumulation of proline (an osmoprotectant) and glutathione (an antioxidant) and maintained a higher GSH/GSSG (reduced glutathione to oxidized glutathione) ratio during cold stress compared to WT. In addition to these effects in response to cold stress, we observed the kinase activity of OsCPK24 varied under different calcium concentrations. Further, OsCPK24 phosphorylated OsGrx10, a glutathione-dependent thioltransferase, at rates modulated by changes in calcium concentration. Together, our results support the hypothesis that OsCPK24 functions as a positive regulator of cold stress tolerance in rice, a process mediated by calcium signaling and involving phosphorylation and the inhibition of OsGrx10 to sustain higher glutathione levels.

Enhanced Formation of Methylglyoxal-Derived Advanced Glycation End Products in Arabidopsis Under Ammonium Nutrition

Nitrate (NO₃^-) and ammonium (NH₄⁺) are prevalent nitrogen (N) sources for plants. Although NH₄⁺ should be the preferred form of N from the energetic point of view, ammonium nutrition often exhibits adverse effects on plant physiological functions and induces an important growth-limiting stress referred as ammonium syndrome. The effective incorporation of NH₄⁺ into amino acid structures requires high activity of the mitochondrial tricarboxylic acid cycle and the glycolytic pathway. An unavoidable consequence of glycolytic metabolism is the production of methylglyoxal (MG), which is very toxic and inhibits cell growth in all types of organisms. Here, we aimed to investigate MG metabolism in Arabidopsis thaliana plants grown on NH₄⁺ as a sole N source. We found that changes in activities of glycolytic enzymes enhanced MG production and that markedly elevated MG levels superseded the detoxification capability of the glyoxalase pathway. Consequently, the excessive accumulation of MG was directly involved in the induction of dicarbonyl stress by introducing MG-derived advanced glycation end products (MAGEs) to proteins. The severe damage to proteins was not within the repair capacity of proteolytic enzymes. Collectively, our results suggest the impact of MG (mediated by MAGEs formation in proteins) in the contribution to NH₄⁺ toxicity symptoms in Arabidopsis.

Insights into citric acid-induced cadmium tolerance and phytoremediation in Brassica juncea L.: Coordinated functions of metal chelation, antioxidant defense and glyoxalase systems

Cadmium (Cd) is a serious environmental threat because it accumulates in plants from soil and is subsequently transported into the food cycle. Increased Cd uptake in plants disrupts plant metabolism and hampers crop growth and development. Therefore, remediation of Cd from soil and enhancing plant tolerance to metal toxicity is vital. In the present study, we investigated the function of different doses of citric acid (CA) on Cd toxicity in terms of metal accumulation and stress tolerance in mustard (Brassica juncea L.). Brassica juncea seedlings (12-day-old) were treated with Cd (0.5mMCd and 1.0mM CdCl₂) alone and in combination with CA (0.5mM and 1.0mM) in a semi-hydroponic medium for three days. Cadmium accumulation in the roots and shoots of the mustard seedlings increased in a dose-dependent manner and was higher in the roots. Increasing the Cd concentration led to reduced growth, biomass, water status, and chlorophyll (chl) content resulting from increased oxidative damage (elevated malondialdehyde, MDA content; hydrogen peroxide, H₂O₂ level; superoxide, O₂^•- generation; lipoxygenase, LOX activity; and methylglyoxal, MG content) and downregulating of the major enzymes of the antioxidant defense and glyoxalase systems. Under Cd stress, both doses of CA improved the growth of the plants by enhancing leaf relative water content (RWC) and chl content; reducing oxidative damage; enhancing the pool of ascorbate (AsA) and glutathione (GSH) and the activities of the antioxidant enzymes (ascorbate peroxidase, APX; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione reductase, GR; glutathione peroxidase, GPX; superoxide dismutase, SOD; catalase, CAT); improving the performance of the glyoxalase system (glyoxalase I, Gly I and glyoxalase II, Gly II activity); and increasing the phytochelatin (PC) content. Exogenous CA also increased the root and shoot Cd content and Cd translocation from the roots to the shoots in a dose-dependent manner. Our findings suggest that CA plays a dual role in mustard seedlings by increasing phytoremediation and enhancing stress tolerance through upregulating the antioxidant defense and glyoxalase systems.

Molecular Profiling of Pierce's Disease Outlines the Response Circuitry of Vitis vinifera to Xylella fastidiosa Infection

Pierce's disease is a major threat to grapevines caused by the bacterium Xylella fastidiosa. Although devoid of a type 3 secretion system commonly employed by bacterial pathogens to deliver effectors inside host cells, this pathogen is able to influence host parenchymal cells from the xylem lumen by secreting a battery of hydrolytic enzymes. Defining the cellular and biochemical changes induced during disease can foster the development of novel therapeutic strategies aimed at reducing the pathogen fitness and increasing plant health. To this end, we investigated the transcriptional, proteomic, and metabolomic responses of diseased Vitis vinifera compared to healthy plants. We found that several antioxidant strategies were induced, including the accumulation of gamma-aminobutyric acid (GABA) and polyamine metabolism, as well as iron and copper chelation, but these were insufficient to protect the plant from chronic oxidative stress and disease symptom development. Notable upregulation of phytoalexins, pathogenesis-related proteins, and various aromatic acid metabolites was part of the host responses observed. Moreover, upregulation of various cell wall modification enzymes followed the proliferation of the pathogen within xylem vessels, consistent with the intensive thickening of vessels' secondary walls observed by magnetic resonance imaging. By interpreting the molecular profile changes taking place in symptomatic tissues, we report a set of molecular markers that can be further explored to aid in disease detection, breeding for resistance, and developing therapeutics.

Maleic acid assisted improvement of metal chelation and antioxidant metabolism confers chromium tolerance in Brassica juncea L

Chromium (Cr) is a highly toxic environmental pollutant that negatively affects plant growth and development. Thus, remediating Cr from soil or increasing plant tolerance against Cr stress is urgent. Organic acids are recognized as agents of phytoremediation and as exogenous protectants, but using maleic acid (MA) to attain these results has not yet been studied. Therefore, our study investigated the effects of MA on Cr uptake and mitigation of Cr toxicity. We treated 8-d-old Indian mustard (Brassica juncea L.) seedlings with Cr (0.15mM and 0.3mM K₂CrO₄, 5 days) alone and in combination with MA (0.25mM) in a semi-hydroponic medium. Under Cr stress, plants accumulated Cr in both the roots and shoots in a dose-dependent manner, where the roots showed higher accumulation. Chromium stress reduced the growth and biomass of the Indian mustard plants by reducing water status and photosynthetic pigments, and increased oxidative damage due to generation of toxic reactive oxygen species (ROS) and methylglyoxal (MG). Chromium stress also interfered with the function of the antioxidant defense and glyoxalase systems. However, using MA in the Cr-stressed plants further increased Cr uptake in the roots, but it slightly reduced the translocation of Cr from the roots to the shoots at a lower dose of Cr and significantly at a higher dose. Moreover, MA also increased the other non-protein thiols (NPTs) containing phytochelatin (PC) content of the seedlings, which reduced Cr toxicity. Supplementing the stressed plants with MA upregulated the non-enzymatic antioxidants (ascorbate, AsA; glutathione, GSH); the activities of the enzymatic antioxidants including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT); and the enzymes of the glyoxalase system including glyoxalase I (Gly I) and glyoxalase II (Gly II); and finally reduced oxidative damage and increased the chlorophyll content and water status as well the growth and biomass of the plants. Our findings suggested two potential uses of MA: first, enhancing phytoremediation, principally phytostabilization and second, working as an exogenous protectant to enhance Cr tolerance.

γ-aminobutyric acid (GABA) confers chromium stress tolerance in Brassica juncea L. by modulating the antioxidant defense and glyoxalase systems

Chromium (Cr) toxicity is hazardous to the seed germination, growth, and development of plants. γ-aminobutyric acid (GABA) is a non-protein amino acid and is involved in stress tolerance in plants. To investigate the effects of GABA in alleviating Cr toxicity, we treated eight-d-old mustard (Brassica juncea L.) seedlings with Cr (0.15 and 0.3 mM K₂CrO₄, 5 days) alone and in combination with GABA (125 µM) in a semi-hydroponic medium. The roots and shoots of the seedlings accumulated Cr in a dose-dependent manner, which led to an increase in oxidative damage [lipid peroxidation; hydrogen peroxide (H₂O₂) content; superoxide (O₂^•-) generation; lipoxygenase (LOX) activity], methylglyoxal (MG) content, and disrupted antioxidant defense and glyoxalase systems. Chromium stress also reduced growth, leaf relative water content (RWC), and chlorophyll (chl) content but increased phytochelatin (PC) and proline (Pro) content. Furthermore, supplementing the Cr-treated seedlings with GABA reduced Cr uptake and upregulated the non-enzymatic antioxidants (ascorbate, AsA; glutathione, GSH) and the activities of the enzymatic antioxidants including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), superoxide dismutase (SOD), catalase (CAT), glyoxalase I (Gly I), and glyoxalase II (Gly II), and finally reduced oxidative damage. Adding GABA also increased leaf RWC and chl content, decreased Pro and PC content, and restored plant growth. These findings shed light on the effect of GABA in improving the physiological mechanisms of mustard seedlings in response to Cr stress.

Exogenous Silicon Attenuates Cadmium-Induced Oxidative Stress in Brassica napus L. by Modulating AsA-GSH Pathway and Glyoxalase System

Cadmium (Cd) brings a devastating health hazard to human being as a serious consequence of agricultural and environmental contamination. We demonstrated the protective effect of silicon (Si) on cadmium (Cd)-stressed rapeseed (Brassica napus L. cv. BINA Sharisha 3) plants through regulation of antioxidant defense and glyoxalase systems. Twelve-day-old seedlings were exposed to Cd stress (0.5 and 1.0 mM CdCl2) separately and in combination with Si (SiO2, 1.0 mM) for 2 days. Cadmium toxicity was evident by an obvious oxidative stress through sharp increases in H2O2 content and lipid peroxidation (malondialdehyde, MDA content), and visible sign of superoxide and H2O2. Cadmium stress also decreased the content of ascorbate (AsA) and glutathione (GSH) as well as their redox pool. The activities of monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and catalase (CAT) were decreased by Cd while ascorbate peroxidase (APX) and glutathione S-transferase (GST) activities were increased. The enzymes of glyoxalase system (glyoxalase I, Gly I and glyoxalase II, Gly II) were also inefficient under Cd stress. However, exogenous application of Si in Cd treated seedlings reduced H2O2 and MDA contents and improved antioxidant defense mechanism through increasing the AsA and GSH pools and activities of AsA-GSH cycle (APX, MDHAR, DHAR and GR) and glyoxalase system (Gly I and Gly II) enzymes and CAT. Thus Si reduced oxidative damage in plants to make more tolerant under Cd stress through augmentation of different antioxidant components and methylglyoxal detoxification system.

Glutathione in plants: biosynthesis and physiological role in environmental stress tolerance

Glutathione (GSH; γ-glutamyl-cysteinyl-glycine) is a small intracellular thiol molecule which is considered as a strong non-enzymatic antioxidant. Glutathione regulates multiple metabolic functions; for example, it protects membranes by maintaining the reduced state of both α-tocopherol and zeaxanthin, it prevents the oxidative denaturation of proteins under stress conditions by protecting their thiol groups, and it serves as a substrate for both glutathione peroxidase and glutathione S-transferase. By acting as a precursor of phytochelatins, GSH helps in the chelating of toxic metals/metalloids which are then transported and sequestered in the vacuole. The glyoxalase pathway (consisting of glyoxalase I and glyoxalase II enzymes) for detoxification of methylglyoxal, a cytotoxic molecule, also requires GSH in the first reaction step. For these reasons, much attention has recently been directed to elucidation of the role of this molecule in conferring tolerance to abiotic stress. Recently, this molecule has drawn much attention because of its interaction with other signaling molecules and phytohormones. In this review, we have discussed the recent progress in GSH biosynthesis, metabolism and its role in abiotic stress tolerance.