Increasing the glutathione content in a chilling-sensitive maize genotype using safeners increased protection against chilling-induced injury

A natural single-nucleotide polymorphism variant in sulfite reductase influences sulfur assimilation in maize

Plants absorb sulfur from the environment and assimilate it into suitable forms for the biosynthesis of a broad range of molecules. Although the biochemical pathway of sulfur assimilation is known, how genetic differences contribute to natural variation in sulfur assimilation remains poorly understood. Here, using a genome-wide association study, we uncovered a single-nucleotide polymorphism (SNP) variant in the sulfite reductase (SiR) gene that was significantly associated with SiR protein abundance in a maize natural association population. We also demonstrated that the synonymous C to G base change at SNP69 may repress translational activity by altering messenger RNA secondary structure, which leads to reduction in ZmSiR protein abundance and sulfur assimilation activity. Population genetic analyses showed that the SNP69C allele was likely a variant occurring after the initial maize domestication and accumulated with the spread of maize cultivation from tropical to temperate regions. This study provides the first evidence that genetic polymorphisms in the exon of ZmSiR could influence the protein abundance through a posttranscriptional mechanism and in part contribute to natural variation in sulfur assimilation. These findings provide a prospective target to improve maize varieties with proper sulfur nutrient levels assisted by molecular breeding and engineering.

Determining the ROS and the Antioxidant Status of Leaves During Cold Acclimation

Cold slows down Calvin cycle activity stronger than photosynthetic electron transport, which supports production of reactive oxygen species (ROS). Even under extreme temperature conditions, most ROS are detoxified by the combined action of low-molecular weight antioxidants and antioxidant enzymes. Subsequent regeneration of the low-molecular weight antioxidants by NAD(P)H and thioredoxin/thiol-dependent pathways relaxes the electron pressure in the photosynthetic electron transport chain. In general, the chloroplast antioxidant system protects plants from severe damage of enzymes, metabolites, and cellular structures by both ROS detoxification and antioxidant recycling. Various methods have been developed to quantify ROS and antioxidant levels in photosynthetic tissues. Here, we summarize a series of exceptionally fast and easily applicable methods that show local ROS accumulation and provide information on the overall availability of reducing sugars, mainly ascorbate, and of thiols.

Maize Tolerance against Drought and Chilling Stresses Varied with Root Morphology and Antioxidative Defense System

Maize belongs to a tropical environment and is extremely sensitive to drought and chilling stress, particularly at early developmental stages. The present study investigated the individual and combined effects of drought (15% PEG-Solution) and chilling stress (15/12 °C) on morpho-physiological growth, osmolyte accumulation, production of reactive oxygen species (ROS), and activities/levels of enzymatic and non-enzymatic antioxidants in two maize hybrids (i.e., "XD889" and "XD319") and two inbred cultivars (i.e., "Yu13" and "Yu37"). Results revealed that individual and combined exposure of drought and chilling stresses hampered the morpho-physiological growth and oxidative status of maize cultivars, nevertheless, the interactive damage caused by drought + chilling was found to be more severe for all the studied traits. Between two individual stress factors, chilling-induced reductions in seedling length and biomass of maize cultivars were more compared with drought stress alone. Greater decrease in root length and biomass under chilling stress ultimately decreased the volume and surface area of the root system, and restricted the shoot growth. All the stress treatments, particularly chilling and drought + chilling, triggered the oxidative stress by higher accumulation of superoxide anion, hydrogen peroxide, hydroxyl ion, and malondialdehyde contents compared with the control. Variations in response of maize cultivars were also apparent against different stress treatments, and XD889 performed comparatively better than the rest of the cultivars. The better growth and greater stress tolerance of this cultivar was attributed to the vigorous root system architecture, as indicated by higher root biomass, root surface area, and root volume under drought and chilling stresses. Moreover, efficient antioxidant defense system in terms of higher total antioxidant capability, superoxide dismutase, peroxidase, catalase, and glutathione reductase activities also contributed in greater stress tolerance of XD889 over other cultivars.

Nitrate reductase rather than nitric oxide synthase activity is involved in 24-epibrassinolide-induced nitric oxide synthesis to improve tolerance to iron deficiency in strawberry (Fragaria × annassa) by up-regulating the ascorbate-glutathione cycle

Involvement of nitrate reductase (NR) and nitric oxide synthase (NOS)-like enzyme in 24-epibrassinolide (EB)-triggered nitric oxide (NO) synthesis to improve iron deficiency (ID) tolerance in strawberry plants was studied. EB was sprayed to strawberry plants every two days for two weeks. Then, the EB-treated plants were pre-treated with inhibitors of NR, tungstate, or NOS, L-NAME for 3 h. During the first three weeks, Fe was supplied as 100 μM EDTA-Fe or FeSO₄ to Fe-sufficient or Fe-deficient plants, respectively. Thereafter, plants were subjected for further three weeks to control (100 μM EDTA-Fe) and Fe deficiency (ID; without Fe). ID reduced biomass, chlorophyll, and chlorophyll fluorescence, while increased oxidative stress parameters, ascorbate (AsA), glutathione (GSH), endogenous NO, and the activities of NR, NOS, and antioxidant enzymes. Pre-treatments with EB and EB + SNP improved ID tolerance of strawberry by improving leaf Fe²⁺, plant growth, and antioxidant enzyme activities, and causing a further elevation in AsA, GSH, NO, NR and NOS. L-NAME application reversed NOS activity, but it did not eliminate NO, however, tungstate application reversed both NR activity and NO synthesis in plants exposed to ID + EB, suggesting that NR is the main contributor of EB-induced NO synthesis to improve ID tolerance in strawberry plants.

Phytoremediation and detoxification of xenobiotics in plants: herbicide-safeners as a tool to improve plant efficiency in the remediation of polluted environments. A mini-review

Phytoremediation is a widely studied and applied technology, based on the use of plants and their associated microorganisms to decontaminate polluted sites. In recent years, different strategies have been investigated to improve the phytoremediation efficiency of the selected plants. In this context, some studies have shown that herbicide-safeners, chemicals applied to crops to enhance their tolerance to herbicides, can increase the phytoremediation of soils and water polluted by organic and inorganic contaminants. Safeners, by inducing the xenobiotic detoxification and the antioxidant metabolism in plants, can enhance their removal potential in the cleaning process. In this review, after a short survey of phytoremediation technologies and the biochemical mechanisms activated by plants to tolerate and detoxify heavy metals and herbicides, the use of herbicide-safeners as a tool to increase the phytoremediation performance is reviewed and discussed.

Comparative Proteomic Analysis of Plant-Pathogen Interactions in Resistant and Susceptible Poplar Ecotypes Infected with Botryosphaeria dothidea

Poplar are important forestry species in China, but the Botryosphaeria dothidea pathogen causes serious economic losses worldwide. To identify candidate B. dothidea resistance proteins and explore the molecular mechanisms involved in poplar-pathogen interactions, proteomic responses of stem samples from resistant and susceptible poplar ecotypes to B. dothidea were investigated using nanoflow liquid chromatography-tandem mass spectrometry with label-free quantitative analysis. We identified 588 proteins, divided into 21 biological process categories including 48 oxidoreductases, 72 hydrolytic enzymes, 80 metabolic enzymes, and 29 proteins of unknown function. Differential proteome analysis revealed large differences between resistant Populus tomentosa Carr and susceptible Populus beijingensis Hsu ecotypes before and after inoculation. Among 102 identified proteins, 22 were highly upregulated in the resistant genotype but downregulated in the susceptible genotype. Proteins induced in P. tomentosa Carr in response to B. dothidea are associated with plant defenses including oxidoreductase activity (catalase, isocitrate dehydrogenase, and superoxide dismutase), phenylpropanoid biosynthesis and phenylalanine metabolism (alcohol dehydrogenase), photosynthesis (ATP synthase subunit alpha, ATP synthase gamma chain, photosystem I P700 chlorophyll a apoprotein A2, photosystem II CP47 chlorophyll apoprotein), carbon fixation (pyruvate kinase, triosephosphate isomerase, malic enzyme, phosphoglycerate kinase, ribulose-1,5-bisphosphate carboxylase, and ribulose bisphosphate carboxylase small chain), and glycolysis/gluconeogenesis (fructose-bisphosphate aldolase). Kyoto Encyclopedia of Genes and Genomes pathway analysis identified 168 proteins related to metabolic pathways, 41 proteins related to the biosynthesis of phenylpropanoids, and 36 proteins related to the biosynthesis of plant hormones, the biosynthesis of alkaloids derived from ornithine, lysine, and nicotinic acid, and photosynthesis in response to B. dothidea. Our findings provide insight into plant-pathogen interactions in resistant and susceptible poplar ecotypes infected with B. dothidea and could assist the development of novel strategies for fighting poplar canker disease.

Interactive effects of drought and heat stresses on morpho-physiological attributes, yield, nutrient uptake and oxidative status in maize hybrids

Maize is a sensitive crop to drought and heat stresses, particularly at the reproductive stages of development. The present study investigated the individual and interactive effects of drought (50% field capacity) and heat (38 °C/30 °C) stresses on morpho-physiological growth, yield, nutrient uptake and oxidative metabolism in two maize hybrids i.e., 'Xida 889' and 'Xida 319'. The stress treatments were applied at tasseling stage for 15 days. Drought, heat and drought + heat stress caused oxidative stress by the over-production of ROS (O2-, H2O2, OH-) and enhanced malondialdehyde contents, which led to reduced photosynthetic components, nutrients uptake and yield attributes. The concurrent occurrence of drought and heat was more severe for maize growth than the single stress. However, both stresses induced the metabolites accumulation and enzymatic and non-enzymatic antioxidants to prevent the oxidative damage. The performance of Xida 899 was more prominent than the Xida 319. The greater tolerance of Xida 889 to heat and drought stresses was attributed to strong antioxidant defense system, higher osmolyte accumulation, and maintenance of photosynthetic pigments and nutrient balance compared with Xida 319.

Shared and genetically distinct Zea mays transcriptome responses to ongoing and past low temperature exposure

BACKGROUND

Cold temperatures and their alleviation affect many plant traits including the abundance of protein coding gene transcripts. Transcript level changes that occur in response to cold temperatures and their alleviation are shared or vary across genotypes. In this study we identify individual transcripts and groups of functionally related transcripts that consistently respond to cold and its alleviation. Genes that respond differently to temperature changes across genotypes may have limited functional importance. We investigate if these genes share functions, and if their genotype-specific gene expression levels change in magnitude or rank across temperatures.

RESULTS

We estimate transcript abundances from over 22,000 genes in two unrelated Zea mays inbred lines during and after cold temperature exposure. Genotype and temperature contribute to many genes' abundances. Past cold exposure affects many fewer genes. Genes up-regulated in cold encode many cytokinin glucoside biosynthesis enzymes, transcription factors, signalling molecules, and proteins involved in diverse environmental responses. After cold exposure, protease inhibitors and cuticular wax genes are newly up-regulated, and environmentally responsive genes continue to be up-regulated. Genes down-regulated in response to cold include many photosynthesis, translation, and DNA replication associated genes. After cold exposure, DNA replication and translation genes are still preferentially downregulated. Lignin and suberin biosynthesis are newly down-regulated. DNA replication, reactive oxygen species response, and anthocyanin biosynthesis genes have strong, genotype-specific temperature responses. The ranks of genotypes' transcript abundances often change across temperatures.

CONCLUSIONS

We report a large, core transcriptome response to cold and the alleviation of cold. In cold, many of the core suite of genes are up or downregulated to control plant growth and photosynthesis and limit cellular damage. In recovery, core responses are in part to prepare for future stress. Functionally related genes are consistently and greatly up-regulated in a single genotype in response to cold or its alleviation, suggesting positive selection has driven genotype-specific temperature responses in maize.

Bud Dormancy in Perennial Fruit Tree Species: A Pivotal Role for Oxidative Cues

For perennial plants, bud dormancy is a crucial step as its progression over winter determines the quality of bud break, flowering, and fruiting. In the past decades, many studies, based on metabolic, physiological, subcellular, genetic, and genomic analyses, have unraveled mechanisms underlying bud dormancy progression. Overall, all the pathways identified are interconnected in a very complex manner. Here, we review early and recent findings on the dormancy processes in buds of temperate fruit trees species including hormonal signaling, the role of plasma membrane, carbohydrate metabolism, mitochondrial respiration and oxidative stress, with an effort to link them together and emphasize the central role of reactive oxygen species accumulation in the control of dormancy progression.

Polyamines may influence phytochelatin synthesis during Cd stress in rice

Although the metabolism of phytochelatins and higher polyamines are linked with each other, the direct relationship between them under heavy metal stress has not yet been clarified. Two approaches were used to reveal the influence of polyamine content on cadmium stress responses, particularly with regard to phytochelatin synthesis: putrescine pre-treatment of rice plants followed by cadmium stress, and treatment with the putrescine synthesis inhibitor, 2-(difluoromethyl)ornithine combined with cadmium treatment. The results indicated that putrescine pre-treatment enhanced the adverse effect of cadmium, while the application of 2-(difluoromethyl)ornithine reduced it to a certain extent. These differences were associated with increased polyamine content, more intensive polyamine metabolism, but decreased thiol and phytochelatin contents. The gene expression level and enzyme activity of phytochelatin synthase also decreased in rice treated with putrescine prior to cadmium stress, compared to cadmium treatment alone. In contrast, the inhibition of putrescine synthesis during cadmium treatment resulted in higher gene expression level of phytochelatin synthase. The results suggest that polyamines may have a substantial influence on phytochelatin synthesis at several levels under cadmium stress in rice.

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.

Transcriptomic response of maize primary roots to low temperatures at seedling emergence

BACKGROUND

Maize (Zea mays) is a C₄ tropical cereal and its adaptation to temperate climates can be problematic due to low soil temperatures at early stages of establishment.

METHODS

In the current study we have firstly investigated the physiological response of twelve maize varieties, from a chilling condition adapted gene pool, to sub-optimal growth temperature during seedling emergence. To identify transcriptomic markers of cold tolerance in already adapted maize genotypes, temperature conditions were set below the optimal growth range in both control and low temperature groups. The conditions were as follows; control (18 °C for 16 h and 12 °C for 8 h) and low temperature (12 °C for 16 h and 6 °C for 8 h). Four genotypes were identified from the condition adapted gene pool with significant contrasting chilling tolerance.

RESULTS

Picker and PR39B29 were the more cold-tolerant lines and Fergus and Codisco were the less cold-tolerant lines. These four varieties were subjected to microarray analysis to identify differentially expressed genes under chilling conditions. Exposure to low temperature during establishment in the maize varieties Picker, PR39B29, Fergus and Codisco, was reflected at the transcriptomic level in the varieties Picker and PR39B29. No significant changes in expression were observed in Fergus and Codisco following chilling stress. A total number of 64 genes were differentially expressed in the two chilling tolerant varieties. These two varieties exhibited contrasting transcriptomic profiles, in which only four genes overlapped.

DISCUSSION

We observed that maize varieties possessing an enhanced root growth ratio under low temperature were more tolerant, which could be an early and inexpensive measure for germplasm screening under controlled conditions. We have identified novel cold inducible genes in an already adapted maize breeding gene pool. This illustrates that further varietal selection for enhanced chilling tolerance is possible in an already preselected gene pool.

Physiological and Biochemical Mechanisms of Seed Priming-Induced Chilling Tolerance in Rice Cultivars

Rice belongs to tropical and subtropical environments and is extremely sensitive to chilling stress particularly during emergence and early stages of seedling development. Seed priming can be a good approach to enhance rice germination and stand establishment under chilling stress. The present study examined the role of different seed priming techniques viz., hydropriming, osmopriming, redox priming, chemical priming, and hormonal priming, in enhancing the chilling tolerance in rice. The most effective reagents and their pre-optimized concentrations based on preliminary experiments were used in this study. Two different rice cultivars were sown under chilling stress (18°C) and normal temperatures (28°C) in separate growth chambers. A non-primed control treatment was also maintained for comparison. Chilling stress caused erratic and delayed germination, poor seedling growth, reduced starch metabolism, and lower respiration rate, while higher lipid peroxidation and hydrogen peroxide accumulation in rice seedlings of both cultivars. Nevertheless, all the seed priming treatments effectively alleviated the negative effects of chilling stress. In addition, seed priming treatments triggered the activities of superoxide dismutase, peroxidase, and catalase, and enhanced the accumulations of glutathione and free proline in rice seedlings, which suggests that these measures help prevent the rice seedlings from chilling induced oxidative stress. Chemical priming with selenium and hormonal priming with salicylic acid remained more effective treatments for both rice cultivars under chilling stress than all other priming treatments. The better performance and greater tolerance of primed rice seedlings was associated with enhanced starch metabolism, high respiration rate, lower lipid peroxidation, and strong antioxidative defense system under chilling stress.

Transcriptomic profiling of Arabidopsis thaliana mutant pad2.1 in response to combined cold and osmotic stress

The contribution of glutathione (GSH) in stress tolerance, defense response and antioxidant signaling is an established fact. In this study transcriptome analysis of pad2.1, an Arabidopsis thaliana mutant, after combined osmotic and cold stress treatment has been performed to explore the intricate position of GSH in the stress and defense signaling network in planta. Microarray data revealed the differential regulation of about 1674 genes in pad2.1 amongst which 973 and 701 were significantly up- and down-regulated respectively. Gene enrichment, functional pathway analysis by DAVID and MapMan analysis identified various stress and defense related genes viz. members of heat shock protein family, peptidyl prolyl isomerase (PPIase), thioredoxin peroxidase (TPX2), glutathione-S-transferase (GST), NBS-LRR type resistance protein etc. as down-regulated. The expression pattern of the above mentioned stress and defense related genes and APETALA were also validated by comparative proteomic analysis of combined stress treated Col-0 and pad2.1. Functional annotation noted down-regulation of UDP-glycosyl transferase, 4-coumarate CoA ligase 8, cinnamyl alcohol dehydrogenase 4 (CAD4), ACC synthase and ACC oxidase which are the important enzymes of phenylpropanoid, lignin and ethylene (ET) biosynthetic pathway respectively. Since the only difference between Col-0 (Wild type) and pad2.1 is the content of GSH, so, this study suggested that in addition to its association with specific stress responsive genes and proteins, GSH provides tolerance to plants by its involvement with phenylpropanoid, lignin and ET biosynthesis under stress conditions.

Synthesis and role of salicylic acid in wheat varieties with different levels of cadmium tolerance

Wheat genotypes with different endogenous SA contents were investigated, in order to reveal how cadmium influences salicylic acid (SA) synthesis, and to find possible relationships between SA and certain protective compounds (members of the antioxidants and the heavy metal detoxification system) and between the SA content and the level of cadmium tolerance. Cadmium exposure induced SA synthesis, especially in the leaves, and it is suggested that the phenyl-propanoid synthesis pathway is responsible for the accumulation of SA observed after cadmium stress. Cadmium influenced the synthesis and activation of protective compounds to varying extents in wheat genotypes with different levels of tolerance; the roots and leaves also responded differently to cadmium stress. Although a direct relationship was not found between the initial SA levels and the degree of cadmium tolerance, the results suggest that the increase in the root SA level during cadmium stress in the Mv varieties could be related with the enhancement of the internal glutathione cycle, thus inducing the antioxidant and metal detoxification systems, which promote Cd stress tolerance in wheat seedlings. The positive correlation between certain SA-related compounds and protective compounds suggests that SA-related signalling may also play a role in the acclimation to heavy metal stress.

Intra-specific variability in the response of maize to arsenic exposure

The response of maize (Zea mays L.) to inorganic arsenic exposure was studied, at the seedling stage under hydroponic conditions, preliminarily in sixteen lines (fourteen hybrids and two inbred lines) and then, more deeply, in six of these lines, selected by showing contrasting differences in their sensitivity to the metalloid. The results indicated that (i) maize is rather tolerant to arsenic toxicity, (ii) arsenite is more phytotoxic than arsenate, (iii) roots are less sensitive than shoots to the metalloid, (iv) a great accumulation of non-protein thiols (probably phytochelatins), without substantial effect on the glutathione content, is produced in roots but not in shoots of arsenic-exposed plants and (v) maize is able to accumulate high levels of arsenic in roots with very low translocation to shoots. The study, thus, suggests that maize, for its very low rate of acropetal transport of arsenic from roots to shoots, may be a safe crop in relation to the risk of entry of metalloid in the food chain and, for being an important bioenergy crop capable of expressing high levels of arsenic tolerance and accumulation in roots, may represent an interesting opportunity for the exploitation of agricultural useless arsenic contaminated lands.

Central role of the flowering repressor ZCCT2 in the redox control of freezing tolerance and the initial development of flower primordia in wheat

BACKGROUND

As both abiotic stress response and development are under redox control, it was hypothesised that the pharmacological modification of the redox environment would affect the initial development of flower primordia and freezing tolerance in wheat (Triticum aestivum L.).

RESULTS

Pharmacologically induced redox changes were monitored in winter (T. ae. ssp. aestivum cv. Cheyenne, Ch) and spring (T. ae. ssp. spelta; Tsp) wheat genotypes grown after germination at 20/17°C for 9 d (chemical treatment: last 3 d), then at 5°C for 21 d (chemical treatment: first 4 d) and subsequently at 20/17°C for 21 d (recovery period). Thiols and their disulphide forms were measured and based on these data reduction potentials were calculated. In the freezing-tolerant Ch the chemical treatments generally increased both the amount of thiol disulphides and the reduction potential after 3 days at 20/17°C. In the freezing-sensitive Tsp a similar effect of the chemicals on these parameters was only observed after the continuation of the treatments for 4 days at 5°C. The applied chemicals slightly decreased root fresh weight and increased freezing tolerance in Ch, whereas they increased shoot fresh weight in Tsp after 4 days at 5°C. As shown after the 3-week recovery at 20/17°C, the initial development of flower primordia was accelerated in Tsp, whereas it was not affected by the treatments in Ch. The chemicals differently affected the expression of ZCCT2 and that of several other genes related to freezing tolerance and initial development of flower primordia in Ch and Tsp after 4 d at 5°C.

CONCLUSIONS

Various redox-altering compounds and osmotica had differential effects on glutathione disulphide content and reduction potential, and consequently on the expression of the flowering repressor ZCCT2 in the winter wheat Ch and the spring wheat Tsp. We propose that the higher expression of ZCCT2 in Ch may be associated with activation of genes of cold acclimation and its lower expression in Tsp with the induction of genes accelerating initial development of flower primordia. In addition, ZCCT2 may be involved in the coordinated control of the two processes.

S-nitrosylation analysis in Brassica juncea apoplast highlights the importance of nitric oxide in cold-stress signaling

Reactive nitrogen species (RNS) including nitric oxide (NO) are important components of stress signaling. However, RNS-mediated signaling in the apoplast remains largely unknown. NO production measured in the shoot apoplast of Brassica juncea seedlings showed nonenzymatic nitrite reduction to NO. Thiol pool quantification showed cold-induced increase in the protein (including S-nitrosothiols) as well as non protein thiols. Proteins from the apoplast were resolved as 109 spots on the 2-D gel, while S-nitrosoglutathione-treated (a NO donor), neutravidin-agarose affinity chromatography-purified S-nitrosylated proteins were resolved as 52 spots. Functional categorization after MALDI-TOF/TOF identification showed 41 and 38% targets to be metabolic/cell-wall-modifying and stress-related, respectively, suggesting the potential role(s) of S-nitrosylation in regulating these responses. Additionally, identification of cold-stress-modulated putative S-nitrosylated proteins by nLC-MS/MS showed that only 38.4% targets with increased S-nitrosylation were secreted by classical pathway, while the majority (61.6%) of these were secreted by unknown/nonclassical pathways. Cold-stress-increased dehydroascorbate reductase and glutathione S-transferase activity via S-nitrosylation and promoted ROS detoxification by ascorbate regeneration and hydrogen peroxide detoxification. Taken together, cold-mediated NO production, thiol pool enrichment, and identification of the 48 putative S-nitrosylated proteins, including 25 novel targets, provided the preview of RNS-mediated cold-stress signaling in the apoplast.

QTLs for seed vigor-related traits identified in maize seeds germinated under artificial aging conditions

High seed vigor is important for agricultural production due to the associated potential for increased growth and productivity. However, a better understanding of the underlying molecular mechanisms is required because the genetic basis for seed vigor remains unknown. We used single-nucleotide polymorphism (SNP) markers to map quantitative trait loci (QTLs) for four seed vigor traits in two connected recombinant inbred line (RIL) maize populations under four treatment conditions during seed germination. Sixty-five QTLs distributed between the two populations were identified and a meta-analysis was used to integrate genetic maps. Sixty-one initially identified QTLs were integrated into 18 meta-QTLs (mQTLs). Initial QTLs with contribution to phenotypic variation values of R²>10% were integrated into mQTLs. Twenty-three candidate genes for association with seed vigor traits coincided with 13 mQTLs. The candidate genes had functions in the glycolytic pathway and in protein metabolism. QTLs with major effects (R²>10%) were identified under at least one treatment condition for mQTL2, mQTL3-2, and mQTL3-4. Candidate genes included a calcium-dependent protein kinase gene (302810918) involved in signal transduction that mapped in the mQTL3-2 interval associated with germination energy (GE) and germination percentage (GP), and an hsp20/alpha crystallin family protein gene (At5g51440) that mapped in the mQTL3-4 interval associated with GE and GP. Two initial QTLs with a major effect under at least two treatment conditions were identified for mQTL5-2. A cucumisin-like Ser protease gene (At5g67360) mapped in the mQTL5-2 interval associated with GP. The chromosome regions for mQTL2, mQTL3-2, mQTL3-4, and mQTL5-2 may be hot spots for QTLs related to seed vigor traits. The mQTLs and candidate genes identified in this study provide valuable information for the identification of additional quantitative trait genes.

Proteomic profiling of γ-ECS overexpressed transgenic Nicotiana in response to drought stress

The contribution of Glutathione (GSH) in drought stress tolerance is an established fact. However, the proteins which are directly or indirectly related to the increased level of GSH in response to drought stress are yet to be known. To explore this, here, transgenic tobacco plants (NtGp11) overexpressing gamma-glutamylcysteine synthetase (γ-ECS) was tested for tolerance against drought stress. NtGp11 conferred tolerance to drought stress by increased germination rate, water retention, water recovery, chlorophyll, and proline content compared with wild-type plants. Semi-quantitative RT-PCR analysis revealed that the transcript levels of stress-responsive genes were higher in NtGp11 compared with wild-type in response to drought stress. Two-dimensional gel electrophoresis (2-DE) coupled with MALDI TOF-TOF MS/MS analysis has been used to identify 43 differentially expressed proteins in response to drought in wild-type and NtGp11 plants. The results demonstrated the up-accumulation of 58.1% of proteins among which 36%, 24%, and 20% of them were related to stress and defense, carbon metabolism and energy metabolism categories, respectively. Taken together, our results demonstrated that GSH plays an important role in combating drought stress in plants by inducing stress related genes and proteins like HSP70, chalcone synthase, glutathione peroxidase, thioredoxin peroxidase, ACC oxidase, and heme oxygenase I.

Nitric oxide affects salt-induced changes in free amino acid levels in maize

It was assumed that salt-induced redox changes affect amino acid metabolism in maize (Zea mays L.), and this influence may be modified by NO. The applied NaCl treatment reduced the fresh weight of shoots and roots. This decrease was smaller after the combined application of NaCl and an NO-donor ((Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, DETA/NO) in the shoots, while it was greater after simultaneous treatment with NaCl and nitro-L-arginine (L-NNA, inhibitor of NO synthesis) in the roots. The quantum yield efficiency of photosystem II was not influenced by the treatments. NaCl had a significant effect on the redox environment in the leaves as it was shown by the increase in the amount of glutathione disulphide and in the redox potential of the glutathione/glutathione disulphide redox pair. This influence of NaCl was modified by DETA/NO and L-NNA. Pharmacological modification of NO levels affected salt-induced changes in both the total free amino acid content and in the free amino acid composition. NaCl alone increased the concentration of almost all amino acids which effect was strengthened by DETA/NO in the case of Pro. L-NNA treatment resulted in a significant increase in the Ala, Val, Gly and Tyr contents. The Ile, Lys and Val concentrations rose considerably after the combined application of NaCl and DETA/NO compared to NaCl treatment alone in the recovery phase. NaCl also increased the expression of several genes related to the amino acid and antioxidant metabolism, and this effect was modified by DETA/NO. In conclusion, modification of NO levels affected salt-induced, glutathione-dependent redox changes and simultaneously the free amino acid composition and the level of several free amino acids. The observed much higher Pro content in plants treated with both NaCl and DETA/NO during recovery may contribute to the protective effect of NO against salt stress.

Proteome Analysis of Rice (Oryza sativa L.) Mutants Reveals Differentially Induced Proteins during Brown Planthopper (Nilaparvata lugens) Infestation

Although rice resistance plays an important role in controlling the brown planthopper (BPH), Nilaparvata lugens, not all varieties have the same level of protection against BPH infestation. Understanding the molecular interactions in rice defense response is an important tool to help to reveal unexplained processes that underlie rice resistance to BPH. A proteomics approach was used to explore how wild type IR64 and near-isogenic rice mutants with gain and loss of resistance to BPH respond during infestation. A total of 65 proteins were found markedly altered in wild type IR64 during BPH infestation. Fifty-two proteins associated with 11 functional categories were identified using mass spectrometry. Protein abundance was less altered at 2 and 14 days after infestation (DAI) (T1, T2, respectively), whereas higher protein levels were observed at 28 DAI (T3). This trend diminished at 34 DAI (T4). Comparative analysis of IR64 with mutants showed 22 proteins that may be potentially associated with rice resistance to the brown planthopper (BPH). Ten proteins were altered in susceptible mutant (D1131) whereas abundance of 12 proteins including S-like RNase, Glyoxalase I, EFTu1 and Salt stress root protein “RS1” was differentially changed in resistant mutant (D518). S-like RNase was found in greater quantities in D518 after BPH infestation but remained unchanged in IR64 and decreased in D1131. Taken together, this study shows a noticeable level of protein abundance in the resistant mutant D518 compared to the susceptible mutant D1131 that may be involved in rendering enhanced level of resistance against BPH.

Metabolism of reactive oxygen species and reactive nitrogen species in pepper (Capsicum annuum L.) plants under low temperature stress

Low temperature is an environmental stress that affects crop production and quality and regulates the expression of many genes, and the level of a number of proteins and metabolites. Using leaves from pepper (Capsicum annum L.) plants exposed to low temperature (8 °C) for different time periods (1 to 3 d), several key components of the metabolism of reactive nitrogen and oxygen species (RNS and ROS, respectively) were analysed. After 24 h of exposure at 8 °C, pepper plants exhibited visible symptoms characterized by flaccidity of stems and leaves. This was accompanied by significant changes in the metabolism of RNS and ROS with an increase of both protein tyrosine nitration (NO(2) -Tyr) and lipid peroxidation, indicating that low temperature induces nitrosative and oxidative stress. During the second and third days at low temperature, pepper plants underwent cold acclimation by adjusting their antioxidant metabolism and reverting the observed nitrosative and oxidative stress. In this process, the levels of the soluble non-enzymatic antioxidants ascorbate and glutathione, and the activity of the main NADPH-generating dehydrogenases were significantly induced. This suggests that ascorbate, glutathione and the NADPH-generating dehydrogenases have a role in the process of cold acclimation through their effect on the redox state of the cell.

Drought and salt stress tolerance of an Arabidopsis glutathione S-transferase U17 knockout mutant are attributed to the combined effect of glutathione and abscisic acid

Although glutathione S-transferases (GSTs) are thought to play major roles in oxidative stress metabolism, little is known about the regulatory functions of GSTs. We have reported that Arabidopsis (Arabidopsis thaliana) GLUTATHIONE S-TRANSFERASE U17 (AtGSTU17; At1g10370) participates in light signaling and might modulate various aspects of development by affecting glutathione (GSH) pools via a coordinated regulation with phytochrome A. Here, we provide further evidence to support a negative role of AtGSTU17 in drought and salt stress tolerance. When AtGSTU17 was mutated, plants were more tolerant to drought and salt stresses compared with wild-type plants. In addition, atgstu17 accumulated higher levels of GSH and abscisic acid (ABA) and exhibited hyposensitivity to ABA during seed germination, smaller stomatal apertures, a lower transpiration rate, better development of primary and lateral root systems, and longer vegetative growth. To explore how atgstu17 accumulated higher ABA content, we grew wild-type plants in the solution containing GSH and found that they accumulated ABA to a higher extent than plants grown in the absence of GSH, and they also exhibited the atgstu17 phenotypes. Wild-type plants treated with GSH also demonstrated more tolerance to drought and salt stresses. Furthermore, the effect of GSH on root patterning and drought tolerance was confirmed by growing the atgstu17 in solution containing l-buthionine-(S,R)-sulfoximine, a specific inhibitor of GSH biosynthesis. In conclusion, the atgstu17 phenotype can be explained by the combined effect of GSH and ABA. We propose a role of AtGSTU17 in adaptive responses to drought and salt stresses by functioning as a negative component of stress-mediated signal transduction pathways.

Low temperature and oxidative stress in cereals

Low temperature stress results in significant yield losses in cereals. Cereals of subtropical origin like maize and rice are severely damaged at temperatures below 10°C and are killed at subzero temperatures. This stress effect is called chilling. In contrast, cereals originating from the temperate zone (wheat, barley, rye and oat) may survive short periods even between −10 and −20°C, depending on the species and varieties, so they are freezing-tolerant to various extents. For the winter type of these cereals a gradual decrease in temperature up to −4°C results in cold acclimation, which increases their freezing tolerance. In addition, it fulfils their vernalization requirement, which is necessary for the correct timing of the vegetative to generative transition. During both chilling and freezing, oxidative stress is induced. Although the accumulation of high concentrations of reactive oxygen species may be lethal, a moderate increase in their level may activate various defence mechanisms. In this review the role of reactive oxygen species, antioxidants, carbohydrates, free amino acids, polyamines and hormones in the response to low temperature stress in cereals will be described. The effect of light and the use of the model plant Brachypodium distachyon L. to reveal the biochemical and molecular biological background of this response will also be discussed.

Stress hormones and abiotic stresses have different effects on antioxidants in maize lines with different sensitivity

The effect of stress hormones and abiotic stress treatments on reactive oxygen species and on antioxidants was compared in two maize (Zea mays L.) lines (Penjalinan and Z7) having different stress tolerance. Following treatment with abscisic acid, salicylic acid or hydrogen peroxide, the amount of hydrogen peroxide and lipid peroxides increased, while after osmotic stress or cultivation in continuous darkness, the levels were unchanged or decreased. The higher amount of lipid peroxides in Penjalinan indicated its greater sensitivity compared to Z7. The level of the examined antioxidants was increased by nearly all treatments. Glutathione and cysteine contents were higher after salicylic acid, hydrogen peroxide and polyethylene glycol treatments and lower after application of abscisic acid, NaCl and growth in darkness in Z7 than in Penjalinan. The activity of glutathione reductase, ascorbate peroxidase, catalase and glutathione S-transferase was higher after almost all treatments in Z7. The expression of the glutathione synthetase (EC 6.3.2.3) gene was not affected by the treatments, while the level of gamma-glutamylcysteine synthetase (EC 6.3.2.2) and glutathione reductase (EC 1.6.4.2) transcripts increased after most treatments. The two stress hormones and the stress treatments resulted in different changes in antioxidant levels in the two maize lines, which indicates the specific, stress tolerance-dependent response of plants to the various growth regulators and adverse environmental effects that were examined.

Inhibition of glutathione synthesis decreases chilling tolerance in Chorispora bungeana callus

The possible roles of reduced glutathione (GSH) in chilling tolerance were studied in callus generated from a representative alpine plant, Chorispora bungeana Fisch. & C.A. Mey (C. bungeana). The callus grew well under low-temperature and chilling treatment led only to slight injury, as indicated by a low level of ion leakage (IL). Malondialdehyde measurements also were not elevated, however GSH was. Exogenously application of l-buthionine-(S R)-sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase (gamma-ECS), arrested the GSH accumulation induced by chilling and resulted in a significant decrease in cell growth and an increase in IL and malondialdehyde. These results implied that C. bungeana is a plant with a strong low-temperature tolerance mechanism, and the tolerance of C. bungeana may be associated with GSH accumulation. Under chilling treatment, the proportion of unsaturated fatty acid in the plasma membrane (PM) increased significantly in callus of C. bungeana mainly due to increases in C18:2 and C18:3, the membrane fluidity (indicated by DPH fluorescent polarization) however was maintained. High PM H(+)-ATPase activities were also induced by chilling. Exogenously application of BSO blocked the effects of chilling treatments on the changes of fatty acids and PM H(+)-ATPase activities, reducing the PM membrane fluidity. On the other hand, simultaneous application of GSH and BSO to callus under chilling treatments reversed the effects of BSO on the changes of fatty acids, PM fluidity and PM H(+)-ATPase activities. These results suggested that GSH induced by low-temperature treatments may confer chilling tolerance to C. bungeana, probably by increasing unsaturated fatty acid compositions and maintaining PM fluidity and high enzymatic activity.

Complex signaling network in regulation of adenosine 5'-phosphosulfate reductase by salt stress in Arabidopsis roots

Sulfur-containing compounds play an important role in plant stress defense; however, only a little is known about the molecular mechanisms of regulation of sulfate assimilation by stress. Using known Arabidopsis (Arabidopsis thaliana) mutants in signaling pathways, we analyzed regulation of the key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase (APR), by salt stress. APR activity and mRNA levels of all three APR isoforms increased 3-fold in roots after 5 h of treatment with 150 mm NaCl. The regulation of APR was not affected in mutants deficient in abscisic acid (ABA) synthesis and treatment of the plants with ABA did not affect the mRNA levels of APR isoforms, showing that APR is regulated by salt stress in an ABA-independent manner. In mutants deficient in jasmonate, salicylate, or ethylene signaling, APR mRNA levels were increased upon salt exposure similar to wild-type plants. Surprisingly, however, APR enzyme activity was not affected by salt in these plants. The same result was obtained in mutants affected in cytokinin and auxin signaling. Signaling via gibberellic acid, on the other hand, turned out to be essential for the increase in APR mRNA by salt treatment. These results demonstrate an extensive posttranscriptional regulation of plant APR and reveal that the sulfate assimilation pathway is controlled by a complex network of multiple signals on different regulatory levels.

The role of 5'-adenylylsulfate reductase in the sulfur assimilation pathway of soybean: molecular cloning, kinetic characterization, and gene expression

Soybean seeds are a major source of protein, but contain low levels of sulfur-containing amino acids. With the objective of studying the sulfur assimilation pathway of soybean, a full-length cDNA clone for 5'-adenylylsulfate reductase (APS reductase) was isolated and characterized. The cDNA clone contained an open reading frame of 1414 bp encoding a 52 kDa protein with a N-terminal chloroplast/plastid transit peptide. Southern blot analysis of genomic DNA indicated that the APS reductase in soybean is encoded by a small multigene family. Biochemical characterization of the heterologously expressed and purified protein shows that the clone encoded a functional APS reductase. Although expressed in tissues throughout the plant, these analyses established an abundant expression of the gene and activity of the encoded protein in the early developmental stages of soybean seed, which declined with seed maturity. Sulfur and phosphorus deprivation increased this expression level, while nitrogen starvation repressed APS reductase mRNA transcript and protein levels. Cold-treatment increased expression and the total activity of APS reductase in root tissues. This study provides insight into the sulfur assimilation pathway of this nutritionally important legume.

Differential responses of antioxidative system to chilling and drought in four rice cultivars differing in sensitivity

Responses of antioxidative defense systems to chilling and drought stresses were comparatively studied in four cultivars of rice (Oryza sativa L.) differing in sensitivity, two of them (Xiangnuo no. 1 and Zimanuo) are tolerant to chilling but sensitive to drought and the other two (Xiangzhongxian no. 2 and IR50) are tolerant to drought but sensitive to chilling. The seedlings of rice were transferred into growth chamber for 5 d at 8 degrees C as chilling treatment, or at 28 degrees C as control, or at 28 degrees C but cultured in 23% PEG-6000 solution as drought stress treatment. Under drought stress the elevated levels of electrolyte leakage, contents of H(2)O(2) and total thiobarbituric acid-reacting substances (TBARS) in Xiangzhongxian no. 2 and IR50 are lower than those in Xiangnuo no. 1 and Zimanuo. On the contrary, Xiangnuo no. 1 and Zimanuo have much lower level of electrolyte leakage, H(2)O(2) and TBARS than Xiangzhongxian no. 2 and IR50 under chilling stress. Activities of antioxidant enzymes (superoxide dismutase (SOD), catalase, and ascorbate-peroxidase (APX)) and contents of antioxidants (ascorbaic acid and reduced glutathione) were measured during the stress treatments. All of them were enhanced greatly until 3 d after drought stress in the two drought-tolerant cultivars, or after chilling stress in the two chilling-tolerant cultivars. They all were decreased at 5 d after stress treatments. On the other hand, activities of antioxidant enzymes and contents of antioxidants were decreased greatly in the drought-sensitive cultivars after drought stress, or in the chilling-sensitive cultivars after chilling stress. The results indicated that tolerance to drought or chilling in rice is well associated with the enhanced capacity of antioxidative system under drought or chilling condition, and that the sensitivity of rice to drought or chilling is linear correlated to the decreased capacity of antioxidative system.

Calcium-regulated phosphorylation of soybean serine acetyltransferase in response to oxidative stress

Glycine max serine acetyltransferase 2;1 (GmSerat2;1) is a member of a family of enzymes that catalyze the first reaction in the biosynthesis of cysteine from serine. It was identified by interaction cloning as a protein that binds to calcium-dependent protein kinase. In vitro phosphorylation assays showed that GmSerat2;1, but not GmSerat2;1 mutants (S378A or S378D), were phosphorylated by soybean calcium-dependent protein kinase isoforms. Recombinant GmSerat2;1 was also phosphorylated by soybean cell extract in a Ca2+-dependent manner. Phosphorylation of recombinant GmSerat2;1 had no effect on its catalytic activity but rendered the enzyme insensitive to the feedback inhibition by cysteine. In transient expression analyses, fluorescently tagged GmSerat2;1 localized in the cytoplasm and with plastids. Phosphorylation state-specific antibodies showed that an increase in GmSerat2;1 phosphorylation occurred in vivo within 5 min of treatment of soybean cells with 0.5 mM hydrogen peroxide, whereas GmSerat2;1 protein synthesis was not significantly induced until 1 h after oxidant challenge. Internal Ca2+ was required in the induction of both GmSerat2;1 phosphorylation and synthesis. Treatment of cells with calcium antagonists showed that externally derived Ca2+ was important for retaining GmSerat2;1 at a basal level of phosphorylation but was not necessary for its hydrogen peroxide-induced synthesis. Protein phosphatase type 1, but not type 2A or alkaline phosphatase, dephosphorylated native GmSerat2;1 in vitro. These results support the hypothesis that GmSerat2;1 is regulated by calcium-dependent protein kinase phosphorylation in vivo and suggest that increased GmSerat2;1 synthesis and phosphorylation in response to active oxygen species could play a role in anti-oxidative stress response.

Soybean ATP sulfurylase, a homodimeric enzyme involved in sulfur assimilation, is abundantly expressed in roots and induced by cold treatment

Soybeans are a rich source of protein and a key feed ingredient in livestock production, but lack sufficient levels of cysteine and methionine to meet the nutritional demands of swine or poultry as feed components. Although engineering the sulfur assimilatory pathway could lead to increased sulfur-containing amino acid content, little is known about this pathway in legumes. Here, we describe the cloning and characterization of soybean ATP sulfurylase (ATPS), which acts as the metabolic entry point into the sulfur assimilation pathway. Analysis of the ATPS clone isolated from a soybean seedling cDNA library revealed an open-reading frame, encoding a 52 kDa polypeptide with an N-terminal chloroplast/plastid transit peptide, which was related to the enzymes from Arabidopsis, potato, human, and yeast. Soybean ATP sulfurylase was expressed in Escherichia coli and purified to apparent homogeneity. Based on gel-filtration chromatography, the enzyme functions as a 100 kDa homodimer. Analysis of genomic DNA by Southern blotting revealed that multiple genes encode ATP sulfurylase in soybean. Analysis of the transcript profiles retrieved from a soybean EST database indicated that ATP sulfurylase mRNA was most abundant in root tissue. Cold treatment induced mRNA accumulation and enhanced the specific activity of ATP sulfurylase in root tissue. Northern blot analysis indicated a decline in the ATP sulfurylase transcript levels during seed development. Likewise, ATP sulfurylase specific activity also declined in the later stages of seed development. Increasing the expression levels of this key enzyme during soybean seed development could lead to an increase in the availability of sulfur amino acids, thereby enhancing the nutritional value of the crop.

Zea mays L. protein changes in response to potassium dichromate treatments

The plant metabolic response to heavy metal stress is largely unknown. The present investigation was undertaken to examine the influence of different concentrations of potassium dichromate on the Zea mays L. plantlets. A clear effect of chromium on maize plantlets growth and seed germination was observed strating from 100-300 ppm up to 1500 ppm. In this concentration range, chromium uptake was dependent on the concentration in the medium. Metallothioneins, involved in heavy metal binding, were measured by capillary electrophoresis (CE), and showed a dose-response induction. Protein profile analyzed by two-dimensional gel electrophoresis showed differential expression of several proteins. Identification of spots of upregulated proteins was performed by MALDI mass spectrometry. Results showed that proteins induced by heavy metal exposure are principally involved in oxidative stress tolerance or in other stress pathways. Induction of proteins implicated in sugar metabolism was also observed. Identification of factors involved in plant response may lead to a better understanding of the mechanisms involved in cell protection and tolerance. This information could be used to improve agricultural production and environmental quality.

Intercellular distribution of glutathione synthesis in maize leaves and its response to short-term chilling

To investigate the intercellular control of glutathione synthesis and its influence on leaf redox state in response to short-term chilling, genes encoding gamma-glutamylcysteine synthetase (gamma-ECS) and glutathione synthetase (GSH-S) were cloned from maize (Zea mays) and specific antibodies produced. These tools were used to provide the first information on the intercellular distribution of gamma-ECS and GSH-S transcript and protein in maize leaves, in both optimal conditions and chilling stress. A 2-d exposure to low growth temperatures (chill) had no effect on leaf phenotype, whereas return to optimal temperatures (recovery) caused extensive leaf bleaching. The chill did not affect total leaf GSH-S transcripts but strongly induced gamma-ECS mRNA, an effect reversed during recovery. The chilling-induced increase in gamma-ECS transcripts was not accompanied by enhanced total leaf gamma-ECS protein or extractable activity. In situ hybridization and immunolocalization of leaf sections showed that gamma-ECS and GSH-S transcripts and proteins were found in both the bundle sheath (BS) and the mesophyll cells under optimal conditions. Chilling increased gamma-ECS transcript and protein in the BS but not in the mesophyll cells. Increased BS gamma-ECS was correlated with a 2-fold increase in both leaf Cys and gamma-glutamylcysteine, but leaf total glutathione significantly increased only in the recovery period, when the reduced glutathione to glutathione disulfide ratio decreased 3-fold. Thus, while there was a specific increase in the potential contribution of the BS cells to glutathione synthesis during chilling, it did not result in enhanced leaf glutathione accumulation at low temperatures. Return to optimal temperatures allowed glutathione to increase, particularly glutathione disulfide, and this was associated with leaf chlorosis.

An intracellular mechanism of aluminum tolerance associated with high antioxidant status in cultured tobacco cells

An aluminum (Al) tolerance mechanism, together with oxidative stress tolerance, was investigated in an Al tolerant cell line (ALT301) and the parental Al sensitive cell line (SL) of tobacco. During Al exposure in a simple calcium solution for 24 h, Al triggered the evolution of a reactive oxygen species (ROS) in SL much higher than ALT301 [Plant Physiol. 128 (2002) 63]. Under the conditions, Al enhanced comparable rates of citrate secretion from both cell lines to the same extent. Al enhanced the gene expression of manganese superoxide dismutase (MnSOD) in both cell lines, but at a significantly higher rate in SL than in ALT301, and also enhanced the enzyme activity of MnSOD in both cell lines to nearly the same level. These results suggest that the extracellular chelation of Al with organic acids and MnSOD is not involved in the mechanism of Al tolerance of ALT301. ALT301 contained ascorbate (ASA) and glutathione (GSH) levels that were higher than SL under normal growth conditions. During 24 h of post-Al treatment culture in growth medium, but not during 24-h Al exposure in a simple Ca(2+) solution, lipid peroxidation was enhanced in SL much higher than in ALT301, and the average SL amounts of ASA and GSH were exhausted compared to ALT301. Pre-loading of ASA prior to Al treatment improved the growth of SL during the post-Al treatment culture. ALT301 also exhibited cross-tolerance to H(2)O(2), Fe(2+) and Cu(2+). Under these oxidant exposures, ALT301 contained lower levels of intracellular H(2)O(2) or lipid peroxides, and maintained higher amounts of ASA and GSH than SL. Taken together, we conclude that the accumulation of Al in cells enhances the peroxidation of lipids exclusively under growing conditions, and that the higher content of ASA and GSH in ALT301 than in SL seems to be in part responsible for the tolerance mechanism of ALT301 to Al by protecting cells from either lipid peroxidation or H(2)O(2) commonly enhanced by Al or other oxidants.

Induction of glutathione S-transferases in Arabidopsis by herbicide safeners

Herbicide safeners increase herbicide tolerance in cereals but not in dicotyledenous crops. The reason(s) for this difference in safening is unknown. However, safener-induced protection in cereals is associated with increased expression of herbicide detoxifying enzymes, including glutathione S-transferases (GSTs). Treatment of Arabidopsis seedlings growing in liquid medium with various safeners similarly resulted in enhanced GST activities toward a range of xenobiotics with benoxacor, fenclorim, and fluxofenim being the most effective. Safeners also increased the tripeptide glutathione content of Arabidopsis seedlings. However, treatment of Arabidopsis plants with safeners had no effect on the tolerance of seedlings to chloroacetanilide herbicides. Each safener produced a distinct profile of enhanced GST activity toward different substrates suggesting a differential induction of distinct isoenzymes. This was confirmed by analysis of affinity-purified GST subunits by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. AtGSTU19, a tau class GST, was identified as a dominant polypeptide in all samples. When AtGSTU19 was expressed in Escherichia coli, the recombinant enzyme was highly active toward 1-chloro-2,4-dinitrobenzene, as well as chloroacetanilide herbicides. Immunoblot analysis confirmed that AtGSTU19 was induced in response to several safeners. Differential induction of tau GSTs, as well as members of the phi and theta classes by safeners, was demonstrated by RNA-blot analysis. These results indicate that, although Arabidopsis may not be protected from herbicide injury by safeners, at least one component of their detoxification systems is responsive to these compounds.