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.

Transgenic barley lines prove the involvement of TaCBF14 and TaCBF15 in the cold acclimation process and in frost tolerance

The enhancement of winter hardiness is one of the most important tasks facing breeders of winter cereals. For this reason, the examination of those regulatory genes involved in the cold acclimation processes is of central importance. The aim of the present work was the functional analysis of two wheat CBF transcription factors, namely TaCBF14 and TaCBF15, shown by previous experiments to play a role in the development of frost tolerance. These genes were isolated from winter wheat and then transformed into spring barley, after which the effect of the transgenes on low temperature stress tolerance was examined. Two different types of frost tests were applied; plants were hardened at low temperature before freezing, or plants were subjected to frost without a hardening period. The analysis showed that TaCBF14 and TaCBF15 transgenes improve the frost tolerance to such an extent that the transgenic lines were able to survive freezing temperatures several degrees lower than that which proved lethal for the wild-type spring barley. After freezing, lower ion leakage was measured in transgenic leaves, showing that these plants were less damaged by the frost. Additionally, a higher Fv/Fm parameter was determined, indicating that photosystem II worked more efficiently in the transgenics. Gene expression studies showed that HvCOR14b, HvDHN5, and HvDHN8 genes were up-regulated by TaCBF14 and TaCBF15. Beyond that, transgenic lines exhibited moderate retarded development, slower growth, and minor late flowering compared with the wild type, with enhanced transcript level of the gibberellin catabolic HvGA2ox5 gene.

Different approaches involving testing methods, gene mapping and transformation reveal new insights into cereal frost tolerance

Freezing tolerance is a quantitative trait, determined by many genes and also influenced by environmental factors. Thus, the development of reliable testing methods is a prerequisite both for the identification of quantitative trait loci (QTLs) and for the identification of the genes behind the QTLs. Transformation methods proved to be effective in the direct verification of isolated genes involved in low temperature stress responses. In order to develop freezing tolerance, winter cereals must be adapted through a cold hardening period, which not only influences cold adaptation but also initiates the vernalization process necessary for flowering. Recent and ongoing studies are endeavouring to uncover the relationship between freezing tolerance and vernalization response at the genetic and molecular levels. This review aims to explain cereal freezing tolerance on the basis of recent discoveries in the areas outlined above.

Redox changes during cold acclimation affect freezing tolerance but not the vegetative/reproductive transition of the shoot apex in wheat

Cold acclimation is necessary for winter wheat (Triticum aestivum L.) to achieve its genetically determined maximum freezing tolerance, and cold also fulfils the vernalisation requirement. Chromosome 5A is a major regulator of these traits. The aim of the present study was to discover whether changes in the half-cell redox potential of the glutathione/glutathione disulphide (GSH/GSSG) and ascorbate/dehydroascorbate (AA/DHA) couples induced by cold acclimation are related to freezing tolerance and vernalisation requirement in a specific genetic system including chromosome 5A substitution lines. The amounts of H₂O₂ and AA, and the AA/DHA ratio showed a rapid and transient increase in the crown of all genotypes during the first week of acclimation, followed by a gradual increase during the subsequent 2 weeks. The amount of GSH and its ratio compared to GSSG quickly decreased during the first day, while later these parameters showed a continuous slow increase. The H₂O₂, AA and GSH concentrations, AA/DHA and GSH/GSSG ratios and the half-cell reduction potential of the GSH/GSSG couple were correlated with the level of freezing tolerance after 22 days at 2 °C; hence these parameters may have an important role in the acclimation process. In contrast to H₂O₂ and the non-enzymatic antioxidants, the lipid peroxide concentration and activity of the four antioxidant enzymes exhibited a transient increase during the first week, with no significant difference between genotypes. None of the parameters studied showed any relationship with the vegetative/generative transition state monitored as apex morphology and vernalisation gene expression.

Differential effects of cold acclimation and abscisic acid on free amino acid composition in wheat

The effect of cold acclimation and abscisic acid (ABA) treatment on the free amino acid composition was compared in Chinese Spring chromosome 5A substitution lines with different levels of freezing tolerance. The total amino acid content gradually increased during the 3-week cold acclimation period, while the effect of ABA became visible only after 7 d. The ratio of members of the glutamate family increased during cold acclimation and the ratio of amino acids belonging to the aspartate family decreased. Opposite changes were observed after treatment with ABA. Consistently with these results, ABA only induced a major increase in the Asn content, while the Asp, Glu, Gln and Pro levels were greatly induced by cold. A corresponding alteration at the gene expression level was only found for Pro and Glu. With the exception of Pro, cold- or ABA-induced changes in the amino acid levelsor Pro, did not correlate with the freezing tolerance of the three genotypes examined and were not affected by chromosome 5A. Since cold acclimation induced the accumulation of most of the amino acids, while ABA had a significant effect only on Asn, the cold-induced changes in free amino acid levels were probably not mediated by ABA.

Effect of radiation dosage on efficiency of chloroplast transfer by protoplast fusion in Nicotiana

Chloroplasts of Nicotiana tabacum SR1 were transferred into Nicotiana plumbaginifolia by protoplast fusion. The protoplasts of the organelle donor were irradiated with different lethal doses using a (60)Co source, to facilitate the elimination of their nuclei from the fusion products. After fusion induction, clones derived from fusion products and containing streptomycin-resistant N. tabacum SR1 chloroplasts were selected by their ability to green on a selective medium. When N. tabacum protoplasts were inactivated by iodoacetate instead of irradiation, the proportion of N. plumbaginifolia nuclear segregant clones was low (1-2%). Irradiation markedly increased this value: Using 50, 120, 210 and 300 J kg(-1) doses, the frequency of segregant clones was 44, 57, 84 and 70 percent, respectively. Regeneration of resistant N. plumbaginifolia plants with SR1 chloroplasts indicated that plastids can be rescued from the irradiated cells by fusion with untreated protoplasts. Resistant N. plumbaginifolia plants that were regenerated (43 clones studied) had diploid (2n = 2X = 20) or tetraploid chromosome numbers and were identical morphologically to parental plants. The absence of aneuploids suggests that in these clones irradiation resulted in complete elimination of the irradiated N. tabacum nuclei. Resistance is inherited maternally (five clones tested). The demonstration of chloroplast transfer and the presence of N. tabacum plastids in the N. plumbaginifolia plants was confirmed by chloroplast DNA fragmentation patterns after EcoRI digestion.

A simplified method to test cereal frost tolerance

Testing cereal frost tolerance goes back for decades in the Agricultural Research Institute, Martonvásár, Hungary. The climatic programmes used in the plant growth chamber have proved to be fairly efficient, but these methods are time-consuming and have become quite expensive in recent years. An attempt was made to shorten this process by reducing the cold hardening phase, and the freezing test has been simplified and shortened by measuring the relative conductance of leaf segments frozen in a liquid freezer. Frost-tolerant and sensitive wheat lines were tested, and the sensitivity of the system was checked by testing single chromosome substitution lines. Differences were found for all lines frozen at different temperatures. To reduce the costs of the experiment it was attempted to cold-harden the plants not only in a growth chamber but also in a cold room under very low light intensity and it was found that even under thess unfavourable conditions the plants developed a certain level of frost tolerance. The simplified frost tolerance test has proved to be effective, but requires further improvement due to the unsatisfactory significance levels.

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.

Effect of drought stress at supraoptimal temperature on polyamine concentrations in transgenic soybean with increased proline levels

The effect of drought stress at supraoptimal temperature on free proline and polyamine levels was compared in wild type and transgenic soybean (Glycine max cv. Ibis) plants having increased proline levels. Since glutamate and arginine are precursors of both proline and polyamines, it was assumed that the genetic manipulation of proline levels would also affect the polyamine levels. The proline and spermine concentrations increased, while the putrescine concentration generally decreased or did not change after the treatments in both genotypes. Following drought higher proline and lower spermine levels were detected in the transgenic plants compared to the wild type ones, which could be explained by the increased use of their common precursors for proline biosynthesis in the transgenic plants.

The expression of several Cbf genes at the Fr-A2 locus is linked to frost resistance in wheat

The C-repeat binding factor (Cbf) gene family has been shown to have a critical role in the regulation of low-temperature stress response in Arabidopsis. In Triticum monococcum, a locus carrying a family of Cbf-like genes, orthologs of Arabidopsis Cbf genes, is tightly linked to the frost tolerance locus Fr-A ( m ) 2, representing candidates for the differences in frost tolerance mapped at this locus. In this work we show that several Cbf genes have dramatically different levels of induction after cold exposure in hexaploid wheat. The Cbf-transcription levels differ between substitution and single chromosome recombinant lines carrying different 5A chromosomes or chromosome segments of the chromosome 5A from frost-tolerant and frost-sensitive wheat varieties. When the expression of eight Cbf genes, previously mapped at the Fr-A2 locus was investigated with gene specific primers using real-time RT-PCR, three Cbf sequences (Cbf1A, Cbf1C, Cbf7) showed a significantly higher relative transcription level (more than fourfold change) in lines differing for the Fr-A2 region. Differences in Cbf expression were also associated with a variation in frost tolerance. These results suggest that the amount of some Cbf mRNAs might be a critical factor for determining the level of frost tolerance in wheat.

Genetic manipulation of proline accumulation influences the concentrations of other amino acids in soybean subjected to simultaneous drought and heat stress

The effect of simultaneous drought and heat stress on free amino acid levels was compared in wild type and transgenic soybean (Glycine max (L.) Merr cv Ibis) plants transformed with the cDNA coding for the last enzyme of Pro biosynthesis, l-Delta(1)-pyrroline-5-carboxylate reductase (EC 1.5.1.2), in sense and antisense directions. The most rapid increase in Pro content was found in the sense transformants that exhibited the least water loss, while the slowest elevation of Pro levels was detected in the antisense transformants that exhibited the greatest water loss during stress. Correspondingly, the level of the Pro precursors Glu and Arg was higher in sense transformants and lower in antisense ones compared to the wild type plants during the initial part of the stress. Interestingly, genetic manipulation of Pro levels also affected the stress-induced changes in the concentration of several other amino acids, which indicates the coordinated regulation of their metabolic pathways.

Physiological and morphological responses to water stress in Aegilops biuncialis and Triticum aestivum genotypes with differing tolerance to drought

The physiological and morphological responses to water stress induced by polyethylene glycol (PEG) or by withholding water were investigated in Aegilops biuncialis Vis. genotypes differing in the annual rainfall of their habitat (1050, 550 and 225 mm year^-1) and in Triticum aestivum L. wheat genotypes differing in drought tolerance. A decrease in the osmotic pressure of the nutrient solution from -0.027 to -1.8 MPa resulted in significant water loss, a low degree of stomatal closure and a decrease in the intercellular CO₂ concentration (C_i) in Aegilops genotypes originating from dry habitats, while in wheat genotypes high osmotic stress increased stomatal closure, resulting in a low level of water loss and high C_i. Nevertheless, under saturating light at normal atmospheric CO₂ levels, the rate of CO₂ assimilation was higher for the Aegilops accessions, under high osmotic stress, than for the wheat genotypes. Moreover, in the wheat genotypes CO₂ assimilation exhibited less or no O₂ sensitivity. These physiological responses were manifested in changes in the growth rate and biomass production, since Aegilops (Ae550, Ae225) genotypes retained a higher growth rate (especially in the roots), biomass production and yield formation after drought stress than wheat. These results indicate that Aegilops genotypes, originating from a dry habitat have better drought tolerance than wheat, making them good candidates for improving the drought tolerance of wheat through intergeneric crossing.

Effect of osmotic stress on glutathione and hydroxymethylglutathione accumulation in wheat

The effect of osmotic stress on glutathione and hydroxymethylglutathione levels was compared in three wheat genotypes and two 5A chromosome substitution lines. Freezing-tolerant genotypes seemed also to be tolerant to osmotic stress induced by polyethylene glycol (PEG), since their fresh weight was not affected by the treatment. However, the growth of freezing-sensitive genotypes was reduced by 7-day PEG treatment and they had greater injuries after osmotic stress. The reduced forms of the two glutathione precursors, cysteine and gamma-glutamylcysteine, and of hydroxymethylglutathione (hmGSH) and glutathione (GSH) were present in greater quantities after PEG treatment in the two tolerant genotypes than in the sensitive ones. Similarly, osmotic stress resulted in a higher ratio of the reduced to the oxidised form of these thiols and in greater activity of gamma-glutamylcysteine synthetase and glutathione reductase in the tolerant genotypes compared to the sensitive ones. Following in vivo glutathione synthesis, a greater incorporation of radioactivity from [35S]sulphate into the four thiols was found in the tolerant genotypes than in the sensitive ones during osmotic stress. The present results indicate that hmGSH and GSH may contribute to the improvement of tolerance against osmotic stress in wheat and that the 5A chromosome influences the stress-induced changes in GSH and hmGSH levels.

Two loci on chromosome 5H determine low-temperature tolerance in a 'Nure' (winter) x 'Tremois' (spring) barley map

Barley ( Hordeum vulgare subsp. vulgare) is an economically important diploid model for the Triticeae; and a better understanding of low-temperature tolerance mechanisms could significantly improve the yield of fall-sown cereals. We developed a new resource for genetic analysis of winter hardiness-related traits, the 'Nure' x 'Tremois' linkage map, based on a doubled-haploid population that is segregating for low-temperature tolerance and vernalization requirement. Three measures of low-temperature tolerance and one measure of vernalization requirement were used and, for all traits, QTLs were mapped on chromosome 5H. The vernalization response QTL coincides with previous reports at the Vrn-1/Fr1 region of the Triticeae. We also found coincident QTLs at this position for all measures of low-temperature tolerance. Using Composite Interval Mapping, a second proximal set, of coincident QTLs for low-temperature tolerance, and the accumulation of two different COR proteins (COR14b and TMC-Ap3) was identified. The HvCBF4 locus, or another member of the CBF loci clustered in this region, is the candidate gene underlying this QTL. There is a CRT/DRE recognition site in the promoter of cor14b with which a CBF protein could interact. These results support the hypothesis that highly conserved regulatory factors, such as members of the CBF gene family, may regulate the stress responses of a wide range of plant species.

Cold acclimation and abscisic acid induced alterations in carbohydrate content in calli of wheat genotypes differing in frost tolerance

The effect of cold and abscisic acid (ABA) treatment on soluble carbohydrate content was compared in callus cultures of wheat genotypes differing in frost tolerance. The effect of 5A chromosome substituted from the frost tolerant <<Cheyenne>> to the sensitive <<Chinese Spring>> on cold-induced carbohydrate accumulation was also determined. Following cold hardening, the increase in sucrose and fructan level in calli of tolerant varieties was significantly higher than those of the sensitive ones. In 5A substitution line higher sucrose and fructan content was detected than in recipient <<Chinese Spring>>. Tendentiously, cold stress caused higher degree of changes in carbohydrate content than the exogenously applied ABA did. Comparing the accumulation pattern of the components of WSC measured in vitro to the previously published in vivo results it can be concluded that in the case of sucrose and fructans it was similar, while for the reducing sugars it was different. The regulatory role of chromosome 5A either in the development of freezing tolerance or carbohydrate accumulation was confirmed in dedifferentiated calli, as well.

Substitution analysis of seedling stage copper tolerance in wheat

The relatively copper-tolerant wheat variety Chinese Spring (recipient), the copper-sensitive variety Cappelle Desprez (donor) and their substitution lines were screened for copper tolerance in a soil pot experiment under artificial growth conditions. Chromosomes 5A, 5B, 5D and 7D of Cappelle Desprez significantly decreased the copper tolerance of the recipient variety to varying extents.  By contrast, the 6B and 3D chromosomes significantly increased the copper tolerance of Chinese Spring, suggesting that a wide range of allelic differences could be expected between wheat genotypes for this character. The significant role of homologous group 5 in copper tolerance was confirmed by testing wheat-rye substitution lines. The substitution of rye chromosome 5R (5R/5A substitution line) into a wheat genetic background significantly increased the copper tolerance of the recipient wheat genotype. The results suggest that chromosomes 5R and 5A probably carry major genes or gene complexes responsible for copper tolerance, and that the copper tolerance of wheat can be improved through the substitution of a single chromosome carrying the responsible genes. At the same time, it is also possible that the effect of homologous group 5 is not specific to copper tolerance, but that the genes located on these chromosomes belong to a general stress adaptation (frost, cold, vernalisation requirements, etc.) complex, which has already been detected on this chromosome. To answer this question further studies are needed to determine the real effect of these chromosome regions and loci on copper tolerance.

Genetic analysis of the expression of the cold-regulated gene cor14b: a way toward the identification of components of the cold response signal transduction in Triticeae

Extensive molecular biological studies have led to the cloning of many cold-regulated genes (cor) whose expression is up-regulated in winter cereals during exposure to cold. Among them is cor14b, a nuclear gene coding for a chloroplast-localized protein whose expression level has been associated with frost resistance in Triticeae. Although frost tolerance is recognised as a complex quantitative character, members of homologous group 5 in Triticeae are known to carry major loci determining frost tolerance. In this work we summarize a number of recent experiments where the cold-regulated gene cor14b was used as a tool (i) to understand the genetic relationship between the expression of cor genes and the frost resistance loci and (ii) to identify barley mutants revealing a chloroplast role in the signal transduction pathway leading to the molecular cold response.Key words: Triticeae, cold hardening, frost resistance, cor genes.

Mapping genes affecting flowering time and frost resistance on chromosome 5B of wheat

Two populations of single chromosome recombinant lines were used to map genes controlling flowering time on chromosome 5B of wheat, and one of the populations was also used to map a new frost resistance gene. Genetic maps were developed, mainly using microsatellite markers, and QTL analysis was applied to phenotypic data on the performance of each population collected from growth-room tests of flowering time and frost tolerance. Using a recombinant substitution-line mapping population derived from a cross between the substitution-line 'Chinese Spring' ('Cheyenne' 5B) and 'Chinese Spring' (CS), the gene Vrn-B1, affecting vernalization response, an earliness per se locus, Eps-5BL1, and a gene, Fr-B1, affecting frost resistance, were mapped. Using a 'Hobbit Sib' ('Chinese Spring' 5BL) x 'Hobbit Sib' recombinant substitution line mapping population, an earliness per se locus, Eps-5BL2 was mapped. The Vrn-B1 locus was mapped on the distal portion of the long arm of chromosome 5B, to a region syntenous with the segments of chromosomes 5A and 5D containing Vrn-A1 and Vrn-D1 loci, respectively. The two Eps-5BL loci were mapped close to the centromere with a 16-cM distance from each other, one in agreement with the position of a homoeologous locus previously mapped on chromosome 5H of barley, and suggested by the response of 'Chinese Spring' deletion lines. The Fr-B1 gene was mapped on the long arm of chromosome 5B, 40 cM from the centromeric marker. Previous comparative mapping data with rice chromosome 9 would suggest that this gene could be orthologous to the other Fr genes mapped previously by us on chromosomes 5A or 5D of wheat, although in a more proximal position. This study completes the mapping of these homoeoallelic series of vernalization requirement genes and frost resistance genes on the chromosomes of the homoeologous group 5 in wheat.

The cold-regulated transcriptional activator Cbf3 is linked to the frost-tolerance locus Fr-A2 on wheat chromosome 5A

Wheat chromosome 5A plays a key role in cold acclimation and frost tolerance. The major frost tolerance gene Fr-A1(formerly Fr1) and two loci that regulate the transcription of cold- regulated genes (Cor) have previously been mapped on the long arm of this chromosome. In this study we report the discovery of a new locus for frost tolerance designated Fr-A2. This new locus was mapped on the long arm of chromosome 5A of diploid wheat (T. monococcum), 40 cM from the centromere and 30 cM proximal to the major frost tolerance locus Fr-A1. We found also that frost-tolerant and frost-susceptible T. monococcum parental lines differed in the transcription level of the cold induced gene Cor14b when plants were grown at 15 degrees C. Transcription levels of this gene were measured in each of the recombinant inbred lines and mapped as a QTL that perfectly overlapped the QTL for frost survival at the Fr-A2 locus. This result suggested that frost tolerance in this cross was mediated by differential regulation of the expression of the Corgenes. In a previous study in hexaploid wheat (T. aestivum) we had shown that Cor14b was regulated by two loci located on chromosome 5A, one in the same chromosome region as the T. monococcum Fr-A2 locus and the other one closely linked to Fr-A1. Since CBF transcriptional activators in Arabidopsis regulate Corgenes and are involved in frost tolerance, we decided to localize the cold-regulated CBF-like barley gene Cbf3 on the T. monococcum map. This gene was mapped on the peak of the Fr-A2 QTL for frost tolerance. This result suggests that the observed differential regulation of Cor14b at the Fr-A2 locus is due to allelic variation at the XCbf3 locus, and that this transcriptional activator gene might be a candidate gene for the Fr-A2 frost tolerance locus on wheat chromosome 5A.

Mapping of genes regulating abiotic stress tolerance in cereals

The location of major QTLs or even genes controlling abiotic stress tolerance is now possible by the application of marker-mediated techniques. This is achieved by exploiting precise genetic stocks, such as doubled haploids (DHs), recombinant substitution lines (RSLs) and recombinant inbred lines (RILs), along with the comprehensive genetic maps now available through the application of molecular marker techniques. These strategies are illustrated here showing how QTLs/genes affecting vernalization response, cold tolerance, osmotic adjustment, osmolite accumulation (free amino acids, polyamines and carbohydrates), salt tolerance and cold-regulated protein accumulation have been identified and located. Also, an example of marker-assisted selection (MAS) for frost tolerance is presented. Major loci and QTLs affecting stress tolerance inTriticeaehave been mapped on the group 5 chromosomes, where the highest concentration of abiotic stress-related QTLs (vernalization response, frost tolerance, salt tolerance and osmolite accumulation) was located. A conserved region with a major role in osmotic adjustment has been located on the group 7 chromosomes.

Induction of glutathione synthesis and glutathione reductase activity by abiotic stresses in maize and wheat

The effect of different abiotic stresses (extreme temperatures and osmotic stress) on the synthesis of glutathione and hydroxymethylglutathione, on the ratio of the reduced to oxidised forms of these thiols (GSH/GSSG, hmGSH/hmGSSG), and on the glutathione reductase (GR) activity was studied in maize and wheat genotypes having different sensitivity to low temperature stress. Cold treatment induced a greater increase in total glutathione (TG) content and in GR activity in tolerant genotypes of both species than in sensitive ones. The GSH/GSSG and hmGSH/hmGSSG ratios were increased by this treatment only in the frost-tolerant wheat variety. High-temperature stress increased the TG content and the GSH/GSSG ratio only in the chilling-sensitive maize genotype, but GR activity was greater after this treatment in both maize genotypes. Osmotic stress resulted in a great increase in the TG content in wheat and the GR activity in maize. The amount of total hydroxymethylglutathione increased following all stress treatments. These results indicate the involvement of these antioxidants in the stress responses of wheat and maize.

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

With the aim of analyzing their protective function against chilling-induced injury, the pools of glutathione and its precursors, cysteine (Cys) and gamma-glutamyl-Cys, were increased in the chilling-sensitive maize (Zea mays) inbred line Penjalinan using a combination of two herbicide safeners. Compared with the controls, the greatest increase in the pool size of the three thiols was detected in the shoots and roots when both safeners were applied at a concentration of 5 microM. This combination increased the relative protection from chilling from 50% to 75%. It is interesting that this increase in the total glutathione (TG) level was accompanied by a rise in glutathione reductase (GR; EC 1.6.4.2) activity. When the most effective safener combination was applied simultaneously with increasing concentrations of buthionine sulfoximine, a specific inhibitor of glutathione synthesis, the total gamma-glutamyl-Cys and TG contents and GR activity were decreased to very low levels and relative protection was lowered from 75% to 44%. During chilling, the ratio of reduced to oxidized thiols first decreased independently of the treatments, but increased again to the initial value in safener-treated seedlings after 7 d at 5 degrees C. Taking all results together resulted in a linear relationship between TG and GR and a biphasic relationship between relative protection and GR or TG, thus demonstrating the relevance of the glutathione levels in protecting maize against chilling-induced injury.

Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants

Glutathione is an important component of the ascorbate-glutathione cycle, which is involved in the regulation of hydrogen peroxide (H2O2) concentrations in plants. During chilling and cold acclimation, i.e. exposure to temperatures between 0 and 15 degrees C, H2O2 may accumulate. Excess electrons from the photosynthetic and respiratory electron transport chains can be used for the reduction of oxygen, thus producing superoxide radicals (O2.-); these are subsequently transformed to H2O2 via superoxide dismutase (SOD; EC 1.15.1.1). During the removal of excess H2O2, reduced glutathione (GSH) is converted to its oxidised form (GSSG), and GSH is regenerated by the activity of NADPH-dependent glutathione reductase (GR; EC 1.6.4.2). At low non-freezing temperatures, high GSH content and GR activity were detected in several plant species, indicating a possible contribution to chilling tolerance and cold acclimation. Changes in H2O2 concentration and GSH/GSSG ratio alter the redox state of the cells and may activate special defence mechanisms through a redox signalling chain. The finding that several defence genes have antioxidant responsive elements or GSSG binding sites in their regulatory regions supports the idea that redox signalling is involved in regulating gene expression in response to low temperature.

Glutathione reductase activity and chilling tolerance are induced by a hydroxylamine derivative BRX-156 in maize and soybean

The possible contribution of antioxidants in the improvement of stress tolerance induced by the hydroxylamine derivative BRX-156 was studied in two thermophilic crops, soybean (Glycine max (L.) Merr.) and maize (Zea mays L.) both during germination and at the seedling stage. The most effective concentration of BRX-156 for an increase in stress tolerance was determined by the complex stressing vigour test (CSVT), in which seeds were germinated under simultaneous anoxia and chilling (5 degrees C) stresses. Under CSVT conditions the activity of glutathione reductase (GR, EC 1.6.4.2), was increased by BRX-156 by up to 200 and 150% in soybean and maize, respectively. Treatment with BRX-156 only resulted in a significantly greater activity of glutathione S-transferase (GST, EC 2.5.1.18) in maize. When young seedlings were chilled at 5 degrees C for a week, the increase in recovery induced by BRX-156 was accompanied by increased GR activity. The GSH synthesis was not affected by BRX-156 under these conditions. Induction of GR activity contributes to the improvement of abiotic stress tolerance by BRX-156 in maize and soybean.