Involvement of hydrogen peroxide in the regulation of senescence in pear

Antioxidant response system in the short-term post-wounding effect in Mesembryanthemum crystallinum leaves

Mechanical wounding of Mesembryanthemum crystallinum leaves in planta induced a fast decrease in stomatal conductance, which was related to accumulation of hydrogen peroxide (H(2)O(2)). Higher levels of H(2)O(2) were accompanied by an increase in total activity of superoxide dismutase (SOD) and a decrease in catalase (CAT) activity. Among SOD forms, manganese SOD (MnSOD) and copper/zinc SOD (Cu/ZnSOD) seem to be especially important sources of H(2)O(2) at early stages of wounding response. Moreover, NADP-malic enzyme (NADP-ME), one of the key enzymes of primary carbon metabolism, which is also involved in stress responses, showed a strong increase in activity in wounded leaves. All these symptoms: high accumulation of H(2)O(2), high activities of Cu/ZnSOD and NADP-ME, together with the decrease of CAT activity, were also observed in the major veins of unwounded leaves. The potential role of veinal tissues as an important source of H(2)O(2) during wounding response is discussed.

Excessive copper induces the production of reactive oxygen species, which is mediated by phospholipase D, nicotinamide adenine dinucleotide phosphate oxidase and antioxidant systems

Tobacco BY-2 suspension cells were used to study the chemical damage and its associated mechanisms caused by Cu2+. Treatment with 100 micromol/L Cu2+ generated a large amount of H2O2 and thiobarbituric acid-reactive substances (TBARS) in cells. Using phospholipase D (PLD) specific inhibitor (1-butanol) or phosphatidic acid (PA), we demonstrated that PLD plays an important role in the generation of H2O2 and TBARS. Semi-quantitative reverse-transcriptase polymerase chain reaction and enzyme activity assays with wild type and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-overexpressing BY-2 cells revealed that PLD and PA are the key factors leading to NADPH oxidase activation, which is responsible for H2O2 and TBARS production induced by Cu2+. Moreover, the content of ascorbic acid (AsA), an effective antioxidant, was sharply reduced in BY-2 cells exposed to excessive Cu2+. Furthermore, a significant downregulation of the enzymes of AsA biosynthesis and the antioxidant system was found. This evidence suggests that excessive Cu2+-elevated reactive oxygen species (ROS) production is caused by upregulated PLD that elevates the activity of NADPH oxidase and its collapsed antioxidant systems that scavenges ROS.

Modulation of copper toxicity-induced oxidative damage by nitric oxide supply in the adventitious roots of Panax ginseng

Nitric oxide (NO) is a highly reactive, membrane-permeable free radical, which has recently emerged as an important signalling molecule and antioxidant. Here we investigated the protective effect of NO against the toxicity caused by excess CuSO(4) (50 microM) in the adventitious roots of mountain ginseng. It was found that NO donor, sodium nitroprusside (SNP), was effective in reducing Cu-induced toxicity in the mountain ginseng adventitious roots. Protective effect of SNP, as indicated by extent of lipid peroxidation, was reversed by incorporation of 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (CPTIO), a NO scavenger, in the medium suggesting that the protective effect of SNP is attributable to NO released, which was revealed from in situ confocal laser scanning microscopic localization of NO in the adventitious roots of mountain ginseng. Results obtained in the present study suggest that reduction of excess Cu-induced toxicity by SNP is most likely mediated through the modulation in the activities of antioxidant enzymes involved in H(2)O(2) detoxification (catalase, peroxidase, ascorbate peroxidase) and in the maintenance of cellular redox couples (glutathione reductase), and contents of molecular antioxidants (particularly non-protein thiol, ascorbate and its redox status). Exogenous NO supply also improved the activity of superoxide dismutase, an enzyme responsible for O*(2) (-) dismutation, and NADPH oxidase, an enzyme responsible for O*(2) (-) generation, in excess Cu supplied adventitious roots of mountain ginseng.

Differential changes in photosynthetic capacity, 77 K chlorophyll fluorescence and chloroplast ultrastructure between Zn-efficient and Zn-inefficient rice genotypes (Oryza sativa) under low zinc stress

The relationship of zinc (Zn) efficiency in rice to differential tolerance of photosynthetic capacity and chloroplast function to low Zn stress was studied using Zn-efficient (IR8192) and Zn-inefficient (Erjiufeng) rice genotypes (Oryza sativa L.). Zinc deficiency caused extensive declines in leaf chlorophyll (Chl) content, ratios of chl a:b, Pn, Fv/Fm and Fv/Fo, indicating that the intrinsic quantum efficiency of the photosystem II (PSII) units was damaged. A greater decline was observed in the inefficient genotype (Erjiufeng) than the efficient genotype (IR8192). The 77 K chl fluorescence emission spectrum revealed that Zn deficiency blocked energy spillover from PSII to PSI and more excitation energy was distributed to PSII in IR8192 than Erjiufeng. The spectrum of Zn-deficient Erjiufeng was completely disordered, implying that the photosynthetic centers were seriously damaged. Electron microscopy showed that Zn deficiency caused a severe damage to the fine structure of chloroplasts, but IR8192 had a better preserved chloroplast ultrastructure as compared with Erjiufeng. These differences may result from the higher levels of the antioxidant enzyme activities and lower oxidant stress level in IR8192. These results indicate that Zn deficiency decreases leaf photosynthetic capacity primarily by reducing the number of PSII units per unit leaf area, and also reducing the photochemical capacity of the remaining PSII units. Therefore, the maintenance of more efficient photochemical capacity under low Zn stress is a key factor for the high Zn efficiency in rice, which may result from less antioxidant damage caused by low Zn to the chloroplast ultrastructure.

Delayed leaf senescence induces extreme drought tolerance in a flowering plant

Drought, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, leading to a decrease in canopy size, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying drought-induced leaf senescence. We generated transgenic plants expressing an isopentenyltransferase gene driven by a stress- and maturation-induced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth after a long drought period that killed the control plants. The transgenic plants maintained high water contents and retained photosynthetic activity (albeit at a reduced level) during the drought. Moreover, the transgenic plants displayed minimal yield loss when watered with only 30% of the amount of water used under control conditions. The production of drought-tolerant crops able to grow under restricted water regimes without diminution of yield would minimize drought-related losses and ensure food production in water-limited lands.

Delayed leaf senescence induces extreme drought tolerance in a flowering plant

Drought, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, leading to a decrease in canopy size, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying drought-induced leaf senescence. We generated transgenic plants expressing an isopentenyltransferase gene driven by a stress- and maturation-induced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth after a long drought period that killed the control plants. The transgenic plants maintained high water contents and retained photosynthetic activity (albeit at a reduced level) during the drought. Moreover, the transgenic plants displayed minimal yield loss when watered with only 30% of the amount of water used under control conditions. The production of drought-tolerant crops able to grow under restricted water regimes without diminution of yield would minimize drought-related losses and ensure food production in water-limited lands.

Genome-wide transcriptional analysis of grapevine berry ripening reveals a set of genes similarly modulated during three seasons and the occurrence of an oxidative burst at vèraison

Background

Grapevine (Vitis species) is among the most important fruit crops in terms of cultivated area and economic impact. Despite this relevance, little is known about the transcriptional changes and the regulatory circuits underlying the biochemical and physical changes occurring during berry development.

Results

Fruit ripening in the non-climacteric crop species Vitis vinifera L. has been investigated at the transcriptional level by the use of the Affymetrix Vitis GeneChip^® which contains approximately 14,500 unigenes. Gene expression data obtained from berries sampled before and after véraison in three growing years, were analyzed to identify genes specifically involved in fruit ripening and to investigate seasonal influences on the process. From these analyses a core set of 1477 genes was found which was similarly modulated in all seasons. We were able to separate ripening specific isoforms within gene families and to identify ripening related genes which appeared strongly regulated also by the seasonal weather conditions. Transcripts annotation by Gene Ontology vocabulary revealed five overrepresented functional categories of which cell wall organization and biogenesis, carbohydrate and secondary metabolisms and stress response were specifically induced during the ripening phase, while photosynthesis was strongly repressed. About 19% of the core gene set was characterized by genes involved in regulatory processes, such as transcription factors and transcripts related to hormonal metabolism and signal transduction. Auxin, ethylene and light emerged as the main stimuli influencing berry development. In addition, an oxidative burst, previously not detected in grapevine, characterized by rapid accumulation of H₂O₂ starting from véraison and by the modulation of many ROS scavenging enzymes, was observed.

Conclusion

The time-course gene expression analysis of grapevine berry development has identified the occurrence of two well distinct phases along the process. The pre-véraison phase represents a reprogramming stage of the cellular metabolism, characterized by the expression of numerous genes involved in hormonal signalling and transcriptional regulation. The post-véraison phase is characterized by the onset of a ripening-specialized metabolism responsible for the phenotypic traits of the ripe berry. Between the two phases, at véraison, an oxidative burst and the concurrent modulation of the anti-oxidative enzymatic network was observed. The large number of regulatory genes we have identified represents a powerful new resource for dissecting the mechanisms of fruit ripening control in non-climacteric plants.

LeERF1 positively modulated ethylene triple response on etiolated seedling, plant development and fruit ripening and softening in tomato

To study the function of LeERF1 in ethylene triple response on etiolated seedling, plant development and fruit ripening and softening, LeERF1 gene was introduced into tomato (Lycopersicon esculentum cv. No. 4 Zhongshu) through Agrobacterium-mediated transformation. The sense LeERF1 and anti-sense LeERF1 transgenic tomato were obtained. Overexpression of LeERF1 in tomato caused the typical ethylene triple response on etiolated seedling. In the adult stage, 35S::LeERF1 resulted in morphological changes in the leaves of the LeERF1-sn lines. Anti-sense LeERF1 fruits had longer shelf life compared with wild-type tomato. The results of this manuscript indicated that LeERF1 positively mediated the ethylene signals, while the function of LeERF1 was verified for the first time to be positively related with ethylene triple response on etiolated seedling, plant development and fruit ripening and softening using LeERF1-sn, wt and LeERF1-as tomato.

Drought tolerance and antioxidant activities in lavender plants colonized by native drought-tolerant or drought-sensitive Glomus Species

This study compared the effectiveness of four arbuscular mycorrhizal (AM) fungal isolates (two autochthonous presumably drought-tolerant Glomus sp and two allochthonous presumably drought-sensitive strains) on a drought-adapted plant (Lavandula spica) growing under drought conditions. The autochthonous AM fungal strains produced a higher lavender biomass, specially root biomass, and a more efficient N and K absorption than with the inoculation of similar allochthonous strains under drought conditions. The autochthonous strains of Glomus intraradices and Glomus mosseae increased root growth by 35% and 100%, respectively, when compared to similar allochthonous strains. These effects were concomitant with an increase in water content and a decline in antioxidant compounds: 25% glutathione, 7% ascorbate and 15% H(2)O(2) by G. intraradices, and 108% glutathione, 26% ascorbate and 43% H(2)O(2) by G. mosseae. Glutathione and ascorbate have an important role in plant protection and metabolic function under water deficit; the low cell accumulation of these compounds in plants colonized by autochthonous AM fungal strains is an indication of high drought tolerance. Non-significant differences between antioxidant activities such as glutathione reductase (GR), catalase (CAT) and superoxide dismutase (SOD) in colonized plants were found. Thus, these results do not allow the generalization that GR, CAT and SOD were correlated with the symbiotic efficiency of these AM fungi on lavender drought tolerance. Plants colonized by allochthonous G. mosseae (the less efficient strain under drought conditions) had less N and K content than those colonized by similar autochthonous strain. These ions play a key role in osmoregulation. The AM symbiosis by autochthonous adapted strains also produced the highest intraradical and arbuscular development and extraradical mycelial having the greatest fungal SDH and ALP-ase activities in the root systems. Inoculation of autochthonous drought tolerant fungal strains is an important strategy that assured the greatest tolerance water stress contributing to the best lavender growth under drought.

Cell wall degradation and modification during programmed cell death in lace plant, Aponogeton madagascariensis (Aponogetonaceae)

An unusual form of leaf morphogenesis occurs in the aquatic, lace plant, Aponogeton madagascariensis (Aponogetonaceae). Early in development, discrete patches of cells undergo programmed cell death (PCD) and form perforations during leaf expansion. In addition to the protoplasts, walls of the dying cells are degraded during PCD. The cuticle of the perforation site is eroded first, followed by dissolution of cell wall matrix components, so that walls appear as loose fibrillar networks as perforations form. Gel diffusion assays of wall-degrading enzyme activity indicated that pectinases are active throughout leaf development, while cellulase activity was restricted to early stages of perforation formation. Alcian blue staining showed that degrading walls remain rich in pectin, and immunolocalization of pectin epitopes indicated that the proportions of esterified and de-esterifed pectins do not change significantly. Walls of perforation border cells are modified by suberin deposition late in development, and reactive oxygen species, thought to have a role in polymerization of phenolic suberin monomers, are present at the same stage. This timing suggests that suberization may limit the spread of PCD and provide an apoplastic barrier against microbial invasion but does not initiate PCD.

Physiological and biochemical responses to high Mn concentrations in two contrasting Populus cathayana populations

We exposed the cuttings of Populus cathayana to Hoagland's solution containing four different manganese (Mn) concentrations (0, 0.1, 0.5 and 1mM) in a greenhouse to characterize the physiological and biochemical basis of Mn resistance in woody plants. Two contrasting populations of P. cathayana were used in our study, which were from the wet and dry climate regions in western China, respectively. The results showed that Mn treatments significantly decreased chlorophyll content and growth characteristics, including shoot height, basal diameter, biomass accumulation and total leaf area in the two populations. Mn treatments also significantly increased the levels of abscisic acid (ABA), polyamines and free amino acids especially proline (Pro), histidine (His) and phenylalanine (Phe) available for cellular signaling and heavy metal chelation. In addition, high Mn concentrations also caused oxidative stress indicated as the accumulation of hydrogen peroxide (H(2)O(2)) and malondialdehyde (MDA) contents. On the other hand, there were different responses to Mn stress between the two contrasting populations. Compared with the dry climate population, the wet climate population accumulated more Mn in plant tissues especially in leaves; it showed lower tolerance index and more pronounced decrease in growth and chlorophyll contents. The wet climate population not only accumulated less ABA, putrescine and free amino acids, but also exhibited lower activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX), thus suffering from more serious oxidative damage. Therefore, our results showed that the wet climate population was more susceptible to Mn stress than the dry climate population.

Genetic engineering of the biosynthesis of glycinebetaine enhances thermotolerance of photosystem II in tobacco plants

Genetically engineered tobacco (Nicotiana tabacum L.) with the ability to accumulate glycinebetaine was established. The wild type and transgenic plants were exposed to heat treatment (25-50 degrees C) for 4 h in the dark and under growth light intensity (300 mumol m(-2) s(-1)). The analyses of oxygen-evolving activity and chlorophyll fluorescence demonstrated that photosystem II (PSII) in transgenic plants showed higher thermotolerance than in wild type plants in particular when heat stress was performed in the light, suggesting that the accumulation of glycinebetaine leads to increased tolerance to heat-enhanced photoinhibition. This increased tolerance was associated with an improvement on thermostability of the oxygen-evolving complex and the reaction center of PSII. The enhanced tolerance was caused by acceleration of the repair of PSII from heat-enhanced photoinhibition. Under heat stress, there was a significant accumulation of H(2)O(2), O (2) (-) and catalytic Fe in wild type plants but this accumulation was much less in transgenic plants. Heat stress significantly decreased the activities of catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monodehydroascorbate reductase in wild type plants whereas the activities of these enzymes either decreased much less or maintained or even increased in transgenic plants. In addition, heat stress increased the activity of superoxide dismutase in wild type plants but this increase was much greater in transgenic plants. Furthermore, transgenic plants also showed higher content of ascorbate and reduced glutathione than that of wild type plants under heat stress. The results suggest that the increased thermotolerance induced by accumulation of glycinebetaine in vivo was associated with the enhancement of the repair of PSII from heat-enhanced photo inhibition, which might be due to less accumulation of reactive oxygen species in transgenic plants.

A cotton ascorbate peroxidase is involved in hydrogen peroxide homeostasis during fibre cell development

Reactive oxygen species (ROS) play important roles in multiple physiological processes such as cellular signalling and stress responses, whereas, the hydrogen peroxide (H(2)O(2)) scavenging enzyme ascorbate peroxidase (APX) participates in the regulation of intracellular ROS levels. Here, a cotton (Gossypium hirsutum) cytosolic APX1 (GhAPX1) was identified to be highly accumulated during cotton fibre elongation by proteomic analysis. GhAPX1 cDNA contained an open reading frame of 753-bp encoding a protein of 250 amino acid residues. When GhAPX1 was expressed in Escherichia coli, the purified GhAPX1 was a dimer consisting of two identical subunits with a molecular mass of 28 kDa. GhAPX1 showed the highest substrate specificity for ascorbate. Quantitative real-time polymerase chain reaction (PCR) analyses showed that GhAPX1 was highly expressed in wild-type 5-d postanthesis fibres with much lower transcript levels in the fuzzless-lintless mutant ovules. Treating in vitro cultured wild-type cotton ovules with exogenous H(2)O(2) or ethylene induced the expression of GhAPX1 and hence increased total APX activity proportionally, followed by extended fibre cell elongation. These data suggest that GhAPX1 expression is upregulated in response to an increase in cellular H(2)O(2) and ethylene. GhAPX1 encodes a functional enzyme that is involved in hydrogen peroxide homeostasis during cotton fibre development.

Dynamics and localization of H2O2 production in elicited plant cells

H(2)O(2) produced in plant cells plays a dual role. In addition to its antimicrobial effect, it also acts as a secondary messenger initiating and modulating responses of plants exposed to unfavorable external signals. A suspension culture of Rubia tinctorum cells challenged with elicitors was used as a model system to investigate H(2)O(2) formation. Cellular H(2)O(2) was measured by a modified titanium(IV) method, while that in the medium was detected with scopoletin fluorescence. Localization of H(2)O(2) production at the ultrastructural level was carried out by the CeCl(3) reaction. A fungal elicitor induced H(2)O(2) production with transient maxima, the first of which appeared 4 min after treatment. Three subsequent maxima appeared in the cells up to 48 h after treatment. Exposure of cells to exogenous jasmonic acid and salicylic acid also changed the H(2)O(2) concentration maxima over 48 h; however, their timing was slightly shifted. Fungal-elicitor, jasmonic acid, and salicylic acid treatments had different effects on the H(2)O(2) concentration in the medium. Ultrastructural investigations revealed that electron-dense precipitates were present at the plasmalemma and in some nearby vesicular cytoplasmic structures 30 min after treatment. Later samples showed cytochemical-precipitate accumulation in the cell walls. These deposits appeared to be local and independent of the direction of the external signal. We could not detect the presence of H(2)O(2) in peroxisomes, mitochondria, plastids, or the central vacuolar space. Electron energy loss spectroscopy investigations distinguished between the cerium-containing precipitates and other electrondense particles, thereby proving that H(2)O(2) generation occurs locally.

Peroxynitrite as an alternative donor of oxygen in HIF-1alpha proline hydroxylation under low oxygen availability

In the last years, nitric oxide (NO) mediated signaling became an integral component in understanding physiological and pathophysiological processes of cell proliferation, death or cellular adaptation. Among other activities, NO affects multiple targets that allow regulation of gene expression. Recently, NO was found to attenuate accumulation of hypoxia inducible factor-1alpha (HIF-1alpha) under hypoxic conditions because of several mechanisms: redistribution of oxygen toward non-respiratory oxygen-dependent targets (like HIF-1alpha proline hydroxylases--PHDs, which perform hydroxylation of Pro402/564 of HIF-1alpha leading to its proteasomal degradation); in addition, peroxynitrite formed during interactions between NO and mitochondria derived superoxide leads to an increase in cytosolic iron/2-oxoglutarate (2-OG), which required for PHD activation. Here, we propose a hypothesis that peroxynitrite, formed in the cells upon exposure to NO under low oxygen availability, serves as an alternative donor of oxygen for activated PHDs so they can perform HIF-1alpha proline hydroxylation to de-accumulate the protein.

Development of NaCl-tolerant strain in Chrysanthemum morifolium Ramat. through in vitro mutagenesis

One NaCl-tolerant chrysanthemum (Chrysanthemum morifolium Ramat.) variant (E2) has been developed in a stable form through IN VITRO mutagenesis using ethylmethane sulfonate (EMS) as the chemical mutagen. Salt tolerance was evaluated by the capacity of the plant to maintain both flower quality and yield under stress conditions. Enhanced tolerance of the E2 variant has been attributed to the increased activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), and dehydroascorbate reductase (DHAR), and, to a lesser extent of membrane damage than NaCl-treated control plants. Isoform analysis revealed that an increase in total SOD activity in the E2 variant was solely due to significant activation of the Cu/Zn isoform. Elevated levels of carotenoids and ascorbate in E2 leaves have been reflected in their higher free radical scavenging capacity (RSC) expressed in terms of DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging ability. Data reflect that a proper balance between enzymatic and non-enzymatic defence systems is required for combating salinity stress in chrysanthemum. Better performance of the E2 progeny under same salinity stress condition, even in the second year, confirms the genetic stability of the salt-tolerance character. On the whole, the E2 variant, developed through 0.025 % EMS treatment, might be considered as a NaCl-tolerant strain showing positive characters towards NaCl stress.

Proteomic analysis of tomato fruits from two ecotypes during ripening

Ripening of climacteric fruits is a complex physiological phenomenon, which makes berries attractive/palatable and also determines their nutritional characteristics. We report here a comparative proteomic investigation on tomato fruits from a regional and commercial elite ecotype during maturation. Several hundreds of protein components were resolved on 2-DE gels (pH range 4-7) stained with colloidal Coomassie; almost 57% presented overlapping gel coordinates between the two compared varieties. Specific proteins were recognized in each ecotype as differentially expressed during ripening. Constant and variable components were excised, in-gel digested and analysed by MALDI-TOF-MS and microLC-ESI-IT-MS/MS approaches. Peptide MS and MS/MS data were searched against publicly available protein and EST databases, and 83 protein spots were unambiguously identified by MS. Gel matching procedures allowed further identification of proteins between ecotypes. In general, common variably expressed proteins in both ecotypes during maturation were associated to important physiological processes such as redox status control, defence, stress, carbon metabolism, energy production and cellular signalling. Protein components differentially expressed in each variety were also identified. The role of some of the identified proteins in the berries is discussed in relation to tomato fruit ripening. Accordingly, this investigation provides the first characterization of the tomato fruit proteome and description of its variation during maturation.

Antioxidant responses to enhanced generation of superoxide anion radical and hydrogen peroxide in the copper-stressed mulberry plants

The aim of the study was to implicate the generation of reactive oxygen species (ROS) and altered cellular redox environment with the effects of Cu-deficiency or Cu-excess in mulberry (Morus alba L.) cv. Kanva 2 plants. A study of antioxidative responses, indicators of oxidative damage and cellular redox environment in Cu-deficient or Cu-excess mulberry plants was undertaken. While the young leaves of plants supplied with nil Cu showed chlorosis and necrotic scorching of laminae, the older and middle leaves of plants supplied with nil or 0.1 microM Cu showed purplish-brown pigmented interveinal areas that later turned necrotic along the apices and margins of leaves. The Cu-excess plants showed accelerated senescence of the older leaves. The Cu-deficient plants showed accumulation of hydrogen peroxide and superoxide anion radical. The accumulation of hydrogen peroxide was strikingly intense in the middle portion of trichomes on Cu-deficient leaves. Though the concentration of total ascorbate increased with the increasing supply of Cu, the ratio of the redox couple (DHA/ascorbic acid) increased in Cu-deficient or Cu-excess plants. The activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) increased in both Cu-deficient and Cu-excess plants. The results suggest that deficiency of Cu aggravates oxidative stress through enhanced generation of ROS and disturbed redox couple. Excess of Cu damaged roots, accelerated the rate of senescence in the older leaves, induced antioxidant responses and disturbed the cellular redox environment in the young leaves of mulberry plants.

Decline in ascorbate peroxidase activity--a prerequisite factor for tepal senescence in gladiolus

Flower senescence was studied in Gladiolus cv. "Snow Princess" over five arbitrarily divided developmental stages (stage 1, half bloom; stage 2, full bloom; stage 3, beginning of wilting; stage 4, 50% wilting; stage 5, complete wilting) in terms of changes in fresh weight, antioxidant enzymes (superoxide dismutase, SOD; ascorbate peroxidase, APX; glutathione reductase, GR) activities and membrane integrity. A significant decrease in tepal fresh weight was observed over the senescence period (after stage 2). Membrane integrity was studied by measuring lipid peroxidation [in terms of thiobarbituric acid reactive substances (TBARS) content] and membrane stability index (MSI) percentage. Maximum TBARS content was recorded in stage 4 (50% wilting). This increase in lipid peroxidation over the senescence period was in close association with high degree of membrane deterioration expressed as decrease in membrane stability index percentage. A significant decrease (two and half-fold) in MSI% in stage 5 (as compared to stage 1) indicates complete membrane deterioration. Progressive increase in endogenous H2O2 level was recorded over senescence period. Maximum H2O2 content (19.7+/-1.4 micromol g(-1) DW) was recorded at stage 5 (complete wilting). Three different patterns were observed in antioxidant enzymes behavior over the senescence period. APX activity was declined significantly as, the flower entered stage 3 (beginning of wilting) from full bloom condition (stage 2). Progressive and significant increase in SOD activity was measured as a function of time. Maximum SOD activity (24.2+/-0.8 U mg(-1) DW) was recorded in stage 5 (three-fold increase over stage 1). GR activity initially increased up to stage 4 (50% wilting) and declined significantly thereafter (approximately seven-fold). An increase in endogenous H2O2 level during senescence may be the result of a programmed down-regulation of APX enzyme activity, which seems to be the prerequisite factor for initiating senescence process in gladiolus tepal.

Involvement of a glycerol-3-phosphate dehydrogenase in modulating the NADH/NAD+ ratio provides evidence of a mitochondrial glycerol-3-phosphate shuttle in Arabidopsis

A mitochondrial glycerol-3-phosphate (G-3-P) shuttle that channels cytosolic reducing equivalent to mitochondria for respiration through oxidoreduction of G-3-P has been extensively studied in yeast and animal systems. Here, we report evidence for the operation of such a shuttle in Arabidopsis thaliana. We studied Arabidopsis mutants defective in a cytosolic G-3-P dehydrogenase, GPDHc1, which, based on models described for other systems, functions as the cytosolic component of a G-3-P shuttle. We found that the gpdhc1 T-DNA insertional mutants exhibited increased NADH/NAD+ ratios compared with wild-type plants under standard growth conditions, as well as impaired adjustment of NADH/NAD+ ratios under stress simulated by abscisic acid treatment. The altered redox state of the NAD(H) pool was correlated with shifts in the profiles of metabolites concerning intracellular redox exchange. The impairment in maintaining cellular redox homeostasis was manifest by a higher steady state level of reactive oxygen species under standard growth conditions and by a significantly augmented hydrogen peroxide production under stress. Loss of GPDHc1 affected mitochondrial respiration, particularly through a diminished capacity of the alternative oxidase respiration pathway. We propose a model that outlines potential involvements of a mitochondrial G-3-P shuttle in plant cells for redox homeostasis.

Involvement of a glycerol-3-phosphate dehydrogenase in modulating the NADH/NAD+ ratio provides evidence of a mitochondrial glycerol-3-phosphate shuttle in Arabidopsis

A mitochondrial glycerol-3-phosphate (G-3-P) shuttle that channels cytosolic reducing equivalent to mitochondria for respiration through oxidoreduction of G-3-P has been extensively studied in yeast and animal systems. Here, we report evidence for the operation of such a shuttle in Arabidopsis thaliana. We studied Arabidopsis mutants defective in a cytosolic G-3-P dehydrogenase, GPDHc1, which, based on models described for other systems, functions as the cytosolic component of a G-3-P shuttle. We found that the gpdhc1 T-DNA insertional mutants exhibited increased NADH/NAD+ ratios compared with wild-type plants under standard growth conditions, as well as impaired adjustment of NADH/NAD+ ratios under stress simulated by abscisic acid treatment. The altered redox state of the NAD(H) pool was correlated with shifts in the profiles of metabolites concerning intracellular redox exchange. The impairment in maintaining cellular redox homeostasis was manifest by a higher steady state level of reactive oxygen species under standard growth conditions and by a significantly augmented hydrogen peroxide production under stress. Loss of GPDHc1 affected mitochondrial respiration, particularly through a diminished capacity of the alternative oxidase respiration pathway. We propose a model that outlines potential involvements of a mitochondrial G-3-P shuttle in plant cells for redox homeostasis.

Isolation of a NaCl-tolerant mutant of Chrysanthemum morifolium by gamma radiation: in vitro mutagenesis and selection by salt stress

A stable NaCl-tolerant mutant (R1) of Chrysanthemum morifolium Ramat has been developed by in vitro mutagenesis with gamma radiation (5 gray; Gy). Salt tolerance was evaluated by the capacity of the plant to maintain both flower quality and yield under NaCl stress. Enhanced salt tolerance of the R1 mutant was attributed to increased activities of reactive oxygen species (ROS)-scavenging enzymes, namely superoxide dismutase (SOD), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR), and to reduced membrane damage, higher relative water content (RWC), chlorophyll and carotenoids contents. RAPD analysis revealed two polymorphic bands (956 and 1093 bp) for the R1 mutant that might be considered as specific RAPD markers associated with salt tolerance. Better performance of the R1 progeny under identical salinity stress conditions, even in the second year, confirmed the genetic stability of the induced salt tolerance character. The R1 mutant developed by gamma ray treatment can be considered a salt-tolerant mutant showing all the positive characteristics of tolerance to NaCl stress.

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-beta-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10-100 microm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, alpha-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-beta-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Salicylic acid (SA) has been proposed to antagonize jasmonic acid (JA) biosynthesis and signaling. We report, however, that in salicylate hydroxylase-expressing tobacco (Nicotiana tabacum) plants, where SA levels were reduced, JA levels were not elevated during a hypersensitive response elicited by Pseudomonas syringae pv phaseolicola. The effects of cotreatment with various concentrations of SA and JA were assessed in tobacco and Arabidopsis (Arabidopsis thaliana). These suggested that there was a transient synergistic enhancement in the expression of genes associated with either JA (PDF1.2 [defensin] and Thi1.2 [thionin]) or SA (PR1 [PR1a-beta-glucuronidase in tobacco]) signaling when both signals were applied at low (typically 10-100 microm) concentrations. Antagonism was observed at more prolonged treatment times or at higher concentrations. Similar results were also observed when adding the JA precursor, alpha-linolenic acid with SA. Synergic effects on gene expression and plant stress were NPR1- and COI1-dependent, SA- and JA-signaling components, respectively. Electrolyte leakage and Evans blue staining indicated that application of higher concentrations of SA + JA induced plant stress or death and elicited the generation of apoplastic reactive oxygen species. This was indicated by enhancement of hydrogen peroxide-responsive AoPR10-beta-glucuronidase expression, suppression of plant stress/death using catalase, and direct hydrogen peroxide measurements. Our data suggests that the outcomes of JA-SA interactions could be tailored to pathogen/pest attack by the relative concentration of each hormone.

Light promotes the synthesis of lignin through the production of H2O2 mediated by diamine oxidases in soybean hypocotyls

In order to analyze the relationship between polyamine oxidative degradation induced by light and the Lignin synthesis in cell walls, the activities of diamine oxidases and peroxidase, the contents of H2O2 and lignin, and the growth of hypocotyls in soybean [Glycine max (Linn.) Merr.] grown under tight or in darkness were investigated. In comparison with the dark treatment, light irradiation significantly inhibited the growth of soybean hypocotyls and promoted the activities of diamine oxidases and peroxidase as well as the accumulation of H2O2 and lignin. Treatments with the different concentrations of diamine oxidase inhibitors (2-hydroxyethylhydrazine and aminoguanidine) under the light condition inhibited diamine oxidase activity, and decreased the contents of H2O2 and lignin. The results provide evidence for the hypothesis that light irradiation could promote the accumulation of H2O2 and lignin in cell walls by activating polyamine oxidative degradation mediated by diamine oxidases.

Differences in the activities of some antioxidant enzymes and in H2O2 content during rhizogenesis and somatic embryogenesis in callus cultures of the ice plant

Callus was obtained from hypocotyls of Mesembryanthemum crystallinum seedlings cultured on two types of medium-germination medium (GM) and callus induction medium (CIM). Following subculture on shoot induction medium SIM1, the callus formed on CIM medium regenerated roots or somatic embryos, while that obtained on GM medium was non-regenerative. The activities of CuZn-superoxidase dismutase (SOD) were comparable in all calli, but the activities of FeSOD and MnSOD varied according to the activity of photosystem II and the regenerative potential of the tissues. Catalase (CAT) activity was related to H2O2 concentration and affected by both the culture conditions and the morphogenic potential of the calli. The possible role of CAT, SODs and H2O2 in the regeneration of M. crystallinum from callus is discussed.

Induction of H2O2 in transgenic rice leads to cell death and enhanced resistance to both bacterial and fungal pathogens

Oxidative burst, mediated by hydrogen peroxide (H2O2), has been recognized as a key component of plant defense response during an incompatible interaction. To determine if elevated levels of H2O2 lead to cell death, activation of defense genes and enhanced resistance to diverse pathogens, transgenic rice plants expressing a fungal glucose oxidase gene (GOX) were generated using both constitutive and inducible expression systems. Constitutive or wound/pathogen-induced expression of GOX also allowed us to determine the effectiveness of these systems in conferring long lasting resistance to various pathogens. Both constitutive and wound/pathogen-induced expression of GOX lead to increases in the endogenous levels of H2O2, which in turn caused cell death. Elevated levels of H2O2 also activated the expression of several defense genes and these transgenic plants showed enhanced resistance to both bacterial and fungal pathogens. In comparison to inducible expression, constitutive expression of GOX resulted in 3-10-fold higher levels of the GOX transcript and the corresponding enzymatic activity. Such increased levels of GOX, which would result in elevated levels of H2O2, caused improper seed set and decreased seed viability in transgenic plants constitutively expressing GOX. Our results suggest that pathogen inducible expression of heterologous genes may be a practical and robust way of generating broad spectrum disease resistance.

Evolution of robusta green coffee redox enzymatic activities with maturation

Oxidation reactions in coffee involve redox-sensitive polyphenols and appear to control the fragmentation of coffee storage proteins both in solution and during roasting. Coffee-specific nitrogenous flavor precursors may derive from this process. Accordingly, data converge to suggest that the redox status of the green bean before roasting might control the development of subsequent redox reactions during roasting. Consequently, we decided to identify biological events that may trigger or prevent oxidation during maturation of the coffee cherry and set the final redox status of the green bean. In a previous study, we observed that the sensitivity of green coffee to oxidative processes decreased along maturation. By using the very same samples originating from open-pollinated Robusta clones, we followed the activity of three essential redox enzymes: catalase (CAT), peroxidase (POD) and polyphenoloxidase (PPO). While CAT and POD activities increased with maturation, PPO activities decreased. Thanks to the identification of an atypical immature subclass, it appeared that CAT might be an essential factor in setting the final redox status of the green bean before the roasting event.

Relationships between fruit exocarp antioxidants in the tomato (Lycopersicon esculentum) high pigment-1 mutant during development

Development-dependent changes in fruit antioxidants were examined in the exocarp (epidermal and hypodermal tissues) of the monogenic recessive tomato (Lycopersicon esculentum L.) mutant high pigment (hp-1) and its wild-type parent 'Rutgers' grown under non-stress conditions in a greenhouse. The hp-1 mutant was chosen for this study because the reportedly higher lycopene and ascorbic acid (AsA) contents of the fruit may alter its tolerance to photooxidative stress. Throughout most of fruit development, reduced AsA concentrations in the exocarp of hp-1 were 1.5 to 2.0 times higher than in 'Rutgers', but total glutathione concentrations were similar in both genotypes. Only in ripe red fruit were reduced AsA and total glutathione concentrations lower in hp-1 than in 'Rutgers'. The redox ratios (reduced : reduced + oxidized) of AsA in hp-1 and 'Rutgers' exocarps were similar and usually > 0.9, however, the redox ratio of glutathione was lower in hp-1 than in 'Rutgers' throughout development. Lycopene concentrations in ripe red fruit were about 5 times higher in hp-1 than in 'Rutgers'. Large increases in the specific enzyme activities of superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), and monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) occurred during fruit development in both genotypes, with an inverse relationship between the activities of these enzymes and chlorophyll content. Glutathione reductase (EC 1.6.4.2) and MDHAR-specific activities were higher in hp-1 than 'Rutgers' only at the later stages of fruit development. Dehydroascorbate reductase (EC 1.8.5.1) activities, however, were usually higher in 'Rugters' than in hp-1. Catalase (CAT, EC 1.11.1.6) activities increased with fruit development until the fruit were orange/light red, when CAT was higher in 'Rutgers' than in hp-1, but then declined in the ripe red fruit of both genotypes. These results suggest that elevated AsA in the exocarp of hp-1 fruit early in fruit development may increase the tolerance of hp-1 fruit to photooxidative injury at that time, but the increasing activities of antioxidant enzymes appear to be developmentally associated with fruit ripening.

Involvement of NADPH oxidase in hydrogen peroxide accumulation by Aspergillus niger elicitor-induced Taxus chinensis cell cultures

After determining that hydrogen peroxide (H2O2) accumulation induced by a fungal elicitor from Aspergillus niger was from the superoxide dismutase-catalyzed dismutation of superoxide radical, the site of H2O2 generation in cell suspension cultures of Taxus chinensis was studied. The results showed that 90% and 10% of the elicitor-induced H2O2 accumulation respectively appeared in intracellular and extracellular fractions of cells, and that the elicitor-induced H2O2 accumulation in protoplasts and plasma membranes was similar to that in intact cells, indicating that the site of H2O2 accumulation was plasma membranes but not in extracellular fraction of Taxus cells. The H2O2 forming enzyme was also investigated. The elicitor-induced H2O2 accumulation in intact cells was not changed by loss of apoplastic peroxidase (POD) by the washing, and the H2O2 accumulation in plasma membranes was inhibited by the mammalian neutrophil NAD(P)H oxidase inhibitor diphenylene iodonium (DPI), but was slightly affected by exogenous POD and its inhibitor. Furthermore, in plasma membranes, the H2O2 accumulation was more significantly enhanced by NADPH than by NADH, and the former was more obviously decreased by DPI than the latter. The present results show that NADPH oxidase in plasma membranes is involved in H2O2 accumulation in fungal elicitor-induced Taxus chinensis cell cultures.

Solubilisation of tomato fruit pectins by ascorbate: a possible non-enzymic mechanism of fruit softening

The aim of this work was to test the hypothesis that endogenous ascorbate, released into the apoplast by membrane permeabilisation early in fruit ripening, could promote the solubilisation and depolymerisation of polysaccharides, and thus contribute to fruit softening. In vitro, ascorbate (1 mM), especially in the presence of traces of either Cu2+ or H2O2, solubilised up to 40% of the total pectin from the alcohol-insoluble residue of mature-green tomato (Lycopersicon esculentum Mill.) fruit. Solubilisation was due to the action of ascorbate-generated hydroxyl radicals (*OH), which can cause non-enzymic scission of polysaccharides. The pectins solubilised by ascorbate in vitro were polydisperse (4-1,000 kDa), partially esterified and galactose-rich. Excised pieces of living tomato fruit released ascorbate into the medium (apoplast); the ability of different tissues to do this increased in the order pericarp < placenta < locule. In all three tissues, but especially in the locule, the ability to release ascorbate increased during ripening. The Cu content of each tissue also increased during ripening, whereas neither Fe nor Mn showed a similar trend. We suggest that progressively increasing levels of Cu and ascorbate in the fruit apoplast would lead to elevated *OH production there and thus to non-enzymic scission of pectins during ripening. Such scission could contribute to the natural softening of the fruit. De-esterified citrus pectin was more susceptible to ascorbate-induced scission in vitro than methylesterified pectin, suggesting a possible new significance for pectin methylesterase activity in fruit ripening. In conclusion, non-enzymic mechanisms of fruit softening should be considered alongside the probable roles of hydrolases, xyloglucan endotransglucosylases and expansins.

Influence of temperature on biomass, iron metabolism and some related bioindicators in tomato and watermelon plants

Tomato, Lycopersicon esculentum L. cv. RX-335, and watermelon plants, Citrullus lanatus [Thom.] Mansf. cv. F-90 were grown under controlled conditions at three different temperatures (10 degrees, 25 degrees and 35 degrees C) for 30 days. The aim of the experiment was to analyse the effect of the different temperatures on Fe uptake and distribution, as well as the behaviour of the main bioindicators of this element. Thus, we analysed the total and free Fe concentrations and H2O2 concentrations, as well as enzymatic activities of Fe-chelate reductase (FeCH-R), aconitase (Aco), guaiacol peroxidase (GPX), catalase (CAT), and Fe-superoxide dismutase (FeSOD), and the dry weight of the plants. The effect caused by each temperature varied according to the species of plant. Our results indicate that heat stress appears in tomato plants when grown at 35 degrees C (above the optimal temperature for growth), while in watermelon plants, which need more heat than do tomatoes, cold stress appears at 10 degrees C (below the optimal temperature for growth). Despite these differences between the two species, the results under conditions of thermal stress were the same: 1) decreased shoot weight, 2) reduced Fe uptake, 3) depressed activities of FeCH-R, Aco, GPX, CAT and 4) boosted SOD activity. In short, our results appear to indicate that, whether heat in tomato plants or cold stress in watermelon plants, Fe uptake was diminished, as were the enzymatic activities related to the levels of this micronutrient in the plant. The high FeSOD activity in these plants could be explained by a defensive response to heat or cold stress.

Enzymatic and non-enzymatic antioxidant responses of Carrizo citrange, a salt-sensitive citrus rootstock, to different levels of salinity

In several plant species, oxidative stress has been shown to be one of the causes of damage produced by salinity. In order to assess the implication of oxidative stress in the reported sensitivity of the citrus rootstock Carrizo citrange to salt stress, 5-month-old seedlings were grown with increasing NaCl concentrations added to the watering solution. As an indicator of oxidative damage, malondialdehyde content was measured. The antioxidant capability of the plants was determined by measuring superoxide dismutase, ascorbate peroxidase, catalase and glutathione reductase activities together with the non-enzymatic antioxidant activity. As additional physiological responses to the stress, 1-aminocyclopropane-1-carboxilic acid and proline accumulation were assessed. Data indicate that Carrizo citrange responded to salt-induced oxidative stress by increasing enzymatic and non-enzymatic antioxidant defenses proportionally to the extent of the stress imposed, and that in all plants the malondialdehyde content remained at a moderate level. We suggest that the important deleterious effects reported in Carrizo citrange grown under high NaCl concentrations are mainly due to a cellular intoxication by Cl(-) ions and not to the salt-induced oxidative stress.

Effect of abscisic acid on active oxygen species, antioxidative defence system and oxidative damage in leaves of maize seedlings

Leaves of maize (Zea mays L.) seedlings were supplied with different concentrations of abscisic acid (ABA). Its effects on the levels of superoxide radical (O(2)(-)), hydrogen peroxide (H(2)O(2)) and the content of catalytic Fe, the activities of several antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), the contents of several non-enzymatic antioxidants such as ascorbate (ASC), reduced glutathione (GSH), alpha-tocopherol (alpha-TOC) and carotenoid (CAR), and the degrees of the oxidative damage to the membrane lipids and proteins were examined. Treatment with 10 and 100 microM ABA significantly increased the levels of O(2)(-) and H(2)O(2), followed by an increase in activities of SOD, CAT, APX and GR, and the contents of ASC, GSH, alpha-TOC and CAR in a dose- and time-dependent pattern in leaves of maize seedlings. An oxidative damage expressed as lipid peroxidation, protein oxidation, and plasma membrane leakage did not occur except for a slight increase with 100 microM ABA treatment for 24 h. Treatment with 1,000 microM ABA led to a more abundant generation of O(2)(-) and H(2)O(2) and a significant increase in the content of catalytic Fe, which is critical for H(2)O(2)-dependent hydroxyl radical production. The activities of these antioxidative enzymes and the contents of alpha-TOC and CAR were still maintained at a higher level, but no longer further enhanced when compared with the treatment of 100 microM ABA. The contents of ASC and GSH had no changes in leaves treated with 1,000 microM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 microM) induced an antioxidative defence response against oxidative damage, but a high concentration of ABA (1,000 microM) induced an excessive generation of AOS and led to an oxidative damage in plant cells.

Fingerprinting of polysaccharides attacked by hydroxyl radicals in vitro and in the cell walls of ripening pear fruit

Hydroxyl radicals (*OH) may cause non-enzymic scission of polysaccharides in vivo, e.g. in plant cell walls and mammalian connective tissues. To provide a method for detecting the action of endogenous *OH in vivo, we investigated the products formed when polysaccharides were treated with *OH (generated in situ by ascorbate-H(2)O(2)-Cu(2+) mixtures) followed by NaB(3)H(4). Treatment with *OH increased the number of NaB(3)H(4)-reacting groups present in citrus pectin, homogalacturonan and tamarind xyloglucan. This increase is attributed partly to the formation of glycosulose and glycosulosuronic acid residues, which are then reduced back to the original (but radioactive) sugar residues and their epimers by NaB(3)H(4). The glycosulose and glycosulosuronic acid residues were stable for >16 h at 20 degrees C in ethanol or buffer (pH 4.7), but were destroyed in alkali. Driselase-digestion of the radiolabelled polysaccharides yielded characteristic patterns of (3)H-products, which included galactose and galacturonate from pectin, and isoprimeverose, galactose, glucose and arabinose from xyloglucan. Pectin yielded at least eight (3)H-labelled anionic products, separable by electrophoresis at pH 3.5. The patterns of radioactive products form useful 'fingerprints' by which *OH-attacked polysaccharides may be recognized. Applied to the cell walls of ripening pear (Pyrus communis) fruit, the method gave evidence for progressive *OH radical attack on polysaccharides during the softening process.

Iron deficiency differently affects peroxidase isoforms in sunflower

The response of both specific (ascorbate peroxidase, APX) and unspecific (POD) peroxidases and H(2)O(2) content of sunflower plants (Helianthus annuus L. cv. Hor) grown hydroponically with (C) or without (-Fe) iron in the nutrient solution were analysed to verify whether iron deficiency led to cell oxidative status. In -Fe leaves a significant increase of H(2)O(2) content was detected, a result confirmed by electron microscopy analysis. As regards extracellular peroxidases, while APX activity significantly decreased, no change was observed in either soluble guaiacol or syringaldazine-dependent POD activity following iron starvation. Moreover, guaiacol-dependent POD activity was found to decrease in both ionically and covalently-cell-wall bound fractions, while syringaldazine-POD activity decreased only in the covalently-bound fraction. At the intracellular level both guaiacol-POD and APX activities underwent a significant decrease. The overall reduction of peroxidase activity was confirmed by the electrophoretic separation of POD isoforms and, at the extracellular level, by cytochemical localization of peroxidases by diaminobenzidine staining. The electrophoretic separation, besides quantitative differences, also revealed quantitative changes, particularly evident for ionically and covalently-bound fractions. Therefore, in sunflower plants, iron deficiency seems to affect the different peroxidase isoenzymes to different extents and to induce a secondary oxidative stress, as indicated by the increased levels of H(2)O(2). However, owing to the almost completely lack of catalytic iron capable of triggering the Fenton reaction, iron-deficient sunflower plants are probably still sufficiently protected against oxidative stress.

Effect of freezing on the activity of catalase in apple flesh tissue

Catalase (CAT, EC 1.11.1.6) activity was measured in flesh tissue of six apple cultivars (Malus domestica Borkh. cvs. Braeburn, Gala, Jonagold, McIntosh, Red Delicious, and Spartan). Activity of CAT was determined for fresh and frozen tissue of the same fruit. Freezing resulted in reductions of 50 to 90% in CAT activity compared with the activity measured in crude extracts from fresh tissues. The rate of freezing had an impact on the level of reduction of CAT activity, with slower freezing procedures leading to greater losses in activity. Six additives to the extraction buffer were tested to evaluate their potential to reduce the inactivation of CAT from frozen tissue, but only EDTA and Tween 20 showed any benefit. However, EDTA and Tween 20 provided only partial recovery in CAT activity. In contrast, crude extracts prepared from fresh tissue showed no appreciable loss in CAT activity after frozen storage for two weeks at -80 degrees C. Gel electrophoresis and immunological analysis indicated that the loss in CAT activity from tissue freezing could be attributed to loss of both the tetrameric CAT enzyme structure and total CAT protein. The implications of using freezing to preserve apple tissue samples prior to catalase activity analysis is discussed.

Involvement of polyamines in the chilling tolerance of cucumber cultivars

The possible involvement of polyamines (PAs) in the chilling tolerance of cucumber (Cucumis sativus L. cv Jinchun No. 3 and cv Suyo) was investigated. Plants with the first expanded leaves were exposed to 3 degrees C or 15 degrees C in the dark for 24 h (chilling), and then transferred to 28 degrees C/22 degrees C under a 12-h photoperiod for another 24 h (rewarming). Chilling-tolerant cv Jinchun No. 3 showed a marked increase of free spermidine (Spd) in leaves, once during chilling and again during rewarming. Putrescine increased significantly during rewarming, but the increase of spermine was slight. Any of these PAs did not increase in chilling-sensitive cv Suyo during either period. PA-biosynthetic enzyme activities appear to mediate these differences between cultivars. Pretreatment of Spd to cv Suyo prevented chill-induced increases in the contents of hydrogen peroxide in leaves and activities of NADPH oxidases and NADPH-dependent superoxide generation in microsomes and alleviated chilling injury. Pretreatment of methylglyoxal-bis-(guanylhydrazone), a PA biosynthesis inhibitor, to chilled cv Jinchun No. 3 prevented Spd increase and enhanced microsomal NADPH oxidase activity and chilling injury. The results suggest that Spd plays important roles in chilling tolerance of cucumber, probably through prevention of chill-induced activation of NADPH oxidases in microsomes.

Induction and control of chromoplast-specific carotenoid genes by oxidative stress

The differentiation of chloroplasts into chromoplasts involves a series of biochemical changes that culminate with the intense accumulation of long chain chromophore carotenoids such as lycopene, rhodoxanthin, astaxanthin, anhydroeschsoltzxanthin, capsanthin, and capsorubin. The signal pathways mediating these transformations are unknown. Chromoplast carotenoids are known to accumulate in green tissues experiencing stress conditions, and studies indicate that they provide efficient protection against oxidative stress. We tested the role of reactive oxygen species (ROS) as regulators of chromoplast carotenoid biosynthesis in vivo. The addition of ROS progenitors, such as menadione, tert-butylhydroperoxide, or paraquat and prooxidants such as diamide or buthionine sulfoximine to green pericarp discs of pepper fruits rapidly and dramatically induce the simultaneous expression of multiple carotenogenic gene mRNAS that give rise to capsanthin. Similarly, down-regulation of catalase by amitrole induces expression of carotenogenic gene mRNAs leading to the synthesis of capsanthin in excised green pericarp discs. ROS signals from plastids and mitochondria also contribute significantly to this process. Analysis of the capsanthin-capsorubin synthase promoter in combination with a beta-glucuronidase reporter gene reveals strong activation in transformed pepper protoplasts challenged with the above ROS. Collectively these data demonstrate that ROS act as a novel class of second messengers that mediate intense carotenoid synthesis during chromoplast differentiation.

Oxidative scission of plant cell wall polysaccharides by ascorbate-induced hydroxyl radicals

Scission of plant cell wall polysaccharides in vivo has generally been assumed to be enzymic. However, in the presence of l-ascorbate, such polysaccharides are shown to undergo non-enzymic scission under physiologically relevant conditions. Scission of xyloglucan by 1 mM ascorbate had a pH optimum of 4.5, and the maximum scission rate was reached after a 10-25-min delay. Catalase prevented the scission, whereas added H2O2 (0.1-10 mM) increased the scission rate and shortened the delay. Ascorbate caused detectable xyloglucan scission above approx. 5 microM. Dehydroascorbate was much less effective. Added Cu2+ (>0.3 microM) also increased the rate of ascorbate-induced scission; EDTA was inhibitory. The rate of scission in the absence of added metals appeared to be attributable to the traces of Cu (2.8 mg.kg-1) present in the xyloglucan. Ascorbate-induced scission of xyloglucan was inhibited by radical scavengers; their effectiveness was proportional to their rate constants for reaction with hydroxyl radicals (.OH). It is proposed that ascorbate non-enzymically reduces O2 to H2O2, and Cu2+ to Cu+, and that H2O2 and Cu+ react to form .OH, which causes oxidative scission of polysaccharide chains. Evidence is reviewed to suggest that, in the wall of a living plant cell, Cu+ and H2O2 are formed by reactions involving ascorbate and its products, dehydroascorbate and oxalate. Systems may thus be in place to produce apoplastic .OH radicals in vivo. Although .OH radicals are often regarded as detrimental, they are so short-lived that they could act as site-specific oxidants targeted to play a useful role in loosening the cell wall, e.g. during cell expansion, fruit ripening and organ abscission.

Influence of salicylic acid on H2O2 production, oxidative stress, and H2O2-metabolizing enzymes. Salicylic acid-mediated oxidative damage requires H2O2

We investigated how salicylic acid (SA) enhances H2O2 and the relative significance of SA-enhanced H2O2 in Arabidopsis thaliana. SA treatments enhanced H2O2 production, lipid peroxidation, and oxidative damage to proteins, and resulted in the formation of chlorophyll and carotene isomers. SA-enhanced H2O2 levels were related to increased activities of Cu,Zn-superoxide dismutase and were independent of changes in catalase and ascorbate peroxidase activities. Prolonging SA treatments inactivated catalase and ascorbate peroxidase and resulted in phytotoxic symptoms, suggesting that inactivation of H2O2-degrading enzymes serves as an indicator of hypersensitive cell death. Treatment of leaves with H2O2 alone failed to invoke SA-mediated events. Although leaves treated with H2O2 accumulated in vivo H2O2 by 2-fold compared with leaves treated with SA, the damage to membranes and proteins was significantly less, indicating that SA can cause greater damage than H2O2. However, pretreatment of leaves with dimethylthiourea, a trap for H2O2, reduced SA-induced lipid peroxidation, indicating that SA requires H2O2 to initiate oxidative damage. The relative significance of the interaction among SA, H2O2, and H2O2-metabolizing enzymes with oxidative damage and cell death is discussed.

Dynamics of Acetaldehyde Production during Anoxia and Post-Anoxia in Red Bell Pepper Studied by Photoacoustic Techniques

Acetaldehyde (AA), ethanol, and CO2 production in red bell pepper (Capsicum annum L.) fruit has been measured in a continuous flow system as the fruit was switched between 20% O2 and anaerobic conditions. Minimum gas phase concentrations of 0.5 nL L-1, 10 nL L-1, and 1 mL L-1, respectively, can be detected employing a laser-based photoacoustic technique. This technique allows monitoring of low production rates and transient features in real time. At the start of anaerobic treatment respiration decreases by 60% within 0.5 h, whereas AA and ethanol production is delayed by 1 to 3 h. This suggests a direct slow-down of the tricarboxylic acid cycle and a delayed onset of alcoholic fermentation. Reexposure of the fruit to oxygen results in a 2- to 10-fold upsurge in AA production. A short anoxic period leads to a sharp transient peak lasting about 40 min, whereas after numerous and longer anoxic periods, post-anoxic AA production stays high for several hours. High sensitivity of the fruit tissue to oxygen is further evidenced by a sharp decrease in post-anoxic AA production upon an early return to anaerobic conditions. Ethanol oxidation by the "peroxidatic" action of catalase is proposed to account for the immediate post-anoxic AA upsurge.

Changes in Isozyme Profiles of Catalase, Peroxidase, and Glutathione Reductase during Acclimation to Chilling in Mesocotyls of Maize Seedlings

The response of antioxidants to acclimation and chilling in various tissues of dark-grown maize (Zea mays L.) seedlings was examined in relation to chilling tolerance and protection from chilling-induced oxidative stress. Chilling caused an accumulation of H2O2 in both the coleoptile + leaf and the mesocotyl (but not roots), and acclimation prevented this accumulation. None of the antioxidant enzymes were significantly affected by acclimation or chilling in the coleoptile + leaf or root. However, elevated levels of glutathione in acclimated seedlings may contribute to an enhanced ability to scavenge H2O2 in the coleoptile + leaf. In the mesocotyl (visibly most susceptible to chilling), catalase3 was elevated in acclimated seedlings and may represent the first line of defense from mitochondria-generated H2O2. Nine of the most prominent peroxidase isozymes were induced by acclimation, two of which were located in the cell wall, suggesting a role in lignification. Lignin content was elevated in mesocotyls of acclimated seedlings, likely improving the mechanical strength of the mesocotyl. One cytosolic glutathione reductase isozyme was greatly decreased in acclimated seedlings, whereas two others were elevated, possibly resulting in improved effectiveness of the enzyme at low temperature. When taken together, these responses to acclimation illustrate the potential ways in which chilling tolerance may be improved in preemergent maize seedlings.

Disease resistance conferred by expression of a gene encoding H2O2-generating glucose oxidase in transgenic potato plants

Plant defense responses to pathogen infection involve the production of active oxygen species, including hydrogen peroxide (H2O2). We obtained transgenic potato plants expressing a fungal gene encoding glucose oxidase, which generates H2O2 when glucose is oxidized. H2O2 levels were elevated in both leaf and tuber tissues of these plants. Transgenic potato tubers exhibited strong resistance to a bacterial soft rot disease caused by Erwinia carotovora subsp carotovora, and disease resistance was sustained under both aerobic and anaerobic conditions of bacterial infection. This resistance to soft rot was apparently mediated by elevated levels of H2O2, because the resistance could be counteracted by exogenously added H2O2-degrading catalase. The transgenic plants with increased levels of H2O2 also exhibited enhanced resistance to potato late blight caused by Phytophthora infestans. The development of lesions resulting from infection by P. infestans was significantly delayed in leaves of these plants. Thus, the expression of an active oxygen species-generating enzyme in transgenic plants represents a novel approach for engineering broad-spectrum disease resistance in plants.

Ethylene Production during Development of Mustard (Brassica juncea) and Canola (Brassica napus) Seed

An open, continuous flow system was used to investigate ethylene production during degreening of maturing seed of mustard (Brassica juncea cv Cutlass and cv Lethbridge 22A) and canola (Brassica napus cv Westar and cv Alto). Isolated mustard seed evolved higher amounts of ethylene than those of canola, and this was particularly evident both early in embryogeny and later during the desiccation phase of seed maturation. The silique walls produced negligible amounts of ethylene in both species. The concentrations of ethylene surrounding seed as they matured within siliques were significantly higher in mustard than in canola, and this interspecies difference was greatest during the seed desiccation phase. In mustard, a 4-fold increase in silique internal ethylene levels was apparent during desiccation. In comparison, only a moderate increase in silique-derived ethylene occurred in canola.

Evidence for Chilling-Induced Oxidative Stress in Maize Seedlings and a Regulatory Role for Hydrogen Peroxide

We have taken advantage of an acclimation phenomenon in a chilling-sensitive maize inbred to investigate the molecular, biochemical, and physiological responses to chilling in preemergent maize seedlings. Three-day-old seedlings were exposed to 4[deg]C for 7 days and did not survive chilling stress unless they were preexposed to 14[deg]C for 3 days. cDNAs representing three chilling acclimation-responsive (CAR) genes were isolated by subtraction hybridization and differential screening and found to be differentially expressed during acclimation. Identification of one of these CAR genes as cat3, which encodes the mitochondrial catalase3 isozyme, led us to hypothesize that chilling imposes oxidative stress in the seedlings. Hydrogen peroxide levels were elevated during both acclimation and chilling of nonacclimated seedlings. Further molecular and biochemical analyses indicated that whereas superoxide dismutase activity was not affected, the levels of cat3 transcripts and the activities of catalase3 and guaiacol peroxidase were elevated in mesocotyls during acclimation. Accumulation of H2O2 following a short treatment with aminotriazole, a catalase inhibitor, indicated that catalase3 seems to be an important H2O2-scavenging enzyme in the seedlings. Control 3-day-old seedlings pretreated with H2O2 or menadione, a superoxide-generating compound, at 27[deg]C induced chilling tolerance. Both of these chemical treatments also increased cat3 transcripts and catalase3 and guaiacol peroxidase activities. We suggest that peroxide has dual effects at low temperatures. During acclimation, its early accumulation signals the production of antioxidant enzymes such as catalase3 and guaiacol peroxidase. At 4[deg]C, in nonacclimated seedlings, it accumulates to damaging levels in the tissues due to low levels of these, and perhaps other, antioxidant enzymes.