Fusarium response to oxidative stress by H2O2 is trichothecene chemotype-dependent

The present study aims at clarifying the impact of oxidative stress on type B trichothecene production. The responses to hydrogen peroxide (H₂O₂) of an array of Fusarium graminearum and Fusarium culmorum strains were compared, both species carrying either the chemotype deoxynivalenol (DON) or nivalenol (NIV). In both cases, levels of in vitro toxin production are greatly influenced by the oxidative parameters of the medium. A 0.5 mM H₂O₂ stress induces a two- to 50-fold enhancement of DON and acetyldeoxynivalenol production, whereas the same treatment results in a 2.4- to sevenfold decrease in NIV and fusarenone X accumulation. Different effects of oxidative stress on toxin production are the result of a variation in Fusarium 's antioxidant defence responses according to the chemotype of the isolate. Compared with DON strains, NIV isolates have a higher H₂O₂-destroying capacity, which partially results from a significant enhancement of catalase activity induced by peroxide stress. A 0.5 mM H₂O₂ treatment leads to a 1.3- to 1.7-fold increase in the catalase activity of NIV isolates. Our data, which show the higher adaptation to oxidative stress developed by NIV isolates, are consistent with the higher virulence of these Fusarium strains on maize compared with DON isolates.     

Generation of H2O2 in Brain Mitochondria

Generation of H2O2 by rat brain mitochondria using succinate and glycerol-1-phosphate as substrates has been demonstrated. Earlier workers were unable to detect this activity in sucrose-Tris buffer. We found that this was due to a lag in the expression of activity in sucrose medium. Using phosphate buffer (50 mM), good rates are now obtained. Generation of H2O2 by rat brain mitochondria required the presence of antimycin A and was dependent on the substrates succinate and glycerol-1-phosphate. Low rates were obtained with NAD+-linked substrates and none with choline, glutamate, and NADH. The Km and Vmax values for H2O2 generation were considerably lower than the corresponding values for the respective dehydrogenase activity, measured by dye reduction. Oxygen-radical scavengers inhibited H2O2 generation, suggesting oxygen radical involvement. Depletion of ubiquinone from mitochondria resulted in loss of H2O2 generation. Reconstitution of such depleted particles with ubiquinone restored the capacity to generate H2O2 in a concentration-dependent manner. Levels of H2O2 production were found to be maximal in cerebellum. Brain mitochondria from rabbit, hamster, mouse, and guinea pig also have the capacity to generate H2O2 on oxidation of glycerol-1-phosphate.     

The cryptic-growth response of maize coleoptiles and its relationship to H2O2-dependent cell wall stiffening

Auxin-mediated elongation growth of maize ( Zea mays L.) coleoptile segments can be nullified by lowering the turgor pressure by 0.45 MPa. Under these conditions irreversible segment length (l_in) measured after freezing/thawing increases steadily over a period of 8 h although the in vivo length (l_tot) remains constant. This phenomenon, designated as 'cryptic growth', is an indication of a wall-stiffening process which appears to be an intrinsic component of irreversible cell wall extension. Using a range of metabolic inhibitors it is demonstrated that cryptic growth is caused by a temperature-sensitive biochemical process in the cell wall which depends on the presence of O₂ and active peroxidase, but not on ATP and protein synthesis. Inhibition of cryptic growth by anaerobic conditions can be alleviated by extermal H₂O₂. Moreover, cryptic growth can be partially inhibited by the antioxidant ascorbate. It is concluded that cryptic growth represents a wall-stiffening reaction mediated by peroxidase-catalyzed, H₂O₂-dependent cross-linking of phenolic residues of wall polymers. The experimental demonstration of a wall-stiffening reaction in a rapidly growing organ supports the concept that irreversible cell elongation (growth) is caused by an interplay of two chemorheological reactions, a turgor-dependent wall-loosening reaction and a separate wall-stiffening reaction which fixes the viscoelastically extended wall structure through oxidative cross-linking and thus conferring irreversibility to wall extension.     

Sinorhizobium meliloti rpoE2 is necessary for H2O2 stress resistance during the stationary growth phase

RpoE2 is an extracytoplasmic σ factor produced by Sinorhizobium meliloti during stationary growth phase. Its inactivation affected the synthesis of the superoxide dismutase, SodC, and catalase, KatC. The absence of SodC within the cell did not result in an increased sensitivity to extracellular superoxides. In contrast, the absence of KatC affected the resistance of S. meliloti to H₂O₂ during the stationary growth phase. A katC strain behaved as an rpoE2 strain during an H₂O₂ challenge, suggesting that the H₂O₂ sensitivity of the rpoE2 strain resulted only from the lack of KatC in this strain.     

Subdivision of equine Tf into H1 and H2

Subdivision of equine TfH into two variants, designated H1 (faster) and H2 (slower), has been accomplished by high voltage, thin layer polyacrylamide gel electrophoresis at pH 7.9. Transferrin H1 and H2 have been shown to be controlled by codominant alleles and gene frequencies of the Tf alleles have been determined in the Australian Thoroughbred, Standardbred. Quarter Horse and Arabian Horse breeds.     

Cadmium-induced subcellular accumulation of O2·− and H2O2 in pea leaves

Cadmium is a toxic metal that produces disturbances in plant antioxidant defences giving rise to oxidative stress. The effect of this metal on H₂O₂ and O₂^·? production was studied in leaves from pea plants growth for 2 weeks with 50 µm Cd, by histochemistry with diaminobenzidine (DAB) and nitroblue tetrazolium (NBT), respectively. The subcellular localization of these reactive oxygen species (ROS) was studied by cytochemistry with CeCl₃ and Mn/DAB staining for H₂O₂ and O₂^·?, respectively, followed by electron microscopy observation. In leaves from pea plants grown with 50 µm CdCl₂ a rise of six times in the H₂O₂ content took place in comparison with control plants, and the accumulation of H₂O₂ was observed mainly in the plasma membrane of transfer, mesophyll and epidermal cells, as well as in the tonoplast of bundle sheath cells. In mesophyll cells a small accumulation of H₂O₂ was observed in mitochondria and peroxisomes. Experiments with inhibitors suggested that the main source of H₂O₂ could be a NADPH oxidase. The subcellular localization of O₂^·? production was demonstrated in the tonoplast of bundle sheath cells, and plasma membrane from mesophyll cells. The Cd‐induced production of the ROS, H₂O₂ and O₂^·?, could be attributed to the phytotoxic effect of Cd, but lower levels of ROS could function as signal molecules in the induction of defence genes against Cd toxicity. Treatment of leaves from Cd‐grown plants with different effectors and inhibitors showed that ROS production was regulated by different processes involving protein phosphatases, Ca²⁺ channels, and cGMP.     

HISTAMINE-SENSITIVE ADENYLATE CYCLASE IN HYPOTHALAMUS OF RAT BRAIN: H1 AND H2 RECEPTORS

Abstract— In in vitro experiments on rat hypothalamic homogenates the effects of biogenic amines such as histamine (HA), noradrenaline (NA), dopamine (DA), serotonin (5-HT) and drugs such as isoprenaline (ISP), 2-(2-pyridyl)ethylamine (H₂ stimulant—H_ls), 4-methyl-histamine (H₂ stimulant H_2s), mepyramine (H₁ antagonistp H_la), cimetidine (H₂ antagonist—H_2a) were tested on adenylate cyclase activity. HA possessed a powerful stimulating effect on hypothalamic adenylate cyclase activity, higher than that shown by the other substances.
The stimulating effect of HA was greatest in hypothalamic tissue from male rats, while tissue from females showed only a modest stimulation. H_2s, induced a greater stimulation of adenylate cyclase than H_ls. On the other hand, the H_2a inhibited HA stimulation to a greater extent than the H_la, H_la and H_2a, when used together, completely inhibited the HA stimulation. HA may have a neurotrans-mitter role in the hypothalamus, and in this area there appears to be a mixed population of H₁ and H₂ receptors, with a majority of H₂ receptors.     

Plant responses to H2S and SO2 fumigation. II. Differences in metabolism of H2S and SO2 in spinach

Fumigation of spinach (Spinacia oleracea L. cvs Estivato and Monosa) with H₂S or SO, for 1 to 6 days resulted in accumulation of sulfhydryl (SH) compounds in the shoots of both H₂S- and SO₂-exposed plants. The sulfate concentration in shoots of SO₂-exposed plants increased linearly with time. SH accumulation showed saturation kinetics as a function of time as well as H₂S concentration, ascribed to the internal H₂S concentration in the plant and the availability of substrates for glutathione synthesis, respectively. SH compounds accumulated more at lower exposure temperatures, whereas sulfate accumulation was more pronounced at higher temperatures. These results are discussed in relation to the possible foliar uptake of H₂S and SO₂, the temperature dependence of uptake and the water solubility of these gases. The possibility of SO₂-induced H₂S emission rather than sulfate accumulation as a source for SH accumulation is also discussed. Cessation of fumigation resulted in a decrease in SH compounds and sulfate content that could be accounted for by sulfur metabolism and growth, respectively.     

Microbial H2/CO2 acetogenesis in animal guts: nature and nutritional significance

Abstract The intestinal tract of invertebrate and vertebrate animals, including man, is an anoxic habitat wherein microbial formation of acetate from H₂+ CO₂ is often a major H₂-consuming reaction. This paper will discuss the magnitude and microbiology of H₂/CO₂ acetogenesis in animal guts, its impact on host animal nutrition, competition for H₂ between anaerobic microbes, and the global significance of intestinal H₂/CO₂ acetogenesis.     

Differential effects of nitric oxide on peroxidase and H2O2 production by the xylem of Zinnia elegans

IWF, intercellular washing fluid
pCMB, p-chloromercuribenzoic acid
SNAP, S-nitroso-N-acetyl-penicillamine SNP, sodium nitroprusside
TMB, 3,3’,5,5’- tetramethylbenzidine

Sodium nitroprusside (SNP) and S-nitroso-N-acetyl-penicillamine (SNAP) are two nitric oxide (NO)-releasing compounds that, when used at 5·0 mol m^–3 concentrations, are capable of releasing NO in the aqueous phase at a rate of 35 ± 4 and 47 ± 5 μmol m^–3 s^–1, respectively. For this reason, the effect of SNP and SNAP on coniferyl alcohol peroxidase and on H₂O₂ production by the lignifying xylem of Zinnia elegans (L.) has been studied in order to ascertain whether NO, which is a synchronizing chemical messenger in animals and an air pollutant, has any effect on these plant-specific metabolic aspects. The results showed that both SNP and SNAP provoke an inhibition in the mol m^–3 concentration range of the coniferyl alcohol peroxidase activity of a basic peroxidase isoenzyme present in the intercellular washing fluid of Z. elegans. The effect of these NO-releasing compounds on peroxidase was confirmed through histochemical studies, which showed that xylem peroxidase was totally inhibited by treatment with these NO donors at 5·0 mol m^–3, and by NO at a concentration change rate of 55 ± 5 and 110 ± 9 μmol m^–3 s^–1. However, SNP, at 5·0 mol m^–3, does not have any effect on H₂O₂ production by the xylem of Z. elegans. The fact that SNP and SNAP are two structurally dissimilar compounds which only share the common ability to release NO in aqueous buffer, and that similar results were obtained when using NO itself, suggest that NO could be considered as an inhibitor of coniferyl alcohol peroxidase which does not affect H₂O₂ production in the xylem of Z. elegans.     

Relationship between C2H2 reduction, H2 evolution and 15N2 fixation in root nodules of pea (Pisum sativum)

The quantitative relationship between C₂H₂ reduction, H₂ evolution and ¹⁵N₂ fixation was investigated in excised root nodules from pea plants ( Pisum sativum L. cv. Bodil) grown under controlled conditions. The C₂H₂/N₂ conversion factor varied from 3.31 to 5.12 between the 32nd and the 67th day after planting. After correction for H₂ evolution in air, the factor (C₂H₂-H₂)/N₂ decreased to values near the theoretical value 3, or in one case to a value significantly ( P < 0.05) below 3. The proportion of the total electron flow through nitrogenase, which is not wasted in H₂ production but used for N₂ reduction, is often stated as the relative efficiency (1-H₂/C₂H₂). This factor varied significantly ( P < 0.05) during the growth period. The actual allocation of electrons to H₂ and N₂, expressed as the H₂/N₂ ratio, was independent of plant age, however. This discrepancy and the observation that the (C₂H₂-H₂)/N₂ conversion factor tended to be lower than 3, suggests that the C₂H₂reduction assay underestimates the total electron flow through nitrogenase.     

Acclimation of photosynthesis, H2O2 content and antioxidants in maize (Zea mays) grown at sub-optimal temperatures

Maize plants were grown at 14, 18 and 20 °C until the fourth leaf had emerged. Leaves from plants grown at 14 and 18 °C had less chlorophyll than those grown at 20 °C. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was decreased at 14 °C compared with 20 °C, but the activation state was highest at 14 °C. Growth at 14 °C increased the abundance (but not the number) of Rubisco breakdown products. Phosphoenolpyruvate carboxylase (PEPC) activity was decreased at 14 °C compared with 20 °C but no chilling-dependent effects on the abundance of the PEPC protein were observed. Maximal extractable NADP-malate dehydrogenase activity increased at 14 °C compared with 20 °C whereas the glutathione pool was similar in leaves from plants grown at both temperatures. Foliar ascorbate and hydrogen peroxide were increased at 14 °C compared with 20 °C. The foliar hydrogen peroxide content was independent of irradiance at both growth temperatures. Plants grown at 14 °C had decreased rates of CO₂ fixation together with decreased quantum efficiencies of photosystem (PS) II in the light, although there was no photo-inhibition. Growth at 14 °C decreased the abundance of the D1 protein of PSII and the PSI psaB gene product but the psaA gene product was largely unaffected by growth at low temperatures. The relationships between the photosystems and the co-ordinate regulation of electron transport and CO₂ assimilation were maintained in plants grown at 14 °C.     

Biological oxidation of hydrogen in soils flushed with a mixture of H2, CO2, O2 and N2

Abstract A stainless steel cylinder filled with soil was flushed upstream with a H₂/CO₂/air mixture. The consequence was a strong enrichment of the aerobic, autotrophic hydrogen-oxidising microflora, which reached densities enabling them to oxidize 84.5 ml H₂· dm⁻²· h⁻¹ in the first 25-cm layer. H₂ concentration profiles, hydrogen uptake activity and cell numbers correlated well with each other. Most of the organisms isolated were dinitrogen fixers. Thus, soils containing hydrogen-oxidising bacteria may act as a biological shield between H₂-rich environments and air, and may be utilized as biofilters, e.g., in the waste-processing industry.