When plants are exposed to stressful environmental conditions, the production of Reactive Oxygen Species (ROS) increases and can cause significant damage to the cells. Antioxidant defenses, which can detoxify ROS, are present in plants. A major hydrogen peroxide detoxifying system in plant cells is the ascorbate-glutathione cycle, in which, ascorbate peroxidase (APX) enzymes play a key role catalyzing the conversion of H2O2 into H2O, using ascorbate as a specific electron donor. Different APX isoforms are present in distinct subcellular compartments, such as chloroplasts, mitochondria, peroxisome, and cytosol. The expression of APX genes is regulated in response to biotic and abiotic stresses as well as during plant development. The APX responses are directly involved in the protection of plant cells against adverse environmental conditions. Furthermore, mutant plants APX genes showed alterations in growth, physiology and antioxidant metabolism revealing those enzymes involvement in the normal plant development.
Current studies, particularly in Arabidopsis, have demonstrated that mutants deficient in cytosolic ascorbate peroxidases (APXs) are susceptible to the oxidative damage induced by abiotic stress. In contrast, we demonstrate here that rice mutants double silenced for cytosolic APXs (APx1/2s) up-regulated other peroxidases, making the mutants able to cope with abiotic stress, such as salt, heat, high light and methyl viologen, similar to non-transformed (NT) plants. The APx1/2s mutants exhibited an altered redox homeostasis, as indicated by increased levels of H2O2 and ascorbate and glutathione redox states. Both mutant and NT plants exhibited similar photosynthesis (CO2 assimilation and photochemical efficiency) under both normal and stress conditions. Overall, the antioxidative compensatory mechanism displayed by the mutants was associated with increased expression of OsGpx genes, which resulted in higher glutathione peroxidase (GPX) activity in the cytosolic and chloroplastic fractions. The transcript levels of OsCatA and OsCatB and the activities of catalase (CAT) and guaiacol peroxidase (GPOD; type III peroxidases) were also up-regulated. None of the six studied isoforms of OsApx were up-regulated under normal growth conditions. Therefore, the deficiency in cytosolic APXs was effectively compensated for by up-regulation of other peroxidases. We propose that signalling mechanisms triggered in rice mutants could be distinct from those proposed for Arabidopsis.
Using a voltage biased electrode inside the edge of the tokamak TCABR, H-mode discharges with strong, Regime I, and partial or total suppressed MHD activity, Regime II, were obtained. In this paper we present the results of the study of these two regimes of operation. The experiment was carried out adjusting the tokamak parameters to obtain discharges with strong or weak MHD activity, without biasing in both cases. During the shots the plasma current varied to cover a range of safety factor from 2.8 up to 3.6, allowing the Hmode barrier to interact with the magnetic islands. Subsequently, the bias was applied and shots with and without MHD activity were obtained. An array of 22 Mirnov coils was used to detect the magnetic oscillations and a triple Langmuir probe to measure plasma floating potentials and ion saturation currents. The results show that the dominant modes are m=2, n=1 and m=3, n=1 for partial excitation and suppression, respectively. In both cases strong decrease of the radial electric field is detected with destruction of the transport barrier and of the H mode regime. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of H α line intensity. The explanation of the excitation and suppression processes is discussed in the text.
Carbon nanotubes with different nitrogen contents were produced by the arc-discharge technique. The samples were first submitted to a concentration process ͑purification͒ and analyzed by x-ray photoelectron spectroscopy, electron-energy-loss spectroscopy, electron transmission, and scanning electron microscopy to study the materials structure and morphology. Measured values of nitrogen concentration were below 5 at. % and varied with the nitrogen partial pressure inside the arc-discharge chamber. Using an optical microscope, highly localized regions of the samples (ϳ1 mm 2 ) were irradiated by an Ar ion laser. Controlling the laser intensity, further local purification was induced and information about the evolution of the structural order of the nanotube samples with different contents of nitrogen was obtained.
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