Vladimir D. Kreslavski

Reactive oxygen species: Re-evaluation of generation, monitoring and role in stress-signaling in phototrophic organisms

This review provides an overview about recent developments and current knowledge about monitoring, generation and the functional role of reactive oxygen species (ROS) - H2O2, HO2, HO, OH(-), (1)O2 and O2(-) - in both oxidative degradation and signal transduction in photosynthetic organisms including microscopic techniques for ROS detection and controlled generation. Reaction schemes elucidating formation, decay and signaling of ROS in cyanobacteria as well as from chloroplasts to the nuclear genome in eukaryotes during exposure of oxygen-evolving photosynthetic organisms to oxidative stress are discussed that target the rapidly growing field of regulatory effects of ROS on nuclear gene expression.

Signaling role of reactive oxygen species in plants under stress

The review considers the role of H2O2, 1O2, O2·−, and the products of lipid peroxidation as signaling molecules in the processes of stress signal transduction in plants. The data concerning possible ROS participation in transduction of stress signals from chloroplasts to the nuclear genome, H2O2 involvement in transduction stress signals in cyanobacteria, and also the interactions between ROS and other signaling systems within the cell are presented. It is suggested that redox regulators, protein kinases/protein phosphatases, and transcription factors play a crucial role in the functioning of ROS-dependent signaling systems in the plant cell.

Molecular Mechanisms of Stress Resistance of the Photosynthetic Apparatus

The mechanisms of action of environmental stress-inducing factors on the photosynthetic apparatus (PA) of plants are considered. The basic targets for stress produced by heat, cold, salinity, osmotic imbalance, and high irradiance are analyzed. It is suggested that stress factors have an influence on the composition of thylakoid membranes and inhibit photosynthetic processes. However, recent studies demonstrated that strong light induces the photodamage to photosystem II (PS II) due to direct action of light on the oxygen-evolving complex. Stress-induced accumulation of reactive oxygen species (ROS) leads to inhibition of the recovery of the PSII by suppressing thede novo synthesis of photosynthetic proteins. In addition, stress stimulates the synthesis of protective low-molecular weight compounds (e.g., glycine betaine) and stress proteins. The major mechanisms of acclimation and protection of the PA against damaging effects of environmental stress-inducing factors are analyzed with special reference to cyanobacterial cells and mutants with high or low stress resistance.

Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities.

The nature of photosynthetic recovery was investigated in 10-d-old wheat (Triticum aestivum L., cv. Moskovskaya-35) seedlings exposed to temperatures of 40 and 42 degrees C for 20 min and to temperature 42 degrees C for 40 min in the dark. The aftereffect of heat treatment was monitored by growing the heat-treated plants in low/moderate/high light at 20 degrees C for 72h. The net photosynthetic rates (P(N)) and the fluorescence ratios F(v)/F(m) were evaluated in intact primary leaves and the rates of cyclic and non-cyclic photophosphorylation were measured in the isolated thylakoids. At least two temporally separated steps were identified in the path of recovery from heat stress at 40 and 42 degrees C in the plants growing in high and moderate/high light, respectively. Both photochemical activity of the photosystem II (PSII) and the activity of CO(2) assimilation system were lowered during the first step in comparison with the corresponding activities immediately after heat treatment. During the second step, the photosynthetic activities completely or partly recovered. Recovery from heat stress at 40 degrees C was accompanied by an appreciably higher rate of cyclic photophosphorylation in comparison with control non-heated seedlings. In pre-heated seedlings, the tolerance of the PSII to photoinhibition was higher than in non-treated ones. The mode of acclimation to different light intensities after heat exposures is analyzed.

Effect of preillumination with red light on photosynthetic parameters and oxidant-/antioxidant balance in Arabidopsis thaliana in response to UV-A

The effect of preillumination with low intensity (10μmol quanta m(-2)s(-1), 10min) light of different wavelengths in the spectral range of 550-730nm on photosynthesis and activity of PSII, the content of photosynthetic pigments and H2O2, as well as the peroxidase activity in the leaves of 26-d-old Arabidopsis thaliana wild-type (WT) plants in response to UV-A radiation was studied. UV-A decreased the activity of the PSII, the content of Chl a, Chl b and carotenoids, as well as increased the peroxidase activity and H2O2 level in the WT leaves. Preillumination of the leaves with red light (RL, λmax=664nm) reduced the inhibitory effect of UV radiation on photosynthesis and activity of the PSII, indicated by delayed light emission as well as the H2O2 level, but increased the peroxidase activity in the leaves compared to illumination by UV radiation only. Illumination with RL alone and the subsequent exposure of plants to darkness increased the peroxidase activity and the transcription activity of genes of the transcription factors APX1 and HYH. Preillumination of leaves with RL, then far red light (FRL, λmax=727nm) partially compensated the effect of the RL for all studied parameters, suggesting that the active form of phytochrome (PFR) is involved in these processes. Preillumination with the wavelengths of 550, 594 and 727nm only did not have a marked effect on photosynthesis. The hy2 mutant of Arabidopsis with reduced synthesis of the phytochrome B chromophore showed decreased resistance of PSII to UV-A compared with the WT of Arabidopsis. UV radiation reduced Chl a fluorescence much faster in the hy2 mutant compared to the WT. Preillumination of the hy2 mutant with RL did not affect the PSII activity and H2O2 level in UV-irradiated leaves. It is assumed that the formation of the increased resistance of the photosynthetic apparatus of Arabidopsis to UV-A radiation involves PFR and the antioxidant system of plants, partly by inducing transcriptional activity of some antioxidant and transcription factors genes.

Preillumination of lettuce seedlings with red light enhances the resistance of photosynthetic apparatus to UV-A

Seedlings of 10-day-old lettuce (Lactuca sativa L., cultivar Berlin) were preilluminated by low intensity red light (λmax=660 nm, 10 min, 5 μmol quanta m(-2) s(-1)) and far-red light (λmax=730 nm, 10 min, 5 μmol quanta m(-2) s(-1)) to study the effect of pre-treatment on photosynthesis, photochemical activity of photosystem II (PSII), the contents of photosynthetic and UV-A-absorbing pigments (UAPs) and H2O2, as well as total and ascorbate peroxidase activities in cotyledonary leaves of seedlings exposed to UV-A. UV radiation reduced the photosynthetic rate (Pn), the activity of PSII, and the contents of Chl a and b, carotenoids and UAPs in the leaves, but increased the content of H2O2 and the total peroxidase activity. Preillumination with red light removed these effects of UV. In turn, the illumination with red light, then far-red light removed the effect of the red light. Illumination with red light alone increased the content of UAPs, as well as peroxidase activity. It is suggested that higher resistance of the lettuce photosynthetic apparatus to UV-A radiation is associated with involvement of the active form of phytochrome B, thereby increasing peroxidase activities as well as UAPs and saving preservation of photosynthetic pigment contents due to pre-illumination with red light.

Photosynthetic Energy Conversion: Hydrogen Photoproduction by Natural and Biomimetic Means

The main function of the photosynthetic process is to capture solar energy and to store it in the form of chemical fuels. Many fuel forms such as coal, oil and gas have been intensively used and are becoming limited. Hydrogen could become an important clean fuel for the future. Among different technologies for hydrogen production, oxygenic natural and artificial photosynthesis using direct photochemistry in synthetic complexes have a great 3 Biomimetics, Learning from Nature 50 potential to produce hydrogen as both use clean and cheap sources water and solar energy. Photosynthetic organisms capture sunlight very efficiently and convert it into organic molecules. Artificial photosynthesis is one way to produce hydrogen from water using sunlight by employing biomimetic complexes. However, splitting of water into protons and oxygen is energetically demanding and chemically difficult. In oxygenic photosynthetic microorganisms water is splitted into electrons and protons during primary photosynthetic processes. The electrons and protons are redirected through the photosynthetic electron transport chain to the hydrogen-producing enzymes-hydrogenase or nitrogenase. By these enzymes, eand H+ recombine and form gaseous hydrogen. Biohydrogen activity of hydrogenase can be very high but it is extremely sensitive to photosynthetic O2. At the moment, the efficiency of biohydrogen production is low. However, theoretical expectations suggest that the rates of photon conversion efficiency for H2 bioproduction can be high enough (> 10%). Our review examines the main pathways of H2 photoproduction using photosynthetic organisms and biomimetic photosynthetic systems and focuses on developing new technologies based on the effective principles of photosynthesis.

Heat-induced impairments and recovery of photosynthetic machinery in wheat seedlings. Role of light and prooxidant-antioxidant balance

The extent of damage caused to the photosynthetic machinery of 10-d-old wheat seedlings by short-term exposure to mild heat, their capacity to recover from it and the possible roles of H2O2, SOD, catalase and ascorbate peroxidase on the recovery process were investigated. Seedlings were subjected to heat treatments at 40/42/44 °C for 20 min in the dark and allowed to grow for 72 h in light of different irradiances (40–800 μE m−2 s−1) at 20 °C for recovery from heat induced damage. Complete or partial recovery of photosynthetic activities was observed in the seedlings treated at 40 °C and 42 °C, but not at 44 °C. Our data suggest that the balance between (pro)oxidant and antioxidant levels poised by heat stress subsequent light is the crucial factor for the extent of recovery from heat induced damage.

Effect of naphthalene on photosystem 2 photochemical activity of pea plants.

The effect of a typical polyaromatic hydrocarbon, naphthalene (Naph), on photosystem 2 (PS-2) photochemical activity in thylakoid membrane preparations and 20-day-old pea leaves was studied. Samples were incubated in water in the presence of Naph (0.078, 0.21, and 0.78 mM) for 0.5–24 h under white light illumination (15 μmol photons·m−2·s−1). The PS-2 activity was determined by studying fast and delayed chlorophyll (Chl) a fluorescence. Incubation of samples in water solutions at Naph concentrations of 0.21 and 0.78 mM led to a decrease in the maximum PS-2 quantum efficiency (Fv/Fm), noticeable changes in the polyphasic induction kinetics of fluorescence (OJIP), and a decrease in the amplitudes of the fast and slow components of delayed fluorescence of Chl a. The rate of release of electrolytes from leaves that were preliminarily incubated with Naph (0.21 mM) was also increased. Significant decrease in the fluorescence parameters in thylakoid membrane preparations was observed at Naph concentration of 0.03 mM and 12-min exposure of the samples. Chlorophyll (a and b) and carotenoid content (mg per gram wet mass) was insignificantly changed. The quantum yields of electron transfer from QA to QB (φET2o) and also to the PS-1 acceptors (φRE1o) were reduced. These results are explained by the increase in the number of QB-non-reducing centers of PS-2, which increased with increasing Naph concentration and exposure time of leaves in Naph solution. The suppression of PS-2 activity was partly abolished in the presence of the electron donor sodium ascorbate. Based on these results, it is suggested that Naph distorts cell membrane intactness and acts mainly on the PS-2 acceptor and to a lesser degree on the PS-2 donor side.

Response of the photosynthetic apparatus to UV-A and red light in the phytochrome B-deficient Arabidopsis thaliana L. hy3 mutant

The effect of UV-A radiation (365 nm) and the protective effect of preillumination with red light (RL, 664 nm, 10 min) or with a combination of red and far-red light (FRL, 727 nm, 10 min) on the activity of the PSII as well as the expression levels of selected genes, especially those encoding chloroplast proteins (sAPX, tAPX, CAB1, and D1), were studied in leaves of the 26-d-old hy3 mutant of Arabidopsis thaliana, which is deficient in the phytochrome B apoprotein. The effects were compared with corresponding effects observed in the hy2 mutant of A. thaliana, which is deficient in the phytochrome chromophore. Illumination with UV-A decreased the photosynthetic pigment content, the maximum photochemical quantum yield of PSII (Fv/Fm), and the effective quantum yield of PSII (ΦPSII). The reduction of the Fv/Fm ratio and ΦPSII was more pronounced in the mutants as compared to wild-type plants (WT). The preillumination of the leaves with RL caused a significant reduction in the inhibitory effect of UV-radiation on the PSII activity in the WT plants, but it caused only a small decrease in the hy3 mutant. The preillumination of leaves with RL and FRL combination compensated the protective effect of RL on the UV-induced decrease of the fluorescence parameters in the WT. Such reversibility is typical for involvement of red/far-red reversible phytochromes at low intensity light. The results suggest an important role of red/far-red reversible phytochromes (phytochrome B) in the resistance of PSII to UV-A radiation caused by changes in contents of either carotenoids or other UV-absorbing pigments probably through biosynthesis of these pigments. The data also demonstrated that phytochrome B and other phytochromes can affect the PSII stress resistance by the fast regulation of the expression of genes encoding antioxidant enzymes and transcription factors at the step of gene transcription.