Hydrogen Peroxide and Superoxide Anion Radical Photoproduction in PSII Preparations at Various Modifications of the Water-Oxidizing Complex

Protective Role of Vitamin D Receptor in Cerebral Ischemia/Reperfusion Injury In Vitro and In Vivo Model

BACKGROUND

Vitamin D receptor (VDR) can prevent myocardial ischemia reperfusion injury (MIRI). Hence, we aimed to illuminate the effect of VDR on cerebral ischemia/reperfusion injury (CIRI).

METHODS

C57BL/6 mice and SK-N-SH cells were utilized to establish CIRI and cellular oxygen deprivation/reoxygenation (OGD/R) models. Mice were injected with 1 μg/kg Calcitriol or 1 μg/kg Paricalcitol (PC) and adenovirus-mediated VDR overexpression or knockdown plasmids. 2,3,5-triphenyl-tetrazolium chloride (TTC) and Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays were performed to measure the brain infarct volume and the apoptosis of cerebral cells. SK-N-SH cells were treated with 5 mM N-acetyl-L-cysteine (NAC) and transfected with VDR knockdown plasmid. Flow cytometry and Cell Counting Kit-8 (CCK-8) assays were employed to assess the apoptosis and cell viability. Enzyme-Linked Immunosorbent Assay (ELISA), quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR) and Western blot were exploited to quantify the levels of reactive species oxygen (ROS), other oxidative stress-related factors, VDR and apoptosis-related factors.

RESULTS

The level of VDR in mouse cerebral tissue was elevated by CIRI (p < 0.001). CIRI-induced cerebral infarction (p < 0.001) and the apoptosis of cerebral cells (p < 0.001) in mice were mitigated by the activation of VDR. VDR overexpression abrogated while VDR silencing enhanced CIRI-induced infarction, oxidative stress and apoptosis of cerebral cells (p < 0.05). Furthermore, VDR silencing aggravated the oxidative stress and apoptosis in OGD/R-treated SK-N-SH cells (p < 0.05). NAC, a scavenger of oxidative stress, could reverse the effects of VDR silencing on apoptosis and oxidative stress in OGD/R-treated SK-N-SH cells (p < 0.01).

CONCLUSION

VDR alleviates the oxidative stress to protect against CIRI.

The synchronicity of bloom-forming cyanobacteria transcription patterns and hydrogen peroxide dynamics

Hydrogen peroxide is a reactive oxygen species (ROS) naturally occurring at low levels in aquatic environments and production varies widely across different ecosystems. Oxygenic photosynthesis generates hydrogen peroxide as a byproduct, of which some portion can be released to ambient water. However, few studies have examined hydrogen peroxide dynamics in relation to cyanobacterial harmful algal blooms (cHABs). A year-long investigation of algal succession and hydrogen peroxide dynamics was conducted at the Caloosahatchee River, Florida, USA. We aimed to identify potential biological mechanisms responsible for elevated hydrogen peroxide production during cHAB events through the exploration of the freshwater microbial metatranscriptome. Hydrogen peroxide concentrations were elevated from February to September of 2021 when cyanobacteria were active and abundant. We observed one Microcystis cHAB event in spring and one in winter. Both had distinct nutrient uptake and cyanotoxin gene expression patterns. While meaningful levels of microcystin were only detected during periods of elevated hydrogen peroxide, cyanopeptolin was by far the most expressed cyanotoxin during the spring bloom when hydrogen peroxide was at its yearly maxima. Gene expressions of five microbial enzymes (Rubisco, superoxide dismutase, cytochrome b559, pyruvate oxidase, and NADH dehydrogenase) positively correlated to hydrogen peroxide concentrations. Additionally, there was higher nitrogen-fixing gene (nifDKH) expression by filamentous cyanobacteria after the spring bloom but no secondary bloom formation occurred. Overall, elevated environmental hydrogen peroxide concentrations were linked to cyanobacterial dominance and greater expression of specific enzymes in the photosynthesis of cyanobacteria. This implicates cyanobacterial photosynthesis and growth results in increased hydrogen peroxide generation as reflected in measured environmental concentrations.

A possible relationship between the effect of factors on photoactivation of photosystem II depleted of functional Mn and cytochrome b559

The photoassembly of the Mn4CaO5 cluster in Mn-depleted photosystem II preparations (photoactivation) was studied under the influence of oxidants, reductants and pH. New data on the effect of these factors on the photoactivation yield are presented. The presence of the oxidant, ferricyanide, negatively affected the photoactivation yield over the entire concentration range studied (0-1 mM). In contrast to ferricyanide, the addition of the reductant, ferrocyanide, up to 1 mM resulted in an increase in the photoactivation yield. Other reductants either did not significantly affect (diphenylcarbazide) or suppressed (ascorbate) the photoactivation yield. The effect of ferrocyanide on photoactivation were found to be similar dichlorophenolindophenol. Investigation of the photoactivation yield as a function of pH revealed that the maximum yield was observed at pH 6.5 in the presence of ferrocyanide and DCPIP, and at pH 5.5 without additives. In addition, the photoactivation yield at pH 5.5 was the same without and with the addition of ferrocyanide or dichlorophenolindophenol. Although ferricyanide suppressed the photoactivation, the photoactivation yield increased in the presence of ferricyanide by shifting the pH to the acidic region. The samples contained approximately 25 % of the HP cyt b559, which was in the reduced state, as the absorbance at 559 nm was decreased upon addition of ferricyanide and subsequent addition of ferrocyanide returned the spectrum to the baseline. A possible relationship between the effect of factors on the photoactivation and the involvement of cyt b559 in the protection of PSII from oxidative damage on the donor side is discussed.

Superoxide Anion Radical Generation in Photosynthetic Electron Transport Chain

This review analyzes data available in the literature on the rates, characteristics, and mechanisms of oxygen reduction to a superoxide anion radical at the sites of photosynthetic electron transport chain where this reduction has been established. The existing assumptions about the role of the components of these sites in this process are critically examined using thermodynamic approaches and results of the recent studies. The process of O2 reduction at the acceptor side of PSI, which is considered the main site of this process taking place in the photosynthetic chain, is described in detail. Evolution of photosynthetic apparatus in the context of controlling the leakage of electrons to O2 is explored. The reasons limiting application of the results obtained with the isolated segments of the photosynthetic chain to estimate the rates of O2 reduction at the corresponding sites in the intact thylakoid membrane are discussed.

Tomato responses to salinity stress: From morphological traits to genetic changes

Tomato is an essential annual crop providing human food worldwide. It is estimated that by the year 2050 more than 50% of the arable land will become saline and, in this respect, in recent years, researchers have focused their attention on studying how tomato plants behave under various saline conditions. Plenty of research papers are available regarding the effects of salinity on tomato plant growth and development, that provide information on the behavior of different cultivars under various salt concentrations, or experimental protocols analyzing various parameters. This review gives a synthetic insight of the recent scientific advances relevant into the effects of salinity on the morphological, physiological, biochemical, yield, fruit quality parameters, and on gene expression of tomato plants. Notably, the works that assessed the salinity effects on tomatoes were firstly identified in Scopus, PubMed, and Web of Science databases, followed by their sifter according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline and with an emphasis on their results. The assessment of the selected studies pointed out that salinity is one of the factors significantly affecting tomato growth in all stages of plant development. Therefore, more research to find solutions to increase the tolerance of tomato plants to salinity stress is needed. Furthermore, the findings reported in this review are helpful to select, and apply appropriate cropping practices to sustain tomato market demand in a scenario of increasing salinity in arable lands due to soil water deficit, use of low-quality water in farming and intensive agronomic practices.

ROS production and signalling in chloroplasts: cornerstones and evolving concepts

Reactive oxygen species (ROS) such as singlet oxygen, superoxide (O₂^●- ) and hydrogen peroxide (H₂ O₂ ) are the markers of living cells. Oxygenic photosynthesis produces ROS in abundance, which act as a readout of a functional electron transport system and metabolism. The concept that photosynthetic ROS production is a major driving force in chloroplast to nucleus retrograde signalling is embedded in the literature, as is the role of chloroplasts as environmental sensors. The different complexes and components of the photosynthetic electron transport chain (PETC) regulate O₂^●- production in relation to light energy availability and the redox state of the stromal Cys-based redox systems. All of the ROS generated in chloroplasts have the potential to act as signals and there are many sulphhydryl-containing proteins and peptides in chloroplasts that have the potential to act as H₂ O₂ sensors and function in signal transduction. While ROS may directly move out of the chloroplasts to other cellular compartments, ROS signalling pathways can only be triggered if appropriate ROS-sensing proteins are present at or near the site of ROS production. Chloroplast antioxidant systems serve either to propagate these signals or to remove excess ROS that cannot effectively be harnessed in signalling. The key challenge is to understand how regulated ROS delivery from the PETC to the Cys-based redox machinery is organised to transmit redox signals from the environment to the nucleus. Redox changes associated with stromal carbohydrate metabolism also play a key role in chloroplast signalling pathways.

Cooperative pathway of O2 reduction to H2O2 in chloroplast thylakoid membrane: new insight into the Mehler reaction

Oxygen reduction in chloroplasts in the light was discovered by (Mehler Arch Biochem Biophys 33:65-77, 1951) as production of hydrogen peroxide. Later, it was shown that the primary product of the oxygen reduction is superoxide radical produced in thylakoids by one-electron transfer from reduced components of photosynthetic electron transport chain to O2 molecule. For a long time, the formation of hydrogen peroxide was considered to be a result of disproportionation of superoxide radicals in chloroplast stroma. Here, we overview a growing number of evidence indicating on another one, additional to disproportionation, pathway of hydrogen peroxide formation in chloroplasts, namely its formation in thylakoid membrane due to reaction of superoxide radical generated in the membrane with the reduced plastoquinone molecule, plastohydroquinone. Since various components of photosynthetic electron transport chain (primarily photosystem I) can supply superoxide radicals to this reaction, we refer this two-step O2 photoreduction to H2O2 as a cooperative process. The significance of hydrogen peroxide production via this pathway for redox signaling and scavenging of reactive oxygen species is discussed.

True oxygen reduction capacity during photosynthetic electron transfer in thylakoids and intact leaves

Reactive oxygen species (ROS) are generated in electron transport processes of living organisms in oxygenic environments. Chloroplasts are plant bioenergetics hubs where imbalances between photosynthetic inputs and outputs drive ROS generation upon changing environmental conditions. Plants have harnessed various site-specific thylakoid membrane ROS products into environmental sensory signals. Our current understanding of ROS production in thylakoids suggests that oxygen (O2) reduction takes place at numerous components of the photosynthetic electron transfer chain (PETC). To refine models of site-specific O2 reduction capacity of various PETC components in isolated thylakoids of Arabidopsis thaliana, we quantified the stoichiometry of oxygen production and consumption reactions associated with hydrogen peroxide (H2O2) accumulation using membrane inlet mass spectrometry and specific inhibitors. Combined with P700 spectroscopy and electron paramagnetic resonance spin trapping, we demonstrate that electron flow to photosystem I (PSI) is essential for H2O2 accumulation during the photosynthetic linear electron transport process. Further leaf disc measurements provided clues that H2O2 from PETC has a potential of increasing mitochondrial respiration and CO2 release. Based on gas exchange analyses in control, site-specific inhibitor-, methyl viologen-, and catalase-treated thylakoids, we provide compelling evidence of no contribution of plastoquinone pool or cytochrome b6f to chloroplastic H2O2 accumulation. The putative production of H2O2 in any PETC location other than PSI is rapidly quenched and therefore cannot function in H2O2 translocation to another cellular location or in signaling.

Oxidative stress in moss Bryum caespiticium (Bryaceae) under the influence of high temperature and light intensity in a technogenically transformed environment

Mosses are pioneer plants in post-technogenic areas. Therefore, the question of adaptive reactions of mosses from these habitats represents a scientific interest. The research is devoted to the study of adaptive changes in the metabolism of the dominant moss species Bryum caespiticium Hedw., collected in the devastated territories of the Novoyavorivsk State Mining and Chemical Enterprise (SMCE) “Sirka (Sulfur)” exposed to hyperthermia and insolation, which cause oxidative stress in plants. The influence of these stressors on the activity and thermal stability of antioxidant enzymes, hydrogen peroxide content, anion radical generation and accumulation of prooxidant components in moss shoots was studied. The activity and thermal stability of peroxidase and superoxide dismutase (SOD) were analysed forB. caespiticium moss from different locations of northern exposure at the sulfur mining dump No 1 in summer and autumn. We established the dependence of the activity of antioxidant enzymes of moss on the intensity of light and temperature on the experimental plots of the dump No 1. In summer, the highest activity and thermal stability rates of peroxidase and SOD were observed. Under the conditions of the experiment in shoots of В. caespiticium from the northern peak of the dump under the influence of 2 hours temperature action (+ 42 ºС) the most significant increase in peroxidase activity was found by 1.78 times and SOD by 1.89 times, as well as increase in its thermal stability by 1.35–1.42 times, respectively. The increase in peroxidase and SOD activity, as well as the increase in their thermal stability caused by hyperthermia were negated by pre-processing with a protein biosynthesis inhibitor cyclohexamide, which may indicate the participation of the protein-synthesizing system in this process. The effect of increasing the thermal stability of enzymes can be considered as a mechanism of adaptation of the protein-synthesizing system to the action of high temperatures. Increase in the activity and thermal stability of antioxidant enzymes is caused primarily by changes in the expression of stress protein genes, which control the synthesis of specific adaptogens and protectors. The obtained results indicate that the extreme conditions of the anthropogenically transformed environment contribute to the development of forms with the highest potential abilities. The mechanism of action of high temperatures is associated with the development of oxidative stress, which is manifested in the intensification of lipid peroxidation and the generation of superoxide anion radical. It was found that temperature stress and high insolation caused an increased generation of superoxide anion radical as the main inducers of protective reactions in the samples of B. caespiticium from the experimental transect of the sulfur mining heap. It is known that the synthesis of Н2О2 occurs under stress and is a signal to start a number of molecular, biochemical and physiological processes of cells, including adaptation of plants to extreme temperatures. It is shown that high temperatures initiate the generation of hydrogen peroxide. Increased reactive oxygen species (ROS) formation, including Н2О2, under the action of extreme temperatures, can cause the activation of signaling systems. Therefore, the increase in the content of Н2О2 as a signaling mediator is a component of the antioxidant protection system. It is determined that adaptive restructuring of the metabolism of the moss В. caespiticium is associated with the accumulation of signaling prooxidant components (diene and triene conjugates and dienketones). The increase in primary lipid peroxidation products, detected by us, under the action of hyperthermia may indicate the intensification of free radical oxidation under adverse climatic conditions in the area of the sulfur production dump, which leads to the intensification of lipid peroxidation processes. The accumulation of radical and molecular lipid peroxidation products are signals for the activation of protective systems, activators of gene expression and processes that lead to increased resistance of plants.

Involvement of Carbonic Anhydrase CAH3 in the Structural and Functional Stabilization of the Water-Oxidizing Complex of Photosystem II from Chlamydomonas reinhardtii

The involvement of carbonic anhydrases (CA) and CA activity in the functioning of photosystem II (PSII) has been studied for a long time and has been shown in many works. However, so far only for CAH3 from Chlamydomonas reinhardtii there is evidence for its association with the donor side of PSII, where the CA activity of CAH3 can influence the functioning of the water-oxidizing complex (WOC). Our results suggest that CAH3 is also involved in the organization of the native structure of WOC independently of its CA activity. It was shown that in PSII preparations from wild type (WT) the high O2-evolving activity of WOC was observed up to 100 mM NaCl in the medium and practically did not decrease with increasing incubation time with NaCl. At the same time, the WOC function in PSII preparations from CAH3-deficient mutant cia3 is significantly inhibited already at NaCl concentrations above 35 mM, reaching 50% at 100 mM NaCl and increased incubation time. It is suggested that the absence of CAH3 in PSII from cia3 causes disruption of the native structure of WOC, allowing more pronounced conformational changes of its proteins and, consequently, suppression of the WOC active center function, when the ionic strength of the medium is increased. The results of Western blot analysis indicate a more difficult removal of PsbP protein from PSII of cia3 at higher NaCl concentrations, apparently due to the changes in the intermolecular interactions between proteins of WOC in the absence of CAH3. At the same time, the values of the maximum quantum yield of PSII did not practically differ between preparations from WT and cia3, indicating no effect of CAH3 on the photoinduced electron transfer in the reaction center of PSII. The obtained results indicate the involvement of the CAH3 protein in the native organization of the WOC and, as a consequence, in the stabilization of its functional state in PSII from C. reinhardtii.

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.

Minimizing an Electron Flow to Molecular Oxygen in Photosynthetic Electron Transfer Chain: An Evolutionary View

Recruitment of H₂O as the final donor of electrons for light-governed reactions in photosynthesis has been an utmost breakthrough, bursting the evolution of life and leading to the accumulation of O₂ molecules in the atmosphere. O₂ molecule has a great potential to accept electrons from the components of the photosynthetic electron transfer chain (PETC) (so-called the Mehler reaction). Here we overview the Mehler reaction mechanisms, specifying the changes in the structure of the PETC of oxygenic phototrophs that probably had occurred as the result of evolutionary pressure to minimize the electron flow to O₂. These changes are warranted by the fact that the efficient electron flow to O₂ would decrease the quantum yield of photosynthesis. Moreover, the reduction of O₂ leads to the formation of reactive oxygen species (ROS), namely, the superoxide anion radical and hydrogen peroxide, which cause oxidative stress to plant cells if they are accumulated at a significant amount. From another side, hydrogen peroxide acts as a signaling molecule. We particularly zoom in into the role of photosystem I (PSI) and the plastoquinone (PQ) pool in the Mehler reaction.