Photodamage of a Mn(III/IV)-oxo mixed-valence compound and photosystem II: evidence that a high-valent manganese species is responsible for UV-induced photodamage of the oxygen-evolving complex in photosystem II

Photosystems under high light stress: throwing light on mechanism and adaptation

High light stress decreases the photosynthetic rate in plants due to photooxidative damage to photosynthetic apparatus, photoinhibition of PSII, and/or damage to PSI. The dissipation of excess energy by nonphotochemical quenching and degradation of the D1 protein of PSII and its repair cycle help against photooxidative damage. Light stress also activates stress-responsive nuclear genes through the accumulation of phosphonucleotide-3'-phosphoadenosine-5'-phosphate, methylerythritol cyclodiphosphate, and reactive oxygen species which comprise the chloroplast retrograde signaling pathway. Additionally, hormones, such as abscisic acid, cytokinin, brassinosteroids, and gibberellins, play a role in acclimation to light fluctuations. Several alternate electron flow mechanisms, which offset the excess of electrons, include activation of plastid or plastoquinol terminal oxidase, cytochrome b 6/f complex, cyclic electron flow through PSI, Mehler ascorbate peroxidase pathway or water-water cycle, mitochondrial alternative oxidase pathway, and photorespiration. In this review, we provided insights into high light stress-mediated damage to photosynthetic apparatus and strategies to mitigate the damage by decreasing antennae size, enhancing NPQ through the introduction of mutants, expression of algal proteins to improve photosynthetic rates and engineering ATP synthase.

Abscisic acid plays a key role in the mechanism of photosynthetic and physiological response effect of Tetrabromobisphenol A on tobacco

The toxicity of bromide to animals and microorganisms has been widely studied, but the mechanism by which bromide toxicity affects plants is rarely studied. This study used the bromophenol compound Tetrabromobisphenol A (TBBPA) as a representative of bromide to explore the physiological and molecular response mechanism of tobacco leaves to TBBPA. In addition, physiological determination, transcriptomics, weighted gene co-expression network analysis (WGCNA) analysis, and random forest prediction model were conducted. The findings from this study indicated that TBBPA limited the photoreaction process by destroying the light-catching antenna protein of tobacco leaves, the activity of the photosystem reaction centers (PSII and PSI), and the linear electron transport efficiency. TBBPA also reduced the rate of the Calvin-Benson cycle by inhibiting the activities of gene such as Rubisco, PGK, and TPI, and finally destroyed the photosynthesis process. Although cyclic electron transport was enhanced under stress conditions, it could not reverse the damage caused by TBBPA on photosynthesis. TBBPA exposure resulted in the accumulation of reactive oxygen species (ROS) in tobacco leaves, and the activities of Superoxide dismutase (SOD), Ascorbate peroxidase (APX), and Glutathione peroxidase (GPX) and their coding genes were significantly down-regulated. Although POD activity and proline (Pro) content were increased, they were insufficient to remove excess O₂^·- free radicals to relieve ROS stress. WCGNA and random forest models predicted that the damage of TBBPA to the above processes in tobacco was closely related to the increase in abscisic acid (ABA) content. TBBPA affects the Calvin cycle by inducing ABA signal transduction and stomatal closure, which leads to a series of chain reactions, such as electron transport chain obstruction, excess of ROS, decrease in chlorophyll synthesis, and photosystem reaction center damage.

A theoretical framework of the hybrid mechanism of photosystem II photodamage

Photodamage of photosystem II is a significant physiological process that is prevalent in the fields of photobiology, photosynthesis research and plant/algal stress. Since its discovery, numerous efforts have been devoted to determine the causes and mechanisms of action of photosystem II photodamage. There are two contrasting hypotheses to explain photodamage: (1) the excitation pressure induced by light absorption by the photosynthetic pigments and (2) direct photodamage of the Mn cluster located at the water-splitting site, which is independent of excitation pressure. While these two hypotheses seemed mutually exclusive, during the last decade, several independent works have proposed an alternative approach indicating that both hypotheses are valid. This was termed the dual hypothesis of photosystem II photodamage, and it postulates that both excess excitation and direct Mn photodamage operate at the same time, independently or in a synergic manner, depending on the type of sample, temperature, light spectrum, or other environmental stressors. In this mini-review, a brief summary of the contrasting hypotheses is presented, followed by recapitulation of key discoveries in the field of photosystem II photodamage of the last decade, and a synthesis of how these works support a full hybrid framework (operation of several mechanisms and their permutations) to explain PSII photodamage. All these are in recognition of Prof. Wah Soon Chow (the Australian National University), one of the key proposers of the dual hypothesis.

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions of trinuclear mixed-valence Mn(ii)/Mn(iii) complexes

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions in two new mixed-valence trinuclear Mn(iii)–Mn(ii)–Mn(iii) complexes has been investigated.

Cytotoxicity of Manganese (III) Complex in Human Breast Adenocarcinoma Cell Line Is Mediated by the Generation of Reactive Oxygen Species Followed by Mitochondrial Damage

Manganese (Mn) complexes are widely studied because of their important catalytic properties in synthetic and biochemical reactions. A Mn (III) complex of an amidoamine ligand was synthesized using a tetradentate amidoamine ligand. In this study, the Mn (III) complex was evaluated for its biological activity by measuring its cytotoxicity in human breast adenocarcinoma cell line (MCF-7). Cytotoxic effects of the Mn (III) complex were determined using established biomarkers in an attempt to delineate the mechanism of action and the utility of the complex as a potential anticancer drug. The Mn (III) complex induces cell death in a dose- and time-dependent manner as shown by microculture tetrazolium assay, a measure of cytotoxic cell death. Our results demonstrated that cytotoxic effects were significantly increased at higher concentrations of Mn (III) complex and with longer time of treatment. The IC50 (Inhibitor concentration that results in 50% cell death) value of Mn (III) complex in MCF-7 cells was determined to be 2.5 mmol/L for 24 hours of treatment. In additional experiments, we determined the Mn (III) complex–mediated cell death was due to both apoptotic and nonspecific necrotic cell death mechanisms. This was assessed by ethidium bromide/acridine orange staining and flow cytometry techniques. The Mn (III) complex produced reactive oxygen species (ROS) triggering the expression of manganese superoxide dismutase 1 and ultimately damaging the mitochondrial function as is evident by a decline in mitochondrial membrane potential. Treatment of the cells with free radical scavenger, N, N-dimethylthiourea decreased Mn (III) complex–mediated generation of ROS and attenuated apoptosis. Together, these results suggest that the Mn (III) complex–mediated MCF-7 cell death utilizes combined mechanism involving apoptosis and necrosis perhaps due to the generation of ROS.

Photoinactivation of Photosystem II in wild-type and chlorophyll b-less barley leaves: which mechanism dominates depends on experimental circumstances

Action spectra of photoinactivation of Photosystem II (PS II) in wild-type and chlorophyll b-less barley leaf segments were obtained. Photoinactivation of PS II was monitored by the delivery of electrons from PS II to PS I following single-turnover flashes superimposed on continuous far-red light, the time course of photoinactivation yielding a rate coefficient k i. Susceptibility of PS II to photoinactivation was quantified as the ratio of k i to the moderate irradiance (I) of light at each selected wavelength. k i/I was very much higher in blue light than in red light. The experimental conditions permitted little excess light energy absorbed by chlorophyll (not utilized in photochemical conversion or dissipated in controlled photoprotection) that could lead to photoinactivation of PS II. Therefore, direct absorption of light by Mn in PS II, rather than by chlorophyll, was more likely to have initiated the much more severe photoinactivation in blue light than in red light. Mutant leaves were ca. 1.5-fold more susceptible to photoinactivation than the wild type. Neither the excess-energy mechanism nor the Mn mechanism can explain this difference. Instead, the much lower chlorophyll content of mutant leaves could have exerted an exacerbating effect, possibly partly due to less mutual shading of chloroplasts in the mutant leaves. In general, which mechanism dominates depends on the experimental conditions.

The combined effects of UV-C radiation and H2O2 on Microcystis aeruginosa, a bloom-forming cyanobacterium

In order to get insight into the impacts of UVC/H2O2 on Microcystis aeruginosa, physiological and morphological changes as well as toxicity were detected under different UVC/H2O2 treatments. In the presence of sole UVC or H2O2, the net oxygen evolution rate decreased significantly (p<0.05) since activity of photosystem II (PSII) was inhibited. Meanwhile, increase of intracellular reactive oxygen species (ROS), degradation of microcystin (MC) and ultrastructure destructions were observed. Under sole UVC treatment, no changes happened in the activity of photosystem I (PSI), but the degradation of D1 protein was observed. Under sole H2O2 treatment, an increase of malondialdehyde, aggregation of D1 protein and deformation of the thylakoid membrane were observed. ROS content under H2O2 treatment was about 5 times than that under UVC treatment. Combined use of UVC and H2O2, as well as 20mJcm(-2) UVC and 60μM H2O2, showed high synergetic effects. Obvious damage to membrane systems, the marked degradation of MC and inhibition of the photosystems were observed. It could be deduced that UVC worked on intracellular membrane components directly and the damaged oxygen-evolving complex, which was followed by the D1 protein degradation. H2O2 oxidised the membrane lipids via an ROS-mediated process, with thylakoid injury and the aggregation of D1 protein being the lethal mechanisms, and both PSII and PSI being the attacking targets. With regard towards the effective inactivation of M. aeruginosa and high removal of MC, UVC/H2O2 proposed a novel practical method in controlling cyanobacterial blooms.

A model for the water-oxidation and recovery systems of the oxygen-evolving complex

We propose a model for the water-oxidation and recovery systems of the oxygen-evolving complex (OEC) of the photosystem II (PSII) enzyme. The whole system is constructed from two catalytic cycles, conducted as a tandem reaction: (i) a water-oxidation loop uses cerium(IV) ammonium nitrate as an oxidant to activate a dimanganese complex for water-oxidation and thereby liberate a molecule of O2 and (ii) a recovery loop begins with photoinhibition of the dimanganese complex but then uses O2 to reactivate the manganese centre. The net result is a catalytic water-oxidation catalyst that can use self-generated O2 for recovery.

Response of chlorophyll d-containing cyanobacterium Acaryochloris marina to UV and visible irradiations

We have previously investigated the response mechanisms of photosystem II complexes from spinach to strong UV and visible irradiations (Wei et al J Photochem Photobiol B 104:118-125, 2011). In this work, we extend our study to the effects of strong light on the unusual cyanobacterium Acaryochloris marina, which is able to use chlorophyll d (Chl d) to harvest solar energy at a longer wavelength (740 nm). We found that ultraviolet (UV) or high level of visible and near-far red light is harmful to A. marina. Treatment with strong white light (1,200 μmol quanta m(-2) s(-1)) caused a parallel decrease in PSII oxygen evolution of intact cells and in extracted pigments Chl d, zeaxanthin, and α-carotene analyzed by high-performance liquid chromatography, with severe loss after 6 h. When cells were irradiated with 700 nm of light (100 μmol quanta m(-2) s(-1)) there was also bleaching of Chl d and loss of photosynthetic activity. Interestingly, UVB radiation (138 μmol quanta m(-2) s(-1)) caused a loss of photosynthetic activity without reduction in Chl d. Excess absorption of light by Chl d (visible or 700 nm) causes a reduction in photosynthesis and loss of pigments in light harvesting and photoprotection, likely by photoinhibition and inactivation of photosystem II, while inhibition of photosynthesis by UVB radiation may occur by release of Mn ion(s) in Mn4CaO5 center in photosystem II.

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.

Potential for green microalgae to produce hydrogen, pharmaceuticals and other high value products in a combined process

Green microalgae for several decades have been produced for commercial exploitation, with applications ranging from health food for human consumption, aquaculture and animal feed, to coloring agents, cosmetics and others. Several products from green algae which are used today consist of secondary metabolites that can be extracted from the algal biomass. The best known examples are the carotenoids astaxanthin and β-carotene, which are used as coloring agents and for health-promoting purposes. Many species of green algae are able to produce valuable metabolites for different uses; examples are antioxidants, several different carotenoids, polyunsaturated fatty acids, vitamins, anticancer and antiviral drugs. In many cases, these substances are secondary metabolites that are produced when the algae are exposed to stress conditions linked to nutrient deprivation, light intensity, temperature, salinity and pH. In other cases, the metabolites have been detected in algae grown under optimal conditions, and little is known about optimization of the production of each product, or the effects of stress conditions on their production. Some green algae have shown the ability to produce significant amounts of hydrogen gas during sulfur deprivation, a process which is currently studied extensively worldwide. At the moment, the majority of research in this field has focused on the model organism, Chlamydomonas reinhardtii, but other species of green algae also have this ability. Currently there is little information available regarding the possibility for producing hydrogen and other valuable metabolites in the same process. This study aims to explore which stress conditions are known to induce the production of different valuable products in comparison to stress reactions leading to hydrogen production. Wild type species of green microalgae with known ability to produce high amounts of certain valuable metabolites are listed and linked to species with ability to produce hydrogen during general anaerobic conditions, and during sulfur deprivation. Species used today for commercial purposes are also described. This information is analyzed in order to form a basis for selection of wild type species for a future multi-step process, where hydrogen production from solar energy is combined with the production of valuable metabolites and other commercial uses of the algal biomass.

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.

Photoinhibition of Photosystem II

Photoinhibition of Photosystem II (PSII) is the light-induced loss of PSII electron-transfer activity. Although photoinhibition has been studied for a long time, there is no consensus about its mechanism. On one hand, production of singlet oxygen ((1)O(2)) by PSII has promoted models in which this reactive oxygen species (ROS) is considered to act as the agent of photoinhibitory damage. These chemistry-based models have often not taken into account the photophysical features of photoinhibition-like light response and action spectrum. On the other hand, models that reproduce these basic photophysical features of the reaction have not considered the importance of data about ROS. In this chapter, it is shown that the evidence behind the chemistry-based models and the photophysically oriented models can be brought together to build a mechanism that confirms with all types of experimental data. A working hypothesis is proposed, starting with inhibition of the manganese complex by light. Inability of the manganese complex to reduce the primary donor promotes recombination between the oxidized primary donor and Q(A), the first stable quinone acceptor of PSII. (1)O(2) production due to this recombination may inhibit protein synthesis or spread the photoinhibitory damage to another PSII center. The production of (1)O(2) is transient because loss of activity of the oxygen-evolving complex induces an increase in the redox potential of Q(A), which lowers (1)O(2) production.

The mechanism of photoinhibition in vivo: re-evaluation of the roles of catalase, α-tocopherol, non-photochemical quenching, and electron transport

Photoinhibition of photosystem II (PSII) occurs when the rate of light-induced inactivation (photodamage) of PSII exceeds the rate of repair of the photodamaged PSII. For the quantitative analysis of the mechanism of photoinhibition of PSII, it is essential to monitor the rate of photodamage and the rate of repair separately and, also, to examine the respective effects of various perturbations on the two processes. This strategy has allowed the re-evaluation of the results of previous studies of photoinhibition and has provided insight into the roles of factors and mechanisms that protect PSII from photoinhibition, such as catalases and peroxidases, which are efficient scavengers of H(2)O(2); α-tocopherol, which is an efficient scavenger of singlet oxygen; non-photochemical quenching, which dissipates excess light energy that has been absorbed by PSII; and the cyclic and non-cyclic transport of electrons. Early studies of photoinhibition suggested that all of these factors and mechanisms protect PSII against photodamage. However, re-evaluation by the strategy mentioned above has indicated that, rather than protecting PSII from photodamage, they stimulate protein synthesis, with resultant repair of PSII and mitigation of photoinhibition. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Manganese-based Materials Inspired by Photosynthesis for Water-Splitting

In nature, the water-splitting reaction via photosynthesis driven by sunlight in plants, algae, and cyanobacteria stores the vast solar energy and provides vital oxygen to life on earth. The recent advances in elucidating the structures and functions of natural photosynthesis has provided firm framework and solid foundation in applying the knowledge to transform the carbon-based energy to renewable solar energy into our energy systems. In this review, inspired by photosynthesis robust photo water-splitting systems using manganese-containing materials including Mn-terpy dimer/titanium oxide, Mn-oxo tetramer/Nafion, and Mn-terpy oligomer/tungsten oxide, in solar fuel production are summarized and evaluated. Potential problems and future endeavors are also discussed.