Decrease In Oxygen Evolution Research Articles

Enriched d‐Band Holes Enabling Fast Oxygen Evolution Kinetics on Atomic‐Layered Defect‐Rich Lithium Cobalt Oxide Nanosheets

AbstractDeveloping a reliable synthesis strategy to concurrently realize electronic structure modulation and two‐dimensionalization of materials is of paramount significance yet still challenging. Herein, a facile and universal strategy is reported to fabricate defect‐abundant atomic‐layered materials with unique electronic structures by mechanical shear‐assisted exfoliation. As a proof‐of‐concept demonstration, atomic‐layered defect‐rich LiCoO2 nanosheets (AD‐LCO) are successfully synthesized, which enable accelerated oxygen evolution kinetics with a substantially decreased oxygen evolution reaction overpotential by 184 and 216 mV at 10 and 50 mA cm–2, respectively. X‐ray absorption spectroscopy suggests that AD‐LCO possesses more d‐band holes and enhanced Co‐O covalency. Density functional theory calculations reveal that the presence of Co lattice vacancies can optimize the adsorption kinetics of intermediates, consequently lowering the energy barrier of the rate‐determining step. Importantly, this method has universal applicability to the fabrication of other ultrathin defect‐rich 2D materials such as BN, WS2, and MoS2. The study has potential implications for offering novel insights into the rational design of ultrathin 2D materials with abundant surface defects for various applications.

Proteomic insights into the photosynthetic divergence between bark and leaf chloroplasts in Salix matsudana.

Bark chloroplasts play important roles in carbon balancing by recycling internal stem CO2 into assimilated carbon. The photosynthetic response of bark chloroplasts to interior stem environments has been studied recently in woody plants. However, the molecular regulatory mechanisms underlying specific characteristics of bark photosynthesis remain unclear. To address this knowledge gap, differences in the structure, photosynthetic activity and protein expression profiles between bark and leaf chloroplasts were investigated in Salix matsudana in this study. Bark chloroplasts exhibited broader and lower grana stacks and higher levels of starch relative to leaf chloroplasts. Concomitantly, decreased oxygen evolution rates and decreased saturated radiation point were observed in bark chloroplasts. Furthermore, a total of 293 differentially expressed proteins (DEPs) were identified in bark and leaf chloroplast profile comparisons. These DEPs were significantly enriched in photosynthesis-related biological processes or Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with photosynthesis. All 116 DEPs within the KEGG pathways associated with photosynthesis light reactions were downregulated in bark chloroplasts, including key proteins responsible for chlorophyll synthesis, light energy harvesting, nonphotochemical quenching, linear electron transport and photophosphorylation. Interestingly, seven upregulated proteins involved in dark reactions were identified in bark chloroplasts that comprised two kinds of malic enzymes typical of C4-type photosynthesis. These results provide comprehensive proteomic evidence to understand the low photochemical capability of bark chloroplasts and suggest that bark chloroplasts might fix CO2 derived from malate decarboxylation.

Effects of low temperature on photoinhibition and singlet oxygen production in four natural accessions of Arabidopsis

Main conclusionsLow temperature decreases PSII damage in vivo, confirming earlier in vitro results. Susceptibility to photoinhibition differs among Arabidopsis accessions and moderately decreases after 2-week cold-treatment. Flavonols may alleviate photoinhibition.The rate of light-induced inactivation of photosystem II (PSII) at 22 and 4 °C was measured from natural accessions of Arabidopsis thaliana (Rschew, Tenela, Columbia-0, Coimbra) grown under optimal conditions (21 °C), and at 4 °C from plants shifted to 4 °C for 2 weeks. Measurements were done in the absence and presence of lincomycin (to block repair). PSII activity was assayed with the chlorophyll a fluorescence parameter Fv/Fm and with light-saturated rate of oxygen evolution using a quinone acceptor. When grown at 21 °C, Rschew was the most tolerant to photoinhibition and Coimbra the least. Damage to PSII, judged from fitting the decrease in oxygen evolution or Fv/Fm to a first-order equation, proceeded more slowly or equally at 4 than at 22 °C. The 2-week cold-treatment decreased photoinhibition at 4 °C consistently in Columbia-0 and Coimbra, whereas in Rschew and Tenela the results depended on the method used to assay photoinhibition. The rate of singlet oxygen production by isolated thylakoid membranes, measured with histidine, stayed the same or slightly decreased with decreasing temperature. On the other hand, measurements of singlet oxygen from leaves with Singlet Oxygen Sensor Green suggest that in vivo more singlet oxygen is produced at 4 °C. Under high light, the PSII electron acceptor QA was more reduced at 4 than at 22 °C. Singlet oxygen production, in vitro or in vivo, did not decrease due to the cold-treatment. Epidermal flavonols increased during the cold-treatment and, in Columbia-0 and Coimbra, the amount correlated with photoinhibition tolerance.

Carbon black nanoparticles to sense algae oxygen evolution for herbicides detection: Atrazine as a case study

A novel amperometric algae-based biosensor was developed for the detection of photosynthetic herbicides in river water. The green photosynthetic algae Chlamydomonas reinhardtii was immobilized on carbon black modified screen-printed electrodes, exploiting carbon black as smart nanomaterial to monitor changes in algae oxygen evolution during the photosynthetic process. The decrease of oxygen evolution, occurring in the presence of herbicides, results in a decrease of current signals by means of amperometric measurements, in an analyte concentration dependent manner. Atrazine as case study herbicide was detected in a concentration range of 0.1 and 50 μM, with a linear range from 0.1 to 5 μM and a detection limit of 1 nM. No interference was observed in presence of 100 ppb arsenic, 20 ppb copper, 5 ppb cadmium, 10 ppb lead, 10 ppb bisphenol A, and 1 ppb paraoxon, tested as safety limits. A ~25% matrix effect and satisfactory recovery values of 107 ± 10% and 96 ± 8% were obtained in river water for 3 and 5 μM of atrazine, respectively. Stability studies were also performed obtaining a high working stability up to 10 h and repeatability with an RSD of 1.1% (n = 12), as well as a good storage stability up to 3 weeks.

Improving the Electrochemical Performance of Li2RuO3 through Chemical Substitution: A Case Study of (x)LiCoO2‐(1‐x)Li2RuO3 Solid Solution (x≤0.4)

AbstractThe present work reports an enhancement in the electrochemical properties of Li2RuO3 by Co+3 substitution. We explore the Li1.33‐x/3Ru0.66‐2x/3CoxO2 series (in the solid solution notation (x)LiCoO2‐(1‐x)Li2RuO3) for x≤0.4 by partial substitution of Co+3 for both Ru+4 and Li+ in the metal layer; in this way, creation of oxygen non‐stoichiometry is avoided. Powder XRD reveals that an increase in the cobalt content in the series leads to a structural transformation from monoclinic to rhombohedral phase, owing to increased cation disorder in the metal layer. Cyclic voltammetric scans shows clear evidence for decreased oxygen evolution for Co‐substituted samples. The x=0.3 member delivers an initial discharge capacity of about 232 mAh g−1 and, after the 100th cycle, 85 % capacity retention is observed. Furthermore, the rate capability is also significantly improved by Co substitution. Li+ removal during the first charge is more than expected by counting on cation oxidation alone, demonstrating the participation of anion redox. Interestingly, the x=0.2 and 0.3 members of the series Li1.33‐x/3Ru0.66‐2x/3CoxO2 show better Li reversibility upon long‐term cycling, endorsing the beneficial effect of Co+3 in stabilizing the layered structure during electrochemical cycling.

Confined growth of porous nitrogen-doped cobalt oxide nanoarrays as bifunctional oxygen electrocatalysts for rechargeable zinc–air batteries

Cobalt oxides are promising bifunctional oxygen electrocatalysts due to their intrinsic activities toward both dissociation and formation of oxygen molecules. Unfortunately, their practical utilization is seriously hindered by the limited accessible active sites and inferior activity. Herein, vertically-aligned porous nanoarrays composed of ultrafine nitrogen-doped cobalt oxide (NP-Co3O4) nanoparticles (4-5 ​nm) are in-situ fabricated on carbon cloth (CC) via mild oxidation of Co-based zeolitic-imidazolate-framework (Co-ZIF) nanoarrays. The oxidation decomposition of 2-methylimidazolate ligands between Co nodes not only confines the growth of cobalt oxides, but also provides the nitrogen sources for realizing low-temperature (200 ​°C) nitrogen doping. In alkaline electrolyte, the as-built NP-Co3O4/CC manifests an extremely high oxygen reduction reaction (ORR) half-wave potential of ~0.9 ​V and a decreased oxygen evolution reaction (OER) overpotential of 330 ​mV ​at 10 ​mA ​cm−2, displaying a record low potential gap of 0.66 ​V. As bifunctional oxygen electrodes in rechargeable aqueous and all-solid-state flexible zinc-air batteries, the NP-Co3O4/CC-based batteries exhibit ultrahigh power densities up to 200 ​mW ​cm−2 and 99.8 ​mW ​cm−3, respectively, which are much higher than those of reported bifunctional oxygen electrocatalysts. This work paves a new avenue for the fabrication of porous nitrogen doped metal oxides towards high-performance zinc-air batteries.

Nitrogen Dioxide at Ambient Concentrations Induces Nitration and Degradation of PYR/PYL/RCAR Receptors to Stimulate Plant Growth: A Hypothetical Model.

Exposing Arabidopsis thaliana (Arabidopsis) seedlings fed with soil nitrogen to 10–50 ppb nitrogen dioxide (NO2) for several weeks stimulated the uptake of major elements, photosynthesis, and cellular metabolisms to more than double the biomass of shoot, total leaf area and contents of N, C P, K, S, Ca and Mg per shoot relative to non-exposed control seedlings. The 15N/14N ratio analysis by mass spectrometry revealed that N derived from NO2 (NO2-N) comprised < 5% of the total plant N, showing that the contribution of NO2-N as N source was minor. Moreover, histological analysis showed that leaf size and biomass were increased upon NO2 treatment, and that these increases were attributable to leaf age-dependent enhancement of cell proliferation and enlargement. Thus, NO2 may act as a plant growth signal rather than an N source. Exposure of Arabidopsis leaves to 40 ppm NO2 induced virtually exclusive nitration of PsbO and PsbP proteins (a high concentration of NO2 was used). The PMF analysis identified the ninth tyrosine residue of PsbO1 (9Tyr) as a nitration site. 9Tyr of PsbO1 was exclusively nitrated after incubation of the thylakoid membranes with a buffer containing NO2 and NO2− or a buffer containing NO2− alone. Nitration was catalyzed by illumination and repressed by photosystem II (PSII) electron transport inhibitors, and decreased oxygen evolution. Thus, protein tyrosine nitration altered (downregulated) the physiological function of cellular proteins of Arabidopsis leaves. This indicates that NO2-induced protein tyrosine nitration may stimulate plant growth. We hypothesized that atmospheric NO2 at ambient concentrations may induce tyrosine nitration of PYR/PYL/RCAR receptors in Arabidopsis leaves, followed by degradation of PYR/PYL/RCAR, upregulation of target of rapamycin (TOR) regulatory complexes, and stimulation of plant growth.

Effects of PSII Manganese-Stabilizing Protein Succinylation on Photosynthesis in the Model Cyanobacterium Synechococcus sp. PCC 7002

Lysine succinylation is a newly identified protein post-translational modification and plays important roles in various biological pathways in both prokaryotes and eukaryotes, but its extent and function in photosynthetic organisms remain largely unknown. Here, we performed the first systematic studies of lysine succinylation in cyanobacteria, which are the only prokaryotes capable of oxygenic photosynthesis and the established model organisms for studying photosynthetic mechanisms. By using mass spectrometry analysis in combination with the enrichment of succinylated peptides from digested cell lysates, we identified 1,704 lysine succinylation sites on 691 proteins in a model cyanobacterium Synechococcus sp. PCC 7002. Bioinformatic analysis revealed that a large proportion of the succinylation sites were present on proteins in photosynthesis and metabolism. Among all identified succinylated proteins involved in photosynthesis, the PSII manganese-stabilizing protein (PsbO) was found to be succinylated on Lys99 and Lys234. Functional studies of PsbO were performed by site-directed mutagenesis, and mutants mimicking either constitutively succinylated (K99E and K234E) or non-succinylated states (K99R and K234R) were constructed. The succinylation-mimicking K234E mutant exhibited a decreased oxygen evolution rate of the PSII center and the efficiency of energy transfer during the photosynthetic reaction. Molecular dynamics simulations suggested a mechanism that may allow succinylation to influence the efficiency of photosynthesis by altering the conformation of PsbO, thereby hindering the interaction between PsbO and the PSII core. Our findings suggest that reversible succinylation may be an important regulatory mechanism during photosynthesis in Synechococcus, as well as in other photosynthetic organisms.

Selective nitration of PsbO1, PsbO2, and PsbP1 decreases PSII oxygen evolution and photochemical efficiency in intact leaves of Arabidopsis

ABSTRACTExposure of intact Arabidopsis leaves to 40 ppm nitrogen dioxide (NO2) in light resulted almost exclusively in nitration of PsbO1, PsbO2, and PsbP1 of photosystem II (PSII), with minor nitration of four non-PS II proteins, including peroxiredoxin II E, as reported previously. Our previous findings that light-triggered selective nitration of PsbO1 decreased oxygen evolution and that inhibition of photoelectric electron transport inhibited nitration of PsbO1 implied that the nitratable tyrosine residue of PsbO1 is redox-active. However, whether the nitratable tyrosine residues of PsbO2 and PsbP1 are redox-active is unknown. In this study, we determined the oxygen evolution and maximal photochemical efficiency of PSII in intact Arabidopsis leaves following exposure to 40 ppm NO2 in light and found that these parameters were decreased to 60 and 70% of the non-exposed control, respectively. Because PsbO1, PsbO2, and PsbP1 accounted for > 80% of anti-3-nitrotyrosine antibody signal intensities, observed decreases in the oxygen evolution and maximal photochemical efficiency of PSII were mainly attributable to nitration of the tyrosine residues of these PSII proteins. Thus, it is postulated that nitratable tyrosine residues of PsbO2 and PsbP1 are redox-active, as in the case of PsbO1. A new hypothetical model is proposed.

Lysine Acetylome Analysis Reveals Photosystem II Manganese-stabilizing Protein Acetylation is Involved in Negative Regulation of Oxygen Evolution in Model Cyanobacterium Synechococcus sp. PCC 7002.

Nε-Acetylation of lysine residues represents a frequently occurring post-translational modification widespread in bacteria that plays vital roles in regulating bacterial physiology and metabolism. However, the role of lysine acetylation in cyanobacteria remains unclear, presenting a hurdle to in-depth functional study of this post-translational modification. Here, we report the lysine acetylome of Synechococcus sp. PCC 7002 (hereafter Synechococcus) using peptide prefractionation, immunoaffinity enrichment, and coupling with high-precision liquid chromatography-tandem mass spectrometry analysis. Proteomic analysis of Synechococcus identified 1653 acetylation sites on 802 acetylproteins involved in a broad range of biological processes. Interestingly, the lysine acetylated proteins were enriched for proteins involved in photosynthesis, for example. Functional studies of the photosystem II manganese-stabilizing protein were performed by site-directed mutagenesis and mutants mimicking either constitutively acetylated (K99Q, K190Q, and K219Q) or nonacetylated states (K99R, K190R, and K219R) were constructed. Mutation of the K190 acetylation site resulted in a distinguishable phenotype. Compared with the K190R mutant, the K190Q mutant exhibited a decreased oxygen evolution rate and an enhanced cyclic electron transport rate in vivo Our findings provide new insight into the molecular mechanisms of lysine acetylation that involved in the negative regulation of oxygen evolution in Synechococcus and creates opportunities for in-depth elucidation of the physiological role of protein acetylation in photosynthesis in cyanobacteria.

Selective nitration of PsbO1 inhibits oxygen evolution from isolated Arabidopsis thylakoid membranes

ABSTRACTTreatment of isolated Arabidopsis thaliana thylakoid membranes with nitrogen dioxide (NO2) induces selective nitration of the tyrosine residue at the ninth amino acid (9Tyr) of PsbO1. This selective nitration is triggered by light and is inhibited by photosynthetic electron transport inhibitors. Therefore, we postulated that, similar to 161Tyr of D1 (YZ), 9Tyr of PsbO1 is redox active and is selectively oxidized by photosynthetic electron transport in response to illumination to a tyrosyl radical that is highly susceptible to nitration. This tyrosyl radical may combine rapidly at diffusion-controlled rates with NO2 to form 3-nitrotyrosine. If this postulation is correct, the nitration of 9Tyr of PsbO1 should decrease oxygen evolution activity. We investigated the effects of PsbO1 nitration on oxygen evolution from isolated thylakoid membranes, and found that nitration decreased oxygen evolution to ≥ 0% of the control. Oxygen evolution and nitration were significantly negatively correlated. This finding is consistent with redox active properties of the 9Tyr gene of PsbO1, and suggests that PsbO1 9Tyr acts as an electron relay, such as YZ in the photosystem II oxygenic electron transport chain.

Electrochemical Evaluation of Lead Base Composite Anodes Fabricated by Accumulative Roll Bonding Technique

Accumulative roll bonding is used for the first time in lead systems to fabricate advanced lead base composite anodes. For this purpose, Ag as the most common and effective additive, Co as the best metallic immiscible substitution for Ag, and MnO2 as the ceramic and electrocatalytic agent have been used as additives to produce anodes. The accumulative roll bonding processed sheets have been fabricated under determined conditions. The electrochemical properties of the prepared samples are investigated by Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, Polarization tests, electrowinning tests, and Scanning Electron Microscopy. The results indicate that the ARB-processed composite lead sheets can be perfectly used as novel developed anodes. The advantages include 5.51 times increase of current density, in the Pb-pct0.5Ag 9-pass sample compared to pure lead anode, decreased charge transfer resistance from 56.31 (Ω cm2) in pure lead anode to 17.5 (Ω cm2) in the Pb-pct2MnO2 8-pass sample (72 pct lower), and decreased oxygen evolution potential from 1.95 (V/SHE) in pure lead anode to 1.77 (V/SHE) in the Pb-pct2MnO2 8-pass sample (0.18 (V/SHE) lower). Electrowinning tests results reveal Pb-2 pctMnO2 8-pass showed best anodic performance withsignificant lower compared corrosion rate (75 pct), product and electrolyte contamination, slime formation, energy consumption and higher Zn deposit and energy conservation (to 294 kWh/t-Zn). Finest Zn deposit morphology (effective reduced grain size corresponding to smoothness and compaction) has been supplied by Pb-2 pctMnO2 8-pass sample resulted from enhanced growth rate of Zn in lack of Pb contaminations that could act as suitable nucleation sites.

Psb30 is a photosystem II reaction center subunit and is required for optimal growth in high light in Chlamydomonas reinhardtii

The psb30 ( ycf12) gene is conserved in a wide variety of oxygenic-photosynthetic organisms except angiosperms and some marine cyanobacteria. Psb30 protein is found in cyanobacterial photosystem II (PSII) core complexes and is dispensable for PSII structure and function. The most recent three-dimensional structure of cyanobacterial PSII core complex has revealed that Psb30 is located in proximity of PsbJ, PsbK, and PsbZ. However Psb30 has not yet been detected in PSII complexes from eukaryotic photosynthetic organisms. Here we found the expression of the chloroplast psb30 gene in the green alga Chlamydomonas reinhardtii by immunoblotting and Psb30 is exclusively co-purifies with PSII core complex and is significantly reduced in PSII-deficient mutants. Partial disintegration of PSII core complex and subsequent fractionation of the resulting subcomplexes revealed that Psb30 is exclusively associated with PSII reaction center. We have generated chloroplast transformants in which the psb30 gene is disrupted and the resulting ΔPsb30 cells showed decreased oxygen evolution activity by 15%, grew photosynthetically under moderate light, and displayed increased sensitivity to high light relative to wild type. We conclude that Psb30 is a PSII reaction center subunit and is required for optimal PSII function under high light environments.

Low temperature aqueous synthesis of highly dispersed Co 3O 4 nanocubes and their electrocatalytic activity studies

We report here a simple approach to the synthesis of highly dispersed Co 3O 4 nanocubes in higher yield by aqueous precipitation method at low temperatures (70–90 °C). The nanosized Co 3O 4 powders were directly achieved from Co(II) aqueous solution under alkaline and oxidizing conditions. TEM results indicate that the as-prepared Co 3O 4 has a cubic nanostructure with uniform size of about 20 nm and narrow size distribution. Powder X-ray diffraction (XRD) and infrared spectrum (IR) results show that the samples are Co 3O 4 in spinel structure. The objective Co 3O 4 nanocubes can be achieved in a wide range of experimental parameters, which can be preserved as stable suspension in distilled water or ethanol for several months with no obvious change. Appropriate reaction time and temperature should be controlled to get better crystallized Co 3O 4 nanocubes with perfect cubic appearance and good dispersibility. It is essential to achieve cubic-like Co 3O 4 particles by using ammonia as precipitator. The as-prepared Co 3O 4 nanocubes, combined with PbO 2 as electrodes, extend improved electrocatalytic activities and decreased oxygen evolution potential in electrochemical reactions, so they can be used as potential electrode materials or electrocatalytic materials. This facile method for the synthesis of Co 3O 4 nanocubes is a simple and general process without any seed, catalyst, or template, thus it is promising for large-scale and low-cost production of Co 3O 4 with high-quality.

Effects of phospholipase and lipase treatments on photosystem II core dimer from a thermophilic cyanobacterium

Lipids are important components of transmembrane protein complexes. In order to study the roles of lipids in photosystem II (PSII), we treated the PSII core dimer complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus with phospholipase A(2) (PLA(2)) and lipase, and examined their effects on PSII structure and function. PLA(2)-treatment decreased the content of phospholipid, phosphatidylglycerol (PG) by 59%, leading to a decrease of oxygen evolution by 40%. On the other hand, although treatment with lipase specifically decreased the content of monogalactosyldiacylglycerol (MGDG) by 52%, it decreased oxygen evolution only by 16%. This indicates that PG plays a more important role in PSII than MGDG. Both PLA(2)- and lipase-treatments induced neither the dissociation of PSII dimer, nor any loss of polypeptides. The degradation of PG resulted in a damage to the Q(B)-binding site as demonstrated from photoreduction activity of 2,6-dichlorophenolindophenol and chlorophyll fluorescence yields in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and the dependencies of oxygen evolution on various electron acceptors before and after PLA(2)- or lipase-treatments. However, there were approximately three and five molecules of PG and MGDG per PSII reaction center left in the PSII dimeric complex after the PLA(2)- and lipase-treatments. These lipids are therefore bound to the interior of the protein matrix and resistant to the lipase treatments. The resistance of these lipids against PLA(2)- and lipase-treatments may be a specific feature of PSII from the thermophilic cyanobacterium, suggesting a possible correlation between binding of lipids and thermostability of PSII.

Foliar and root Cu supply affect differently Fe- and Zn-uptake and photosynthetic activity in soybean plants

A comparative study on the effect of excess copper (Cu) supplied through leaves and root treatments in soybean plants is presented. Although Cu was accumulated in leaves upon both treatments, important differences were observed between Cu treatment of leaves and Cu supplementation in hydroponic medium. The application of 50 μM CuSO 4 on leaves increased chlorophyll content, oxygen evolution activity of thylakoids and fluorescence emission at 685 and 695 nm with respect to emission at 735 nm compared with control plants. In contrast, soybean plants grown in hydroponic medium supplemented with 50 μM CuSO 4 exhibited lower chlorophyll content, decreased oxygen evolution activity of thylakoids and decreased fluorescence emission at 685 and 695 nm compared with control plants. On the other hand, the elemental analysis of leaves showed differences between the two treatments investigated. The results indicated that no antagonist interaction between Cu- and Fe-uptake took place in plants where excess Cu was applied on leaves but Cu compete with Fe-uptake in plants grown with excess Cu in the hydroponic medium. Furthermore, differences in Zn-uptake were also observed. Zn content decreased upon Cu treatment of leaves whereas the opposite was observed upon Cu treatment through roots. Interestingly, plants with Cu-treated leaves behaved similarly as cell suspensions grown in the presence of excess Cu. The results strongly indicate that different Cu-uptake and transport pathways must operate in leaf cells compared with root cells.

Absence of the PsbQ Protein Results in Destabilization of the PsbV Protein and Decreased Oxygen Evolution Activity in Cyanobacterial Photosystem II

We have previously reported that cyanobacterial photosystem II (PS II) contains a protein homologous to PsbQ, the extrinsic 17-kDa protein found in higher plant and green algal PS II (Kashino, Y., Lauber, W. M., Carroll, J. A., Wang, Q., Whitmarsh, J., Satoh, K., and Pakrasi, H. B. (2002) Biochemistry 41, 8004-8012) and that it has regulatory role(s) on the water oxidation machinery (Thornton, L. E., Ohkawa, H., Roose, J. L., Kashino, Y., Keren, N., and Pakrasi, H. B. (2004) Plant Cell 16, 2164-2175). In this work, the localization and the function of PsbQ were assessed using the cyanobacterium Synechocystis sp. PCC 6803. From the predicted sequence, cyanobacterial PsbQ is expected to be a lipoprotein on the luminal side of the thylakoid membrane. Indeed, experiments in this work show that upon Triton X-114 fractionation of thylakoid membranes, PsbQ partitioned in the hydrophobic phase, and trypsin digestion revealed that PsbQ was highly exposed to the luminal space of thylakoid membranes. Detailed functional assays were conducted on the psbQ deletion mutant (DeltapsbQ) to analyze its water oxidation machinery. PS II complexes purified from DeltapsbQ mutant cells had impaired oxygen evolution activity and were remarkably sensitive to NH(2)OH, which indicates destabilization of the water oxidation machinery. Additionally, the cytochrome c(550) (PsbV) protein partially dissociated from purified DeltapsbQ PS II complexes, suggesting that PsbQ contributes to the stability of PsbV in cyanobacterial PS II. Therefore, we conclude that the major function of PsbQ is to stabilize the PsbV protein, thereby contributing to the protection of the catalytic Mn(4)-Ca(1)-Cl(x) cluster of the water oxidation machinery.

Evidence for PSII donor-side damage and photoinhibition induced by cadmium treatment on rice ( Oryza sativa L.)

The effects of cadmium (from 7.5 to 75 μM) on chloroplasts of rice were studied at the structural and biochemical level. Loss of pigments, reduction of thylakoids and decrease in oxygen evolution and Fv/Fm ratio occur in leaves following cadmium treatment. However, the amount of photosystem II reaction center proteins and that of its light harvesting complex is not affected, indicating that cadmium does not adversely influence the structural organization of this photosystem. In thylakoids isolated from cadmium-treated plants a loss in the capability to reduce 2,6-dichlorophenolindophenol is observed, which is partially restored if diphenylcarbazide is used as an electron donor, indicating that cadmium affects water splitting activity. In thylakoids isolated from control plants and treated with cadmium, diphenylcarbazide preserves most of the photosystem II activity lost after incubation with cadmium; most of the S 2 multiline electron paramagnetic resonance signal from the manganese cluster is lost, whereas the TyrD + and other signals are retained. Light-induced photosystem II damage, in vitro, is promoted by Cd-treatment as deduced from the mobility shift of the D1 protein observed by immunoblot.

Function of mitochondria during the transition of barley protoplasts from low light to high light

Mitochondrial contribution to photosynthetic metabolism during the transition from low light (25-100 micromol quanta m(-2) s(-1), limiting photosynthesis) to high light (500 micromol quanta m(-2) s(-1), saturating photosynthesis) was investigated in protoplasts from barley (Hordeum vulgare) leaves. After the light shift, photosynthetic oxygen evolution rate increased rapidly during the first 30-40 s and then declined up to 60-70 s after which the rate increased to a new steady-state after 80-110 s. Rapid fractionation of protoplasts was used to follow changes in sub-cellular distribution of key metabolites during the light shift and the activation state of chloroplastic NADP-dependent malate dehydrogenase (EC 1.1.1.82) was measured. Although oligomycin (an inhibitor of the mitochondrial ATP synthase) affected the metabolite content of protoplasts following the light shift, the first oxygen burst was not affected. However, the transition to the new steady-state was delayed. Rotenone (an inhibitor of mitochondrial complex I) had similar, but less pronounced effect as oligomycin. From the analysis of metabolite content and sub-cellular distribution we suggest that the decrease in oxygen evolution following the first oxygen burst is due to phosphate limitation in the chloroplast stroma. For the recovery the control protoplasts can utilize ATP supplied by mitochondrial oxidative phosphorylation to quickly overcome the limitation in stromal phosphate and to increase the content of Calvin cycle metabolites. The oligomycin-treated protoplasts were deficient in cytosolic ATP and thereby unable to support Calvin cycle operation. This resulted in a delayed capacity to adjust to a sudden increase in light intensity.

Cadmium-induced plant stress investigated by scanning electrochemical microscopy

In vivo oxygen evolution above single stomata in Brassica juncea has been used to investigate, for the first time, the effect of Cd-induced stress as imaged by scanning electrochemical microscopy (SECM). SECM images showed a clear stomatal structure-a pore, whose aperture is modulated by two guard cells, serving as the conduit for the oxygen produced. Lower stomatal density and larger stoma size were found in plants treated with 0.2 mM CdCl2 compared with control plants. Either the introduction of Cd caused a slower cell replication in the plane of the epidermis, hence fewer stomata, and/or the number of open stomata was reduced when plants were under Cd-stress. Oxygen evolution above individual stomatal complexes in Cd-treated plants was lower than that from control plants, as determined from the electrochemical current above the middle of each stoma. All guard cells under illumination were swollen, indicating that the stomata were open in both control and treated plants. Thus, decreased oxygen evolution in response to Cd cannot be attributed to simple closing of the stomata, but to a lower photosynthetic yield. SECM provides an excellent tool for monitoring the effects of Cd on photosynthetic activity at the scale of individual stomata.