Identification of the pheophytin-QA-Fe domain of the reducing side of the photosystem II as the Cu(II)-inhibitory binding site

Environmentally relevant copper concentrations stimulate photosynthesis in Monoraphidium sp

Microalgae require copper (Cu) in trace levels for their growth and metabolism, it is a vital component of certain metalloproteins. Although this element has been widely studied concerning microalgae physiology, the effects of environmentally relevant levels have been less studied. We studied the photosynthesis and growth of the Chlorophyte Monoraphidium sp. exposed to Cu ranging from low (1.7 nM) to high (589.0 nM) free Cu ions (Cu²⁺) concentrations. The growth rate was unaffected by Cu concentrations in the range of 1.7-7.4 nM Cu²⁺, but decreased beyond it. The relative maximum electron transport rate (rETR_m), saturation irradiance (E_k), photochemical quenching (qP and qL), and PSII operating efficiency [Formula: see text] were stimulated in the 3.4-7.4 nM Cu²⁺ range, concentrations slightly higher than the control, whereas non-photochemical quenching (NPQ) gradually increased with increasing Cu²⁺. The photosystem II antenna size [Sigma (II)₄₄₀] increased under high Cu (589.0 nM), which resulted in a decrease in the quinone A (Q_A) reduction time (tau). In contrast, the Q_A re-oxidation time was unaffected by Cu exposure. These findings show that a slight increase in Cu stimulated photosynthesis in Monoraphidium sp., whereas high Cu reduced photosynthesis and increased the dissipation of captured light energy. This research is a contribution to the understanding of the dynamic photo-physiological responses of Monoraphidium sp. to Cu ions.

Validating the predictions of murburn model for oxygenic photosynthesis: Analyses of ligand-binding to protein complexes and cross-system comparisons

In this second half of our treatise on oxygenic photosynthesis, we provide support for the murburn model of the light reaction of photosynthesis and ratify key predictions made in the first part. Molecular docking and visualization of various ligands of quinones/quinols (and their derivatives) with PS II/Cytochrome b₆f complexes did not support chartered 2e-transport role of quinols. A broad variety of herbicides did not show any affinity/binding-based rationales for inhibition of photosynthesis. We substantiate the proposal that disubstituted phenolics (perceived as protonophores/uncouplers or affinity-based inhibitors in the classical purview) serve as interfacial modulators of diffusible reactive (oxygen) species or DR(O)S. The DRS-based murburn model is evidenced by the identification of multiple ADP-binding sites on the extra-membraneous projection of protein complexes and structure/distribution of the photo/redox catalysts. With a panoramic comparison of the redox metabolic machinery across diverse organellar/cellular systems, we highlight the ubiquitous one-electron murburn facets (cofactors of porphyrin, flavin, FeS, other metal centers and photo/redox active pigments) that enable a facile harnessing of the utility of DRS. In the summative analyses, it is demonstrated that the murburn model of light reaction explains the structures of membrane supercomplexes recently observed in thylakoids and also accounts for several photodynamic experimental observations and evolutionary considerations. In toto, the work provides a new orientation and impetus to photosynthesis research. Communicated by Ramaswamy H. Sarma.

Effects of Novel Photosynthetic Inhibitor [CuL2]Br2 Complex on Photosystem II Activity in Spinach

The effects of the novel [CuL₂]Br₂ complex (L = bis{4H-1,3,5-triazino [2,1-b]benzothiazole-2-amine,4-(2-imidazole)}copper(II) bromide complex) on the photosystem II (PSII) activity of PSII membranes isolated from spinach were studied. The absence of photosynthetic oxygen evolution by PSII membranes without artificial electron acceptors, but in the presence of [CuL₂]Br₂, has shown that it is not able to act as a PSII electron acceptor. In the presence of artificial electron acceptors, [CuL₂]Br₂ inhibits photosynthetic oxygen evolution. [CuL₂]Br₂ also suppresses the photoinduced changes of the PSII chlorophyll fluorescence yield (F_V) related to the photoreduction of the primary quinone electron acceptor, Q_A. The inhibition of both characteristic PSII reactions depends on [CuL₂]Br₂ concentration. At all studied concentrations of [CuL₂]Br₂, the decrease in the F_M level occurs exclusively due to a decrease in Fv. [CuL₂]Br₂ causes neither changes in the F₀ level nor the retardation of the photoinduced rise in F_M, which characterizes the efficiency of the electron supply from the donor-side components to Q_A through the PSII reaction center (RC). Artificial electron donors (sodium ascorbate, DPC, Mn²⁺) do not cancel the inhibitory effect of [CuL₂]Br₂. The dependences of the inhibitory efficiency of the studied reactions of PSII on [CuL₂]Br₂ complex concentration practically coincide. The inhibition constant Ki is about 16 µM, and logKi is 4.8. As [CuL₂]Br₂ does not change the aromatic amino acids’ intrinsic fluorescence of the PSII protein components, it can be proposed that [CuL₂]Br₂ has no significant effect on the native state of PSII proteins. The results obtained in the present study are compared to the literature data concerning the inhibitory effects of PSII Cu(II) aqua ions and Cu(II)-organic complexes.

Azospirillum brasilense reduces oxidative stress in the green microalgae Chlorella sorokiniana under different stressors

In this study, we investigated oxidative stress in the green microalgae, Chlorella sorokiniana, in co-culture with the plant growth promoting bacteria (PGPB), Azospirillum brasilense. This relationship was studied in the absence of an exogenous stressor, under copper stress, and under nitrogen limitation stress. We confirmed that copper and nitrogen limitation induced algal oxidative stress and reductions in chlorophyll content. In all cases, the presence of A. brasilense lowered the accumulation of intracellular reactive oxygen species (ROS) while promoting chlorophyll content. This effect was driven, in part, by A. brasilense's secretion of the auxin hormone, indole-3-acetic acid, which is known to mitigate stress in higher plants. The findings of the present study show that stress mitigation by A. brasilense resulted in suppressed starch accumulation under nitrogen limitation stress and neutral lipid accumulation under copper stress. In fact, A. brasilense could almost completely mitigate oxidative stress in C. sorokiniana resulting from nitrogen limitation, with ROS accumulation rates comparable to the axenic control cultures. The biotechnological implication of these findings is that co-culture strategies with A. brasilense (and similar PGPB) are most effective for high growth applications. A second growth stage may be needed to induce accumulation of desired products.

Effects of Acetogenins from Annona coriacea on the in Vitro Reactions of Photosynthesis

Our search for candidates for photosynthesis inhibitors is allowing us to report the effect of two acetogenins identified in Annona coriacea Mart. leaves, ACG-A and ACG-B, a non-adjacent bis-THF and a mono-THF types, respectively. This is an important class of natural products which presents biological properties such as anticancer, neurotoxic, larvicidal and insecticidal. However, this is only the second report associated to its herbicidal activity. Their mechanisms of action on the light reactions of the photosynthesis were elucidated by polarographic techniques. Compounds inhibited the noncyclic electron transport on basal, phosphorylating, and uncoupled conditions from H₂ O to methyl viologen (MV); therefore, they act as Hill reaction inhibitors. Studies on fluorescence of chlorophyll a (ChL a) indicated that they inhibited the acceptor side of PSII between P680 and PQ-pool, exactly as the commercial herbicide DCMU does.

Evaluation of Alkaloids Isolated from Ruta graveolens as Photosynthesis Inhibitors

Eight alkaloids (1⁻8) were isolated from Ruta graveolens, and their herbicide activities were evaluated through in vitro, semivivo, and in vivo assays. The most relevant results were observed for Compounds 5 and 6⁻8 at 150 μM, which decreased dry biomass by 20% and 23%, respectively. These are significant results since they presented similar values with the positive control, commercial herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Based on the performed assays, Compound 5 (graveoline) is classified as an electron-transport inhibitor during the light phase of photosynthesis, as well as a plant-growth regulator. On the other hand, Compounds 6⁻8 inhibited electron and energy transfers, and are also plant-growth inhibitors. These phytotoxic behaviors based on acridone and quinolone alkaloids may serve as a valuable tool in the further development of a new class of herbicides since natural products represent an interesting alternative to replace commercial herbicides, potentially due their low toxicity.

Live cyanobacteria produce photocurrent and hydrogen using both the respiratory and photosynthetic systems

Oxygenic photosynthetic organisms perform solar energy conversion of water and CO₂ to O₂ and sugar at a broad range of wavelengths and light intensities. These cells also metabolize sugars using a respiratory system that functionally overlaps the photosynthetic apparatus. In this study, we describe the harvesting of photocurrent used for hydrogen production from live cyanobacteria. A non-harmful gentle physical treatment of the cyanobacterial cells enables light-driven electron transfer by an endogenous mediator to a graphite electrode in a bio-photoelectrochemical cell, without the addition of sacrificial electron donors or acceptors. We show that the photocurrent is derived from photosystem I and that the electrons originate from carbohydrates digested by the respiratory system. Finally, the current is utilized for hydrogen evolution on the cathode at a bias of 0.65 V. Taken together, we present a bio-photoelectrochemical system where live cyanobacteria produce stable photocurrent that can generate hydrogen.

Biologically ### produced electrical currents and hydrogen are new energy sources. Here, the authors find that low presser microfluidizer treatment produced cyanobacterium that can utilize electrons from respiratory and photosynthesis to promote current and hydrogen generation, without the addition of exogenous electron mediators.

The Effect of Copper and Selenium Nanocarboxylates on Biomass Accumulation and Photosynthetic Energy Transduction Efficiency of the Green Algae Chlorella Vulgaris

Nanoaquachelates, the nanoparticles with the molecules of water and/or carboxylic acids as ligands, are used in many fields of biotechnology. Ultra-pure nanocarboxylates of microelements are the materials of spatial perspective. In the present work, the effects of copper and selenium nanoaquachelates carboxylated with citric acid on biomass accumulation of the green algae Chlorella vulgaris were examined. Besides, the efficiency of the reactions of the light stage of photosynthesis was estimated by measuring chlorophyll a fluorescence. The addition of 0.67-4 mg L^-1 of Cu nanocarboxylates resulted in the increase in Chlorella biomass by ca. 20%; however, their concentrations ranging from 20 to 40 mg L^-1 strongly inhibited algal growth after the 12th day of cultivation. Se nanocarboxylates at 0.4-4 mg L^-1 concentrations also stimulated the growth of C. vulgaris, and the increase in biomass came up to 40-45%. The addition of Se nanocarboxylates at smaller concentrations (0.07 or 0.2 mg L^-1) at first caused the retardation of culture growth, but that effect disappeared after 18-24 days of cultivation. The addition of 2-4 mg L^-1 of Cu nanocarboxylates or 0.4-4 mg L^-1 of Se nanocarboxylates caused the evident initial increase in such chlorophyll a fluorescence parameters as maximal quantum yield of photosystem II photochemistry (F _v/F _m) and the quantum yield of photosystem II photochemistry in the light-adapted state (F _v'/F _m'). Photochemical fluorescence quenching coefficients declined after 24 days of growth with Cu nanocarboxylates, but they increased after 6 days of the addition of 2 or 4 mg L^-1 Se nanocarboxylates. Those alterations affected the overall quantum yield of the photosynthetic electron transport in photosystem II.

Evaluation of antidesmone alkaloid as a photosynthesis inhibitor

Antidesmone, isolated from Waltheria brachypetala Turcz., owns special structural features as two α,β-unsaturated carbonyl groups and a side alkyl chain that can compete with the quinones involved in the pool of plastoquinones at photosystem II (PSII). In this work, we showed that the alkaloid is an inhibitor of Hill reaction and its target was located at the acceptor side of PSII. Studies of chlorophyll (Chl) a fluorescence showed a J-band that indicates direct action of antidesmone in accumulation of Q_A^- (reduced plastoquinone A) due to the electron transport blocked at the Q_B (plastoquinone B) level similar to DCMU. In vivo assays indicated that antidesmone is a selective post-emergent herbicide probe at 300μM by reducing the biomass production of Physalis ixacarpa plants. Furthermore, antidesmone also behaves as pre-emergent herbicide due to inhibit Physalis ixacarpa plant growth about 60%. Antidesmone, a natural product containing a 4(1H)-pyridones scaffold, will serve as a valuable tool in further development of a new class of herbicides.

Mechanisms of metal toxicity in plants

Metal toxicity in plants is still a global problem for the environment, agriculture and ultimately human health.

Flavonoids Affect the Light Reaction of Photosynthesis in Vitro and in Vivo as Well as the Growth of Plants

Flavonoids retusin (5-hydroxy-3,7,3',4'-tetramethoxyflavone) (1) and pachypodol (5,4'-dihydroxy-3,7,3'-trimethoxyflavone) (2) were isolated from Croton ciliatoglanduliferus Ort. Pachypodol acts as a Hill reaction inhibitor with its target on the water splitting enzyme located in PSII. In the search for new herbicides from natural compounds, flavonoids 1 and 2 and flavonoid analogues quercetin (3), apigenin (4), genistein (5), and eupatorin (6) were assessed for their effect in vitro on the photosynthetic electron transport chain and in vivo on the germination and growth of the plants Physalis ixocarpa, Trifolium alexandrinum and Lolium perenne. Flavonoid 3 was the most active inhibitor of the photosynthetic uncoupled electron flow (I50 = 114 μM) with a lower log P value (1.37). Results in vivo suggest that 1, 2, 3, and 5 behave as pre- and postemergent herbicides, with 3 and 5 being more active.

Mexican propolis flavonoids affect photosynthesis and seedling growth

As a continuous effort to find new natural products with potential herbicide activity, flavonoids acacetin (1), chrysin (2) and 4',7-dimethylnarangenin (3) were isolated from a propolis sample collected in the rural area of Mexico City and their effects on the photosynthesis light reactions and on the growth of Lolium perenne, Echinochloa crus-galli and Physalis ixocarpa seedlings were investigated. Acacetin (1) acted as an uncoupler by enhancing the electron transport under basal and phosphorylating conditions and the Mg(2+)-ATPase. Chrysin (2) at low concentrations behaved as an uncoupler and at concentrations up to 100 μM its behavior was as a Hill reaction inhibitor. Finally, 4',7-dimethylnarangenin (3) in a concentration-dependent manner behaved as a Hill reaction inhibitor. Flavonoids 2 and 3 inhibited the uncoupled photosystem II reaction measured from water to 2,5-dichloro-1,4-benzoquinone (DCBQ), and they did not inhibit the uncoupled partial reactions measured from water to sodium silicomolybdate (SiMo) and from diphenylcarbazide (DPC) to diclorophenol indophenol (DCPIP). These results indicated that chrysin and 4',7-dimethylnarangenin inhibited the acceptor side of PS II. The results were corroborated with fluorescence of chlorophyll a measurements. Flavonoids also showed activity on the growth of seedlings of Lolium perenne and Echinochloa crus-galli.

Cyclopent-4-ene-1,3-diones: a new class of herbicides acting as potent photosynthesis inhibitors

In a recent paper, we reported the synthesis and photosynthesis-inhibitory activity of a series of analogues of rubrolides. From quantitative structure-activity relationship (QSAR) studies, we found that the most efficient compounds are those having higher ability to accept electrons. On the basis of those findings, we directed our effort to synthesize new analogues bearing a strong electron-withdrawing group (nitro) in the benzylidene ring and evaluate their effects on photosynthesis. However, the employed synthetic approach led to novel cyclopent-4-ene-1,3-diones as major products. Here, we report the synthesis and mechanism of action of such cyclopent-4-ene-1,3-diones as a new class of photosynthesis inhibitors. These compounds block the electron transport at the QB level by interacting at the D1 protein at the reducing side of Photosystem II and act as Hill reaction inhibitors, with higher activity than the corresponding rubrolides. To the best of our knowledge, this is the first report on the photosynthesis inhibitory activity of cyclopentenediones.

Furoquinoline alkaloids isolated from Balfourodendron riedelianum as photosynthetic inhibitors in spinach chloroplasts

In the search for natural inhibitors of plant growth, we investigate the mechanism of action of the natural furoquinoline alkaloids isolated from Balfourodendron riedelianum (Rutaceae): evolitrine (1), kokusaginine (2), γ-fagarine (3), skimmianine (4) and maculosidine (5) on the photosynthesis light reactions. Their effect on the electron transport chain on thylakoids was analyzed. Alkaloids 1, 2, 4 and 5 inhibited ATP synthesis, basal, phosphorylating and uncoupled electron transport acting as Hill reaction inhibitors on spinach chloroplasts. Alkaloid 3 was not active. The inhibition and interaction site of alkaloids 1, 2, 4 and 5 on the non-cyclic electron transport chain was studied by polarography and fluorescence of the chlorophyll a (Chl a). The results indicate that the target for 1 was localized on the donor and acceptor side of PS II. In addition alkaloids 2 and 5 affect the PS I electron acceptors on leaf discs.

Protecting effect of phosphorylation on oxidative damage of D1 protein by down-regulating the production of superoxide anion in photosystem II membranes under high light

The physiological significance of photosystem II (PSII) core protein phosphorylation has been suggested to facilitate the migration of oxidative damaged D1 and D2 proteins, but meanwhile the phosphorylation seems to be associated with the suppression of reactive oxygen species (ROS) production, and it also relates to the degradation of PSII reaction center proteins. To more clearly elucidate the possible protecting effect of the phosphorylation on oxidative damage of D1 protein, the degradation of oxidized D1 protein and the production of superoxide anion in the non-phosphorylated and phosphorylated PSII membranes were comparatively detected using the Western blotting and electron spin resonance spin-trapping technique, respectively. Obviously, all of three ROS components, including superoxide anion, hydrogen peroxide and hydroxyl radical are responsible for the degradation of oxidized D1 protein, and the protection of the D1 protein degradation by phosphorylation is accompanied by the inhibition of superoxide anion production. Furthermore, the inhibiting effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a competitor to Q(B), on superoxide anion production and its protecting effect on D1 protein degradation are even more obvious than those of phosphorylation. Both DCMU effects are independent of whether PSII membranes are phosphorylated or not, which reasonably implies that the herbicide DCMU and D1 protein phosphorylation probably share the same target site in D1 protein of PSII. So, altogether it can be concluded that the phosphorylation of D1 protein reduces the oxidative damage of D1 protein by decreasing the production of superoxide anion in PSII membranes under high light.

Friedelane triterpenes from Celastrus vulcanicola as photosynthetic inhibitors

Five friedelane triterpenoids, epifriedelinol (1), friedelin (2), canophyllol (3), pulpononic acid (4) and 3-oxo-29-hydroxyfriedelane (5), were isolated from Celastrus vulcanicola (Celastraceae), and were identified by spectroscopic methods, comparison with authentic samples and reported data. In the search for potential herbicides, compounds 1-5 were evaluated for their photosynthetic inhibitory activity. Compound 1 acts as an energy transfer inhibitor, interacting and enhancing the light-activated Mg(2+)-ATPase, while 3 behaves as a Hill reaction inhibitor. The in vivo assays indicated that 1 and 3 act as selective postemergence herbicides at 100 μM by reducing biomass production in the weed Physalis ixocarpa. Moreover, results from Chl a fluorescence transients in leaves of Lolium perenne and P. ixocarpa suggest that both compounds affect photosynthesis efficiency of the chloroplasts as a response to a process of detoxification and repair. Thus, 1 and 3 reduce biomass by more complex mechanisms than only the damaging of the photosynthetic apparatus.

Monitoring copper toxicity in natural phytoplankton assemblages: application of Fast Repetition Rate fluorometry

Four experiments were conducted with natural coastal phytoplankton assemblages exposed to [Cu] within the range 5-80 microg L(-1). The effect of Cu on several biological variables such as chlorophyll a concentration, particle size distribution, O2-production and fluorescence variables recorded by a Fast Repetition Rate fluorometer was monitored during 72 h. Variable fluorescence (Fv) was the most sensitive and rapid among all the variables tested. This work contributes to reinforce the use of fluorescence endpoints in ecotoxicological studies by proving their ecological relevance through relationships found between fluorescence and population-level responses as growth rate and gross O2 production. The lowest calculated EC10 was 2.65 microg L(-1), concentration commonly exceeded in polluted waters.

Effect of 2, 5-substituents on the stability of cyclic nitrone superoxide spin adducts: A density functional theory approach

In the present study, five cyclic nitrone superoxide spin adducts, i.e. DMPO-OOH, M(3)PO-OOH, EMPO-OOH, DEPMPO-OOH and DEPDMPO-OOH, were chosen as model compounds to investigate the effect of 2,5-subsitituents on their stability, through structural analysis and decay thermodynamics using density functional theory (DFT) calculations. Analysis of the optimized geometries reveals that none of the previously proposed stabilizing factors, including intramolecular H-bonds, intramolecular non-bonding interactions, bulky steric protection nor the C(2)-N(1) bond distance can be used to clearly explain the effect of 2,5-substituents on the stability of the spin adducts. Subsequent study found that spin densities on the nitroxyl nitrogen and oxygen are well correlated with the half-lives of the spin adducts and consequently are the proper parameters to characterize the effect of 2,5-substituents on their stability. Examination of the decomposition thermodynamics further supports the effect of the substituents on the persistence of cyclic nitrone superoxide spin adducts.

Adaptation to high light irradiances enhances the photosynthetic Cu2+ resistance in Cu2+ tolerant and non-tolerant populations of the brown macroalgae Fucus serratus

The relationship between light acclimation and Cu(2+) tolerance was studied in two populations of Fucus serratus known to be naturally non-tolerant and tolerant to Cu(2+). Acclimation to high irradiances increased the photosynthetic tolerance to Cu(2+). The xanthophyll cycle was apparently not involved in protecting the photosynthetic apparatus against Cu(2+) toxicity, as results showed that Cu(2+) did not induce dynamic photoinhibition. The higher photosynthetic Cu(2+) resistance of high light algae did not result in increased growth. The excess energy acquired by high light-adapted algae appeared to be utilized in Cu(2+) defense mechanisms in the Cu(2+) non-tolerant population. The polyphenol content of the algae was reciprocal to the Cu(T) content, suggesting that polyphenol may be the primary Cu(2+) defense of non-tolerant low light algae, acting through secretion and extracellular chelating of Cu(2+), while the compounds do not seem to be involved in the primary Cu(2+) tolerance mechanism in Cu(2+) tolerant algae.

Effects of copper on the photosynthesis of intact chloroplasts: interaction with manganese

Highly purified, intact chloroplasts were prepared from pea (Pisum sativum L.) and spinach (Spinacia oleracea L.) following an identical procedure, and were used to investigate the cupric cation inhibition on the photosynthetic activity. In both species, copper inhibition showed a similar inhibitor concentration that decreases the enzyme activity by 50% (IC(50) approximately 1.8 microM) and did not depend on the internal or external phosphate (Pi) concentration, indicating that copper did not interact with the Pi translocator. Fluorescence analysis suggested that the presence of copper did not facilitate photoinhibition, because there were no changes in maximal fluorescence (F(m)) nor in basal fluorescence (F(o)) of copper-treated samples. The electron transport through the photosystem II (PSII) was also not affected (operating efficiency of PSII-F'v/F'm similar in all conditions). Yet, under Cu(2+) stress, the proportion of open PSII reaction centers was dramatically decreased, and the first quinone acceptor (Q(A)) reoxidation was fully inhibited, as demonstrated by the constant photochemical quenching (q(P)) along experiment time. The quantum yield of PSII electron transport (Phi(PSII)) was also clearly affected by copper, and therefore reduced the photochemistry efficiency. Manganese, when added simultaneously with copper, delayed the inhibition, as measured by oxygen evolution and chlorophyll fluorescence, but neither reversed the copper effect when added to copper-inhibited plastids, nor prevented the inhibition of the Hill activity of isolated copper-treated thylakoids. Our results suggest that manganese competed with copper to penetrate the chloroplast envelope. This competition seems to be specific because other divalent cations e.g. magnesium and calcium, did not interfere with the copper action in intact chloroplasts. All results do suggest that, under these conditions, the stroma proteins, such as the Calvin-Benson cycle enzymes or others are the most probable first target for the Cu(2+) action, resulting in the total inhibition of chloroplast photosynthesis and in the consequent unbalanced rate of production and consumption of the reducing power.

Toxicity of copper in natural marine picoplankton populations

Standard microalgae toxicity tests should be able to establish responses in real ecosystems. Natural marine picoplankton samples collected during the months of March, June, August, October 2007 and January 2008, where exposed to 72 h copper toxicity tests. Results analysed by flow cytometry distinguished two groups, with different cytometric characteristics that can match with two of Synechococcus populations. EC(50) values for these two populations resulted low, ranging from 0.62 to 26.28 microg L(-1), this converts copper in a very powerful contaminant and Synechococcus in one of the most sensitive groups of phytoplankton. Differences in EC(50) values for a same population can be related to the month of collection including different initial cellular densities and oceanographic parameters that can affect the picoplankton's tolerance and distribution.

THE ROLE OF REACTIVE OXYGEN SPECIES IN COPPER TOXICITY TO TWO FRESHWATER GREEN ALGAE(1)

The role of reactive oxygen species (ROS) in copper (Cu) toxicity to two freshwater green algal species, Pseudokirchneriella subcapitata (Korshikov) Hindák and Chlorella vulgaris Beij., was assessed to gain a better mechanistic understanding of this toxicity. Cu-induced formation of ROS was investigated in the two algal species and linked to short-term effects on photosynthetic activity and to long-term effects on cell growth. A light- and time-dependent increase in ROS concentrations was observed upon exposure to environmentally relevant Cu concentrations of 50 and 250 nM and was comparable in both algal species. However, effects of 250 nM Cu on photosynthesis were different, leading to a 12% reduction in photosynthetic activity in P. subcapitata, but not in C. vulgaris. These results indicate that differences in species-specific sensitivities measured as photosynthetic activity were not caused by differences in the cellular ROS content of the algae, but probably by different species-specific ROS defense systems. To investigate the role of ROS in Cu-mediated inhibition of photosynthesis, the ROS scavenger N-tert-butyl-α-phenylnitrone (BPN) was used, resulting in a reduction of Cu-induced ROS production up to control level and a complete restoration of photosynthetic activity of Cu-exposed P. subcapitata. This finding implied that ROS play a primary role in Cu toxicity to algae. Furthermore, we observed a time-dependent ROS release process across the plasma membrane. More than 90% of total ROS were determined to be extracellular in P. subcapitata, indicating an efficient method of cellular protection against oxidative stress.

Damage to the oxygen-evolving complex by superoxide anion, hydrogen peroxide, and hydroxyl radical in photoinhibition of photosystem II

Under strong illumination of a photosystem II (PSII) membrane, endogenous superoxide anion, hydrogen peroxide, and hydroxyl radical were successively produced. These compounds then cooperatively resulted in a release of manganese from the oxygen-evolving complex (OEC) and an inhibition of oxygen evolution activity. The OEC inactivation was initiated by an acceptor-side generated superoxide anion, and hydrogen peroxide was most probably responsible for the transportation of reactive oxygen species (ROS) across the PSII membrane from the acceptor-side to the donor-side. Besides ROS being generated in the acceptor-side induced manganese loss; there may also be a ROS-independent manganese loss in the OEC of PSII. Both superoxide anion and hydroxyl radical located inside the PSII membrane were directly identified by a spin trapping-electron spin resonance (ESR) method in combination with a lipophilic spin trap, 5-(diethoxyphosphoryl)-5-phenethyl-1-pyrroline N-oxide (DEPPEPO). The endogenous hydrogen peroxide production was examined by oxidation of thiobenzamide.

Copper in plants

Copper is an essential metal for normal plant growth and development, although it is also potentially toxic. Copper participates in numerous physiological processes and is an essential cofactor for many metalloproteins, however, problems arise when excess copper is present in cells. Excess copper inhibits plant growth and impairs important cellular processes (i.e., photosynthetic electron transport). Since copper is both an essential cofactor and a toxic element, involving a complex network of metal trafficking pathways, different strategies have evolved in plants to appropriately regulate its homeostasis as a function of the environmental copper level. Such strategies must prevent accumulation of the metal in the freely reactive form (metal detoxification pathways) and ensure proper delivery of this element to target metalloproteins. The mechanisms involved in the acquisition of this essential micronutrient have not been clearly defined although a number of genes have recently been identified which encode potential copper transporters. This review gives a briefly overview of the current understanding of the more important features concerning copper toxicity and tolerance in plants, and brings information of recent findings on copper trafficking including copper detoxification factors, copper transporters and copper chaperones.

Copper effect on cytochrome b of photosystem II under photoinhibitory conditions

Toxic Cu (II) effect on cytochrome b(559) under aerobic photoinhibitory conditions was examined in two different photosystem II (PSII) membrane preparations active in oxygen evolution. The preparations differ in the content of cytochrome b(559) redox potential forms. Difference absorption spectra showed that the presence of Cu (II) induced the oxidation of the high-potential form of cytochrome b(559) in the dark. Addition of hydroquinone reduced the total oxidized high-potential form of cytochrome b(559) present in Cu (II)-treated PSII membranes indicating that no conversion to the low-potential form took place. Spectroscopic determinations of cytochrome b(559) during photoinhibitory treatment showed slower kinetics of Cu (II) effect on cytochrome b(559) in comparison with the rapid loss of oxygen evolution activity in the same conditions. This result indicates that cytochrome b(559) is affected after PSII centres are photoinhibited. The high-potential form was more sensitive to toxic Cu (II) action than the low-potential form under illumination at pH 6.0. The content of the high-potential form of cytochrome b(559) was completely lost; however, the low-potential content was unaffected in these conditions. This loss did not involve cytochrome protein degradation. The results are discussed in terms of different binding properties of the heme iron to the protonated or unprotonated histidine ligand in the high-potential and low-potential forms of cytochrome b(559), respectively.

Inter-population differences in inherited copper tolerance involve photosynthetic adaptation and exclusion mechanisms in Fucus serratus

•  A comparative study of copper (Cu) toxicity and tolerance in three populations of Fucus serratus was conducted by examining Cu²⁺ effects on various physiological parameters. •  Chlorophyll fluorescence, oxygen evolution, copper content, and relative growth rate of embryos and adults were measured on Cu²⁺ -exposed material. •  Algae naturally exposed to elevated total Cu concentration (Cu_T ), were more Cu²⁺ resistant than those from clean sites, as indicated by higher embryo and adult growth rates and lower copper contents. The Cu²⁺ tolerance of F. serratus is at least partly inherited and relies partly on metal exclusion. •  There were inhibitory effects of Cu²⁺ on oxygen exchange rates in both tolerant and non-tolerant algae. By contrast to sensitive algae, the maximum efficiency of photosystem II (F_v /F_m ), maximum fluorescence (F_m ) and zero fluorescence (F_o ) of resistant algae were unaffected by Cu²⁺ , whereas decreased quantum yield (ΦPSII) and increased nonphotochemical quenching (NPQ) were most pronounced in resistant algae. Inhibitory effects of Cu²⁺ on ΦPSII may result in the excitation energy being dissipated through xanthophyll-dependent quenching mechanisms in tolerant algae. In nontolerant algae, lower energy dissipation may result in chlorophyll degradation.

Increased tolerance to thermal inactivation of oxygen evolution in spinach Photosystem II membranes by substitution of the extrinsic 33-kDa protein by its homologue from a thermophilic cyanobacterium

Photosynthetic oxygen evolution is an extremely heat-sensitive process and incubation of spinach Photosystem II (PSII) membranes at 40 degrees C for only several minutes leads to its complete inactivation. Substitution experiments of the spinach 33-kDa manganese stabilizing protein by a homologue protein, isolated either from the thermophilic cyanobacterium Phormidium laminosum, or from Escherichia coli as a recombinant thermophilic cyanobacterial protein, showed a significant increase in tolerance to heat inactivation of the oxygen-evolving activity. The results allow us to suggest that thermal inactivation of oxygen evolution in higher plant PSII membranes is due to dissociation of the 33-kDa protein as a consequence of temperature-induced conformational changes, and stabilization can be provided by substitution by a thermostable homologue whose secondary structure and binding to PSII remain unaltered at moderately high temperatures.

Two inhibition targets by [Cr(2gb)(3)](3+) and [Co(2gb)(3)](3+) on redox enzymes of spinach thylakoids

[Cr(2gb)(3)]Cl(ZnCl(4)), [Cr(2gb)(3)]Cl(3), and [Co(2gb)(3)]Cl(3) were synthesized and characterized. Their chemical structures and the oxidation states of their metal centers remained unchanged in solution. The effects of these compounds, CrCl(3) and [Co(NH(3))(6)]Cl(3), on photosynthesis were investigated. The coordination compounds inhibit ATP synthesis and electron flow (basal, phosphorylating, and uncoupled), behaving as Hill reaction inhibitors. The target for [Cr(2gb)(3)]Cl(ZnCl(4)) is located at the Q(B) level. In contrast, the interaction sites of [Cr(2gb)(3)]Cl(3) and [Co(2gb)(3)]Cl(3) are located in the span from P(680) to Q(A) and at the b(6)f complex. Neither CrCl(3) nor [Co(NH(3))(6)]Cl(3) inhibited photosynthesis. The 100% inhibition on PS II of [Cr(2gb)(3)]Cl(ZnCl(4)) is explained in terms of a synergystic effect between the 2gb-chromium(III) coordination compound and the ZnCl(4)(2)(-) anion.

Oxygen-evolving diatom thylakoid membranes

Two protocols were developed that yielded purified oxygen-evolving thylakoid membranes from the diatom Cylindrotheca fusiformis. One protocol employed sonication, while the second involved French press lysis of protoplasts formed by brief culture of cells in a cation-depleted medium. Regardless of the method of cell breakage, some damage to electron transport components occurred. For preservation of both light-dependent electron transport activity and in vivo fluorescence properties, 2 M sorbitol proved to be more effective than 1 M sorbitol, regardless of the method used for cell lysis. Thylakoids purified in 2 M sorbitol using the protoplast/French press method showed the best preservation of in vivo fluorescence emission signals and Photosystem II activity with ferricyanide was completely inhibited by DCMU. Thylakoids purified in 2 M sorbitol using sonication had higher rates of Photosystem II activity with ferricyanide, but this activity was less sensitive to DCMU. Whole-chain electron transport activity was low in all preparations. This is the first report of O2 evolution and of long-wavelength fluorescence in purified thylakoids of any chromophytic alga.

Increase in the quantum yield of photoinhibition contributes to copper toxicity in vivo

The effect of copper on photoinhibition of photosystem II in vivo was studied in bean (Phaseolus vulgaris L. cv Dufrix). The plants were grown hydroponically in the presence of various concentrations of Cu2+ ranging from the optimum 0.3 &mgr;m (control) to 15 &mgr;m. The copper concentration of leaves varied according to the nutrient medium from a control value of 13 mg kg-1 dry weight to 76 mg kg-1 dry weight. Leaf samples were illuminated in the presence and absence of lincomycin at different light intensities (500-1500 &mgr;mol photons m-2 s-1). Lincomycin prevents the concurrent repair of photoinhibitory damage by blocking chloroplast protein synthesis. The photoinhibitory decrease in the light-saturated rate of O2 evolution measured from thylakoids isolated from treated leaves correlated well with the decrease in the ratio of variable to maximum fluorescence measured from the leaf discs; therefore, the fluorescence ratio was used as a routine measurement of photoinhibition in vivo. Excess copper was found to affect the equilibrium between photoinhibition and repair, resulting in a decrease in the steady-state concentration of active photosystem II centers of illuminated leaves. This shift in equilibrium apparently resulted from an increase in the quantum yield of photoinhibition (PhiPI) induced by excess copper. The kinetic pattern of photoinhibition and the independence of PhiPI on photon flux density were not affected by excess copper. An increase in PhiPI may contribute substantially to Cu2+ toxicity in certain plant species.

Bicarbonate is an essential constituent of the water-oxidizing complex of photosystem II

It is shown that restoration of photoinduced electron flow and O2 evolution with Mn2+ in Mn-depleted photosystem II (PSII) membrane fragments isolated from spinach chloroplasts is considerably increased with bicarbonate in the region pH 5.0-8.0 in bicarbonate-depleted medium. In buffered solutions equilibrated with the atmosphere (nondepleted of bicarbonate), the bicarbonate effect is observed only at pH lower than the pK of H2CO3 dissociation (6.4), which indicates that HCO3- is the essential species for the restoration effect. The addition of just 2 Mn2+ atoms per one PSII reaction center is enough for the maximal reactivation when bicarbonate is present in the medium. Analysis of bicarbonate concentration dependence of the restoration effect reveals two binding sites for bicarbonate with apparent dissociation constant (Kd) of approximately 2.5 microM and 20-34 microM when 2,6-dichloro-p-benzoquinone is used as electron acceptor, while in the presence of silicomolybdate only the latter one remains. Similar bicarbonate concentration dependence of O2 evolution was obtained in untreated Mn-containing PSII membrane fragments. It is suggested that the Kd of 20-34 microM is associated with the donor side of PSII while the location of the lower Kd binding site is not quite clear. The conclusion is made that bicarbonate is an essential constituent of the water-oxidizing complex of PSII, important for its assembly and maintenance in the functionally active state.

Photoinhibition of photosystem II from higher plants. Effect of copper inhibition

Strong illumination of Cu(II)-inhibited photosystem II membranes resulted in a faster loss of oxygen evolution activity compared with that of the intact samples. The phenomenon was oxygen- and temperature-dependent. However, D1 protein degradation rate was similar in both preparations and slower than that found in non-oxygen evolving PSII particles (i.e. Mn-depleted photosystem II). These results seem to indicate that during illumination Cu(II)-inhibited samples do not behave as a typical non-oxygen evolving photosystem II. Cytochrome b559 was functional in the presence of Cu(II). The effect of Cu(II) inhibition decreased the amount of photoreduced cytochrome b559 and slowed down the rate of its photoreduction. The presence of Cu(II) during illumination seems to protect P680 against photodamage as occurs in photosystem II reaction centers when the acceptor side is protected. The data were consistent with the finding that production of singlet oxygen was highly reduced in the preparations treated with Cu(II). EPR spin trapping experiments showed that inactivation of Cu(II)-treated samples was dominated by hydroxyl radical, and the loss of oxygen evolution activity was diminished by the presence of superoxide dismutase and catalase. These results indicate that the rapid loss of oxygen evolution activity in the presence of Cu(II) is mainly due to the formation of .OH radicals from superoxide ion via a Cu(II)-catalyzed Haber-Weiss mechanism. Considering that this inactivation process was oxygen-dependent, we propose that the formation of superoxide occurs in the acceptor side of photosystem II by interaction of molecular oxygen with reduced electron acceptor species, and thus, the primarily Cu(II)-inhibitory site in photosystem II is on the acceptor side.

Cu(II)-inhibitory effect on photosystem II from higher plants. A picosecond time-resolved fluorescence study

The influence of Cu(II) inhibition on the primary reactions of photosystem II (PSII) electron transport was studied by picosecond time-resolved fluorescence on isolated PSII membranes. The fluorescence decay from Cu(II)-inhibited PSII centers showed a dominant amplitude of a fast phase (100-300 ps) similar to PSII centers in the uninhibited "open state" and minor contributions of components around 600 ps and 2.6 ns. These data indicate efficient primary charge separation in PSII membranes incubated with Cu(II). The quantum yield of primary reactions in the inhibited PSII centers was similar to that of "open" PSII centers. Kinetic analysis of the decay curves in the framework of the exciton/radical pair equilibrium model showed no significant changes in the rate constants associated with the charge separation/recombination equilibrium. However, in closed centers (QA reduced), a decrease in the rate constant K23, associated with the back-reaction of a relaxed radical pair, by a factor of 4 was calculated. The free energy losses upon primary charge separation (delta G1) and during subsequent radical pair relaxation (delta G2) were also determined in Cu(II)-inhibited centers and were compared with uninhibited centers. No changes in the delta G1 values and a significant decrease in the delta G2 values were found as compared with those of control PSII centers in the "closed" state. These data indicate that Cu(II) does not affect primary radical pair formation, but strongly affects the formation of a relaxed radical pair, by neutralizing the negative charge on QA- and eliminating the repulsive interaction between Pheo- and QA- and/or by modifying the general dielectric properties of the protein region, surrounding these cofactors. Moreover, a close attractive interaction between Pheo-, QA-, and Cu2+ can be proposed. Our results are in good agreement with very recent EPR results indicating an additional effect of Cu2+ on the acceptor side [Jegerschöld et al. (1995) Biochemistry 34, 12747-12758].

Biochemically active sesquiterpene lactones from Ratibida mexicana

Bioactivity-directed fractionation of the methanol extract of the roots of Ratibida mexicana resulted in the isolation of two bioactive sesquiterpene lactones, isoalloalantolactone and elema-1,3,11-trien-8,12-olide. Both compounds caused a significant inhibition of the radicle growth of Amaranthus hypochondriacus and Echinochloa crus-galli, exerted moderate cytotoxic activity against three different solid tumour cell lines and inhibited significantly the radial growth of three phytopathogenic fungi. Isoalloalantolactone also caused the inhibition of ATP synthesis, proton uptake and electron transport (basal, phosphorylating and uncoupled) from water to methylviologen, therefore acting as a Hill's reaction inhibitor. The lactone did not affect photosystem I but inhibited photosystem II. The site of inhibition of isoalloalantolactone is located in the span of P680 to QA redox enzymes because the uncoupled electron transport from water to silicomolybdate and, from DPC to DCIP are inhibited approximately to the same extent.

The donor side of Photosystem II as the copper-inhibitory binding site : Fluorescence and polarografic studies

We have measured, under Cu (II) toxicity conditions, the oxygen-evolving capacity of spinach PS II particles in the Hill reactions H2O→SiMo (in the presence and absence of DCMU) and H2O→PPBQ, as well as the fluorescence induction curve of Tris-washed spinach PS II particles. Cu (II) inhibits both Hill reactions and, in the first case, the DCMU-insensitive H2O → SiMo activity. In addition, the variable fluorescence is lowered by Cu (II). We have interpreted our results in terms of a donor side inhibition close to the reaction center. The same polarographic and fluorescence measurements carried out at different pHs indicate that Cu (II) could bind to amino acid residues that can be protonated and deprotonated. In order to reverse the Cu (II) inhibition by a posterior EDTA treatment, in experiments of preincubation of PS II particles with Cu (II) in light we have demonstrated that light is essential for the damage due to Cu (II) and that this furthermore is irreversible.

The inhibitory mechanism of Cu(II) on the Photosystem II electron transport from higher plants

In a previous paper, we reported that Cu(II) inhibited the photosynthetic electron transfer at the level of the pheophytin-QA-Fe domain of the Photosystem II reaction center. In this paper we characterize the underlying mechanism of Cu(II) inhibition. Cu(II)-inhibition effect was more sensitive with high pH values. Double-reciprocal plot of the inhibition of oxygen evolution by Cu(II) is shown and its corresponding inhibition constant, Ki, was calculated. Inhibition by Cu(II) was non-competitive with respect to 2,6-dichlorobenzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea and competitive with respect to protons. The non-competitive inhibition indicates that the Cu(II)-binding site is different from that of the 2,6-dichlorobenzoquinone electron acceptor and 3-(3,4-dichlorophenyl)-1,1-dimethylurea sites, the QB niche. On the other hand, the competitive inhibition with respect to protons may indicate that Cu(II) interacts with an essential amino acid group(s) that can be protonated or deprotonated in the inhibitory-binding site.