A fluorescence-based bioassay for aquatic macrophytes and its suitability for effect analysis of non-photosystem II inhibitors

BACKGROUND, GOALS AND SCOPE: During the last years the miniaturization of toxicity test systems for rapid and parallel measurements of large quantities of samples has often been discussed. For unicellular algae as well as for aquatic macrophytes, fluorescence-based miniaturized test systems have been introduced to analyze photosystem II (PSII) inhibitors. Nevertheless, high-throughput screening should also guarantee the effect detection of a broad range of toxicants in order to ensure routinely applicable, high-throughput measuring device experiments which can cover a broad range of toxicants and modes of action others than PSII inhibition. Thus, the aim of this study was to establish a fast and reproducible measuring system for non-PSII inhibitors for aquatic macrophyte species to overcome major limitations for use.

METHODS

A newly developed imaging pulse-amplitude-modulated chlorophyll fluorometer (I-PAM) was applied as an effect detector in short-term bioassays with the aquatic macrophyte species Lemna minor. This multiwell-plate based measuring device enabled the incubation and measurement of up to 24 samples in parallel. The chemicals paraquat-dichloride, alizarine and triclosan were chosen as representatives for the toxicant groups of non-PSII herbicides, polycyclic aromatic hydrocarbons (PAHs) and pharmaceuticals and personal care products (PPCPs), which are often detected in the aquatic environment. The I-PAM was used (i) to establish and validate the sensitivity of the test system to the three non-PSII inhibitors, (ii) to compare the test systems with standardized and established biotests for aquatic macrophytes, and (iii) to define necessary time scales in aquatic macrophyte testing. For validation of the fluorescence-based assay, the standard growth test with L. minor (ISO/DIS 20079) was performed in parallel for each chemical.

RESULTS

The results revealed that fluorescence-based measurements with the I-PAM allow rapid and parallel analysis of large amounts of aquatic macrophyte samples. The I-PAM enabled the recording of concentration-effect-curves with L. minor samples on a 24-well plate with single measurements. Fluorescence-based concentration-effect-curves could be detected for all three chemicals after only 1 h of incubation. After 4-5 h incubation time, the maximum inhibition of fluorescence showed an 80-100% effect for the chemicals tested. The EC50 after 24 h incubation were estimated to be 0.06 mg/L, 0.84 mg/L and 1.69 mg/L for paraquat-dichloride, alizarine and triclosan, respectively.

DISCUSSION

The results obtained with the I-PAM after 24 h for the herbicide paraquat-dichloride and the polycyclic aromatic hydrocarbon alizarine were in good accordance with median effective concentrations (EC50s) obtained by the standardized growth test for L. minor after 7 d incubation (0.09 mg/L and 0.79 mg/L for paraquat-dichloride and alizarine, respectively). Those results were in accordance with literature findings for the two chemicals. In contrast, fluorescence-based EC50 of the antimicrobial agent triclosan proved to be two orders of magnitude greater when compared to the standard growth test with 7 d incubation time (0.026 mg/L) as well as with literature findings.

CONCLUSION

Typically, aquatic macrophyte testing is very time consuming and relies on laborious experimental set-ups. The I-PAM measuring device enabled fast effect screening for the three chemicals tested. While established test systems for aquatic macrophytes need incubation times of > or = 7 d, the I-PAM can detect inhibitory effects much earlier (24 h), even if inhibition of chemicals is not specifically associated with PSII. Thus, the fluorescence-based bioassay with the I-PAM offers a promising approach for the miniaturization and high-throughput testing of chemicals with aquatic macrophytes. For the chemical triclosan, however, the short-term effect prediction with the I-PAM has been shown to be less sensitive than with long-term bioassays, which might be due to physicochemical substance properties such as lipophilicity.

RECOMMENDATIONS AND PERSPECTIVES

The results of this study show that the I-PAM represents a promising tool for decreasing the incubation times of aquatic macrophyte toxicity testing to about 24 h as a supplement to existing test batteries. The applicability of this I-PAM bioassay on emergent and submerged aquatic macrophyte species should be investigated in further studies. Regarding considerations that physicochemical properties of the tested substances might play an important role in microplate bioassays, the I-PAM bioassay should either be accompanied by evaluating physicochemical properties modeled from structural information prior to an experimental investigation, or by intensified chemical analyses to identify and determine nominal concentrations of the toxicants tested. The chemicals paraquat-dichloride, alizarine and triclosan were chosen as representatives for the toxicant groups of non-PSII herbicides, PAHs and PPCPs which are often detected in the aquatic environment. Nevertheless, in order to ensure a routinely applicable measuring device, experiments with a broader range of toxicants and samples of surface and/or waste waters are necessary.

Effects of carotenoid inhibition on the photosynthetic RC-LH1 complex in purple sulphur bacterium Thiorhodospira sibirica

Core complexes (LH1-RC) were isolated using preparative gel electrophoresis from photosynthetic membranes of the purple bacterium, Thiorhodospira sibirica, grown in the absence or presence of the carotenoid biosynthesis inhibitor, diphenylamine. The biosynthesis of carotenoids is affected by diphenylamine both quantitavely and qualitatively: after inhibition, the level of carotenoids in core complexes reaches only 10% of the normal content, as analyzed by HPLC and absorption spectroscopy. The normally grown bacterium biosynthesizes spirilloxanthin, rhodopin, anhydrorhodovibrin and lycopene, whereas after inhibition only neurosporene, zeta-carotene and their derivatives are found in the complexes. There is no concomitant accumulation of appreciable amounts of colorless carotenoid precursors. Interestingly, the main absorption band of the core light harvesting complex isolated from carotenoid-inhibited cells, shows a red shift to 889 nm, instead of a blue shift observed in many carotenoid-deficient species of purple photosynthetic bacteria. The stability of isolated core complexes against n-octyl-beta-D: -glucopyranoside clearly depends on the presence of carotenoids. Subcomplexes resulting from the detergent treatment, were characterized by non-denaturating gel electrophoresis combined with in situ absorption spectroscopy. Core complexes with the native carotenoid complement dissociate into three subcomplexes: (a) LH1 complexes partially depleted of carotenoids, with an unusual spectrum in the NIR region (lambdamax = 791, 818, 847 and 875 nm), (b) reaction centers associated with fragments of LH1, (c) small amounts of a carotenoidless B820 subcomplex. The core complex from the carotenoid-deficient bacterium is much less stable and yields only the two sub-complexes (b) and (c). We conclude that carotenoids contribute critically to stability and interactions of the core complexes with detergents.

Formation of a Semiquinone at the QB Site by A- or B-Branch Electron Transfer in the Reaction Center from Rhodobacter sphaeroides

In Rhodobacter sphaeroides reaction centers containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the A-branch of cofactors is prevented by the loss of the QA ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulate the P(+)QB- radical pair following transmembrane electron transfer along the B-branch of cofactors (Wakeham, M. C., Goodwin, M. G., McKibbin, C., and Jones, M. R. (2003) Photoaccumulation of the P(+)QB- radical pair state in purple bacterial reaction centers that lack the QA ubiquinone. FEBS Lett. 540, 234-240). The yield of the P(+)QB- state appears to depend upon which additional mutations are present. In the present paper, Fourier transform infrared (FTIR) difference spectroscopy was used to demonstrate that photooxidation of the reaction center's primary donor in QA-deficient reaction centers results in formation of a semiquinone at the QB site by B-branch electron transfer. Reduction of QB by the B-branch pathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, in contrast to the QA- to QB reaction in the wild-type reaction center, which is not active at cryogenic temperatures. These FTIR results suggest that the conformational changes that "gate" the QA- to QB reaction do not necessarily have the same influence on QB reduction when the electron donor is the HB anion, at least in a minority of reaction centers.

Synthesis and herbicidal activity of 2-cyano-3-substituted-pyridinemethylaminoacrylates

Two series of 2-cyano-3-substituted-pyridinemethylaminoacrylates, namely 12 new (Z)-2-cyano-3-methylthio-3-substituted-pyridinemethaneaminoacrylates and 10 new (Z)-2-cyano-3-alkyl-3-substituted-pyridinemethaneaminoacrylates, were synthesized as herbicidal inhibitors of photosystem II (PSII) electron transport. All of these compounds were confirmed by (1)H NMR, elemental, IR, and mass spectrum analyses. Their herbicidal activities were evaluated. Some compounds exhibited excellent herbicidal activities, even at a dose of 75 g/ha. A suitable substituent at the 2-position of the pyridine ring and the well-fit group at the 3-position of acrylate were essential for high herbicidal activity. 2-Cyanoacrylates containing a substituted pyridine ring provide higher herbicidal activities than parent compounds containing phenyl. These PSII inhibitor herbicides are safe to corn, which is a major crop in China.

Rapid and specific detection of herbicides using a self-assembled photosynthetic reaction center from purple bacterium on an SPR chip

In this study, a direct detection system for herbicides inhibiting photosynthetic electron transfer was developed using the photosynthetic reaction center (RC) from the purple bacterium, Rhodobacter sphaeroides, and surface plasmon resonance (SPR) apparatus. The heavy-subunit-histidine-tagged RCs (HHisRCs) were immobilized on an SPR sensor chip via nickel chelation chemistry as a binder for one of the triazine herbicides, atrazine. Immediately after injection of atrazine solution on the HHisRCs-immobilized chip, the SPR responses increased and reached plateaus within 1 min. The SPR signals were proportional to the sample concentrations of atrazine in the range 1-100 microg/ml. To evaluate the binding specificity to atrazine, chlorinated aromatic herbicides, DCMU and MCPP, were investigated using the HHisRCs-immobilized chip. An RC inhibitor, DCMU, could also be detected with a higher detection limit of 20 microg/ml than atrazine (1 microg/ml). MCPP showed no signals because its inhibition mechanism against plants is different from that of atrazine and DCMU. These results indicated that the sensor chip immobilized RCs could be used for the specific detection of photosynthetic inhibitors.

Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II

Certain phenolic compounds represent a distinct class of Photosystem (PS) II Q(B) site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90-95% of Q(B) sites even at saturating concentrations. The remaining PS II reaction centers (5-10%) showed modified Q(A) to Q(B) electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of Q(B) site inhibitors, DCMU and atrazine. In the sensitive centers, the S(2)Q(A)(-) state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S(2)Q(A)(-) state than the S(1)Q(A) state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S(2)-multiline electron paramagnetic resonance (EPR) signal and the 'split' EPR signal, originating from the S(2)Y(Z) state showed no significant changes upon binding of phenolic inhibitors at the Q(B) site. We thus propose a working model where Q(A) redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the Q(B) niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the Q(B) site.

Cold-acclimation protects photosystem II against freezing damage in the halotolerant alga Dunaliella salina

Cold-acclimation (CA) of the halotolerant alga Dunaliella was inhibited by light and by high salt. CA was associated with enhanced resistance to freezing in saline growth solutions, as manifested by protection of photosynthetic oxygen evolution and by reduced permeabilisation of the plasma membrane. Oxygen evolution activity in isolated chloroplasts was not affected by freezing, but was inhibited by high salt and the inhibition could be reversed or protected by glycerol. The activity of chloroplasts from cold-acclimated cells was more resistant to salt than of non-acclimated cells. Electron transport measurements in chloroplasts indicated that high salt inhibited PS-II, but not PS-I electron transport. High salt also inhibited PS-II thermoluminescence (TL) activity in chloroplasts. Similar inhibition of PS-II TL was observed by freezing intact cells in saline solutions. Chloroplasts from cold-acclimated cells had enhanced resistance to inhibition of PS-II electron transport and of PS-II TL by high salt. These results suggest that inhibition of oxygen evolution upon freezing Dunaliella cells may result from inactivation of PS-II due to massive influx of salt and loss of glycerol. The enhanced freeze-resistance of cold-acclimated cells to inhibition of oxygen evolution can be accounted for partly by protection of PS-II against high salt.

Binding of novel inhibitors of electron transfer in photosystem 2, derivatives of perfluoroisopropyldinitrobenzene, with polypeptide D2 of the reaction center

A binding site for novel inhibitors of K15 type (derivatives of perfluoroisopropyldinitrobenzene) with the components of reaction center (RC) of photosystem 2 (PS-2) of higher plants has been investigated. It has been shown that multiple washing the PS-2 submembrane chloroplast fragments (BBY-particles) treated with the K15 inhibitor, including multiple dilution in buffer in the presence of high concentrations of mono- and divalent ions, prolonged (up to 2-5 h) incubation, centrifugation, and subsequent resuspension in buffer deprived of the inhibitor, does not lead to restoration of functional activity of the PS-2. After addition of dithionite, inducing reduction and consequent decomposition of the inhibitor, and subsequent removal of dithionite by washing, the functional activity of PS-2 was completely restored. Incubation in the presence of sodium dodecyl sulfate (SDS), leading to solubilization of the sample to the level of protein components, induced the appearance of a fraction of free K15 retaining the initial inhibitory efficiency. To create a covalent binding of the inhibitor with protein, retained under the conditions of denaturing SDS polyacrylamide gel electrophoresis, the azido-containing analog of K15 (K15-N(3)) was used. The need for radioactive label for identification of K15 was avoided by the revealed ability of K15-type inhibitors to emit fluorescence, which retained its features under the experimental conditions. With the technique of photoaffinity binding and denaturing SDS-PAGE in the presence of 6 M urea of submembrane chloroplast fragments enriched in PS-2 the D2-polypeptide, an integral component of the reaction center of PS-2, has been shown to be a binding site for K15-type inhibitors. This conclusion is in agreement with a suggestion (put forward in our earlier publications) that K15-type inhibitors are bound to PS-2 reaction center, replacing Q(A) in its binding site. Hence, an agent specifically binding to polypeptide D2 has been found for the first time. The data are compared with information about inhibitory action and binding sites of the known inhibitors of electron transfer in PS-2.

Glycinebetaine protects the D1/D2/Cytb559 complex of photosystem II against photo-induced and heat-induced inactivation

The presence of 1.0 mol/L glycinebetaine during isolation of D1/D2/Cytb559 reaction centre (RC) complexes from photosystem II (PSII) membrane fragments preserved the photochemical activity, monitored as the light-induced reduction of pheophytin and electron transport from diphenylcarbazide to 2.6-dichlorophenol-indophenol.-Glycinebetaine also protected the D1/D2/Cytb559 complexes against strong light-induced damage to the photochemical reactions and the irreversible bleaching of beta-carotene and chlorophyll. The presence of glycinebetaine also enhanced thermotolerance of the D1/D2/Cytb559 complexes isolated in the presence of 1.0 mol/L betaine with an increase in the temperature for 50% inactivation from 29 degrees C to 35 degrees C. The results indicate an increased supramolecular structural stability in the presence of glycinebetaine.

Polyphenolic allelochemicals from the aquatic angiosperm Myriophyllum spicatum inhibit photosystem II

Myriophyllum spicatum (Haloragaceae) is a highly competitive freshwater macrophyte that produces and releases algicidal and cyanobactericidal polyphenols. Among them, beta-1,2,3-tri-O-galloyl-4,6-(S)-hexahydroxydiphenoyl-D-glucose (tellimagrandin II) is the major active substance and is an effective inhibitor of microalgal exoenzymes. However, this mode of action does not fully explain the strong allelopathic activity observed in bioassays. Lipophilic extracts of M. spicatum inhibit photosynthetic oxygen evolution of intact cyanobacteria and other photoautotrophs. Fractionation of the extract provided evidence for tellimagrandin II as the active compound. Separate measurements of photosystem I and II activity with spinach (Spinacia oleracea) thylakoid membranes indicated that the site of inhibition is located at photosystem II (PSII). In thermoluminescence measurements with thylakoid membranes and PSII-enriched membrane fragments M. spicatum extracts shifted the maximum temperature of the B-band (S(2)Q(B)(-) recombination) to higher temperatures. Purified tellimagrandin II in concentrations as low as 3 microM caused a comparable shift of the B-band. This demonstrates that the target site of this inhibitor is different from the Q(B)-binding site, a common target of commercial herbicides like 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Measurements with electron paramagnetic resonance spectroscopy suggest a higher redox midpoint potential for the non-heme iron, located between the primary and the secondary quinone electron acceptors, Q(A) and Q(B). Thus, tellimagrandin II has at least two modes of action, inhibition of exoenzymes and inhibition of PSII. Multiple target sites are a common characteristic of many potent allelochemicals.

Chloroplast Cu/Zn-superoxide dismutase is a highly sensitive site in cucumber leaves chilled in the light

Light-chilling stress, the combination of low-light illumination and low temperature, preferentially inactivated photosystem I (PSI) of cucumber (Cucumis sativus L.) leaves, resulting in the photoinhibition of photosynthesis. The extent of PSI photoinhibition, determined in vivo by monitoring absorption changes around 810 nm (induced by far-red light), was closely correlated with the redox state of the PSII electron acceptor Q(A), measured as the chlorophyll fluorescence parameter, 1-qP, where qP is a photochemical quenching coefficient. In contrast, the decrease in the far-red-induced leaf absorptance signal was not well correlated with the limited fragmentation of the PsaA/B gene products in the PSI reaction center after the light-chilling stress. Amongst various enzymes involved in the photooxidative damage such as superoxide dismutase (SOD), ascorbate peroxidase, and NAD(P)H dehydrogenase, only SOD was inhibited by light-chilling treatment. Further, an approximately 3-fold increase in the leaf content of H(2)O(2), a potent inhibitor of Cu/Zn-SOD, was observed after light-chilling stress. From these results, we suggest that Cu/Zn-SOD is the primary target of the light-chilling stress, followed by subsequent inactivation of PSI by reactive oxygen species.

Target sites for herbicides: entering the 21st century

At present the use-rate of modern herbicides is in the range of 100-300 g AI ha-1, with a tendency to decline. The low use-rate (ca 10 g AI ha-1) of the original sulfonylurea and cyclic imide herbicides prompted agrochemical scientists to look for even more active compounds which led to the successive discoveries of many new herbicidal acetolactate synthase inhibitors and no less than 18 cyclic imides in the class of protoporphyrinogen-IX oxidase inhibitors in the 1990s. In this paper, mechanisms of action related to function and biosynthesis of chlorophylls, carotenoids, plastoquinone, amino acids, fatty acids and photosynthetic electron transport and other metabolic processes are discussed as plant-specific herbicidal target domains.

Role of protective and repair mechanisms in the inhibition of photosynthesis in marine macroalgae

The mechanism of photoinhibition was investigated in three representative macroalgal species growing on the coast of Patagonia: the chlorophyte Ulva rigida C. Agardh, the rhodophyte Porphyra columbina Montagne and the phaeophyte Dictyota dichotoma (Huds.) Lamour. Dark adapted specimens were exposed to 15 min unfiltered solar radiation to induce photoinhibition, and subsequently the recovery of the photosynthetic quantum yield was followed for up to 6 h. Photoinhibition is believed to be due to the damage and proteolysis of the D1 protein in the reaction center of Photosystem II. During recovery this protein is resynthesized. In order to prove this hypothesis, inhibitors of the chloroplast protein synthesis, streptomycin and chloramphenicol were applied. Both retarded the repair process indicating an inhibition of the D1 protein resynthesis during recovery after the damage they experienced during light exposure. Some algal groups use the xanthophyll cycle to ameliorate the inhibition by excessive light. Dithiothreitol, an inhibitor of violaxanthin de-epoxidase, was administered, to impair the xanthophyll cycle. It strongly affected both photoinhibition and recovery even in the red algal species, which do not have the xanthophyll cycle, indicating that this drug has significant side effects and should be used with caution for the study of the involvement of this protective cycle in algae. Pigmentation was followed in the three species using absorption spectroscopy of thallus extracts at 665 nm during continuous exposure to natural solar radiation or radiation deprived of the UV component during two days. The results indicated a pronounced variation in pigmentation over time due to bleaching and resynthesis. Solar radiation was monitored during the experiments in three channels (UV-B, UV-A and PAR) using an ELDONET instrument on site.

Inhibition of photosystem II of spinach by the respiration inhibitors piericidin A and thenoyltrifluoroacetone

The effects of several respiration inhibitors on photosystem II (PS II) were investigated. Among the agents tested, piericidin A and thenoyltrifluoroacetone (TTFA) inhibited the photosynthetic electron transport of spinach as measured from chlorophyll (Chl) fluorescence parameters (Fm'-F)/Fm' and Fv/Fm. Using specific donors and acceptors of electrons, we identified the sites of inhibition in and around the PS II complex; the site of inhibition by TTFA was between QA, primary quinone acceptor in PS II, and QB, secondary quinone acceptor, in the acceptor side of P680, the reaction center Chl of PS II, while inhibition by piericidin A of the acceptor side was downstream of Q(B), out of the PS II complex. Both agents also inhibited the donor side of P680, probably between tyrosine-161 of the reaction center protein of PS II and P680.

The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence

The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).

The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence

The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.

Photoinhibition in mutants of Arabidopsis deficient in thylakoid unsaturation

Thylakoid lipid composition in higher plants is characterized by a high level of fatty acid unsaturation. We have screened four mutants of Arabidopsis that have reduced levels of fatty acid unsaturation. Three of the mutant lines tested, fad5, fad6, and the fad3-2 fad7-2 fad8 triple mutant, were more susceptible to photoinhibition than wild-type Arabidopsis, whereas one mutant, fab1, was indistinguishable from wild type. The fad3-2 fad7-2 fad8 triple mutant, which contains no trienoic fatty acids in its thylakoid membranes, was most susceptible to photoinhibition. Detailed investigation of photoinhibition in the triple mutant revealed that the rate of photoinactivation of PSII was the same in wild-type and mutant plants. However, the recovery of photoinactivated PSII was slower in fad3-2 fad7-2 fad8, relative to wild type, at all temperatures below 27 degrees C. These results indicate that trienoic fatty acids of thylakoid membrane lipids are required for low-temperature recovery from photoinhibition in Arabidopsis.

A critical role for the Var2 FtsH homologue of Arabidopsis thaliana in the photosystem II repair cycle in vivo

Using a var2-2 mutant of Arabidopsis thaliana, which lacks a homologue of the zinc-metalloprotease, FtsH, we demonstrate that this protease is required for the efficient turnover of the D1 polypeptide of photosystem II and protection against photoinhibition in vivo. We show that var2-2 leaves are much more susceptible to light-induced photosystem II photoinhibition than wild-type leaves. Furthermore, the rate of photosystem II photoinhibition in untreated var2-2 leaves is equivalent to that of var2-2 and wild-type leaves, which have been treated with lincomycin, an inhibitor of the photosystem II repair cycle at the level of D1 synthesis. This is in contrast to untreated wild-type leaves, which show a much slower rate of photosystem II photoinhibition due to an efficient photosystem II repair cycle. The recovery of var2-2 leaves from photosystem II photoinhibition is also impaired relative to wild-type. Using Western blot analysis in the presence of lincomycin we show that the D1 polypeptide remains stable in leaves of the var2-2 mutant under photoinhibitory conditions that lead to D1 degradation in wild-type leaves and that the abundance of DegP2 is not affected by the var2-2 mutation. We conclude, therefore, that the Var2 FtsH homologue is required for the cleavage of the D1 polypeptide in vivo. In addition, we identify a conserved lumenal domain in Var2 that is unique to FtsH homologues from oxygenic phototrophs.

Action of an algicide from a cyanobacterium, Oscillatoria laetevirens, on photosystem II

Oscillatoria laetevirens produces an algicide, named oscillatorin (OS), which inhibits growth of higher plants. Effect of purified oscillatorin and some 'urea-triazine type' herbicides was studied on photosystem II activity and composition of pigment protein complex in spinach thylakoid membrane. For oscillatorin the I50 at 10 microg chlorophyll concentration, inhibitor constant (Ki), specific binding sites and Hill coefficient were calculated to be 1.45, 0.15, 2.3 and 0.2 microM respectively. Metribuzin and oscillatorin affected towards the donor side and brought about identical changes in polypeptide composition of PSII complex. Further, metribuzin and atrazine exerted antagonistic and synergistic responses on oscillatorin action. Some of these parameters were also studied on weed plants to assess upon the weedicidal potential of oscillatorin.

Modulation of photosystem II chlorophyll fluorescence by electrogenic events generated by photosystem I

In an attempt to uncover electric field interactions between PS I and PS II during their functioning, fluorescence induction curves were measured on hydroxylamine-treated thylakoids of Chenopodium album under conditions ensuring low and high levels of photogenerated membrane potentials. In parallel experiments with Peperomia metallica chloroplasts, the photocurrents were measured with patch-clamp electrodes and served as indicator of electrogenic activity of thylakoid membranes in continuous light. Inhibition of linear electron flow at PS II donor side by hydroxylamine (0.1 mM) eliminated a slow rise of chlorophyll fluorescence to a peak level and suppressed photoelectrogenesis. Activation of PS I-dependent electron transport using cofactors of either cyclic (phenazine methosulfate) or noncyclic electron transport (reduced TMPD or DCPIP in combination with methyl viologen) restored photoelectrogenesis in hydroxylamine-treated chloroplasts and led to reappearance of slow components in the fluorescence induction curve. Exposure of thylakoids to valinomycin reduced the peak fluorescence in the presence of KCl but not in the absence of KCl. Combined application of valinomycin and nigericin in the presence of KCl exerted stronger suppression of fluorescence than valinomycin alone but was ineffective in the absence of KCl. In samples treated with hydroxylamine and PS I cofactors (DCPIP/ascorbate and methyl viologen), preillumination with a single-turnover flash or a multiturnover pulse shifted the induction curves of both membrane potential and chlorophyll fluorescence to shorter times, which confirms the supposed influence of PS I-generated electrical field on PS II fluorescence. A model is presented that describes modulating effect of the membrane potential on chlorophyll fluorescence and roughly simulates the fluorescence induction curves measured at low and high membrane potentials.

QSAR study of 1,8-naphthyridin-4-ones as inhibitors of photosystem II

The effect of substituents on in vitro activity of 20 1,8-naphthyridin-4-ones, a novel class of photosystem II inhibitors, was studied. A four-parameter QSAR model based on the molecular connectivity indices was developed which accounts for about 87% of the variations in inhibitory potencies of these compounds. The model suggests that the position, size, and polarity of substituents are factors that predominantly control their activity. By using physicochemical constants, a quantitative model for both 1,8-naphthyridin-4-ones and structurally related 2-trifluoromethyl-4-hydroxyquinoline derivatives was proposed.

The properties of the chlorophyll a/b-binding proteins Lhca2 and Lhca3 studied in vivo using antisense inhibition

The specific functions of the light-harvesting proteins Lhca2 and Lhca3 were studied in Arabidopsis ecotype Colombia antisense plants in which the proteins were individually repressed. The antisense effect was specific in each plant, but levels of Lhca proteins other than the targeted products were also affected. The contents of Lhca1 and Lhca4 were unaffected, but Lhca3 (in Lhca2-repressed plants) was almost completely depleted, and Lhca2 decreased to about 30% of wild-type levels in Lhca3-repressed plants. This suggests that the Lhca2 and Lhca3 proteins are in physical contact with each other and that they require each other for stability. Photosystem I fluorescence at 730 nm is thought to emanate from pigments bound to Lhca1 and Lhca4. However, fluorescence emission and excitation spectra suggest that Lhca2 and Lhca3, which fluoresce in vitro at 680 nm, also could contribute to far-red fluorescence in vivo. Spectral forms with absorption maxima at 695 and 715 nm, apparently with emission maxima at 702 and 735 nm, respectively, might be associated with Lhca2 and Lhca3.

Quinolones and their N-oxides as inhibitors of photosystem II and the cytochrome b(6)/f-complex

4(1H)-quinolones (2-alkyl- (1), 2-alkyl-3-methyl- (2), 2-methyl-3-alkyl- (3), 1-hydroxy-2-methyl-3-alkyl- (4) and 1-hydroxy-2-alkyl- (5)) with n-alkyl side chains varying from C(5) to C(17) have been synthesized and tested for biological activity in photosystem II and the cytochrome b(6)/f-complex. In photosystem II, quinolones 1 and 2 showed only moderate activity, whereas 3<5<4 (increasing activity) were potent inhibitors. Displacement experiments with [(14)C]atrazine indicated that the quinolones share an identical binding site with other photosystem II commercial herbicides. In the cytochrome b(6)/f-complex, only 3<4 showed enhanced activity. Maximal inhibitory potency was achieved at a carbon chain length of 12-14 A. Further increase of the chain length decreased activity. In a quantitative structure-activity relationship inhibitory activity in photosystem II and the cytochrome b(6)/f-complex could be correlated to the physicochemical parameters lipophilicity pi and/or to STERIMOL L.

Inhibition of photosystems I and II and enhanced back flow of photosystem I electrons in cucumber leaf discs chilled in the light

Pre-illumination of cucumber leaf discs at a chilling temperature in low-irradiance white light resulted in accelerated re-reduction of P700(+) [the special Chl pair in the photosystem I (PSI) reaction centre] when the far-red measuring light was turned off. Measurements (in +/- methyl viologen or +/- DCMU conditions) of the re-reduction half time suggest that accelerated re-reduction of P700(+) appeared to be predominantly due to charge recombination and only partly due to reductants sustained by previous cyclic electron flow around PSI. Apparently, charge recombination in PSI was greatly enhanced by inhibition of forward, linear electron flow. Inhibition of PSII electron transport was observed to occur to a lesser extent than that of PSI, but only if the measurement of PSII functionality was free from complications due to downstream accumulation of electrons in pools. We suggest that promotion of controlled charge recombination and cyclic electron flow round PSI during chilling of leaves in the light may partly prevent further damage to both photosystems.

Role of the core region of the PufX protein in inhibition of reconstitution of the core light-harvesting complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus

PufX, the protein encoded by the pufX gene of Rhodobacter capsulatus and Rhodobacter sphaeroides, has been further characterized. The mature forms of these proteins contain 9 and 12 fewer amino acids, respectively, at the C-terminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconstitution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of Rb. sphaeroides and Rb. capsulatus PufX were chemically synthesized. Neither the N- nor C-terminal polypeptides of Rb. sphaeroides were inhibitory to LH1 reconstitution. However, all core segments were active, causing 50% inhibition at a concentration ratio of between 3:1 and 6:1 relative to the LH1 alpha-polypeptides whose concentrations were 3-4 microM. CD measurements indicated that the core segment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha-helix in trifluoroethanol while the core segment containing 43 amino acids of Rb. capsulatus PufX exhibited 59 and 55% alpha-helix in trifluoroethanol and in 0.80% octylglucoside in water, respectively. Approximately 50% alpha-helix was also indicated by a PHD (Burkhard-Rost) structure prediction. Binding of bacteriochlorophyll to these PufX core segments is implicated.

Effects of tail-like substituents on the binding of competitive inhibitors to the Q(B) site of photosystem II

The QB quinone-binding site of photosystem II is an important target for herbicides. Two major classes of herbicides are based on s-triazine and phenylurea moieties. A small library of triazine and phenylurea compounds has been synthesized which have tail-like substituents in order to test the effects of charge, hydrophobicity and size of the tail on binding properties. It is found that a tail can be attached to one of the alkylamino groups of triazine-type herbicides or to the para position of phenylurea-type herbicides without loss of binding, provided that the tail is hydrophobic. This indicates that the herbicides must be oriented in the QB site such that these positions point toward the natural isoprenyl tail-binding pocket that extends out of the Q(B) site. In turn, the requirement that the tail must extend out of the QB site constrains the size of the other herbicide substituents in the pocket. This is in agreement with the presumed orientation and fit of ligands in the QB site. When longer hydrophobic tails are used, the binding penalty that occurs upon adding a charged substituent at the distal end is reduced. This allows the use of a series of tail substituents possessing a distal charge as an approximate molecular ruler to measure the distance from the QB site to the aqueous phase. Even a 10-carbon alkyl chain still shows a 4-fold effect from the presence or absence of a distal charge. Such a chain does not appear to be long enough to extend from the bulk aqueous phase to the QB site because binding is completely lost when a large hydrophilic domain (PEG(4000)) is attached to the distal end. Longer tails are effective only if they are sufficiently hydrophobic. An effort was made to use tailed herbicides for affinity binding of photosystem II. It was found that hydrophobic linkers promote nonspecific binding, but careful choice of solvent conditions, such as the use of excess nonionic detergent well above its critical micelle concentration, might obviate this problem during affinity-binding applications.

Heterocyclic ortho-quinones, a novel type of Photosystem II inhibitors

Members of the new chemical class of 7-substituted 6-bromo-benzo[4,5]imidazo[1,2alpha]pyridin-8,9-diones were found to be excellent inhibitors at the Q(B) site of the photosystem II D1 reaction center protein. The best inhibitors with pI(50)-values of >7 are: dimethyl-propyl, 7.05; i-pentyl, 7.36; t. butyl, 7.47; and i-propyl, 7.51. Displacement experiments with [14C]atrazine revealed that the 8,9-diones behave non-competitively in respect of Photosystem II herbicides and, hence, have to be considered as a new type of Photosystem II inhibitors. This notion is further corroborated by their inhibitory activity in D1 mutants of Chlamydomonas reinhardtii.

Cl- channel inhibitors of the arylaminobenzoate type act as photosystem II herbicides: a functional and structural study

The Cl- channel blocker NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) inhibited photosynthetic oxygen evolution of isolated thylakoid membranes in a pH-dependent manner with a K(i) of about 2 microM at pH 6. Applying different electron acceptors, taking electrons either directly from photosystem II (PS II) or photosystem I (PS I), the site of inhibition was localized within PS II. Measurements of fluorescence induction kinetics and thermoluminescence suggest that the binding of NPPB to the QB binding site of PS II is similar to the herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). The effects of different arylaminobenzoate derivatives and other Cl- channel inhibitors on photosynthetic electron transport were investigated. The structure--activity relationship of the inhibitory effect on PS II shows interesting parallels to the one observed for the arylaminobenzoate block of mammalian Cl- channels. A molecular modeling approach was used to fit NPPB into the QB binding site and to identify possible molecular interactions between NPPB and the amino acid residues of the binding site in PS II. Taken together, these data give a detailed molecular picture of the mechanism of NPPB binding.

Equilibrium and kinetic parameters for the binding of inhibitors to the QB pocket in bacterial chromatophores: dependence on the state of QA

The equilibrium and kinetic parameters for the binding of various inhibitors to the Q(B) pocket of the bacterial reaction center were investigated in chromatophores from Rhodobacter capsulatus and Rhodobacter sphaeroides. By monitoring the near-IR absorption changes specific to Q(A)(-) and Q(B)(-), we measured the fraction of inhibited centers in the dark and the kinetics and extent of inhibitor displacement after one flash due to the formation of the Q(A)Q(B)(-) state. The inhibitor release rate was much faster for triazines and o-phenanthroline (t(1/2) in the 50 ms to 1 s range) than for stigmatellin (t(1/2) approximately 20 s). For inhibitors with a rapid release rate, the fast phase of P(+) decay observed in the absence of secondary donor reflects the competition between P(+)Q(A)(-) recombination and inhibitor release: it is thus faster than the P(+)Q(A)(-) recombination, and its relative extent is smaller than the fraction of initially inhibited centers. At appropriate inhibitor concentrations, one can have almost total binding in the dark and almost total inhibitor displacement after one flash. Under such conditions, a pair of closely spaced flashes resets the two-electron gate in a single state (Q(A)Q(B)(-)), irrespective of the initial state. The apparent dissociation constant of terbutryn was significantly increased (by a factor of 4-7) in the presence of Q(A)(-), in agreement with the conclusion of Wraight and co-workers [Stein, R. R., et al. (1984) J. Cell. Biochem. 24, 243-259]. We suggest that this effect is essentially due to a tighter binding of ubiquinone in the Q(A)(-) state.

Retardation of proton transfer caused by binding of the transition metal ion to the bacterial reaction center is due to pKa shifts of key protonatable residues

Transition metal ions bind to the reaction center (RC) protein of the photosynthetic bacterium Rhodobacter sphaeroides and slow the light-induced electron and proton transfer to the secondary quinone, Q(B). We studied the properties of the metal ion-RC complex by measuring the pH dependence of the dissociation constant and the stoichiometry of proton release upon ligand formation. We investigated the mechanism of inhibition by measuring the stoichiometry and kinetics of flash-induced proton binding, the transfer of (first and second) electrons to Q(B), and the rate of steady-state turnover of the RC in the absence and presence of Cd(2+) and Ni(2+) on a wide pH range. The following results were obtained. (1) The complexation of transition metal ions Cd(2+) and Ni(2+) with the bacterial RC showed strong pH dependence. This observation was explained by different (pH-dependent) states of the metal-ligand cluster: the complex formation was strong when the ligand (Asp and His residues) was deprotonated and was much weaker if the ligand was partly (or fully) protonated. A direct consequence of the model was the pH-dependent proton release upon complexation. (2) The retardation of transfer of electrons and protons to Q(B) was also strongly pH-dependent. The effect was large in the neutral pH range and decreased toward the acidic and alkaline pH values. (3) Steady-state turnover measurements indicated that the rate of the second proton transfer was much less inhibited than that of the first one, which became the rate-limiting step in continuous turnover of the RC. (4) Sodium azide partly recovered the proton transfer rate. The effect is not due to removal of the bound metal ion by azide but probably by formation of a proton-transporting azide network similarly as water molecules may build up proton pathways. (5) We argue that the inhibition comes mainly from pK(a) shifts of key protonatable residues that control the proton transfer along the H-bond network to Q(B). The electrostatic interaction between the metal ion and these residues may result in acidic pK(a) shifts between 1.5 and 2.0 that account for the observed retardation of the electron and proton transfer.

Iron stress restricts photosynthetic intersystem electron transport in Synechococcus sp. PCC 7942

Although exposure of Synechococcus sp. PCC 7942 to iron stress induced the accumulation of the isiA gene product (CP43') compared with non-stressed controls, immunodetection of the N-terminus of cytochrome (Cyt) f indicated that iron stress not only reduced the content of the 40 kDa, heme-binding, Cyt f polypeptide by 32% but it also specifically induced the accumulation of a new, 23 kDa, non-heme-binding, putative Cyt f polypeptide. Concomitantly, iron stress restricted intersystem electron transport based on the in vivo reduction of P700(+), monitored as delta A(820)/A(820) in the presence and absence of electron transport inhibitors, as well as the inhibition of the Emerson enhancement effect on O(2) evolution. However, iron stress appeared to be associated with enhanced rates of PS I cyclic electron transport, low rates of PS I-driven photoreduction of NADP(+) but comparable rates for PS II+PS I photoreduction of NADP(+) relative to controls. We hypothesize that Synechococcus sp. PCC 7942 exhibits a dynamic capacity to uncouple PS II and PS I electron transport, which may allow for the higher than expected growth rates observed during iron stress.

An overnight chill induces a delayed inhibition of photosynthesis at midday in mango (Mangifera indica L.)

The effect of a cold night on photosynthesis in herbaceous chilling-sensitive crops, like tomato, has been extensively studied and is well characterized. This investigation examined the behaviour of the sub-tropical fruit tree, mango, to enable comparison with these well-studied systems. Unlike tomato, chilling between 5 degrees C and 7 degrees C overnight produced no significant inhibition of light-saturated CO(2) assimilation (A:) during the first hours following rewarming, measured either under controlled environment conditions or in the field. By midday, however, there was a substantial decline in A:, which could not be attributed to photoinhibition of PSII, but rather was associated with an increase in stomatal limitation of A: and lower Rubisco activity. Overnight chilling of tomato can cause severe disruption in the circadian regulation of key photosynthetic enzymes and is considered to be a major factor underlying the dysfunction of photosynthesis in chilling-sensitive herbaceous plants. Examination of the gas exchange of mango leaves maintained under constant conditions for 2 d, demonstrated that large depressions in A: during the subjective night were primarily the result of stomatal closure. Chilling did not disrupt the ability of mango leaves to produce a circadian rhythm in stomatal conductance. Rather, the midday increase in stomatal limitation of A: appeared to be the result of altered guard cell sensitivity to CO(2) following the dark chill.

Pattern recognition analysis of endogenous cell metabolites for high throughput mode of action identification: removing the postscreening dilemma associated with whole-organism high throughput screening

Although whole-organism HTS can give clear indications of in vivo activity, typically few clues are given as to the mechanism of action (MOA), and determining the MOA for large numbers of active compounds can be costly and complex-an alternative approach is required. This report demonstrates that it is possible to conduct relatively high throughput MOA characterization of HTS hits utilizing a single sample preparation and analytical method. By monitoring a wide range of endogenous cellular metabolites via (1)H nuclear magnetic resonance spectroscopy, the MOA of herbicides can be predicted using computational methods to compare the metabolite perturbation patterns. Herbicides that induce a characteristic pattern of metabolic perturbation in maize include inhibitors of acetolactate synthase, acetyl co-enzyme A carboxylase, protoporphyrinogen oxidase, 5-enolpyruvylshikimate-3-phosphate synthase, and phytoene desaturase. In soya, photosystem II inhibitors can also be detected, further demonstrating that this method is not limited to inhibitors of enzymes that directly act upon endogenous metabolites, or a single species. The methods, including data analysis, can be readily automated, enabling relatively high throughput MOA elucidation of whole-organism screen hits. Additionally, for compounds with a novel MOA, this approach may lead to MOA identification faster than traditional methods. It is envisaged that application of these data analysis methods to other data types-for example, transcription (mRNA) or translation (protein) profiles-is likely to permit higher throughput with smaller sample requirements, along with ability to discriminate MOAs that are not adequately discriminated based upon endogenous metabolite profiles.

Inhibition of photosynthetic electron transport by UV-A radiation targets the photosystem II complex

We have studied the inhibition of photosynthetic electron transport by UV-A (320-400 nm) radiation in isolated spinach thylakoids. Measurements of Photosystem II (PSII) and Photosystem I activity by Clark-type oxygen electrode demonstrated that electron flow is impaired primarily in PSII. The site and mechanism of UV-A induced damage within PSII was assessed by flash-induced oxygen and thermoluminescence (TL) measurements. The flash pattern of oxygen evolution showed an increased amount of the S0 state in the dark, which indicate a direct effect of UV-A in the water-oxidizing complex. TL measurements revealed the UV-A induced loss of PSII centers in which charge recombination between the S2 state of the water oxidizing complex and the semireduced Q(A)- and Q(B)- quinone electron acceptors occur. Flash-induced oscillation of the B TL band, originating from the S2Q(B)- recombination, showed a decreased amplitude after the second flash relative to that after the first one, which is consistent with a decrease in the amount of Q(B)- relative to Q(B) in dark adapted samples. The efficiency of UV-A light in inhibiting PSII electron transport exceeds that of visible light 45-fold on the basis of equal energy and 60-fold on the basis of equal photon number, respectively. In conclusion, our data show that UV-A radiation is highly damaging for PSII, whose electron transport is affected both at the water oxidizing complex, and the binding site of the Q(B) quinone electron acceptor in a similar way to that caused by UV-B radiation.

The natural product capsaicin inhibits photosynthetic electron transport at the reducing side of photosystem II and purple bacterial reaction center: structural details of capsaicin binding

Capsaicin, a natural quinone analog, was found to block electron transport, in both plant photosystem II (PSII) and bacterial reaction center (RC) from Rhodobacter sphaeroides, at the QB site. The mode of action of capsaicin was investigated by O2 evolution measurements and fluoresence induction studies in the case of PSII, and flash-induced absorbance spectroscopy in the case of the bacterial RC. Structural details of capsaicin binding to the bacterial RC complex were determined by X-ray crystallographic analysis.

Evidence for the presence of a component of the Mn complex of the photosystem II reaction centre which is exposed to water in the S(2) state of the water oxidation complex

The interaction of water oxidising photosystem II preparations with the aqueous environment has been investigated using electron spin echo envelope modulation spectroscopy in the presence of 2H(2)O. The spectra show interaction of 2H of 2H(2)O with the preparation in the S(2) state. The component interacting with water decays during 1-4 weeks storage at 77 K. No interaction of water with the classical multiline S(2) Mn signal, which is more stable on storage at 77 K, was detected. The results show that a component of the water oxidation complex, possibly involving the Mn centre, is accessible to water and may be the water binding site for photosynthetic water oxidation.

Ionic and osmotic effects of NaCl-induced inactivation of photosystems I and II in Synechococcus sp

We report here that osmotic effects and ionic effects are both involved in the NaCl-induced inactivation of the photosynthetic machinery in the cyanobacterium Synechococcus sp. PCC 7942. Incubation of the cyanobacterial cells in 0.5 M NaCl induced a rapid and reversible decline and subsequent slow and irreversible loss of the oxygen-evolving activity of photosystem (PS) II and the electron transport activity of PSI. An Na(+)-channel blocker protected both PSII and PSI against the slow, but not the rapid, inactivation. The rapid decline resembled the effect of 1.0 M sorbitol. The presence of both an Na(+)-channel blocker and a water-channel blocker protected PSI and PSII against the short- and long-term effects of NaCl. Salt stress also decreased cytoplasmic volume and this effect was enhanced by the Na(+)-channel blocker. Our observations suggested that NaCl had both osmotic and ionic effects. The osmotic effect decreased the amount of water in the cytosol, rapidly increasing the intracellular concentration of salts. The ionic effect was caused by an influx of Na(+) ions through potassium/Na(+) channels that also increased concentrations of salts in the cytosol and irreversibly inactivated PSI and PSII.

Formate-induced inhibition of the water-oxidizing complex of photosystem II studied by EPR

The effects of various formate concentrations on both the donor and the acceptor sides in oxygen-evolving PS II membranes (BBY particles) were examined. EPR, oxygen evolution and variable chlorophyll fluorescence have been observed. It was found that formate inhibits the formation of the S(2) state multiline signal concomitant with stimulation of the Q(A)(-)Fe(2+) signal at g = 1.82. The decrease and the increase in intensities of the multiline and Q(A)(-)Fe(2+) signals, respectively, had a linear relation for formate concentrations between 5 and 500 mM. The g = 4.1 signal formation measured in the absence of methanol was not inhibited by formate up to 250 mM in the buffer. In the presence of 3% methanol the g = 4.1 signal evolved as formate concentration increased. The evolved signal could be ascribed to the inhibited centers. Oxygen evolution measured in the presence of an electron acceptor, phenyl-p-benzoquinone, was also inhibited by formate proportionally to the decrease in the multiline signal intensity. The inhibition seemed to be due to a retarded electron transfer from the water-oxidizing complex to Y(Z)(+), which was observed in the decay kinetics of the Y(Z)(+) signal induced by illumination above 250 K. These results show that formate induces inhibition of water oxidation reactions as well as electron transfer on the PS II acceptor side. The inhibition effects of formate in PS II were found to be reversible, indicating no destructive effect on the reaction center induced by formate.

Inactivation of photosystems I and II in response to osmotic stress in Synechococcus. Contribution of water channels

The effects of osmotic stress due to sorbitol on the photosynthetic machinery were investigated in the cyanobacterium Synechococcus R-2. Incubation of cells in 1.0 M sorbitol inactivated photosystems I and II and decreased the intracellular solute space by 50%. These effects of sorbitol were reversible: Photosynthetic activity and cytoplasmic volume returned to the original values after removal of the osmotic stress. A blocker of water channels prevented the osmotic-stress-induced inactivation and shrinkage of the intracellular space. It also prevented the recovery of photosynthetic activity and cytoplasmic volume when applied just before release from osmotic stress. Inhibition of protein synthesis by lincomycin had no significant effects on the inactivation and recovery processes, an observation that suggests that protein synthesis was not involved in these processes. Our results suggest that osmotic stress decreased the amount of water in the cytoplasm via the efflux of water through water channels (aquaporins), with resultant increases in intracellular concentrations of ions and a decrease in photosynthetic activity.

Determination of the binding sites of the proton transfer inhibitors Cd2+ and Zn2+ in bacterial reaction centers

The reaction center (RC) from Rhodobacter sphaeroides couples light-driven electron transfer to protonation of a bound quinone acceptor molecule, Q(B), within the RC. The binding of Cd(2+) or Zn(2+) has been previously shown to inhibit the rate of reduction and protonation of Q(B). We report here on the metal binding site, determined by x-ray diffraction at 2.5-A resolution, obtained from RC crystals that were soaked in the presence of the metal. The structures were refined to R factors of 23% and 24% for the Cd(2+) and Zn(2+) complexes, respectively. Both metals bind to the same location, coordinating to Asp-H124, His-H126, and His-H128. The rate of electron transfer from Q(A)(-) to Q(B) was measured in the Cd(2+)-soaked crystal and found to be the same as in solution in the presence of Cd(2+). In addition to the changes in the kinetics, a structural effect of Cd(2+) on Glu-H173 was observed. This residue was well resolved in the x-ray structure-i.e., ordered-with Cd(2+) bound to the RC, in contrast to its disordered state in the absence of Cd(2+), which suggests that the mobility of Glu-H173 plays an important role in the rate of reduction of Q(B). The position of the Cd(2+) and Zn(2+) localizes the proton entry into the RC near Asp-H124, His-H126, and His-H128. Based on the location of the metal, likely pathways of proton transfer from the aqueous surface to Q(B) are proposed.

Homology model directed alignment selection for comparative molecular field analysis: application to photosystem II inhibitors

The use of a computational docking protocol in conjunction with a protein homology model to derive molecular alignments for Comparative Molecular Field Analysis (CoMFA) was examined. In particular, the DOCK program and a model of the herbicidal target site, photosystem II (PSII), was used to derive alignments for two PSII inhibitor training sets, a set of benzo- and napthoquinones and a set of butenanilides. The protein design software in the QUANTA molecular modeling package was used to develop a homology model of spinach PSII based on the reported amino acid sequence and the X-ray crystal structure of the purple bacterium reaction center. The model is very similar to other reported PSII protein homology models. DOCK was then used to derive alignments for CoMFA modeling by docking the inhibitors in the PSII binding pocket. The molecular alignments produced from docking yielded highly predictive CoMFA models. As a comparison, the more traditional atom-atom alignments of the same two training sets failed to produce predictive CoMFA models. The general utilities of this application for homology model refinement and as an alternative scoring method are discussed.

Tolerance of Oscillatoria limnetica Lemmermann to atrazine in natural phytoplankton populations and in pure culture: influence of season and temperature

The responses of algae to herbicides depend on the sensitivity of each species, but competition within algal communities may be an important regulator of the effects of herbicides on aquatic systems. The impact of herbicides on algae also depends on abiotic factors like nutrients, light, and temperature. We examined the tolerance of the alga Oscillatoria limnetica Lemmermann to the photosystem II inhibitor herbicide atrazine under different culture conditions to assess those interactions between herbicides and biotic and abiotic factors. The density of the cyanobacterium O. limnetica was determined in natural phytoplanktonic assemblages and in unialgal cultures in medium containing 10 microg/L atrazine. Experiments (total of 13) were conducted in spring and early and late summer, during which the effect of atrazine varied in nature and intensity. The growth of the cyanobacterium was always inhibited in cold experiments, whereas it was stimulated in warm experiments within the natural phytoplankton assemblage, but unaffected in pure culture. Laboratory experiments with unialgal culture showed that the sensitivity to atrazine increased as the temperature decreased. Phytoplankton community structure, interactions between species, and environmental parameters (e.g., temperature) are important factors controlling the responses of cyanobacteria to the herbicide. These interactions between sensitivity to herbicides and environmental factors may reduce or emphasize the effects of pollution in aquatic systems. Thus, the ecotoxicological relevance of herbicides in aquatic systems is quite complex and cannot be assessed by single-species short-term laboratory toxicological tests.http://link. springer-ny.com/link/service/journals/00244/bibs/37n4p472.++ +html</HEA

Light-induced degradation of cytochrome b559 during photoinhibition of the photosystem II reaction center

The behaviour of cytochrome (cyt) b559 during acceptor- and donor-side photoinhibition has been investigated in oxygen-evolving and non-evolving photosystem II (PSII) membranes. Strong illumination at 20 degrees C under aerobiosis induced a strong decrease in the absorbance of the cyt b559 alpha-band in the two preparations. This absorbance decline was observed only in non-oxygen-evolving PSII samples when illumination was performed under aerobiosis but at 4 degrees C, or under anaerobiosis at 20 degrees C. These results suggest that acceptor-side photoinhibition induces the degradation of cyt b559 by a mechanism related to an enzymatic reaction mediated by singlet oxygen. Donor-side photoinhibition may induce, however, a non-enzymatic photocleavage of the protein.

Voltammetric detection of superoxide production by photosystem II

Oxygen radicals play both pathological and physiological roles in biological systems. The detection of such radicals is difficult due to their transient nature and the presence of highly efficient antioxidant mechanisms. In plants the physiological role of oxygen is twofold, oxygen is produced by the oxidation of water and consumed as an electron acceptor. The direct involvement of oxygen in photosynthetic events exposes the photosynthetic apparatus to a high probability of damage by oxygen radicals. We report here a direct, simple and rapid method for the measurement of superoxide in vitro based on voltammetric detection. It has potential applications for other in vitro systems investigating superoxide production. We show that in addition to the well established production of superoxide from photosystem I, under reducing conditions superoxide is also produced by photosystem II, probably from the Q(A) site.

The violaxanthin cycle protects plants from photooxidative damage by more than one mechanism

When light energy absorbed by plants becomes excessive relative to the capacity of photosynthesis, the xanthophyll violaxanthin is reversibly deepoxidized to zeaxanthin (violaxanthin cycle). The protective function of this phenomenon was investigated in a mutant of Arabidopsis thaliana, npq1, that has no functional violaxanthin deepoxidase. Two major consequences of the npq1 mutation are the absence of zeaxanthin formation in strong light and the partial inhibition of the quenching of singlet excited chlorophylls in the photosystem II light-harvesting complexes. Prolonged exposure of whole plants to bright light resulted in a limited photoinhibition of photosystem II in both npq1 and wild-type leaves, although CO(2) fixation and the linear electron transport in npq1 plants were reduced substantially. Lipid peroxidation was more pronounced in npq1 compared with the wild type, as measured by chlorophyll thermoluminescence, ethane production, and the total hydroperoxy fatty acids content. Lipid peroxidation was amplified markedly under chilling stress, and photooxidative damage ultimately resulted in leaf bleaching and tissue necrosis in npq1. The npq4 mutant, which possesses a normal violaxanthin cycle but has a limited capacity of quenching singlet excited chlorophylls, was rather tolerant to lipid peroxidation. The double mutant, npq4 npq1, which differs from npq4 only by the absence of the violaxanthin cycle, exhibited an increased susceptibility to photooxidative damage, similar to that of npq1. Our results demonstrate that the violaxanthin cycle specifically protects thylakoid membrane lipids against photooxidation. Part of this protection involves a mechanism other than quenching of singlet excited chlorophylls.

Remarkable affinity and selectivity for Cs+ and uranyl (UO22+) binding to the manganese site of the apo-water oxidation complex of photosystem II

The size and charge density requirements for metal ion binding to the high-affinity Mn2+ site of the apo-water oxidizing complex (WOC) of spinach photosystem II (PSII) were studied by comparing the relative binding affinities of alkali metal cations, divalent metals (Mg2+, Ca2+, Mn2+, Sr2+), and the oxo-cation UO22+. Cation binding to the apo-WOC-PSII protein was measured by: (1) inhibition of the rate and yield of photoactivation, the light-induced recovery of O2 evolution by assembly of the functional Mn4Ca1Clx, core from its constituent inorganic cofactors (Mn2+, Ca2+, and Cl-); and by (2) inhibition of the PSII-mediated light-induced electron transfer from Mn2+ to an electron acceptor (DCIP). Together, these methods enable discrimination between inhibition at the high- and low-affinity Mn2+ sites and the Ca2+ site of the apo-WOC-PSII. Unexpectedly strong binding of large alkali cations (Cs+ >> Rb+ > K+ > Na+ > Li+) was found to smoothly correlate with decreasing cation charge density, exhibiting one of the largest Cs+/Li+ selectivities (>/=5000) for any known chelator. Both photoactivation and electron-transfer measurements at selected Mn2+ and Ca2+ concentrations reveal that Cs+ binds to the high-affinity Mn2+ site with a slightly greater affinity (2-3-fold at pH 6.0) than Mn2+, while binding about 10(4)-fold more weakly to the Ca2+-specific site required for reassembly of functional O2 evolving centers. In contrast to Cs+, divalent cations larger than Mn2+ bind considerably more weakly to the high-affinity Mn2+ site (Mn2+ >> Ca2+ > Sr2+). Their affinities correlate with the hydrolysis constant for formation of the metal hydroxide by hydrolysis of water: Me2+aq --> [MeOH]+aq + H+aq. Along with the strong stimulation of the rate of photoactivation by alkaline pH, these metal cation trends support the interpretation that [MnOH]+ is the active species that forms upon binding of Mn2+aq to apo-WOC. Further support for this interpretation is found by the unusually strong inhibition of Mn2+ photooxidation by the linear uranyl cation (UO22+). The intrinsic binding constant for [MnOH]+ to apo-WOC was determined using a thermodynamic cycle to be K = 4.0 x 10(15) M-1 (at pH 6.0), consistent with a high-affinity, preorganized, multidentate coordination site. We propose that the selectivity for binding [MnOH]+, a linear low charge-density monocation, vs symmetrical Me2+ dications is functionally important for assembly of the WOC by enabling: (1) discrimination against higher charge density alkaline earth cations (Mg2+ and Ca2+) and smaller alkali metal cations (Na+ and K+) that are present in considerably greater abundance in vivo, and thus would suppress photoactivation; and (2) higher affinity binding of the one Ca2+ ion or the remaining three Mn2+ ions via coordination to form mu-hydroxo-bridged intermediates, apo-WOC-[Mn(mu-OH)2Mn]3+ or apo-WOC-[Mn(mu-OH)Ca]3+, during subsequent assembly steps of the native Mn4Ca1Clx core. In contrast to more acidic Me2+ divalent ion inhibitors of the high-affinity Mn2+ site, like Ca2+ and Sr2+, Cs+ does not accelerate the decay of the first light-induced intermediate, IM1, formed during photoactivation (attributed to apo-WOC-[Mn(OH)2]+). The inability of Cs+ to promote decay of IM1, despite having comparable affinity as Mn2+, is consistent with its considerably weaker Lewis acidity, resulting in the reprotonation of IM1 by water becoming the rate-limiting step for decay prior to displacement of Mn2+. All four different lines of evidence provide a self-consistent picture indicating that the initial step in assembly of the WOC involves high-affinity binding of [MnOH]+.

Mutation of Phe-363 in the photosystem II protein CP47 impairs photoautotrophic growth, alters the chloride requirement, and prevents photosynthesis in the absence of either PSII-O or PSII-V in Synechocystis sp. PCC 6803

The deletion of the amino acids between Gly-351 and Thr-365 within the large, lumen-exposed, hydrophilic region (loop E) of the photosystem II (PSII) chlorophyll a-binding protein CP47 produced a strain of Synechocystis sp. PCC 6803 that failed to assemble stable PSII centers [Eaton-Rye, J. J., and Vermaas, W. F. J. (1991) Plant Mol. Biol. 17, 1165-1177]. The importance of two conserved Phe residues at positions 362 and 363 within this deletion has been investigated. The F363R strain had impaired photoautotrophic growth and an enhanced sensitivity to photoinactivation, demonstrating that Phe is required at position 363 for normal PSII function. In contrast, photoautotrophic growth in strains N361K and F362R was unaffected. Uniquely, among the mutant strains tested, F363R was unable to grow under chloride-limiting conditions, and this effect was reversed by replacing chloride with bromide. The removal of the manganese-stabilizing protein (PSII-O), the 12 kDa extrinsic protein (PSII-U), and cytochrome c-550 (PSII-V) was investigated in each mutant in vivo. In N361K and F362R, removal of PSII-V produced a more deleterious effect than the removal of PSII-O, but even so, all strains remained photoautotrophic. In contrast, the absence of PSII-V and PSII-O in F363R produced obligate photoheterotrophic strains. The removal of PSII-U increased the susceptibility of PSII to heat inactivation and further decreased the stability of PSII in F363R, demonstrating that PSII-U can contribute to the stabilization of mutations that have been introduced into CP47. The order of importance of the selective removal of the extrinsic proteins in strains carrying mutations in loop E of CP47 was found to be as follows: DeltaPSII-V >/= DeltaPSII-O > DeltaPSII-U.

Lipophilicity determination of some potential photosystem II inhibitors on reversed-phase high-performance thin-layer chromatography

The retention characteristics of 25 2-cyano-3-methylthio-3-substituted amine-acrylates are determined using reversed-phase thin-layer chromatography (RP-TLC) with methanol-water mixtures as eluents. The relationship between Rm values and partition coefficients (C log P) are established. The Rm values decrease linearly with increasing methanol concentration in the eluent. The Rm values extrapolated to zero organic modifier concentration (Rm0) in the eluent are highly related to C log P. The Rm0 value can be used to evaluate the lipophilicity of this kind of compound.