The herbicide binding niche of photosystem II-a model

GO promotes detoxification of nicosulfuron in sweet corn by enhancing photosynthesis, chlorophyll fluorescence parameters, and antioxidant enzyme activity

Although graphene oxide (GO) has extensive recognized application prospects in slow-release fertilizer, plant pest control, and plant growth regulation, the incorporation of GO into nano herbicides is still in its early stages of development. This study selected a pair of sweet corn sister lines, nicosulfuron (NIF)-resistant HK301 and NIF-sensitive HK320, and sprayed them both with 80 mg kg-1 of GO-NIF, with clean water as a control, to study the effect of GO-NIF on sweet corn seedling growth, photosynthesis, chlorophyll fluorescence, and antioxidant system enzyme activity. Compared to spraying water and GO alone, spraying GO-NIF was able to effectively reduce the toxic effect of NIF on sweet corn seedlings. Compared with NIF treatment, 10 days after of spraying GO-NIF, the net photosynthetic rate (A), stomatal conductance (Gs), transpiration rate (E), photosystem II photochemical maximum quantum yield (Fv/Fm), photochemical quenching coefficient (qP), and photosynthetic electron transfer rate (ETR) of GO-NIF treatment were significantly increased by 328.31%, 132.44%, 574.39%, 73.53%, 152.41%, and 140.72%, respectively, compared to HK320. Compared to the imbalance of redox reactions continuously induced by NIF in HK320, GO-NIF effectively alleviated the observed oxidative pressure. Furthermore, compared to NIF treatment alone, GO-NIF treatment effectively increased the activities of superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) in both lines, indicating GO induced resistance to the damage caused by NIF to sweet corn seedlings. This study will provides an empirical basis for understanding the detoxification promoting effect of GO in NIF and analyzing the mechanism of GO induced allogeneic detoxification in cells.

Silica Hydrogels as Entrapment Material for Microalgae

Despite being a promising feedstock for food, feed, chemicals, and biofuels, microalgal production processes are still uneconomical due to slow growth rates, costly media, problematic downstreaming processes, and rather low cell densities. Immobilization via entrapment constitutes a promising tool to overcome these drawbacks of microalgal production and enables continuous processes with protection against shear forces and contaminations. In contrast to biopolymer gels, inorganic silica hydrogels are highly transparent and chemically, mechanically, thermally, and biologically stable. Since the first report on entrapment of living cells in silica hydrogels in 1989, efforts were made to increase the biocompatibility by omitting organic solvents during hydrolysis, removing toxic by-products, and replacing detrimental mineral acids or bases for pH adjustment. Furthermore, methods were developed to decrease the stiffness in order to enable proliferation of entrapped cells. This review aims to provide an overview of studied entrapment methods in silica hydrogels, specifically for rather sensitive microalgae.

Recent Advances in the Pesticide Activities of 2-Cyanoacrylate Derivatives

2-Cyanoacrylates are a unique class of compounds with high photosystem II electron transport inhibitory activity. Through the in-depth study of molecular design, synthesis, biological activity, structure-activity relationship, and action mechanism, a large number of 2-cyanoacrylate derivatives with excellent herbicidal activity were found. In addition, 2-cyanoacrylate derivatives with different substituents have also been found to have good fungicidal or anti-plant virus activities. To further guide the design and development of new pesticide candidates, the representative research work of 2-cyanoacrylate derivatives with herbicidal, fungicidal, and anti-plant virus activities was reviewed.

Target-Site Mutations Conferring Herbicide Resistance

Mutations conferring evolved herbicide resistance in weeds are known in nine different herbicide sites of action. This review summarizes recently reported resistance-conferring mutations for each of these nine target sites. One emerging trend is an increase in reports of multiple mutations, including multiple amino acid changes at the glyphosate target site, as well as mutations involving two nucleotide changes at a single amino acid codon. Standard reference sequences are suggested for target sites for which standards do not already exist. We also discuss experimental approaches for investigating cross-resistance patterns and for investigating fitness costs of specific target-site mutations.

Do plants pay a fitness cost to be resistant to glyphosate?

We reviewed the literature to understand the effects of glyphosate resistance on plant fitness at the molecular, biochemical and physiological levels. A number of correlations between enzyme characteristics and glyphosate resistance imply the existence of a plant fitness cost associated with resistance-conferring mutations in the glyphosate target enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). These biochemical changes result in a tradeoff between the glyphosate resistance of the EPSPS enzyme and its catalytic activity. Mutations that endow the highest resistance are more likely to decrease catalytic activity by reducing the affinity of EPSPS for its natural substrate, and/or slowing the velocity of the enzyme reaction, and are thus very likely to endow a substantial plant fitness cost. Prediction of fitness costs associated with EPSPS gene amplification and overexpression can be more problematic. The validity of cost prediction based on the theory of evolution of gene expression and resource allocation has been cast into doubt by contradictory experimental evidence. Further research providing insights into the role of the EPSPS cassette in weed adaptation, and estimations of the energy budget involved in EPSPS amplification and overexpression are required to understand and predict the biochemical and physiological bases of the fitness cost of glyphosate resistance.

A novel psbA mutation (Phe274-Val) confers resistance to PSII herbicides in wild radish (Raphanus raphanistrum)

BACKGROUND

Wild radish (Raphanus raphanistrum) is a globally important weed of crops. Two atrazine-resistant wild radish populations (R1 and R2), collected from the Western Australia grain belt, were investigated for resistance to photosystem II (PSII) herbicides.

RESULTS

Sequencing of the full-length psbA gene revealed the well-known Ser264-Gly substitution in population R1, whereas population R2 displayed a novel Phe274-Val substitution. Herbicide dose-response studies confirmed that the population with the Ser264-Gly mutation exhibited high-level resistance to atrazine, but super-sensitivity to bromoxynil. Plants possessing the novel Phe274-Val mutation exhibited a modest level of resistance to atrazine, metribuzin and diuron, and were bromoxynil susceptible. Structural modelling of the mutant D1 proteins predicts that the Ser264-Gly mutation endows atrazine resistance by abolishing H-bonds, but confers bromoxynil super-sensitivity by enhancing hydrogen bonding. The Phe274-Val substitution provides resistance to atrazine and diuron by indirectly affecting H-bond formation between the Ser264 residue and the herbicides.

CONCLUSION

The results demonstrate that the Phe274-Val mutation is likely responsible for resistance to PSII-inhibiting triazine and urea herbicides. To our knowledge, this is the first evidence of the psbA Phe274-Val mutation in wild radish conferring resistance to PSII herbicides. © 2018 Society of Chemical Industry.

Extreme irgarol tolerance in an Ulva lactuca L. population on the Swedish west coast

The herbicide irgarol 1051 is commonly used on ship hulls to prevent growth of algae, but as a component of self-eroding paints it can also spread in the surrounding waters and affect non-target organisms. The effect of irgarol on settlement and growth of zoospores from the marine macro algae Ulva lactuca from the Gullmar fjord on the Swedish west coast was investigated in the present study. The zoospores were allowed to settle and grow in the presence of irgarol, but neither settlement - nor growth inhibition was observed at concentrations of up to 2000 nmol l(-1). This is between 10 and 100 times higher than effect concentrations reported earlier for algae. Irgarol also induced the greening effect (4-fold increase in chlorophyll a content) in the settled zoospore/germling population, typical for photosystem II inhibitors like irgarol. This study support previous findings that irgarol constitutes a selection pressure in the marine environment.

The nonheme iron in photosystem II

Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs QA and QB that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO2, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.

Design, synthesis, and biological activities of arylmethylamine substituted chlorotriazine and methylthiotriazine compounds

Heterocyclic rings were introduced into the core structure of s-triazine to design and synthesize a series of novel triazines containing arylmethylamino moieties. These compounds were characterized by using spectroscopic methods and elemental analysis. Their herbicidal, insecticidal, fungicidal, and antitumor activities were evaluated. Most of these compounds exhibited good herbicidal activity, especially against the dicotyledonous weeds, and compound F8 was almost at the same level as the control compound atrazine. Their structure-activity relationships were discussed. At the same time, some triazines had interesting fungicidal and insecticidal activities, of which F4 exhibited 100% efficacy against Puccinia triticina even at 20 ppm, and F5 showed Lepidopteran-specific activity in both leaf-piece and artificial diet assays. Moreover, these compounds showed antitumor activities against leukemia HL-60 cell line and lung adenocarcinoma A-549 cell line.

Growth rate of Pseudokirchneriella subcapitata exposed to herbicides found in surface waters in the Alqueva reservoir (Portugal): a bottom-up approach using binary mixtures

Previous work showed the existence of ecotoxicity of water samples from the Alqueva reservoir due to the presence of the herbicides atrazine, simazine, terbuthylazine and metolachlor. In the present study we examine the effects of these herbicides singly and as binary mixtures on the growth rate of the microalgae Pseudokirchneriella subcapitata. Usually, the toxicity of mixtures is evaluated in relation to the reference models CA (concentration addiction) and IA (independent action). In this study CA model was selected to evaluate the joint effects of s-triazine herbicides on the growth of algae due to their similar mode of action. Moreover, IA reference model was chosen to evaluate the joint toxicity of the chloroacetanilide metolachlor and the s-triazine herbicides due to their different mode of action. In this study dose ratio was the common deviation obtained on both reference models. In the binary mixtures between atrazine/simazine and atrazine/terbuthylazine the increase of the mixtures toxicity (synergism) was mainly due to atrazine. Also, in the binary mixture between atrazine and metolachlor, atrazine was responsible for the increase (synergism) of the mixture toxicity. In the cases of the binary mixtures between simazine/metolachlor and terbuthylazine/metolachlor, the increase of the mixtures toxicity (synergism) was detected when metolachlor was the herbicide dominant, and antagonism was detected when simazine and terbuthylazine were dominant in both mixtures. This study represents an important step to understand the interactions among herbicides detected previously in the waters of the Alqueva reservoir.

Structural and mechanistic investigations of photosystem II through computational methods

The advent of oxygenic photosynthesis through water oxidation by photosystem II (PSII) transformed the planet, ultimately allowing the evolution of aerobic respiration and an explosion of ecological diversity. The importance of this enzyme to life on Earth has ironically been paralleled by the elusiveness of a detailed understanding of its precise catalytic mechanism. Computational investigations have in recent years provided more and more insights into the structural and mechanistic details that underlie the workings of PSII. This review will present an overview of some of these studies, focusing on those that have aimed at elucidating the mechanism of water oxidation at the CaMn₄ cluster in PSII, and those exploring the features of the structure and dynamics of this enzyme that enable it to catalyse this energetically demanding reaction. This article is part of a Special Issue entitled: Photosystem II.

Differential proteome analysis of mature and germinated embryos of Araucaria angustifolia

Araucaria angustifolia is an endangered Brazilian native conifer tree. The aim of the present work was to identify differentially expressed proteins between mature and germinated embryos of A. angustifolia, using one and two dimensional gel electrophoresis approaches followed by protein identification by tandem mass spectrometry. The identities of 32 differentially expressed protein spots from two dimensional gel maps were successfully determined, including proteins and enzymes involved in storage mobilization such as the vicilin-like storage protein and proteases. A label free approach, based on spectral counts, resulted in detection of 10 and 14 mature and germinated enriched proteins, respectively. Identified proteins were mainly related to energetic metabolism pathways, translational processes, oxidative stress regulation and cellular signaling. The integrated use of both strategies permitted a comprehensive protein expression overview of changes in germinated embryos in relation to matures, providing insights into the this process in a recalcitrant seed species. Applications of the data generated on the monitoring and control of in vitro somatic embryos were discussed.

Structural basis of cyanobacterial photosystem II Inhibition by the herbicide terbutryn

Herbicides that target photosystem II (PSII) compete with the native electron acceptor plastoquinone for binding at the Q(B) site in the D1 subunit and thus block the electron transfer from Q(A) to Q(B). Here, we present the first crystal structure of PSII with a bound herbicide at a resolution of 3.2 Å. The crystallized PSII core complexes were isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The used herbicide terbutryn is found to bind via at least two hydrogen bonds to the Q(B) site similar to photosynthetic reaction centers in anoxygenic purple bacteria. Herbicide binding to PSII is also discussed regarding the influence on the redox potential of Q(A), which is known to affect photoinhibition. We further identified a second and novel chloride position close to the water-oxidizing complex and in the vicinity of the chloride ion reported earlier (Guskov, A., Kern, J., Gabdulkhakov, A., Broser, M., Zouni, A., and Saenger, W. (2009) Nat. Struct. Mol. Biol. 16, 334-342). This discovery is discussed in the context of proton transfer to the lumen.

Estimating the capability of different phytoplankton groups to adapt to contamination: herbicides will affect phytoplankton species differently

• Investigating the differential capacity of the response of phytoplankton to human-induced environmental forcing has become a key issue to understanding further the future repercussions on the functioning of aquatic ecosystems. • The initial tolerance to the widely dispersed herbicide simazine was measured in diverse phytoplankton species. An experimental ratchet system maintaining large populations of dividing cells (which ensures the occurrence of rare spontaneous mutations that confer adaptation) and a strong selection pressure (which ensures the preservation of such mutations within the population) was later applied to estimate the capability of different groups of phytoplankton to adapt to simazine. • Initially, simazine doses between 0.05 and 0.15 ppm were able to inhibit 100% growth in all the species tested. However, a significant increase in simazine resistance was achieved in all derived populations during the ratchet experiment. The differential capacity for simazine adaptation was observed among the different species. • The capacity of different species to adapt to simazine can be explained in relation to taxonomic group, ploidy, growth rate and habitat preference. Haploid populations of continental Chlorophyta showed the greatest capacity to adapt to simazine. By contrast, populations of Haptophyta of open ocean regions were the group least capable of adapting to the herbicide.

Phytoremediation potential of the novel atrazine tolerant Lolium multiflorum and studies on the mechanisms involved

Atrazine impact on human health and the environment have been extensively studied. Phytoremediation emerged as a low cost, environmental friendly biotechnological solution for atrazine pollution in soil and water. In vitro atrazine tolerance assays were performed and Lolium multiflorum was found as a novel tolerant species, able to germinate and grow in the presence of 1 mg kg(-1) of the herbicide. L. multiflorum presented 20% higher atrazine removal capacity than the natural attenuation, with high initial degradation rate in microcosms. The mechanisms involved in atrazine tolerance such as mutation in psbA gene, enzymatic detoxification via P(450) or chemical hydrolysis through benzoxazinones were evaluated. It was demonstrated that atrazine tolerance is conferred by enhanced enzymatic detoxification via P(450). Due to its atrazine degradation capacity in soil and its agronomical properties, L. multiflorum is a candidate for designing phytoremediation strategies for atrazine contaminated agricultural soils, especially those involving run-off avoiding.

Structure-based design of novel Chlamydomonas reinhardtii D1-D2 photosynthetic proteins for herbicide monitoring

The D1-D2 heterodimer in the reaction center core of phototrophs binds the redox plastoquinone cofactors, Q(A) and Q(B), the terminal acceptors of the photosynthetic electron transfer chain in the photosystem II (PSII). This complex is the target of the herbicide atrazine, an environmental pollutant competitive inhibitor of Q(B) binding, and consequently it represents an excellent biomediator to develop biosensors for pollutant monitoring in ecosystems. In this context, we have undertaken a study of the Chlamydomonas reinhardtii D1-D2 proteins aimed at designing site directed mutants with increased affinity for atrazine. The three-dimensional structure of the D1 and D2 proteins from C. reinhardtii has been homology modeled using the crystal structure of the highly homologous Thermosynechococcus elongatus proteins as templates. Mutants of D1 and D2 were then generated in silico and the atrazine binding affinity of the mutant proteins has been calculated to predict mutations able to increase PSII affinity for atrazine. The computational approach has been validated through comparison with available experimental data and production and characterization of one of the predicted mutants. The latter analyses indicated an increase of one order of magnitude of the mutant sensitivity and affinity for atrazine as compared to the control strain. Finally, D1-D2 heterodimer mutants were designed and selected which, according to our model, increase atrazine binding affinity by up to 20 kcal/mol, representing useful starting points for the development of high affinity biosensors for atrazine.

Physiological modes of action of fluoxetine and its human metabolites in algae

Fluoxetine, the active ingredient of many antidepressants, was identified as specifically toxic toward algae in a quantitative structure-activity relationship (QSAR) analysis with literature data for algae, daphnia, and fish. The goal of this study was to elucidate the mode of action in algae and to evaluate the toxicity of the major human metabolites of fluoxetine using two different algae tests. The time dependence and sensitivity of thedifferenteffectendpointsyield information on the physiological mode of action. Baseline toxicity was predicted with QSARs based on measured liposome-water partition coefficients. The ratio of predicted baseline toxicity to experimental toxicity (toxic ratio TR) gives information on the intrinsic potency (extent of specificity of effect). The metabolite p-trifluoromethylphenol was classified to act as baseline toxicant Fluoxetine (TR 60-150) and its pharmacologically active metabolite norfluoxetine (TR 10-80) exhibited specific toxicity. By comparison with reference compounds we conclude that fluoxetine and norfluoxetine have an effect on the energy budget of algal cells since the time pattern of these two compounds is most similar to that observed for norflurazon, but they act less specifically as indicated by lower TR values and the similarity of the effect pattern to baseline toxicants. The mixture toxicity of fluoxetine and its human metabolites norfluoxetine and p-TFMP can be predicted using the model of concentration addition for practical purposes of risk assessment despite small deviations from this model for the specific endpoints like PSII inhibition because the integrative endpoints like growth rate and reproduction in all cases gave agreement with the predictions for concentration addition.

Comparison between the short-term and the long-term toxicity of six triazine herbicides on photobacteria Q67

The bioluminescence inhibition of six triazine herbicides including desmetryne (DES), simetryn (SIM), velpar (VEL), prometon (PRO), metribuzin (MET), and aminotriazine (AMI) on Vibrio qinghaiensis sp.-Q67 (Q67) was determined to investigate the effects of exposure duration on the ecotoxicological relevance of triazine herbicides. Based on the short-term microplate toxicity analysis (MTA), a long-term MTA was established to assess the impact of exposure time on the toxicities of the herbicides. The results show that the long-term toxicities of DES and SIM are similar to their short-term toxicities, and the long-term toxicities of VEL, PRO, and MET are higher than their short-term toxicities, while AMI without short-term toxicity has a high long-term toxicity. In addition, a parabolic relationship was found between the pEC(50) (the negative logarithm of the EC(50), log 1/EC(50)) and the logarithm of octanol-water partition coefficient (logK(ow)). To better understand their toxicity process, the time-dependent toxicities of the six herbicides on Q67 were determined over a period of 12 h during which measurements were taken every 30 min to generate an integral effect surface related to both concentration and duration.

QSAR analysis and specific endpoints for classifying the physiological modes of action of biocides in synchronous green algae

We propose the use of additional physiological endpoints in the 24h growth inhibition test with synchronous cultures of Scenedesmus vacuolatus for the classification of physiological modes of toxic action of chemicals in green algae. The classification scheme is illustrated on the example of one baseline toxicant (3-nitroaniline) and five biocides (irgarol, diuron, Sea-Nine, tributyltin (TBT) and norflurazon). The well-established endpoint of inhibition of reproduction is used for an analysis of the degree of specificity of toxicity by comparing the experimental data with predictions from a quantitative structure-activity relationship (QSAR) for baseline toxicity (narcosis). For those compounds with a toxic ratio greater than 10, i.e. a 10 times higher effect in reproduction than predicted by baseline toxicity, additionally the physiological endpoints inhibition of photosynthesis, cell division and cell volume growth were experimentally assessed. Depending on the relative sensitivity of the different endpoints the chemicals were classified into five different classes of modes of toxic action using a flow chart that was developed in the present study. The advantage of the novel classification scheme is the simplicity of the experimental approach. For the determination of the inhibition of reproduction, the cell size and numbers are quantified with a particle analyzer. This information can be used to derive also the physiological endpoints of cell volume growth and inhibition of cell division. The only additional measurement is the inhibition of the photosynthesis efficiency, which can be easily performed using the non-invasive saturation pulse method and pulse-modulated chlorophyll fluorometry with the Tox-Y-PAM instrument. This mechanistic approach offers a great future potential in ecotoxicology for the physiological mode of action classification of chemicals in algae, which should be a crucial step considered in the risk assessment of chemicals.

Inhibitors in the functional dissection of the photosynthetic electron transport system

The significance of inhibitors and artificial electron acceptor and donor systems as experimental tools for studying the photosynthetic system is described by reviewing early classical articles. The historical development in unravelling the role and sequence of electron carriers and energy conserving sites in the electron transport chain is acknowledged. Emphasis is given to inhibitors of the acceptor side of photosystem II and of the plastoquinol oxidation site in the cytochrome b6/f complex. Their role in regulatory processes under redox control is introduced.

Molecular identification and expression of differentially regulated genes of the European flounder, Platichthys flesus, submitted to pesticide exposure

Effects of pesticide exposure on the European flounder, Platichthys flesus, were investigated using a suppression subtractive hybridization method (SSH) to identify up- and down-regulated genes after a 30-day exposure to herbicides (a cocktail of atrazine, diuron, and isoproturon, and a single herbicide, glyphosate). A total of 256 expressed gene sequences were identified as having the potential for being differentially expressed, of which 116 could be identified by homology with databased sequences. The metabolic functions with which they are associated include energy production, general metabolism, signaling, transport, immune system, and structure. Expression of 14 of these genes was analyzed in liver, muscle, and gills by reverse transcriptase-polymerase chain reaction (RT-PCR) under experimental conditions (0, 15, and 30 days of exposure) and under field conditions (sampling in two estuaries displaying different levels of pesticide contamination). This study provides a first basis for studying the response of fish to pesticide exposure and allows the characterization of new potential genetic markers of pesticide contamination in the field.

A psbA mutation in Kochia scoparia (L) Schrad from railroad rights-of-way with resistance to diuron, tebuthiuron and metribuzin

Kochia [Kochia scoparia (L) Schrad] has become resistant to many herbicides used in cropland and railroad rights-of-way in North Dakota and Minnesota. Kochia scoparia plants that had survived annual treatments with diuron and tebuthiuron were sampled along railroad rights-of-way in North Dakota and Minnesota. The samples were screened in the greenhouse for resistance to diuron, tebuthiuron, metribuzin and bromoxynil from 0.5x to 32x the recommended use rates. A resistant K scoparia accession (MN-3R) was confirmed with resistance up to 16-fold higher than recommended use rates for tebuthiuron and diuron and up to 4-fold higher for metribuzin. However, the resistant K scoparia accession was susceptible to bromoxynil even at 50% of the recommended use rate. The herbicide binding region of the psbA gene fragment of eight resistant (R) and seven susceptible (S) K scoparia accessions was PCR-amplified and sequenced for detection of mutations. The psbA gene of four R K scoparia accessions was mutated at residue 219 with substitution of isoleucine for valine (GenBank accession number AY251265). The seven S K scoparia accession sequences were wild-type at this residue (GenBank accession number AY251266). The other four R accessions sequences showed a previously known triazine R mutation with substitution of glycine for serine at residue 264. All 15 K scoparia accessions were wild-type at all other psbA residues within the region analyzed. Resistance to diuron, tebuthiuron and metribuzin among the railroad rights-of-way K scoparia is probably due to the mutation at residue 219 of the psbA gene in some plants, but due to the previously reported Ser(264)Gly substitution in other plants. Target-site resistance associated with a change of valine to isoleucine at residue 219 of the psbA target-site in weeds has previously been reported for Poa annua L selected in diuron-treated grass seed fields, and for Amaranthus powelli S Wats selected in linuron-treated carrot fields. This is the first report of the mutation in herbicide-resistant K scoparia.

Molecular and immunochemical characteristics of monoclonal and recombinant antibodies selective for the triazine herbicide simetryn and application to environmental analysis

A monoclonal antibody (mab) selective for the thiomethyl-s-triazine herbicide simetryn was obtained and characterized in enzyme-linked immunosorbent assay (ELISA). An IC(50) value for simetryn was 8.5 ng/mL, and the detection range extended from 1.1 to 70 ng/mL in ELISA. The cDNAs encoding variable heavy chain (VH) and variable light chain (VL) of the mab were cloned to produce various recombinant antibodies. Single-chain variable fragment (scFv) antibodies derived from the mab were characterized in ELISA and showed similar reactivities and specificities to the parent mab. A urea denaturation test revealed that the scFv antibodies bound to simetryn were more stable than those in the absence of antigen. A sandwich ELISA based on VH and VL fragments of the mab was successfully developed and showed similar sensitivity to those based on the mab and scFv antibodies in ELISA. In the recovery experiments using spiked environmental samples, the results obtained in ELISA based on the mab were favorably correlated with those by HPLC.

Molecular evolution of herbicide resistance to phytoene desaturase inhibitors in Hydrilla verticillata and its potential use to generate herbicide-resistant crops

Hydrilla [Hydrilla verticillata (Lf) Royle] is one of the most serious invasive aquatic weed problems in the USA. This plant possesses numerous mechanisms of vegetative reproduction that enable it to spread very rapidly. Management of this weed has been achieved by the systemic treatment of water bodies with the herbicide fluridone. At least three dioecious fluridone-resistant biotypes of hydrilla with two- to fivefold higher resistance to the herbicide than the wild-type have been identified. Resistance is the result of one of three independent somatic mutations at the arginine 304 codon of the gene encoding phytoene desaturase, the molecular target site of fluridone. The specific activities of the three purified phytoene desaturase variants are similar to the wild-type enzyme. The appearance of these herbicide-resistant biotypes may jeopardize the ability to control the spread of this non-indigenous species to other water bodies in the southern USA. The objective of this paper is to provide general information about the biology and physiology of this aquatic weed in relation to its recent development of resistance to the herbicide fluridone, and to discuss how this discovery might lead to a new generation of herbicide-resistant crops.

Molecular identification and expression study of differentially regulated genes in the Pacific oyster Crassostrea gigas in response to pesticide exposure

The effects of pesticide contamination on the metabolism of marine molluscs are poorly documented. We investigated the response of a marine bivalve, the Pacific oyster, Crassostrea gigas, using a suppression subtractive hybridization method to identify up- and down-regulated genes after a 30-day exposure period to herbicides (a cocktail of atrazine, diuron and isoproturon, and to the single herbicide glyphosate). A total of 137 unique differentially expressed gene sequences was identified, as well as their associated physiological process. The expression of 18 of these genes was analyzed by RT-PCR under laboratory experimental conditions. The metabolic functions they are associated with include xenobiotic detoxification, energy production, immune system response and transcription. This study provides a preliminary basis for studying the response of marine bivalves to long-term herbicide exposure in terms of regulated gene expression and characterizes new potential genetic markers of herbicide contamination.

The effect of atrazine on Atlantic salmon (Salmo salar) smolts in fresh water and after sea water transfer

Groups of Atlantic salmon smolts were exposed to low levels of the pesticide atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) (0-22.7microgl(-1)) in fresh water and the physiological effects of exposure were measured. Further experiments exposed salmon smolts to similar levels of atrazine in fresh water, and then exposed them to full strength sea water. Atrazine in fresh water resulted in a significant reduction in gill Na(+)K(+)ATPase activity at concentrations of 2.0, 5.0 and 10.0microgl(-1). There were few other physiological changes in the smolts except for slightly elevated plasma cortisol concentrations and monovalent ion concentrations at and above 5.0microgl(-1). However, a sea water challenge caused mortalities in smolts that had been pre-exposed to atrazine in fresh water at concentrations of 1.0, 2.0, 5.0, 10.0 and 22.7microgl(-1). Moreover, surviving fish showed signs of major physiological stress: elevated plasma cortisol, thyroxine, osmolality, and monovalent ion concentrations. However, atrazine exposure had no effect on muscle or plasma water contents. The data suggests that exposure of salmon smolts to atrazine in fresh water may compromise their physiological capabilities to survive in saline conditions.

Synthesis of a group-selective antibody library against haptens

A generic strategy is described for the generation of libraries comprising hapten-selective antibody genes against a group of structurally related low molecular weight target molecules. Hapten antibody libraries are frequently suffering from high background levels of irrelevant antibody genes as a consequence of the immunization, where the small non-immunogenic target molecule is coupled to a large immunogenic carrier protein. In order to elevate the percentage of hapten-specific genes in the library, B cells harboring antibody genes against the group of triazine herbicides were enriched from 21 individual splenocyte populations by means of immunomagnetic separation (IMS). IMS utilizes the specific binding of membrane-associated immunoglobulin receptors on the B cell surface to hapten-coated paramagnetic beads. The variable genes of the specifically enriched subpopulation were cloned into a phagemid vector. The corresponding library yielded up to 75% triazine binding antibody clones after three rounds of phage selection. At least half of these antibodies (abs) were displaceable by triazines resulting in quantitative assays with nanomolar sensitivities. In contrast, no displaceable clone was obtained at the same selection level in a control library, where IMS was omitted. Due to the elevated percentage of relevant antibody genes, the library can be utilized either for the direct isolation of functional antibodies against various triazine herbicides or as group-specific gene source for evolutionary antibody optimization.

Predicting the joint algal toxicity of multi-component s-triazine mixtures at low-effect concentrations of individual toxicants

Herbicidal s-triazines are widespread contaminants of surface waters. They are highly toxic to algae and other primary producers in aquatic systems. This results from their specific interference with photosynthetic electron transport. Risk assessment for aquatic biota has to consider situations of simultaneous exposure to various of these toxicants. In tests with freshwater algae we predicted and determined the toxicity of multiple mixtures of 18 different s-triazines. The toxicity parameter was the inhibition of reproduction of Scenedesmus vacuolatus. Concentration-response analyses were performed for single toxicants and for mixtures containing all 18 s-triazines in two different concentration ratios. Experiments were designed to allow a valid statistical description of the entire concentration-response relationships, including the low concentration range down to EC1. Observed effects and effect concentrations of mixtures were compared to predictions of mixture toxicity. Predictions were calculated from the concentration-response functions of individual s-triazines by applying the concepts of concentration addition and independent action (response addition) alternatively. Predictions based on independent action tend to underestimate the overall toxicity of s-triazine mixtures. In contrast, the concept of concentration addition provides highly accurate predictions of s-triazine mixture toxicity, irrespective of the effect level under consideration and the concentration ratio of the mixture components. This also holds true when the mixture components are present in concentrations below their individual NOEC values. Concentrations statistically estimated to elicit non-significant effects of only 1% still contribute to the overall toxicity. When present in a multi-component mixture they can co-operate to give a severe joint effect. Applicability of the findings obtained with s-triazines to mixtures of other contaminants in aquatic systems and consequences for risk assessment procedures are discussed.

QSAR approach for the selection of congeneric compounds with a similar toxicological mode of action

The selection of compounds with a similar toxicological mode of action is a key problem in the study of chemical mixtures. In this paper, an approach for the selection of chemicals with similar mode of action, based on the analysis of structural similarities by means of QSAR and chemometric methods, is described. As a first step, a complete representation of chemical structures for examined chemicals (phenylureas and triazines) by different sets of molecular descriptors allows a preliminary exploration of similarity using multi-dimensional scaling (MDS). The use of genetic algorithm (GA) to select the most relevant molecular descriptors in modeling toxicity data makes it possible to develop predictive toxicity models. The final step is a similarity analysis, based again on MDS, using selected molecular descriptors, really relevant in describing the toxicological effect.

Amino- and urea-substituted thiazoles inhibit photosynthetic electron transfer

Amino- and urea-substituted thiazoles exhibited in vivo herbicidal activity on duckweed (Lemna paucicostata Hegelm. strain 6746) cultures and appeared to act via inhibition of photosynthetic electron transport system. A small number of the thiazole derivatives tested were active but only at relatively high concentrations. The most active structures were the amino-substituted thiazoles with isopropyl and n-butyl side chains and the urea-substituted thiazole with p-chlorophenyl side chain. Decreasing the length of the side chain had a negative effect on the PSII inhibitory activity. The urea-substituted series was as a group less active than the amino series, and the free acid series had no biological activity. The most active compounds competed for the same binding site as atrazine on PSII. Computer modeling highlighted the structural similarities between some of the thiazoles and the commercial herbicides diuron and atrazine.

A motif for quinone binding sites in respiratory and photosynthetic systems

Many of the membrane-bound protein complexes of respiratory and photosynthetic systems are reactive with quinones. To date, no clear structural relationship between sites that bind quinone has been defined, apart from that in the homologous family of "type II" photosynthetic reaction centres. We show here that a structural element containing a weak sequence motif is common to the Q(A) and Q(B) sites of bacterial reaction centres and the Q(i) site of the mitochondrial bc(1) complex. Analyses of sequence databases indicate that this element may also be present in the PsaA/B subunits of photosystem I, in the ND4 and ND5 subunits of complex I and, possibly, in the mitochondrial alternative quinol oxidase. This represents a first step in the structural classification of quinone binding sites.

Reduced turnover of the D1 polypeptide and photoactivation of electron transfer in novel herbicide resistant mutants of Synechocystis sp. PCC 6803

Two missense mutants, A263P and S264P, and a deletion mutant des-Ala263, Ser264, have been constructed in the D1 protein of the cyanobacterium Synechocystis sp PCC 6803. All were expected to induce a significant conformational change in the QB-binding region of photosystem II (PSII). Although the des-Ala263, Ser264-D1 mutant accumulated some D1 protein in the thylakoid membrane it was unable to grow photoautotrophically or evolve oxygen. Thermoluminescence and chlorophyll fluorescence studies confirmed that this deletion mutant did not show any functional PSII activity. In contrast, [S264P]D1 was able to grow photoautotrophically and give light-saturated rates of oxygen evolution at 60% of the rate of the wild-type control strain, TC31. The A263P missense mutant was also able to evolve oxygen at 50% of TC31 rates although it did not readily grow photoautotrophically. Thermoluminescence, flash oxygen yield and chlorophyll fluorescence measurements indicated that in both missense mutants electron transfer from QA to QB was significantly impaired in dark adapted cells. However, QA to QB electron transfer could be photoactivated in the mutants by background illumination. Both the A263P and S264P mutants also showed an increase in resistance to the s-triazine family of herbicides although this feature did not hold for the phenolic herbicide, ioxynil. Of particular interest was that the two missense mutants, especially S264P, possessed a slower rate of turnover of the D1 protein compared with TC31 and in vivo contained detectable levels of a 41-kDa adduct consisting of D1 and the alpha subunit of cytochrome b559. When protein synthesis was blocked by the addition of lincomycin, D1 degradation was again slower in S264P than TC31. The results are discussed in terms of structural changes in the QB-binding region which affect herbicide and plastoquinone binding and perturb the normal regulatory factors that control the degradation of the D1 protein and its synchronisation with the synthesis of a replacement D1 protein.

Mechanism of photosystem II photoinactivation and D1 protein degradation at low light: the role of back electron flow

Light intensities that limit electron flow induce rapid degradation of the photosystem II (PSII) reaction center D1 protein. The mechanism of this phenomenon is not known. We propose that at low excitation rates back electron flow and charge recombination between the QB*- or QA*- semiquinone acceptors and the oxidized S(2,3) states of the PSII donor side may cause oxidative damage via generation of active oxygen species. Therefore, damage per photochemical event should increase with decreasing rates of PSII excitation. To test this hypothesis, the effect of the dark interval between single turnover flashes on the inactivation of water oxidation, charge separation and recombination, and the degradation of D1 protein were determined in spinach thylakoids. PSII inactivation per flash increases as the dark interval between the flashes increases, and a plateau is reached at dark intervals, allowing complete charge recombination of the QB*-/S2,3 or QA*-/S2 states (about 200 and 40 s, respectively). At these excitation rates: (i) 0.7% and 0.4% of PSII is inactivated and 0.4% and 0.2% of the D1 protein is degraded per flash, respectively, and (ii) the damage per flash is about 2 orders of magnitude higher than that induced by equal amount of energy delivered by excess continuous light. No PSII damage occurs if flashes are given in anaerobic conditions. These results demonstrate that charge recombination in active PSII is promoted by low rates of excitation and may account for a the high quantum efficiency of the rapid turnover of the D1 protein induced by limiting light.

A model for the photosystem II reaction center core including the structure of the primary donor P680

For a detailed understanding of the function of photosystem II (PSII), a molecular structure is needed. The crystal structure has not yet been determined, but the PSII reaction center proteins D1 and D2 show homology with the L and M subunits of the photosynthetic reaction center from purple bacteria. We have modeled important parts of the D1 and D2 proteins on the basis of the crystallographic structure of the reaction center from Rhodopseudomonas viridis. The model contains the central core of the PSII reaction center, including the protein regions for the transmembrane helices B, C, D, and E and loops B-C and C-D connecting the helices. In the model, four chlorophylls, two pheophytins, and the nonheme Fe2+ ion are included. We have applied techniques from computational chemistry that incorporate statistical data on side-chain rotameric states from known protein structure and that describe interactions within the model using an empirical potential energy function. The conformation of chlorophyll pigments in the model was optimized by using exciton interaction calculations in combination with potential energy calculations to find a solution that agrees with experimentally determined exciton interaction energies. The model is analyzed and compared with experimental results for the regions of P680, the redox active pheophytin, the acceptor side Fe2+, and the tyrosyl radicals TyrD and TyrZ. P680 is proposed to be a weakly coupled chlorophyll a pair which makes three hydrogen bonds with residues on the D1 and D2 proteins. In the model the redox-active pheophytin is hydrogen bonded to D1-Glu130 and possibly also to D1-Tyr126 and D1-Tyr147. TyrD is hydrogen bonded to D2-His190 and also interacts with D2-Gln165. TyrZ is bound in a hydrophilic environment which is partially constituted by D1-Gln165, D1-Asp170, D1-Glu189, and D1-His190. These polar residues are most likely involved in proton transfer from oxidized TyrZ or in metal binding.

Modeling of the D1/D2 proteins and cofactors of the photosystem II reaction center: implications for herbicide and bicarbonate binding

A three-dimensional model of the photosystem II (PSII) reaction center from the cyanobacterium Synechocystis sp. PCC 6803 was generated based on homology with the anoxygenic purple bacterial photosynthetic reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis, for which the X-ray crystallographic structures are available. The model was constructed with an alignment of D1 and D2 sequences with the L and M subunits of the bacterial reaction center, respectively, and by using as a scaffold the structurally conserved regions (SCRs) from bacterial templates. The structurally variant regions were built using a novel sequence-specific approach of searching for the best-matched protein segments in the Protein Data Bank with the "basic local alignment search tool" (Altschul SF, Gish W, Miller W, Myers EW, Lipman DJ, 1990, J Mol Biol 215:403-410), and imposing the matching conformational preference on the corresponding D1 and D2 regions. The structure thus obtained was refined by energy minimization. The modeled D1 and D2 proteins contain five transmembrane alpha-helices each, with cofactors (4 chlorophylls, 2 pheophytins, 2 plastoquinones, and a non-heme iron) essential for PSII primary photochemistry embedded in them. A beta-carotene, considered important for PSII photoprotection, was also included in the model. Four different possible conformations of the primary electron donor P680 chlorophylls were proposed, one based on the homology with the bacterial template and the other three on existing experimental suggestions in literature. The P680 conformation based on homology was preferred because it has the lowest energy. Redox active tyrosine residues important for P680+ reduction as well as residues important for PSII cofactor binding were analyzed. Residues involved in interprotein interactions in the model were also identified. Herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modeled in the plastoquinone QB binding niche using the structural information available from a DCMU-binding bacterial reaction center. A bicarbonate anion, known to play a role in PSII, but not in anoxygenic photosynthetic bacteria, was modeled in the non-heme iron site, providing a bidentate ligand to the iron. By modifying the previous hypothesis of Blubaugh and Govindjee (1988, Photosyn Res 19:85-128), we modeled a second bicarbonate and a water molecule in the QB site and we proposed a hypothesis to explain the mechanism of QB protonation mediated by bicarbonate and water. The bicarbonate, stabilized by D1-R257, donates a proton to QB2- through the intermediate of D1-H252; and a water molecule donates another proton to QB2-. Based on the discovery of a "water transport channel" in the bacterial reaction center, an analogous channel for transporting water and bicarbonate is proposed in our PSII model. The putative channel appears to be primarily positively charged near QB and the non-heme iron, in contrast to the polarity distribution in the bacterial water transport channel. The constructed model has been found to be consistent with most existing data.

Modeling the quinone-B binding site of the photosystem-II reaction center using notions of complementarity and contact-surface between atoms

Functional identity and significant similarities in cofactors and sequence exist between the L and M reaction center proteins of the photosynthetic bacteria and the D1 and D2 photosystem-II reaction center proteins of cyanobacteria, algae, and plants. A model of the quinone (QB) binding site of the D1 protein is presented based upon the resolved structure of the QB binding pocket of the L subunit, and introducing novel quantitative notions of complementarity and contact surface between atoms. This model, built without using traditional methods of molecular mechanics and restricted to residues in direct contact with QB, accounts for the experimentally derived functional state of mutants of the D1 protein in the region of QB. It predicts the binding of both the classical and phenol-type PSII herbicides and rationalizes the relative levels of tolerance of mutant phenotypes.

The D-E region of the D1 protein is involved in multiple quinone and herbicide interactions in photosystem II

The region between helices D and E (D-E region) of the D1 protein of photosystem II (PSII) is exposed at the stromal side of the photosynthetic membrane, contains the secondary plastoquinone (QB) binding niche, and is involved in processes at the reducing side of PSII. The role of the D-E region was studied in 27 site-directed mutants generated in the psbAII gene of the cyanobacterium Synechocystis sp. PCC 6803. The photochemical performance of the modified PSII reaction centers was assessed with respect to photoautotrophic growth, oxygen evolution, fluorescence induction, and herbicide inhibition. A few mutations, located at positions presumably involved in essential interactions in the QB binding niche, greatly interfered with PSII performance. On the other hand, mutations in the presumptive loop region between helices D and de resulted in relatively minor effects, indicating a flexible region not critical for photochemical function. Indeed, although more than 80% of the D-E region is phylogenetically invariant, the bulk of the mutations affected the measured parameters only moderately. The significance of the conserved residues appears to be in subtle interactions that optimize the thermodynamic balance between some of the redox components of PSII, as indicated by mild changes in the steady state fluorescence. Many mutations modified tolerances to PSII herbicides. The dispersion of these mutations throughout the D-E region indicates the complex nature of the interactions, direct and indirect, affecting herbicide binding in the QB niche. Mutation of codons Ser221 and Ser222 to Leu221 and Ala222 revealed a new location coordinating the herbicide diuron in the D1 protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial cytochrome b: evolution and structure of the protein

Cytochrome b is the central redox catalytic subunit of the quinol: cytochrome c or plastocyanin oxidoreductases. It is involved in the binding of the quinone substrate and it is responsible for the transmembrane electron transfer by which redox energy is converted into a protonmotive force. Cytochrome b also contains the sites to which various inhibitors and quinone antagonists bind and, consequently, inhibit the oxidoreductase. Ten partial primary sequences of cytochrome b are presented here and they are compared with sequence data from over 800 species for a detailed analysis of the natural variation in the protein. This sequence information has been used to predict some aspects of the structure of the protein, in particular the folding of the transmembrane helices and the location of the quinone- and heme-binding pockets. We have observed that inhibitor sensitivity varies greatly among species. The comparison of inhibition titrations in combination with the analysis of the primary structures has enabled us to identify amino acid residues in cytochrome b that may be involved in the binding of the inhibitors and, by extrapolation, quinone/quinol. The information on the quinone-binding sites obtained in this way is expected to be both complementary and supplementary to that which will be obtained in the future by mutagenesis and X-ray crystallography.

Construction of a Chlamydomonas reinhardtii mutant with an intronless psbA gene

Efficient chloroplast transformation systems now available allow the manipulation of the evolutionarily highly conserved psbA gene in the eucaryotic organism Chlamydomonas reinhardtii. Two copies of this gene in the inverted repeat region of the chloroplast genome contain four large group I introns. To analyse possible functions of these introns and to generate a mutant for simplified psbA gene manipulations, a psbA cDNA fragment was introduced into a psbA deletion mutant using the biolistic transformation method. A transformant with no introns in the psbA gene has been obtained and represents the first example of the removal of a complete set of introns from a chloroplast gene. The newly generated strain is photosynthetically competent and contains no detectable recipient genome copies. The loss of all four introns appears to be phenotypically silent.

Thermoluminescence and flash-induced oxygen yield in herbicide resistant mutants of the D1 protein in Synechococcus PCC7942

Several strains of Synechococcus PCC7942 carrying point mutations in the gene psbA were studied by thermoluminescence and polarographic measurement of flash-induced oxygen yield. The following results were obtained: (a) Replacement of Ser-264 in D1 by Ala (mutant Di1) or Gly (mutant G264) resulting in DCMU and atrazine resistance leads to a downshift of the thermoluminescence (TL) B-band peak temperature from 40 degrees C in wild-type thylakoids to about 30 degrees C. In dark adapted samples of both mutants the TL and oxygen yield pattern induced by a train of single turnover flashes were strongly damped indicative of a high miss factor. (b) In contrast to Ser-264 mutants, replacement of Phe-255 in D1 by Tyr (mutant Tyr5) induced strong resistance to atrazine but not to DCMU and did not affect the peak termperature of the B-band and the flash-induced TL and oxygen yield patterns. In this respect mutant Tyr5 resembles the wild type. (c) No significant differences have been found between strains with single site mutations in psbAI and normal psbAII/psbAIII genes, and strains with same mutations in psbAI but additional deletion of psbAII and psbAIII. Obviously in strains were psbAI is present, PS II complexes containing gene products of psbAII and psbAIII are not assembled in detectable amounts. (d) Strains with double mutations at positions 264 and 255 display a downshift of the B-band peak temperature. Their oscillatory patterns of B-band intensity and oxygen yield are highly damped. This behaviour is similar to strains D1 and G264 which are modified at position 264 only. We extend reports on additivity of mutation effects on herbicide binding to binding of QB. (e) Mutations at the QB site not only influence the binding of QB and herbicides but also change the thermoluminescence quantum yield and the lifetimes of the redox states S2 and S3 of the water oxidase. This finding might indicate long ranging effects on Photosystem II exerted by structural modifications of the QB site. From these data we conclude that Ser-264 is essential for binding of atrazine, DCMU and QB, whereas Phe-255 is involved in atrazine binding and its substitution by Tyr does not markedly affect QB or DCMU binding in Synechococcus PCC7942.

Analysis of a herbicide resistant mutant obtained by transformation of the Chlamydomonas chloroplast

A herbicide resistant Chlamydomonas double mutant (I219A264) has been obtained by transforming the psbA deletion mutant FuD7 with a cloned psbA gene fragment containing mutations in codons 219 and 264. Copies from both the recipient (FuD7) genome and the genome carrying the mutated psbA gene persist in the transformant. This stable heteroplasmic state appears to be required for photoautotrophic growth. Comparison of resistance profiles for classical and phenol-type inhibitors of the double mutant and the corresponding single mutants demonstrates independent, additive contributions of both amino acids to herbicide binding. The approach chosen here to modify the psbA gene should be useful in those cases where consequences of psbA gene manipulations are not predictable with respect to inhibitor resistance.