Singlet oxygen imaging in Arabidopsis thaliana leaves under photoinhibition by excess photosynthetically active radiation

Viable protoplast formation of the coral endosymbiont alga Symbiodinium spp. in a microfluidics platform

Symbiodiniaceae is an important dinoflagellate family which lives in endosymbiosis with reef invertebrates, including coral polyps, making them central to the holobiont. With coral reefs currently under extreme threat from climate change, there is a pressing need to improve our understanding on the stress tolerance and stress avoidance mechanisms of Symbiodinium spp. Reactive oxygen species (ROS) such as singlet oxygen are central players in mediating various stress responses; however, the detection of ROS using specific dyes is still far from definitive in intact Symbiodinium cells due to the hindrance of uptake of certain fluorescent dyes because of the presence of the cell wall. Protoplast technology provides a promising platform for studying oxidative stress with the main advantage of removed cell wall, however the preparation of viable protoplasts remains a significant challenge. Previous studies have successfully applied cellulose-based protoplast preparation in Symbiodiniaceae; however, the protoplast formation and regeneration process was found to be suboptimal. Here, we present a microfluidics-based platform which allowed protoplast isolation from individually trapped Symbiodinium cells, by using a precisely adjusted flow of cell wall digestion enzymes (cellulase and macerozyme). Trapped single cells exhibited characteristic changes in their morphology, cessation of cell division and a slight decrease in photosynthetic activity during protoplast formation. Following digestion and transfer to regeneration medium, protoplasts remained photosynthetically active, regrew cell walls, regained motility, and entered exponential growth. Elevated flow rates in the microfluidic chambers resulted in somewhat faster protoplast formation; however, cell wall digestion at higher flow rates partially compromised photosynthetic activity. Physiologically competent protoplasts prepared from trapped cells in microfluidic chambers allowed for the first time the visualization of the intracellular localization of singlet oxygen (using Singlet Oxygen Sensor Green dye) in Symbiodiniaceae, potentially opening new avenues for studying oxidative stress.

Measuring ROS and redox markers in plant cells

Reactive oxygen species (ROS) are produced throughout plant cells as a by-product of electron transfer processes. While highly oxidative and potentially damaging to a range of biomolecules, there exists a suite of ROS-scavenging antioxidant strategies that maintain a redox equilibrium. This balance can be disrupted in the event of cellular stress leading to increased ROS levels, which can act as a useful stress signal but, in excess, can result in cell damage and death. As crop plants become exposed to greater degrees of multiple stresses due to climate change, efforts are ongoing to engineer plants with greater stress tolerance. It is therefore important to understand the pathways underpinning ROS-mediated signalling and damage, both through measuring ROS themselves and other indicators of redox imbalance. The highly reactive and transient nature of ROS makes this challenging to achieve, particularly in a way that is specific to individual ROS species. In this review, we describe the range of chemical and biological tools and techniques currently available for ROS and redox marker measurement in plant cells and tissues. We discuss the limitations inherent in current methodology and opportunities for advancement.

Oxygen and ROS in Photosynthesis

Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Exogenous calcium alleviates low night temperature stress on the photosynthetic apparatus of tomato leaves

The effect of exogenous CaCl2 on photosystem I and II (PSI and PSII) activities, cyclic electron flow (CEF), and proton motive force of tomato leaves under low night temperature (LNT) was investigated. LNT stress decreased the net photosynthetic rate (Pn), effective quantum yield of PSII [Y(II)], and photochemical quenching (qP), whereas CaCl2 pretreatment improved Pn, Y(II), and qP under LNT stress. LNT stress significantly increased the non-regulatory quantum yield of energy dissipation [Y(NO)], whereas CaCl2 alleviated this increase. Exogenous Ca2+ enhanced stimulation of CEF by LNT stress. Inhibition of oxidized PQ pools caused by LNT stress was alleviated by CaCl2 pretreatment. LNT stress reduced zeaxanthin formation and ATPase activity, but CaCl2 pretreatment reversed both of these effects. LNT stress caused excess formation of a proton gradient across the thylakoid membrane, whereas CaCl2 pretreatment decreased the said factor under LNT. Thus, our results showed that photoinhibition of LNT-stressed plants could be alleviated by CaCl2 pretreatment. Our findings further revealed that this alleviation was mediated in part by improvements in carbon fixation capacity, PQ pools, linear and cyclic electron transports, xanthophyll cycles, and ATPase activity.

Reactive oxygen species: re-evaluation of generation, monitoring and role in stress-signaling in phototrophic organisms

This review provides an overview about recent developments and current knowledge about monitoring, generation and the functional role of reactive oxygen species (ROS) - H2O2, HO2, HO, OH(-), (1)O2 and O2(-) - in both oxidative degradation and signal transduction in photosynthetic organisms including microscopic techniques for ROS detection and controlled generation. Reaction schemes elucidating formation, decay and signaling of ROS in cyanobacteria as well as from chloroplasts to the nuclear genome in eukaryotes during exposure of oxygen-evolving photosynthetic organisms to oxidative stress are discussed that target the rapidly growing field of regulatory effects of ROS on nuclear gene expression.

Flavodiiron protein Flv2/Flv4-related photoprotective mechanism dissipates excitation pressure of PSII in cooperation with phycobilisomes in Cyanobacteria

Oxygenic photosynthesis evolved with cyanobacteria, the ancestors of plant chloroplasts. The highly oxidizing chemistry of water splitting required concomitant evolution of efficient photoprotection mechanisms to safeguard the photosynthetic machinery. The role of flavodiiron proteins (FDPs), originally called A-type flavoproteins or Flvs, in this context has only recently been appreciated. Cyanobacterial FDPs constitute a specific protein group that evolved to protect oxygenic photosynthesis. There are four FDPs in Synechocystis sp. PCC 6803 (Flv1 to Flv4). Two of them, Flv2 and Flv4, are encoded by an operon together with a Sll0218 protein. Their expression, tightly regulated by CO2 levels, is also influenced by changes in light intensity. Here we describe the overexpression of the flv4-2 operon in Synechocystis sp. PCC 6803 and demonstrate that it results in improved photochemistry of PSII. The flv4-2/OE mutant is more resistant to photoinhibition of PSII and exhibits a more oxidized state of the plastoquinone pool and reduced production of singlet oxygen compared with control strains. Results of biophysical measurements indicate that the flv4-2 operon functions in an alternative electron transfer pathway from PSII, and thus alleviates PSII excitation pressure by channeling up to 30% of PSII-originated electrons. Furthermore, intact phycobilisomes are required for stable expression of the flv4-2 operon genes and for the Flv2/Flv4 heterodimer-mediated electron transfer mechanism. The latter operates in photoprotection in a complementary way with the orange carotenoid protein-related nonphotochemical quenching. Expression of the flv4-2 operon and exchange of the D1 forms in PSII centers upon light stress, on the contrary, are mutually exclusive photoprotection strategies among cyanobacteria.

Production, detection, and signaling of singlet oxygen in photosynthetic organisms

SIGNIFICANCE

In photosynthetic organisms, excited chlorophylls (Chl) can stimulate the formation of singlet oxygen ((1)O(2)), a highly toxic molecule that acts in addition to its damaging nature as an important signaling molecule. Thus, due to this dual role of (1)O(2), its production and detoxification have to be strictly controlled.

RECENT ADVANCES

Regulation of pigment synthesis is essential to control (1)O(2) production, and several components of the Chl synthesis and pigment insertion machineries to assemble and disassemble protein/pigment complexes have recently been identified. Once produced, (1)O(2) activates a signaling cascade from the chloroplast to the nucleus that can involve multiple mechanisms and stimulate a specific gene expression response. Further, (1)O(2) signaling was shown to interact with signal cascades of other reactive oxygen species, oxidized carotenoids, and lipid hydroperoxide-derived reactive electrophile species.

CRITICAL ISSUES

Despite recent progresses, hardly anything is known about how and where the (1)O(2) signal is sensed and transmitted to the cytoplasm. One reason for that is the limitation of available detection methods challenging the reliable quantification and localization of (1)O(2) in plant cells. In addition, the process of Chl insertion into the reaction centers and antenna complexes is still unclear.

FUTURE DIRECTIONS

Unraveling the mechanisms controlling (1)O(2) production and signaling would help clarifying the specific role of (1)O(2) in cellular stress responses. It would further enable to investigate the interaction and sensitivity to other abiotic and biotic stress signals and thus allow to better understand why some stressors activate an acclimation, while others provoke a programmed cell death response.

Photoinhibition of Photosystem II

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

Magnetic field protects plants against high light by slowing down production of singlet oxygen

Recombination of the primary radical pair of photosystem II (PSII) of photosynthesis may produce the triplet state of the primary donor of PSII. Triplet formation is potentially harmful because chlorophyll triplets can react with molecular oxygen to produce the reactive singlet oxygen (¹O₂). The yield of ¹O₂ is expected to be directly proportional to the triplet yield and the triplet yield of charge recombination can be lowered with a magnetic field of 100-300 mT. In this study, we illuminated intact pumpkin leaves with strong light in the presence and absence of a magnetic field and found that the magnetic field protects against photoinhibition of PSII. The result suggests that radical pair recombination is responsible for significant part of ¹O₂ production in the chloroplast. The magnetic field effect vanished if leaves were illuminated in the presence of lincomycin, an inhibitor of chloroplast protein synthesis, or if isolated thylakoid membranes were exposed to light. These data, in turn, indicate that ¹O₂ produced by the recombination of the primary charge pair is not directly involved in photoinactivation of PSII but instead damages PSII by inhibiting the repair of photoinhibited PSII. We also found that an Arabidopsis thaliana mutant lacking α-tocopherol, a scavenger of ¹O₂, is more sensitive to photoinhibition than the wild-type in the absence but not in the presence of lincomycin, confirming that the target of ¹O₂ is the repair mechanism.

In vivo imaging of Ca2+, pH, and reactive oxygen species using fluorescent probes in plants

Changes in the levels of Ca(2+), pH, and reactive oxygen species (ROS) are recognized as key cellular regulators involved in diverse physiological and developmental processes in plants. Critical to understanding how they exert such widespread control is an appreciation of their spatial and temporal dynamics at levels from organ to organelle and from seconds to many hours. With appropriate controls, fluorescent sensors can provide a robust approach with which to quantify such changes in Ca(2+), pH, and ROS in real time, in vivo. The fluorescent cellular probes available for visualization split into two broad classes: (a) dyes and (b) an increasingly diverse set of genetically encoded sensors based around green fluorescent proteins (GFPs). The GFP probes in particular can be targeted to well-defined subcellular locales, offering the possibility of high-resolution mapping of these signals within the cell.

Singlet oxygen: there is indeed something new under the sun

Singlet oxygen, O(2)(a(1)Delta(g)), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for approximately 80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O(2)(X(3)Sigma), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a "mature citizen", singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

Putative function of cytochrome b559 as a plastoquinol oxidase

The function of cytochrome b559 (cyt b559) in photosystem II (PSII) was studied in a tobacco mutant in which the conserved phenylalanine at position 26 in the beta-subunit was changed to serine. Young leaves of the mutant showed no significant difference in chloroplast ultra structure or in the amount and activity of PSII, while in mature leaves the size of the grana stacks and the amount of PSII were significantly reduced. Mature leaves of the mutant showed a higher susceptibility to photoinhibition and a higher production of singlet oxygen, as shown by spin trapping electron paramagnetic resonance (EPR) spectroscopy. Oxygen consumption and superoxide production were studied in thylakoid membranes in which the Mn cluster was removed to ensure that all the cyt b559 was present in its low potential form. In thylakoid membranes, from wild-type plants, the larger fraction of superoxide production was 3-(3,4-dichlorophenyl)-1,1-dimethylurea-sensitive. This type of superoxide formation was absent in thylakoid membranes from the mutant. The physiological importance of the plastoquinol oxidation by cyt b559 for photosynthesis is discussed.

Monitoring reactive oxygen species formation and localisation in living cells by use of the fluorescent probe CM-H(2)DCFDA and confocal laser microscopy

Reactive oxygen species (ROS) develop as a consequence of wounding, light stress and chemical imbalances but act also as signals in living cells. The integrity of cells is seriously endangered, if ROS cannot be controlled by scavenging molecules and other repair mechanisms of the cell. For studying ROS development and signalling under stress, a reliable indicator is needed. We have tested the ROS sensitive dye 5-(and-6) chloromethyl-2',7' dichlorodihydrofluorescein diacetate acetyl ester (CM-H(2)DCFDA) using onion bulb scale and leaf epidermis as well as Arabidopsis leaves and protoplasts. ROS were generated by several fundamentally different methods-externally applied hydrogen peroxide, heat shock, high light or wounding. Confocal microscopy and fluorescence quantification over time showed that the indicator responds in an additive and dose-dependent manner. The response to externally applied hydrogen peroxide followed saturation kinetics, consistent with a channel-mediated uptake of the stressor across the plasma membrane. An inherent problem of the tested indicator was the uneven uptake in tissues, as compared with protoplasts, making it difficult to discriminate an uneven indicator distribution from an uneven ROS distribution. However, in protoplasts and under carefully designed preparation conditions CM-H(2)DCFDA is a useful general ROS indicator. Subcellularly, the de-esterified probe localised to the cytosol, to mitochondria and to chloroplasts.

Singlet oxygen in plants: production, detoxification and signaling

Singlet oxygen ((1)O(2)) is a singular reactive oxygen species (ROS) that is produced constitutively in plant leaves in light via chlorophylls that act as photosensitizers. This (1)O(2) production is spatially resolved within thylakoid membranes and is enhanced under light stress conditions. (1)O(2) can also be produced by phytotoxins during plant-pathogen interactions. (1)O(2) is highly reactive, can be toxic to cells and can be involved in the signaling of programmed cell death or acclimation processes. Here, we summarize current knowledge on (1)O(2) management in plants and on the biological effects of this peculiar ROS. Compared with other ROS, (1)O(2) has received relatively little attention, but recent developments indicate that it has a crucial role in the responses of plants to light.

Singlet oxygen production in photosystem II and related protection mechanism

High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P(680) of the primary acceptor pheophytin or of the quinone acceptor Q(A), modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by beta-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress.

A comparative study of fluorescent singlet oxygen probes in plant leaves

Four fluorescent singlet oxygen sensors: DanePy, its oxalate salt, Singlet Oxygen Sensor Green and MVP, were infiltrated into tobacco leaves and tested for toxicity, subcellular localization, light sensitivity and capacity to trap the singlet oxygen produced in photoinhibition. For reference, a broad sensitivity free radical probe, TEMPO-9-AC, was also included. Photochemical yield was approximately 15% and 10% inhibited by Singlet Oxygen Sensor Green and MVP, respectively, but was not significantly affected by the other probes. Under photoinhibitory conditions, brought about by irradiating lincomycin-treated leaves with strong photosynthetically active radiation, DanePy and Singlet Oxygen Sensor Green were responsive. Singlet Oxygen Sensor Green was also reactive to low, non-photoinhibitory light exposure of the leaf, which was not characteristic to the other probes. MVP did not respond to singlet oxygen which can partly be explained by a possible attenuation of its blue emission in the leaf, as shown by the example TEMPO-9-AC. DanePy-oxalate did not respond to photosynthetic singlet oxygen due to lack of its penetration into photosynthetic tissue and hence could be useful in detecting any singlet oxygen which escapes from a chloroplast initiation site. DanePy was localized in the chloroplasts, while Singlet Oxygen Sensor Green was mainly found in the epidermal cells preferentially associated with the nucleus.

Spatial and temporal electrochemical control of singlet oxygen production and decay in photosensitized experiments

Active spatial and temporal modulation of domains of singlet oxygen activity is demonstrated using electrochemical tools. Using singlet oxygen microscopy in photosensitized experiments, it is shown that singlet oxygen concentrations around an ultramicroelectrode can be controlled by applying a bias voltage to the electrode. Two phenomena that can be exploited separately or collectively are examined: (1) the singlet oxygen concentration can be altered by local oxidation or reduction of the photosensitizer, which is the precursor to singlet oxygen, and (2) the reduction of oxygen to produce the superoxide anion which, among other things, is an effective singlet oxygen quencher, results in a local decrease in the concentration of singlet oxygen around the electrode. Both of these phenomena depend significantly on the diffusion of molecules along concentration gradients established by the biased electrode. The results reported herein demonstrate that one can indeed exert local electrochemical control and readily manipulate the population of singlet oxygen produced in a photosensitized process.

Photosystem II damage induced by chemically generated singlet oxygen in tobacco leaves

In the present work, we investigated the role of chemically generated singlet oxygen, produced by photodynamic effect of rose bengal, in damaging the PSII complex in tobacco leaves in which protein synthesis-dependent repair was inhibited by infiltration with lincomycin. A 30-min exposure to low-intensity (150 micromol m(-2) s(-1)) photosynthetically active radiation (PAR) induced singlet oxygen production as detected by quenching of 3-[N-(beta-diethylaminoethyl)-N-dansyl]aminomethyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole fluorescence in leaves infiltrated with both lincomycin and rose bengal. This light treatment caused photoinhibition of PSII, as revealed by the marked loss both of the photochemical yield and the amount of D1 protein in PSII reaction center. When rose bengal was not present in the leaves, these symptoms of photodamage were not induced by the same low-intensity PAR. However, when excitation pressure on PSII was increased to 1500 micromol m(-2) s(-1), irreversible photodamage of PSII was also observed, showing that the lincomycin treatment applied in vivo was sufficiently inhibiting protein repair. Our results show that singlet oxygen is able to cause oxidative damage in PSII directly, as suggested earlier and argue against its recently hypothesized role exclusive to inhibiting PSII protein repair (Nishiyama et al. 2006).

Chloroplast-located flavonoids can scavenge singlet oxygen

* The hypothesis was tested that flavonoids may scavenge singlet oxygen ((1)O(2)) in mesophyll cells of Phillyrea latifolia exposed to excess-light stress. * In cross-sections taken from leaves developed at 10% (shade) or 100% (sun) solar irradiance, we evaluated the excess photosynthetically active radiation (PAR)-induced accumulation of (1)O(2) in mesophyll cells by imaging the fluorescence quenching of the specific (1)O(2) probe N-[2-(diethylamino)ethyl]-N-[(2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrol-3-yl)methyl]-5-(dimethylamino)-1-naphthalenesulfonamide (DanePy). The intracellular location of flavonoids was also analyzed using three-dimensional deconvolution microscopy. * Photo-induced quenching of DanePy fluorescence was markedly greater in the mesophyll of shade leaves than in that of sun leaves, the former showing a negligible accumulation of mesophyll flavonoids. The photo-induced generation of (1)O(2) was inversely related to the content of flavonoids in the mesophyll cells of sun leaves. Flavonoids were located in the chloroplasts, and were likely associated with the chloroplast envelope. * Here we provide relevant evidence for the potential scavenger activity of chloroplast-located flavonoids against (1)O(2) and new insights into the photo-protective role of flavonoids in higher plants.

Regulation of peroxiredoxin expression versus expression of Halliwell-Asada-Cycle enzymes during early seedling development of Arabidopsis thaliana

During early seedling development of oil seed plants, the transition from lipid based heterotrophic to photoautotrophic carbohydrate metabolism is accompanied with a biphasic control of the chloroplast antioxidant system. In continuous light, organellar peroxiredoxins (Prx) and thylakoid-bound ascorbate peroxidase (tAPx) are activated early in seedling development, while stromal ascorbate peroxidase (sAPx), Cu/Zn-superoxide dismutase-2 (Csd2) and monodehydroascorbate reductase (MDHAR) and the cytosolic peroxiredoxins PrxIIB, PrxIIC and PrxIID are fully activated between 2.5 and 3 days after radicle emergence (DARE). Discontinuous light synchronized the expression of chloroplast antioxidant enzymes, but defined diurnally specific typeII-Prx-patterns in the cytosol and initiated chloroplast senescence around 2.5 DARE. Carbohydrate feeding uncoupled sAPx expression from the light pattern. In contrast, sucrose-feeding did not significantly impact on Prx transcript amounts. It is concluded that upon post-germination growth Prxs are activated endogenously to provide early antioxidant protection, which is supported by the Halliwell-Asada-Cycle, whose expressional activation depends on metabolic signals provided only later in development or in day-night-cycles.

Light and singlet oxygen in plant defense against pathogens: phototoxic phenalenone phytoalexins

Plants defend themselves from pathogen infections or mechanical injury by a number of mechanisms, including the induced biosynthesis of antimicrobial secondary metabolites. These compounds, termed phytoalexins, represent a very economical way to counteract hazard, because the carbon and energy resources are diverted to phytoalexin synthesis only at the early period of attack and only at its site. The occurrence of phenalenone chromophores in phytoalexins of plants originally nonphototoxic suggests that these plants respond to pathogen attacks by biosynthesizing singlet oxygen photosensitizers able to use solar energy for defense. This concept may have implications for the development of novel crop protection strategies.

Singlet Oxygen in Plants—Its Significance and Possible Detection with Double (Fluorescent and Spin) Indicator Reagents

Direct detection of reactive oxygen species (ROS), especially singlet oxygen, in plants under stress conditions is of special importance, not only to identify primary events of oxidative damage, but also in studies exploring the potential role of ROS as signal molecules. Due to short life-times and diffusion distances of ROS, these tasks require highly reactive and selective indicator reagents, localized at the presumed site of production. In the present study, we compared four double sensors: ROS indicator reagents in which partial fluorescence quenching of a dansyl moiety occurs as a result of nitroxide radical formation from a sterically hindered amine constituent. Our experiments support the idea that shorter donor-acceptor distances within these molecules result in higher reactivity to ROS. The presence of a diethylaminoethyl side chain resulted in better selectivity to singlet oxygen: reagents lacking such substituent had an additional reactivity to superoxide anions, probably as a result of the formation of zwitterionic structures. Fluorescence localization studies of the indicator reagents in tobacco leaves and in Chlamydomonas cells show promising perspectives of their applications to plant stress studies.

Two-Photon Singlet Oxygen Microscopy: The Challenges of Working with Single Cells

A microscope is described in which singlet molecular oxygen, O2(a1deltag), is produced in a femtoliter focal volume via a nonlinear two-photon photosensitized process, and the 1270 nm phosphorescence from this population of O2(a1deltag) is detected in a photon counting experiment. Although two-photon excitation of a sensitizer is less efficient than excitation by a one-photon process, nonlinear excitation has several distinct advantages with respect to the spatial resolution accessible. Pertinent aspects of this two-photon O2(a1deltag) microscope were characterized using bulk solutions of photosensitizers. These data were compared to those obtained from a single biological cell upon linear one-photon excitation of a sensitizer incorporated in the cell. On the basis of the results obtained, we outline the challenges of using nonlinear optical techniques to create O2(aldeltag) at the single cell level and to then optically detect the O2(aldeltag) thus produced in a time-resolved experiment.

Optical detection of singlet oxygen from single cells

The lowest excited electronic state of molecular oxygen, singlet molecular oxygen, O(2)(a (1)Delta(g)), is a reactive species involved in many chemical and biological processes. To better understand the roles played by singlet oxygen in biological systems, particularly at the sub-cellular level, optical tools have been developed to create and directly detect this transient state in time- and spatially-resolved experiments from single cells. Data obtained indicate that, contrary to common perception, this reactive species can be quite long-lived in a cell and, as such, can diffuse over appreciable distances including across the cell membrane into the extracellular environment. On one hand, these results demonstrate that the behavior of singlet oxygen in an intact cell can be significantly different from that inferred from model bulk studies. More generally, these results provide a new perspective for mechanistic studies of intra- and inter-cellular signaling and events that ultimately lead to photo-induced cell death.

Photoinactivation of photosystem II by flashing light

Inhibition of Photosystem II (PS II) activity by single turnover visible light flashes was studied in thylakoid membranes isolated form spinach. Flash illumination results in decreased oxygen evolving activity of PS II, which effect is most pronounced when the water-oxidizing complex is in the S2 and S3 states, and increases with increasing time delay between the subsequent flashes. By applying the fluorescent spin-trap DanePy, we detected the production of singlet oxygen, whose amount was increasing with increasing flash spacing. These findings were explained in the framework of a model, which assumes that recombination of the S2QB - and S3QB - states generate the triplet state of the reaction center chlorophyll and lead to the production of singlet oxygen.

Synthesis and evaluation of the permeability transition inhibitory characteristics of paramagnetic and diamagnetic amiodarone derivatives

Several amiodarone analogues were synthesized varying the 2-substituent on the benzofuran ring and diethylaminoethyl side chain of phenolether by introducing 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole and 1,2,5,6-tetrahydropyridine nitroxides or their amino or hydroxylamino precursors. The new compounds were screened on isolated mitochondria and perfused heart and their toxicity was evaluated on WRL-68 liver cells and H9C2 cardiomyocytes. Most of the newly synthesized derivatives exerted uncoupling effect on the mitochondrial oxidative phosphorilation at higher concentrations, compared to amiodarone and one of the modified amiodarone analogues showed an effect similar to that of amiodarone on the mitochondrial permeability transition and on restoring of mitochondrial high-energy phosphate metabolites in perfused hearts. This amiodarone analogue can be new leading compound among the experimental amiodarone analogues with the same or enhanced efficiency of amiodarone, but with less side effects.

Production of reactive oxygen species in chloride- and calcium-depleted photosystem II and their involvement in photoinhibition

Mixed photosystem II (PSII) samples consisting of Cl(-)-depleted and active, or Ca(2+)-depleted and active PSII enriched membrane fragments, respectively, were investigated with respect to their susceptibility to light. In the presence of Cl(-)-depleted PSII, active centers were damaged more severely, most likely caused by a higher amount of reactive oxygen species formed in the nonfunctional centers. Cl(-) depletion led to an increased H(2)O(2) production, which seemed to be responsible for the stimulation of PSII activity loss. To distinguish between direct H(2)O(2) formation by partial water oxidation and indirect H(2)O(2) formation by oxygen reduction involving the prior formation of O(2)(-?), the production of reactive oxygen species was followed by spin trapping EPR spectroscopy. All samples investigated, i.e. PSII with a functional water splitting complex, Ca(2+)- and Cl(-)-depleted PSII, produced upon illumination O(2)(-?) and OH(?) radicals on the acceptor side, while Cl(-)-depleted PSII produced additionally OH(?) radicals originating from H(2)O(2) formed on the donor side of PSII.

Transgenic tobacco plants expressing antisense ferredoxin-NADP(H) reductase transcripts display increased susceptibility to photo-oxidative damage

Ferredoxin-NADP(H) reductase (FNR) catalyses the final step of the photosynthetic electron transport in chloroplasts. Using an antisense RNA strategy to reduce expression of this flavoenzyme in transgenic tobacco plants, it has been demonstrated that FNR mediates a rate-limiting step of photosynthesis under both limiting and saturating light conditions. Here, we show that these FNR-deficient plants are abnormally prone to photo-oxidative injury. When grown under autotrophic conditions for 3 weeks, specimens with 20-40% extant reductase undergo leaf bleaching, lipid peroxidation and membrane damage. The magnitude of the effect was proportional to the light intensity and to the extent of FNR depletion, and was accompanied by morphological changes involving accumulation of aberrant plastids with defective thylakoid stacking. Damage was initially confined to chloroplast membranes, whereas Rubisco and other stromal proteins began to decline only after several weeks of autotrophic growth, paralleled by partial recovery of NADPH levels. Exposure of the transgenic plants to moderately high irradiation resulted in rapid loss of photosynthetic capacity and accumulation of singlet oxygen in leaves. The collected results suggest that the extensive photo-oxidative damage sustained by plants impaired in FNR expression was caused by singlet oxygen building up to toxic levels in these tissues, as a direct consequence of the over-reduction of the electron transport chain in FNR-deficient chloroplasts.

Interaction of singlet molecular oxygen with double fluorescent and spin sensors

Double fluorescent and spin sensors were recently used to detect transient oxidants via simultaneous fluorescence change and production of the nitroxide radical detected by electron paramagnetic resonance. One such oxidant, singlet molecular oxygen ((1)O(2)), was detected in thylakoid membrane using these probes. In the present study, we investigated the total (physical and chemical) quenching of (1)O(2) phosphorescence by sensors composed of the 2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrole moiety attached to xanthene or dansyl fluorophores. We found that the quenching rate constants were in the range (2-7) x 10(7) M(-1)s(-1) in acetonitrile and D(2)O. Quenching of (1)O(2) is usually an additive process in which different functional groups may contribute. We estimated that the (1)O(2) quenching by the amine fragments was ca. one to two orders of magnitude lower than that for the complete molecules. Our data suggest that the incorporation of a fluorescent chromophore results in additional strong quenching of (1)O(2), which may in turn decrease the nitroxide yield via the (1)O(2) chemical path, possibly having an effect on quantitative interpretations. We have also found that probes with the dansyl fluorophore photosensitized (1)O(2) upon UV excitation with the quantum yield of 0.087 in acetonitrile at 366 nm. This result shows that care must be taken when the dansyl-based sensors are used in experiments requiring UV irradiation. We hope that our results will contribute to a better characterization and wider use of these novel double sensors.

Singlet oxygen production in herbicide-treated photosystem II

Photo-generated reactive oxygen species in herbicide-treated photosystem II were investigated by spin-trapping. While the production of .OH and O2-* was herbicide-independent, 1O2 with a phenolic was twice that with a urea herbicide. This correlates with the reported influence of these herbicides on the redox properties of the semiquinone QA-* and fits with the hypothesis that 1O2 is produced by charge recombination reactions that are stimulated by herbicide binding and modulated by the nature of the herbicide. When phenolic herbicides are bound, charge recombination at the level of P+*Pheo-* is thermodynamically favoured forming a chlorophyll triplet and hence 1O2. With urea herbicides this pathway is less favourable.

Reactive oxygen species and UV-B: effect on cyanobacteria

Reactive oxygen species (ROS) are involved in the damage and response of cyanobacteria to UV-B irradiation. In cyanobacteria, there are several targets for the potentially toxic ROS such as lipids, DNA and protein. The damage to photosynthetic apparatus induces the inhibition of photosynthesis that is mediated partially by ROS. UV-B-induced oxidative stress and oxidative damage increases with irradiation time and can be reversed after long-term irradiation. This raises the interesting question of whether cyanobacteria can acclimatize to the present UV-B stress. On one hand, ROS may also act as signal molecules and mediate the genetic regulation of photosynthetic genes and the induction of antioxidant enzymes. On the other hand, the efficient defense and repair system allows cyanobacteria to recover from the oxidative damage under moderate UV-B irradiation. In addition, the following methods are discussed: the fluorogenic probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA), used to detect oxidative stress induced by UV-B; thiobarbituric acid reactive substances (TBARS), used to determine lipid peroxidation in cyanobacteria; fluorimetric analysis of DNA unwinding (FADU), used to quantify DNA strand breaks induced by ROS formation under UV-B stress.

Detection of singlet oxygen and superoxide with fluorescent sensors in leaves under stress by photoinhibition or UV radiation

In order to understand the physiological functions of reactive oxygen species (ROS) generated in leaves, their direct measurement in vivo is of special importance. Here we report experiments with two dansyl-based ROS sensors, the singlet oxygen specific DanePy and HO-1889NH, which is reactive to both singlet oxygen and superoxide radicals. Here we report in vivo detection of (1)O(2) and O(2)(-*) by fluorescence quenching of two dansyl-based ROS sensors, the (1)O(2) specific DanePy and HO-1889NH, which was reactive with both (1)O(2) and O(2)(-*). The ROS sensors were administered to spinach leaves through a pinhole, and then the leaves were exposed to either excess photosynthetically active radiation or UV (280-360 nm) radiation. Microlocalization of the sensors' fluorescence and its ROS-induced quenching was followed with confocal laser scanning microscopy and with fluorescence imaging. These sensors were specifically localized in chloroplasts. Quenching analysis indicated that the leaves exposed to strong light produced (1)O(2), but hardly any O(2)(-*). On the other hand, the dominant ROS in UV-irradiated leaves was O(2)(-*), while (1)O(2) was minor.

Imaging of photo-oxidative stress responses in leaves

High resolution digital imaging was used to identify sites of photo-oxidative stress responses in Arabidopsis leaves non-invasively, and to demonstrate the potential of using a suite of imaging techniques for the study of oxidative metabolism in planta. Tissue-specific photoinhibition of photosynthesis in individual chloroplasts in leaves was imaged by chlorophyll fluorescence microscopy. Singlet oxygen production was assessed by imaging the quenching of the fluorescence of dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole (DanePy) that results from its reaction with singlet oxygen. Superoxide and hydrogen peroxide accumulation were visualized by the reduction of nitroblue tetrazolium (NBT) to formazan deposits and by polymerization with 3,3'-diaminobenzidine (DAB), respectively. Stress-induced expression of a gene involved with antioxidant metabolism was imaged from the bioluminescence from leaves of an Arabidopsis APX2-LUC transformant, which co-expresses an ascorbate peroxidase (APX2) with firefly luciferase. Singlet oxygen and superoxide production were found to be primarily located in mesophyll tissues whereas hydrogen peroxide accumulation and APX2 gene expression were primarily localized in the vascular tissues.