SINGLET OXYGEN IS NOT PRODUCED IN PHOTOSYSTEM I UNDER PHOTOINHIBITORY CONDITIONS

Zeaxanthin Epoxidase Activity Is Downregulated by Hydrogen Peroxide

The xanthophyll zeaxanthin (Zx) serves important photoprotective functions in chloroplasts and is particularly involved in the dissipation of excess light energy as heat in the antenna of photosystem II (PSII). Zx accumulates under high-light (HL) conditions in thylakoid membranes and is reconverted to violaxanthin by Zx epoxidase (ZEP) in low light or darkness. ZEP activity is completely inhibited under long-lasting HL stress, and the ZEP protein becomes degraded along with the PSII subunit D1 during photoinhibition of PSII. This ZEP inactivation ensures that high levels of Zx are maintained under harsh HL stress. The mechanism of ZEP inactivation is unknown. Here, we investigated ZEP inactivation by reactive oxygen species (ROS) under in vitro conditions. Our results show that ZEP activity is completely inhibited by hydrogen peroxide (H2O2), whereas inhibition by singlet oxygen or superoxide seems rather unlikely. Due to the limited information about the amount of singlet oxygen and superoxide accumulating under the applied experimental conditions, however, a possible inhibition of ZEP activity by these two ROS cannot be generally excluded. Despite this limitation, our data support the hypothesis that the accumulation of ROS, in particular H2O2, might be responsible for HL-induced inactivation of ZEP under in vivo conditions.

Acclimation of Chlamydomonas reinhardtii to extremely strong light

Most photosynthetic organisms are sensitive to very high light, although acclimation mechanisms enable them to deal with exposure to strong light up to a point. Here we show that cultures of wild-type Chlamydomonas reinhardtii strain cc124, when exposed to photosynthetic photon flux density 3000 μmol m^-2 s^-1 for a couple of days, are able to suddenly attain the ability to grow and thrive. We compared the phenotypes of control cells and cells acclimated to this extreme light (EL). The results suggest that genetic or epigenetic variation, developing during maintenance of the population in moderate light, contributes to the acclimation capability. EL acclimation was associated with a high carotenoid-to-chlorophyll ratio and slowed down PSII charge recombination reactions, probably by affecting the pre-exponential Arrhenius factor of the rate constant. In agreement with these findings, EL acclimated cells showed only one tenth of the ¹O₂ level of control cells. In spite of low ¹O₂ levels, the rate of the damaging reaction of PSII photoinhibition was similar in EL acclimated and control cells. Furthermore, EL acclimation was associated with slow PSII electron transfer to artificial quinone acceptors. The data show that ability to grow and thrive in extremely strong light is not restricted to photoinhibition-resistant organisms such as Chlorella ohadii or to high-light tolerant mutants, but a wild-type strain of a common model microalga has this ability as well.

Photobleaching of Chlorophyll in Light-Harvesting Complex II Increases in Lipid Environment

Excess light causes damage to the photosynthetic apparatus of plants and algae primarily via reactive oxygen species. Singlet oxygen can be formed by interaction of chlorophyll (Chl) triplet states, especially in the Photosystem II reaction center, with oxygen. Whether Chls in the light-harvesting antenna complexes play direct role in oxidative photodamage is less clear. In this work, light-induced photobleaching of Chls in the major trimeric light-harvesting complex II (LHCII) is investigated in different molecular environments - protein aggregates, embedded in detergent micelles or in reconstituted membranes (proteoliposomes). The effects of intense light treatment were analyzed by absorption and circular dichroism spectroscopy, steady-state and time-resolved fluorescence and EPR spectroscopy. The rate and quantum yield of photobleaching was estimated from the light-induced Chl absorption changes. Photobleaching occurred mainly in Chl a and was accompanied by strong fluorescence quenching of the remaining unbleached Chls. The rate of photobleaching increased by 140% when LHCII was embedded in lipid membranes, compared to detergent-solubilized LHCII. Removing oxygen from the medium or adding antioxidants largely suppressed the bleaching, confirming its oxidative mechanism. Singlet oxygen formation was monitored by EPR spectroscopy using spin traps and spin labels to detect singlet oxygen directly and indirectly, respectively. The quantum yield of Chl a photobleaching in membranes and detergent was found to be 3.4 × 10-5 and 1.4 × 10-5, respectively. These values compare well with the yields of ROS production estimated from spin-trap EPR spectroscopy (around 4 × 10-5 and 2 × 10-5). A kinetic model is proposed, quantifying the generation of Chl and carotenoid triplet states and singlet oxygen. The high quantum yield of photobleaching, especially in the lipid membrane, suggest that direct photodamage of the antenna occurs with rates relevant to photoinhibition in vivo. The results represent further evidence that the molecular environment of LHCII has profound impact on its functional characteristics, including, among others, the susceptibility to photodamage.

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.

Sensing β-carotene oxidation in photosystem II to master plant stress tolerance

Stressful environmental conditions lead to the production of reactive oxygen species in the chloroplasts, due to limited photosynthesis and enhanced excitation pressure on the photosystems. Among these reactive species, singlet oxygen (¹ O₂ ), which is generated at the level of the PSII reaction center, is very reactive, readily oxidizing macromolecules in its immediate surroundings, and it has been identified as the principal cause of photooxidative damage in plant leaves. The two β-carotene molecules present in the PSII reaction center are prime targets of ¹ O₂ oxidation, leading to the formation of various oxidized derivatives. Plants have evolved sensing mechanisms for those PSII-generated metabolites, which regulate gene expression, putting in place defense mechanisms and alleviating the effects of PSII-damaging conditions. A new picture is thus emerging which places PSII as a sensor and transducer in plant stress resilience through its capacity to generate signaling metabolites under excess light energy. This review summarizes new advances in the characterization of the apocarotenoids involved in the PSII-mediated stress response and of the pathways elicited by these molecules, among which is the xenobiotic detoxification.

Diversity of strategies for escaping reactive oxygen species production within photosystem I among land plants: P700 oxidation system is prerequisite for alleviating photoinhibition in photosystem I

In land plants, photosystem I (PSI) photoinhibition limits carbon fixation and causes growth defects. In addition, recovery from PSI photoinhibition takes much longer than PSII photoinhibition when the PSI core-complex is degraded by oxidative damage. Accordingly, PSI photoinhibition should be avoided in land plants, and land plants should have evolved mechanisms to prevent PSI photoinhibition. However, such protection mechanisms have not yet been identified, and it remains unclear whether all land plants suffer from PSI photoinhibition in the same way. In the present study, we focused on the susceptibility of PSI to photoinhibition and investigated whether mechanisms of preventing PSI photoinhibition varied among land plant species. To assess the susceptibility of PSI to photoinhibition, we used repetitive short-pulse (rSP) illumination, which specifically induces PSI photoinhibition. Subsequently, we found that land plants possess a wide variety of tolerance mechanisms against PSI photoinhibition. In particular, gymnosperms, ferns and mosses/liverworts exhibited higher tolerance to rSP illumination-induced PSI photoinhibition than angiosperms, and detailed analyses indicated that the tolerance of these groups could be partly attributed to flavodiiron proteins, which protected PSI from photoinhibition by oxidizing the PSI reaction center chlorophyll (P700) as an electron acceptor. Furthermore, we demonstrate, for the first time, that gymnosperms, ferns and mosses/liverworts possess a protection mechanism against photoinhibition of PSI that differs from that of angiosperms.

Effects of exogenous β-carotene, a chemical scavenger of singlet oxygen, on the millisecond rise of chlorophyll a fluorescence of cyanobacterium Synechococcus sp. PCC 7942

Singlet-excited oxygen (¹O ₂^* ) has been recognized as the most destructive member of the reactive oxygen species (ROS) which are formed during oxygenic photosynthesis by plants, algae, and cyanobacteria. ROS and ¹O ₂^* are known to damage protein and phospholipid structures and to impair photosynthetic electron transport and de novo protein synthesis. Partial protection is afforded to photosynthetic organism by the β-carotene (β-Car) molecules which accompany chlorophyll (Chl) a in the pigment-protein complexes of Photosystem II (PS II). In this paper, we studied the effects of exogenously added β-Car on the initial kinetic rise of Chl a fluorescence (10-1000 μs, the OJ segment) from the unicellular cyanobacterium Synechococcus sp. PCC7942. We show that the added β-Car enhances Chl a fluorescence when it is excited at an intensity of 3000 μmol photons m^-2 s^-1 but not when excited at 1000 μmol photons m^-2 s^-1. Since β-Car is an efficient scavenger of ¹O ₂^* , as well as a quencher of ³Chl a ^* (precursor of ¹O ₂^* ), both of which are more abundant at higher excitations, we assume that the higher Chl a fluorescence in its presence signifies a protective effect against photo-oxidative damages of Chl proteins. The protective effect of added β-Car is not observed in O₂-depleted cell suspensions. Lastly, in contrast to β-Car, a water-insoluble molecule, a water-soluble scavenger of ¹O ₂^* , histidine, provides no protection to Chl proteins during the same time period (10-1000 μs).

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.

The action spectrum of Photosystem II photoinactivation in visible light

Photosynthesis is always accompanied by light induced damage to the Photosystem II (PSII) which is compensated by its subsequent repair. Photoinhibition of PSII is a complex process, balancing between photoinactivation, protective and repair mechanisms. Current understanding of photoinactivation is limited with competing hypotheses where the photosensitiser is either photosynthetic pigments or the Mn4CaO5 cluster itself, with little consensus on the mechanisms and consequences of PSII photoinactivation. The mechanism of photoinactivation should be reflected in the action spectrum of PSII photoinactivation, but there is a great diversity of the action spectra reported thus far. The only consensus is that PSII photoinactivation is greatest in the UV region of the electromagnetic spectrum. In this review, the authors revisit the methods, technical constraints and the different action spectra of PSII photoinactivation reported to date and compare them against the diverse mechanisms proposed. Upon critical examination of the reported action spectra, a hybrid mechanism of photoinactivation, sensitised by both photosynthetic pigments and the Mn4CaO5 appears to be the most plausible rationalisation.

Experimental evidence suggesting that H2O2 is produced within the thylakoid membrane in a reaction between plastoquinol and singlet oxygen

Plastoquinol (PQH2-9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH2-9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH2-9 to singlet oxygen ((1)O2) and analyzed the products. The main reaction of (1)O2 with PQH2-9 in methanol was found to result in formation of PQ-9 and H2O2, and the amount of H2O2 produced was essentially the same as the amount of oxidized PQH2-9. Formation of H2O2 in the reaction between (1)O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane.

A security network in PSI photoprotection: regulation of photosynthetic control, NPQ and O2 photoreduction by cyclic electron flow

Cyclic electron flow (CEF) around PSI regulates acceptor-side limitations and has multiple functions in the green alga, Chlamydomonas reinhardtii. Here we draw on recent and historic literature and concentrate on its role in Photosystem I (PSI) photoprotection, outlining causes and consequences of damage to PSI and CEF's role as an avoidance mechanism. We outline two functions of CEF in PSI photoprotection that are both linked to luminal acidification: firstly, its action on Photosystem II with non-photochemical quenching and photosynthetic control and secondly, its action in poising the stroma to overcome acceptor-side limitation by rebalancing NADPH and ATP ratios for carbon fixation.

Singlet oxygen production by PSII under light stress: mechanism, detection and the protective role of β-carotene

In this review, I outline the indirect evidence for the formation of singlet oxygen ((1)O(2)) obtained from experiments with the isolated PSII reaction center complex. I also review the methods we used to measure singlet oxygen directly, including luminescence at 1,270 nm, both steady state and time resolved. Other methods we used were histidine-catalyzed molecular oxygen uptake (enabling (1)O(2) yield measurements), and dye bleaching and difference absorption spectroscopy to identify where quenchers of (1)O(2) can access this toxic species. We also demonstrated the protective behavior of carotenoids bound within Chl-protein complexes which bring about a substantial amount of (1)O(2) quenching within the reaction center complex. Finally, I describe how these techniques have been used and expanded in research on photoinhibition and on the role of (1)O(2) as a signaling molecule in instigating cellular responses to various stress factors. I also discuss the current views on the role of (1)O(2) as a signaling molecule and the distance it might be able to travel within cells.

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.

Chemical quenching of singlet oxygen by carotenoids in plants

Carotenoids are considered to be the first line of defense of plants against singlet oxygen ((1)O(2)) toxicity because of their capacity to quench (1)O(2) as well as triplet chlorophylls through a physical mechanism involving transfer of excitation energy followed by thermal deactivation. Here, we show that leaf carotenoids are also able to quench (1)O(2) by a chemical mechanism involving their oxidation. In vitro oxidation of β-carotene, lutein, and zeaxanthin by (1)O(2) generated various aldehydes and endoperoxides. A search for those molecules in Arabidopsis (Arabidopsis thaliana) leaves revealed the presence of (1)O(2)-specific endoperoxides in low-light-grown plants, indicating chronic oxidation of carotenoids by (1)O(2). β-Carotene endoperoxide, but not xanthophyll endoperoxide, rapidly accumulated during high-light stress, and this accumulation was correlated with the extent of photosystem (PS) II photoinhibition and the expression of various (1)O(2) marker genes. The selective accumulation of β-carotene endoperoxide points at the PSII reaction centers, rather than the PSII chlorophyll antennae, as a major site of (1)O(2) accumulation in plants under high-light stress. β-Carotene endoperoxide was found to have a relatively fast turnover, decaying in the dark with a half time of about 6 h. This carotenoid metabolite provides an early index of (1)O(2) production in leaves, the occurrence of which precedes the accumulation of fatty acid oxidation products.

Early transcriptional defense responses in Arabidopsis cell suspension culture under high-light conditions

The early transcriptional defense responses and reactive oxygen species (ROS) production in Arabidopsis (Arabidopsis thaliana) cell suspension culture (ACSC), containing functional chloroplasts, were examined at high light (HL). The transcriptional analysis revealed that most of the ROS markers identified among the 449 transcripts with significant differential expression were transcripts specifically up-regulated by singlet oxygen ((1)O(2)). On the contrary, minimal correlation was established with transcripts specifically up-regulated by superoxide radical or hydrogen peroxide. The transcriptional analysis was supported by fluorescence microscopy experiments. The incubation of ACSC with the (1)O(2) sensor green reagent and 2',7'-dichlorofluorescein diacetate showed that the 30-min-HL-treated cultures emitted fluorescence that corresponded with the production of (1)O(2) but not of hydrogen peroxide. Furthermore, the in vivo photodamage of the D1 protein of photosystem II indicated that the photogeneration of (1)O(2) took place within the photosystem II reaction center. Functional enrichment analyses identified transcripts that are key components of the ROS signaling transduction pathway in plants as well as others encoding transcription factors that regulate both ROS scavenging and water deficit stress. A meta-analysis examining the transcriptional profiles of mutants and hormone treatments in Arabidopsis showed a high correlation between ACSC at HL and the fluorescent mutant family of Arabidopsis, a producer of (1)O(2) in plastids. Intriguingly, a high correlation was also observed with ABA deficient1 and more axillary growth4, two mutants with defects in the biosynthesis pathways of two key (apo)carotenoid-derived plant hormones (i.e. abscisic acid and strigolactones, respectively). ACSC has proven to be a valuable system for studying early transcriptional responses to HL stress.

Photoinhibition of photosystem I

The photoinhibition of Photosystem I (PSI) drew less attention compared with that of Photosystem II (PSII). This could be ascribed to several reasons, e.g. limited combinations of plant species and environmental conditions that cause PSI photoinhibition, the non-regulatory aspect of PSI photoinhibition, and methodological difficulty to determine the accurate activity of PSI under stress conditions. However, the photoinhibition of PSI could be more dangerous than that of PSII because of the very slow recovery rate of PSI. This article is intended to introduce such characteristics of PSI photoinhibition with special emphasis on the relationship between two photosystems as well as the protective mechanism of PSI in vivo. Although the photoinhibition of PSI could be induced only in specific conditions and specific plant species in intact leaves, PSI itself is quite susceptible to photoinhibition in isolated thylakoid membranes. PSI seems to be well protected from photoinhibition in vivo in many plant species and many environmental conditions. This is quite understandable because photoinhibition of PSI is not only irreversible but also the potential cause of many secondary damages. This point would be different from the case of PSII photoinhibition, which could be regarded as one of the regulatory mechanisms under stressed as well as non-stressed conditions.

Direct detection of free radicals and reactive oxygen species in thylakoids

In plants, reactive oxygen species (ROS), also known as active oxygen species (AOS), are associated with normal, physiologic processes as well as with responses to adverse conditions. ROS are connected to stress in many ways: as primary elicitors, as products and propagators of oxidative damage, or as signal molecules initiating defense or adaptation. The photosynthetic electron transport is a major site of oxidative stress by visible or ultraviolet light, high or low temperature, pollutants or herbicides. ROS production can be presumed from detecting oxidatively damaged lipids, proteins, or pigments as well as from the alleviating effects of added antioxidants. On the contrary, measuring ROS by special sensor molecules provides more direct information. This chapter focuses on the application of spin trapping electron paramagnetic resonance (EPR) spectroscopy for detecting ROS: singlet oxygen and oxygen free radicals in thylakoid membrane preparations.

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.

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).

Limited sensitivity of pigment photo-oxidation in isolated thylakoids to singlet excited state quenching in photosystem II antenna

Light-induced pigment oxidation and its relation to excited state quenching in photosystems antennae have been investigated in isolated thylakoids. The results indicate that (i) chlorophyll oxidation takes place in two sequential steps. A slow initial phase is followed by a steep increase in the bleaching rate when more than one quarter of the chromophores are oxidised. (ii) During the initial slow phase, the carotenoid pool is bleached with an apparent rate which is about three times faster than that found for chlorophyll a and more than six times faster than that of chlorophyll b. (iii) Pigment bleaching has been observed both in photosystem I and photosystem II, and it has been possible to estimate a similar carotenoid bleaching rate in the two photosystems. (iv) The protection conferred by singlet state quenchers in the initial slow phase of pigment oxidation is modest. Taking into consideration that both the photosystems are subjected to the oxidative treatment, a somewhat larger protective effect than those estimated for photo-inhibition in thylakoids [S. Santabarbara, F.M. Garlaschi, G. Zucchelli, R.C. Jennings, Biochim. Biophys. Acta 1409 (1999) 165-170] can be computed, although it is less than 50% of the expected level on the basis of the observed reciprocity to the number of incident photons. (v) Pigment oxidation is associated with the loss of membrane ultra-structure, which is interpreted as originating from a decrease in grana stacking. The dynamics of loss of membrane ultra-structure parallel the phases observed for chlorophyll photo-bleaching.

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.

Improved survival of very high light and oxidative stress is conferred by spontaneous gain-of-function mutations in Chlamydomonas

Investigations into high light and oxidative stress in photosynthetic organisms have focussed primarily on genetic impairment of different photoprotective functions. There are few reports of "gain-of-function" mutations that provide enhanced resistance to high light and/or oxidative stress without reduced productivity. We have isolated at least four such very high light resistant (VHL(R)) mutations in the green alga, Chlamydomonas reinhardtii, that permit near maximal growth rates at light intensities lethal to wild type. This resistance is not due to an alteration in electron transport rate or quantity and functionality of the two photosystems that could have enhanced photochemical quenching. Nor is it due to reduced excitation pressure by downregulation of the light harvesting antennae or increased nonphotochemical quenching. In fact, photosynthetic activity is unaffected in more than 30 VHL(R) isolates. Instead, the basis of the VHL(R) phenotype is a combination of traits, which appears to be dominated by enhanced capacity to tolerate reactive oxygen species generated by excess light, methylviologen, rose bengal or hydrogen peroxide. This is further evidenced in lower levels of ROS after exposure to very high light in the VHL(R)-S9 mutant. Additionally, the VHL(R) phenotype is associated with increased zeaxanthin accumulation, maintenance of fast synthesis and degradation rates of the D1 protein, and sustained balanced electron flow into and out of PSI under very high light. We conclude that the VHL(R) mutations arose from a selection pressure that favors changes to the regulatory system(s) that coordinates several photoprotective processes amongst which repair of PSII and enhanced detoxification of reactive oxygen species play seminal roles.

Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II

Electron spin resonance spectroscopy and liquid chromatography have been used to detect radical formation and fragmentation of polypeptides during photoinhibition of purified major antenna proteins, free of protease contaminants. In the absence of oxygen and light, no radicals were observed and there was no damage to the proteins. Similarly illumination of the apoproteins did not induce any polypeptide fragmentation, suggesting that chlorophyll, light and atmospheric oxygen are all participating in antenna degradation. The use of TEMP and DMPO as spin traps showed that protein damage initiates with generation of (1)O(2), presumably from a triplet chlorophyll, acting as a Type II photosensitizer which attacks directly the amino acids causing a complete degradation of protein into small fragments, without the contribution of proteases. Through the use of scavengers, it was shown that superoxide and H(2)O(2) were not involved initially in the reaction mechanism. A higher production of radicals was observed in trimers than in monomeric antenna, while radical production is strongly reduced when antennae were organized in the photosystem II (PSII) complex. Thus, monomerization of antennae as well as their incorporation into the PSII complex seem to represent physiologically protected forms. A comparison is made of the photoinhibition mechanisms of different photosynthetic systems.

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

Arabidopsis thaliana leaves were infiltrated with DanePy (3-(N-diethylaminoethyl)-N-dansyl)aminomethyl-2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrole), a double, fluorescent and spin sensor of singlet oxygen. DanePy fluorescence was imaged by laser scanning microscopy. We found that DanePy penetrated into chloroplasts but did not alter the functioning of the photosynthetic electron transport as assessed by chlorophyll fluorescence induction. In imaging, DanePy fluorescence was well distinct from chlorophyll fluorescence. Photoinhibition by excess photosynthetically active radiation caused quenching of DanePy fluorescence in the chloroplasts but not in other cell compartments. When leaves were infiltrated with dansyl, the fluorescent group in DanePy, there was no fluorescence quenching during photoinhibition. This shows that the fluorescence quenching of DanePy is caused by the conversion of its pyrrol group into nitroxide, i.e. it was caused by the reaction of singlet oxygen with the double sensor and not by artifacts. These data provide direct experimental evidence for the localization of singlet oxygen production to chloroplasts in vivo.

The water-water cycle as alternative photon and electron sinks

The water-water cycle in chloroplasts is the photoreduction of dioxygen to water in photosystem I (PS I) by the electrons generated in photosystem II (PS II) from water. In the water-water cycle, the rate of photoreduction of dioxygen in PS I is several orders of magnitude lower than those of the disproportionation of superoxide catalysed by superoxide dismutase, the reduction of hydrogen peroxide to water catalysed by ascorbate peroxidase, and the reduction of the resulting oxidized forms of ascorbate by reduced ferredoxin or catalysed by either dehydroascorbate reductase or monodehydroascorbate reductase. The water-water cycle therefore effectively shortens the lifetimes of photoproduced superoxide and hydrogen peroxide to suppress the production of hydroxyl radicals, their interactions with the target molecules in chloroplasts, and resulting photoinhibition. When leaves are exposed to photon intensities of sunlight in excess of that required to support the fixation of CO2, the intersystem electron carriers are over-reduced, resulting in photoinhibition. Under such conditions, the water-water cycle not only scavenges active oxygens, but also safely dissipates excess photon energy and electrons, in addition to downregulation of PS II and photorespiration. The dual functions of the water-water cycle for protection from photoinhibition under photon excess stress are discussed, along with its functional evolution.