Oscillations of reaction center II-D1 protein degradation in vivo induced by repetitive light flashes. Correlation between the level of RCII-QB- and protein degradation in low light

The energy cost of repairing photoinactivated photosystem II: an experimental determination in cotton leaf discs

Photosystem II (PSII), which splits water molecules at minimal excess photochemical potential, is inevitably photoinactivated during photosynthesis, resulting in compromised photosynthetic efficiency unless it is repaired. The energy cost of PSII repair is currently uncertain, despite attempts to calculate it. We experimentally determined the energy cost of repairing each photoinactivated PSII in cotton (Gossypium hirsutum) leaves, which are capable of repairing PSII in darkness. As an upper limit, 24 000 adenosine triphosphate (ATP) molecules (including any guanosine triphosphate synthesized at the expense of ATP) were required to repair one entire PSII complex. Further, over a 7-h illumination period at 526-1953 μmol photons m^-2  s^-1 , the ATP requirement for PSII repair was on average up to 4.6% of the ATP required for the gross carbon assimilation. Each of these two measures of ATP requirement for PSII repair is two- to three-fold greater than the respective reported calculated value. Possible additional energy sinks in the PSII repair cycle are discussed.

Photosystem II photoinhibition and photoprotection in a lycophyte, Selaginella martensii

The Lycophyte Selaginella martensii efficiently acclimates to diverse light environments, from deep shade to full sunlight. The plant does not modulate the abundance of the Light Harvesting Complex II, mostly found as a free trimer, and does not alter the maximum capacity of thermal dissipation (NPQ). Nevertheless, the photoprotection is expected to be modulatable upon long-term light acclimation to preserve the photosystems (PSII, PSI). The effects of long-term light acclimation on PSII photoprotection were investigated using the chlorophyll fluorometric method known as "photochemical quenching measured in the dark" (qP_d ). Singularly high-qP_d values at relatively low irradiance suggest a heterogeneous antenna system (PSII antenna uncoupling). The extent of antenna uncoupling largely depends on the light regime, reaching the highest value in sun-acclimated plants. In parallel, the photoprotective NPQ (pNPQ) increased from deep-shade to high-light grown plants. It is proposed that the differences in the long-term modulation in the photoprotective capacity are proportional to the amount of uncoupled LHCII. In deep-shade plants, the inconsistency between invariable maximum NPQ and lower pNPQ is attributed to the thermal dissipation occurring in the PSII core.

Impact of energy limitations on function and resilience in long-wavelength Photosystem II

Photosystem II (PSII) uses the energy from red light to split water and reduce quinone, an energy-demanding process based on chlorophyll a (Chl-a) photochemistry. Two types of cyanobacterial PSII can use chlorophyll d (Chl-d) and chlorophyll f (Chl-f) to perform the same reactions using lower energy, far-red light. PSII from Acaryochloris marina has Chl-d replacing all but one of its 35 Chl-a, while PSII from Chroococcidiopsis thermalis, a facultative far-red species, has just 4 Chl-f and 1 Chl-d and 30 Chl-a. From bioenergetic considerations, the far-red PSII were predicted to lose photochemical efficiency and/or resilience to photodamage. Here, we compare enzyme turnover efficiency, forward electron transfer, back-reactions and photodamage in Chl-f-PSII, Chl-d-PSII, and Chl-a-PSII. We show that: (i) all types of PSII have a comparable efficiency in enzyme turnover; (ii) the modified energy gaps on the acceptor side of Chl-d-PSII favour recombination via P_D1⁺Phe^- repopulation, leading to increased singlet oxygen production and greater sensitivity to high-light damage compared to Chl-a-PSII and Chl-f-PSII; (iii) the acceptor-side energy gaps in Chl-f-PSII are tuned to avoid harmful back reactions, favouring resilience to photodamage over efficiency of light usage. The results are explained by the differences in the redox tuning of the electron transfer cofactors Phe and Q_A and in the number and layout of the chlorophylls that share the excitation energy with the primary electron donor. PSII has adapted to lower energy in two distinct ways, each appropriate for its specific environment but with different functional penalties.

Partially Dissecting Electron Fluxes in Both Photosystems in Spinach Leaf Disks during Photosynthetic Induction

Photosynthetic induction, a gradual increase in photosynthetic rate on a transition from darkness or low light to high light, has ecological significance, impact on biomass accumulation in fluctuating light and relevance to photoprotection in strong light. However, the experimental quantification of the component electron fluxes in and around both photosystems during induction has been rare. Combining optimized chlorophyll fluorescence, the redox kinetics of P700 [primary electron donor in Photosystem I (PSI)] and membrane inlet mass spectrometry in the absence/presence of inhibitors/mediator, we partially estimated the components of electron fluxes in spinach leaf disks on transition from darkness to 1,000 �mol photons�m-2�s-1 for up to 10 min, obtaining the following findings: (i) the partitioning of energy between both photosystems did not change noticeably; (ii) in Photosystem II (PSII), the combined cyclic electron flow (CEF2) and charge recombination (CR2) to the ground state decreased gradually toward 0 in steady state; (iii) oxygen reduction by electrons from PSII, partly bypassing PSI, was small but measurable; (iv) cyclic electron flow around PSI (CEF1) peaked before becoming somewhat steady; (v) peak magnitudes of some of the electron fluxes, all probably photoprotective, were in the descending order: CEF1 > CEF2 + CR2 > chloroplast O2 uptake; and (vi) the chloroplast NADH dehydrogenase-like complex appeared to aid the antimycin A-sensitive CEF1. The results are important for fine-tuning in silico simulation of in vivo photosynthetic electron transport processes; such simulation is, in turn, necessary to probe partial processes in a complex network of interactions in response to environmental changes.

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.

The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance

Excess illumination damages the photosynthetic apparatus with severe implications with regard to plant productivity. Unlike model organisms, the growth of Chlorella ohadii, isolated from desert soil crust, remains unchanged and photosynthetic O2 evolution increases, even when exposed to irradiation twice that of maximal sunlight. Spectroscopic, biochemical and molecular approaches were applied to uncover the mechanisms involved. D1 protein in photosystem II (PSII) is barely degraded, even when exposed to antibiotics that prevent its replenishment. Measurements of various PSII parameters indicate that this complex functions differently from that in model organisms and suggest that C. ohadii activates a nonradiative electron recombination route which minimizes singlet oxygen formation and the resulting photoinhibition. The light-harvesting antenna is very small and carotene composition is hardly affected by excess illumination. Instead of succumbing to photodamage, C. ohadii activates additional means to dissipate excess light energy. It undergoes major structural, compositional and physiological changes, leading to a large rise in photosynthetic rate, lipids and carbohydrate content and inorganic carbon cycling. The ability of C. ohadii to avoid photodamage relies on a modified function of PSII and the dissipation of excess reductants downstream of the photosynthetic reaction centers. The biotechnological potential as a gene source for crop plant improvement is self-evident.

Photoinactivation of Photosystem II in wild-type and chlorophyll b-less barley leaves: which mechanism dominates depends on experimental circumstances

Action spectra of photoinactivation of Photosystem II (PS II) in wild-type and chlorophyll b-less barley leaf segments were obtained. Photoinactivation of PS II was monitored by the delivery of electrons from PS II to PS I following single-turnover flashes superimposed on continuous far-red light, the time course of photoinactivation yielding a rate coefficient k i. Susceptibility of PS II to photoinactivation was quantified as the ratio of k i to the moderate irradiance (I) of light at each selected wavelength. k i/I was very much higher in blue light than in red light. The experimental conditions permitted little excess light energy absorbed by chlorophyll (not utilized in photochemical conversion or dissipated in controlled photoprotection) that could lead to photoinactivation of PS II. Therefore, direct absorption of light by Mn in PS II, rather than by chlorophyll, was more likely to have initiated the much more severe photoinactivation in blue light than in red light. Mutant leaves were ca. 1.5-fold more susceptible to photoinactivation than the wild type. Neither the excess-energy mechanism nor the Mn mechanism can explain this difference. Instead, the much lower chlorophyll content of mutant leaves could have exerted an exacerbating effect, possibly partly due to less mutual shading of chloroplasts in the mutant leaves. In general, which mechanism dominates depends on the experimental conditions.

A reversible decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase carboxylation activity caused by the aggregation of the enzyme's large subunit is triggered in response to the exposure of moderate irradiance-grown plants to low irradiance

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is highly regulated in response to fluctuations in the environment, including changes in irradiance. However, no complex data are available on Rubisco regulatory mechanisms triggered in plants which are submitted to moderate-low irradiance shift. Therefore, we investigated in a comprehensive way the changes at the level of amount of Rubisco protein, its structural organization and carboxylase activity of the holoenzyme as triggered by exposure of moderate irradiance-grown Arabidopsis thaliana plants to low irradiance conditions. An exposure of moderate irradiance-grown plants to low irradiance for a single photoperiod caused the exclusion of a certain pool of Rubisco under altered conditions owing to oxidative modifications resulting in the formation of protein aggregates involving Rubisco large subunit (LS). As a result, both initial and total Rubisco carboxylase activities were reduced, whereas Rubisco activation state remained largely unchanged. The results of the determination of reactive oxygen species indicated that a moderate/low irradiance transition had stimulated (1) O2 accumulation and we strongly suggest that Rubisco oxidative modifications leading to formation of aggregates encompassing Rubisco-LS were triggered by (1) O2 . When moderate irradiance regime was resumed, the majority of Rubisco-LS containing aggregates tended to be resolubilized, and this allowed Rubisco carboxylation activities to be largely recovered, without changes in the activation state of the enzyme. In the longer term, these results allow us to better understand a complexity of Rubisco regulatory mechanisms activated in response to abiotic stresses and during recovery from the stresses.

Translational regulation in chloroplasts for development and homeostasis

Chloroplast genomes encode 100-200 proteins which function in photosynthesis, the organellar genetic system, and other pathways and processes. These proteins are synthesized by a complete translation system within the chloroplast, with bacterial-type ribosomes and translation factors. Here, we review translational regulation in chloroplasts, focusing on changes in translation rates which occur in response to requirements for proteins encoded by the chloroplast genome for development and homeostasis. In addition, we delineate the developmental and physiological contexts and model organisms in which translational regulation in chloroplasts has been studied. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Quality control of PSII: behavior of PSII in the highly crowded grana thylakoids under excessive light

The grana thylakoids of higher plant chloroplasts are crowded with PSII and the associated light-harvesting complexes (LHCIIs). They constitute supercomplexes, and often form semi-crystalline arrays in the grana. The crowded condition of the grana may be necessary for efficient trapping of excitation energy by LHCII under weak light, but it might hinder proper movement of LHCII necessary for reversible aggregation of LHCII in the energy-dependent quenching of Chl fluorescence under moderate high light. When the thylakoids are illuminated with extreme high light, the reaction center-binding D1 protein of PSII is photodamaged, and the damaged protein migrates to the grana margins for degradation and subsequent repair. In both moderate and extreme high-light conditions, fluidity of the thylakoid membrane is crucial. In this review, we first provide an overview of photoprotective processes, then discuss changes in membrane fluidity and mobility of the protein complexes in the grana under excessive light, which are closely associated with photoprotection of PSII. We hypothesize that reversible aggregation of LHCII, which is necessary to avoid light stress under moderate high light, and swift turnover of the photodamaged D1 protein under extreme high light are threatened by irreversible protein aggregation induced by reactive oxygen species in photochemical reactions.

Regulation of photosynthetic electron transport and photoinhibition

Photosynthetic organisms and isolated photosystems are of interest for technical applications. In nature, photosynthetic electron transport has to work efficiently in contrasting environments such as shade and full sunlight at noon. Photosynthetic electron transport is regulated on many levels, starting with the energy transfer processes in antenna and ending with how reducing power is ultimately partitioned. This review starts by explaining how light energy can be dissipated or distributed by the various mechanisms of non-photochemical quenching, including thermal dissipation and state transitions, and how these processes influence photoinhibition of photosystem II (PSII). Furthermore, we will highlight the importance of the various alternative electron transport pathways, including the use of oxygen as the terminal electron acceptor and cyclic flow around photosystem I (PSI), the latter which seem particularly relevant to preventing photoinhibition of photosystem I. The control of excitation pressure in combination with the partitioning of reducing power influences the light-dependent formation of reactive oxygen species in PSII and in PSI, which may be a very important consideration to any artificial photosynthetic system or technical device using photosynthetic organisms.

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.

Herbicide effect on the photodamage process of photosystem II: fourier transform infrared study

The photodamage process of photosystem II by strong illumination was investigated by examining the herbicide effects on the photoinactivation of redox cofactors. O(2)-evolving photosystem II membranes from spinach in the absence of herbicide and in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and bromoxynil were subjected to strong white-light illumination at 298K, and the illumination-time dependence of the activities of Q(A), the Mn cluster, and P680 were monitored using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The decrease in the Q(A) activity was suppressed and accelerated by DCMU and bromoxynil, respectively, in comparison with the sample without herbicide. The intensity change in the S(2)/S(1) FTIR signal of the Mn cluster exhibited a time course virtually identical to that in the Q(A) signal in all the three samples, suggesting that the loss of the S(1)→S(2) transition was ascribed to the Q(A) inactivation and hence the Mn cluster was inactivated not faster than Q(A). The decrease in the P680 signal was always slower than that of Q(A) keeping the tendency of the herbicide effect. Degradation of the D1 protein occurred after the P680 inactivation. These observations are consistent with the acceptor-side mechanism, in which double reduction of Q(A) triggers the formation of (1)O(2)* to promote further damage to other cofactors and the D1 protein, rather than the recently proposed mechanism that inactivation of the Mn cluster initiates the photodamage. Thus, the results of the present study support the view that the acceptor-side mechanism dominantly occurs in the photodamage to PSII by strong white-light illumination.

Molecular mechanisms of photodamage in the Photosystem II complex

Light induced damage of the photosynthetic apparatus is an important and highly complex phenomenon, which affects primarily the Photosystem II complex. Here the author summarizes the current state of understanding of the molecular mechanisms, which are involved in the light induced inactivation of Photosystem II electron transport together with the relevant mechanisms of photoprotection. Short wavelength ultraviolet radiation impairs primarily the Mn₄Ca catalytic site of the water oxidizing complex with additional effects on the quinone electron acceptors and tyrosine donors of PSII. The main mechanism of photodamage by visible light appears to be mediated by acceptor side modifications, which develop under conditions of excess excitation in which the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation of excess excitation facilitates the reduction of intersystem electron carriers and Photosystem II acceptors, and thereby induces the formation of reactive oxygen species, especially singlet oxygen whose production is sensitized by triplet chlorophyll formation in the reaction center of Photosystem II. The highly reactive singlet oxygen and other reactive oxygen species, such as H₂O₂ and O₂⁻, which can also be formed in Photosystem II initiate damage of electron transport components and protein structure. In parallel with the excess excitation dependent mechanism of photodamage inactivation of the Mn₄Ca cluster by visible light may also occur, which impairs electron transfer through the Photosystem II complex and initiates further functional and structural damage of the reaction center via formation of highly oxidizing radicals, such as P 680(+) and Tyr-Z(+). However, the available data do not support the hypothesis that the Mn-dependent mechanism would be the exclusive or dominating pathway of photodamage in the visible spectral range. This article is part of a Special Issue entitled: Photosystem II.

Photoinactivation of photosystem II: is there more than one way to skin a cat?

We briefly review the main mechanisms proposed for photodamage to photosystem II (PSII), at the donor and acceptor sides, and then discuss the mechanism whereby filamentous cyanobacteria inhabiting biological sand crusts such as Microcoleus sp. are able to avoid serious damage to their photosynthetic machinery. We show that the decline in fluorescence following exposure to excess light does not reflect a reduction in PSII activity but rather the activation of a non-radiative charge recombination in PSII. Furthermore, we show that the difference in the thermoluminescent peak temperature intensities in these organisms, in the presence and absence of inhibitors such as dichlorophenyl-dimethylurea (DCMU), is smaller than observed in model organisms suggesting that the redox gap between Q(A)⁻ and P₆₈₀+ is smaller. On the basis of these data, we propose that this could enable an alternative, pheophytin-independent recombination, thereby minimizing the damaging ¹O₂ production associated with radiative recombination.

Superoxide anion radicals generated by methylviologen in photosystem I damage photosystem II

The effect of superoxide anion radicals on the photosynthetic electron transport chain was studied in leaves and isolated thylakoids from tobacco. Superoxide was generated by methylviologen (MV) in the light at the acceptor side of photosystem I (PSI). In isolated thylakoids, the largest damage was observed at the level of the water-splitting activity in photosystem II (PSII), whereas PSI was hardly affected at the light intensities used. Addition of reactive oxygen scavengers protected PSII against damage. In leaves in the presence of MV, the quantum yield of PSII decreased during illumination whereas the size of the P(700) signal remained constant. There was no D1 protein loss in leaves illuminated in the presence of MV and lincomycin, but a modification to a slightly higher molecular mass was observed. These data show that PSII is more sensitive to superoxide or superoxide-derived reactive oxygen species (ROS) than PSI. In our experiments, this susceptibility was not because of any action of the ROS on the translation of the D1 protein or on the repair cycle of photosystem.

Role of charge recombination processes in photodamage and photoprotection of the photosystem II complex

Light-induced damage of the photosynthetic apparatus is an important and complex phenomenon, which affects primarily the photosystem II (PSII) complex. Here, the author summarizes the current state of understanding, which concerns the role of charge recombination reactions in photodamage and photoprotection. The main mechanism of photodamage induced by visible light appears to be mediated by acceptor side modifications, which develop under light intensity conditions when the capacity of light-independent photosynthetic processes limits the utilization of electrons produced in the initial photoreactions. This situation facilitates triplet chlorophyll formation and singlet oxygen production in the reaction center of PSII, which initiates the damage of electron transport components and protein structure. This mechanism is an important, but not exclusive, pathway of photodamage, and light-induced inactivation of the Mn cluster of water oxidation may occur in parallel with the singlet oxygen-dependent pathway.

Short flashes and continuous light have similar photoinhibitory efficiency in intact leaves

Lincomycin-treated pumpkin leaves were illuminated with either continuous light or saturating single-turnover xenon flashes to study the dependence of photoinactivation of photosystem II (PSII) on the mode of delivery of light. The flash energy and the time interval between the flashes were varied between the experiments, and photoinactivation was measured with oxygen evolution and the ratio of variable to maximum fluorescence (F(v)/F(m)). The photoinhibitory efficiency of saturating xenon flashes was found to be directly proportional to flash energy and independent of the time interval between the flashes. These findings indicate that a low-light-specific mechanism, based on charge recombination between PSII electron acceptors and the oxygen-evolving complex, is not the main cause of photoinactivation caused by short flashes in vivo. Furthermore, the relationship between the rate constant of photoinactivation and photon flux density was similar for flashes and continuous light when F(v)/F(m) was used to quantify photoinactivation, suggesting that continuous-light photoinactivation has a mechanism in which the quantum yield does not depend on the mode of delivery of light. A similar quantum yield of photoinhibition for flashes and continuous light is compatible with the manganese-based photoinhibition mechanism and with mechanisms in which singlet oxygen, produced via a direct photosensitization reaction, is the agent of damage. However, the classical acceptor-side and donor-side mechanisms do not predict a similar quantum yield for flashes and continuous light.

In search of the main properties of phthalocyanines participating in toxicity against cyanobacteria

Phthalocyanines are promising photosensitizers for use in various branches of science including nanotechnology. In the presence of visible light and diatomic oxygen, phthalocyanines can react to produce singlet oxygen (1O2*), which has known inhibitory effects on cellular growth and metabolic activity, although other mechanisms may be involved. The present work focuses on the properties of phthalocyanines (atom charge densities, singlet oxygen production, inhibition effects at various irradiances) contributing to toxicity against the cyanobacteria, Synechococcus nidulans. Our results indicate that positive charge densities at peripheral parts of substituents exhibit greater inhibitory effects against S. nidulans than the amount of singlet oxygen produced, potentially by binding to negatively charged membranes on the cell surface. The weak effect of 1O2* was further demonstrated by a 10% increase in phthalocyanine toxicity (the maximal inhibition detected) when the irradiance increased 3-fold from 1200 to 4000 lux.

Plastid alternative oxidase (PTOX) promotes oxidative stress when overexpressed in tobacco

Photoinhibition and production of reactive oxygen species were studied in tobacco plants overexpressing the plastid terminal oxidase (PTOX). In high light, these plants was more susceptible to photoinhibition than wild-type plants. Also oxygen-evolving activity of isolated thylakoid membranes from the PTOX-overexpressing plants was more strongly inhibited in high light than in thylakoids from wild-type plants. In contrast in low light, in the PTOX overexpressor, the thylakoids were protected against photoinhibition while in wild type they were significantly damaged. The production of superoxide and hydroxyl radicals was shown by EPR spin-trapping techniques in the different samples. Superoxide and hydroxyl radical production was stimulated in the overexpressor. Two-thirds of the superoxide production was maintained in the presence of DNP-INT, an inhibitor of the cytochrome b(6)f complex. No increase of the SOD content was observed in the overexpressor compared with the wild type. We propose that superoxide is produced by PTOX in a side reaction and that PTOX can only act as a safety valve under stress conditions when the generated superoxide is detoxified by an efficient antioxidant system.

Replacement of alpha-tocopherol by beta-tocopherol enhances resistance to photooxidative stress in a xanthophyll-deficient strain of Chlamydomonas reinhardtii

Tocopherols (vitamin E) comprise a class of lipid-soluble antioxidants synthesized only in plants, algae, and some cyanobacteria. The majority of tocopherols in photosynthetic cells is in the alpha form, which has the highest vitamin E activity in humans, whereas the beta, gamma, and delta forms normally account for a small percentage of total tocopherols. The antioxidant activities of these forms of tocopherol differ depending on the experimental system, and their relative activities in vivo are unclear. In a screen for suppressors of the xanthophyll-deficient npq1 lor1 double mutant of Chlamydomonas reinhardtii, we isolated a vte3 mutant lacking alpha-tocopherol but instead accumulating beta-tocopherol. The vte3 mutant contains a mutation in the homolog of a 2-methyl-6-phytyl-1,4-benzoquinone methyltransferase gene found in plants. The vte3 npq1 lor1 triple mutant with beta-tocopherol survived better under photooxidative stress than did the npq1 lor1 mutant, but the vte3 mutant on its own did not have an obvious phenotype. Following transfer from low light to high light, the triple mutant showed a higher efficiency of photosystem II, a higher level of cell viability, and a lower level of lipid peroxide, a marker for oxidative stress, than did the npq1 lor1 mutant. After high-light transfer, the level of the photosystem II reaction center protein, D1, was also higher in the vte3 npq1 lor1 mutant, but the rate of D1 photodamage was not significantly different from that of the npq1 lor1 mutant. Taken together, these results suggest that the replacement of alpha-tocopherol by beta-tocopherol in a xanthophyll-deficient strain of Chlamydomonas reinhardtii contributes to better survival under conditions of photooxidative stress.

Characterization of soldat8, a suppressor of singlet oxygen-induced cell death in Arabidopsis seedlings

The flu mutant of Arabidopsis thaliana overaccumulates in the dark the immediate precursor of chlorophyllide, protochlorophyllide (Pchlide), a potent photosensitizer, that upon illumination generates singlet oxygen ((1)O2). Once (1)O2 has been released in plastids of the flu mutant, mature plants stop growing, while seedlings die. Several suppressor mutations, dubbed singlet oxygen-linked death activator (soldat), were identified that specifically abrogate (1)O2-mediated stress responses in young flu seedlings without grossly affecting (1)O2-mediated stress responses of mature flu plants. One of the soldat mutations, soldat8, was shown to impair a gene encoding the SIGMA6 factor of the plastid RNA polymerase. Reintroduction of a wild-type copy of the SOLDAT8 gene into the soldat8/flu mutant restored the phenotype of the flu parental line. In contrast to flu, seedlings of soldat8/flu did not bleach when grown under non-permissive dark/light conditions, despite their continuous overaccumulation of the photosensitizer Pchlide in the dark. The activity of SIGMA6 is confined primarily to the very early stage of seedling development. Inactivation of SIGMA6 in soldat8 mutants disturbed plastid homeostasis, drastically reduced the non-photochemical quenching capacity and enhanced the light sensitivity of young soldat8 seedlings. Surprisingly, after being grown under very low light, soldat8 seedlings showed an enhanced resistance against a subsequent severe light stress that was significantly higher than in wild-type seedlings. In order to reach a similar enhanced stress resistance, wild-type seedlings had to be exposed to a brief higher light treatment that triggered an acclimatory response. Such a mild pre-stress treatment did not further enhance the stress resistance of soldat8 seedlings. Suppression of (1)O2-mediated cell death in young flu/soldat8 seedlings seems to be due to a transiently enhanced acclimation at the beginning of seedling development caused by the initial disturbance of plastid homeostasis.

D1-protein dynamics in photosystem II: the lingering enigma

The D1/D2 heterodimer core is the heart of the photosystem II reaction center. A characteristic feature of this heterodimer is the differentially rapid, light-dependent degradation of the D1 protein. The D1 protein is possibly the most researched photosynthetic polypeptide, with aspects of structure-function, gene, messenger and protein regulation, electron transport, reactive oxygen species, photoinhibition, herbicide binding, stromal-granal translocations, reversible phosphorylation, and specific proteases, all under intensive investigation more than three decades after the protein's debut in the literature. This review will touch on some treaded areas of D1 research that have, so far, defied clear resolution, as well as cutting edge research on mechanisms and consequences of D1 protein degradation.

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.

Engineered ectopic expression of the psbA gene encoding the photosystem II D1 protein in Synechocystis sp. PCC6803

A genetic vector-recipient system was developed to engineer expression of the wild-type psbA2 gene encoding the photosystem II (PSII) D1 protein only from a non-native location (ectopic) in the Synechocystis sp. PCC6803 and the result was a new strain, designated MK1. While MK1 accumulates near normal levels of PSII under low light conditions, it is very sensitive to photoinhibition. This phenotype can be traced to impaired PSII repair capacity. Based upon the hypothesis that the non-native transcriptional activity of the re-introduced psbA gene was insufficient to sustain the translation rate necessary for normal PSII repair rates, we conducted a quantitative analysis of the relationship between psbA transcript abundance on the rate of recovery from photoinhibition. Analysis of MK1 and two other engineered strains, with intermediate levels of psbA mRNA, indicated that transcript levels are indeed limiting the engineered strains. Furthermore, transcript levels may become limiting even in the wild-type, but only under very high light conditions when the demands for D1 replacement synthesis are maximal. The work extends the original studies of Komenda and colleagues (Komenda et al. (2000) Plant Mol Biol 42(4):635-645) and sets the stage for alternative approaches to engineering non-native expression of the D1 protein.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q(A), in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q(A) by approximately -60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q(A). Thus the influence of the redox potential of Q(A) on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q(A) was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q(A) causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q(A). The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form.

The glutathione peroxidase homologous gene Gpxh in Chlamydomonas reinhardtii is upregulated by singlet oxygen produced in photosystem II

The expression of the glutathione peroxidase homologous gene Gpxh, known to be specifically induced by the formation of singlet oxygen (1O2), was analyzed in cells of Chlamydomonas reinhardtii exposed to environmental conditions causing photoinhibition. Illumination with high light intensities, leading to an increased formation of 1O2 in photosystem II, continuously induced the expression of Gpxh in cell for at least 2 h. Phenolic herbicides like dinoterb, raise the rate of 1O2 formation by increasing the probability of charge recombination in photosystem II via the formation of the primary radical pair and thereby 3P680 formation (Fufezan C et al. 2002, FEBS Letters 532, 407-410). In the presence of dinoterb the light-induced loss of the D1 protein in C. reinhardtii was increased and the high light-induced Gpxh expression was further stimulated. DCMU, a urea-type herbicide, causing reduced 1O2 generation in photosystem II, protected the D1 protein slightly against degradation and downregulated the expression of the Gpxh gene compared to untreated cells exposed to high light intensities. This indicates that the Gpxh expression is induced by 1O2 under environment conditions causing photoinhibition.

A genetic approach towards elucidating the biological activity of different reactive oxygen species in Arabidopsis thaliana

Plants are often exposed to external conditions that adversely affect their growth, development or productivity. Such unfavourable environmental stress factors may result in rapid and transient increases of intracellular concentrations of reactive oxygen species (ROS) that are chemically distinct and impact plants either by being cytotoxic or by acting as a signal. Because different ROS are generated simultaneously in different cellular and extracellular compartments, it is almost impossible to link a particular ROS to a specific stress response and to determine its mode of action. The conditional flu mutant of Arabidopsis has been used to determine the biological role of singlet oxygen. Immediately after a dark/light shift of the flu mutant, singlet oxygen is generated within the plastids activating several stress responses that include growth inhibition of mature plants and seedling lethality. These stress responses do not result from physicochemical damage caused by singlet oxygen, but are attributable to the activation of a genetically determined stress response programme triggered by the Executer1 protein. Singlet oxygen-mediated stress responses at the transcriptional level necessitate a retrograde transduction of signals from the chloroplast to the nucleus that activate distinct sets of genes different from those that are induced by superoxide/hydrogen peroxide. Hence, the biological activities of these two types of ROS are distinct from each other. Whether they act independently or interact is not known yet and is the topic of our current research.

A simple chlorophyll fluorescence parameter that correlates with the rate coefficient of photoinactivation of photosystem II

A method of partitioning the energy in a mixed population of active and photoinactivated Photosystem II (PS II) complexes based on chlorophyll fluorescence measurements is presented. There are four energy fluxes, each with its quantum efficiency: a flux associated with photochemical electron flow in active PS II reaction centres (JPS II), thermal dissipation in photoinactivated, non-functional PS IIs (JNF), light-regulated thermal dissipation in active PS IIs (JNPQ) and a combined flux of fluorescence and constitutive, light-independent thermal dissipation (Jf,D). The four quantum efficiencies add up to 1.0, without the need to introduce an 'excess' term E, which in other studies has been claimed to be linearly correlated with the rate coefficient of photoinactivation of PS II (kpi). We examined the correlation of kpi with various fluxes, and found that the combined flux (JNPQ + Jf,D= Jpi) is as well correlated with kpi as is E. This combined flux arises from Fs/Fm ', the ratio of steady-state to maximum fluorescence during illumination, which represents the quantum efficiency of combined non-photochemical dissipation pathways in active PS IIs. Since Fs/Fm ' or its equivalent, Jpi, is a likely source of events leading to photoinactivation of PS II, we conclude that Fs/Fm ' is a simple predictor of kpi.

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.

Photoinactivation of photosystem II in leaves

Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p = kDeltax, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N = Noexp(-kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N = Noexp(-kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N = Noexp(-kx), the quantum yield of photoinactivation of PS II can be defined as -dN/dx = kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1micromol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10(7) photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely-photoinactivated PS II complexes, able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.

Photosystem II proteins PsbL and PsbJ regulate electron flow to the plastoquinone pool

The psbEFLJ operon of tobacco plastids encodes four bitopic low molecular mass transmembrane components of photosystem II. Here, we report the effect of inactivation of psbL on the directional forward electron flow of photosystem II as compared to that of the wild type and the psbJ deletion mutant, which is impaired in PSII electron flow to plastoquinone [Regel et al. (2001) J. Biol. Chem. 276, 41473-41478]. Exposure of Delta psbL plants to a saturating light pulse gives rise to the maximal fluorescence emission, Fm(L), which is followed within 4-6 s by a broader hitherto not observed second fluorescence peak in darkness, Fm(D). Conditions either facilitating oxidation or avoiding reduction of the plastoquinone pool do not affect the Fm(L) level of Delta psbL plants but prevent the appearance of Fm(D). The level of Fm(D) is proportional to the intensity and duration of the light pulse allowing reduction of the plastoquinone pool in dark-adapted leaves prior to the activation of PSI and oxidation of plastoquinol. Lowering the temperature decreases the Fm(D) level in the Delta psbL mutant, whereas it increases considerably the lifetime of Q(A)*- in the Delta psbJ mutant. The thermoluminescence signal generated by Q(A)*-/S(2) charge recombination is not affected; on the other hand, charge recombination of Q(B)*-/S(2,3) could not be detected in Delta psbL plants. PSII is highly sensitive to photoinhibition in Delta psbL. We conclude that PsbL prevents reduction of PSII by back electron flow from plastoquinol protecting PSII from photoinactivation, whereas PsbJ regulates forward electron flow from Q(A)*- to the plastoquinone pool. Therefore, both proteins contribute substantially to ensure unidirectional forward electron flow from PSII to the plastoquinone pool.

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.

Photosystem II damage and repair cycle in the green alga Dunaliella salina: involvement of a chloroplast-localized HSP70

The involvement of HSP70B in the photosystem II damage and repair process in Dunaliella salina was investigated. A full-length cDNA of the D. salina hsp70B gene was cloned and sequenced. Expression patterns of the hsp70B gene were investigated upon shifting a D. salina culture from low-light to high-light growth conditions, designed to significantly accelerate the rate of PSII photodamage. Northern blot analyses and nuclear run-on transcription assays revealed a significant but transient induction of hsp70B gene transcription, followed by a subsequent increase in HSP70B protein synthesis and accumulation. Mild detergent solubilization of photoinhibited thylakoid membranes, in which photodamaged PSII centers had accumulated, followed by native gel electrophoresis revealed the formation of a 320 kDa protein complex that contained, in addition to the HSP70B, the photodamaged but as yet undegraded D1 protein as well as D2 and CP47. Evidence suggested that the 320 kDa complex is a transiently forming PSII repair intermediate. Denaturing solubilization of the 320 kDa PSII repair intermediate by SDS-urea resulted in cross-linking of its polypeptide constituents, yielding a 160 kDa protein complex. The role of the HSP70B in the repair of photodamaged PSII centers, e.g. in stabilizing the disassembled PSII-core complex and in facilitating the D1 degradation and replacement process, is discussed.

Deregulation of electron flow within photosystem II in the absence of the PsbJ protein

The photosystem II (PSII) complex of photosynthetic oxygen evolving membranes comprises a number of small proteins whose functions remain unknown. Here we report that the low molecular weight protein encoded by the psbJ gene is an intrinsic component of the PSII complex. Fluorescence kinetics, oxygen flash yield, and thermoluminescence measurements indicate that inactivation of the psbJ gene in Synechocystis 6803 cells and tobacco chloroplasts lowers PSII-mediated oxygen evolution activity and increases the lifetime of the reduced primary acceptor Q(A)(-) (more than a 100-fold in the tobacco DeltapsbJ mutant). The decay of the oxidized S(2,3) states of the oxygen-evolving complex is considerably accelerated, and the oscillations of the Q(B)(-)/S(2,3) recombination with the number of exciting flashes are damped. Thus, PSII can be assembled in the absence of PsbJ. However, the forward electron flow from Q(A)(-) to plastoquinone and back electron flow to the oxidized Mn cluster of the donor side are deregulated in the absence of PsbJ, thereby affecting the efficiency of PSII electron flow following the charge separation process.

Herbicide-induced oxidative stress in photosystem II

Some herbicides act by binding to the exchangeable quinone site in the photosystem II (PSII) reaction centre, thus blocking electron transfer. In this article, it is hypothesized that the plant is killed by light-induced oxidative stress initiated by damage caused by formation of singlet oxygen in the reaction centre itself. This occurs when light-induced charge pairs in herbicide-inhibited PSII decay by a charge recombination route involving the formation of a chlorophyll triplet state that is able to activate oxygen. The binding of phenolic herbicides favours this pathway, thus increasing the efficiency of photodamage in this class of herbicides.

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.

GTP enhances the degradation of the photosystem II D1 protein irrespective of its conformational heterogeneity at the Q(B) site

The light exposure history and/or binding of different herbicides at the Q(B) site may induce heterogeneity of photosystem II acceptor side conformation that affects D1 protein degradation under photoinhibitory conditions. GTP was recently found to stimulate the D1 protein degradation of photoinactivated photosystem II (Spetea, C. , Hundal, T., Lohmann, F., and Andersson, B. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6547-6552). Here we report that GTP enhances the cleavage of the D1 protein D-E loop following exposure of thylakoid membranes to either high light, low light, or repetitive single turnover flashes but not to trypsin. GTP does not stimulate D1 protein degradation in the presence of herbicides known to affect the accessibility of the cleavage site to proteolysis. However, GTP stimulates degradation that can be induced even in darkness in some photosystem II conformers following binding of the PNO8 herbicide (Nakajima, Y., Yoshida, S., Inoue, Y., Yoneyama, K., and Ono, T. (1995) Biochim. Biophys. Acta 1230, 38-44). Both the PNO8- and the light-induced primary cleavage of the D1 protein occur in the grana membrane domains. The subsequent migration of photosytem II containing the D1 protein fragments to the stroma domains for secondary proteolysis is light-activated. We conclude that the GTP effect is not confined to a specific photoinactivation pathway nor to the conformational state of the photosystem II acceptor side. Consequently, GTP does not interact with the site of D1 protein cleavage but rather enhances the activity of the endogenous proteolytic system.

Turnover of the aggregates and cross-linked products of the D1 protein generated by acceptor-side photoinhibition of photosystem II

It is known that the reaction-center binding protein D1 in photosystem (PS) II is degraded significantly during photoinhibition. The D1 protein also cross-links covalently or aggregates non-covalently with the nearby polypeptides in PS II complexes by illumination. In the present study, we detected the adducts between the D1 protein and the other reaction-center binding protein D2 (D1/D2), the alpha-subunit of cyt b(559) (D1/cyt b(559)), and the antenna chlorophyll-binding protein CP43 (D1/CP43) by SDS/urea-polyacrylamide gel electrophoresis and Western blotting with specific antibodies. The adducts were observed by weak and strong illumination (light intensity: 50-5000 microE m(-2) s(-1)) of PS II membranes, thylakoids and intact chloroplasts from spinach, under aerobic conditions. These results indicate that the cross-linking or aggregation of the D1 protein is a general phenomenon which occurs in vivo as well as in vitro with photodamaged D1 proteins. We found that the formation of the D1/D2, D1/cyt b(559) and D1/CP43 adducts is differently dependent on the light intensity; the D1/D2 heterodimers and D1/cyt b(559) were formed even by illumination with weak light, whereas generation of the D1/CP43 aggregates required strong illumination. We also detected that these D1 adducts were efficiently removed by the addition of stromal components, which may contain proteases, molecular chaperones and the associated proteins. By two-dimensional SDS/urea-polyacrylamide gel electrophoresis, we found that several stromal proteins, including a 15-kDa protein are effective in removing the D1/CP43 aggregates, and that their activity is resistant to SDS.

D1-D2 protein degradation in the chloroplast. Complex light saturation kinetics

The D1 and D2 proteins of the photosystem II (PSII) reaction center are stable in the dark, while rapid degradation occurs in the light. Thus far, a quantitative correlation between degradation and photon fluences has not been determined. In Spirodela oligorrhiza, D1-D2 degradation increases with photon flux. We find that kinetics for D2 degradation mirror those for D1, except that the actual half-life times of the D2 protein are about three times larger than those of the D1. The degradation ratio, D2/D1, is fluence independent, supporting the proposal [Jansen, M.A.K., Greenberg, B.M., Edelman, M., Mattoo, A.K. & Gaba, V. (1996), Photochem. Photobiol. 63, 814-817] that degradation of the two proteins is coupled. It is commonly conceived that D1 degradation is predominantly associated with photon fluences that are supersaturating for photosynthesis. We now show that a fluence as low as 5 mumol.m-2.s-1 elicited a reaction constituting > 25% of the total degradation response, while > 90% of the degradation potential was attained at intensities below saturation for photosynthesis (approximately 750 mumol.m-2.s-1). Thus, in intact plants, D1 degradation is overwhelmingly associated with fluences limiting for photosynthesis. D1 degradation increases with photon flux in a complex, multiphasic manner. Four phases were uncovered over the fluence range from 0-1600 mumol.m-2.s-1. The multiphasic saturation kinetics underscore that the D1 and D2 degradation response is complex, and emanates from more than one parameter. The physiological processes associated with each phase remain to be determined.

Influence of herbicide binding on the redox potential of the quinone acceptor in photosystem II: relevance to photodamage and phytotoxicity

Here we show that herbicide binding influences the redox potential (Em) of the plastoquinone QA/QA- redox couple in Photosystem II (PSII). Phenolic herbicides lower the Em by approximately 45 mV, while DCMU raises the Em by 50 mV. These shifts are reflected in changes in the peak temperature of thermoluminescence bands arising from the recombination of charge pairs involving QA-. The herbicide-induced changes in the Em of QA/QA- correlate with earlier work showing that phenolic herbicides increase the sensitivity of PSII to light, while DCMU protects against photodamage. This correlation is explained in terms of the following hypothesis which is based on reactions occurring in the bacterial reaction center. The back-reaction pathway for P680+QA- is assumed to be modulated by the free-energy gap between the P680+QA- and the P680+Ph- radical pairs. When this gap is small (i.e., when the Em of QA/QA- is lowered), a true back-reaction is favored in which P680+Ph- is formed, a state which decays forming a significant yield of P680 triplet. This triplet state of chlorophyll reacts with oxygen, forming singlet oxygen, a species likely to be responsible for photodamage. When the free-energy gap is increased (i.e., when the Em of QA/QA- is raised), the yield of the P680+Ph- is diminished and a greater proportion of the P680+QA- radical pair decays by an alternative, less damaging, route. We propose that at least some of the phytotoxic properties of phenolic herbicides may be explained by the fact that they render PSII ultrasensitive to light due to this mechanism.

Exposure of Synechocystis 6803 cells to series of single turnover flashes increases the psbA transcript level by activating transcription and down-regulating psbA mRNA degradation

Exposure of Synechocystis sp. PCC 6803 cells to series of single turnover flashes increases specifically the level of psbA and psbD2 messages, encoding the D1 and D2 proteins of photosystem II, as compared to light exposed cells. This increase is due to maintenance the transcription rate as high as in growth light and to the down-regulation of transcript degradation as in darkness. Inhibition of the plastoquinone pool reduction by DCMU or its oxidation by DBMIB does not diminish the transcription of the psbA gene under growth conditions. However, the degradation rate of psbA transcript, as well as of other transcripts encoding proteins of thylakoid complexes, is down-regulated in all conditions leading to the oxidation of the plastoquinone pool. We conclude that single turnover flashes are sensed as 'light' by transcription machinery of the cells irrespective of the plastoquinone pool reduction state and as 'dark' by the transcript degradation system.

Photoinhibition of photosynthesis in vivo results in singlet oxygen production detection via nitroxide-induced fluorescence quenching in broad bean leaves

In plants experiencing environmental stress, the formation of reactive oxygen is often presumed. In this study, singlet oxygen was detected in broad bean (Vicia faba) leaves that were photoinhibited in vivo. Detection was based on the reaction of singlet oxygen with DanePy (dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole) yielding a nitroxide radical (DanePyO) which is EPR active and also features lower fluorescence compared to DanePy. The two (fluorescent and spin) sensor fuctions of DanePy are commensurate, which makes detecting singlet oxygen possible with a spectrofluorimeter in samples hard to measure with EPR spectroscopy [Kálai, T., Hideg, E., Vass, I., and Hideg, K. (1998) Free Radical Biol. Med. 24, 649-652]. We found that in leaves saturated with DanePy, the fluorescence of this double sensor was decreased when the leaves were photoinhibited by 1500 micromol m-2 s-1 photosynthetically active radiation. This fluorescence quenching is the first direct experimental evidence that photoinhibition of photosynthesis in vivo is accompanied by 1O2 production and is, at least partly, governed by the process characterized as acceptor side-induced photoinhibition in vitro.

Reaction centre photochemistry in cyanide-treated photosystem II

EPR was used to study the triplet state of chlorophyll generated by radical pair recombination in the photosystem II (PSII) reaction centre. The spin state of the non-haem Fe2+ was varied using the CN--binding method (Y. Sanakis, V. Petrouleas, B.A. Diner, Biochemistry 33 (1994) 9922-9928) and the redox state of the quinone acceptor (QA) was changed from semi-reduced to fully reduced (F.J.E. van Mieghem, W. Nitschke, P. Mathis, A.W. Rutherford, Biochim. Biophys. Acta 977 (1989) 207-214). It was found that the triplet was not detectable using continuous wave EPR when QA- was present irrespective of the spin-state of the Fe2+. It was also found that the triplet state became detectable by EPR when the semiquinone was removed (by reduction to the quinol) and that the triplet observed was not influenced by the spin state of the Fe2+. Since it is known from earlier work that the EPR detection of the triplet reflects a change in the triplet lifetime, it is concluded that the redox state of the quinone determines the triplet lifetime (at least in terms of its detectability by continuous wave EPR) and that the magnetic state of the iron, (through the weakly exchange-coupled QA- Fe2+ complex) is not a determining factor. In addition, we looked for polarisation transfer from the radical pair to QA- in PSII where the Fe2+ was low spin. Such polarisation is seen in bacterial reaction centres under comparable conditions. In PSII, however, we were unable to find evidence for such polarisation of the semiquinone. It is suggested that both the short triplet lifetime in the presence of QA- and the lack of polarised QA- might be explained in terms of the electron transfer mechanism for triplet quenching involving the semiquinone which was proposed previously (F.J.E. van Mieghem, K. Brettel, B. Hillmann, A. Kamlowski, A.W. Rutherford, E. Schlodder, Biochemistry 34 (1995) 4798-4813). It is suggested that this mechanism may occur in PSII (but not in purple bacterial reaction centres) due the triplet-bearing chlorophyll being adjacent to the pheophytin at low temperature as suggested from structural studies (F.J.E. van Mieghem, K. Satoh, A.W. Rutherford, Biochim. Biophys. Acta 1058 (1992) 379-385).