A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO₂ is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.     

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO(2) is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.     

How do environmental stresses accelerate photoinhibition?

Environmental stress enhances the extent of photoinhibition, a process that is determined by the balance between the rate of photodamage to photosystem II (PSII) and the rate of its repair. Recent investigations suggest that exposure to environmental stresses, such as salt, cold, moderate heat and oxidative stress, do not affect photodamage but inhibit the repair of PSII through suppression of the synthesis of PSII proteins. In particular, production of D1 protein is downregulated at the translation step by the direct inactivation of the translation machinery and/or by primarily interrupting the fixation of CO2. The latter results in the creation of reactive oxygen species (ROS), which in turn block the synthesis of PSII proteins in chloroplasts.     

Application of low temperatures during photoinhibition allows characterization of individual steps in photodamage and the repair of photosystem II

Recent investigations of photoinhibition have revealed that photodamage to photosystem II (PSII) involves two temporally separated steps: the first is the inactivation of the oxygen-evolving complex by light that has been absorbed by the manganese cluster and the second is the impairment of the photochemical reaction center by light that has been absorbed by chlorophyll. Our studies of photoinhibition in Synechocystis sp. PCC 6803 at various temperatures demonstrated that the first step in photodamage is not completed at low temperatures, such as 10°C. Further investigations suggested that an intermediate state, which is stabilized at low temperatures, might exist at the first stage of photodamage. The repair of PSII involves many steps, including degradation and removal of the D1 protein, synthesis de novo of the precursor to the D1 protein, assembly of the PSII complex, and processing of the precursor to the D1 protein. Detailed analysis of photodamage and repair at various temperatures has demonstrated that, among these steps, only the synthesis of the precursor to D1 appears to proceed at low temperatures. Investigations of photoinhibition at low temperatures have also indicated that prolonged exposure of cyanobacterial cells or plant leaves to strong light diminishes their ability to repair PSII. Such non-repairable photoinhibition is caused by inhibition of the processing of the precursor to the D1 protein after prolonged illumination with strong light at low temperatures.     

The mechanism of photoinhibition in vivo: Re-evaluation of the roles of catalase, α-tocopherol, non-photochemical quenching, and electron transport

Photoinhibition of photosystem II (PSII) occurs when the rate of light-induced inactivation (photodamage) of PSII exceeds the rate of repair of the photodamaged PSII. For the quantitative analysis of the mechanism of photoinhibition of PSII, it is essential to monitor the rate of photodamage and the rate of repair separately and, also, to examine the respective effects of various perturbations on the two processes. This strategy has allowed the re-evaluation of the results of previous studies of photoinhibition and has provided insight into the roles of factors and mechanisms that protect PSII from photoinhibition, such as catalases and peroxidases, which are efficient scavengers of H(2)O(2); α-tocopherol, which is an efficient scavenger of singlet oxygen; non-photochemical quenching, which dissipates excess light energy that has been absorbed by PSII; and the cyclic and non-cyclic transport of electrons. Early studies of photoinhibition suggested that all of these factors and mechanisms protect PSII against photodamage. However, re-evaluation by the strategy mentioned above has indicated that, rather than protecting PSII from photodamage, they stimulate protein synthesis, with resultant repair of PSII and mitigation of photoinhibition. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Systematic analysis of the relation of electron transport and ATP synthesis to the photodamage and repair of photosystem II in Synechocystis

The photosynthetic machinery and, in particular, the photosystem II (PSII) complex are susceptible to strong light, and the effects of strong light are referred to as photodamage or photoinhibition. In living organisms, photodamaged PSII is rapidly repaired and, as a result, the extent of photoinhibition represents a balance between rates of photodamage and the repair of PSII. In this study, we examined the roles of electron transport and ATP synthesis in these two processes by monitoring them separately and systematically in the cyanobacterium Synechocystis sp. PCC 6803. We found that the rate of photodamage, which was proportional to light intensity, was unaffected by inhibition of the electron transport in PSII, by acceleration of electron transport in PSI, and by inhibition of ATP synthesis. By contrast, the rate of repair was reduced upon inhibition of the synthesis of ATP either via PSI or PSII. Northern blotting and radiolabeling analysis with [(35)S]Met revealed that synthesis of the D1 protein was enhanced by the synthesis of ATP. Our observations suggest that ATP synthesis might regulate the repair of PSII, in particular, at the level of translation of the psbA genes for the precursor to the D1 protein, whereas neither electron transport nor the synthesis of ATP affects the extent of photodamage.     

Revised scheme for the mechanism of photoinhibition and its application to enhance the abiotic stress tolerance of the photosynthetic machinery

When photosynthetic organisms are exposed to abiotic stress, their photosynthetic activity is significantly depressed. In particular, photosystem II (PSII) in the photosynthetic machinery is readily inactivated under strong light and this phenomenon is referred to as photoinhibition of PSII. Other types of abiotic stress act synergistically with light stress to accelerate photoinhibition. Recent studies of photoinhibition have revealed that light stress damages PSII directly, whereas other abiotic stresses act exclusively to inhibit the repair of PSII after light-induced damage (photodamage). Such inhibition of repair is associated with suppression, by reactive oxygen species (ROS), of the synthesis of proteins de novo and, in particular, of the D1 protein, and also with the reduced efficiency of repair under stress conditions. Gene-technological improvements in the tolerance of photosynthetic organisms to various abiotic stresses have been achieved via protection of the repair system from ROS and, also, by enhancing the efficiency of repair via facilitation of the turnover of the D1 protein in PSII. In this review, we summarize the current status of research on photoinhibition as it relates to the effects of abiotic stress and we discuss successful strategies that enhance the activity of the repair machinery. In addition, we propose several potential methods for activating the repair system by gene-technological methods.     

Protection by α-tocopherol of the repair of photosystem II during photoinhibition in Synechocystis sp. PCC 6803

α-Tocopherol is a lipophilic antioxidant that is an efficient scavenger of singlet oxygen. We investigated the role of α-tocopherol in the protection of photosystem II (PSII) from photoinhibition using a mutant of the cyanobacterium Synechocystis sp. PCC 6803 that is deficient in the biosynthesis of α-tocopherol. The activity of PSII in mutant cells was more sensitive to inactivation by strong light than that in wild-type cells, indicating that lack of α-tocopherol enhances the extent of photoinhibition. However, the rate of photodamage to PSII, as measured in the presence of chloramphenicol, which blocks the repair of PSII, did not differ between the two lines of cells. By contrast, the repair of PSII from photodamage was suppressed in mutant cells. Addition of α-tocopherol to cultures of mutant cells returned the extent of photoinhibition to that in wild-type cells, without any effect on photodamage. The synthesis de novo of various proteins, including the D1 protein that plays a central role in the repair of PSII, was suppressed in mutant cells under strong light. These observations suggest that α-tocopherol promotes the repair of photodamaged PSII by protecting the synthesis de novo of the proteins that are required for recovery from inhibition by singlet oxygen.     

A change in the sensitivity of elongation factor G to oxidation protects photosystem II from photoinhibition in Synechocystis sp. PCC 6803

The repair of photosystem II (PSII) after photodamage is particularly sensitive to oxidative stress and inhibition of such repair is associated with the oxidation of specific cysteine residues in elongation factor G (EF-G), a key translation factor, in the cyanobacterium Synechocystis sp. PCC 6803. Expression of mutated EF-G with a target cysteine residue replaced by serine in Synechocystis resulted in the protection of PSII from photoinhibition. This protection was attributable to the enhanced repair of PSII via acceleration of the synthesis of the D1 protein, which might have been due to reduced sensitivity of protein synthesis to oxidative stress.     

Genomic investigation of the system for selenocysteine incorporation in the bacterial domain

The Photosystem II complex (PSII) is susceptible to inactivation by strong light, and the inactivation caused by strong light is referred to as photoinactivation or photoinhibition. In photosynthetic organisms, photoinactivated PSII is rapidly repaired and the extent of photoinactivation reflects the balance between the light-induced damage (photodamage) to PSII and the repair of PSII. In this study, we examined these two processes separately and quantitatively under stress conditions in the cyanobacterium Synechocystis sp. PCC 6803. The rate of photodamage was proportional to light intensity over a range of light intensities from 0 to 2000 microE m(-2) s(-1), and this relationship was not affected by environmental factors, such as salt stress, oxidative stress due to H2O2, and low temperature. The rate of repair also depended on light intensity. It was high under weak light and reached a maximum of 0.1 min(-1) at 300 microE m(-2) s(-1). By contrast to the rate of photodamage, the rate of repair was significantly reduced by the above-mentioned environmental factors. Pulse-labeling experiments with radiolabeled methionine revealed that these environmental factors inhibited the synthesis de novo of proteins. Such proteins included the D1 protein which plays an important role in the photodamage-repair cycle. These observations suggest that the repair of PSII under environmental stress might be the critical step that determines the outcome of the photodamage-repair cycle.     

Cold transiently activates calcium-permeable channels in Arabidopsis mesophyll cells

Oxygenation of ribulose-1,5-bisphosphate catalyzed by Rubisco produces glycolate-2-P. The photorespiratory pathway, which consists of photorespiratory carbon and nitrogen cycles, metabolizes glycolate-2-P to the Calvin cycle intermediate glycerate-3-P and is proposed to be important for avoiding photoinhibition of photosystem II (PSII), especially in C3 plants. We show here that mutants of Arabidopsis (Arabidopsis thaliana) with impairment of ferredoxin-dependent glutamate synthase, serine hydroxymethyltransferase, glutamate/malate transporter, and glycerate kinase had accelerated photoinhibition of PSII by suppression of the repair of photodamaged PSII and not acceleration of the photodamage to PSII. We found that suppression of the repair process was attributable to inhibition of the synthesis of the D1 protein at the level of translation. Our results suggest that the photorespiratory pathway helps avoid inhibition of the synthesis of the D1 protein, which is important for the repair of photodamaged PSII upon interruption of the Calvin cycle.     

Environmental stress inhibits the synthesis de novo of proteins involved in the photodamage-repair cycle of Photosystem II in Synechocystis sp. PCC 6803

The Photosystem II complex (PSII) is susceptible to inactivation by strong light, and the inactivation caused by strong light is referred to as photoinactivation or photoinhibition. In photosynthetic organisms, photoinactivated PSII is rapidly repaired and the extent of photoinactivation reflects the balance between the light-induced damage (photodamage) to PSII and the repair of PSII. In this study, we examined these two processes separately and quantitatively under stress conditions in the cyanobacterium Synechocystis sp. PCC 6803. The rate of photodamage was proportional to light intensity over a range of light intensities from 0 to 2000 μE m⁻² s⁻¹, and this relationship was not affected by environmental factors, such as salt stress, oxidative stress due to H₂O₂, and low temperature. The rate of repair also depended on light intensity. It was high under weak light and reached a maximum of 0.1 min⁻¹ at 300 μE m⁻² s⁻¹. By contrast to the rate of photodamage, the rate of repair was significantly reduced by the above-mentioned environmental factors. Pulse-labeling experiments with radiolabeled methionine revealed that these environmental factors inhibited the synthesis de novo of proteins. Such proteins included the D1 protein which plays an important role in the photodamage-repair cycle. These observations suggest that the repair of PSII under environmental stress might be the critical step that determines the outcome of the photodamage-repair cycle.     

Expression of a highly active catalase VktA in the cyanobacterium Synechococcus elongatus PCC 7942 alleviates the photoinhibition of photosystem II

The repair of photosystem II (PSII) after photodamage is particularly sensitive to reactive oxygen species—such as H₂O₂, which is abundantly produced during the photoinhibition of PSII. In the present study, we generated a transformant of the cyanobacterium Synechococcus elongatus PCC 7942 that expressed a highly active catalase, VktA, which is derived from a facultatively psychrophilic bacterium Vibrio rumoiensis, and examined the effect of expression of VktA on the photoinhibition of PSII. The activity of PSII in transformed cells declined much more slowly than in wild-type cells when cells were exposed to strong light in the presence of H₂O₂. However, the rate of photodamage to PSII, as monitored in the presence of chloramphenicol, was the same in the two lines of cells, suggesting that the repair of PSII was protected by the expression of VktA. The de novo synthesis of the D1 protein, which is required for the repair of PSII, was activated in transformed cells under the same stress conditions. Similar protection of the repair of PSII in transformed cells was also observed under strong light at a relatively low temperature. Thus, the expression of the highly active catalase mitigates photoinhibition of PSII by protecting protein synthesis against damage by H₂O₂ with subsequent enhancement of the repair of PSII.     

Photoinhibition of photosystem II under environmental stress

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to an imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. In the "classical" scheme for the mechanism of photoinhibition, strong light induces the production of reactive oxygen species (ROS), which directly inactivate the photochemical reaction center of PSII. By contrast, in a new scheme, we propose that photodamage is initiated by the direct effect of light on the oxygen-evolving complex and that ROS inhibit the repair of photodamaged PSII by suppressing primarily the synthesis of proteins de novo. The activity of PSII is restricted by a variety of environmental stresses. The effects of environmental stress on damage to and repair of PSII can be examined separately and it appears that environmental stresses, with the exception of strong light, act primarily by inhibiting the repair of PSII. Studies have demonstrated that repair-inhibitory stresses include CO(2) limitation, moderate heat, high concentrations of NaCl, and low temperature, each of which suppresses the synthesis of proteins de novo, which is required for the repair of PSII. We postulate that most types of environmental stress inhibit the fixation of CO(2) with the resultant generation of ROS, which, in turn, inhibit protein synthesis.     

Photoinhibition of photosystem II under environmental stress

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to an imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. In the “classical” scheme for the mechanism of photoinhibition, strong light induces the production of reactive oxygen species (ROS), which directly inactivate the photochemical reaction center of PSII. By contrast, in a new scheme, we propose that photodamage is initiated by the direct effect of light on the oxygen-evolving complex and that ROS inhibit the repair of photodamaged PSII by suppressing primarily the synthesis of proteins de novo. The activity of PSII is restricted by a variety of environmental stresses. The effects of environmental stress on damage to and repair of PSII can be examined separately and it appears that environmental stresses, with the exception of strong light, act primarily by inhibiting the repair of PSII. Studies have demonstrated that repair-inhibitory stresses include CO₂ limitation, moderate heat, high concentrations of NaCl, and low temperature, each of which suppresses the synthesis of proteins de novo, which is required for the repair of PSII. We postulate that most types of environmental stress inhibit the fixation of CO₂ with the resultant generation of ROS, which, in turn, inhibit protein synthesis.     

Glycinebetaine alleviates the inhibitory effect of moderate heat stress on the repair of photosystem II during photoinhibition

Transformation with the bacterial gene codA for choline oxidase allows Synechococcus sp. PCC 7942 cells to accumulate glycinebetaine when choline is supplemented exogenously. First, we observed two types of protective effect of glycinebetaine against heat-induced inactivation of photosystem II (PSII) in darkness; the codA transgene shifted the temperature range of inactivation of the oxygen-evolving complex from 40-52 degrees C (with half inactivation at 46 degrees C) to 46-60 degrees C (with half inactivation at 54 degrees C) and that of the photochemical reaction center from 44-55 degrees C (with half inactivation at 51 degrees C) to 52-63 degrees C (with half inactivation at 58 degrees C). However, in light, PSII was more sensitive to heat stress; when moderate heat stress, such as 40 degrees C, was combined with light stress, PSII was rapidly inactivated, although these stresses, when applied separately, did not inactivate either the oxygen-evolving complex or the photochemical reaction center. Further our studies demonstrated that the moderate heat stress inhibited the repair of PSII during photoinhibition at the site of synthesis de novo of the D1 protein but did not accelerate the photodamage directly. The codA transgene and, thus, the accumulation of glycinebetaine alleviated such an inhibitory effect of moderate heat stress on the repair of PSII by accelerating the synthesis of the D1 protein. We propose a hypothetical scheme for the cyanobacterial photosynthesis that moderate heat stress inhibits the translation machinery and glycinebetaine protects it against the heat-induced inactivation.     

Photoprotection in plants: a new light on photosystem II damage

Sunlight damages photosynthetic machinery, primarily photosystem II (PSII), and causes photoinhibition that can limit plant photosynthetic activity, growth and productivity. The extent of photoinhibition is associated with a balance between the rate of photodamage and its repair. Recent studies have shown that light absorption by the manganese cluster in the oxygen-evolving complex of PSII causes primary photodamage, whereas excess light absorbed by light-harvesting complexes acts to cause inhibition of the PSII repair process chiefly through the generation of reactive oxygen species. As we review here, PSII photodamage and the inhibition of repair are therefore alleviated by photoprotection mechanisms associated with avoiding light absorption by the manganese cluster and successfully consuming or dissipating the light energy absorbed by photosynthetic pigments, respectively.     

Inhibition of Calvin–Benson cycle suppresses the repair of photosystem II in Symbiodinium: implications for coral bleaching

Inhibition of Calvin–Benson cycle (CBC) activity by thermal stress has been hypothesized to cause photoinhibition of photosystem II (PSII) in zooxanthellae of reef-building corals and consequently lead to bleaching. This study tests whether the interruption of CBC by glycolaldehyde (GA) leads to photoinhibition and subsequent coral bleaching in Stylophora pistillata. When S. pistillata was incubated with GA, the O₂ evolution rate declined in a dose-dependent manner and the extent of photoinhibition, reflected by a decreased maximum quantum yield of PSII (F _v/F _m), was enhanced. The effect of GA on photoinhibition was similar to that of chloramphenicol (CAP), an inhibitor of protein synthesis in chloroplasts. When S. pistillata was incubated in weak light following a high-light-induced photoinhibitory treatment, the recovery of PSII from photoinhibition was suppressed in a similar manner to both GA- and CAP-treated samples. After incubation in moderate light at 26°C, S. pistillata showed a bleaching response only in presence of GA. These results suggest that coral bleaching-like responses are caused by interruption of the CBC activity in S. pistillata and are associated with accelerated photoinhibition through suppression of the protein synthesis-dependent repair of PSII but not to an increase in photodamage to PSII.     

Cost and benefit of the repair of photodamaged photosystem II in spinach leaves: roles of acclimation to growth light

When visible light is excess, the photosynthetic machinery is photoinhibited. The extent of net photoinhibition of photosystem II (PSII) is determined by a balance between the rate of photodamage to D1 and some other PSII proteins and the rate of the turnover cycle of these proteins. It is widely believed that the protein turnover requires much energy cost. The aims of this study are to (1) evaluate the energy cost of PSII repair, (2) measure the benefit in terms of photosynthetic gain realized by the repairing of the photodamaged PSII, and (3) know whether acclimation of photosynthesis to growth light affects the rates of the photodamage and repair. We grew spinach in high-light (HL) and low-light (LL) and measured the rates of D1 photodamage and repair in these leaves. We determined the rate constants of photodamage (k (pi)) and repair (k (rec)) by the PAM fluorometry in the presence or in the absence of lincomycin, an inhibitor of 70S protein synthesis. HL leaves showed smaller k (pi) and greater k (rec) than LL leaves. The energy cost of the repairing of the photodamaged D1 protein was <0.5?% of ATP produced by photophosphorylation at PPFDs ranging from 400 to 1600?μmol?m(-2)?s(-1) and was greater in HL leaves than in LL leaves. The benefits brought about by the repair were more than from 35 to 270 times the cost at PPFDs ranging from 400 to 1600?μmol?m(-2)?s(-1). The benefits of HL leaves were greater than those of LL leaves because of the higher photosynthesis rates in HL leaves. Running a simple simulation of daily photosynthesis using the parameters obtained in this study, we discuss why the plants need to pay the cost of D1 protein turnover to repair the photodamaged PSII.