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.     

Very strong UV-A light temporally separates the photoinhibition of photosystem II into light-induced inactivation and repair

When organisms that perform oxygenic photosynthesis are exposed to strong visible or UV light, inactivation of photosystem II (PSII) occurs. However, such organisms are able rapidly to repair the photoinactivated PSII. The phenomenon of photoinactivation and repair is known as photoinhibition. Under normal laboratory conditions, the rate of repair is similar to or faster than the rate of photoinactivation, preventing the detailed analysis of photoinactivation and repair as separate processes. We report here that, using strong UV-A light from a laser, we were able to analyze separately the photoinactivation and repair of photosystem II in the cyanobacterium Synechocystis sp. PCC 6803. Very strong UV-A light at 364 nm and a photon flux density of 2600 μmol photons m⁻² s⁻¹ inactivated the oxygen-evolving machinery and the photochemical reaction center of PSII within 1 or 2 min before the first step in the repair process, namely, the degradation of the D1 protein, occurred. During subsequent incubation of cells in weak visible light, the activity of PSII recovered fully within 30 min and this process depended on protein synthesis. During subsequent incubation of cells in darkness for 60 min, the D1 protein of the photoinactivated PSII was degraded. Further incubation in weak visible light resulted in the rapid restoration of the activity of PSII. These observations suggest that very strong UV-A light is a useful tool for the analysis of the repair of PSII after photoinactivation.     

Temperature/light dependent development of selective resistance to photoinhibition of photosystem I

Exposure of winter rye leaves grown at 20°C and an irradiance of either 50 or 250 μmol m⁻² s⁻¹ to high light stress (1600 μmol m⁻² s⁻¹, 4 h) at 5°C resulted in photoinhibition of PSI measured in vivo as a 34% and 31% decrease in ΔA₈₂₀/A₈₂₀ (P700⁺). The same effect was registered in plants grown at 5°C and 50 μmol m⁻² s⁻¹. This was accompanied by a parallel degradation of the PsaA/PsaB heterodimer, increase of the intersystem e⁻ pool size as well as inhibition of PSII photochemistry measured as F_v/F_m. Surprisingly, plants acclimated to high light (800 μmol m⁻² s⁻¹) or to 5°C and moderate light (250 μmol m⁻² s⁻¹) were fully resistant to photoinhibition of PSI and did not exhibit any measurable changes at the level of PSI heterodimer abundance and intersystem e⁻ pool size, although PSII photochemistry was reduced to 66% and 64% respectively. Thus, we show for the first time that PSI, unlike PSII, becomes completely resistant to photoinhibition when plants are acclimated to either 20°C/800 μmol m⁻² s⁻¹ or 5°C/250 μmol m⁻² s⁻¹ as a response to growth at elevated excitation pressure. The role of temperature/light dependent acclimation in the induction of selective tolerance to PSI photoinactivation is discussed.     

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.     

‘Birth defects’ of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water‐oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane‐inlet mass spectrometry and O₂‐polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de‐etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂‐polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B‐inhibitor binding, and thermoluminescence studies indicate that the decline of the high‐light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A^? → Q_B during de‐etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer‐range energy transfer.     

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 °C (with half inactivation at 46 °C) to 46-60 °C (with half inactivation at 54 °C) and that of the photochemical reaction center from 44-55 °C (with half inactivation at 51 °C) to 52-63 °C (with half inactivation at 58 °C). However, in light, PSII was more sensitive to heat stress; when moderate heat stress, such as 40 °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.     

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.     

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.     

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates (A_max) in not flooded, high light treatments (6.1 μmol CO₂ m⁻² s⁻¹). Individuals of P. glomerata showed similar patterns, with A_max increasing from 1.9 μmol CO₂ m⁻² s⁻¹ under total flooding to 7.1 μmol CO₂ m⁻² s⁻¹ in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII (F_v/F_m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F_v/F_m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding (C. latiloba) and shade (P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.     

Photoinhibition and repair in Dunaliella salina acclimated to different growth irradiances

The light-dependent rate of photosystem-II (PSII) damage and repair was measured in photoautotrophic cultures of Dunaliella salina Teod. grown at different irradiances in the range 50–3000 mol photons · m^–2· s^–1. Rates of cell growth increased in the range of 50–800 mol photons·m^–2·s^–1, remained constant at a maximum in the range of 800–1,500 mol photons·m^–2 ·s^–1, and declined due to photoinhibition in the range of 1500–3000 mol photons·m^–2·s^–1. Western blot analyses, upon addition of lincomycin to the cultures, revealed first-order kinetics for the loss of the PSII reaction-center protein (D1) from the 32-kDa position, occurring as a result of photodamage. The rate constant of this 32-kDa protein loss was a linear function of cell growth irradiance. In the presence of lincomycin, loss of the other PSII reaction-center protein (D2) from the 34-kDa position was also observed, occurring with kinetics similar to those of the 32-kDa form of D1. Increasing rates of photodamage as a function of irradiance were accompanied by an increase in the steady-state level of a higher-molecular-weight protein complex ( 160-kDa) that cross-reacted with D1 antibodies. The steady-state level of the 160-kDa complex in thylakoids was also a linear function of cell growth irradiance. These observations suggest that photodamage to D1 converts stoichiometric amounts of D1 and D2 (i.e., the D1/D2 heterodimer) into a 160-kDa complex. This complex may help to stabilize the reaction-center proteins until degradation and replacement of D1 can occur. The results indicated an intrinsic half-time of about 60 min for the repair of individual PSII units, supporting the idea that degradation of D1 after photodamage is the rate-limiting step in the PSII repair process.Abbreviations Chl chlorophyll - PSI photosystem I - PSII photosystem II - D1 the 32-kDa reaction-center protein of PSII, encoded by the chloroplast psbA gene - D2 the 34-kDa reactioncenter protein of PSII, encoded by the chloroplast psbD gene - Q_A primary electron-accepting plastoquinone of PSII The work was supported by grant 94-37100-7529 from the US Department of Agriculture, National Research Initiative Competitive Grants Program.     

Protein synthesis is the primary target of reactive oxygen species in the photoinhibition of photosystem II

Photoinhibition of photosystem II (PSII) occurs when the rate of photodamage to PSII exceeds the rate of the repair of photodamaged PSII. Recent examination of photoinhibition by separate determinations of photodamage and repair has revealed that the rate of photodamage to PSII is directly proportional to the intensity of incident light and that the repair of PSII is particularly sensitive to the inactivation by reactive oxygen species (ROS). The ROS-induced inactivation of repair is attributable to the suppression of the synthesis de novo of proteins, such as the D1 protein, that are required for the repair of PSII at the level of translational elongation. Furthermore, molecular analysis has revealed that the ROS-induced suppression of protein synthesis is associated with the specific inactivation of elongation factor G via the formation of an intramolecular disulfide bond. Impairment of various mechanisms that protect PSII against photoinhibition, including photorespiration, thermal dissipation of excitation energy, and the cyclic transport of electrons, decreases the rate of repair of PSII via the suppression of protein synthesis. In this review, we present a newly established model of the mechanism and the physiological significance of repair in the regulation of the photoinhibition of PSII.     

Very strong UV-A light temporally separates the photoinhibition of photosystem II into light-induced inactivation and repair

When organisms that perform oxygenic photosynthesis are exposed to strong visible or UV light, inactivation of photosystem II (PSII) occurs. However, such organisms are able rapidly to repair the photoinactivated PSII. The phenomenon of photoinactivation and repair is known as photoinhibition. Under normal laboratory conditions, the rate of repair is similar to or faster than the rate of photoinactivation, preventing the detailed analysis of photoinactivation and repair as separate processes. We report here that, using strong UV-A light from a laser, we were able to analyze separately the photoinactivation and repair of photosystem II in the cyanobacterium Synechocystis sp. PCC 6803. Very strong UV-A light at 364 nm and a photon flux density of 2600 micromol photons m(-2) s(-1) inactivated the oxygen-evolving machinery and the photochemical reaction center of PSII within 1 or 2 min before the first step in the repair process, namely, the degradation of the D1 protein, occurred. During subsequent incubation of cells in weak visible light, the activity of PSII recovered fully within 30 min and this process depended on protein synthesis. During subsequent incubation of cells in darkness for 60 min, the D1 protein of the photoinactivated PSII was degraded. Further incubation in weak visible light resulted in the rapid restoration of the activity of PSII. These observations suggest that very strong UV-A light is a useful tool for the analysis of the repair of PSII after photoinactivation.     

Comparison of stress susceptibility of in hospite and isolated zooxanthellae among five coral species

Coral species in a similar habitat often show different bleaching susceptibilities. It is not understood which partner of coral-zooxanthellae complexes is responsible for differential stress susceptibility. Stress susceptibilities of in hospite and isolated zooxanthellae from five species of corals collected from shallow water in Okinawa were compared. To estimate stress susceptibility, we measured the maximum quantum yields (F_v/F_m) of in hospite and isolated zooxanthellae after 3-h exposure to either 28 or 34 °C at various light intensities and their recovery after 12 h under dim light at 26 °C. Significant reduction in photochemical efficiency (F_v/F_m) of photosystem II (PSII) was observed in in hospite zooxanthellae exposed to high light intensity (1000 μmol quanta m⁻² s⁻¹), while PSII activity of isolated zooxanthellae decreased significantly even at a lower light intensity (70 μmol quanta m⁻² s⁻¹). The recovery of the PSII activity after 12 h was incomplete in both in hospite and isolated zooxanthellae, indicating the presence of chronic photoinhibition. The stress susceptibility of isolated zooxanthellae was more variable among species than in hospite zooxanthellae. The order of stress susceptibility among the five coral species was different between in hospite and isolated zooxanthellae. The present results suggest that the host plays a significant role in determining bleaching susceptibility of corals, though zooxanthellae from different host have different stress susceptibilities.     

Photoinactivation of Catalase Occurs under Both High- and Low-Temperature Stress Conditions and Accompanies Photoinhibition of Photosystem II

Severe photoinactivation of catalase (EC 1.11.1.6) and a decline of variable fluorescence (F_v), indicating photoinhibition of photosynthesis, were observed as rapid and specific symptoms in leaves exposed to a high heat-shock temperature of 40°C as well as in leaves exposed to low chilling temperatures in white light of only moderately high photosynthetic photon flux density of 520 μE m⁻² s⁻¹. Other parameters, such as peroxidase (EC 1.11.1.7), glycolate oxidase (EC 1.1.3.1), glutathione reductase (EC 1.6.4.2), or the chlorophyll content, were hardly affected under these conditions. At a compatible temperature of 22°C, the applied light intensity did not induce severe photoinactivations. In darkness, exposures to high or low temperatures did not affect catalase levels. Also, decline of F_v in light was not related to temperature sensitivity in darkness. The effective low-temperature ranges inducing photoinactivation of catalase differed significantly for chilling-tolerant and chilling-sensitive plants. In leaves of rye (Secale cereale L.) and pea (Pisum sativum L.), photoinactivation occurred only below 15°C, whereas inactivation occurred at 15°C in cucumber (Cucumis sativus L.) and maize (Zea mays L.). The behavior of F_v was similar, but the difference between chilling-sensitive and chilling-tolerant plants was less striking. Whereas the catalase polypeptide, although photoinactivated, was not cleaved at 0 to 4°C, the D1 protein of photosystem II was greatly degraded during the low-temperature treatment of rye leaves in light. Rye leaves did not exhibit symptoms of any major general photodamage, even when they were totally depleted of catalase after photoinactivation at 0 to 4°C, and catalase recovered rapidly at normal temperature. In cucumber leaves, the decline of catalase after exposures to bright light at 0 to 4°C was accompanied by bleaching of chlorophyll, and the recovery observed at 25°C was slow and required several days. Similar to the D1 protein of photosystem II, catalase differs greatly from other proteins by its inactivation and high turnover in light. Inasmuch as catalase and D1 protein levels depend on continuous repair synthesis, preferential and rapid declines are generally to be expected in light whenever translation is suppressed by stress actions, such as heat or chilling, and recovery will reflect the repair capacity of the plants.     

The interaction of visible and UV-B light during photodamage and repair of Photosystem II

In order to understand the mechanism of photodamage induced by solar radiation under natural conditions, we studied the interaction of visible and ultraviolet-B light in the inactivation and repair of the Photosystem II complex by using oxygen evolution and flash-induced chlorophyll fluorescence measurements. In isolated spinach thylakoids and Synechocystis 6803 cells, in which de novo protein synthesis is blocked by lincomycin, photodamage of Photosystem II by visible and UV-B light is characterized by linear semilogarithmic inactivation curves for both separate and combined illumination protocols. The extent of PS II inactivation obtained after combined illumination can be well simulated by assuming independent damaging events induced by visible and UV-B photons. In intact Synechocystis cells capable of protein repair, simultaneous illumination by visible and UV-B light impairs Photosystem II activity to a smaller extent than expected from the independent damaging events. This protective effect is pronounced at low visible light (130 μE m⁻² s⁻¹), but becomes negligible at high intensities (1300 μE m⁻² s⁻¹). Exposure of intact Synechocystis 6803 cells to direct sunlight leads to a rapid inactivation of PS II, accompanied by the accumulation of donor side inhibited centers. This phenomenon, which shows the impairment of the manganese cluster of water oxidation was not observed when the ultraviolet components of sunlight were filtered out. We conclude that visible and UV-B photons inactivate PS II via non-interacting mechanisms, which affect different target sites. In intact cells, the two spectral regions do interact, and results in synergistically enhanced protein repair capacity when UV-B radiation is accompanied by low intensity visible light, which provides protection against photodamage. However, this ameliorating effect becomes insignificant at high light intensities characteristic of direct sunlight. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen (¹O₂)-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F_v/F_m (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH₂OH failed to restore the F_v/F_m ratio suggesting that the reduction of F_v/F_m reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q_A⁻ with S₂) and B (due to recombination of Q_B⁻ with S₂/S₃) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F₆₉₅ band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by ¹O₂ produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.     

Photoinhibitory damage is modulated by the rate of photosynthesis and by the photosystem II light-harvesting chlorophyll antenna size

We investigated the effect of photosynthetic electron transport and of the photosystem II (PSII) chlorophyll (Chl) antenna size on the rate of PSII photoinhibitory damage. To modulate the rate of photosynthesis and the light-harvesting capacity in the unicellular chlorophyte Dunaliella salina Teod., we varied the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO₃, or with supplemental CO₂ bubbled in the form of 3% CO₂ in air. The NaHCO₃-grown cells displayed slow rates of photosynthesis and had a small PSII light-harvesting Chl antenna size (60 Chl molecules). The half-time of PSII photodamage was 40 min. When switched to supplemental CO₂ conditions, the rate of photodamage was retarded to a t_1/2 = 70 min. Conversely, CO₂-supplemented cells displayed faster rates of photosynthesis and a larger PSII light-harvesting Chl antenna size (500 Chl molecules). They also showed a rate of photodamage with t_1/2 = 40 min. When depleted of CO₂, the rate of photodamage was accelerated (t_1/2  = 20 min). These results indicate that the in-vivo susceptibility to photodamage is modulated by the rate of forward electron transport through PSII. Moreover, a large Chl antenna size enhances the rate of light absorption and photodamage and, therefore, counters the mitigating effect of forward electron transport. We propose that under steady-state photosynthesis, the rate of light absorption (determined by incident light intensity and PS Chl antenna size) and the rate of forward electron transport (determined by CO₂ availability) modulate the oxidation/reduction state of the primary PSII acceptor Q_A, which in turn defines the low/high probability for photodamage in the PSII reaction center. Received: 14 August 1997 / Accepted: 26 September 1997     

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.     

Interruption of the Calvin cycle inhibits the repair of Photosystem II from photodamage

In photosynthetic organisms, impairment of the activities of enzymes in the Calvin cycle enhances the extent of photoinactivation of Photosystem II (PSII). We investigated the molecular mechanism responsible for this phenomenon in the unicellular green alga Chlamydomonas reinhardtii. When the Calvin cycle was interrupted by glycolaldehyde, which is known to inhibit phosphoribulokinase, the extent of photoinactivation of PSII was enhanced. The effect of glycolaldehyde was very similar to that of chloramphenicol, which inhibits protein synthesis de novo in chloroplasts. The interruption of the Calvin cycle by the introduction of a missense mutation into the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) also enhanced the extent of photoinactivation of PSII. In such mutant 10-6C cells, neither glycolaldehyde nor chloramphenicol has any additional effect on photoinactivation. When wild-type cells were incubated under weak light after photodamage to PSII, the activity of PSII recovered gradually and reached a level close to the initial level. However, recovery was inhibited in wild-type cells by glycolaldehyde and was also inhibited in 10-6C cells. Radioactive labelling and Northern blotting demonstrated that the interruption of the Calvin cycle suppressed the synthesis de novo of chloroplast proteins, such as the D1 and D2 proteins, but did not affect the levels of psbA and psbD mRNAs. Our results suggest that the photoinactivation of PSII that is associated with the interruption of the Calvin cycle is attributable primarily to the inhibition of the protein synthesis-dependent repair of PSII at the level of translation in chloroplasts.