Photoinactivation Of Photosystem II Research Articles

Drought stress modulates expression of aquaporin gene and photosynthetic efficiency in Pennisetum glaucum (L.) R. Br. genotypes

Photosynthetic efficiency and availability of water have a close relationship which determines the growth and development of plants. Drought has a remarkable effect on photosynthetic efficiency by affecting the functioning of Photosystem II (PSII). In this study, two drought genotypes of pearl millet were taken and assessed for photosynthetic performance of PSII (electron transport rate (ETR) and maximum yield) under a simulated drought stress condition. Drought stress caused a gradual downregulation in the chlorophyll content in both genotypes. The elevated content of β-carotene was recorded on the 12th day of the experiment. The photo inactivation of PSII was also assessed in both the genotypes. On the basis of an overall comparison, it is evident that the genotype IP14599 has better adapted to drought than the genotype IP14222. Furthermore, the results were confirmed by the analysis of transcript expression level of two aquaporin candidate genes (PIP2;3 and PIP1;5) and it was found that genotype IP14599 was better to maintain homeostasis by increasing in water channeling and movement in response to drought as compared to IP14222. Therefore, it was confirmed that PIP2; 3 and PIP1;5 gene modulate expression of mRNA to provide tolerance against drought in millet plants.

The trade‐off between the light‐harvesting and photoprotective functions of fucoxanthin‐chlorophyll proteins dominates light acclimation in Emiliania huxleyi (clone CCMP 1516)

Mechanistic understanding of the costs and benefits of photoacclimation requires knowledge of how photophysiology is affected by changes in the molecular structure of the chloroplast. We tested the hypothesis that changes in the light dependencies of photosynthesis, nonphotochemical quenching and PSII photoinactivation arises from changes in the abundances of chloroplast proteins in Emiliania huxleyi strain CCMP 1516 grown at 30 (Low Light; LL) and 1000 (High Light; HL) μmol photons m(-2) s(-1) photon flux densities. Carbon-specific light-saturated gross photosynthesis rates were not significantly different between cells acclimated to LL and HL. Acclimation to LL benefited cells by increasing biomass-specific light absorption and gross photosynthesis rates under low light, whereas acclimation to HL benefited cells by reducing the rate of photoinactivation of PSII under high light. Differences in the relative abundances of proteins assigned to light-harvesting (Lhcf), photoprotection (LI818-like), and the photosystem II (PSII) core complex accompanied differences in photophysiology: specifically, Lhcf:PSII was greater under LL, whereas LI818:PSII was greater in HL. Thus, photoacclimation in E. huxleyi involved a trade-off amongst the characteristics of light absorption and photoprotection, which could be attributed to changes in the abundance and composition of proteins in the light-harvesting antenna of PSII.

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

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

Physiological basis for high resistance to photoinhibition under nitrogen depletion in Emiliania huxleyi

We quantified and compared the photophysiological characteristics of Emiliania huxleyi with two marine diatoms Thalassiosira pseudonana and Coscinodiscus sp. under nitrogen‐ and phosphorus‐replete and ‐depleted conditions. Under nitrogen‐depleted conditions, E. huxleyi maintained significant photosystem II (PSII) function over at least 38 d, whereas for both diatoms, PSII function declined to marginal levels within 8‐10 d of nitrogen depletion. In contrast, under phosphorus‐depleted conditions, PSII function in E. huxleyi declined sharply within 7 d and in T. pseudonana within 3 d. Under nutrient‐replete conditions, E. huxleyi had one of the highest PSII repair rate constants among phytoplankton examined to date. Under nitrogen depletion, E. huxleyi exhibited a decrease in susceptibility to photoinactivation of PSII, while the diatoms showed no significant change in susceptibility. The superior ability of E. huxleyi relative to the diatoms to limit PSII photoinactivation as well as to maintain PSII repair for many days of nitrogen depletion could help to explain why E. huxleyi is able to form blooms under the low‐nitrate, high‐light conditions that frequently occur in stratified surface waters.

Rapid photoprotection in sea‐ice diatoms from the East Antarctic pack ice

Photoinhibition and D1 protein re‐synthesis were investigated in bottom‐dwelling sea‐ice microalgal communities from the East Antarctic pack ice during early spring. Bottom‐dwelling sea‐ice microalgal communities were dominated by diatoms that exhibited rapid photoprotection when exposed to a range of different light levels (10 µmol photons m−2 s−1, 50 µmol photons m−2 s−1, 100 µmol photons m−2 s−1, and 200 µ;mol photons m−2 s−1). Photosynthetic capacity of photosystem II (PSII) dropped significantly over 3 h under 200 −mol photons m−2 s−1, but largely recovered when placed in a low‐light environment (10 µmol photons m−2 s−1) for an additional 3 h. PSII repair rates increased with increasing irradiance, and the D1‐protein pool remained steady even under high light (200 µmol photons m−2 s−1). Sea‐ice diatoms showed a low intrinsic susceptibility to photoinactivation of PSII across all the light treatments, and a strong and irradiance‐dependent induction of nonphotochemical quenching, which did not depend upon chloroplast protein synthesis, was also seen. These highly plastic organisms, once thought to be adapted to shade, are in fact well equipped to withstand rapid and relatively large changes in light at low temperatures with minimal long‐term effect on their photosynthetic machinery.

The Involvement of Dual Mechanisms of Photoinactivation of Photosystem II in Capsicum annuum L. Plants

For plants, light is an indispensable resource. However, it also causes a loss of photosynthetic activity associated with photoinactivation of photosystem II (PSII). In studies of the mechanism of this photoinactivation, there are two conflicting hypotheses at present. One is that excess energy received by leaves, being neither utilized by photosynthesis nor dissipated safely in non-photochemical quenching, causes the photoinactivation. The other involves a two-step mechanism in which excitation of Mn by photons is the primary cause. In the former hypothesis, photoinactivation of PSII should not occur in low light that provides little excess energy, but in the latter hypothesis it should. Therefore, we tested these two hypotheses in different irradiances. We used a system that can measure the fraction of functional PSII complexes under natural conditions and over a long period in intact leaves, which were attached to a plant treated with lincomycin taken up via the roots. The leaves were photoinactivated in low, medium or high light (30, 60 or 950 micromol m(-2) s(-1)) with white, blue, green or red light-emitting diode arrays. Our results showed that the extent of photoinactivation per photon exposure was higher in high light than in low light, consistent with the abundance of excess energy. However, photoinactivation did occur in low light with little excess energy, and blue light caused the greatest extent of photoinactivation followed by white, green and red light in this order, an order that can be predicted from the Mn absorbance spectrum. These results suggest that both mechanisms occur in the photoinactivation process.

No single way to understand singlet oxygen signalling in plants

When plant cells are under environmental stress, several chemically distinct reactive oxygen species (ROS) are generated simultaneously in various intracellular compartments and these can cause oxidative damage or act as signals. The conditional flu mutant of Arabidopsis, which generates singlet oxygen in plastids during a dark-to-light transition, has allowed the biological activity of singlet oxygen to be determined, and the criteria to distinguish between cytotoxicity and signalling of this particular ROS to be defined. The genetic basis of singlet-oxygen-mediated signalling has been revealed by the mutation of two nuclear genes encoding the plastid proteins EXECUTER (EX)1 and EX2, which are sufficient to abrogate singlet-oxygen-dependent stress responses. Conversely, responses due to higher cytotoxic levels of singlet oxygen are not suppressed in the ex1/ex2 background. Whether singlet oxygen levels lower than those that trigger genetically controlled cell death activate acclimation is now under investigation.

Deletion of PsbM in Tobacco Alters the QB Site Properties and the Electron Flow within Photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.

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.

The rate coefficient of repair of photosystem II after photoinactivation

During photosynthesis, photoinactivation and repair of photosystem II (PSII) occur simultaneously, resulting in a net loss of functional PSII under a given irradiance. This study determines the rate coefficients for the partial processes, allowing the calculation of the partial rates at any concentration of functional/non‐functional PSII. The rate coefficient of photoinactivation was obtained from the onset of photoinactivation of PSII in leaf segments of Capsicum annuum L. in the absence of repair, and was in turn used to obtain the rate coefficient (kr) of repair of PSII when repair was occurring. The value of kr was found to be near maximum at an irradiance as low as 29 µmol photons m−2 s−1 and peaked at or somewhat above the growth irradiance; however, it declined on further increasing the irradiance, possibly due to oxidative stress. The value of kr was considerably decreased by elevating the CO2 to about 1%, particularly at low irradiance, probably due to acidification of the stroma to a pH outside the range that is optimal for protein synthesis. The method of determining kr is convenient to apply, not relying on radiolabelling and pulse‐chase experiments.

The role of inactive photosystem-II-mediated quenching in a last-ditch community defence against high light stress in vivo.

Photoinactivation of photosystem II (PSII), the light-induced loss of ability to evolve oxygen, is an inevitable event during normal photosynthesis, exacerbated by saturating light but counteracted by repair via new protein synthesis. The photoinactivation of PSII is dependent on the dosage of light: in the absence of repair, typically one PSII is photoinactivated per 10(7) photons, although the exact quantum yield of photoinactivation is modulated by a number of factors, and decreases as fewer active PSII targets are available. PSII complexes initially appear to be photoinactivated independently; however, when less than 30% functional PSII complexes remain, they seem to be protected by strongly dissipative PSII reaction centres in several plant species examined so far, a mechanism which we term 'inactive PSII-mediated quenching'. This mechanism appears to require a pH gradient across the photosynthetic membrane for its optimal operation. The residual fraction of functional PSII complexes may, in turn, aid in the recovery of photoinactivated PSII complexes when conditions become less severe. This mechanism may be important for the photosynthetic apparatus in extreme environments such as those experienced by over-wintering evergreen plants, desert plants exposed to drought and full sunlight and shade plants in sustained sunlight.

The role of ΔpH-dependent dissipation of excitation energy in protecting photosystem II against light-induced damage in Arabidopsis thaliana

The Arabidopsis thaliana subunit PsbS of photosystem II (PSII) is essential for the non-photochemical quenching of chlorophyll fluorescence and thus for ΔpH-dependent energy dissipation (qE). As a result of the excision of an En-transposon, a frameshift mutation in the psbS gene was obtained, which results in the complete absence of the PsbS protein and of qE. Two-dimensional gel analyses of thylakoid membranes indicated that the depletion of PsbS has no effect on PSII composition, excluding a structural role for PsbS in the organization of the PSII antenna. The susceptibility of mutant plants to photoinactivation of PSII was significantly increased during exposure to high light for up to 8 h. Divergence of mutant plants from wild-type levels of photoinactivation were most pronounced during the first 2 h of illumination, while after longer exposure times the rate of PSII inactivation were similar in both genotypes. The increased PSII inactivation in the mutant was not accompanied by an increased rate of D1 protein degradation, and recovery of PSII activity in the mutant under low light was similar or even faster in comparison to wild-type plants. However, growth under high light intensities resulted in decreased growth rates of psbs mutant plants. We conclude that energy dissipation in PSII related to qE is not primarily required for the protection of PSII against light-induced destruction, but may rather be involved in reducing the electron pressure on the photosynthetic electron transport chain at saturating light intensities.

A very early event of photoinactivation of photsystem II at low temperature

When the photosynthetic machinery is irridiated by strong light the photosystem II (PSII) complex is photodamaged although PSII is repaired by a process which requires protein synthesis. We investigated the photoinactivation and recovery of PSII at low temperatures in Synechocystis sp. PCC 6803 cells and a mutant defective in unsaturation of membrane lipids. When the cells were exposed to light of 2000 µE m-2 s-2 at 0°C and then were incubated at 34°C in darkness, approximately 50% of the PSII activity recovered after 60 min. This recovery occurred in the presence of lincomycin which inhibits translation of D1 protein suggesting that the recovery in darkness did not involve synthesis de novo of proteins. This observation together with other results lead us to hypothesize of the involvement of an intermediately photoinactivated PSII prior to the stable photodamaged PSII, which is currently well discussed.

Role of two forms of the D1 protein in the recovery from photoinhibition of photosystem II in the cyanobacterium Synechococcus PCC 7942

The study of turnover of two distinct forms of the photosystem II (PSII) D1 protein in cells of the cyanobacterium Synechococcus PCC 7942 showed that the ‘high-light’ form D1:2 is degraded significantly faster at 500 μE m −2 s −1 as compared with 50 μE m −2 s −1 while the degradation rates of the ‘low-light’ form D1:1 under low and high irradiance are not substantially different. Consequently, the D1:1 turnover does not match photoinactivation of PSII under increased irradiance and therefore the cells containing this D1 form exhibit a decrease in the PSII activity. Monitoring of the content of each D1 form during a recovery from growth–temperature photoinhibition showed a good correlation between the synthesis of D1:2 and restoration of the PSII activity. In contrast, when photoinhibitory treatment was conducted at low temperature, a fast recovery was not accompanied by the D1:2 accumulation. The data suggest that photoinactivation at growth temperature results in a modification of PSII that inhibits insertion of D1:1 and, therefore, for restoration of the photochemical activity in the photoinactivated PSII complexes the D1:2 synthesis is needed. This may represent the primary reason for the requirement of psbAII/ psbAIII expression under increased irradiance.

Effects of solar ultraviolet radiation on the potential efficiency of photosystem II in leaves of tropical plants

The effects of solar ultraviolet (UV)-B and UV-A radiation on the potential efficiency of photosystem II (PSII) in leaves of tropical plants were investigated in Panama (9 degrees N). Shade-grown tree seedlings or detached sun leaves from the outer crown of mature trees were exposed for short periods (up to 75 min) to direct sunlight filtered through plastic or glass filters that absorbed either UV-B or UV-A+B radiation, or transmitted the complete solar spectrum. Persistent changes in potential PSII efficiency were monitored by means of the dark-adapted ratio of variable to maximum chlorophyll a fluorescence. In leaves of shade-grown tree seedlings, exposure to the complete solar spectrum resulted in a strong decrease in potential PSII efficiency, probably involving protein damage. A substantially smaller decline in the dark-adapted ratio of variable to maximum chlorophyll a fluorescence was observed when UV-B irradiation was excluded. The loss in PSII efficiency was further reduced by excluding both UV-B and UV-A light. The photoinactivation of PSII was reversible under shade conditions, but restoration of nearly full activity required at least 10 d. Repeated exposure to direct sunlight induced an increase in the pool size of xanthophyll cycle pigments and in the content of UV-absorbing vacuolar compounds. In sun leaves of mature trees, which contained high levels of UV-absorbing compounds, effects of UV-B on PSII efficiency were observed in several cases and varied with developmental age and acclimation state of the leaves. The results show that natural UV-B and UV-A radiation in the tropics may significantly contribute to photoinhibition of PSII during sun exposure in situ, particularly in shade leaves exposed to full sunlight.

Resistance to photoinhibition of photosystem II and catalase and antioxidative protection in high mountain plants

ABSTRACTIn leaves of three alpine high mountain plants, Homogyne alpina, Ranunculus glacialis and Soldanella alpina, both photosystem II (PSII) and the enzyme catalase appeared to he highly resistant to photoinactivation under natural field conditions. While the Dl protein of PSII and catalase have a rapid turnover in light and require continuous new protein synthesis in non‐adapted plants, little apparent photoinactivation of PSII or catalase was induced in the alpine plants by translation inhibitors or at low temperature, suggesting that turnover of the Dl protein and catalase was slow in these leaves. In vitro PSII was rapidly inactivated in light in isolated thylakoids from H. alpina and R. glacialis. In isolated intact chloroplasts from R. glacialis, photoinactivation of PSII was slower than in thylakoids. Partially purified catalase from R. glacialis and S. alpina was as sensitive to photoinactivation in vitro as catalases from other sources. Catalase from H. alpina had, however, a 10‐fold higher stability in light. The levels of xanthophyll cycle carotenoids, of the antioxidants ascorbate and glulathione, and of the activities of catalase, superoxide dismutase and glutathione reductase were very high in S. alpina, intermediate in H. alpina, but very low in R. glacialis. However, isolated chloroplasts from all three alpine species contained much higher concentrations of ascorbate and glutathione than chloroplasts from lowland plants.

ELIPs – Light‐induced stress proteins

Exposure of plants to light intensities higher than those required to saturate photosynthesis leads to a reduction in photosynthetic capacity. This effect is known as photoinhibition. Photoinhibition is followed by destruction of carotenoids, bleaching of chlorophylls and increased lipid peroxidation due to damage by oxygen‐derivatives. The oxygen concentration in chloroplasts in the light is high because of oxygen production by photosystem II (PSII). This can result in the release of reactive intermediates of reduced dioxygen such as superoxide radicals, hydroxyl radicals, hydrogen peroxide or singlet oxygen. In order to maintain their normal function under light stress conditions, chloroplasts have developed multiple repair and protection systems. The induction of specific light stress proteins, the ELIPs (for early light‐induced proteins) can be considered to be part of these protective responses. The accumulation of ELIPs under light stress conditions is correlated with the photoinactivation of PSII, degradation of the Dl‐protein of PSII reaction centre and changes in the level of pigments. Futhermore, the accumulation of ELIPs in the thylakoids is strictly controlled by the pigment content, especially by chlorophylls. Isolation of ELIPs in a native form and analysis of pigments bound to these proteins revealed that ELIPs can bind chlorophyll a and lutein. These data indicate that ELIPs might represent unique chlorophyll‐binding proteins which have a transient function(s) during light stress. A transient ‘pigment‐carrier’ function is postulated for ELIPs.

Photoinactivation and photoprotection of photosystem II in nature

Photosystem II plays a central role not only in energy transduction, but also in monitoring the molecular redox mechanisms involved in signal transduction for acclimation to environmental stresses. Central to the regulation of photosystem II (PSII) function as a light‐driven molecular machine in higher plant leaves, is an inevitable photo‐inactivation of one PSII after 106–107 photons have been delivered to the leaf, although the act of photoinactivation per se requires only one photon. PSII function in acclimated pea leaves shows a reciprocity between irradiance and the time of illumination, demonstrating that the photoinactivation of PSII is a light dosage effect, depending on the number of photons absorbed rather than the rate of photon absorption. Hence, PSII photoinactivation will occur at low as well as high irradiance. There is a heterogeneity of PSII functional stability, possibly with less stable PSII monomers being located in grana margins and more stable PSII dimers in appressed granal domains. Matching the inevitable photoinactivation of PSII, green plants have an intrinsic capacity for D1 protein synthesis to restore PSII function which is saturated at very low light. Photoinhibition of PSII in vivo is often a photoprotective strategy rather than a damaging process.