The physiological links of the increased photosystem II activity in moderately desiccated Porphyra haitanensis (Bangiales, Rhodophyta) to the cyclic electron flow during desiccation and re-hydration

Photosynthetic electron flow changed considerably during desiccation and re-hydration of the intertidal macroalgae Porphyra haitanensis. Activities of both photosystem (PSI) and photosystem (PSII) increased significantly at moderate desiccation levels. Whereas PSII activity was abolished at an absolute water content (AWC) <24 %, PSI remained active with progressive decreases in AWC to values as low as 16 %. This result suggested that cyclic electron flow around PSI was still active after inactivation of linear electron flow following severe desiccation. Moreover, the PSI activity was restored more rapidly than that of PSII upon re-hydration. Pretreatment of the blades with 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) suppressed PSII activity following desiccation to an AWC of ~16 % AWC. Cyclic electron flow around PSI decreased markedly in blades pretreated with DCMU than in blades without pretreatment of DCMU during re-hydration in seawater containing DCMU. All results suggested that the activity of PSII under desiccation conditions plays an important role in the operation of cyclic electron flow during desiccation and its recovery during re-hydration. Therefore, we proposed the PSII activity during desiccation could eventually lead to the accumulation of NADPH, which could serve as electron donor for P700⁺ and promote its recovery during re-hydration, thereby favoring the operation of cyclic electron flow.     

Dark chilling increases glucose-6-phosphate dehydrogenase activity in soybean leaves

Available evidence suggests that the stress‐induced increase in the activity of glucose‐6‐phosphate dehydrogenase (G6PDH, EC 1.1.1.49), the key regulatory enzyme of the oxidative pentose phosphate pathway, might often be related to the presence of plant water deficit. The response of G6PDH to dark chilling in chilling sensitive plant species is still unknown. In this communication we report on this response and its dependence on the presence of chill‐induced drought stress. A chilling sensitive soybean (Glycine max L. Merr.) genotype was exposed to dark chilling of the entire plant (whole‐chilled) or only the shoots and leaves (shoot‐chilled). The development of chill‐induced drought stress upon illumination was quantified by measurement of proline and relative water content (RWC). Chill‐induced drought stress (decrease in RWC and increase in proline content) developed with time in whole‐chilled plants, but not in shoot‐chilled plants. The response of the above‐mentioned treatments on G6PDH activity in fully expanded leaves was assessed. In parallel, the effects on CO₂ assimilation, PSII activity and chloroplast fructose‐1,6‐bisphosphatase (FBPase EC 3.1.3.11) and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco EC 4.1.1.39) activity were quantified. A decrease in CO₂ assimilation rate, FBPase activity and ribulose‐1,5‐bisphosphate (RuBP) content was observed in whole‐chilled but not in shoot‐chilled plants. However, in shoot‐chilled plants regulation of diurnal PSII activity was altered. The increase in the activation state of NADP‐dependent malate dehydrogenase (NADP‐MDH EC 1.1.1.82) in shoot‐chilled plants suggests an increase in stromal redox state. Although the two different dark chilling treatments resulted in distinct physiological and biochemical effects, both induced an increase in foliar G6PDH activity, suggesting an important role of this enzyme during and following dark chilling stress, irrespective of the presence of chill‐induced drought stress.     

Preliminary comparison of atmospheric CO2 enhancement to photosynthesis of Pyropia yezoensis (Bangiales,Rhodophyta) leafy thalli and filamentous thalli

Intertidal macroalgae are submerged in seawater at high tide and exposed to air at low tide. When they are exposed to the air, CO₂ is the main inorganic carbon source. In this study, the photosynthetic performances of PSI and PSII were measured in different generations of Pyropia yezoensis (leafy thalli and filamentous thalli) that had been exposed to air containing different CO₂ concentrations. Changes in photosynthesis during dehydration and salt treatment under the different CO₂ concentrations were also analyzed. The results showed that in leafy thalli, the effective photochemical quantum yield of PSII (YII) was enhanced as CO₂ increased, which suggested that CO₂ assimilation was enhanced and that they can utilize CO₂ in the air directly, even when they are subjected to moderate stress. These findings could explain why, in P. yezoensis aquaculture, moderate exposure to air does not lead to a decrease in crop yield. However, in filamentous thalli, there were no significant differences in YII at the CO₂ concentrations tested. The expression of genes involved in the Calvin cycle in leafy thalli was higher than that in filamentous thalli. CO₂ uptake and biomass of P. yezoensis leafy thalli is larger than filamentous thalli, which may be due to its different carbon utilization mechanism and the adaptation of intertidal environment in the evolutionary process.     

The enhancement of cyclic electron flow around photosystem I improves the recovery of severely desiccated Porphyra yezoensis (Bangiales, Rhodophyta)

Porphyra yezoensis, a representative species of intertidal macro-algae, is able to withstand periodic desiccation at low tide but is submerged in seawater at high tide. In this study, changes in photosynthetic electron flow in P. yezoensis during desiccation and re-hydration were investigated. The results suggested that the cyclic electron flow around photosystem I (PSI) increased significantly during desiccation, continued to operate at times of severe desiccation, and showed greater tolerance to desiccation than the electron flow around PSII. In addition, PSI activity in desiccated blades recovered faster than PSII activity during re-hydration. Even though linear electron flow was suppressed by DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea], cyclic electron flow could still be restored. This process was insensitive to antimycin A and could be suppressed by dibromothymoquinone (DBMIB). The prolonged dark treatment of blades reduced the speed in which the cyclic electron flow around PSI recovered, suggesting that stromal reductants, including NAD(P)H, played an important role in the donation of electrons to PSI and were the main cause of the rapid recovery of cyclic electron flow in desiccated blades during re-hydration. These results suggested that cyclic electron flow in P. yezoensis played a significant physiological role during desiccation and re-hydration and may be one of the most important factors allowing P. yezoensis blades to adapt to intertidal environments.     

Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0–0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport ( Φ _PSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (q_N) and a decrease in photochemical quenching (q_P). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from Q_A⁻ to Q_B. The slope at the origin of the relative variable fluorescence curves (dV/dt_o) and the relative variable fluorescence at phase J (V_J) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.     

Glutamate synthesis in barley roots: the role of the plastidic glucose-6-phosphate dehydrogenase

Evidence is provided for a close link between glutamate (Glu) synthesis and the production of reducing power by the oxidative pentose phosphate pathway (OPPP) in barley ( Hordeum vulgare L. var. Alfeo) root plastids. A rapid procedure for isolating organelles gave yields of plastids of over 30%, 60% of which were intact. The formation of Glu by intact plastids fed with glutamine and 2-oxoglutarate, both substrates of glutamate synthase (GOGAT), depends on glucose-6-phosphate (Glc-6-P) supply. The whole process exhibited an apparent K(m Glc-6-P) of 0.45 mM and is abolished by azaserine, a specific inhibitor of GOGAT; ATP caused a decrease in the rate of Glu formation. Glucose and other sugar phosphates were not as effective in supporting Glu synthesis with respect to Glc-6-P; only ribose-5-phosphate, an intermediate of OPPP, supported rates equivalent to Glc-6-P. Glucose-6-phosphate dehydrogenase (Glc6PDH) rapidly purified from root plastids showed an apparent K(m Glc-6-P) of 0.96 mM and an apparent K(m NADP)(+) of 9 micro M. The enzyme demonstrated high tolerance to NADPH, exhibiting a K(i) (NADPH) of 58.6 micro M and selectively reacted with antibodies against potato plastidic, but not chloroplastic, Glc6PDH isoform. The data support the hypothesis that plastidic OPPP is the main site of reducing power supply for GOGAT within the plastids, and suggest that the plastidic OPPP would be able to sustain Glu synthesis under high NADPH:NADP(+) ratios even if the plastidic Glc6PDH may not be functioning at its highest rates.     

Glucose-6-phosphate dehydrogenase acts as a regulator of cell redox balance in rice suspension cells under salt stress

The reduced coenzyme nicotinamide-adenine dinucleotide phosphate (NADPH) is an important molecule in cellular redox balance. Glucose-6-phosphate dehydrogenase (G6PDH) is a key enzyme in the pentose phosphate pathway, the most important NADPH-generating pathway. In this study, roles of G6PDH in maintaining cell redox balance in rice suspension cells under salt stress were investigated. Results showed that the G6PDH activity decreased in the presence of 80 mM NaCl on day 2. Application of exogenous glucose stimulated the activity of G6PDH and NADPH oxidase under salt stress. Exogenous glucose also increased the ion leakage, thiobarbituric acid reactive substances and hydrogen peroxide (H₂O₂) contents in the presence of 80 mM NaCl on day 2, implying that the reduction of the G6PDH activity was necessary to avoid serious damage caused by salt stress. The NAPDH/NADP⁺ ratio increased on day 2 but decreased on day 4 under 80 mM NaCl plus glucose treatment. Diphenyleneiodonium, an NADPH oxidase inhibitor, decreased the H₂O₂ content under 80 mM NaCl treatment on day 2. These results imply that the H₂O₂ accumulation induced by glucose treatment under salt stress on day 2 was related to the NADPH oxidase. Western-blot analysis showed that the G6PDH expression was slightly induced by glucose and was obviously blocked by DPI on day 2 under salt stress. In conclusion, G6PDH plays a key role in maintaining the cell redox balance in rice suspension cells under salt stress. The coordination of G6PDH and NADPH oxidase is required in maintaining cell redox balance in salt tolerance.     

Desiccation enhances phosphorylation of PSII and affects the distribution of protein complexes in the thylakoid membrane

Desiccation has significant effects on photosynthetic processes in intertidal macro‐algae. We studied an intertidal macro‐alga, Ulva sp., which can tolerate desiccation, to investigate changes in photosynthetic performance and the components and structure of thylakoid membrane proteins in response to desiccation. Our results demonstrate that photosystem II (PSII) is more sensitive to desiccation than photosystem I (PSI) in Ulva sp. Comparative proteomics of the thylakoid membrane proteins at different levels of desiccation suggested that there were few changes in the content of proteins involved in photosynthesis during desiccation. Interestingly, we found that both the PSII subunit, PsbS (Photosystem II S subunit) (a four‐helix protein in the LHC superfamily), and light‐harvesting complex stress‐related (LHCSR) proteins, which are required for non‐photochemical quenching in land plants and algae, respectively, were present under both normal and desiccation conditions and both increased slightly during desiccation. In addition, the results of immunoblot analysis suggested that the phosphorylation of PSII and LHCII increases during desiccation. To investigate further, we separated out a supercomplex formed during desiccation by blue native‐polyacrylamide gel electrophoresis and identified the components by mass spectrometry analysis. Our results show that phosphorylation of the complex increases slightly with decreased water content. All the results suggest that during the course of desiccation, few changes occur in the content of thylakoid membrane proteins, but a rearrangement of the protein complex occurs in the intertidal macro‐alga Ulva sp.     

Specific photosynthetic and morphological characteristics allow macroalgae Gloiopeltis furcata (Rhodophyta) to survive in unfavorable conditions

Gloiopeltis furcata (Postels & Ruprecht) J. Agardh, a macroalga, which grows in an upper, intertidal zone, can withstand drastic environmental changes caused by the periodic tides. In this study, the photosynthetic and morphological characteristics of G. furcata were investigated. The photosynthetic performance and electron flows of the thalli showed significant variations in response to desiccation and salinity compared with the control group. Both PSII and PSI activities declined gradually when the thalli were under stress. However, the electron transport rate of PSI showed still a low value during severe conditions, while the rate of PSII approached zero. Furthermore, PSI activity of the treated thalli recovered faster than PSII after being submerged in seawater. Even though the linear electron flow was inhibited by DCMU [3-(3, 4-dichlorophenyl)-1,1-dimethylurea], the cyclic electron flow could still be restored. The rate of cyclic electron flow recovery declined with the increasing time of dark treatment, which suggested that stromal reductants from starch degradation played an important role in the donation of electrons to PSI. This study demonstrated that PSII was more sensitive than PSI to desiccation and salinity in G. furcata and that the cyclic electron flow around PSI played a significant physiological role. In addition, G. furcata had branches, which were hollow inside and contained considerable quantities of funoran. These might be the most important factors in allowing G. furcata to adapt to adverse intertidal environments.     

Reconstructing a puzzle: existence of cyanophages containing both photosystem‐I and photosystem‐II gene suites inferred from oceanic metagenomic datasets

Cyanobacteria play a key role in marine photosynthesis, which contributes to the global carbon cycle and to the world oxygen supply. Genes encoding the photosystem‐II (PSII) reaction centre are found in many cyanophage genomes, and it was suggested that the horizontal transfer of these genes might be involved in increasing phage fitness. Recently, evidence for the existence of phages carrying Photosystem‐I (PSI) genes was also reported. Here, using a combination of different marine metagenomic datasets and a unique crossing of the datasets, we now describe the finding of phages that, as in plants and cyanobacteria, contain both PSII and PSI genes. In addition, these phages also contain NADH dehydrogenase genes. The presence of modified PSII and PSI genes in the same viral entities in combination with electron transfer proteins like NAD(P)H dehydrogenase (NDH‐1) strongly points to a role in perturbation of the cyanobacterial host photosynthetic electron flow. We therefore suggest that, depending on the physiological condition of the infected cyanobacterial host, the viruses may use different options to maximize survival. The modified PSI may alternate between functioning with PSII in linear electron transfer and contributing to the production of both NADPH and ATP or functioning independently of PSII in cyclic mode via the NDH‐1 complex and thus producing only ATP.     

Differential effects of exposure to parasites and bacteria on stress response in turbot Scophthalmus maximus simultaneously stressed by low water depth

The stress response of turbot Scophthalmus maximus was evaluated in fish maintained 8 days under different water depths, normal (NWD, 30 cm depth, total water volume 40 l) or low (LWD, 5 cm depth, total water volume 10 l), in the additional presence of infection–infestation of two pathogens of this species. This was caused by intraperitoneal injection of sublethal doses of the bacterium Aeromonas salmonicida subsp. salmonicida or the parasite Philasterides dicentrarchi (Ciliophora:Scuticociliatida). The LWD conditions were stressful for fish, causing increased levels of cortisol in plasma, decreased levels of glycogen in liver and nicotinamide adenine dinucleotide phosphate (NADP) and increased activities of G6Pase and GSase. The presence of bacteria or parasites in fish under NWD resulted in increased cortisol levels in plasma whereas in liver, changes were of minor importance including decreased levels of lactate and GSase activity. The simultaneous presence of bacteria and parasites in fish under NWD resulted a sharp increase in the levels of cortisol in plasma and decreased levels of glucose. Decreased levels of glycogen and lactate and activities of GSase and glutathione reductase (GR), as well as increased activities of glucose‐6‐phosphate dehydrogenase (G6PDH), 6‐phosphogluconate dehydrogenase (6PGDH) and levels of nicotinamide adenine dinucleotide phosphate (NADPH) occurred in the same fish in liver. Finally, the presence of pathogens in S. maximus under stressful conditions elicited by LWD resulted in synergistic actions of both type of stressors in cortisol levels. In liver, the presence of bacteria or parasites induced a synergistic action on several variables such as decreased activities of G6Pase and GSase as well as increased levels of NADP and NADPH and increased activities of GPase, G6PDH and 6PGDH.     

The regulation of xanthophyll cycle activity and of non-photochemical fluorescence quenching by two alternative electron flows in the diatoms Phaeodactylum tricornutum and Cyclotella meneghiniana

Intact cells of diatoms are characterized by a rapid diatoxanthin epoxidation during low light periods following high light illumination while epoxidation is severely restricted in phases of complete darkness. The present study shows that rapid diatoxanthin epoxidation is dependent on the availability of the cofactor of diatoxanthin epoxidase, NADPH, which cannot be generated in darkness due to the inactivity of PSI. In the diatom Phaeodactylum tricornutum, NADPH production during low light is dependent on PSII activity, and addition of DCMU consequently abolishes diatoxanthin epoxidation. In contrast to P. tricornutum, DCMU does not affect diatoxanthin epoxidation in Cyclotella meneghiniana, which shows the same rapid epoxidation in low light both in the absence or presence of DCMU. Measurements of the reduction state of the PQ pool and PSI activity indicate that, in the presence of DCMU, NADPH production in C. meneghiniana occurs via alternative electron transport, which includes electron donation from the chloroplast stroma to the PQ pool and, in a second step, from PQ to PSI. Similar electron flow to PQ is also observed during high light illumination of DCMU-treated P. tricornutum cells. In contrast to C. meneghiniana, the electrons are not directed to PSI, but most likely to a plastoquinone oxidase. This chlororespiratory electron transport leads to the establishment of an uncoupler-sensitive proton gradient in the presence of DCMU, which induces diadinoxanthin de-epoxidation and NPQ. In C. meneghiniana, electron flow to the plastoquinone oxidase is restricted, and consequently, diadinoxanthin de-epoxidation and NPQ is not observed after addition of DCMU.     

Light-Harvesting Function in the Diatom Phaeodactylum tricornutum: II. Distribution of Excitation Energy between the Photosystems

The distribution of excitation energy between photosystems I and II (PSI and PSII) was investigated in the marine diatom Phaeodactylum tricornutum (Bohlin) using light-induced changes in fluorescence yield and rate of modulated O₂ evolution. The intensity dependence of the fast fluorescence rise in dark adapted cells (±DCMU) suggests that light absorbed by the major antenna complex was not delivered preferentially to PSII but is more equally distributed between the photosystems. Reversible, slow fluorescence yield changes measured in the absence of DCMU were correlated with decreased initial fluorescence and rate constants for PSII photochemistry, increased variable fluorescence, alteration of the fluorescence excitation and emission spectra, and could be effected by either 510 nm (PSII) or 704 nm (PSI) light. Slow, reversible fluorescence yield changes were also observed in the presence of DCMU, but were characterized by a loss of both initial and variable fluorescence and could not be induced by PSI light. The absence of slow changes in the yield of fluorescence and rate of modulated O₂ evolution, following addition or removal of PSI background light to modulated PSII excitation, does not support regulation of excitation energy density in PSI at the expense of PSII. The results suggest that adjustments are made at the level of excitation energy transfer to the PSII reaction center which prevent prolonged loss of photosynthetic capacity. Energy distribution is regulated by ionic distributions independently of the plastoquinone pool redox state. These differences in light-harvesting function are probably a response to the aquatic light field and may account for the success of diatoms in low and variable light environments.     

Capacity for NADPH regeneration in the leaves of two poplar genotypes differing in ozone sensitivity

Cell capacity for cytosolic NADPH regeneration by NADP‐dehydrogenases was investigated in the leaves of two hybrid poplar (Populus deltoides × Populus nigra) genotypes in response to ozone (O₃) treatment (120 ppb for 17 days). Two genotypes with differential O₃ sensitivity were selected, based on visual symptoms and fallen leaves: Robusta (sensitive) and Carpaccio (tolerant). The estimated O₃ flux (POD₀), that entered the leaves, was similar for the two genotypes throughout the treatment. In response to that foliar O₃ flux, CO₂ assimilation was inhibited to the same extent for the two genotypes, which could be explained by a decrease in Rubisco (EC 4.1.1.39) activity. Conversely, an increase in PEPC (EC 4.1.1.31) activity was observed, together with the activation of certain cytosolic NADP‐dehydrogenases above their constitutive level, i.e. NADP‐G6PDH (EC 1.1.1.49), NADP‐ME (malic enzyme) (EC 1.1.1.40) and NADP‐ICDH (NADP‐isocitrate dehydrogenase) (EC1.1.1.42). However, the activity of non‐phosphorylating NADP‐GAPDH (EC 1.2.1.9) remained unchanged. From the 11th fumigation day, NADP‐G6PDH and NADP‐ME profiles made it possible to differentiate between the two genotypes, with a higher activity in Carpaccio than in Robusta. At the same time, Carpaccio was able to maintain high levels of NADPH in the cells, while NADPH levels decreased in Robusta O₃‐treated leaves. All these results support the hypothesis that the capacity for cells to regenerate the reducing power, especially the cytosolic NADPH pool, contributes to improve tolerance to high ozone exposure.     

Growth of Anacystis in the Presence of Thiosulphate and its Consequences for the Architecture of the Photosynthetic Apparatus

With the aim of obtaining information on the degree of flexibility maintained in cyanobacteria in context with their phylogenetic position, Anacystis was grown in the presence of thiosulphate, oxidized in a photosystem I (PSI) dependent reaction (K_M 7.4 × 10^?3 M thiosulfate). Besides DBMIB, only o-phenanthroline and p-hydroxymercuribenzoate blocked thiosulphate-dependent PSI activity to some extent; iodonitrothymol, DCMU and cyanide had no influence. Growth of Anacystis in the presence of thiosulphate induced a reorganization of the photosynthetic apparatus characterized by a shift in the PSII/PSI ratio in favor of PSI, comparable to low light conditions. Capability for oxygenic photosynthesis never completely disappeared; structural elements of PSII were retained in the membrane to a certain degree. The antenna pigment system signalled high light under conditions of thiosulphate oxidation as judged from the ratio of phycocyanin to chlorophyll. Besides a shift in the ratio of PSII to PSI components, the polypeptide pattern of thylakoids from thiosulphate grown cells shows several additional components compared to the controls and, moreover, higher concentrations of some polypeptides present in the controls, particularly a M_r 41000 polypeptide. The process of thiosulphate oxidation appears bound to the thylakoid membrane.     

Redox of Plastoquinone Pool Regulates the Expression and Activity of NADPH Dehydrogenase Supercomplex in <Emphasis Type="Italic">Synechocystis</Emphasis> sp. Strain PCC 6803

A highly active NADPH dehydrogenase supercomplex, which is essential for cyclic electron transport around photosystem I (cyclic PSI) and respiration, was newly identified in cyanobacteria. Synechocystis sp. strain PCC 6803 cells were treated with exogenous glucose (Glc) or 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); subsequently, active staining of NADPH-nitroblue tetrazolium oxidoreductase, western blot, and the initial rate of P700⁺ dark reduction were assessed in the cyanobacterium at several time points. The expression and enzyme activity levels of NADPH dehydrogenase supercomplex were gradually inhibited and closely associated with the decrease in the rate of cyclic PSI accompanying the addition of exogenous Glc to the cultures. In contrast, the activity levels were significantly stimulated but did not cause an increase in the rate of cyclic PSI as expected in the presence of DCMU. Since Glc results in the partial reduction of the plastoquinone (PQ) pool while DCMU results in the overoxidation of the PQ pool, the present results demonstrate that the expression and activity of NADPH dehydrogenase supercomplex are under the influence of the redox control of the PQ pool while the operation of cyclic PSI as mediated by this supercomplex requires an appropriate redox poise of the PQ pool.     

Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage

Photodamage of Photosystem II (PSII) has been considered as an unavoidable and harmful reaction that decreases plant productivity. PSII, however, has an efficient and dynamically regulated repair machinery, and the PSII activity becomes inhibited only when the rate of damage exceeds the rate of repair. The speed of repair is strictly regulated according to the energetic state in the chloroplast. In contrast to PSII, Photosystem I (PSI) is very rarely damaged, but when occurring, the damage is practically irreversible. While PSII damage is linearly dependent on light intensity, PSI gets damaged only when electron flow from PSII exceeds the capacity of PSI electron acceptors to cope with the electrons. When electron flow to PSI is limited, for example in the presence of DCMU, PSI is extremely tolerant against light stress. Proton gradient (ΔpH)-dependent slow-down of electron transfer from PSII to PSI, involving the PGR5 protein and the Cyt b6f complex, protects PSI from excess electrons upon sudden increase in light intensity. Here we provide evidence that in addition to the ΔpH-dependent control of electron transfer, the controlled photoinhibition of PSII is also able to protect PSI from permanent photodamage. We propose that regulation of PSII photoinhibition is the ultimate regulator of the photosynthetic electron transfer chain and provides a photoprotection mechanism against formation of reactive oxygen species and photodamage in PSI.     

Effects of 5-aminolevulinic acid treatment on photosynthesis of strawberry

Effects of root treatment with 5-aminolevulinic acid (ALA) on leaf photosynthesis in strawberry (Fragaria ananassa Duch.) plants were investigated by rapid chlorophyll fluorescence and modulated 820 nm reflection using 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) and methyl viologen (MV). Our results showed that ALA treatments increased the net photosynthetic rate and decreased the intercelluar CO₂ concentration in strawberry leaves. Under DCMU treatment, trapping energy for Q_A reduction per PSII reaction center increased greatly, indicating DCMU inhibited electron transfer from Q_A ^?. The maximum photochemical efficiency of PSII (F_v/F_m) decreased under the DCMU treatment, while a higher F_v/F_m remained in the ALA-pretreated plants. Not only the parameters related to a photochemical phase, but also that one related to a heat phase remained lower after the ALA pretreatment, compared to the sole DCMU treatment. The MV treatment decreased PSI photochemical capacity. The results of modulated 820 nm reflection analysis showed that DCMU and MV treatments had low re-reduction of P700 and plastocyanin (PSI). However, the strawberry leaf discs pretreated with ALA exhibited high re-reduction of PSI under DCMU and MV treatments. The results of this study suggest that the improvement of photosynthesis by ALA in strawberry was not only related to PSII, but also to PSI and electron transfer chain.     

Spectrally decomposed dark-to-light transitions in <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

Photosynthetic activity and respiration share the thylakoid membrane in cyanobacteria. We present a series of spectrally resolved fluorescence experiments where whole cells of the cyanobacterium Synechocystis sp. PCC 6803 and mutants thereof underwent a dark-to-light transition after different dark-adaptation (DA) periods. Two mutants were used: (i) a PSI-lacking mutant (ΔPSI) and (ii) M55, a mutant without NAD(P)H dehydrogenase type-1 (NDH-1). For comparison, measurements of the wild-type were also carried out. We recorded spectrally resolved fluorescence traces over several minutes with 100 ms time resolution. The excitation light was at 590 nm so as to specifically excite the phycobilisomes. In ΔPSI, DA time has no influence, and in dichlorophenyl-dimethylurea (DCMU)-treated samples we identify three main fluorescent components: PB–PSII complexes with closed (saturated) RCs, a quenched or open PB–PSII complex, and a PB–PSII ‘not fully closed.’ For the PSI-containing organisms without DCMU, we conclude that mainly three species contribute to the signal: a PB–PSII–PSI megacomplex with closed PSII RCs and (i) slow PB → PSI energy transfer, or (ii) fast PB → PSI energy transfer and (iii) complexes with open (photochemically quenched) PSII RCs. Furthermore, their time profiles reveal an adaptive response that we identify as a state transition. Our results suggest that deceleration of the PB → PSI energy transfer rate is the molecular mechanism underlying a state 2 to state 1 transition.     

The effect of Glc6P uptake and its subsequent oxidation within pea root plastids on nitrite reduction and glutamate synthesis

In roots, nitrate assimilation is dependent upon a supply of reductant that is initially generated by oxidative metabolism including the pentose phosphate pathway (OPPP). The uptake of nitrite into the plastids and its subsequent reduction by nitrite reductase (NiR) and glutamate synthase (GOGAT) are potentially important control points that may affect nitrate assimilation. To support the operation of the OPPP there is a need for glucose 6-phosphate (Glc6P) to be imported into the plastids by the glucose phosphate translocator (GPT). Competitive inhibitors of Glc6P uptake had little impact on the rate of Glc6P-dependent nitrite reduction. Nitrite uptake into plastids, using (13)N labelled nitrite, was shown to be by passive diffusion. Flux through the OPPP during nitrite reduction and glutamate synthesis in purified plastids was followed by monitoring the release of (14)CO(2) from [1-(14)C]-Glc6P. The results suggest that the flux through the OPPP is maximal when NiR operates at maximal capacity and could not respond further to the increased demand for reductant caused by the concurrent operation of NiR and GOGAT. Simultaneous nitrite reduction and glutamate synthesis resulted in decreased rates of both enzymatic reactions. The enzyme activity of glucose 6-phosphate dehydrogenase (G6PDH), the enzyme supporting the first step of the OPPP, was induced by external nitrate supply. The maximum catalytic activity of G6PDH was determined to be more than sufficient to support the reductant requirements of both NiR and GOGAT. These data are discussed in terms of competition between NiR and GOGAT for the provision of reductant generated by the OPPP.