Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis

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The relationship between state II to state I transitions and cyclic electron flow around photosystem I

The effects of electron acceptors, inhibitors of electron flow and uncouplers and inhibitors of photophosphorylation on a state II to I transition were studied. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not inhibit the state II to I transition. By contrast, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), methyl viologen and antimycin A inhibited the transition indicating that the cyclic electron flow around photosystem I, but not the oxidation of electron carriers (such as plastoquinone), induced the state II to I transition. Uncouplers, but not inhibitors of photophosphorylation, inhibited the state transition suggesting that the proton transport through the cyclic electron flow was related to the transition.     

The relationship between state II to state I transitions and cyclic electron flow around photosystem I

The effects of electron acceptors, inhibitors of electron flow and uncouplers and inhibitors of photophosphorylation on a state II to I transition were studied. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not inhibit the state II to I transition. By contrast, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), methyl viologen and antimycin A inhibited the transition indicating that the cyclic electron flow around photosystem I, but not the oxidation of electron carriers (such as plastoquinone), induced the state II to I transition. Uncouplers, but not inhibitors of photophosphorylation, inhibited the state transition suggesting that the proton transport through the cyclic electron flow was related to the transition.     

Cyclic electron flow around photosystem I in C(3) plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex

Cyclic electron flow around photosystem (PS) I has been widely described in vitro in chloroplasts or thylakoids isolated from C(3) plant leaves, but its occurrence in vivo is still a matter of debate. Photoacoustic spectroscopy and kinetic spectrophotometry were used to analyze cyclic PS I activity in tobacco (Nicotiana tabacum cv Petit Havana) leaf discs illuminated with far-red light. Only a very weak activity was measured in air with both techniques. When leaf discs were placed in anaerobiosis, a high and rapid cyclic PS I activity was measured. The maximal energy storage in far-red light increased to 30% to 50%, and the half-time of the P(700) re-reduction in the dark decreased to around 400 ms; these values are comparable with those measured in cyanobacteria and C(4) plant leaves in aerobiosis. The stimulatory effect of anaerobiosis was mimicked by infiltrating leaves with inhibitors of mitochondrial respiration or of the chlororespiratory oxidase, therefore, showing that changes in the redox state of intersystem electron carriers tightly control the rate of PS I-driven cyclic electron flow in vivo. Measurements of energy storage at different modulation frequencies of far-red light showed that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco mutant deficient in the plastid Ndh complex was kinetically different from that of the wild type, the cycle being slower in the former leaves. We conclude that the Ndh complex is required for rapid electron cycling around PS I.     

Redox modulation of cyclic electron flow around photosystem I in C3 plants

We have investigated the occurrence of cyclic electron flow in intact spinach leaves. In particular, we have tested the hypothesis that cyclic flow requires the presence of supercomplexes in the thylakoid membrane or other strong associations between proteins. Using biochemical approaches, we found no evidence of the presence of supercomplexes related to cyclic electron flow, making previous structural explanations for the modulation of cyclic flow rather unlikely. On the other hand, we found that the fraction of photosystem I complexes engaged in cyclic flow could be modulated by changes in the redox state of the chloroplast stroma. Our findings support therefore a dynamic model for the occurrence of linear and cyclic electron flow in C3 plants, based on the competition between cytochrome b(6)f and FNR for electrons carried by ferredoxin. This would be ultimately regulated by the balance between the redox state of PSI acceptors and donors during photosynthesis, in a diffusing system.     

Targeted inactivation of the plastid ndhB gene in tobacco results in an enhanced sensitivity of photosynthesis to moderate stomatal closure

The ndh genes encoding for the subunits of NAD(P)H dehydrogenase complex represent the largest family of plastid genes without a clearly defined function. Tobacco (Nicotiana tabacum) plastid transformants were produced in which the ndhB gene was inactivated by replacing it with a mutant version possessing translational stops in the coding region. Western-blot analysis indicated that no functional NAD(P)H dehydrogenase complex can be assembled in the plastid transformants. Chlorophyll fluorescence measurements showed that dark reduction of the plastoquinone pool by stromal reductants was impaired in ndhB-inactivated plants. Both the phenotype and photosynthetic performance of the plastid transformants was completely normal under favorable conditions. However, an enhanced growth retardation of ndhB-inactivated plants was revealed under humidity stress conditions causing a moderate decline in photosynthesis via stomatal closure. This distinctive phenotype was mimicked under normal humidity by spraying plants with abscisic acid. Measurements of CO(2) fixation demonstrated an enhanced decline in photosynthesis in the mutant plants under humidity stress, which could be restored to wild-type levels by elevating the external CO(2) concentration. These results suggest that the plastid NAD(P)H:plastoquinone oxidoreductase in tobacco performs a significant physiological role by facilitating photosynthesis at moderate CO(2) limitation.     

A balanced PGR5 level is required for chloroplast development and optimum operation of cyclic electron transport around photosystem I

PSI cyclic electron transport contributes markedly to photosynthesis and photoprotection in flowering plants. Although the thylakoid protein PGR5 (Proton Gradient Regulation 5) has been shown to be essential for the main route of PSI cyclic electron transport, its exact function remains unclear. In transgenic Arabidopsis plants overaccumulating PGR5 in the thylakoid membrane, chloroplast development was delayed, especially in the cotyledons. Although photosynthetic electron transport was not affected during steady-state photosynthesis, a high level of non-photochemical quenching (NPQ) was transiently induced after a shift of light conditions. This phenotype was explained by elevated activity of PSI cyclic electron transport, which was monitored in an in vitro system using ruptured chloroplasts, and also in leaves. The effect of overaccumulation of PGR5 was specific to the antimycin A-sensitive pathway of PSI cyclic electron transport but not to the NAD(P)H dehydrogenase (NDH) pathway. We propose that a balanced PGR5 level is required for efficient regulation of the rate of antimycin A-sensitive PSI cyclic electron transport, although the rate of PSI cyclic electron transport is probably also regulated by other factors during steady-state photosynthesis.     

Flavodoxin accumulation contributes to enhanced cyclic electron flow around photosystem I in salt-stressed cells of Synechocystis sp. strain PCC 6803

The salt-regulated accumulation of flavodoxin encoded by the isiB gene and its possible function were investigated in the cyanobacterium Synechocystis sp. strain PCC 6803. In Northern blot experiments, a slight increase of the isiB -specific mRNA was observed in salt-shocked and salt-acclimated cells. High levels of flavodoxin protein were detected in cells acclimated to 342 m M NaCl. In order to analyze the function of flavodoxin in cyanobacterial salt acclimation, an insertion null mutant of isiB was constructed. It was possible to adapt this mutant to raised salt concentrations and, as expected, to low iron contents. Salt-acclimated cells of wild type (WT) Synechocystis display increased activity of photosystem I (PSI), primarily used for increased cyclic electron transport capacity (Jeanjean et al. 1993, Plant Cell Physiol 34: 1073–1079). In salt-acclimated cells of the flavodoxin null mutant, the level of cyclic electron flow was lower than in wild type cells. It was concluded that flavodoxin plays a role as an alternative electron carrier, used for cyclic electron flow in salt-treated Synechocystis cells.     

Studies on photosystem I. II. Involvement of ferredoxin in cyclic electron flow

The effects of ferredoxin (Fd) and ferredoxin-NADP reductase on the light-induced spectral changes of cytochrome f (cyt f) were investigated with specific reference to their possible involvement in the cyclic electron transfort pathway of photosystem I (PS I). The steady-state level of photooxidation of reduced cytochrome f is decreased by ferredoxin but unaffected by either ferredoxin-NADP reductase alone or ferredoxin plus ferredoxin-NADP reductase when present in equimolar concentrations. These data are taken as evidence for a cyclic electron transport pathway of: PS I → “X” → Fd → (cyt f) → PC → PS I. The reduced ferredoxin could either reduce directly plastocyanin (PC) or via cytochrome f; the data do not allow differentiation between these two possibilities. However, neither ferredoxin-NADP reductase nor cytochrome b564 appear to serve as electron carriers in this pathway.     

Photosynthetic electron flow during leaf senescence: Evidence for a preferential maintenance of photosystem I activity and increased cyclic electron flow

Limitations in photosystem function and photosynthetic electron flow were investigated during leaf senescence in two field-grown plants, i.e., Euphorbia dendroides L. and Morus alba L., a summer- and winter-deciduous, shrub and tree, respectively. Analysis of fast chlorophyll (Chl) a fluorescence transients and post-illumination fluorescence yield increase were used to assess photosynthetic properties at various stages of senescence, the latter judged from the extent of Chl loss. In both plants, the yield of primary photochemistry of PSII and the content of PSI remained quite stable up to the last stages of senescence, when leaves were almost yellow. However, the potential for linear electron flow along PSII was limited much earlier, especially in E. dendroides, by an apparent inactivation of the oxygen-evolving complex and a lower efficiency of electron transfer to intermediate carriers. On the contrary, the corresponding efficiency of electron transfer from intermediate carriers to final acceptors of PSI was increased. In addition, cyclic electron flow around PSI was accelerated with the progress of senescence in E. dendroides, while a corresponding trend in M. alba was not statistically significant. However, there was no decrease in PSI activity even at the last stages of senescence. We argue that a switch to cyclic electron flow around PSI during leaf senescence may have the dual role of replenishing the ATP and maintaining a satisfactory nonphotochemical energy quenching, since both are limited by hindered linear electron transfer.     

Regulation of pepc gene expression in Anabaena sp. PCC 7120 and its effects on cyclic electron flow around photosystem I and tolerances to environmental stresses

Since pepc gene encoding phosphoenolpyruvate carboxylase(PEPCase) has been cloned from Anabaena sp. PCC7120 and other cyanobacteria, the effects of pepc gene expression on photosynthesis have not been reported yet. In this study, we constructed mutants containing either upregulated(forward) or downregulated(reverse) pepc gene in Anabaena sp. PCC 7120. Results from real-time quantitative polymerase chain reaction(RT-q PCR), Western blot and enzymatic analysis showed that PEPCase activity was significantly reduced in the reverse mutant compared with the wild type, and that of the forward mutant was obviously increased.Interestingly, the net photosynthesis in both the reverse mutant and the forward mutant were higher than that of the wild type, but dark respiration was decreased only in the reverse mutant. The absorbance changes of P700 upon saturation pulse showed the photosystem I(PSI) activity was inhibited, as reflected by Y(I), and Y(NA) was elevated, and Rdark reduction of P700 t was stimulated, indicating enhanced cyclic electron flow(CEF) around PSI in the reverse mutant.Additionally, the reverse mutant photosynthesis was higher than that of the wild type in low temperature, low and high pH,and high salinity, and this implies increased tolerance in the reverse mutant through downregulated pepc gene.     

Cyclic electron flow around photosystem I via chloroplast NAD(P)H dehydrogenase (NDH) complex performs a significant physiological role during photosynthesis and plant growth at low temperature in rice

The role of NAD(P)H dehydrogenase (NDH)-dependent cyclic electron flow around photosystem I in photosynthetic regulation and plant growth at several temperatures was examined in rice (Oryza sativa) that is defective in CHLORORESPIRATORY REDUCTION 6 (CRR6), which is required for accumulation of sub-complex A of the chloroplast NDH complex (crr6). NdhK was not detected by Western blot analysis in crr6 mutants, resulting in lack of a transient post-illumination increase in chlorophyll fluorescence, and confirming that crr6 mutants lack NDH activity. When plants were grown at 28 or 35°C, all examined photosynthetic parameters, including the CO(2) assimilation rate and the electron transport rate around photosystems I and II, at each growth temperature at light intensities above growth light (i.e. 800 μmol photons m(-2) sec(-1)), were similar between crr6 mutants and control plants. However, when plants were grown at 20°C, all the examined photosynthetic parameters were significantly lower in crr6 mutants than control plants, and this effect on photosynthesis caused a corresponding reduction in plant biomass. The F(v)/F(m) ratio was only slightly lower in crr6 mutants than in control plants after short-term strong light treatment at 20°C. However, after long-term acclimation to the low temperature, impairment of cyclic electron flow suppressed non-photochemical quenching and promoted reduction of the plastoquinone pool in crr6 mutants. Taken together, our experiments show that NDH-dependent cyclic electron flow plays a significant physiological role in rice during photosynthesis and plant growth at low temperature.     

An active supercomplex of NADPH dehydrogenase mediated cyclic electron flow around Photosystem I from the panicle chloroplast of Oryza sativa

Chloroplast NAD（P）H dehydrogenase-like complex （NDH） plays a crucial role in the protection of plants against oxida- tive stress. In higher plants, NDH interacts with Photosystem I （PSI） to form an NDH-PSI supercomplex. However, the chloroplast supercomplex with NADPH oxidation activity remains to be identified. Here, we reported the identification of a supercomplex of NDH with NADPH-nitroblue tetrazo- fium oxidoreductase activity in the chloroplast of rice panicle. The active supercomplex from the panicle chloro- plast contained higher amounts of the NDH subunits （NdhH, NdhK, and NdhA） than that from the flag leaf chloroplast. The highly active supercomplex might underlie the high ac- tivity of the NADPH-dependent NDH pathway and the larger proton gradient across thylakoid membranes via cyclic electron flow around PSI, as well as the higher maximal photochemical efficiency of Photosystem II at the flowering to grain-filfing stage. The supercomplex is sug- gested to be essential for the high efficiency of photosynthesis and play a protective role in the grain formation in rice plant.     

Open reading frame ssr2016 is required for antimycin A-sensitive photosystem I-driven cyclic electron flow in the cyanobacterium Synechocystis sp. PCC 6803

Open reading frame ssr2016 encodes a protein with substantial sequence similarities to PGR5 identified as a component of the antimycin A-sensitive ferredoxin:plastoquinone reductase (FQR) in PSI cyclic photophosphorylation in Arabidopsis thaliana. We studied cyclic electron flow in Synechocystis sp. PCC 6803 in vivo in ssr2016 deletion mutants generated either in a wild-type background or in a ndhB deletion mutant. Our results indicate that ssr2016 is required for FQR and that it operates in a parallel pathway to the NDH1 complex. The ssr2016 deletion mutants are high light sensitive, suggesting that FQR might be important in controlling redox poise under adverse conditions.     

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.     

Functional division of f-type and m-type thioredoxins to regulate the Calvin cycle and cyclic electron transport around photosystem I

Redox regulation of chloroplast proteins is necessary to adjust photosynthetic performance with changes in light. The thioredoxin (Trx) system plays a central role in this process. Chloroplast-localized classical Trx is a small redox-active protein that regulates many target proteins by reducing their disulfide bonds in a light-dependent manner. Arabidopsis thaliana mutants lacking f-type Trx (trx f1f2) or m-type Trx (trx m124-2) have been reported to show delayed reduction of Calvin cycle enzymes. As a result, the trx m124-2 mutant exhibits growth defects. Here, we characterized a quintuple mutant lacking both Trx f and Trx m to investigate the functional complementarity of Trx f and Trx m. The trx f1f2 m124-2 quintuple mutant was newly obtained by crossing, and is analyzed here for the first time. The growth defects of the trx m124-2 mutant were not enhanced by the lack of Trx f. In contrast, deficiencies of both Trxs additively suppressed the reduction of Calvin cycle enzymes, resulting in a further delay in the initiation of photosynthesis. Trx f appeared to be necessary for the rapid activation of the Calvin cycle during the early induction of photosynthesis. To perform effective photosynthesis, plants seem to use both Trxs in a coordinated manner to activate carbon fixation reactions. In contrast, the PROTON GRADIENT REGULATION 5 (PGR5)-dependent cyclic electron transport around photosystem I was regulated by Trx m, but not by Trx f. Lack of Trx f did not affect the activity and regulation of the PGR5-dependent pathway. Trx f may have a higher specificity for target proteins, whereas Trx m has a variety of target proteins to regulate overall photosynthesis and other metabolic reactions in the chloroplasts.