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.     

Cyclic electron flow around photosystem I in unicellular green algae

Cyclic electron flow around PSI, or cyclic photophosphorylation, is the photosynthetic process which recycles the reducing equivalents produced by photosystem I in the stroma towards the plastoquinone pool. Through the activity of cytochrome b ₆ f, which also transfers protons across the membrane, it promotes the synthesis of ATP. The literature dealing with cyclic electron flow in unicellular algae is far less abundant than it is for plants. However, in the chloroplast of algae such as Chlorella or Chlamydomonas, an efficient carbohydrate catabolism renders the redox poise much more reducing than in plant chloroplasts. It is therefore worthwhile highlighting the specific properties of unicellular algae because cyclic electron flow is highly dependent upon the accumulation of these stromal reducing equivalents. Such an increase of reducing power in the stroma stimulates the reduction of plastoquinones, which is the limiting step of cyclic electron flow. In anaerobic conditions in the dark, this reaction can lead to a fully reduced plastoquinone pool and induce state transitions, the migration of 80% of light harvesting complexes II and 20% of cytochrome b ₆ f complex from the PSII-enriched grana to the PSI-enriched lamella. These ultrastructural changes have been proposed to further enhance cyclic electron flow by increasing PSI antenna size, and forming PSI-cyt b ₆ f supercomplexes. These hypotheses are discussed in light of recently published data.     

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.     

Chloroplastic NAD(P)H dehydrogenase complex and cyclic electron transport around photosystem I

Recent molecular genetics studies have revealed that cyclic electron transport around photosystem I is essential for normal photosynthesis and growth of plants. Chloroplastic NAD(P)H dehydorgenase (NDH) complex, a homologue of the complex I in respiratory electron transport, is involved in one of two cyclic pathways. Recent studies on the function and structure of the NDH complex are reviewed.     

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.     

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.     

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.     

Inhibition of electron flow around photosystem I in chloroplasts of Cd-treated maize plants is due to Cd-induced iron deficiency

Photosystem I activity of chloroplasts isolated from 21 days old maize seedlings ( Zea mays L. cv. Hidosil) cultivated in a nutrient solution containing different concentrations of Cd (10,20,30μM) was investigated. Cd markedly decreased ferredoxin(Fd)-dependent NADP⁺ photoreduction, while it had no effect on electron transport from 2. 6-dichlorophenolindophenol to methyl viologen, indicating that the metal interferred with electron transport on the reducing side of photosystem I. The decrease in electron transport correlated with a low Fd content, which in turn was correlated with a low Fe concentration, suggesting Cd-induced Fe deficiency. In in vitro experiments direct Cd inhibition of Fd-dependent NADP⁺ photoreduction required much higher Cd concentrations than those observed in Cd-treated plants.     

Heat-stress stimulation of electron flow in a photosystem I submembrane fraction

Oxygen uptake using methyl viologen as the terminal electron acceptor was recorded in digitonin-derived photosystem I submembrane fractions incubated at either 25 or 50 degrees C. A two- to four-fold heat-stress stimulation of electron flow was detected at 50 degrees C when reduced 2,6-dichlorophenol-indophenol was used as the primary electron donor. However, no stimulation was seen with N,N,N',N'-tetramethylphenylenediamine as the donor. The stimulation was enhanced by specific cations (Mg2+, Na+, K+), but not by Mn2 or Ca2+. The enhancement obtained with Mg2+ could be eliminated by incubating for a prolonged period. It is proposed that the observed heat-stress stimulation is due to a conformational change at the level of the cytochrome b6-f complex. This change increased the affinity of the protein complex for 2,6-dichlorophenol-indophenol at its oxidation sites. The involvement of a conformational modification is demonstrated by the absence of heat-stress stimulation in submembrane fractions immobilized in an albumin-glutaraldehyde cross-linked matrix.     

Experimental and theoretical study of processes of cyclical electron transport around photosystem I

The kinetics of photoinduced EPR I signals at different concentrations of ferredoxin was studied on isolated pea chloroplasts. A kinetic model of ferredoxin-dependent electron transport around photosystem I was suggested. A multiparticle model was constructed, which makes it possible to "directly" model the processes of electron transfer in multiprotein complexes and limited diffusion in different compartments of the system (stroma, lumen, and intermembrane space). A comparison of the kinetic and "direct" models revealed an important role of spatial organization of the system in the kinetics of redox turnover of P700.     

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

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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.     

Subunit III (Psa-F) of photosystem I reaction center of the C4 dicotyledon Flaveria trinervia

An immunological survey of C3, C4 and C3-C4-intermediate Flaveria species showed that subunit III (PsaF) of the photosystem I reaction center (PSI-RC) is present in all these species. This was confirmed by the isolation of the gene encoding the PSI-RC subunit III (PsaF) from Flaveria trinervia, the first psaF gene to be isolated from a C4 plant. The deduced amino acid sequence showed a high degree of similarity to the corresponding protein of spinach which is a C3 species. A region of 17 hydrophobic amino acids in the C-terminal part of the F. trinervia protein was found to be especially conserved in all PsaF proteins studied so far (cyanobacteria and Chlamydomonas).Abbreviations PSI-RC Photosystem I reaction center - cTPs chloroplast-targeted-proteins - chl chlorophyll - SDS sodium dodecyl sulfate     

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.