Digalactosyl-diacylglycerol deficiency impairs the capacity for photosynthetic intersystem electron transport and state transitions in Arabidopsis thaliana due to photosystem I acceptor-side limitations

Compared with wild type, the dgd1 mutant of Arabidopsis thaliana exhibited a lower amount of PSI-related Chl-protein complexes and lower abundance of the PSI-associated polypeptides, PsaA, PsaB, PsaC, PsaL and PsaH, with no changes in the levels of Lhca1-4. Functionally, the dgd1 mutant exhibited a significantly lower light-dependent, steady-state oxidation level of P700 (P700(+)) in vivo, a higher intersystem electron pool size, restricted linear electron transport and a higher rate of reduction of P700(+) in the dark, indicating an increased capacity for PSI cyclic electron transfer compared with the wild type. Concomitantly, the dgd1 mutant exhibited a higher sensitivity to and incomplete recovery of photoinhibition of PSI. Furthermore, dgd1 exhibited a lower capacity to undergo state transitions compared with the wild type, which was associated with a higher reduction state of the plastoquinone (PQ) pool. We conclude that digalactosyl-diacylglycerol (DGDG) deficiency results in PSI acceptor-side limitations that alter the flux of electrons through the photosynthetic electron chain and impair the regulation of distribution of excitation energy between the photosystems. These results are discussed in terms of thylakoid membrane domain reorganization in response to DGDG deficiency in A. thaliana.     

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.     

Polyribosome loading of spinach mRNAs for photosystem I subunits is controlled by photosynthetic electron transport

In light-, but not in dark-grown spinach seedlings, the mRNAs for the nuclear-encoded photosystem I subunits D, F and L are associated with polyribosomes and this association is prevented by the application of 3-(3',4'-dichlorophenyl)-1,1'-dimethyl urea (DCMU), an inhibitor of the photosynthetic electron transport. To identify the cis-elements which are responsible for this regulation, we generated a series of chimeric PsaD constructs and tested them in transgenic tobacco. The spinach PsaD 5'-untranslated region is sufficient to confer light- and photosynthesis-dependent polyribosome association onto the uidA reporter gene, while the tobacco PsaD 5'-untranslated region directs constitutive polyribosome association. These results are discussed with regard to signals from photosynthetic electron flow which control processes in the cytoplasm.     

Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

In Arabidopsis thaliana, the D-subunit of photosystem I (PSI-D) is encoded by two functional genes, PsaD1 and PsaD2, which are highly homologous. Knock-out alleles for each of the loci have been identified by a combination of forward and reverse genetics. The double mutant psad1-1 psad2-1 is seedling-lethal, high-chlorophyll-fluorescent and deficient for all tested PSI subunits, indicating that PSI-D is essential for photosynthesis. In addition, psad1-1 psad2-1 plants show a defect in the accumulation of thylakoid multiprotein complexes other than PSI. Of the single-gene mutations, psad2 plants behave like wild-type (WT) plants, whereas psad1-1 markedly affects the accumulation of PsaD mRNA and protein, and photosynthetic electron flow. Additional effects of the psad1-1 mutation include a decrease in growth rate under greenhouse conditions and downregulation of the mRNA expression of most genes involved in the light phase of photosynthesis. In the same mutant, a marked decrease in the levels of PSI and PSII polypeptides is evident, as well as a light-green leaf coloration and increased photosensitivity. Increased dosage of PsaD2 in the psad1-1 background restores the WT phenotype, indicating that PSI-D1 and PSI-D2 have redundant functions.     

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.     

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.     

Trimeric organization of photosystem I is required to maintain the balanced photosynthetic electron flow in cyanobacterium Synechocystis sp. PCC 6803

Photosynthesis Research - In Synechocystis sp. PCC 6803 and some other cyanobacteria photosystem I reaction centres exist predominantly as trimers, with minor contribution of monomeric form, when...     

Cosuppression of photosystem I subunit PSI-H in Arabidopsis thaliana. Efficient electron transfer and stability of photosystem I is dependent upon the PSI-H subunit

PSI-H is an intrinsic membrane protein of 10 kDa that is a subunit of photosystem I (PSI). PSI-H is one of the three PSI subunits found only in eukaryotes. The function of PSI-H was characterized in Arabidopsis plants transformed with a psaH cDNA in sense orientation. Cosuppressed plants containing less than 3% PSI-H are smaller than wild type when grown on sterile media but are similar to wild type under optimal conditions. PSI complexes lacking PSI-H contain 50% PSI-L, whereas other PSI subunits accumulate in wild type amounts. PSI devoid of PSI-H has only 61% NADP+ photoreduction activity compared with wild type and is highly unstable in the presence of urea as determined from flash-induced absorbance changes at 834 nm. Our data show that PSI-H is required for stable accumulation of PSI and efficient electron transfer in the complex. The plants lacking PSI-H compensate for the less efficient PSI with a 15% increase in the P700/chlorophyll ratio, and this compensation is sufficient to prevent overreduction of the plastoquinone pool as evidenced by normal photochemical quenching of fluorescence. Nonphotochemical quenching is approximately 60% of the wild type value, suggesting that the proton gradient across the thylakoid membrane is decreased in the absence of PSI-H.     

Singlet and triplet state transitions of carotenoids in the antenna complexes of higher-plant photosystem I

In this work, the spectroscopic characteristics of carotenoids associated with the antenna complexes of Photosystem I have been studied. Pigment composition, absorption spectra, and laser-induced triplet-minus-singlet (T-S) spectra were determined for native LHCI from the wild type (WT) and lut2 mutant from Arabidopsis thaliana as well as for reconstituted individual Lhca WT and mutated complexes. All WT complexes bind lutein and violaxanthin, while beta-carotene was found to be associated only with the native LHCI preparation and recombinant Lhca3. In the native complexes, the main lutein absorption bands are located at 492 and 510 nm. It is shown that violaxanthin is able to occupy all lutein binding sites, but its absorption is blue-shifted to 487 and 501 nm. The "red" lutein absorbing at 510 nm was found to be associated with Lhca3 and Lhca4 which also show a second carotenoid, peaking around 490 nm. Both these xanthophylls are involved in triplet quenching and show two T-S maxima: one at 507 nm (corresponding to the 490 nm singlet absorption) and the second at 525 nm (with absorption at 510 nm). The "blue"-absorbing xanthophyll is located in site L1 and can receive triplets from chlorophylls (Chl) 1012, 1011, and possibly 1013. The red-shifted spectral component is assigned to a lutein molecule located in the L2 site. A 510 nm lutein was also observed in the trimers of LHCII but was absent in the monomers. In the case of Lhca, the 510 nm band is present in both the monomeric and dimeric complexes. We suggest that the large red shift observed for this xanthophyll is due to interaction with the neighbor Chl 1015. In the native T-S spectrum, the contribution of carotenoids associated with Lhca2 is visible while the one of Lhca1 is not. This suggests that in the Lhca2-Lhca3 heterodimeric complex energy equilibration is not complete at least on a fast time scale.     

AS-1 cyanophage infection inhibits the photosynthetic electron flow of photosystem II in Synechococcus sp. PCC 6301, a cyanobacterium

In Synechococcus sp. cells AS-1 cyanophage infection gradually inhibits the photosystem II mediated photosynthetic electron flow whereas the activity of photosystem I is apparently unaffected by the cyanophage infection. Transient fluorescence induction and flash-induced delayed luminescence decay studies revealed that the inhibition may occur at the level of the secondary acceptor, Q_B of photosystem II. In addition, the breakdown of D₁-protein is inhibited, comparable to DCMU-induced protection of D₁-protein turnover, in AS-1-infected cells.     

Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.     

Stable assembly of PsaE into cyanobacterial photosynthetic membranes is dependent on the presence of other accessory subunits of photosystem I

We studied assembly of the PsaE subunit of photosystem I into photosynthetic membranes of cyanobacterial mutant strains that lack specific photosystem I subunits. Radiolabeled PsaE was incubated with photosynthetic membranes, and their binding and assembly were assayed by resistance to removal by chaotropic agents and proteolytic digestion. PsaE incorporated into the wild-type membranes was resistant to these treatments. In the absence of PsaD, it was resistant to proteolytic digestion, but was removed by NaBr. When the membranes were isolated from a mutant strain in which the psaF and psaJ genes have been inactivated, PsaE assembled in vitro could not be removed. PsaE could associate with the membranes of the strain DF in which the psaD, psaJ and psaF genes have been mutated. However, the radiolabeled PsaE associated with these membranes was removed both by the proteolytic as well as by the chaotropic agents. Characterization of PsaE present in vivo revealed similar results. These observations suggest that PsaD and PsaF/J may interact with PsaE and stabilize it in the photosystem I complex.     

Effects of exogenous phenolic acids on photosystem functions and photosynthetic electron transport rate in strawberry leaves

Our study investigated the physiological and biochemical basis for the effects of exogenous phenolic acids on the function of the photosynthetic apparatus and photosynthetic electron transport rate in strawberry seedlings. Potted seedlings of the strawberry (Fragaria × ananassa Duch.) were used. Syringic acid inhibited net photosynthetic rate and water-use efficiency decreased. Additionally, primary quinone electron acceptor of the PSII reaction centre, the PSII reaction centre and the oxygen evolving complex were also impaired. Both the maximum quantum yield of the PSII primary photochemistry and the performance index on absorption basis were depressed, resulting in reduced function of the photosynthetic electron transport chain. Otherwise, low phthalic acid concentrations enhanced photosynthetic capacity, while high concentrations showed opposite effects. Syringic acid exhibited a higher toxic effect than that of phthalic acid which was more evident at higher concentrations.     

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.     

The role of chloride ion in photosystem II. I. Effects of chloride ion on photosystem II electron transport and on hydroxylamine inhibition.

1. Chloroplasts washed with Cl--free, low-salt media (pH 8) containing EDTA, show virtually no DCMU-insensitive silicomolybdate reduction. The activity is readily restored when 10 mM Cl- is added to the reaction mixture. Very similar results were obtained with the other Photosystem II electron acceptor 2,5-dimethylquinone (with dibromothymoquinone), with the Photosystem I electron acceptor FMN, and also with ferricyanide which accepts electrons from both photosystems. 2. Strong Cl--dependence of Hill activity was observed invariably at all pH values tested (5.5--8.3) and in chloroplasts from three different plants: spinach, tobacco and corn (mesophyll). 3. In the absence of added Cl- the functionally Cl--depleted chloroplasts are able to oxidize, through Photosystem II, artificial reductants such as catechol, diphenylcarbazide, ascorbate and H2O2 at rates which are 4--12 times faster than the rate of the residual Hill reaction. 4. The Cl--concentration dependence of Hill activity with dimethylquinone as an electron acceptor is kinetically consistent with the typical enzyme activation mechanism: E(inactive) + Cl- in equilibrium E . Cl- (active), and the apparent activation constant (0.9 mM at pH 7.2) is unchanged by chloroplast fragmentation. 5. The initial phase of the development of inhibition of water oxidation in Cl--depleted chloroplasts during the dark incubation with NH2OH (1/2 H2SO4) is 5 times slower when the incubation medium contains Cl- than when the medium contains NH2OH alone or NH2OH plus acetate ion. (Acetate is shown to be ineffective in stimulating O2 evolution).     

Effect of exogenous glucose on electron flow to photosystem I and respiration in cyanobacterial cells

The effects of exogenous glucose on the rates of alternative pathways of photosystem II (PSII)-independent electron flow to PSI and of dark respiration in Synechocystis sp. 6803 cells were studied. The presence of glucose was shown to accelerate the electron flow to P700⁺, the PSI primary electron donor oxidized with Far-red light (FRL), which excites specifically only PSI. An increase in the glucose concentration was accompanied by a further activation of electron flow to PSI, which was supported by the dark donation of reducing equivalents to the electron transport chain. An increase in the external glucose concentration resulted also in the disappearance of lag-phase in the kinetics of P700⁺ reduction, which was observed in the cells incubated without glucose after FRL switching off. A similarity of nonphotochemical processes of electron transfer to PSI in cyanobacteria and higher plants was supposed, basing on the earlier observed fact of the occurrence of such lagphase in higher plants and its dependence on the exhausting of stromal reductants in the light. Acceleration of dark electron flow to PSI in the presence of glucose, a major respiratory substrate, may indicate the coupling between nonphotochemical processes in the photosynthetic and respiratory chains of electron transport in cyanobacterial cells. A close correlation between photosynthesis and respiration in cyanobacterial cells is also confirmed by a sharp acceleration of respiration with an increase in the glucose concentration in medium.     

Effects of cell excitation on photosynthetic electron flow and intercellular transport in Chara

Protoplasma - Impact of membrane excitability on fluidic transport of photometabolites and their cell-to-cell passage via plasmodesmata was examined by pulse-modulated chlorophyll (Chl)...