Thylakoid protein phosphorylation,state 1-state 2 transitions,and photosystem stoichiometry adjustment: redox control at multiple levels of gene expression

In photosynthesis in chloroplasts, control of thylakoid protein phosphorylation by redox state of inter-photosystem electron carriers makes distribution of absorbed excitation energy between the two photosystems self-regulating. During operation of this regulatory mechanism, reduction of plastoquinone activates a thylakoid protein kinase which phosphorylates the light-harvesting complex LHC II, causing a change in protein recognition that results in redistribution of energy to photosystem I at the expense of photosystem II, thus tending to oxidise the reduced plastoquinone pool. These events correspond to the transition from light-state 1 to light-state 2. The reverse transition (to light-state 1) is initiated by transient oxidation of plastoquinone, inactivation of the LHC II kinase, and return of dephosphorylated LHC II from photosystem I to photosystem II, supplying excitation energy to photosystem II and thereby reducing plastoquinone. State 1-state 2 transitions therefore operate by means of redox control of reversible, post-translational modification of pre-existing proteins. A balance in the rates of light utilization by photosystem I and photosystem II can also be achieved, on longer time-scales and between wider limits, by adjustment of the relative quantities, or stoichiometry, of photosystem I and photosystem II. Recent evidence suggests that adjustment of photosystem stoichiometry is also a response to perturbation of the redox state of inter-photosystem electron carriers, and involves specific redox control of de novo protein synthesis, assembly, and breakdown. It is therefore suggested that the same redox sensor initiates these different adaptations by control of gene expression at different levels, according to the time-scale and amplitude of the response. This integrated feedback control may serve to maintain redox homeostasis, and, as a result, quantum yield. Evidence for the components required by such systems is discussed.     

State 1-State 2 transitions in the cyanobacterium Synechococcus 6301 are controlled by the redox state of electron carriers between Photosystems I and II

The mechanism by which state 1-state 2 transitions in the cyanobacterium Synechococcus 6301 are controlled was investigated by examining the effects of a variety of chemical and illumination treatments which modify the redox state of the plastoquinone pool. The extent to which these treatments modify excitation energy distribution was determined by 77K fluorescence emission spectroscopy. It was found that treatment which lead to the oxidation of the plastoquinone pool induce a shift towards state 1 whereas treatments which lead to the reduction of the plastoquinone pool induce a shift towards state 2. We therefore propose that state transitions in cyanobacteria are triggered by changes in the redox state of plastoquinone or a closely associated electron carrier. Alternative proposals have included control by the extent of cyclic electron transport around PS I and control by localised electrochemical gradients around PS I and PS II. Neither of these proposals is consistent with the results reported here.Abbreviations DBMIB 2,5-dibromo-3methyl-6-isopropyl-p-benzoquinone - Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DQH₂ duroquinol (tetramethyl-p-hydroquinone) - LHC II light-harvesting chlorophyll a/b-binding protein of PS II - Light 1 light predominantly exciting PS I - Light 2 light predominantly exciting PS II - M.V. methyl viologen - PS photosystem     

State 1-state 2 adaptation in the cyanobacteria Synechocystis PCC 6714 wild type and Synechocystis PCC 6803 wild type and phycocyanin-less mutant

The mechanism of excitation energy distribution between the two photosystems (state transitions) is studied in Synechocystis 6714 wild type and in wild type and a mutant lacking phycocyanin of Synechocystis 6803. (i) Measurements of fluorescence transients and spectra demonstrate that state transitions in these cyanobacteria are controlled by changes in the efficiency of energy transfer from PS II to PS I (spillover) rather than by changes in association of the phycobilisomes to PS II (mobile antenna model). (ii) Ultrastructural study (freeze-fracture) shows that in the mutant the alignment of the PS II associated EF particles is prevalent in state 1 while the conversion to state 2 results in randomization of the EF particle distribution, as already observed in the wild type (Olive et al. 1986). In the mutant, the distance between the EF particle rows is smaller than in the wild type, probably because of the reduced size of the phycobilisomes. Since a parallel increase of spillover is not observed we suggest that the probability of excitation transfer between PS II units and between PS II and PS I depends on the mutual orientation of the photosystems rather than on their distance. (iii) Measurements of the redox state of the plastoquinone pool in state 1 obtained by PS I illumination and in state 2 obtained by various treatments (darkness, anaerobiosis and starvation) show that the plastoquinone pool is oxidized in state 1 and reduced in state 2 except in starved cells where it is still oxidized. In the latter case, no important decrease of ATP was observed. Thus, we propose that in Synechocystis the primary control of the state transitions is the redox state of a component of the cytochrome b ₆/f complex rather than that of the plastoquinone pool.Abbreviations DCCD dicyclohexylcarbodiimide - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - EF exoplasmic face - PQ plasto-quinone - PS photosystem - PBS phycobilisome     

Temporal Changes in State Transitions and Photosystem Organization in the Unicellular,Diazotrophic Cyanobacterium Cyanothece sp. ATCC 51142

The unicellular Cyanobacterium Cyanothece sp. ATCC 51142, grown under alternating 12-h light/12-h dark conditions, temporally separated N2 fixation from photosynthesis. The regulation of photosynthesis was studied using fluorescence spectra and kinetics to determine changes in state transitions and photosystem organization. The redox poise of the plastoquinone (PQ) pool appeared to be central to this regulation. Respiration supported N2 fixation by oxidizing carbohydrate granules, but reduced the PQ pool. This induced state 2 photosystem II monomers and lowered the capacity for O2 evolution. State 2 favored photosystem I trimers and cyclic electron transport, which could stimulate N2 fixation; the stimulation suggested an ATP limitation to N2 and CO2 fixation. The exhaustion of carbohydrate granules at around 6 h in the dark resulted in reduced respiratory electron flow, which led to a more oxidized PQ pool and produced a sharp transition from state 2 to state 1. This transient state 1 returned to state 2 in the remaining hours of darkness. In the light phase, photosystem II dimerization correlated with increased phycobilisome coupling to photosystem II (state 1) and increased rates of O2 evolution. However, dark adaptation did not guarantee state 2 and left photosystem I centers in a mostly monomeric state at certain times.     

Optimizing photosynthesis under fluctuating light: The role of the Arabidopsis STN7 kinase

Optimal photosynthetic performance requires that equal amounts of light are absorbed by photosystem ii (PSii) and photosystem i (PSi), which are functionally linked through the photosynthetic electron transport chain. However, photosynthetic organisms must cope with light conditions that lead to the preferential stimulation of one or the other of the photosystems. Plants react to such imbalances by mounting acclimation responses that redistribute excitation energy between photosystems and restore the photosynthetic redox poise. in the short term, this involves the so-called state transition process, which, over periods of minutes, alters the antennal crosssections of the photosystems through the reversible association of a mobile fraction of light-harvesting complex ii (LHCii) with PSi or PSii. Longer-lasting changes in light quality initiate a long-term response (LTr), occurring on a timescale of hours to days, that redresses imbalances in excitation energy by changing the relative amounts of the two photosystems. Despite the differences in their timescales of action, state transitions and LTr are both triggered by the redox state of the plastoquinone (PQ) pool, via the activation of the thylakoid kinase STN7, which appears to act as a common redox sensor and/or signal transducer for both responses. This review highlights recent findings concerning the role of STN7 in coordinating short- and long-term photosynthetic acclimation responses.Key words: state transitions, long-term acclimation, photosynthesis, STN7, Arabidopsis     

A mechanism for regulation of chloroplast LHC II kinase by plastoquinol and thioredoxin

State transitions are acclimatory responses to changes in light quality in photosynthesis. They involve the redistribution of absorbed excitation energy between photosystems I and II. In plants and green algae, this redistribution is produced by reversible phosphorylation of the chloroplast light harvesting complex II (LHC II). The LHC II kinase is activated by reduced plastoquinone (PQ) in photosystem II-specific low light. In high light, when PQ is also reduced, LHC II kinase becomes inactivated by thioredoxin. Based on newly identified amino acid sequence features of LHC II kinase and other considerations, a mechanism is suggested for its redox regulation.     

Photosynthetic acclimation: structural reorganisation of light harvesting antenna--role of redox-dependent phosphorylation of major and minor chlorophyll a/b binding proteins

In order to carry out photosynthesis, plants and algae rely on the co-operative interaction of two photosystems: photosystem I and photosystem II. For maximum efficiency, each photosystem should absorb the same amount of light. To achieve this, plants and green algae have a mobile pool of chlorophyll a/b-binding proteins that can switch between being light-harvesting antenna for photosystem I or photosystem II, in order to maintain an optimal excitation balance. This switch, termed state transitions, involves the reversible phosphorylation of the mobile chlorophyll a/b-binding proteins, which is regulated by the redox state of the plastoquinone-mediating electron transfer between photosystem I and photosystem II. In this review, we will present the data supporting the function of redox-dependent phosphorylation of the major and minor chlorophyll a/b-binding proteins by the specific thylakoid-bound kinases (Stt7, STN7, TAKs) providing a molecular switch for the structural remodelling of the light-harvesting complexes during state transitions. We will also overview the latest X-ray crystallographic and electron microscopy-derived models for structural re-arrangement of the light-harvesting antenna during State 1-to-State 2 transition, in which the minor chlorophyll a/b-binding protein, CP29, and the mobile light-harvesting complex II trimer detach from the light-harvesting complex II-photosystem II supercomplex and associate with the photosystem I core in the vicinity of the PsaH/L/O/P domain.     

Relationship between the Quantum Efficiencies of Photosystems I and II in Pea Leaves

The irradiance dependence of the efficiencies of photosystems I and II were measured for two pea (Pisum sativum [L.]) varieties grown under cold conditions and one pea variety grown under warm conditions. The efficiencies of both photosystems declined with increasing irradiance for all plants, and the quantum efficiency of photosystem I electron transport was closely correlated with the quantum efficiency of photosystem II electron transport. In contrast to the consistent pattern shown by efficiency of the photosystems, the redox state of photosystem II (as estimated from the photochemical quenching coefficient of chlorophyll fluorescence) exhibited relationships with both irradiance and the reduction of P-700 that varied with growth environment and genotype. This variability is considered in the context of the modulation of photosystem II quantum efficiency by both photochemical and nonphotochemical quenching of excitation energy.     

Two molecular forms of pea ferredoxin in the electron transport chain of chloroplasts

The effects of two molecular forms of water-soluble ferredoxin (Fd I and Fd II) on the kinetics of electron transport in bean chloroplasts (class B) were studied. The light-induced redox transitions of the photosystem I reaction center P700 were measured by the intensity of the EPR signal I produced by P700+. Both forms of ferredoxin, Fd I and Fd II, when added to the chloroplasts in catalytic amounts, stimulate the light-induced electron transfer from P700 to NADP+. Nevertheless, Fd I is a better mediator of the back reactions from NADPH to P700+. This electron transfer pathway is sensitive to the cyclic electron transport inhibitor, antimycin A, and to DCMU inhibitor of electron transport between photosystem II and plastoquinone. It may be concluded that the two molecular forms of ferredoxin, Fd I and Fd II, differ in their ability to catalyze cyclic electron transport in photosystem I. The role of Fd I and Fd II in regulation of electron transport at the acceptor site of photosystem I is discussed.     

A strain of Synechocystis sp. PCC 6803 without photosynthetic oxygen evolution and respiratory oxygen consumption: implications for the study of cyclic photosynthetic electron transport

Cyclic electron transport around photosystem (PS) I is believed to play a role in generation of ATP required for adaptation to stress in cyanobacteria and plants. However, elucidation of the pathway(s) of cyclic electron flow is difficult because of low rates of this electron flow relative to those of linear photosynthetic and respiratory electron transport. We have constructed a strain of Synechocystis sp. PCC 6803 that lacks both PSII and respiratory oxidases and that, consequently, neither evolves nor consumes oxygen. However, this strain is still capable of cyclic electron flow around PSI. The photoheterotrophic growth rate of this strain increased with light intensity up to an intensity of about 25 mumol photons m-2 s-1, supporting the notion that cyclic electron flow contributes to ATP generation in this strain. Indeed, the ATP-generating ability of PSI is demonstrated by the fact that the PSII-less oxidase-less strain is able to grow at much higher salt concentrations than a strain lacking PSI. A quinone electrode was used to measure the redox state of the plastoquinone pool in vivo in the various strains used in this study. In contrast to what is observed in chloroplasts, the plastoquinone pool was rather reduced in darkness and was oxidized in the light. This is in line with significant electron donation by respiratory pathways (NADPH dehydrogenase and particularly succinate dehydrogenase) in darkness. In the light, the pool becomes oxidized due to the presence of much more PSI than PSII. In the oxidase-less strains, the plastoquinone pool was very much reduced in darkness and was oxidized in the light by PSI. Photosystem II activity did not greatly alter the redox state of the plastoquinone pool. The results suggest that cyclic electron flow around PSI can contribute to generation of ATP, and a strain deficient in linear electron transport pathways provides an excellent model for further investigations of cyclic electron flow.     

NADPH dehydrogenase-mediated respiratory electron transport in thylakoid membranes of the cyanobacterium Synechocystis sp. PCC 6803 is inactive in the light

Non-photochemical redox changes of the plastoquinone pools in darkness were investigated in the cyanobacterium Synechocystis sp. PCC 6803 by monitoring changes in Chl fluorescence yield during light-to-dark transitions. The inhibitors rotenone and mercury with or without 1 mM succinate fully suppressed the post-illumination increase in Chl fluorescence in both NADPH dehydrogenase-defective (M55) and deltaCtaI cells. The latter cells lack subunit I of cytochrome aa3-type cytochrome c oxidase. These results strongly suggest that NADPH dehydrogenase plays the major role in electron donation in the non-photo-chemical reduction of plastoquinone. The rising phase of post-illumination Chl fluorescence in both wild type pretreated with KCN, and deltaCtaI cells, was significantly slowed by low light illumination. We detected comparable photochemical levels of both photosystem (PS) II and PSI during steady state illumination in wild type and deltaCtaI cells. From these results, we suggest that respiratory electron flow involved in the non-photochemical redox change of plastoquinone is not likely to occur in the light.     

Electron and proton transport in chloroplasts taking into account lateral heterogeneity of thylakoids. Mathematical model]

A mathematical model of a chloroplast was constructed, which takes into account the inhomogeneous distribution of complexes of photosystems I and II between granal and intergranal thylakoids. The structural and functional complexes of photosystems I and II, which are localized in intergranal and granal thylakoids, respectively, and the b/f complex, which is uniformly distributed in thylakoid membranes, are assumed to be immobile. The interactions between spatially distant electron transport complexes are provided by plastoquinone and plastocyanine, which diffuse in the thylakoid membrane and intrathylakoid space, respectively. The main stages of proton transport associated with the functioning of photosystem II and oxidation-reduction transformations of plastoquinone are considered. The model takes into account the interactions of protons with membrane-bound buffer groups, the lateral diffusion of hydrogen ions in the intrathylakoid space and in the lumen between adjacent granal thylakoids, and the transmembrane proton transport associated with the function of ATP synthase and passive leakage of protons from thylakoids outside. The numerical integration of two systems of differential equations describing the behavior of some variables in two different regions: granal and intergranal thylakoids was performed. The model describes adequately the kinetics of processes being studied and predicts the occurrence of inhomogeneous lateral profiles of proton potentials and redox state of electron carriers. Modeling the electron and proton transport with allowance for the topological features of chloroplasts (lateral heterogeneity of thylakoids) is important for correct interpretation of "power-flux" interactions and the experimentally measured kinetic parameters averaged over the entire spatially inhomogeneous thylakoid system.     

Regulation of photosynthetic electron transport

The photosynthetic electron transport chain consists of photosystem II, the cytochrome b(6)f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO(2) assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO(2) levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled "Regulation of Electron Transport in Chloroplasts".