Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5'-triphosphate

Modeling the Role of LHCII-LHCII, PSII-LHCII, and PSI-LHCII Interactions in State Transitions

The light-dependent reactions of photosynthesis take place in the plant chloroplast thylakoid membrane, a complex three-dimensional structure divided into the stacked grana and unstacked stromal lamellae domains. Plants regulate the macro-organization of photosynthetic complexes within the thylakoid membrane to adapt to changing environmental conditions and avoid oxidative stress. One such mechanism is the state transition that regulates photosynthetic light harvesting and electron transfer. State transitions are driven by changes in the phosphorylation of light harvesting complex II (LHCII), which cause a decrease in grana diameter and stacking, a decrease in energetic connectivity between photosystem II (PSII) reaction centers, and an increase in the relative LHCII antenna size of photosystem I (PSI) compared to PSII. Phosphorylation is believed to drive these changes by weakening the intramembrane lateral PSII-LHCII and LHCII-LHCII interactions and the intermembrane stacking interactions between these complexes, while simultaneously increasing the affinity of LHCII for PSI. We investigated the relative roles and contributions of these three types of interaction to state transitions using a lattice-based model of the thylakoid membrane based on existing structural data, developing a novel algorithm to simulate protein complex dynamics. Monte Carlo simulations revealed that state transitions are unlikely to lead to a large-scale migration of LHCII from the grana to the stromal lamellae. Instead, the increased light harvesting capacity of PSI is largely due to the more efficient recruitment of LHCII already residing in the stromal lamellae into PSI-LHCII supercomplexes upon its phosphorylation. Likewise, the increased light harvesting capacity of PSII upon dephosphorylation was found to be driven by a more efficient recruitment of LHCII already residing in the grana into functional PSII-LHCII clusters, primarily driven by lateral interactions.

Regulation of photosynthetic cyclic electron flow pathways by adenylate status in higher plant chloroplasts

Cylic electron flow (CEF) around Photosystem I in photosynthetic eukaryotes is likely to be necessary to augment ATP production, rapidly- and precisely balancing the plastid ATP/NADPH energy budget to meet the demands of downstream metabolism. Many regulatory aspects of this process are unclear. Here we demonstrate that the higher plant plastid NADH/Fd:plastoquinone reductase (NDH) and proposed PGR5/PGRL1 ferredoxin:plastoquinone reductase (FQR) pathways of CEF are strongly, rapidly and reversibly inhibited in vitro by ATP with K_i values of 670 μM and 240 μM respectively, within the range of physiological changes in ATP concentrations. Control experiments ruled out effects on secondary reactions, e.g. FNR- and cytochrome b₆f activity, nonphotochemical quenching of chlorophyll fluorescence etc., supporting the view that ATP is an inhibitor of CEF and its associated pmf generation and subsequent ATP production. The effects are specific to ATP, with the ATP analog AMP-PNP showing little inhibitory effect, and ADP inhibiting only at higher concentrations. For the FQR pathway, inhibition was found to be classically competitive with Fd, and the NDH pathway showing partial competition with Fd. We propose a straightforward model for regulation of CEF in plants in which CEF is activated under conditions when stromal ATP low, but is downregulated as ATP levels build up, allowing for effective ATP homeostasis. The differences in K_i values suggest a two-tiered regulatory system, where the highly efficient proton pumping NDH is activated with moderate decreases in ATP, with the less energetically-efficient FQR pathway being activated under more severe ATP depletion.

Linking chloroplast relocation to different responses of photosynthesis to blue and red radiation in low and high light-acclimated leaves of Arabidopsis thaliana (L.)

Low light (LL) and high light (HL)-acclimated plants of A. thaliana were exposed to blue (BB) or red (RR) light or to a mixture of blue and red light (BR) of incrementally increasing intensities. The light response of photosystem II was measured by pulse amplitude-modulated chlorophyll fluorescence and that of photosystem I by near infrared difference spectroscopy. The LL but not HL leaves exhibited blue light-specific responses which were assigned to relocation of chloroplasts from the dark to the light-avoidance arrangement. Blue light (BB and BR) decreased the minimum fluorescence ([Formula: see text]) more than RR light. This extra reduction of the [Formula: see text] was stronger than theoretically predicted for [Formula: see text] quenching by energy dissipation but actual measurement and theory agreed in RR treatments. The extra [Formula: see text] reduction was assigned to decreased light absorption of chloroplasts in the avoidance position. A maximum reduction of 30% was calculated. Increasing intensities of blue light affected the fluorescence parameters NPQ and q_P to a lesser degree than red light. After correcting for the optical effects of chloroplast relocation, the NPQ responded similarly to blue and red light. The same correction method diminished the color-specific variations in q_P but did not abolish it; thus strongly indicating the presence of another blue light effect which also moderates excitation pressure in PSII but cannot be ascribed to absorption variations. Only after RR exposure, a post-illumination overshoot of [Formula: see text] and fast oxidation of PSI electron acceptors occurred, thus, suggesting an electron flow from stromal reductants to the plastoquinone pool.

A new paradigm for producing astaxanthin from the unicellular green alga Haematococcus pluvialis

The unicellular green alga Haematococcus pluvialis has been exploited as a cell factory to produce the high-value antioxidant astaxanthin for over two decades, due to its superior ability to synthesize astaxanthin under adverse culture conditions. However, slow vegetative growth under favorable culture conditions and cell deterioration or death under stress conditions (e.g., high light, nitrogen starvation) has limited the astaxanthin production. In this study, a new paradigm that integrated heterotrophic cultivation, acclimation of heterotrophically grown cells to specific light/nutrient regimes, followed by induction of astaxanthin accumulation under photoautotrophic conditions was developed. First, the environmental conditions such as pH, carbon source, nitrogen regime, and light intensity, were optimized to induce astaxanthin accumulation in the dark-grown cells. Although moderate astaxanthin content (e.g., 1% of dry weight) and astaxanthin productivity (2.5 mg L(-1)  day(-1) ) were obtained under the optimized conditions, a considerable number of cells died off when subjected to stress for astaxanthin induction. To minimize the susceptibility of dark-grown cells to light stress, the algal cells were acclimated, prior to light induction of astaxanthin biosynthesis, under moderate illumination in the presence of nitrogen. Introduction of this strategy significantly reduced the cell mortality rate under high-light and resulted in increased cellular astaxanthin content and astaxanthin productivity. The productivity of astaxanthin was further improved to 10.5 mg L(-1)  day(-1) by implementation of such a strategy in a bubbling column photobioreactor. Biochemical and physiological analyses suggested that rebuilding of photosynthetic apparatus including D1 protein and PsbO, and recovery of PSII activities, are essential for acclimation of dark-grown cells under photo-induction conditions. Biotechnol. Bioeng. 2016;113: 2088-2099. © 2016 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Analysis of PSII antenna size heterogeneity of Chlamydomonas reinhardtii during state transitions

PSII antenna size heterogeneity has been intensively studied in the past. Based on DCMU fluorescence rise kinetics, multiple types of photosystems with different properties were described. However, due to the complexity of fluorescence signal analysis, multiple questions remain unanswered. The number of different types of PSII is still debated as well as their degree of connectivity. In Chlamydomonas reinhardtii we found that PSIIα possesses a high degree of connectivity and an antenna 2-3 times larger than PSIIβ, as described previously. We also found some connectivity for PSIIβ in contrast with the majority of previous studies. This is in agreement with biochemical studies which describe PSII mega-, super- and core-complexes in Chlamydomonas. In these studies, the smallest unit of PSII in vivo would be a dimer of two core complexes hence allowing connectivity. We discuss the possible relationships between PSIIα and PSIIβ and the PSII mega-, super- and core-complexes. We also showed that strain and medium dependent variations in the half-time of the fluorescence rise can be explained by variations in the proportions of PSIIα and PSIIβ. When analyzing the state transition process in vivo, we found that this process induces an inter-conversion of PSIIα and PSIIβ. During a transition from state 2 to state 1, DCMU fluorescence rise kinetics are satisfactorily fitted by considering two PSII populations with constant kinetic parameters. We discuss our findings about PSII heterogeneity during state transitions in relation with recent results on the remodeling of the pigment-protein PSII architecture during this process.

Post-genomic insight into thylakoid membrane lateral heterogeneity and redox balance

Photosynthetic machinery requires balanced distribution of excitation energy from the light-harvesting complexes to photosystems. The efficiency of light-harvesting is regulated by thermal dissipation of excess energy, while the distribution of energy between photosystems is dependent on STN7 kinase and phosphorylation of thylakoid proteins. The regulation of excitation energy transfer has been linked to the lateral segregation of photosynthetic complexes along the thylakoid membrane. The study of photosynthetic regulation mechanisms using Arabidopsis mutants, which have been available for the last ten years, has challenged traditional views on regulation of excitation energy distribution. Here, we discuss an urgent need to create a holistic view of the dynamics of the thylakoid membrane using systematic research of the mutants available today.

State transitions--the molecular remodeling of photosynthetic supercomplexes that controls energy flow in the chloroplast

In oxygen-evolving photosynthesis, the two photosystems-photosystem I and photosystem II-function in parallel, and their excitation levels must be balanced to maintain an optimal photosynthetic rate under natural light conditions. State transitions in photosynthetic organisms balance the absorbed light energy between the two photosystems in a short time by relocating light-harvesting complex II proteins. For over a decade, the understanding of the physiological consequences, the molecular mechanism, and its regulation has increased considerably. After providing an overview of the general understanding of state transitions, this review focuses on the recent advances of the molecular aspects of state transitions with a particular emphasis on the studies using the green alga Chlamydomonas reinhardtii. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Evidence that phosphorylation and dephosphorylation regulate the distribution of excitation energy between the two photosystems of photosynthesis in vivo: Photoacoustic and fluorimetric study of an intact leaf

State 1-state 2 transitions in an intact tobacco leaf were monitored by the photoacoustic method. Modulated oxygen evolution yield and its enhancement by continuous far-red light ("Emerson enhancement") were used to characterize the balance of light distribution between the two photosystems. These measurements were additionally supported by fluorimetry. Adaptation of the leaf to far-red light (lambda [unk] 700 nm), mainly absorbed in photosystem I (light 1), results in state 1, where short-wavelength light (light 2) is distributed in favor of photosystem II. This is shown by a low yield of oxygen evolution, a high extent of Emerson enhancement, a concomitantly high extent of fluorescence quenching by far-red light, and a low ratio of the 77 K emission peaks at 730 and 695 nm. The magnitudes of these parameters were reversed when the leaf was adapted to light 2 (state 2), indicating a change towards a more equal distribution of the excitation between the two photosystems. Preincubation of an intact leaf with NaF, a specific phosphatase inhibitor, stimulated the extent of adaptation to light 2, shown by all the above criteria, and completely abolished adaptation to light 1. Light 1 preillumination prior to NaF treatment resulted initially in state 1, but then a transition to state 2 was irreversibly induced by any light. The NaF effect was specific because NaCl did not affect the state 1-state 2 transitions. Leaching out the NaF restored the original physiological transitions of the leaf. NaF presumably acts here in the same way as it acts in isolated thylakoids-by blocking the dephosphorylation of membranal proteins (particularly the chlorophyll a/b-protein complex) phosphorylated by a light 2-activated kinase. Our results give direct support to the suggestion [Allen, J. F., Bennett, J., Steinback, K. E. & Arntzen, C. J. (1981) Nature (London) 291, 25-29] that it is the phosphorylation level of thylakoid proteins that controls the light distribution between the two photosystems in vivo, shown previously in isolated thylakoids.

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of FV/FM on excitation wavelength

The F(0) and F(M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F(V)/F(M) (=(F(M) - F(0))/F (M)) have been derived: within each of the three spectra of F(V)/F(M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the (V)/F(M) differed significantly between leaf types. From spectra of F(V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F(V)/F(M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F(V)/F(M) at these wavelengths. All three leaves exhibited low values of F(V)/F(M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F(V)/F(M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C(4) species, than in barley, a C(3) species. Finally, low values of F(V)/F(M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F(V)/F(M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.

The discovery of state transitions in photosynthesis 40 years ago

In the late 1960s, I identified an aspect of the kinetics of chlorophyll fluorescence in algal cells that I was unable to explain in terms of photochemical quenching. I proposed a novel regulatory mechanism for the distribution of light energy to photosystems I and II, which is now known by the term of "state transitions." I also examined the "cation-dependent redistribution of light energy" to photosystems I and II and the "energy-dependent quenching" of chlorophyll fluorescence. At that time, financial constraints prevented me from measuring the emission and action spectra of chlorophyll fluorescence at liquid-nitrogen temperature and the light quality-dependent changes in the yield of chlorophyll fluorescence at room temperature. The financial problems were solved by constructing several pieces of electronic equipment using skills obtained by repairing radios when I was a high-school and college student.

Environmentally modulated phosphorylation and dynamics of proteins in photosynthetic membranes

Recent advances in vectorial proteomics of protein domains exposed to the surface of photosynthetic thylakoid membranes of plants and the green alga Chlamydomonas reinhardtii allowed mapping of in vivo phosphorylation sites in integral and peripheral membrane proteins. In plants, significant changes of thylakoid protein phosphorylation are observed in response to stress, particularly in photosystem II under high light or high temperature stress. Thylakoid protein phosphorylation in the algae is much more responsive to the ambient redox and light conditions, as well as to CO(2) availability. The light-dependent multiple and differential phosphorylation of CP29 linker protein in the green algae is suggested to control photosynthetic state transitions and uncoupling of light harvesting proteins from photosystem II under high light. The similar role for regulation of the dynamic distribution of light harvesting proteins in plants is proposed for the TSP9 protein, which together with other recently discovered peripheral proteins undergoes specific environment- and redox-dependent phosphorylation at the thylakoid surface. This review focuses on the environmentally modulated reversible phosphorylation of thylakoid proteins related to their membrane dynamics and affinity towards particular photosynthetic protein complexes.

State transitions revisited-a buffering system for dynamic low light acclimation of Arabidopsis

The mobile part of the light-harvesting chlorophyll (chl) a/b protein complex (LHCII), composed of the Lhcb1 and Lhcb2 proteins, is the basic unit of chloroplast state transitions--the short term tuning system in balancing the excitation energy between Photosystem (PS) II and PSI. State transitions are catalysed by the thylakoid associated STN7 kinase, and we show here that besides the phosphorylation of the Lhcb1 and Lhcb2 proteins, also the phosphorylation of Lhcb4.2 (CP29) is under the control of the STN7 kinase. Upon growth of Arabidopsis WT and stn7 mutant plants under low and moderate light conditions, the WT plants favoured state 2 whereas stn7 was locked in state 1. The lack of the STN7 kinase and state transitions in stn7 also modified the thylakoid protein contents upon long-term low light acclimation resulting, for example, in low Lhcb1 and in elevated Lhca1 and Lhca2 protein amounts as compared to WT. Adjustments of thylakoid protein contents probably occurred at post-transcriptional level since the DNA microarray experiments from each growth condition did not reveal any significant differences between stn7 and WT transcriptomes. The resulting high Lhcb2/Lhcb1 ratio in stn7 upon growth at low light was accompanied by lower capacity for NPQ than in WT. On the contrary, higher amounts of PsbS in stn7 under moderate and high light growth conditions resulted in higher NPQ compared to WT and consequently also in a protection of PSII against photoinhibition. STN7 kinase and the state transitions are suggested to have a physiological significance for dynamic acclimation to low but fluctuating growth light conditions. They are shown to function as a buffering system upon short high light illumination peaks by shifting the thylakoids from state 2 to state 1 and thereby down regulating the induction of stress-responsive genes, a likely result from transient over-reduction of PSI acceptors.

Photosynthesis: a blueprint for solar energy capture and biohydrogen production technologies

Solar energy capture, conversion into chemical energy and biopolymers by photoautotrophic organisms, is the basis for almost all life on Earth. A broad range of organisms have developed complex molecular machinery for the efficient conversion of sunlight to chemical energy over the past 3 billion years, which to the present day has not been matched by any man-made technologies. Chlorophyll photochemistry within photosystem II (PSII) drives the water-splitting reaction efficiently at room temperature, in contrast with the thermal dissociation reaction that requires a temperature of ca. 1550 K. The successful elucidation of the high-resolution structure of PSII, and in particular the structure of its Mn(4)Ca cluster provides an invaluable blueprint for designing solar powered biotechnologies for the future. This knowledge, combined with new molecular genetic tools, fully sequenced genomes, and an ever increasing knowledge base of physiological processes of oxygenic phototrophs has inspired scientists from many countries to develop new biotechnological strategies to produce renewable CO(2)-neutral energy from sunlight. This review focuses particularly on the potential of use of cyanobacteria and microalgae for biohydrogen production. Specifically this article reviews the predicted size of the global energy market and the constraints of global warming upon it, before detailing the complex set of biochemical pathways that underlie the photosynthetic process and how they could be modified for improved biohydrogen production.

Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue

The formation of grana in chloroplasts of higher plants is examined in terms of the subtle interplay of physicochemical forces of attraction and repulsion. The attractive forces between two adjacent membranes comprise (1) van der Waals attraction that depends on the abundance and type of atoms in each membrane, on the distance between the membranes and on the dielectric constant, (2) depletion attraction that generates local order by granal stacking at the expense of greater disorder (i.e. entropy) in the stroma, and (3) an electrostatic attraction of opposite charges located on adjacent membranes. The repulsive forces comprise (1) electrostatic repulsion due to the net negative charge on the outer surface of thylakoid membranes, (2) hydration repulsion that operates at small separations between thylakoid membranes due to layers of bound water molecules, and (3) steric hindrance due to bulky protrusions of Photosystem I (PSI) and ATP synthase into the stroma. In addition, specific interactions may occur, but they await experimental demonstration. Although grana are not essential for photosynthesis, they are ubiquitous in higher plants. Grana may have been selected during evolution for the functional advantages that they confer on higher plants. The functional consequences of grana stacking include (1) enhancement of light capture through a vastly increased area-to-volume ratio and connectivity of several PSIIs with large functional antenna size, (2) the ability to control the lateral separation of PSI from PSII and, therefore, the balanced distribution of excitation energy between two photosystems working in series, (3) the reversible fine-tuning of energy distribution between the photosystems by State 1-State 2 transitions, (4) the ability to regulate light-harvesting via controlled thermal dissipation of excess excitation energy, detected as non-photochemical quenching, (5) dynamic flexibility in the light reactions mediated by a granal structure in response to regulation by a trans-thylakoid pH gradient, (6) delaying the premature degradation of D1 and D2 reaction-centre protein(s) in PSII by harbouring photoinactived PSIIs in appressed granal domains, (7) enhancement of the rate of non-cyclic synthesis of adenosine triphosphate (ATP) as well as the regulation of non-cyclic vs. cyclic ATP synthesis, and (8) the potential increase of photosynthetic capacity for a given composition of chloroplast constituents in full sunlight, concomitantly with enhancement of photochemical efficiency in canopy shade. Hence chloroplast ultrastructure and function are intimately intertwined.

Metabolic Flexibility of the Green Alga Chlamydomonas reinhardtii as Revealed by the Link between State Transitions and Cyclic Electron Flow

In this Review we focus on the conversion of linear photosynthetic electron transport from water to NADP to the cyclic pathway around Photosystem I in the green alga Chlamydomonas reinhardtii. We discuss the strict relationship that exists between the changes in pathways of electron transport and state transitions, i.e., the reversible functional association of light harvesting proteins with one of the two photosystems of oxygenic photosynthesis. Such a link has not been reported in the case of other photosynthetic organisms, where the state transitions do not affect the pathway of electron transport. Rather, they provide a tool to optimise the rate of linear flow. We propose a kinetic-structural model that explains the mechanism of this particular relationship in Chlamydomonas, and discuss the advantages that this peculiar situation gives to the energetic metabolism of this alga.

In vivo changes of the oxidation-reduction state of NADP and of the ATP/ADP cellular ratio linked to the photosynthetic activity in Chlamydomonas reinhardtii

The ATP/ADP and NADP/NADPH ratios have been measured in whole-cell extract of the green alga Chlamydomonas reinhardtii, to understand their availability for CO(2) assimilation by the Calvin cycle in vivo. Measurements were performed during the dark-light transition of both aerobic and anaerobic cells, under illumination with saturating or low light intensity. Two different patterns of behavior were observed: (a) In anaerobic cells, during the lag preceding O(2) evolution, ATP was synthesized without changes in the NADP/NADPH ratio, consistently with the operation of cyclic electron flow. (b) In aerobiosis, illumination increased the ATP/ADP ratio independently of the intensity used, whereas the amount of NADPH was decreased at limiting photon flux and regained the dark-adapted level under saturating photon flux. Moreover, under these conditions, the addition of low concentrations of uncouplers stimulated photosynthetic O(2) evolution. These observations suggest that the photosynthetic generation of reducing equivalents rather than the rate of ATP formation limits the photosynthetic assimilation of CO(2) in C. reinhardtii cells. This situation is peculiar to C. reinhardtii, because neither NADPH nor ATP limited this process in plant leaves, as shown by their increase upon illumination in barley (Hordeum vulgare) leaves, independent of light intensity. Experiments are presented and were designed to evaluate the contribution of different physiological processes that might increase the photosynthetic ATP/NADPH ratio-the Mehler reaction, respiratory ATP supply following the transfer of reducing equivalents via the malate/oxaloacetate shuttle, and cyclic electron flow around PSI-to this metabolic situation.

Chimeric fusions of subunit IV and PetL in the b6f complex of Chlamydomonas reinhardtii: structural implications and consequences on state transitions

The cytochrome b(6)f complex of Chlamydomonas reinhardtii contains four large subunits and at least three small ones, PetG, PetL, and PetM, whose role and location are unknown. Chimeric proteins have been constructed, in which the C terminus of subunit IV is fused to either one or the other of the two putative N termini of PetL. Biochemical and functional analysis of the chimeras together with mass spectrometry analysis of the wild-type (WT) complex led to the following conclusions: (i) neither a free subunit IV C terminus nor a free PetL N terminus is required for assembly of the b(6)f complex; (ii) the first AUG codon in the sequence of the gene petL is used for initiation; (iii) the N terminus of WT PetL lies in the lumen; (iv) in the WT complex, the N terminus of PetL and the C terminus of subunit IV are within reach of each other; (v) the purified b(6)f complex from C. reinhardtii contains an eighth, hitherto unrecognized subunit, PetN; and (vi) the ability to perform state transitions is lost in the chimeric mutants, although (vii) the Q-cycle is unaffected. A structural hypothesis is presented to account for this peculiar phenotype.

Involvement of state transitions in the switch between linear and cyclic electron flow in Chlamydomonas reinhardtii

The energetic metabolism of photosynthetic organisms is profoundly influenced by state transitions and cyclic electron flow around photosystem I. The former involve a reversible redistribution of the light-harvesting antenna between photosystem I and photosystem II and optimize light energy utilization in photosynthesis whereas the latter process modulates the photosynthetic yield. We have used the wild-type and three mutant strains of the green alga Chlamydomonas reinhardtii--locked in state I (stt7), lacking the photosystem II outer antennae (bf4) or accumulating low amounts of cytochrome b6f complex (A-AUU)--and measured electron flow though the cytochrome b6f complex, oxygen evolution rates and fluorescence emission during state transitions. The results demonstrate that the transition from state 1 to state 2 induces a switch from linear to cyclic electron flow in this alga and reveal a strict cause-effect relationship between the redistribution of antenna complexes during state transitions and the onset of cyclic electron flow.

A critical role for the Var2 FtsH homologue of Arabidopsis thaliana in the photosystem II repair cycle in vivo

Using a var2-2 mutant of Arabidopsis thaliana, which lacks a homologue of the zinc-metalloprotease, FtsH, we demonstrate that this protease is required for the efficient turnover of the D1 polypeptide of photosystem II and protection against photoinhibition in vivo. We show that var2-2 leaves are much more susceptible to light-induced photosystem II photoinhibition than wild-type leaves. Furthermore, the rate of photosystem II photoinhibition in untreated var2-2 leaves is equivalent to that of var2-2 and wild-type leaves, which have been treated with lincomycin, an inhibitor of the photosystem II repair cycle at the level of D1 synthesis. This is in contrast to untreated wild-type leaves, which show a much slower rate of photosystem II photoinhibition due to an efficient photosystem II repair cycle. The recovery of var2-2 leaves from photosystem II photoinhibition is also impaired relative to wild-type. Using Western blot analysis in the presence of lincomycin we show that the D1 polypeptide remains stable in leaves of the var2-2 mutant under photoinhibitory conditions that lead to D1 degradation in wild-type leaves and that the abundance of DegP2 is not affected by the var2-2 mutation. We conclude, therefore, that the Var2 FtsH homologue is required for the cleavage of the D1 polypeptide in vivo. In addition, we identify a conserved lumenal domain in Var2 that is unique to FtsH homologues from oxygenic phototrophs.

Contrasted effects of inhibitors of cytochrome b6f complex on state transitions in Chlamydomonas reinhardtii: the role of Qo site occupancy in LHCII kinase activation

We have investigated the relationship between the occupancy of the Q(o) site in the cytochrome b(6)f complex and the activation of the LHCII protein kinase that controls state transitions. To this aim, fluorescence emission and LHCII phosphorylation patterns were studied in whole cells of Chlamydomonas reinhardtii treated with different plastoquinone analogues. The analysis of fluorescence induction at room temperature indicates that stigmatellin consistently prevented transition to State 2, whereas 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone behaved as an inhibitor of state transitions only after the cells were preilluminated. The same effects were observed on the phosphorylation patterns of the LHCII proteins, while subunit V of the cytochrome b(6)f complex showed a different behavior. These findings are discussed on the basis of a dynamic structural model of cytochrome b(6)f that relates the activation of the LHCII kinase to the occupancy of the Q(o) site and the movement of the Rieske protein.

State transitions, cyclic and linear electron transport and photophosphorylation in Chlamydomonas reinhardtii

The relationship between state transitions and the kinetic properties of the electron transfer chain has been studied in Chlamydomonas reinhardtii. The same turnover rate of cytochrome f was found in state 1 and 2. However, while DBMIB was inhibitory in both states, DCMU was effective only in state 1. These observations suggest that linear electron transport was active only in state 1, while a cyclic pathway around photosystem (PS) I operated in state 2. The reversible shift from linear to cyclic electron transport was modulated by changes of PSII antenna size, which inactivated the linear pathway, and by oxygen, which inhibited the cyclic one. Attainment of state 2, under anaerobiosis in the dark, was associated with the decline of the ATP/ADP ratio in the cells and the dark reduction of the intersystem carriers. Upon illumination of the cells, the ATP/ADP ratio increased in a few seconds to the aerobic level. Then, several minutes later, the F(m) returned to the state 1 level, and O(2) evolution was reactivated. This suggests that ATP, though required for photosynthesis, is not the rate-limiting factor in the reactivation of photosynthetic O(2) evolution, which is rather controlled by the redox state of the electron carriers.

The Qo site of cytochrome b6f complexes controls the activation of the LHCII kinase

We created a Qo pocket mutant by site-directed mutagenesis of the chloroplast petD gene in Chlamydomonas reinhardtii. We mutated the conserved PEWY sequence in the EF loop of subunit IV into PWYE. The pwye mutant did not grow in phototrophic conditions although it assembled wild-type levels of cytochrome b6f complexes. We demonstrated a complete block in electron transfer through the cytochrome b6f complex and a loss of plastoquinol binding at Qo. The accumulation of cytochrome b6f complexes lacking affinity for plastoquinol enabled us to investigate the role of plastoquinol binding at Qo in the activation of the light-harvesting complex II (LHCII) kinase during state transitions. We detected no fluorescence quenching at room temperature in state II conditions relative to that in state I. The quantum yield spectrum of photosystem I charge separation in the two state conditions displayed a trough in the absorption region of the major chlorophyll a/b proteins, demonstrating that the cells remained locked in state I. 33Pi labeling of the phosphoproteins in vivo demonstrated that the antenna proteins remained poorly phosphorylated in both state conditions. Thus, the absence of state transitions in the pwye mutant demonstrates directly that plastoquinol binding in the Qo pocket is required for LHCII kinase activation.

Control of Photosystem II in spinach leaves by continuous light and by light pulses given in the dark

The light-induced induction of components of non-photochemical quenching of chlorophyll fluorescence which are distinguished by different rates of dark relaxation (qNf, rapidly relaxing and qNs, slowly relaxing or not relaxing at all in the presence brief saturating light pulses which interrupt darkness at low frequencies) was studied in leaves of spinach.After dark adaptation of the leaves, a fast relaxing component developed in low light only after a lag phase. Quenching increased towards a maximum with increasing photon flux density. This 'fast' component of quenching was identified as energy-dependent quenching qE. It required formation of an appreciable transthylakoid ΔpH and was insignificant when darkened spinach leaves received 1 s pulses of light every 30 s even though zeaxanthin was formed from violaxanthin under these conditions.Another quenching component termed qNs developed in low light without a lag phase. It was not dependent on a transthylakoid pH gradient, decayed exponentially with a long half time of relaxation and was about 20% of total quenching irrespective of light intensity. When darkened leaves were flashed at frequencies higher than 0.004 Hz with 1 s light pulses, this quenching also appeared. Its extent was very considerable, and it did not require formation of zeaxanthin. Relaxation was accelerated by far-red light, and this acceleration was abolished by NaF.We suggest that qNs is the result of a so-called state transition, in which LHC II moves after its phosphorylation from fluorescent PS II to nonfluorescent PS I. This state transition was capable of decreasing in darkened leaves the potential maximum quantum efficiency of electron flow through Photosystem II by about 20%.

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (YO2) and photochemical yield of PS II (ΦPS II) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, YO2 and ΦPS II followed each other, suggesting a close coupling between the oxidation of water and QA reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432-443). Following a light-to-dark transition, however, the relationship between QA reduction and the fraction of PS II reaction centers capable of evolving O2 became temporarily uncoupled. ΦPS II recovered to the preillumination levels within 5-10 s, while the YO2 required up to 60 s to recover under aerobic conditions. The recovery of YO2 was independent of the redox state of QA, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (σPS II). The hysteresis between YO2 and the reduction of QA during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.

Xanthophyll cycle-dependent quenching of photosystem II chlorophyll a fluorescence: formation of a quenching complex with a short fluorescence lifetime

Excess light triggers protective nonradiative dissipation of excitation energy in photosystem II through the formation of a trans-thylakoid pH gradient that in turn stimulates formation of zeaxanthin and antheraxanthin. These xanthophylls when combined with protonation of antenna pigment-protein complexes may increase nonradiative dissipation and, thus, quench chlorophyll a fluorescence. Here we measured, in parallel, the chlorophyll a fluorescence lifetime and intensity to understand the mechanism of this process. Increasing the xanthophyll concentration in the presence of a pH gradient (quenched conditions) decreases the fractional intensity of a fluorescence lifetime component centered at approximately 2 ns and increases a component at approximately 0.4 ns. Uncoupling the pH gradient (unquenched conditions) eliminates the 0.4-ns component. Changes in the xanthophyll concentration do not significantly affect the fluorescence lifetimes in either the quenched or unquenched sample conditions. However, there are differences in fluorescence lifetimes between the quenched and unquenched states that are due to pH-related, but nonxanthophyll-related, processes. Quenching of the maximal fluorescence intensity correlates with both the xanthophyll concentration and the fractional intensity of the 0.4-ns component. The unchanged fluorescence lifetimes and the proportional quenching of the maximal and dark-level fluorescence intensities indicate that the xanthophylls act on antenna, not reaction center processes. Further, the fluorescence quenching is interpreted as the combined effect of the pH gradient and xanthophyll concentration, resulting in the formation of a quenching complex with a short (approximately 0.4 ns) fluorescence lifetime.

Protein phosphorylation and Mg2+ influence light harvesting and electron transport in chloroplast thylakoid membrane material containing only the chlorophyll-a/b-binding light-harvesting complex of photosystem II and photosystem I

A material containing only photosystem I (PSI) and the chlorophyll-a/b-binding light-harvesting complex of PSII (LHC-II) has been isolated from the chloroplast thylakoid membrane by solubilization with Triton X-100. Fluorescence spectroscopy shows that, within the material, LHC-II is coupled to PSI for excitation-energy transfer and that this coupling is decreased by the presence of Mg2+, which also decreased PSI electron transport specifically at limiting light intensity. Inclusion of phosphorylated LHC-II within the material did not alter its structure, but gave decreased energy transfer to PSI and inhibition of electron transport which was independent of light intensity, implying effects of phosphorylation on both light harvesting and directly on electron transport. Inclusion of Mg2+ within the phosphorylated material gave decreased energy transfer, but slightly increased PSI electron transport. A cation-induced direct promotion of PSI electron transport was also observed in isolated PSI particles. The PSI/LHC-II material represents a model system for examining protein interactions during light-state adaptations and the possibility that LHC-II can contribute to the antenna of PSI in light state 2 in vivo is discussed.

Dark induction of zeaxanthin-dependent nonphotochemical fluorescence quenching mediated by ATP

Zeaxanthin-dependent nonphotochemical fluorescence quenching is a light-induced activity in plants that apparently protects against the potentially damaging effects of excess light. We report a dark-induced nonphotochemical quenching in thylakoids of Lactuca sativa L. cv. Romaine mediated by ATP. This effect is due to low lumen pH from hydrolysis-dependent proton pumping and hence required an active ATPase. The induction was optimal at 0.3 mM ATP, a physiological concentration, and occurred under conditions of little or no reverse electron flow. The properties of ATP-induced quenching were in all respects examined similar to light-induced quenching, including antimycin inhibition of quenching induction but not delta pH. We conclude that zeaxanthin-dependent quenching depends directly on lumen pH and that the role of light is indirect. Although it is known that zeaxanthin and low lumen pH are insufficient for quenching to occur, the results apparently exclude the redox state of an electron-transport carrier or formation of light-induced carotenoid triplets as a further requirement. We propose that a slow pH-dependent conformational change together with zeaxanthin cause static quenching in the pigment bed; possibly antimycin inhibits this change. Furthermore, we suggest from the ability of ATP to sustain quenching in the dark for extended periods that persistent or slowly reversible zeaxanthin quenching often observed in vivo may be due to sustained delta pH from ATP hydrolysis.

Resolution of components of non-photochemical chlorophyll fluorescence quenching in barley leaves

Non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves has been analysed by monitoring its relaxation in the dark, by applying saturating pulses of light. At least three kinetically distinct phases to qN recovery are observed, which have previously been identified (Quick and Stitt 1989) as being due to high-energy state quenching ('fast'), excitation energy redistribution due to a state transition ('medium') and photoinhibition ('slow'). However, measurements of chlorophyll fluorescence at 77 K from leaf extracts show that state transitions only occur in low light conditions, whereas the 'medium' component of qN is very large in high light. The source of that part of the 'medium' component not accounted for by a state transition is discussed.