Redox potentials of electron acceptors in photosystem IIα and IIβ

Photosystem II fluorescence: slow changes--scaling from the past

With the advent of photoelectric devices (photocells, photomultipliers) in the 1930s, fluorometry of chlorophyll (Chl) a in vivo emerged as a major method in the science of photosynthesis. Early researchers employed fluorometry primarily for two tasks: to elucidate the role in photosynthesis, if any, of other plant pigments, such as Chl b, Chl c, carotenoids and phycobilins; and to use it as a convenient inverse measure of photosynthetic activity. In pursuing the latter task, it became apparent that Chl a fluorescence emission is influenced (i) by redox active Chl a molecules in the reaction center of photosystem (PS) II (photochemical quenching); (ii) by an electrochemical imbalance across the thylakoid membrane (high energy quenching); and (iii) by the size of the peripheral antennae of weakly fluorescent PSI and strongly fluorescent PSII in response to changes in the ambient light (state transitions). In this perspective we trace the historical evolution of our awareness of these concepts, particularly of the so-called 'State Transitions'.

PsbS genotype in relation to coordinated function of PS II and PS I in Arabidopsis leaves

Application of multiple probes to systems that carry specific mutations provides a powerful means for studying how known regulators of light utilization interact in vivo. Two lines of Arabidopsis thaliana were studied, each carrying a unique lesion in the nuclear psbS gene encoding a 22-kDa pigment-binding protein (PS II-S) essential for full expression of photoprotective, rapid-phase, nonphotochemical quenching of chlorophyll fluorescence (NPQ). The PS II-S protein is absent in line npq4-1 due to deletion of psbS. Line npq4-9 expresses normal levels of PS II-S but carries a single amino acid substitution that lowers NPQ capacity by about 50%. A prior report [Peterson RB and Havir EA (2001) Planta 214: 142-152] described an altered pattern of redox states of the acceptor side of Photosystem II (PS II) and donor side of Photosystem I (PS I) for npq4-9 suggesting that interphotosystem electron transport may be restricted by a higher transthylakoid DeltapH in this line. In vivo steady state fluorescence and absorbance measurements (820 nm) confirmed these earlier observations for line npq4-9 but not for npq4-1. Thus, the prior results cannot be correlated simply to a loss of NPQ capacity. Likewise, the kinetics of the 820-nm absorbance change did not indicate a substantial effect of psbS genotype on electron flow from plastoquinol to PS I. A simple model is proposed to relate linear electron transport rate (measured gasometrically) to a parameter (based on fluorescence) that provides a relative measure of the density of excitation available for photochemistry in PS II. Surprisingly, analyses using this model suggested that the in vivo midpoint potential of the primary quinone acceptor in PS II (Q(A)) is lowered in both psbS mutant lines. This heretofore-unsuspected role for PS II-S is discussed with regard to: (1) numerous prior reports indicating plasticity of the redox potential of Q(A) and (2) the basis for the contrasting regulation of quantum yields of PS I and II in npq4-1 and npq4-9.

Non-photochemical quenching of chlorophyll a fluorescence by oxidised plastoquinone: new evidences based on modulation of the redox state of the endogenous plastoquinone pool in broken spinach chloroplasts

Twenty-five years ago, non-photochemical quenching of chlorophyll fluorescence by oxidised plastoquinone (PQ) was proposed to be responsible for the lowering of the maximum fluorescence yield reported to occur when leaves or chloroplasts were treated in the dark with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of electron flow beyond the primary quinone electron acceptor (Q(A)) of photosystem (PS) II. Since then, the notion of PQ-quenching has received support but has also been put in doubt, due to inconsistent experimental findings. In the present study, the possible role of the native PQ-pool as a non-photochemical quencher was reinvestigated, employing measurements of the fast chlorophyll a fluorescence kinetics (from 50 micros to 5 s). The about 20% lowering of the maximum fluorescence yield F(M), observed in osmotically broken spinach chloroplasts treated with DCMU, was eliminated when the oxidised PQ-pool was non-photochemically reduced to PQH(2) by dark incubation of the samples in the presence of NAD(P)H, both under anaerobic and aerobic conditions. Incubation under anaerobic conditions in the absence of NAD(P)H had comparatively minor effects. In DCMU-treated samples incubated in the presence of NAD(P)H fluorescence quenching started to develop again after 20-30 ms of illumination, i.e., the time when PQH(2) starts getting reoxidized by PS I activity. NAD(P)H-dependent restoration of F(M) was largely, if not completely, eliminated when the samples were briefly (5 s) pre-illuminated with red or far-red light. Addition to the incubation medium of HgCl(2) that inhibits dark reduction of PQ by NAD(P)H also abolished NAD(P)H-dependent restoration of F(M). Collectively, our results provide strong new evidence for the occurrence of PQ-quenching. The finding that DCMU alone did not affect the minimum fluorescence yield F(0) allowed us to calculate, for different redox states of the native PQ-pool, the fractional quenching at the F(0) level (Q(0)) and to compare it with the fractional quenching at the F(M) level (Q(M)). The experimentally determined Q(0)/Q(M) ratios were found to be equal to the corresponding F(0)/F(M) ratios, demonstrating that PQ-quenching is solely exerted on the excited state of antenna chlorophylls.

Correlation between lifetime heterogeneity and kinetics heterogeneity during chlorophyll fluorescence induction in leaves: 1. Mono-frequency phase and modulation analysis reveals a conformational change of a PSII pigment complex during the IP thermal phase

The relationship between the fluorescence lifetime (tau) and yield (Phi) obtained in phase and modulation fluorometry at 54 MHz during the chlorophyll fluorescence induction in dark-adapted leaves under low actinic light has been investigated. Three typical phases have been identified: (i) linear during the OI photochemical rise, (ii) convex curvature during the subsequent IP thermal rise, and (iii) linear during the PS slow decay. A similar relationship has been obtained in the fluorescence induction for the fluorescence yield measured at 685 nm plotted versus the fluorescence yield measured at 735 nm. A spectrally resolved analysis shows that the curvature of the tau-Phi relationship is not due to chlorophyll fluorescence reabsorption effects. Several other hypotheses are discussed and we conclude that the curvature of the tau-Phi relationship is due to a variable and transitory nonphotochemical quenching. We tentatively propose that this quenching results from a conformational change of a pigment-protein complex of Photosystem II core antenna during the IP phase and could explain both spectral and temporal transitory changes of the fluorescence. A variable blue shift of the 685 nm peak of the fluorescence spectrum during the IP phase has been observed, supporting this hypothesis.

The light sensitivity of ATP synthase mutants of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii mutants defective in the chloroplast ATP synthase are highly sensitive to light. The ac46 mutant is affected in the MDH1 gene, required for production or stability of the monocistronic atpH mRNA encoding CF(O)-III. In this and other ATP synthase mutants, we show that short-term exposure to moderate light intensities-a few minutes-induces an inhibition of electron transfer after the primary quinone acceptor of photosystem II (PSII), whereas longer exposure-several hours-leads to a progressive loss of PSII cores. An extensive swelling of thylakoids accompanies the initial inhibition of electron flow. Thylakoids deflate as PSII cores are lost. The slow process of PSII degradation involves the participation of ClpP, a chloroplast-encoded peptidase that is part of a major stromal protease Clp. In the light of the above findings, we discuss the photosensitivity of ATP synthase mutants with respect to the regular photoinhibition process that affects photosynthetic competent strains at much higher light intensities.

Characterization of photosystem II activity and heterogeneity during the cell cycle of the green alga scenedesmus quadricauda

The photosynthetic activity of the green alga Scenedesmus quadricauda was investigated during synchronous growth in light/dark cycles. The rate of O2 evolution increased 2-fold during the first 3 to 4 h of the light period, remained high for the next 3 to 4 h, and then declined during the last half of the light period. During cell division, which occurred at the beginning of the dark period, the ability of the cells to evolve O2 was at a minimum. To determine if photosystem II (PSII) controls the photosynthetic capacity of the cells during the cell cycle we measured PSII activity and heterogeneity. Measurements of electron-transport activity revealed two populations of PSII, active centers that contribute to carbon reduction and inactive centers that do not. Measurements of PSII antenna sizes also revealed two populations, PSIIalpha and PSIIbeta, which differ from one another by their antenna size. During the early light period the photosynthetic capacity of the cells doubled, the O2-evolving capacity of PSII was nearly constant, the proportion of PSIIbeta centers decreased to nearly zero, and the proportion of inactive PSII centers remained constant. During the period of minimum photosynthetic activity 30% of the PSII centers were insensitive to the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which may be related to reorganization of the thylakoid membrane. We conclude from these results that PSII does not limit the photosynthetic activity of the cells during the first half of the light period. However, the decline in photosynthetic activity observed during the last half of the light period can be accounted for by limited PSII activity.

Fractionation of the thylakoid membranes from tobacco. A tentative isolation of 'end membrane' and purified 'stroma lamellae' membranes

Thylakoids isolated from tobacco were fragmented by sonication and the vesicles so obtained were separated by partitioning in aqueous polymer two-phase systems. By this procedure, grana vesicles were separated from stroma exposed membrane vesicles. The latter vesicles could be further fractionated by countercurrent distribution, with dextran-polyethylene glycol phase systems, and divided into two main populations, tentatively named 'stroma lamellae' and 'end membrane'. Both these vesicle preparations have high chlorophyll a/b ratio, high photosystem (PS) I and low PS II content, suggesting their origin from stroma exposed regions of the thylakoid. The two vesicle populations have been compared with respect to biochemical composition and photosynthetic activity. The 'end membrane' has a higher chlorophyll a/b ratio (5.7 vs. 4.7), higher P700 content (4.7 vs. 3.3 mmol/mol of chlorophyll). The 'end membrane' has the lowest PS II content, the ratio PS I/PS II being more than 10, as shown by EPR measurements. The PS II in both fractions is of the beta-type. The decay of fluorescence is different for the two populations, the 'stroma lamellae' showing a very slow decay even in the presence of K3Fe(CN)6 as an acceptor. The two vesicle populations have very different surface properties: the end membranes prefer the upper phase much more than the stroma lamellae, a fact which was utilized for their separation. Arguments are presented which support the suggestion that the two vesicle populations originate from the grana end membranes and the stroma lamellae, respectively.

Heterogeneity and photoinhibition of photosystem II studied with thermoluminescence

Thermoluminescence (TL) signals were recorded from grana stacks, margins, and stroma lamellae from fractionated, dark-adapted thylakoid membranes of spinach (Spinacia oleracea L.) in the absence and in the presence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, the TL signal from grana fractions consisted of a homogenous B-band, which originates from recombination of the semi-quinone QB- with the S2 state of the water-splitting complex and reflects active photosystem II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S2QA- recombination. Margin fractions mainly showed two TL-bands, the B- and C-bands, at approximately 50 degreesC in the absence of DCMU, and Q- and C-bands in the presence of DCMU. The C-band is ascribed to a TyrD+-QA- recombination. In the absence of DCMU, the fractions of stromal lamellae mainly gave rise to a TL emission at 42 degreesC. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52 degreesC) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45 degreesC (C-band) slightly increased. It is proposed that the 42 to 52 degreesC band that was observed in marginal and stromal lamellae and in photoinhibited thylakoid membranes reflects inactive PSII centers that are assumed to be equivalent to inactive PSII QB-nonreducing centers.

Inhibition of Photosystem 2 primary photochemistry by photogenerated protons

Photosystem 2 photochemical efficiency, measured as the rate of Qa reduction, was observed to be inhibited by preillumination with single turnover flashes, whilst Fo and Fm were not affected. Such inhibition was reversed by the uncoupler nigericin or by incubating the thylakoids in the dark for ca. 2 min after the preillumination. The presence of ATP in micromolar concentrations increased the time of dark recovery from the inhibition. The inhibition of fluorescence rise was not changed when 70% of the excitation energy available in the antenna was quenched by dinitrobenzene. Quantitative analysis of the observed fluorescence induction indicates that this phenomenon is due to the inhibition of the photochemical reaction itself. Uncouplers such NH4Cl were unable to reverse the inhibition and only a few flashes of saturating intensity (10 or less) were required for the onset of it. This suggests that protons localised in domains rather than a pH gradient between the thylakoid lumen bulk solution and the external one are involved in this regulation of PS 2 efficiency.

The role of calcium in the pH-dependent control of Photosystem II

pH-dependent inactivation of Photosystem (PS) II and related quenching of chlorophyll-a-fluorescence have been investigated in isolated thylakoids and PS II-particles and related to calcium release at the donor side of PS II. The capacity of oxygen evolution (measured under light saturation) decreases when the ΔpH is high and the pH in the thylakoid lumen decreases below 5.5. Oxygen evolution recovers upon uncoupling. The pH-response of inactivation can be described by a 1 H(+)-transition with an apparent pK-value of about 4.7. The yield of variable fluorescence decreases in parallel to the inactivation of oxygen evolution. pH-dependent quenching requires light and can be inhibited by DCMU. In PS II-particles, inactivation is accompanied by a reversible release of Ca(2+)-ions (one Ca(2+) released per 200 Chl). In isolated thylakoids, where a ΔpH was created by ATP-hydrolysis, both inactivation of oxygen evolution (and related fluorescence quenching) by internal acidification and the recovery of that inactivation can be suppressed by calcium-channel blockers. In the presence of the Ca(2+)-ionophore A23187, recovery of Chl-fluorescence (after relaxation of the ΔpH) is stimulated by external Ca(2+) and retarded by EGTA. As shown previously (Krieger and Weis 1993), inactivation of oxygen evolution at low pH is accompanied by an upward shift of the midpoint redox-potential, Em, of QA. Here, we show that in isolated PS II particles the pH-dependent redox-shift (about 160 mV, as measured from redox titration of Chl-fluorescence) is suppressed by Ca(2+)-channel blockers and DCMU. When a redox potential of -80 to -120mV was established in a suspension of isolated thylakoids, the primary quinone acceptor, QA, was largely reduced in presence of a ΔpH (created by ATP-hydrolysis) but oxidized in presence of an uncoupler. Ca(2+)-binding at the lumen side seems to control redox processes at the lumen- and stroma-side of PS II. We discuss Ca(2+)-release to be involved in the physiological process of 'high energy quenching'.

Analysis of fluorescence induction in thylakoids with the method of moments reveals two different active Photosystem II centres

There is presently a debate concerning the number of phases in fluorescence induction and on the identification of the several possible heterogeneities in PS II centres. However, the usual methods of analysis present numerical problems, including a lack of 'robustness' (robustness being defined as the ability to give the correct answer in the presence of distortions or artefacts). We present here the adaptation of the method of moments, which was developed for robustness, to the analysis of fluorescence induction. We were thus able to identify three phases in the fluorescence induction in the presence of DCMU. The slowest phase was attributed to the centres inactive in plastoquinone reduction by using duroquinone as electron acceptor. In order to compare fluorescence with and without DCMU, we introduced the 'rate of photochemistry', defined as the product of the area times the rate constant of an exponential. This quantity is invariant for a given centre no matter what the size of the electron acceptor pool is. The two fastest phases in the presence of DCMU were attributed to active centres because their rate of photochemistry was the same as that of the plastoquinone-reducing phases in the absence of DCMU. Because their reduction of plastoquinone showed different kinetics, these two types of active centres were either separated by more than 250 nm or were associated with discrete plastoquinone pools having restricted diffusion domains.

Photosystem II heterogeneity: the acceptor side

It is well known that two photosystems, I and II, are needed to transfer electrons from H2O to NADP(+) in oxygenic photosynthesis. Each photosystem consists of several components: (a) the light-harvesting antenna (L-HA) system, (b) the reaction center (RC) complex, and (c) the polypeptides and other co-factors involved in electron and proton transport. First, we present a mini review on the heterogeneity which has been identified with the electron acceptor side of Photosystem II (PS II) including (a) L-HA system: the PS IIα and PS IIβ units, (b) RC complex containing electron acceptor Q1 or Q2; and (c) electron acceptor complex: QA (having two different redox potentials QL and QH) and QB (QB-type; Q'B type; and non-QB type); additional components such as iron (Q-400), U (Em,7=-450 mV) and Q-318 (or Aq) are also mentioned. Furthermore, we summarize the current ideas on the so-called inactive (those that transfer electrons to the plastoquinone pool rather slowly) and active reaction centers. Second, we discuss the bearing of the first section on the ratio of the PS II reaction center (RC-II) and the PS I reaction center (RC-I). Third, we review recent results that relate the inactive and active RC-II, obtained by the use of quinones DMQ and DCBQ, with the fluorescence transient at room temperature and in heated spinach and soybean thylakoids. These data show that inactive RC-II can be easily monitored by the OID phase of fluorescence transient and that heating converts active into inactive centers.

Characterization of photosystem II in stroma thylakoid membranes

The functional state of the PS II population localized in the stroma exposed non-appressed thylakoid region was investigated by direct analysis of the PS II content of isolated stroma thylakoid vesicles. This PS II population, possessing an antenna size typical for PS IIβ, was found to have a fully functional oxygen evolving capacity in the presence of an added quinone electron acceptor such as phenyl-p-benzoquinone. The sensitivity to DCMU for this PS II population was the same as for PS II in control thylakoids. However, under more physiological conditions, in the absence of an added quinone acceptor, no oxygen was evolved from stroma thylakoid vesicles and their PS II centers were found to be incapable to pass electrons to PS I and to yield NADPH. By comparison of the effect of a variety of added quinone acceptors with different midpoint potentials, it is concluded that the inability of PS II in the stroma thylakoid membranes to contribute to NADPH formation probably is due to that QA of this population is not able to reduce PQ, although it can reduce some artificial acceptors like phenyl-p-benzoquinone. These data give further support to the notion of a discrete PS II population in the non-appressed stroma thylakoid region, PS IIβ, having a higher midpoint potential of QA than the PS II population in the appressed thylakoid region, PS IIα. The physiological significance of a PS II population that does not produce any NADPH is discussed.

Light saturation response of inactive photosystem II reaction centers in spinach

The effective absorption cross section of inactive photosystem II (PS II) centers, which is the product of the effective antenna size and the quantum yield for photochemistry, was investigated by comparing the light saturation curves of inactive PS II and active reaction centers in intact chloroplasts and thylakoid membranes of spinach (Spinacia oleracea). Inactive PS II centers are defined as the impaired PS II reaction centers that require greater than 50 ms for the reoxidation of QA (-) subsequent to a single turnover flash. Active reaction centers are defined as the rapidly turning over PS II centers (recovery time less than 50 ms) and all of the PS I centers. The electrochromic shift, measured by the flash-induced absorbance increase at 518 nm, was used to probe the activity of the reaction centers. Light saturation curves were generated for inactive PS II centers and active reaction centers by measuring the extent of the absorbance increase at 518 nm induced by red actinic flashes of variable energy. The light saturation curves show that inactive PS II centers required over twice as many photons as active reaction centers to achieve the same yield. The ratio of the flash energy required for 50% saturation for active reaction centers (PS II active + PS I) compared to inactive PS II centers was 0.45±0.04 in intact chloroplasts, and 0.54±0.11 in thylakoid membranes. Analysis of the light saturation curves using a Poisson statistical model in which the ratio of the antenna size of active PS II centers to that of PS I is considered to range from 1 to 1.5, indicates that the effective absorption cross section of inactive PS II centers was 0.54-0.37 times that of active PS II centers. If the quantum yield for photochemistry is assumed to be one, we estimate that the antenna system serving the inactive PS II centers contains approx. 110 chlorophyll molecules.

Chlorophyll a fluorescence transient as an indicator of active and inactive Photosystem II in thylakoid membranes

Upon illumination, a dark-adapted photosynthetic sample shows time-dependent changes in chlorophyll (Chl) a fluorescence yield, known as the Kautsky phenomenon or the OIDPS transient. Based on the differential effects of electron acceptors such as 2,5-dimethyl-p-benzoquinone (DMQ) and 2,6-dichloro-p-benzoquinone (DCBQ) on Chl a fluorescence transients of spinach thylakoids, we suggest that the OID phase reflects the reduction of the electron acceptor QA to QA- in the inactive PS II (see Graan, T. and Ort, D. (1986) Diochim. Biophys. Acta 852, 320-330). In spinach thylakoids, heat-induced increase of the Chl a fluorescence yield is also differentially sensitive to the addition of DMQ and DCBQ suggesting that this increase is mainly on the 'I' level, and thus heating is suggested to convert active PS II to inactive PS II centers. The kinetics of decay of QA-, calculated from variable Chl a fluorescence, was analyzed into three exponential components (365-395 microseconds; 6-7 ms; and 1.4-1.7 s). In heated samples, the decay rate of variable Chl a fluorescence is slower than the normal back-reaction rate; there is a preponderance of the slow component that may be due, partly, to the active centers undergoing slow back reaction between QA- and the S2 state of the oxygen-evolving complex.

Heterogeneity in chloroplast photosystem II

Photosystem-two (PSII) in the chloroplasts of higher plants and green algae is not homogeneous. A review of PSII heterogeneity is presented and a model is proposed which is consistent with much of the data presented in the literature. It is proposed that the non-quinone electron acceptor of PSII is preferentially associated with the sub-population of PSII known as PSIIß.

Quantitation of plastoquinone photoreduction in spinach chloroplasts

The question of plastoquinone (PQ) concentration and its stoichiometry to photosystem I (PSI) and PSII in spinach chloroplasts is addressed here. The results from three different experimental approaches were compared. (a) Quantitation from the light-induced absorbance change at 263 nm (ΔA263) yielded the following ratios (mol:mol); Chl:PQ=70:1, PQ:PSI=9:1 and PQ:PSIIα=7:1. The kinetics of PQ photoreduction were a monophasic but non-exponential function of time. The deviation of the semilogarithmic plots from linearity reflects the cooperativity of several electron transport chains at the PQ pool level. (b) Estimates from the area over the fluorescence induction curve (Afl) tend to exaggerate the PQ pool size because of electron transfer via PSI to molecular oxygen (Mehler reaction) resulting in the apparent increase of the pool of electron acceptors. The reliability of the Afl method is increased substantially upon plastocyanin inhibition by KCN. (c) Quantitation of the number of electrons removed from PQH2 by PSI, either under far-red excitation or after the addition of DCMU to preilluminated chloroplasts, is complicated due to the competitive loss of electrons from PQH2 to molecular oxygen. The latter is biphasic reaction occurring with half-times of about 2 s (30-40% of PQH2) and of about 60 s (60-70% of PQH2).

Thermodynamics of the charge recombination in photosystem II

The temperature dependence of the electric field-induced chlorophyll luminescence in photosystem II was studied in Tris-washed, osmotically swollen spinach chloroplasts (blebs). The system II reaction centers were brought in the state Z(+)P(+)-QA (-)QB (-) by preillumination and the charge recombination to the state Z(+)PQAQB (-) was measured at various temperatures and electrical field strengths. It was found that the activation enthalpy of this back reaction was 0.16 eV in the absence of an electrical field and diminished with increasing field strength. It is argued that this energy is the enthalpy difference between the states IQA (-) and I(-)QA and accounts for about half of the free energy difference between these states. The redox state of QB does not influence this free energy difference within 150 μs after the photoreduction of QA. The consequences for the interpretation of thermodynamic properties of QA are discussed.

Electron transfer in photosystem II

The picture presently emerging from studies on the mechanism of photosystem II electron transport is discussed. The reactions involved in excitation trapping, charge separation and stabilization of the charge pair in the reaction center, followed by the reactions with the substrates, plastoquinone reduction and water oxidation, are described successively. Finally, a brief discussion on photosystem II heterogeneity is presented.