Photosynth Res DOI 10.1007/s11120-008-9349-3
REVIEW
Singlet oxygen production in photosystem II and related protection mechanism Anja Krieger-Liszkay Æ Christian Fufezan Æ Achim Trebst
Received: 30 May 2008 / Accepted: 3 August 2008 ! Springer Science+Business Media B.V. 2008
Abstract High-light illumination of photosynthetic organisms stimulates the production of singlet oxygen by photosystem II (PSII) and causes photo-oxidative stress. In the PSII reaction centre, singlet oxygen is generated by the interaction of molecular oxygen with the excited triplet state of chlorophyll (Chl). The triplet Chl is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary electron donor P680 of the primary acceptor pheophytin or of the quinone acceptor QA, modulate the pathway of charge recombination in PSII and influence the yield of singlet oxygen formation. The involvement of singlet oxygen in the process of photoinhibition is discussed. Singlet oxygen is efficiently quenched by b-carotene, tocopherol or plastoquinone. If not quenched, it can trigger the up-regulation of genes, which are involved in the molecular defence response of photosynthetic organisms against photo-oxidative stress. Keywords Photoinhibition ! Photosystem II ! QA midpoint potential ! Singlet oxygen
A. Krieger-Liszkay (&) CEA, Institut de Biologie et Technologies de Saclay, CNRS URA 2096, Service de Bioe´nerge´tique Biologie Structurale et Me´canisme, 91191 Gif-sur-Yvette Cedex, France e-mail:
[email protected] C. Fufezan Institut fu¨r Biochemie und Biotechnologie der Pflanzen, Westfa¨lische Wilhelms-Universita¨t Mu¨nster, 48143 Mu¨nster, Germany A. Trebst Plant Biochemistry, Ruhr-University Bochum, 44780 Bochum, Germany
Abbreviations Chl Chlorophyll DCMU 3-(3,4-Dichlorophenyl)-1,1-dimethylurea Fv Variable fluorescence Fm Maximal fluorescence P680 Primary electron donor in PSII QA Primary quinone electron acceptor in PSII QB Secondary quinone electron acceptor in PSII Ph Pheophytin—primary electron acceptor PSII Photosystem II
Introduction When plants are exposed to excess light, processes are induced, which protect photosystem II (PSII) against photo-oxidative damage. This includes long-term adaptation processes that lead to an alteration of antenna size depending on the incoming light intensities (Anderson and Osmond 1987; Murchie et al. 2005), short-term changes in the antenna distribution between PSI and PSII by state transitions (Rochaix 2007), and activation of the xanthophyll cycle and conformational changes in the antenna (Holt et al. 2004; Horton and Ruban 2005; Horton et al. 2008), leading to the dissipation of excess energy by nonphotochemical routes. In addition, modifications inside the PSII reaction centre itself play an important role in protecting against photo-oxidative damage (Clarke et al. 1993; Ko´s et al. 2008). Here, we will focus on the modifications of the midpoint redox potential of the primary quinone acceptor, QA, in stress situations, and the consequences this has for the formation of singlet oxygen (1O2). The role of 1 O2 in signalling and defence against 1O2 via carotene, plastoquinone and tocopherol will also be discussed.
123
Photosynth Res
Photo-oxidative damage is defined as the various forms of damage to the cell resulting from light-induced reactions, which form reactive oxygen species (ROS): hydrogen peroxide (H2O2), superoxide (O2•-), hydroxyl radicals (OH•) and singlet oxygen (1O2) (Halliwell and Gutteridge 1999). The photosensitised generation of singlet oxygen was first demonstrated by Kautsky and de Bruijn (1931). The main ROS produced in the reaction centre of PSII is singlet oxygen (Durrant et al. 1990; Macpherson et al. 1993; Hideg et al. 1994; for review, see Krieger-Liszkay 2005). Chlorophyll (Chl), which acts as the main light-absorbing pigment in the light-harvesting complexes (LHC), the inner antenna and also in the reaction centres, is very efficient in absorbing light and has the additional advantage that the excited states are long-lived enough (up to a few nanoseconds) to allow conversion of the excitation energy into an electrochemical potential via charge separation. If the energy is not efficiently used, the spins of the electrons in the excited state can rephase and give rise to a lower energy excited state: the Chl triplet state. The Chl triplet state can react with 3O2 to produce the very reactive 1O2 if no efficient quenchers are close by. The lifetime of 1O2 in a cell has been measured to be approximately 3 ls (Skovsen et al. 2005; Hatz et al. 2007) and in this time, a fraction of 1O2 may be able to diffuse over considerable distances of several hundred nanometres. Chlorophyll triplet states can be populated directly by intersystem crossing (changing of the spin) from singlet excited Chl in the antenna. 1O2 is generated if the triplet state of Chl is not quenched by the neighbouring carotenoid molecules. 1O2 can also be produced in photosensitizing reactions with pigments, which are not bound to a quenching protein. This is the case in heme and Chl synthesis, when intermediates such as protoporphyrin or free Chls (protochlorophyllide), accumulate (op den Camp et al. 2003; von Gromoff et al. 2008). During acclimation of plants to changes in irradiation, the relative amount of LHC and photosystems is adjusted to the environmental condition (Walters 2005). Upon exposure to high light, PSII and LHC disassemble for protein degradation and the pigments are released. Chl degradation reactions, involving chlorophyllase and ring-splitting oxidase (Matile et al. 1999; Eckhardt et al. 2004), quickly deal with this dangerous situation, but singlet oxygen might be intermittently formed and could even act as a signal to the expression system (as discussed in the final chapter). Chlorophyll triplet states can also be generated in the reaction centres by charge recombination reactions (reversal of the charge separation and electron transfer reactions). 1O2 formation by charge recombination is favoured under certain physiological conditions such as exposure to high light intensities or drought, leading to the closure of the stomata and low CO2 concentrations in the chloroplasts (Hideg et al. 2001, 2002; Trebst et al. 2002).
123
Under such conditions, the plastoquinone pool can be in a very highly reduced state and photoinhibition, i.e. the lightinduced loss of PSII activity, occurs (for review, see Adir et al. 2003). 1O2 can react with proteins, pigments, nucleic acids and lipids, and is thought to be the most important species responsible for light-induced loss of PSII activity, the degradation of the D1 polypeptide (PSII reaction centre polypeptide) and pigment bleaching (for reviews on photoinhibition, see Prasil et al. 1992; Aro et al. 1993; Nixon et al. 2005; Vass and Aro 2007; Tyystja¨rvi 2008). Even though the majority of Chl is found in the antenna, 1 O2 is mainly generated in the reaction centre, due to the protective effect of antenna carotenoids. When carotenoids are in close vicinity to the Chl triplet, which is the origin of the observed 1O2, they are able to quench that triplet directly. For this, the edge-to-edge distance between the two molecules must be within the van der Waals distance ˚ ), i.e. the electron orbitals must have some overlap. (3.6 A In this spin exchange reaction, the triplet state of the carotenoid is formed, which can dissipate the excess energy as heat (Edge and Truscott 1999). This is possible in the antenna system but not in the reaction centre, even though two b-carotene molecules are present in the PSII reaction centre (for the location of the b-carotenes in the reaction centre, see Fig. 1 and Loll et al. 2005).
Photosystem II In the reaction centre of PSII (Fig. 1), the first radical pair formed after excitation by light is P680?Ph- (Reaction 1), with P680 being the primary electron donor and pheophytin the primary electron acceptor (for reviews on PSII, see Goussias et al. 2002; Renger 2008; for the X-ray structure of PSII, see Ferreira et al. 2004; Loll et al. 2005). The charge separation in PSII is initiated on ChlD1 rather than on PD1 and PD2 (for a recent review on the nature, localization and identity of P680, see Rappaport and Diner 2008). The next step of electron transfer after the formation of the primary radical pair (P680?Ph-) leads to the reduction of the primary quinone acceptor QA (Reaction 2). The P680? cation is delocalised between PD1 and PD2 with a higher probability being on PD1. Subsequently, P680? is reduced by electron donation by a redox active tyrosine residue, forming the redox active neutral tyrosyl radical (TyrZ, Reaction 3), which by itself, obtains an electron from the water oxidising complex (Reaction 4). The last step (Reaction 5) is the reduction of the secondary quinone QB, which forms a stable semiquinone. In total, QB can accept two electrons and two protons. After double reduction and protonation, QB leaves the binding site and diffuses into the plastoquinone pool. The QB-binding site is then reoccupied by an oxidised plastoquinone molecule.
Photosynth Res Fig. 1 A model of the reaction centre of photosystem II showing only the main subunits D1 (orange), D2 (grey) and Cyt b559 (red), which carry the redox active cofactors. The membrane is indicated as a grey box. The forward electron transfer reactions are indicated by numbers in the order of their occurrence. Abbreviations: ChlZ, peripheral chlorophylls; Car, carotenes; Fe, non-heme iron. OEC, oxygen evolving complex; P, the chlorophylls that become photooxidised; Chl, accessory chlorophylls; Ph, pheophytins. The indices indicate the protein in which the cofactors are located. QA and QB, the first and second plastoquinone acceptors. TyrZ and TyrD, redox active tyrosine residues. The model is based on the X-ray structure by Loll et al. (2005)
The forward electron transfer is much faster than the charge recombination reactions. If the primary quinone acceptor stays reduced, because of a block in the forward electron transport due to a reduced plastoquinone pool (the so-called closed state of the reaction centre), the yield of charge separation is lowered by the presence of the semiquinone anion QA- at room temperature (van Gorkom 1985; Schatz et al. 1988; van Mieghem et al. 1995). In the closed reaction centre, however, if primary charge separation does occur, it is followed by a recombination of charges. Charge recombination of the primary pair (P680?Ph-) will produce either the singlet or the triplet state of P680. At low temperature (40 K), the triplet state (3P680) is mainly localised on one of the accessory chlorophylls, ChlD1 (van Mieghem et al. 1991; Kamlowski et al. 1996; Diner et al. 2001). At higher temperatures (250–300 K), 3P680 is delocalised over ChlD1 and the chlorophylls PD1 and PD2 (Kamlowski et al. 1996). According to Noguchi et al. (2001), up to 30% of the triplet is located at PD1 at room temperature. 3P680 reacts with 3O2 leading to the toxic and very reactive 1O2. Under reducing conditions, i.e. in the presence of dithionite and light (van Mieghem et al. 1989; Vass et al. 1992) or under anaerobiosis and light (Vass et al. 1992), QA becomes doubly reduced, thereby releasing the negative electrostatic effect of QA- on the energy of the primary pair, and a high yield of charge separation, recombination and P680 triplet formation is observed (van Mieghem et al. 1989). The double reduction of QA and the high yield of 3P680 formation in such centres have been
suggested to be relevant for photoinhibition in vivo (van Mieghem et al. 1989; Vass et al. 1992). In the reaction centre, the distance between the carotenes and the triplet Chl is too large to allow a direct triplet quenching. Instead, these b-carotenes can quench 1O2 (Telfer 2002, 2005). b-carotene cannot be located in the close neighbourhood of PD1 because, if it was, it would get immediately oxidised by PD1? instead of TyrZ (for a recent review, see Faller et al. 2006). However, the b-carotenes CarD1 and CarD2 cannot only act as 1O2 quenchers, but also function as electron donors in a side path reaction to protect and disarm P680?, if no electron is available from the donor side (Vrettos et al. 1999; Hanley et al. 1999). This electron donation pathway is slow (t1/2 1–2 ms) and the quantum yield is low and therefore the efficiency of electron donation from the side pathway is low (reviewed in Faller et al. 2006). If the CarD2 gets oxidised by P?(D1, D2), it will be reduced by Cytb559 or by ChlZ,D2. ChlZ? could act as a quencher and dissipate excess energy inside the reaction centre (see Faller et al. 2006). Although 1O2 generation by PSII was experimentally shown to occur (Hideg et al. 1994; Macpherson et al. 1993; Fufezan et al. 2002), neither its concentration at a given light intensity nor its fate is known. Several protective mechanisms exist in PSII, which avoid uncontrolled inactivation of PSII by 1O2 and keep PSII operating. Here, we discuss modifications of the midpoint potential of the cofactors in PSII, the controlled degradation of the D1 protein, the importance of the 1O2 quenchers and scavengers tocopherol and plastoquinol, and the function of 1O2 in signalling.
123
Photosynth Res
Modification of the midpoint potential of cofactors in PSII The yield of charge recombination reactions and thereby the production of 3Chl and 1O2, respectively, can be modified by the midpoint potential of the redox couple QA/QA- (Fig. 2; Krieger-Liszkay and Rutherford 1998; Rutherford and Krieger-Liszkay 2001; Krieger-Liszkay 2005). When the forward electron flow is blocked (completely reduced quinone pool) and QA is in its normal low potential form, the back reaction via the primary radical pair (P680?Ph-) is favoured, leading to a higher probability of 3P680 and 1O2 formation. When the midpoint potential of the redox couple QA/QA- is raised, this back reaction is disfavoured, the yield of 1O2 production is lowered and direct recombination to the ground state may occur. This direct recombination pathway does not involve the repopulation of the primary charge pair (P680?Ph-). Alternatively, the midpoint potential of the redox couple of Ph/Ph- (Rappaport et al. 2002; Cser and Vass 2007) and of P680/P680? (Vavilin and Vermaas 2000) can be altered, also leading to variations in the charge recombination pathway and thereby in the yield of 3P680 and 1 O2 formation (Fig. 2). Changing the molecular properties of PSII at three different sites results in significant modifications of the midpoint potential of QA and subsequent 1O2 formation: (1) removal of calcium or manganese from the donor side; (2) binding of urea-type or phenolic herbicides to the QBbinding pocket; (3) site-directed mutagenesis of amino acid residues in the QA-binding pocket. Shift of the midpoint potential of the QA/QA- redox couple by inactivation of the water-splitting complex In Ca2?- and Mn-depleted PSII, the water-splitting activity is inhibited, and additionally, the midpoint potential of the QA/QA- redox couple is up-shifted by about 150 mV to the so-called high potential form. In PSII that has an active water-splitting complex, the midpoint potential of the QA/ QA- couple was found to be -80 mV, the so-called low potential form, in PSII preparations from spinach (Krieger and Weis 1992; Krieger et al. 1995). In centres with the high potential form of QA (Em about ?65 mV), forward electron flow from QA to QB is energetically disfavoured (Johnson et al. 1995; Krieger et al. 1995; Andre´asson et al. 1995) and direct charge recombination to the ground state of P680 may occur (Krieger-Liszkay and Rutherford 1998; Rutherford and Krieger-Liszkay 2001). In Ca2?- and Mndepleted PSII, no generation of singlet oxygen could be measured by spin trapping EPR with temperature under continuous illumination (Krieger et al. 1998a). When Cl-, the second obligatory cofactor of the Mn cluster, is removed, no shift in the midpoint potential of the QA/QA-
123
Fig. 2 Simplified schematic model of the recombination pathway of the charge separated state P?QA- (P stands for P680). Green arrows: non-radiative charge recombination to the ground state via direct recombination of P?QA- or via indirect recombination involving the repopulation of the primary charge pair P?Ph-. Red arrows: charge recombination leading to the formation of 3P and 1O2. Numbers indicate modifications of the midpoint potentials of the redox compounds P680, Ph, QA. (1) Q130L, (2) Q130E (D1) (Rappaport et al. 2002; Cser and Vass 2007), (3) A294S (D2) (Fufezan et al. 2007), (4) presence of bromoxynil (wt), (5) presence of DCMU (wt) (Krieger-Liszkay and Rutherford 1998), (6) mutation of residues 187– 194 of the D2 protein (C8-2), (7) mutation of residues 179–186 of the D2 protein (C7-3) (Vavilin and Vermaas 2000). The figure illustrates that when the midpoint potential of QA is lowered (low potential QA and 3, 4) the energy gap between P?QA- is decreased, favouring the back reaction and formation of P?Ph- and charge recombination involving the formation of 3P. When the midpoint potential of QA is more positive (high potential QA and 5), the energy gap between P?QA- is increased favouring the safe direct recombination pathway. In centres with QA in the low potential form, when the midpoint potential of Ph is more positive (2), the free energy gap between P?Ph- and P* increases and P?Ph- decays mainly via the indirect non-radiative route. The repopulation of P* is less likely. When the midpoint potential of P is more negative (6) direct charge recombination of P?QA- is favoured and the repopulation of P* is less likely to occur. When the midpoint potential of P is more positive (7), direct charge recombination of P?QA is disfavoured and the probability of the indirect recombination pathway and the repopulation of P* increases
redox couple is observed (Krieger and Rutherford 1997), indicating that the loss of Ca2? and not the loss of activity of the water-splitting complex is responsible for the observed alteration at the acceptor side. The molecular basis of this Ca2? effect is unknown. One possibility is that, upon the release of Ca2?, a structural change in a protein subunit of the reaction centre and especially at the QA-binding site occurs which could be responsible for the observed change in the midpoint potential. It might also be noted that cytochrome b559 transmits structural alterations between the donor and the acceptor side. In inactive and non-photoactivated PSII, cytochrome b559 occurs in a low
Photosynth Res
potential form and changes upon the assembly of the Mn to a high potential form, characteristic of active PSII. The change in the potential form of cytochrome b559 is already observed before the process of photoactivation (lightdependent assembly of the Mn cluster) is fully completed (Mizusawa et al. 1997). Shift of the midpoint potential of the QA/QA- redox couple by the binding of urea-type or phenolic herbicides to the QB-binding pocket Binding of herbicides to the QB-binding site also affects the midpoint potential of QA (indicated by the levels 4 and 5 in Fig. 2). Phenolic herbicides lower the midpoint potential by approximately 45 mV, and 3-(3,4-Dichlorophenyl)-1,1dimethylurea (DCMU) raises it by about 50 mV as was shown by redox titrations of variable Chl fluorescence (Krieger-Liszkay and Rutherford 1998). A smaller difference of 60 mV between the redox potential in the presence of bromoxynil and that in the presence of DCMU was reported by Roberts et al. (2003) when estimated from the back reaction rate of S2QA-. The effect of the different types of herbicides on the midpoint potential of QA is not only observed for the low potential form but also for the high potential form of QA (Krieger-Liszkay and Rutherford 1998). The absolute change in the midpoint potential of QA by these herbicides is much lower (±50 mV) than the shift induced by the inactivation of the water-splitting complex (Ca2?- or Mn-depletion) but it nevertheless has a big effect on 1O2 production. Fufezan et al. (2002) showed that the yield of 1O2 production in the presence of a phenolic herbicide in active PSII-enriched membrane fragments (with QA in the low potential form) is twice as high as in the presence of DCMU. This effect is already seen at relatively low light intensities (400 lmol quanta m-2 s-1) and the amount of 1O2 produced increases linearly with increasing light intensities. The FTIR spectra of QA obtained in the presence of a phenolic herbicide when compared to urea-type herbicides (DCMU) indicate an interaction of the phenolic C–O- with D1-His215 (Takano et al. 2008). This changes the strength of the hydrogen bond between the CO of QA, with D2-His214 via the ironhistidine bridge, causing the decrease of the QA redox potential. In the presence of urea-type herbicides, a strong CO stretching peak of QA- was observed. Ishikita and Knapp (2005) calculated that a change in a simple hydrogen bond in the vicinity of QA can result in a redox potential shift of about 100 mV. The authors’ calculations treat the protein as completely static and since several experimental observations argue for highly dynamic diversity in conformational states, a sum of different factors are more likely to be the overall reason for different QA redox potentials.
Shift of the energetics of PSII by site-directed mutagenesis Site-directed mutagenesis in the immediate protein environment of QA (Fufezan et al. 2007), Ph (Rappaport et al. 2002, Cser and Vass 2007) and P680 (Vavilin and Vermaas 2000) influences the midpoint potentials of the cofactors and the distribution of electrons between the radiative and non-radiative charge recombination pathways (Fig. 2). The midpoint potential of the QA/QA- redox couple is also modulated by the changes in its immediate protein environment. The exchange of Ala to Ser in the D2 protein at position 249, within the binding pocket of QA, via sitedirected mutagenesis in Thermosynechococcus elongatus shifts the redox potential of QA by approximately 60 mV towards a more negative potential (Fufezan et al. 2007). This mutation led to the acceleration of fluorescence decay kinetics and to enhanced photoinhibition and 1O2 production. This was interpreted as an increase in the indirect pathway of the charge recombination, which leads to a repopulation of the primary charge pair (P680?Ph-) and an increased yield of 3P680 formation. In the A294S mutant, the removal of the water-splitting complex again induces a shift of the midpoint potential of QA, to the high potential form (unpublished results, indicated by the two levels for 3 in Fig. 2). In addition to QA, the redox potentials of P680 and Ph do also influence the charge recombination pathway (Fig. 2). With a series of site-directed mutations in the psbA gene, that codes for the D1 protein, exchanging amino acids at positions D1-Q130 and D1-H198 in the vicinity of Ph or P680, respectively, the redox potentials of these cofactors could be changed in Synechocystis (Merry et al. 1998) and Chlamydomonas (Dorlet et al. 2001; Cuni et al. 2004). These mutants have been studied in detail by Rappaport et al. (2002, 2005) and Cser and Vass (2007) using thermoluminescence and measurements of the decay of variable fluorescence. Mutations that lowered the redox potential of Ph, such as Q130L (indicated as level 1 in Fig. 2) and H198 K, slowed down the overall decay of fluorescence yield, most probably due to the increased energy gap between the radical pairs S2/3QA- and P680?Ph-. In contrast, Q130E (indicated as level 2 in Fig. 2) and H198A have a decreased energy gap and the overall fluorescence yield decay is accelerated compared to the wild type (Rappaport et al. 2002; Cser and Vass 2007). These results show that the free-energy difference between P680* and P680?Ph- affects the recombination kinetics. When the midpoint potential of Ph is more positive in reaction centres with Q130E (about 50 mV, Rappaport et al. 2002), the yield of the indirect non-radiative recombination route is largely increased (Rappaport et al. 2002; Cser and Vass 2007). The decay of P680?Ph-
123
Photosynth Res
via the non-radiative pathway is so fast that the probability of the population of the triplet state of P680?Ph- and thereby the formation of 3P is low. By altering the CD-loop in the D2 subunit in Synechocystis, a change in fluorescence decay was reported by Vavilin and Vermaas (2000). The CD-loop is close to the donor side of PSII and the change in recombination kinetics was taken as a change in the midpoint potential of P680. The midpoint potential of P680 was either decreased in a series of mutants (e.g. C8-2, mutation of residues 187–194 of the D2 protein; indicated as level 6 in Fig. 2) or increased in C7-3 (mutation of residues 179–186 of the D2 protein; indicated as level 7 in Fig. 2). The majority of mutants (e.g. C8-2) showed a temperature shift to lower temperatures and a decrease in the intensity of the thermoluminescence band originating from S2QA- recombination, whereas one mutant (C7-3) showed a shift to higher temperatures and an increase in the thermoluminescence intensity. The former effect was interpreted as an increase of the proportion of the direct recombination pathway via the [P680?QA ] radical pair, which does not lead to the formation of 1P680* and 3P680, while the mutations in C7-3 favoured the radiative pathway. Physiological importance of shifts in the midpoint potential of the QA/QA- redox couple The change in the midpoint potential of QA may be regarded as a key molecular switch for changing the charge recombination pathway within PSII. In PSII with low potential, QA charge recombination via the primary radical pair is favoured, leading to the formation of 3P680 and 1O2. By shifting the midpoint potential of QA from the low to the high potential form and thereby changing the recombination pathway to direct recombination, the formation of 1 O2 can be avoided. Under these conditions, forward electron transfer to QB is thermodynamically disfavoured. This regulation mechanism of PSII may be of physiological importance under the following conditions: in non-photoactivated PSII; during high light conditions, when the capacity of quenching processes in the antenna is saturated; in cold-hardened leaves and in desiccated PSII. Photosystem II is assembled without the Mn cluster and with QA in the high potential form (Johnson et al. 1995). In the light, during the so-called photoactivation process, the Mn cluster is assembled and the midpoint potential of QA is switched from the high potential to the low potential form, which allows linear electron flow. In the state prior to complete assembly of the functional water-splitting complex, PSII is protected against photodamage induced by 1O2 formation by the presence of the high potential form of QA. Under high-light conditions, the change of the midpoint potential of QA may also be involved in the pH-dependent control of the PSII activity. In addition to the dissipation of
123
excess energy in the antenna (Demmig-Adams 1990; Holt et al. 2004; Ruban and Horton 2005; Horton et al. 2008), the electron transfer can be altered at the level of the reaction centre of PSII. In excess light, when the pH in the lumen decreases below a certain threshold value, up to one Ca2? per PSII is released and the midpoint potential of QA is thereby switched to the high potential form (Krieger and Weis 1992). This was demonstrated in spinach thylakoid membranes, in which a proton gradient was maintained by ATP-hydrolysis in the dark, by measuring the Chl fluorescence at the Fo-level at different redox potentials as a measure for the reduction state of QA (Krieger and Weis 1993). In vivo, pea plants grown in an intermittent light regime, and therefore lacking most of the Chla/b binding proteins, show a capacity of non-photochemical quenching, which seems to be related to a reversible release of Ca2? and an upshift in the midpoint potential of QA, as shown by thermoluminescence (Johnson and Krieger 1994). After illumination in the presence of DCMU, these plants emitted thermoluminescence with a temperature maximum at 40– 50"C, the so-called C-band reflecting TyrD?QA recombination (Johnson et al. 1994). The C-band is observed in isolated Ca2?-depleted PSII with QA in the high potential form (Krieger et al. 1993, 1998b). Illumination of the green alga Chlorella vulgaris also induced the appearance of a C-band in the presence of DCMU, which was abolished by the addition of an uncoupler (Krieger et al. 1993), indicating that a proton gradient was built up that induced a reversible inactivation of the water-splitting complex. Figure 3 shows an example of the induction of the C-band after 15-min illumination in pea leaves. Pea leaves were illuminated for 15 min with white light (300 lmol quanta m-2 s-1). The illumination induced a strong quenching of variable fluorescence, which relaxed fast in the dark (Fig. 3, top). This indicates that no photoinhibition took place. In the presence of DCMU, darkadapted leaves show mainly a Q-band (S2QA- recombination; see Rutherford et al. 1982), while after illumination the intensity of the Q-band decreased and the intensity of the Cband increased. This indicates that a part of the PSII centres were converted into a state with QA in the high potential form. The C-band, reflecting inactive PSII, is also found in the margin region and the stroma lamellae of the thylakoid membrane (Andre´e et al. 1998). In the non-granal part of the thylakoid membrane, the C-band may reflect newly assembled PSII prior to the process of photoactivation. Longer illumination times or illumination with higher light intensities cause a decrease in the intensity of both the Q-band and the C-band (Vass et al. 1988; Farineau 1990). Photoinhibition of Chlamydomonas reinhardtii results in the destabilisation of the S2QB- charge pair seen by a down-shift of the B-band (S2,3QB- recombination) in thermoluminescence (Ohad et al. 1988). Severely
Fluorescence, a.u.
Photosynth Res
dark
15 min AL
dark
Thermoluminescence, a.u.
dark
15 min AL
Q-band C-band
Temperature, °C
Fig. 3 Chlorophyll fluorescence and thermoluminescence signals on leaf segments of pea. Chlorophyll fluorescence was measured with a pulse-amplitude modulation fluorometer (PAM 101–3, Walz, Effeltrich). The intensity of the measuring light (standard PAM 101set) was sufficiently low (integral intensity about 10-8 mol quanta m-2 s-1, frequency of modulated light: 1.6 kHz) to prevent net reduction of PQ. Saturating flashes (1 s) were given at intervals of 100 s to measure the maximum fluorescence. As actinic white light (AL), a cold light source was used with an intensity of 300 lmol quanta m-2 s-1. Thermoluminescence was measured with a home-built apparatus (heating rate 0.4"C/ s). Leaves were measured either prior to or after 15 min AL. During cooling from 20"C to the starting temperature, a solution of 1 mM DCMU (in 0.3 M sorbitol) was added to the leaf disc. One saturating flash was given at -5"C (dark) and at 1"C (15 min AL). The sample measured after illumination (15 min AL) was not frozen but only cooled to 1"C to prevent the collapse of the proton gradient. The leaf segments were vacuum-infiltrated with 0.3 M sorbitol prior to the thermoluminecence measurement to permit DCMU to diffuse quickly into the leaf segments
photoinhibited cells exhibit emission at the same temperature as the Q-band. It is unclear whether a C-band could have been observed under these conditions, since the glow curves were recorded only up to 45"C. Cold-hardening of plants seems to cause an alteration of the PSII reactions centre, which may also be correlated with an upshift of the midpoint potential of QA. Briantais and coworkers reported a decrease in the intensity of the B-band and a slight increase in the intensities of the Q-band and the Cband. It was also reported by Sane et al. (2003) that an increase in the midpoint potential of QA is involved in the increased resistance of cold-hardened plants to photoinhibition. Desiccated cyanobacteria in desert crusts show no variable fluorescence (Harel et al. 2004). Upon rehydration, variable fluorescence recovers within minutes, indicating that PSII is present in a non-functional, photoprotected state in the dry crust. Shortly after rehydration, thermoluminescence measurements show the presence of centres, in which the QA to QB electron transfer is blocked (Harel et al. 2004). The same phenomenon is observed in dried chloro- and cyanolichens (Heber et al. 2007; Heber and Krieger-Liszkay, unpublished). Experiments are needed to show whether fluorescence quenching and the inhibition of the QA to QB electron transfer is related to a change in the midpoint potential of QA in photosynthetic organisms, which survive desiccation. Taken together, an upshift of the midpoint potential of QA occurs under physiologically relevant stress conditions, and may help to protect the reaction centre against 1O2 generation and subsequent degradation of the D1 protein by changing the probability of the indirect charge recombination pathway in favour to the direct charge recombination pathway. Cyanobacteria have developed an additional strategy to support high-light stress and to lower the probability of dangerous charge recombination reactions within PSII. They have different genes for the D1 protein that are expressed under different stress conditions (Clarke et al. 1993; Tichy´ et al. 2003), and the different forms of D1 have different amino acids at position D1-130. Sequences expressed in low light have D1-Q130, whereas sequences expressed in high light have D1-E130 (Ko´s et al. 2008). The presence of D1-E130 in the high-light D1 isoform is probably related to its effect on the redox potential of Ph, which enhances the charge recombination via the nonradiative pathway and thereby lowers the yield of 1O2 generation (Ko´s et al. 2008; see I.3 and Fig. 2). In all higher plants, a glutamate occupies the position D1-130 as in the high-light sequence of the cyanobacteria. Photoinhibition of PSII and turnover of the D1 protein As described above, it is assumed by most authors (Prasil et al. 1992; Aro et al. 1993; Vass and Aro 2007) that
123
Photosynth Res
123
photochemistry and 1O2 formation, but is caused by light absorption by the Mn cluster and a release of Mn (see Tyystja¨rvi 2008). This mechanism has been shown to occur under UV illumination (Vass et al. 1996). In the visible light, however, Mn absorption bands are very weak, and it is an ongoing debate whether release of Mn is a primary event in photoinhibition in the visible light. Figure 4 shows that the orange light (600–650 nm) induces a faster decrease in PSII activity than the blue-green light (400– 550 nm) in cyanobacteria DOCP strain, which is deficient in non-photochemical quenching, (Wilson et al. 2006). In this strain, quenching of variable fluorescence is correlated with photoinhibition and not with fast relaxing non-photochemical quenching. In cyanobacterial PSII, the bluegreen light is absorbed by the small inner Chl antenna (CP43 and CP47), while the orange light is absorbed by the large extramembranal antenna of PSII, the phycobilisome. At the same light intensity, the orange light induces the closure of more reaction centers than the blue-green light, generating more 1O2, whereas the blue-green light would be expected to damage more Mn clusters. Mn absorbs more light in the range of 400 to 550 nm than in the range of 600 to 650 nm. This indicates that mechanisms other than the absorption of Mn are responsible for photoinhibition in visible light. These other mechanisms are obviously linked to PSII photochemistry.
100
(400-550 nm) 90 80
Fv (% of initial)
photoinhibition and the degradation of the D1 protein is triggered by 1O2. It has been demonstrated clearly by Ohad and coworkers that photoinhibition induced by single turnover visible light flashes was correlated with the charge recombination reactions between QB- or QA and the S2,3 state of the water-splitting complex (Keren et al. 1995, 1997, 2000). This was the case for both the light-induced loss of PSII activity and the loss of the D1 protein. In a more recent study, it was shown that indeed 1O2 was produced by the light flashes, which generated S2,3QBstates (Szilard et al. 2005). PSII is protected against flashinduced photoinhibition by an inactive water-splitting complex and QA in its high potential form (Keren et al. 2000). The hypothesis that 1O2 is the reactive oxygen species responsible for photoinhibition and the triggering of D1 protein degradation is supported by the finding of several groups that, herbicides which modify the midpoint potential of QA, either stimulate or suppress photoinhibition. The urea derivative DCMU and the related herbicides have been reported to retard photodamage (e.g. Kirilovsky et al. 1994; Keren et al. 1995, 1997) and degradation of the D1 protein (e.g. Keren et al. 1995, 1997; Nakajima et al. 1996; Jansen et al. 1993). In contrast to DCMU, phenolic herbicides have the opposite effect and stimulate the susceptibility of PSII to light (Pallet and Dodge 1980; Nakajima et al. 1996) and the degradation of D1 (e.g. Jansen et al. 1993). All measurements on the effects of herbicides on the degradation of D1 were performed in the presence of the protein synthesis inhibitors chloramphenicol or lincomycin. The oxidation of specific amino acid residues by 1O2 seems to be involved in photoinhibition. Mass spectrometry of the isolated PSII reaction centre proteins after photoinhibitory illumination showed successive oxidation of the D1 protein and to a lesser extent of the D2 protein by an increase in mass by the appropriate amount of additional oxygen atoms (Sharma et al. 1997). However, the relevance of these experiments to photoinhibition in vivo is not clear. 1 O2 seems to be not only responsible for a loss of PSII activity and the triggering of D1 protein degradation, but also for inhibiting the translation elongation of the D1 protein (Nishiyama et al. 2004). Murata and coworkers even postulated that the effect of 1O2 on the resynthesis of the D1 protein is the only important effect (e.g. Nishiyama et al. 2004; Takahashi and Murata 2008). They did not observe a loss of PSII activity in the presence of 1O2 producing photosensitizers and concluded that 1O2 does not damage PSII. Hideg et al. (2007), however, reported recently that 1O2 produced by the photosensitizer Rose Bengal did cause damage to PSII. It is also discussed in the literature that the primary event in photoinhibition is not directly due to PSII
70
(600-650 nm)
60 50 40
0
200
400
600
800
1000
Time (sec)
Fig. 4 The decrease in photosystem II activity during exposure of cells to high light intensities. Cells of a mutant of Synechocystis PCC 6803 lacking the NPQ mechanism (DOCP, Wilson et al. 2006) were illuminated with 1,100 lmol photons m-2 s-1 of blue-green light (400–550 nm) (open) or orange light (600–650 nm) (triangles). The decrease of variable fluorescence (Fv = Fm - Fo) was followed using a PAM fluorometer. Fo remained constant and Fm decreased during the illumination. Saturating pulses (1 s) were applied every 30 s to asses the maximal fluorescence. It is well established that the loss of variable fluorescence corresponds to a loss in PSII activity (see e.g. Kirilovsky et al. 1988). The data are provided by courtesy of Diana Kirilovsky
Photosynth Res
Independent of the molecular basis of photoinhibition, the D1 protein is degraded and the pigments are released. Free Chls will act as photosensitizers and will produce high amounts of 1O2. This can be seen when 1O2 is measured by spin trapping EPR in PSII preparations or thylakoid membranes. Up to a certain light intensity and illumination time, the amount of detected 1O2 increases linearly with the light intensity (Fufezan et al. 2002). At very high light intensities or prolonged illumination, the generation of 1O2 increases exponentially with time. This effect is probably caused by the free Chl liberated from its protein environment. In vivo, special proteins such as the early lightinduced proteins (ELIPS) (Heddad et al. 2006) or watersoluble chlorophyll binding proteins (WSCP) are expressed upon exposure of leaves to high light. ELIPs and WSCP bind free Chls and Chl derivates, and thereby prevent the formation of 3Chl and 1O2, as was shown for the WSCP from cauliflower leaves (Schmidt et al. 2003). The D1 protein is degraded rapidly (i.e. rapid turnover), resynthesised and new functional PSII is reassembled (Prasil et al. 1992; Aro et al. 1993; Nixon et al. 2005; Vass and Aro 2007). The turnover of the D1 protein involves numerous steps—the disassembly of PSII after moving it outside the grana stacks followed by protein degradation, protein synthesis and loading of the reaction centre with cofactors and pigments during the reassembly process. The latter involves active, concurrent and rapid biosynthesis of b-carotene (Trebst and Depka 1997; Depka et al. 1998), plastoquinone (Kruk and Trebst 2008) and Chl (Vavilin and Vermaas 2007). All these steps have to be well controlled. Photoinhibition and degradation of the D1 protein may also be regarded as a predetermined breaking point to avoid uncontrollable photo-oxidative damage. By damaging one specific protein subunit with a limited number of cofactors, the damage is well restricted. Photosynthetic electron transfer is disrupted and thereby the danger of further ROS production contained, as long as the degradation process and the fate of the cofactors stay controlled. The expense of the D1 breakdown, viewed as a physiological sacrifice, appears excessive. However, the extent and investment in protein and cofactor biosynthesis should not be overestimated. A net loss of PSII does not occur at low light intensities. In high light conditions, photosynthetic electron transport is not the limiting step of photosynthesis. The photosynthetic electron transport supplies the sufficient reducing power and ATP. Under continuous high light, not only is the size of the LHCII-antenna reduced but also is the number of reaction centres. In shade leaves, there are more PSII reaction centres relative to PSI to compensate for the additional far-red light (Walters 2005). Following this argumentation, the D1 protein degradation is both a fast response to high light stress as well as part of a
necessary reduction in the amount of PSII, which has to be adjusted in response to a changed environment in longterm acclimation. It is suggested that, during acclimation, 1 O2 is not only involved in photoinhibition and D1 degradation but may also be the signal in turning on gene expression and enzyme activation reactions required for resynthesis of the components of PSII maintenance (see below). Photosynthetic organisms, in contrast to Escherichia coli and yeast, are able to acclimate to 1O2, as has been shown recently for C. reinhardtii (Ledford et al. 2007).
Role of tocopherol and plastoquinone as 1O2 scavengers A rapid turnover of low molecular weight components is correlated with D1 protein turnover. Tocopherols and hydroquinones are well known to act as singlet oxygen scavengers (Neely et al.1988; Kruk et al. 1994; Hundal et al. 1995; see further quotes in Kruk and Trebst 2008). In scavenging, in contrast to quenching, the protecting substrate is irreversibly oxidised. In vivo scavenging means that the protective component has to be resynthesised. 1O2 oxidises tocopherol to tocopherylquinone by an irreversible opening of the chromanol ring. Other ROS such as superoxide, however, can be removed by tocopherol via the formation of the tocopheryl radical that can be re-reduced by, for example, ascorbate. The oxidation mechanism in the reaction of hydroquinones with 1O2 is more complex. The role of tocopherol and plastoquinol in the protection of PSII was only recently recognised. A rapid turnover of both a-tocopherol and plastoquinone was observed and had been correlated to the conditions of the D1 protein turnover in high light stress in C. reinhardtii (Trebst 2003; Kruk et al. 2005; Krieger-Liszkay and Trebst 2006; Kruk and Trebst 2008). As chloramphenicol or lincomycin is used to measure the D1 protein degradation rate by blocking the compensating protein resynthesis in the recovery phase of the rapid protein turnover, inhibitors of the biosynthetic pathway for plastoquinone and tocopherols were used to prevent resynthesis when their pools became exhausted in high light. These inhibitors are herbicides, such as isoxaflutole and pyrazolate (see handbooks in weed science), which interfere with a common intermediate step in the biosynthesis of both plastoquinone and tocopherols. The target of these herbicides is the HPP-dioxygenase. This enzyme converts, by oxidation and rearrangement, an hydroxyphenylpyruvate (HPP) derived from tyrosine or aroginate to homogentisate. In the unblocked pathway, homogentisic acid is decarboxylated and isoprenylated to the respective sidechains, appropriately methylated and, in the case of tocopherol, the chromanol ring is closed (see textbooks).
123
Photosynth Res
On incubation of C. reinhardtii in steady state light with these dioxygenase inhibitors, followed by high light, the degradation rates of both tocopherols and plastoquinone are measurable. The pools are exhausted in about 2 h of illumination with high light intensities. The rate of tocopherol and plastoquinol (remember that in high light the plastoquinone pool is largely reduced) turnover in high light (1,500 lmol quanta m-2 s-1) is equivalent to the rate of D1 protein degradation. The calculated turnover rate in high light for plastoquinone A (different for plastoquinone C, see Kruk and Trebst 2008) is in the order of 0.23 nmol h-1 ml-1 cell culture with 18 lmol Chl and 0.11 nmol h-1 ml-1 for a-tocopherol (Kruk and Trebst 2008). This is indeed a rapid turnover of these two compounds when 1O2 is generated in PSII. The role of tocopherol in the protection of PS II has been confirmed by Havaux et al. (2000) and by Golan et al. (2006). However, acclimation of Chlamydomonas to 1O2 stress does not alter the tocopherol content of the cells (Ledford et al. 2007). In spite of the described mechanisms for the fate of singlet oxygen (carotene quenching and scavenging by tocopherol and plastoquinol, reaction with the D1 protein), there is evidence that some 1O2 still escapes. 1O2 is an important signal from the chloroplast to gene expression systems inside and outside the chloroplast.
The role of singlet oxygen in the regulation of gene expression The significance of ROS and singlet oxygen signalling is increasingly appreciated (Beck 2005; Laloi et al. 2006, 2007; Kim et al. 2008). 1O2 is not only produced by charge recombination processes in the PSII reaction centre, but also by the accumulation of precursors in the pathway to heme and Chl accumulate. Tetrapyrrols play an important role in chloroplast-to-nucleus retrograde signalling (von Gromoff et al. 2008). In particular, experiments by Apel and coworkers with flu mutants deficient in Chl biosynthesis demonstrate clearly that 1O2 induces extended changes in the gene expression pattern (op den Camp et al. 2003). In these mutants, precursors of Chl biosynthesis such as protochlorophyllide, accumulate and act as photosensitizers to produce 1O2. Apel and coworkers showed the involvement of two plastid-localised proteins EXECUTER1 and EXECUTER2 in the genetic response to 1 O2, indicating the generation of a more stable second messenger within the chloroplast (Lee et al. 2007). Despite the signalling event induced by 1O2 inside the chloroplast, 1 O2 also acts as a signal outside the chloroplast. In Chlamydomonas, the expression of the nucleus-encoded GPXH gene, a glutathione reductase homologue, was induced by the water-soluble photosensitizer Rose Bengal, which
123
produces 1O2 (Leisinger et al. 2001; Fischer et al. 2006, 2007). The induction of the GPXH gene was higher in low light in the presence of Rose Bengal than that with high light in the presence of a phenolic herbicide, even though high light in the presence of a phenolic herbicide resulted in a higher amount of 1O2, even outside the thylakoid membrane (Fischer et al. 2007). By spin trapping EPR with a hydrophilic and a lipophilic spin trap and fluorescence microscopy, it was shown that PSII-generated 1O2 is detectable in the cytosol (Fischer et al. 2007). 1O2 can either diffuse or is regenerated in the cytosol be the reaction of lipidperoxides. In Chlamydomonas, a nuclearencoded reporter gene construct, which is responsive to 1 O2, shows a similar induction response to GPXH (Shao et al. 2007). The soluble photosensitizer methylene blue produces less 1O2 in the light than the membrane-localised photosensitizer protoporphyrin IX or high-light illumination in the presence of phenolic herbicides, whereas the induction of the reporter gene is the highest in the presence of methylene blue, followed by protoporphyrin IX and, last, high light and phenolic herbicide (Table 1). These data indicate that signalling events not only take place inside the chloroplast but also outside the chloroplast. Under natural conditions in wild-type plants, PSII-generated 1O2 will act as a signal both inside and outside the chloroplast (see EXECUTER), to activate, via changes in the expression levels of responsive genes, defence against extreme photo-oxidative stress, when other protection mechanisms are exhausted. This will only be the case when the production of 1O2 overwhelms the capacity of the lipidsoluble antioxidants (carotene, tocopherol and plastoquinone) in the thylakoid membrane. Recently, acclimation of Chlamydomonas to 1O2 was investigated (Ledford et al. 2007). Microarray analysis of the RNA levels showed that only a small number of genes responded to sublethal levels of 1O2. The increased expression of GPXH or GSTS1 (a glutathione S-transferase) was sufficient to enhance the 1O2 resistance. Table 1 Activation of the expression of a 1O2-responsive reporter gene by different photosensitizers in Chlamydomonas reinhardtii. Data are taken mainly from Shao et al. (2007) Photosensitizer
1
O2 (norm.)
Likely cellular location of 1 O2 formation
Methylene blue
1
Cytosol
4
Protoporphyrin IX
3
Chloroplast envelope membrane
2
HL ? DCMU
1
PSII
0.75
HL ? phenolic herbicides
2.75
PSII
1.5
Fold increase in expression level of reporter gene
Photosynth Res
Acclimation to 1O2 did not alter the composition or content of carotenoids or tocopherol. It remains still unclear how 1O2 can act as a signal, how it is perceived and how, finally, the expression of specific genes is enhanced. One can envisage the following options: 1 O2 could directly modify a protein sensor; alternatively, a product of the reaction of 1O2 with lipids, pigments or proteins could interact with a sensor of 1O2; 1O2 could also modify the redox state of the cell via the production of lipidperoxides and other peroxides, which have to be detoxified in reactions using glutathione as substrate. A change in the redox state may then activate a signalling process. Acknowledgements We would like to thank Diana Kirilovsky (CEA Saclay) for providing the data for Fig. 4 and Bill Rutherford, Arezki Sedoud (both CEA Saclay) and Giles Johnson (University of Manchester) for scientific discussions and critical reading of the manuscript. Financial support by the Alexander von Humboldt foundation to C.F. and by the Deutsche Forschungsgemeinschaft via SFB 480 to AT is gratefully acknowledged.
References Adir N, Zer H, Shochat S, Ohad I (2003) Photoinhibition—a historical perspective. Photosynth Res 76:343–370. doi:10.1023/ A:1024969518145 Anderson JM, Osmond CB (1987) Shade-sun responses: compromises between acclimation and photoinhibition. In: Kyle DJ, Osmond CB, Arntzen CJ (eds) Photoinhibition. Elesevier, Amsterdam, pp 1–36 Andre´asson LE, Vass I, Styring S (1995) Ca2?-depletion modifies the electron transfer on both donor and acceptor sides in photosystem II. Biochim Biophys Acta 1230:155–164. doi:10.1016/00052728(95)00047-M Andre´e S, Weis E, Krieger A (1998) Heterogeneity and photoinhibition of photosystem II studied by thermoluminescence. Plant Physiol 116:1053–1061. doi:10.1104/pp.116.3.1053 Aro EM, Virgin I, Andersson B (1993) Photoinhibition of photosystem II. Inactivation, protein damage and turnover. Biochim Biophys Acta 1143:113–134. doi:10.1016/0005-2728(93)90134-2 Beck CF (2005) Signaling pathways from the chloroplast to the nucleus. Planta 222:743–756. doi:10.1007/s00425-005-0021-2 Briantais JM, Ducruet JM, Hodges M, Krause GH (1992) Effects of high light at chilling temperature on photosystem II in spinach leaves. Photosynth Res 31:1–10. doi:10.1007/BF00049531 ¨ quist G (1993) Rapid Clarke AK, Soitamo A, Gustafsson P, O interchange between two distinct forms of cyanobacterial photosystem II reaction-center protein D1 in response to photoinhibition. Proc Natl Acad Sci USA 90:9973–9977. doi: 10.1073/pnas.90.21.9973 Cser K, Vass I (2007) Radiative and non-radiative charge recombination pathways in photosystem II studied by thermoluminescence and chlorophyll fluorescence in the cyanobacterium Synechocystis 6803. Biochim Biophys Acta 1767:233–243. doi:10.1016/j. bbabio.2007.01.022 Cuni A, Xiong L, Sayre RT, Rappaport F, Lavergne J (2004) Modification of the pheophytin midpoint potential in photosystem II: modulation of the quantum yield of charge separation and charge recombination pathways. Phys Chem Chem Phys 6:4825– 4831. doi:10.1039/b407511k
Demmig-Adams B (1990) Carotenoids and photoprotection in plants: a role of the xanthophyll zeaxanthin. Biochim Biophys Acta 1020:1–24. doi:10.1016/0005-2728(90)90088-L Depka B, Jahns P, Trebst A (1998) Beta-carotene to zeaxanthin conversion in the rapid turnover of the D1 protein of photosystem II. FEBS Lett 424:267–270. doi:10.1016/S0014-5793(98) 00188-4 Diner BA, Schlodder E, Nixon PJ, Coleman WJ, Rappaport F, Lavergne J et al (2001) Site-directed mutations at D1-His198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization. Biochemistry 40:9265–9281. doi:10.1021/ bi010121r Dorlet P, Xiong L, Sayre RT, Un S (2001) High field EPR study of the pheophytin anion radical in wild type and D1-E130 mutants of photosystem II in Chlamydomonas reinhardtii. J Biol Chem 276:22313–22316. doi:10.1074/jbc.M102475200 Durrant JR, Giorgi LB, Barber J, Klug DR, Porter G (1990) Characterization of triplet-states in isolated photosystem II reaction centres—oxygen quenching as a mechanism for photodamage. Biochim Biophys Acta 1017:175–176 Eckhardt U, Grimm B, Ho¨rtensteiner S (2004) Recent advances in chlorophyll biosynthesis and breakdown in higher plants. Plant Mol Biol 56:1–14. doi:10.1007/s11103-004-2331-3 Edge R, Truscott TG (1999) Carotenoid radicals and the interaction of carotenoids with active oxygen species. In: Frank HA, Young AJ, Britton D, Cogdell RJ (eds) Advances in photosynthesis: the photochemistry of carotenoids, vol 8. Kluwer, Dordrecht, pp 223–234 Faller P, Fufezan C, Rutherford AW (2006) Side-path electron donors: cytochrome b559, chlorophyll Z and b-carotene. In: Wydrzynski T, Satoh K (eds) Photosystem II: the water/plastoquinone oxidoreductase in photosynthesis. Kluwer, Dordrecht chap. 15 Farineau J (1990) Photochemical alterations of photosystem II induced by two different photoinhibitory treatments in isolated chloroplasts in pea. A thermoluminescence study. Biochim Biophys Acta 1016:357–363. doi:10.1016/0005-2728(90)90169-5 Ferreira KN, Iverson TM, Maghlaoui K, Barber J, Iwata S (2004) Architecture of the photosynthetic oxygen-evolving centre. Science 303:1831–1838. doi:10.1126/science.1093087 Fischer BB, Eggen RI, Trebst A, Krieger-Liszkay A (2006) The glutathione peroxidase homologous gene Gpxh in Chlamydomonas reinhardtii is upregulated by singlet oxygen produced in photosystem II. Planta 223:583–590. doi:10.1007/s00425-0050108-9 Fischer BB, Krieger-Liszkay A, Hideg E, Snyrychova´ I, Wiesendanger M, Eggen RI (2007) Role of singlet oxygen in chloroplast to nucleus retrograde signaling in Chlamydomonas reinhardtii. FEBS Lett 581:5555–5560. doi:10.1016/j.febslet.2007.11.003 Fufezan C, Rutherford AW, Krieger-Liszkay A (2002) Singlet oxygen production in herbicide-treated photosystem II. FEBS Lett 532:407–410. doi:10.1016/S0014-5793(02)03724-9 Fufezan C, Gross CM, Sjo¨din M, Rutherford AW, Krieger-Liszkay A, Kirilovsky D (2007) Influence of the redox potential of the primary quinone electron acceptor on photoinhibition in photosystem II. J Biol Chem 282:12492–12502. doi:10.1074/jbc. M610951200 Golan T, Mu¨ller-Moule´ P, Niyogi KK (2006) Photoprotection mutants of Arabidopsis thaliana acclimate to high light by increasing photosynthesis and specific antioxidants. Plant Cell Environ 29:879–887. doi:10.1111/j.1365-3040.2005.01467.x Goussias C, Boussac A, Rutherford AW (2002) Photosystem II and photosynthetic oxidation of water: an overview. Philos Trans R Soc Lond B 57:1369–1381. doi:10.1098/rstb.2002.1134 Halliwell B, Gutteridge JMC (1999) Free radicals in biology and medicine. University Press, Oxford, UK
123
Photosynth Res Hanley J, Deligiannakis Y, Pascal A, Faller P, Rutherford AW (1999) Carotenoid oxidation in photosystem II. Biochemistry 38:8189– 8195. doi:10.1021/bi990633u Harel Y, Ohad I, Kaplan A (2004) Activation of photosynthesis and resistance to photoinhibition in cyanobacteria within biological desert crust. Plant Physiol 136:3070–3079. doi:10.1104/pp. 104.047712 Hatz S, Lambert JDC, Ogilby PR (2007) Measuring the lifetime of singlet oxygen in a single cell: addressing the issue of cell viability. Photochem Photobiol Sci 6:1106–1116. doi: 10.1039/b707313e Havaux M, Bonfits J-P, Lu¨tz C, Niyogi KK (2000) Photodamage of the photosynthetic apparatus and its dependence on the leaf development stage in the npq1 Arabidopsis mutant deficient in the xanthophylls cyle enzyme violaxanthin de-epoxidase. Plant Physiol 124:273–284. doi:10.1104/pp.124.1.273 Heber U, Azarkovich M, Shuvalov V (2007) Activation of mechanisms of photoprotection by desiccation and by light: poikilohydric photoautotrophs. J Exp Bot 58:2745–2759. doi: 10.1093/jxb/erm139 Heddad M, Nore´n H, Reiser V, Dunaeva M, Andersson B, Adamska I (2006) Differential expression and localization of early lightinduced proteins in Arabidopsis. Plant Physiol 142:75–87. doi: 10.1104/pp.106.081489 Hideg E, Spetea C, Vass I (1994) Singlet oxygen production in thylakoid membranes during photoinhibition as detected by EPR spectroscopy. Photosynth Res 39:191–199. doi:10.1007/BF000 29386 Hideg E, Ogawa K, Kalai T, Hideg K (2001) Singlet oxygen imaging in Arabidopsis thaliana leaves under photoinhibition by excess photosynthetically active radiation. Physiol Plant 112:10–14. doi:10.1034/j.1399-3054.2001.1120102.x Hideg E, Barta C, Kalai T, Vass I, Hideg K, Asada K (2002) Detection of singlet oxygen and superoxide with fluorescent sensors in leaves under stress by photoinhibition or UV radiation. Plant Cell Physiol 43:1154–1164. doi:10.1093/pcp/ pcf145 Hideg E, Ko´s PB, Vass I (2007) Photosystem II damage induced by chemically generated singlet oxygen in tobacco leaves. Physiol Plant 131:33–40. doi:10.1111/j.1399-3054.2007.00913.x Holt NE, Fleming GR, Niyogi KK (2004) Towards an understanding of the mechanism of non-photochemical quenching in green plants. Biochemistry 43:8281–8289. doi:10.1021/bi0494020 Horton P, Ruban A (2005) Molecular design of the photosystem II light-harvesting antenna: photosynthesis and photoprotection. J Exp Bot 56:365–373. doi:10.1093/jxb/eri023 Horton P, Johnson MP, Perez-Bueno ML, Kiss AZ, Ruban AV (2008) Photosynthetic acclimation: does the dynamic structure and macro-organisation of photosystem II in higher plant grana membranes regulate light harvesting states? FEBS J 275:1069– 1079. doi:10.1111/j.1742-4658.2008.06263.x Hundal T, Forsmark-Andree P, Ernster L, Andersson B (1995) Antioxidant activity of reduced plastoquinone in chloroplast thyalkoid membranes. Arch Biochem Biophys 324:117–122. doi:10.1006/abbi.1995.9920 Ishikita H, Knapp EW (2005) Control of quinone redox potentials in photosystem II: electron transfer and photoprotection. J Am Chem Soc 127:14714–21470. doi:10.1021/ja052567r Jansen MA, Depka B, Trebst A, Edelman M (1993) Engagement of specific sites in the plastoquinone niche regulates degradation of the D1 protein in photosystem II. J Biol Chem 268:21246– 21252 Johnson G, Krieger A (1994) Thermoluminescence as a probe of photosystem II in intact leaves: non-photochemical fluorescence quenching in peas grown in an intermittent light regime. Photosynth Res 41:371–379. doi:10.1007/BF02183039
123
Johnson GN, Boussac A, Rutherford AW (1994) The origin of the 40– 50"C thermoluminescence bands in photosystem II. Biochim Biophys Acta 1184:85–92. doi:10.1016/0005-2728(94)90157-0 Johnson G, Rutherford AW, Krieger A (1995) A change in the midpoint potential of the quinone QA in photosystem II associated with photoactivation of oxygen evolution. Biochim Biophys Acta 1229:202–207. doi:10.1016/0005-2728(95) 00003-2 Kamlowski A, Frankemo¨ller I, van der Est A, Stehlik D, Holzwarth AR (1996) Evidence for the delocalization of the triplet state 3 P680 in the D1D2cytb559-complex of photosystem II. Ber Bunsen-Ges 100:2045–2051 Kautsky H, de Bruijn (1931) Die Aufkla¨rung der Photoluminescenztilgung fluorescierender Systeme durch Sauerstoff: Die Bildung aktiver, diffusionsfa¨higer Sauerstoffmoleku¨le durch Sensibilisierung. Naturwissenschaften 19:1043. doi:10.1007/BF01516190 Keren N, Gong H, Ohad I (1995) Oscillations of reaction centre II-D1 protein degradation in vivo induced by repetitive flashes. J Biol Chem 270:806–814. doi:10.1074/jbc.270.24.14611 Keren N, Berg A, van Kan PJM, Levanon H, Ohad I (1997) Mechanism of photosystem II inactivation and D1 protein degradation at low light intensities: the role of electron back flow. Proc Natl Acad Sci USA 94:1579–1584. doi:10.1073/ pnas.94.4.1579 Keren N, Ohad I, Drepper F, Rutherford AW, Krieger-Liszkay A (2000) Inhibition of photosystem II activity by saturating single turnover flashes in calcium-depleted and active photosystem II. Photosynth Res 63:209–216. doi:10.1023/A:1006435530817 Kim C, Meskauskiene R, Apel K, Laloi C (2008) No single way to understand singlet oxygen signalling in plants. EMBO Rep 9:435–439. doi:10.1038/embor.2008.57 Kirilovsky D, Vernotte C, Astier C, Etienne AL (1988) Reversible and irreversible photoinhibition in herbicide-resistant mutants of Synechocystis 6714. Biochim Biophys Acta 933:124–131. doi: 10.1016/0005-2728(88)90062-X Kirilovsky D, Rutherford AW, Etienne AL (1994) Influence of DCMU and ferricyanide on photodamage in photosystem II. Biochemistry 33:3087–3095. doi:10.1021/bi00176a043 Ko´s PB, Dea´k Z, Cheregi O, Vass I (2008) Differential regulation of psbA and psbD gene expression, and the role of the different D1 protein copies in the cyanobacterium Thermosynechococcus elongatus BP-1. Biochim Biophys Acta 1777:74–83. doi: 10.1016/j.bbabio.2007.10.015 Krieger A, Rutherford AW (1997) Comparison of chloride-depleted and calcium-depleted PSII: the midpoint potential of QA and susceptibility to photodamage. Biochim Biophys Acta 1319:91– 98. doi:10.1016/S0005-2728(96)00117-X Krieger A, Weis E (1992) Energy-dependent quenching of chlorophyll-a-fluorescence: the involvement of a proton-calcium exchange at photosystem II. Photosynthetica 27:89–98 Krieger A, Weis E (1993) The role of calcium in the pH-dependent control of photosystem II. Photosynth Res 37:117–130. doi: 10.1007/BF02187470 Krieger A, Weis E, Demeter S (1993) Low pH-induced Ca ion release in the water-splitting system is accompanied by a shift in the midpoint redox potential of the primary quinone acceptor QA. Biochim Biophys Acta 1144:411–418. doi:10.1016/0005-2728 (93)90128-3 Krieger A, Rutherford AW, Johnson GN (1995) On the determination of the redox mid-point potential of the primary quinone acceptor, QA, in photosystem II. Biochim Biophys Acta 1229:193–201. doi:10.1016/0005-2728(95)00002-Z Krieger A, Rutherford AW, Vass I, Hideg E (1998a) Relationship between activity, D1 loss, and Mn binding in photoinhibition of photosystem II. Biochemistry 37:16262–16269. doi:10.1021/ bi981243v
Photosynth Res Krieger A, Rutherford AW, Jegerscho¨ld C (1998b) Thermoluminescence measurements on chloride-depleted and calcium-depleted photosystem II. Biochim Biophys Acta 1364:46–54. doi: 10.1016/S0005-2728(98)00009-7 Krieger-Liszkay A (2005) Singlet oxygen production in photosynthesis. J Exp Bot 56:337–346. doi:10.1093/jxb/erh237 Krieger-Liszkay A, Rutherford AW (1998) Influence of herbicide binding on the redox potential of the quinone acceptor in photosystem II: relevance to photodamage and phytotoxicity. Biochemistry 37:17339–17344. doi:10.1021/bi9822628 Krieger-Liszkay A, Trebst A (2006) Tocopherol is the scavenger of singlet oxygen produced by the triplet state of chlorophyll in the PSII reaction centre. J Exp Bot 57:1677–1684. doi:10.1093/ jxb/erl002 Kruk J, Trebst A (2008) Plastoquinol as a singlet oxygen scavenger in photosystem II. Biochim Biophys Acta 1777:154–162. doi: 10.1016/j.bbabio.2007.10.008 Kruk J, Strzałka, Schmid GH (1994) Antioxidant properties of plastoquinol and other biological prenylquinols in liposomes qnd solution. Free Radic Res 21:409–416. doi:10.3109/1071576940 9056593 Kruk J, Holla¨nder-Czytko H, Oettmeier W, Trebst A (2005) Tocopherol as singlet oxygen scavenger in photosystem II. J Plant Physiol 162:749–757. doi:10.1016/j.jplph.2005.04.020 Laloi C, Przybyla D, Apel K (2006) A genetic approach towards elucidating the biological activity of different reactive oxygen species in Arabidopsis thaliana. J Exp Bot 57:1719–1724. doi: 10.1093/jxb/erj183 Laloi C, Stachowiak M, Pers-Kamczyc E, Warzych E, Murgia I, Apel K (2007) Cross-talk between singlet oxygen- and hydrogen peroxide-dependent signaling of stress responses in Arabidopsis thaliana. Proc Natl Acad Sci USA 104:672–677. doi:10.1073/ pnas.0609063103 Ledford HK, Chin BL, Niyogi KN (2007) Acclimation to singlet oxygen stress in Chlamydomonas reinhardtii. Eukaryot Cell 6:919–930. doi:10.1128/EC.00207-06 Lee KP, Kim C, Landgraf F, Apel K (2007) EXECUTER1- and EXECUTER2-dependent transfer of stress-related signals from the plastid to the nucleus of Arabidopsis thaliana. Proc Natl Acad Sci USA 104:10270–10275. doi:10.1073/pnas.0702061104 Leisinger U, Ru¨fenacht K, Fischer B, Pesaro M, Spengler A, Zehnder AJB et al (2001) The glutathione peroxidase homologous gene from Chlamydomonas reinhardtii is transcriptionally upregulated by singlet oxygen. Plant Mol Biol 46:395–408. doi: 10.1023/A:1010601424452 Loll B, Kern J, Saenger W, Zouni A, Biesieadka J (2005) Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II. Nature 438:1040–1044. doi:10.1038/nature 04224 Macpherson AN, Telfer A, Truscott TG, Barber J (1993) Direct detection of singlet oxygen from isolated photosystem II reaction centres. Biochim Biophys Acta 1143:301–309. doi:10.1016/ 0005-2728(93)90201-P Matile P, Hortensteiner S, Thomas H (1999) Chlorophyll degradation. Annu Rev Plant Physiol Plant Mol Biol 50:67–95. doi: 10.1146/annurev.arplant.50.1.67 Merry SAP, Nixon PJ, Barter LMC, Schilstra MJ, Porter G, Barber J et al (1998) Modulation of quantum yield of primary radical formation in photosystem II by site-directed mutagenesis affecting radical cations and anions. Biochemistry 37:17439– 17447. doi:10.1021/bi980502d Mizusawa N, Miyao M, Yamashita T (1997) Restoration of the high potential from of cytochrome b559 of photosystem II occurs via a two-step mechanism under illumination in the presence of manganese ions. Biochim Biophys Acta 1318:145–148. doi: 10.1016/S0005-2728(96)00130-2
Murchie EH, Hubbart S, Peng S, Horton P (2005) Acclimation of photosynthesis to high irradiance in rice: gene expression and interactions with leaf development. J Exp Bot 56:449–460. doi: 10.1093/jxb/eri100 Nakajima Y, Yoshida S, Ono T (1996) Differential effects of urea/ triazine-type and phenol-type photosystem II inhibitors on inactivation of the electron transport and degradation of the D1 protein during photoinhibition. Plant Cell Physiol 37:673–680 Neely WC, Martin M, Barker SA (1988) Products and relative reaction rates of the oxidation of tocopherols with singlet molecular oxygen. Photochem Photobiol 48:423–428. doi: 10.1111/j.1751-1097.1988.tb02840.x Nishiyama Y, Allakhverdiev SI, Yamamoto H, Hayashi H, Murata N (2004) Singlet oxygen inhibits the repair of photosystem II by suppressing the translation elongation of the D1 protein in Synechocystis sp. PCC 6803. Biochemistry 43:11321–11330. doi:10.1021/bi036178q Nixon PJ, Barker M, Boehm M, de Vries R, Komenda J (2005) FtsHmediated repair of the photosystem II complex in response to light stress. J Exp Bot 56:357–363. doi:10.1093/jxb/eri021 Noguchi T, Tomo T, Kato C (2001) Fourier transform infrared study of the cation radical of P680 in the photosystem II reaction center: evidence for charge delocalization on the chlorophyll dimer. Biochemistry 40:2176–2185. doi:10.1021/bi0019848 Ohad I, Koike H, Shochat S, Inoue Y (1988) Changes in the properties of reaction center II during the initial stages of photoinhibition as revealed by thermoluminescence measurements. Biochim Biophys Acta 933:288–298. doi:10.1016/ 0005-2728(88)90036-9 op den Camp RG, Przybyla D, Ochsenbein C, Laloi C, Kim C, Danon A, Wagner D, Hideg E, Go¨bel C, Feussner I, Nater M, Apel K (2003) Rapid induction of distinct stress responses after the release of singlet oxygen in Arabidopsis. Plant Cell 15:2320– 2332 Pallet KE, Dodge AD (1980) Studies into the action of some photosynthetic inhibitor herbicides. J Exp Bot 31:1051–1066 Prasil O, Adir N, Ohad I (1992) Dynamics of photosystem II: mechanism of photoinhibition and recovery process. In: Barber J (ed) Topics in photosynthesis, the photosystems: structure, function and molecular biology. Elsevier, Amsterdam, pp 220– 250 Rappaport F, Diner BA (2008) Primary photochemistry and energetics leading to the oxidation of the (Mn)4Ca cluster and to the evolution of molecular oxygen in photosystem II. Coord Chem Rev 252:259–272 Rappaport F, Guergova-Kuras M, Nixon PJ, Diner BA, Lavergne J (2002) Kinetics and pathways of charge recombination in photosystem II. Biochemistry 41:8518–8527 Rappaport F, Cuni A, Xiong L, Sayre R, Lavergne J (2005) Charge recombination and thermoluminescence in photosystem II. Biophys J 88:1948–1958 Renger G (2008) Functional pattern of photosystem II. In: Renger G (ed) Primary processes in photosynthesis. RSC Publishing, Cambridge, UK, pp 237–290 part 2 Roberts AG, Gregor W, Britt RD, Kramer DM (2003) Acceptor and donor-side interactions of phenolic inhibitors in photosystem II. Biochim Biophys Acta 1604:23–32 Rochaix JD (2007) Role of thylakoid protein kinases in photosynthetic acclimation. FEBS Lett 581:2768–2775 Rutherford AW, Krieger-Liszkay A (2001) Herbicide-induced oxidative stress in photosystem II. Trends Biochem Sci 26:648–653 Rutherford AW, Crofts AR, Inoue Y (1982) Thermoluminescence as a probe of photosystem II photochemistry: the origin of the flashinduced glow peaks. Biochim Biophys Acta 682:457–465 ¨ quist G (2003) Changes Sane PV, Ivanov AG, Hurry V, Huner NPA, O in the redox potential of primary and secondary electron-
123
Photosynth Res accepting quinones in photosystem II confer increased resistance to photoinhibition in low-temperature-acclimated Arabidopsis. Plant Phys 132:2144–2151 Schatz GH, Brock H, Holzwarth AR (1988) Kinetic and energetic model for the primary processes in photosystem II. Biophys J 54:397–405 Schmidt K, Fufezan C, Krieger-Liszkay A, Satoh H, Paulsen H (2003) Recombinant water-soluble chlorophyll protein from Brassica oleracea var. botrys binds various chlorophyll derivatives. Biochemistry 42:7427–7433 Shao N, Krieger-Liszkay A, Schroda M, Beck CF (2007) A reporter system for the individual detection of hydrogen peroxide and singlet oxygen: its use for the assay of reactive oxygen species produced in vivo. Plant J 50:475–487 Sharma J, Panico M, Barber J, Morris HR (1997) Characterization of the low molecular weight photosystem II reaction center subunits and their light-induced modifications by mass spectrometry. J Biol Chem 272:3935–3943 Skovsen E, Snyder JW, Lambert JDC, Ogilby PR (2005) Lifetime and diffusion of singlet oxygen in a cell. J Phys Chem B 109:8570– 8573 Szilard A, Sass L, Hideg E, Vass I (2005) Photoinactivation of photosystem II by flashing light. Photosynth Res 84:15–20 Takahashi S, Murata N (2008) How do environmental stresses accelerate photoinhibition? Trends Plant Sci 13:178–182 Takano A, Takahashi R, Suzuki H, Noguchi T (2008) Herbicide effect on the hydrogen-bonding interaction of the primary quinone electron acceptor QA in photosystem II as studied by Fourier transform infrared spectroscopy. Photosynth Res. doi: 10.1007/s11120-008-9302-5 Telfer A (2002) What is b-carotene doing in the photosystem II reaction centre? Phil Trans R Soc Lond B 357:1431–1440 Telfer A (2005) Too much light? How beta-carotene protects the photosystem II reaction centre. Photochem Photobiol Sci 4:950– 956 Tichy´ M, Lupı´nkova´ L, Sicora C, Vass I, Kuvikova´ S, Pra´sil O, Komenda J (2003) Synechocystis 6803 mutants expressing distinct forms of the photosystem II D1 protein from Synechococcus 7942: relationship between the psbA coding region and sensitivity to visible and UV-B radiation. Biochim Biophys Acta 1605:55–66 Trebst A (2003) Function of beta-carotene and tocopherol in photosystem II. Z Naturforsch C 58:609–620 Trebst A, Depka B (1997) Role of carotene in the rapid turnover and assembly of photosystem II in Chlamydomonas reinhardtii. FEBS Lett 400:359–362 Trebst A, Depka B, Hollander-Czytko H (2002) A specific role for tocopherol and of chemical singlet oxygen quenchers in the
123
maintenance of photosystem II structure and function in Chlamydomonas reinhartii. FEBS Lett 516:156–160 Tyystja¨rvi E (2008) Photoinhibition of photosystem II and photodamage of the oxygen evolving manganese cluster. Coord Chem Rev 252:361–376 van Gorkom HJ (1985) Electron transfer in photosystem II. Photosynth Res 6:97–112 van Mieghem FJE, Satoh K, Rutherford AW (1991) A chlorophyll tilted 30 relative to the membrane in the photosystem II reaction center. Biochim Biophys Acta 1058:379–385 Vass I, Aro EM (2007) Photoinhibition of photosynthetic electron transport. In: Renger G (ed) Primary processes in photosynthesis, comprehensive series in photochemical and photobiological sciences. RSC Publishing, The Royal Society of Chemistry, Cambridge, UK, pp 393–425 part 1 Vass I, Mohanty N, Demeter S (1988) Photoinhibition of electron transport activity of photosystem II in isolated thylakoids studied by thermoluminescence and delayed luminescence. Z Naturforsch 43c:871–876 Vass I, Styring S, Hundal T, Koivuniemi A, Aro EM, Andersson B (1992) Reversible and irreversible intermeidates during photoinhibition of photosystem II: stable reduced QA species promote chlorophyll triplet formation. Proc Natl Acad Sci USA 89:1408– 1412 Vass I, Sass L, Spetea C, Bakou A, Ghanotakis DF, Petrouleas V (1996) UV-B-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence. Impairment of donor and acceptor side components. Biochemistry 35:8964–8973 Vavilin DV, Vermaas WFJ (2000) Mutations in the CD-loop region of the D2 protein in Synechocystis sp. PCC 6803 modify charge recombination reaction pathways in photosystem II in vivo. Biochemistry 39:14831–14838 Vavilin D, Vermaas W (2007) Continuous chlorophyll degradation accompanied by chlorophyllide and phytol reutilization for chlorophyll synthesis in Synechocystis sp. PCC 6803. Biochim Biophys Acta 1767:920–929 von Gromoff ED, Alawady A, Meinecke L, Grimm B, Beck CF (2008) Heme, a plastid-derived regulator of nuclear gene expression in Chlamydomonas. Plant Cell 20:552–567 Vrettos JS, Stewart DH, dePaula JC, Brudvig GW (1999) Lowtemperature optical and resonance Raman of carotenoid cation radical in photosystem II. J Phys Chem B 103:6403–6406 Walters RG (2005) Towards an understanding of photosynthetic acclimation. J Exp Bot 56:435–447 Wilson A, Ajlani G, Verbavatz JM, Vass I, Kerfeld CA, Kirilovsky D (2006) A soluble carotenoid protein involved in phycobilisomerelated energy dissipation in cyanobacteria. Plant Cell 18:992– 1007
