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Photosystem II catalyses the splitting of water and the reduction in plastoquinone in thylakoid membranes of all oxygenic photosynthetic organisms. The final step in quinol formation is protonation of the reduced secondary quinone electron acceptor Q B 2 - ( H + ) to give Q B H 2 . The proton for this step is hypothesized to be provided by a hydrogen-bond network incorporating amino acids from the Photosystem II D1 and D2 reaction center proteins, together with several bound waters and a bicarbonate ion ligated to a non-heme iron found between the primary plastoquinone electron acceptor Q A and Q B . The aim of this study was to investigate the role of bicarbonate and the D1:Ser268 residue in the formation of Q B H 2 . Using targeted mutagenesis of the D1 protein in the cyanobacterium Synechocystis sp. PCC 6803, we have created two mutants, S268A and S268T. Our D1:Ser268 mutants exhibited increased sensitivity to formate-induced inhibition of electron transfer between Q A and Q B and indicate that D1:Ser268 and bicarbonate support the second protonation in the formation of Q B H 2 via two different pathways that both lead to the protonation of Q B 2 - ( H + ) by D1:His215., (© 2020 Federation of European Biochemical Societies.)

Here, we developed a method for measuring the in vivo redox state of the plastoquinone (PQ) pool in the cyanobacteria Synechocystis sp. PCC 6803. Cells were illuminated on a glass fiber filter, PQ was extracted with ethyl acetate and determined with HPLC. Control samples with fully oxidized and reduced photoactive PQ pool were prepared by far-red and high light treatments, respectively, or by blocking the photosynthetic electron transfer chemically before or after PQ in moderate light. The photoactive pool comprised 50% of total PQ. We find that the PQ pool of cyanobacteria behaves under light treatments qualitatively similarly as in plant chloroplasts, is less reduced during growth under high than under ambient CO 2 and remains partly reduced in darkness., (© 2019 Federation of European Biochemical Societies.)

In photosynthetic systems of oxygenic type, plastoquinone (PQ) molecules are reduced by photosystem II (PSII). The turnover of PQ determines the rate of PSII operation. PQ molecules are present in surplus with respect to PSII. In this work, using the pulse amplitude modulation-fluorometry technique, we quantified photo-reducible PQ pools in chloroplasts of two contrasting ecotypes of Tradescantia, acclimated either to low light (~ 100 μmol photons·m -2 ·s -1 , LL) or to high light (~ 1000 μmol photons·m -2 ·s -1 , HL). The LL-grown plants are characterized by higher capacity of rapidly reducible PQ pool ([PQ] 0 /[PSII] ≈ 8) as compared to HL-grown plants of both species ([PQ] 0 /[PSII] ≈ 4). The elevated content of PQ in LL plants favours photosynthetic electron flow at low-solar irradiance., (© 2019 Federation of European Biochemical Societies.)

The Arabidopsis vte1 mutant is devoid of tocopherol and plastochromanol (PC-8). When exposed to excess light energy, vte1 produced more singlet oxygen ( 1 O 2 ) and suffered from extensive oxidative damage compared with the wild type. Here, we show that overexpressing the solanesyl diphosphate synthase 1 (SPS1) gene in vte1 induced a marked accumulation of total plastoquinone (PQ-9) and rendered the vte1 SPS1oex plants tolerant to photooxidative stress, indicating that PQ-9 can replace tocopherol and PC-8 in photoprotection. High total PQ-9 levels were associated with a noticeable decrease in 1 O 2 production and higher levels of Hydroxyplastoquinone (PQ-C), a 1 O 2 -specific PQ-9 oxidation product. The extra PQ-9 molecules in the vte1 SPS1oex plants were stored in the plastoglobules and the chloroplast envelopes, rather than in the thylakoid membranes, whereas PQ-C was found almost exclusively in the thylakoid membranes. Upon exposure of wild-type plants to high light, the thylakoid PQ-9 pool decreased, whereas the extrathylakoid pool remained unchanged. In vte1 and vte1 SPS1oex plants, the PQ-9 losses in high light were strongly amplified, affecting also the extrathylakoid pool, and PQ-C was found in high amounts in the thylakoids. We conclude that the thylakoid PQ-9 pool acts as a 1 O 2 scavenger and is replenished from the extrathylakoid stock., (© 2018 John Wiley & Sons Ltd.)

The plastoquinone (PQ)-pool in chloroplast thylakoid membranes is a key electron carrier in the photosynthetic electron transport chain (PETC), and its redox state plays an essential role in the control of plant metabolism. Oxygen reduction in thylakoid membranes produces superoxide anion radicals ( O 2 · - ), which may react with the PQ-pool. Here, using isolated thylakoids, we show for the first time the oxidation of the PQ-pool by O 2 · - . The xanthine-xanthine oxidase system was used to supply O 2 · - externally to the thylakoid membrane and the redox state of the PQ-pool was monitored by tracking chlorophyll a fluorescence. We propose that, in vivo, the reaction of  O 2 · - produced in Photosystem I with reduced PQ (plastohydroquinone) creates hydrogen peroxide, which serves as a messenger that signals the redox state of the PETC., (© 2018 Federation of European Biochemical Societies.)

In the present study, we have identified new prenyllipid metabolites formed during high light stress in Arabidopsis thaliana, whose origin and function remained unknown so far. It was found that plastoquinone-C accumulates mainly in the reduced form under high light conditions, as well as during short-term excess light illumination both in the wild-type and tocopherol biosynthetic vte1 mutant, suggesting that plastoquinone-C, a singlet oxygen-derived prenyllipid, is reduced in chloroplasts by photosystem II or enzymatically, outside thylakoids. Plastoquinone-B, a fatty acid ester of plastoquinone-C, was identified for the first time in Arabidopsis in high light grown wild-type plants and during short-time, excess light illumination of the wild-type plants and the vte1 mutant. The gene expression analysis showed that vte2 gene is most pronouncedly up-regulated among the prenyllipid biosynthetic genes under high light and induction of its expression is mainly caused by an increased level of singlet oxygen, as was demonstrated in experiments with D2 O-treated plants under excess light conditions., (© 2015 John Wiley & Sons Ltd.)

Plastoquinol (PQH2-9) and plastoquinone (PQ-9) mediate photosynthetic electron transfer. We isolated PQH2-9 from thylakoid membranes, purified it with HPLC, subjected the purified PQH2-9 to singlet oxygen ((1)O2) and analyzed the products. The main reaction of (1)O2 with PQH2-9 in methanol was found to result in formation of PQ-9 and H2O2, and the amount of H2O2 produced was essentially the same as the amount of oxidized PQH2-9. Formation of H2O2 in the reaction between (1)O2 and PQH2-9 may be an important source of H2O2 within the lipophilic thylakoid membrane., (Copyright © 2015 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

In the present study, we have shown that hydroxy-plastochromanol and plastoquinone-C, the hydroxy derivatives of plastochromanol and plastoquinone-9, respectively, are specifically formed from the parent compounds upon action of singlet oxygen and can be regarded as stable, specific, natural products of singlet oxygen action during photo-oxidative stress in vivo. The presented data indicate that plastoquinone-C formation dominates mainly during relatively short periods of high light stress where efficient production of singlet oxygen takes place, whereas hydroxy-plastochromanol is rather formed under conditions of long-term, less pronounced generation of singlet oxygen. An interesting observation was that hydroxy-plastochromanol is formed even at very low light conditions (5-10 μmol photons m(-2) s(-1)), indicating that singlet oxygen is generated not only during high light stress but also its formation by photosystem II is inseparably connected with the functioning of this photosystem even at the lowest light intensities., (© 2013 John Wiley & Sons Ltd.)

Since the times of the Bible, an extract of black cumin seeds was used as a medicine to treat many human pathologies. Thymoquinone (2-demethylplastoquinone derivative) was identified as an active antioxidant component of this extract. Recently, it was shown that conjugates of plastoquinone and penetrating cations are potent mitochondria-targeted antioxidants effective in treating a large number of age-related pathologies. This review summarizes new data on the antioxidant and some other properties of membrane-penetrating cationic compounds where 2-demethylplastoquinone substitutes for plastoquinone. It was found that such a substitution significantly increases a window between anti- and prooxidant concentrations of the conjugates. Like the original plastoquinone derivatives, the novel compounds are easily reduced by the respiratory chain, penetrate through model and natural membranes, specifically accumulate in mitochondria in an electrophoretic fashion, and strongly inhibit H2O2-induced apoptosis at pico- and nanomolar concentrations in cell cultures. At present, cationic demethylplastoquinone derivatives appear to be the most promising mitochondria-targeted drugs of the quinone series., (Copyright © 2013 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

State transitions and photosystem stoichiometry adjustment are two oxidation-reduction (redox)-regulated acclimatory responses in photosynthesis. State transitions are short-term adaptations that, in chloroplasts, involve reversible post-translational modification by phosphorylation of light-harvesting complex II (LHC II). Photosystem stoichiometry adjustments are long-term responses involving transcriptional regulation of reaction centre genes. Both responses are initiated by changes in light quality and are regulated by the redox state of plastoquinone (PQ). The LHC II kinase involved in the state 2 transition is a serine/threonine kinase known as STT7 in Chlamydomonas, and as STN7 in Arabidopsis. The phospho-LHC II phosphatase that produces the state 1 transition is a PP2C-type protein phosphatase currently termed both TAP38 and PPH1. In plants and algae, photosystem stoichiometry adjustment is governed by a modified two-component sensor kinase of cyanobacterial origin - chloroplast sensor kinase (CSK). CSK is a sensor of the PQ redox state. Chloroplast sigma factor 1 (SIG1) and plastid transcription kinase (PTK) are the functional partners of CSK in chloroplast gene regulation. We suggest a signalling pathway for photosystem stoichiometry adjustment. The signalling pathways of state transitions and photosystem stoichiometry adjustments are proposed to be distinct, with the two pathways sensing PQ redox state independently of each other., (© 2011 Blackwell Publishing Ltd.)

The effect of the plastoquionone (PQ) pool oxidation state on minimum chlorophyll fluorescence was studied in the green alga Chlamydomonas reinhardtii. In wild type and a mutant strain that lacks both photosystems but retains light harvesting complexes, oxygen depletion induced a rise in minimum chlorophyll fluorescence. An increase in minimum fluorescence yield is also observed when the PQ pool becomes reduced in the presence of oxygen and after application of an ionophore that collapses the transmembrane proton gradient. Together these results indicate that minimum chlorophyll fluorescence is modulated by the PQ oxidation state., (Copyright (c) 2010 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

A conjugate of plastoquinone with decylrhodamine 19 (SkQR1) selectively accumulates in mitochondria of normal and tumor cells. SkQR1 protected the cellular pool of reduced glutathione under oxidative stress. Overexpression of P-glycoprotein (Pgp 170) multidrug resistance pump strongly suppresses accumulation of SkQR1. The inhibitors of Pgp 170 stimulate accumulation of SkQR1 in various cell lines indicating that SkQR1 is a substrate of Pgp 170. The protective effect of SkQR1 against oxidative stress is diminished in the cells overexpressing Pgp 170. It is suggested that mitochondria-targeted antioxidants could selectively protect normal (Pgp 170-negative) cells against the toxic effect of anti-cancer treatments related to oxidative stress., (2009. Published by Elsevier B.V.)

Besides major photosynthetic complexes of oxygenic photosynthesis, new electron carriers have been identified in thylakoid membranes of higher plant chloroplasts. These minor components, located in the stroma lamellae, include a plastidial NAD(P)H dehydrogenase (NDH) complex and a plastid terminal plastoquinone oxidase (PTOX). The NDH complex, by reducing plastoquinones (PQs), participates in one of the two electron transfer pathways operating around photosystem I (PSI), the other likely involving a still uncharacterized ferredoxin-plastoquinone reductase (FQR) and the newly discovered PGR5. The existence of a complex network of mechanisms regulating expression and activity of the NDH complex, and the presence of higher amounts of NDH complex and PTOX in response to environmental stress conditions the phenotype of mutants, indicate that these components likely play a role in the acclimation of photosynthesis to changing environmental conditions. Based on recently published data, we propose that the NDH-dependent cyclic pathway around PSI participates to the ATP supply in conditions of high ATP demand (such as high temperature or water limitation) and together with PTOX regulates cyclic electron transfer activity by tuning the redox state of intersystem electron carriers. In response to severe stress conditions, PTOX associated to the NDH and/or the PGR5 pathway may also limit electron pressure on PSI acceptor and prevent PSI photoinhibition.

The oxidation of the PQ-pool after illumination with 50 or 500 micromol quantam(-2)s(-1) was measured in isolated thylakoids as the increase in DeltaA(263), i.e., as the appearance of PQ. While it was not observed under anaerobic conditions, under aerobic conditions it was biphasic. The first faster phase constituted 26% or 44% of total reappearance of PQ, after weak or strong light respectively. The dependence on oxygen presence as well as the correlation with the rate of oxygen consumption led to conclusion that this phase represents the appearance of PQ from PQ(*-) produced in the course of PQH(2) oxidation by superoxide accumulated in the light within the membrane.

A cDNA of Chlamydomonas reinhardtii encoding a plastidial homogentisate prenyltransferase was identified. Functional expression studies in Escherichia coli revealed that the enzyme possessed properties similar to the prenyltransferase of Arabidopsis thaliana encoded by At3g11950 but different from the phytyltransferases of A. thaliana and Synechocystis. Unlike the phytyltransferases, the C. reinhardtii and the respective A. thaliana enzyme showed highest activities with solanesyl diphosphate, but were hardly active with phytyl diphosphate. Hence, these data provide evidence that the latter represent homogentisate solanesyltransferases involved in plastoquinone-9 biosynthesis. Overexpression of At3g11950 in A. thaliana, however, suggests that the solanesyltransferase can affect tocopherol biosynthesis as well.

High irradiance and moderate heat inhibit the activity of the photosynthetic apparatus of oat (Avena sativa L.) leaves. The incubation of oat leaves under high light intensity in conjunction with high temperatures strongly decreased the maximal quantum yield of photosystem (PS) II, indicating the close synergistic effect of both stress factors on PS II inhibition and the subsequent irreversible damage to the photosynthetic apparatus. The PS I A/B protein levels remained similar to control values in leaves incubated under high light intensity or moderate heat, and decreased only when both stress factors were simultaneously applied. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in response to high light intensity and/or heat treatments. In addition, these stress treatments were seen to stimulate the activity of electron donation by NADPH and ferredoxin to plastoquinone, the PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes. Incubation with n-propyl gallate (an inhibitor of PTOX) inhibited the increase of NDH-K and PTOX levels under high light intensity and heat, and slightly stimulated the activity of electron donation by NADPH and ferredoxin to plastoquinone. Antimycin A (an inhibitor of cyclic electron flow) increased the NADH DH activity and preserved the levels of NDH-K and PTOX in thylakoid membranes from leaves incubated under high light intensity and heat. The up-regulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the protection against high irradiance and moderate heat.

With the aim to enhance the plant vitamin E content, the barley gene encoding 4-hydroxyphenylpyruvate dioxygenase was overexpressed in tobacco plants under control of the 35S promoter. Transgenic lines have a higher capacity for homogentisate biosynthesis as evident by a more than 10-fold higher resistance towards the bleaching herbicide sulcotrione. Seeds from transgenic lines have an up to two-fold enhanced level of vitamin E without a change in the ratio of gamma-tocopherol and gamma-tocotrienol. While the vitamin E content is not affected in leaves, the level of plastoquinone is enhanced in leaves of transgenic lines during leaf senescence., (Copyright 2003 Federation of European Biochemical Societies)

The disruption of the Synechocystis open reading frame Deltaslr0090 encoding a gene with high homology to plant genes encoding 4-hydroxyphenylpyruvate dioxygenase results in an impairment of tocopherol biosynthesis without affecting levels of plastoquinone, carotenoids and chlorophyll as well as cell growth and photosynthesis. Our results indicate that in Synechocystis in contrast to the situation in higher plants the 4-hydroxyphenylpyruvate dioxygenase is not required for the synthesis of plastoquinone.

The pgr1 mutant of Arabidopsis thaliana carries a single point mutation (P194L) in the Rieske subunit of the cytochrome b6/f (cyt b6/f) complex and is characterised by a reduced electron transport activity at saturating light intensities in vivo. We have investigated the electron transport in this mutant under in vitro conditions. Measurements of P700 reduction kinetics and of photosynthetic electron transport rates indicated that electron transfer from cyt b6/f to photosystem I is not generally reduced in the mutant, but that the pH dependence of this reaction is altered. The data imply that the pH-dependent inactivation of electron transport through cyt b6/f is shifted by about 1 pH unit to more alkaline pH values in pgr1 thylakoids in comparison with wild-type thylakoids. This interpretation was confirmed by determination of the transmembrane deltapH at different stromal pH values showing that the lumen pH in pgr1 mutant plants cannot drop below pH 6 reflecting most likely a shift of the pK and/or the redox potential of the oxidised Rieske protein.

The effects of benzoquinone analogues, phenyl-1,4-benzoquinone (PBQ) and 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (DBMIB), on state transitions in Synechocystis sp. PCC 6803 were investigated. PBQ induced a transition from state 2 to state 1 in the absence of actinic light whereas DBMIB caused a state 2 transition. 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea could not eliminate the effects of PBQ and DBMIB. These results imply that the redox state of the plastoquinone pool controls the state transitions in vivo and cytochrome b6f complex is involved in this process. As a working hypothesis, we propose that the occupancy of the quinol oxidation site and the movement of the Rieske protein may be pivotal in this regulation.

A putative 2-methyl-6-phytylbenzoquinone (MPBQ) methyltransferase gene, SLL0418, was identified from the Synechocystis PCC6803 genome based on its homology to previously characterized gamma-tocopherol methyltransferases. Genetic and biochemical evidence confirmed open reading frame (ORF) SLL0418 encodes a MPBQ methyltransferase. An SLL0418 partial knockout mutant accumulated beta-tocopherol with no effect in the overall tocopherol content of the cell. In vitro assays of the SLL0418 gene expressed in Escherichia coli showed the enzyme efficiently catalyzes methylation of ring carbon 3 of MPBQ. In addition, the enzyme also catalyzes the methylation of ring carbon 3 of 2-methyl-6-solanylbenzoquinol in the terminal step of plastoquinone biosynthesis.

The deduced polypeptide sequence of open reading frame slr1736 reveals homology to chlorophyll synthase and 1,4-dihydroxy-2-naphthoic acid phytyltransferase in Synechocystis sp. strain PCC 6803. In tocopherol and plastoquinone biosynthesis, a condensation reaction mechanistically similar to that of these two enzymes is performed. To analyze the function of this novel prenyltransferase, a deletion mutant of slr1736 was generated by homologous recombination. The mutant showed a markedly decreased tocopherol content, while plastoquinone levels remained unchanged. Since the aromatic precursor homogentisic acid accumulated in the mutant, the function of the enzyme was proven to be a novel tocopherol phytyltransferase.

We have examined the effects of heat stress on electron transfer in the thylakoid membrane of an engineered plastid ndh deletion mutant, delta1, incapable of performing the Ndh-mediated reduction of the plastoquinone pool in the chloroplast. Upon heat stress in the dark, the rate of PSII-independent reduction of PSI after subsequent illumination by far-red light is dramatically enhanced in both delta1 and a wild-type control plant (WT). In contrast, in the dark, only the WT shows an increase in the reduction state of the plastoquinone pool. We conclude that the heat stress-induced reduction of the intersystem electron transport chain can be mediated by Ndh-independent pathways in the light but that in the dark the dominant pathway for reduction of the plastoquinone pool is catalysed by the Ndh complex. Our results therefore demonstrate a functional role for the Ndh complex in the dark.

Isopentenyl diphosphate (IPP) is the biological C5 precursor of isoprenoids. By labeling experiments using [1-(13)C]glucose, higher plants were shown to possess two distinct biosynthetic routes for IPP biosynthesis: while the cytoplasmic sterols were formed via the acetate/mevalonate pathway, the chloroplast-bound isoprenoids (beta-carotene, lutein, prenyl chains of chlorophylls and plastoquinone-9) were synthesized via a novel IPP biosynthesis pathway (glyceraldehyde phosphate/pyruvate pathway) which was first found in eubacteria and a green alga. The dichotomy in isoprenoid biosynthesis in higher plants allows a reasonable interpretation of previous odd and inconclusive results concerning the biosynthesis of chloroplast isoprenoids, which so far had mainly been interpreted in the frame of models using compartmentation of the mevalonate pathway.

Plastoquinone-9 (PQ-9)-depleted PSII reaction center core complex, consisting of CP47/D1/D2/Cytb-559/I, was isolated from spinach PSII particles. PQ-9, lipids and several proteins were extracted from the original PSII particles and separated by several steps of chromatography to be reconstituted into the isolated complex. PQ-9 reconstituted in the complex with the help of thylakoid lipids (digalactosyldiglyceride) did not function as QA by itself. However, PQ-9 simultaneously reconstituted with L protein and the thylakoid lipids successfully functioned as QA in the complex. Other proteins of PSII origin, such as CP43, H, K, nuclear encoded 4.1 and 5.0 kDa proteins, are unable to restore the QA activity in the complex.

The plastoquinone content of photosystem II complexes isolated from spinach photosystem II-enriched membranes subjected to strong photoinhibitory illumination under anaerobic conditions was determined by HPLC. A pronounced decrease of the plastoquinone content was found in the photoinactivated complexes. These results corroborate earlier models in which photoinhibitory illumination is suggested to eventually lead to release of doubly reduced and protonated QA from its site on the D2-protein.

Reversible, light-dependent protein phosphorylation was observed in isolated thylakoid membranes of the cyanobacterium Synechococcus 6301. A polypeptide of 15 kDa in particular was phosphorylated under plastoquinone-reducing conditions and was not phosphorylated under plastoquinone-oxidising conditions. Phosphorylation and dephosphorylation reactions involving this and several other membrane polypeptides showed sensitivity to inhibitors of protein kinases and phosphatases. Changes in phosphorylation state correlated with changes in low temperature fluorescence emission characteristic of changes in excitation energy distribution between the photosystems. The 15 kDa phosphopolypeptide is likely to be involved directly in light state adaptations in cyanobacteria.

The isolation of a cytochrome b6-f complex from spinach, which is depleted of plastoquinone (and lipid), is reported. The depleted complex no longer functions as a plastoquinol-plastocyanin oxidoreductase but can be reconstituted with plastoquinone and exogenous lipids. The lipid classes digalactosyldiacylglycerol, phosphatidylglycerol and phosphatidylcholine were active in reconstitution while monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol were not. Neither plastoquinone nor lipid alone fully reconstitutes electron transport in the depleted complex. Saturation of plastoquinol-plastocyanin oxidoreductase activity in the depleted complex occurs at 1 plastoquinone per cytochrome f.