Characterization of factors affecting the activity of photosystem I cyclic electron transport in chloroplasts

PSI cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the antimycin A-sensitive pathway is the main route of electrons in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, its function is still unclear, and there are numerous debates on the rate of electron transport in vivo and its regulation. To assess how PGR5-dependent PSI cyclic electron transport is regulated in vivo, we characterized its activity in ruptured chloroplasts isolated from Arabidopsis thaliana. The activity of ferredoxin (Fd)-dependent plastoquinone (PQ) reduction in the dark is impaired in the pgr5 mutant. Alkalinization of the reaction medium enhanced the activity of Fd-dependent PQ reduction in the wild type. Even weak actinic light (AL) illumination also markedly activated PGR5-dependent PSI cyclic electron transport in ruptured chloroplasts. Even in the presence of linear electron transport [11 mumol O2 (mg Chl)(-1) h(-1)], PGR5-dependent PSI electron transport was detected as a difference in Chl fluorescence levels in ruptured chloroplasts. In the wild type, PGR5-dependent PSI cyclic electron transport competed with NADP+ photoreduction. These results suggest that the rate of PGR5-dependent PSI cyclic electron transport is high enough to balance the production ratio of ATP and NADPH during steady-state photosynthesis, consistently with the pgr5 mutant phenotype. Our results also suggest that the activity of PGR5-dependent PSI cyclic electron transport is regulated by the redox state of the NADPH pool.     

Effects of low CO2 on NAD(P)H dehydrogenase,a mediator of cyclic electron transport around photosystem I in the cyanobacterium synechocystis PCC6803

The expression and activity of type-1 NAD(P)H dehydrogenase (NDH-1) was compared between cells of Synechocystis PCC6803 grown in high (H-cells) and low (L-cells) CO(2) conditions. Western analysis indicated that L-cells contain higher amounts of the NDH-1 subunits, NdhH, NdhI and NdhK. An NADPH-specific subcomplex of NDH-1 showed higher NADPH-nitroblue tetrazolium oxidoreductase activity in L-cells. The activities of both NADPH-menadione oxidoreductase and light-dependent NADPH oxidation driven by photosystem I were much higher in L-cells than in H-cells. The initial rate of re-reduction of P700(+) following actinic light illumination in the presence of DCMU under background far-red light was enhanced in L-cells. In addition, rotenone, a specific inhibitor of NDH-1, suppressed the relative rate of post-illumination increase in Chl fluorescence of L-cells more than that of H-cells, suggesting that the involvement of NDH-1 in cyclic electron flow around photosystem I was enhanced by low CO(2). Taken together, these results suggest that NDH-1 complex and NDH-1-mediated cyclic electron transport are stimulated by low CO(2) and function in the acclimation of cyanobacteria to low CO(2).     

The redox state of the plastoquinone pool controls the level of the light-harvesting chlorophyll a/b binding protein complex II (LHC II) during photoacclimation

A cytochrome b ₆ f deficient mutant of Lemna perpusilla maintains a constant and lower level of the light-harvesting chl a/b-binding protein complex II (LHC II) as compared to the wild type plants at low-light intensities. Inhibition of the plastoquinone pool reduction increases the LHC II content of the mutant at both low- and high-light intensities but only at high-light intensity in the wild type plants. Proteolytic activity against LHC II appears during high-light photoacclimation of wild type plants. However, the acclimative protease is present in the mutant at both light intensities. These and additional results suggest that the plastoquinone redox state serves as the major signal-transducing component in the photoacclimation process affecting both, synthesis and degradation of LHC II and appearance of acclimative LHC II proteolysis. The plastoquinol pool cannot be oxidized by linear electron flow in the mutant plants which are locked in a ‘high light’ acclimation state. The cytochrome b ₆ f complex may be involved indirectly in the regulation of photoacclimation via 1) regulation of the plastoquinone redox state; 2) regulation of the redox-controlled thylakoid protein kinase allowing exposure of the dephosphorylated LHC II to acclimative proteolysis. This revised version was published online in June 2006 with corrections to the Cover Date.     

Reduced levels of cytochrome bf complex in transgenic tobacco leads to marked photochemical reduction of the plastoquinone pool,without significant change in acclimation to irradiance

Tobacco transgenics with decreased amounts of the FeS apoprotein were generated using an antisense RNA construct targeted against the nuclear-encoded Rieske FeS protein of cytochrome bf complex [Price et al. (1995) Aust J Plant Physiol 22: 285–297]. FeS phenotypes ranging from intermediate to low were obtained which had 69% and 26% of the Rieske FeS protein of wild type. Similar reductions in the other subunits of cytochrome bf complex, cytochrome f, cytochrome b₅₆₃and the 17 kDa subunit, were demonstrated in the thylakoids of intermediate and low FeS phenotypes. Confirmation that the levels of assembled cytochrome bf in leaves matched the levels of the FeS protein was demonstrated by laser flash-induced redox absorbance changes in leaves, with the extents of cytochrome f oxidation and cytochrome b₅₆₃reduction being equivalent to the decreased amounts of the subunits in isolated thylakoids of the antisense plants. Despite greatly enhanced photochemical reduction of Q_Aand the plastoquinone pool in the antisense plants, light acclimation of the FeS phenotypes to irradiance did not occur. Furthermore, the state 1–state 2 transitions were identical in wild type and antisense plants. Our results suggest that neither Q_Anor the plastoquinone pool acts alone in either the redox control of gene expression or the regulation of light energy distribution between the photosystems. We suggest rather that reduced plastoquinone acting at the inner Q_psite of cytochrome bf complex is involved in molecular redox signalling.     

Ferredoxin limits cyclic electron flow around PSI (CEF-PSI) in higher plants--stimulation of CEF-PSI enhances non-photochemical quenching of Chl fluorescence in transplastomic tobacco

We tested the hypothesis that ferredoxin (Fd) limits the activity of cyclic electron flow around PSI (CEF-PSI) in vivo and that the relief of this limitation promotes the non-photochemical quenching (NPQ) of Chl fluorescence. In transplastomic tobacco (Nicotiana tabacum cv Xanthi) expressing Fd from Arabidopsis (Arabidopsis thaliana) in its chloroplasts, the minimum yield (F(o)) of Chl fluorescence was higher than in the wild type. F(o) was suppressed to the wild-type level upon illumination with far-red light, implying that the transfer of electrons by Fd-quinone oxidoreductase (FQR) from the chloroplast stroma to plastoquinone was enhanced in transplastomic plants. The activity of CEF-PSI became higher in transplastomic than in wild-type plants under conditions limiting photosynthetic linear electron flow. Similarly, the NPQ of Chl fluorescence was enhanced in transplastomic plants. On the other hand, pool sizes of the pigments of the xanthophyll cycle and the amounts of PsbS protein were the same in all plants. All these results supported the hypothesis strongly. We conclude that breeding plants with an NPQ of Chl fluorescence increased by an enhancement of CEF-PSI activity might lead to improved tolerance for abiotic stresses, particularly under conditions of low light use efficiency.     

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

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

NAD(P)H Dehydrogenase-Dependent Cyclic Electron Flow around Photosystem I in the Cyanobacterium Synechocystis PCC 6803: a Study of Dark-Starved Cells and Spheroplasts

Electron donation to P700⁺ through plastoquinone in the intersystemchain from both respiratory substrates and the photoreductantsin PSI has been shown to be mediated by the NAD(P)H-dehydrogenasecomplex (NDH) in Synechocystis PCC 6803 cells [Mi et al. (1992)Plant Cell Physiol. 33: 1233]. To confirm the participationof NDH in the cyclic electron flow around PSI, the redox kineticsof P700 and Chi fluorescence were analyzed in cells rendereddeficient in respiratory substrates by dark starvation and inspheroplasts. Dark-starved cells showed a high steady-state level of P700⁺under far-red (FR) illumination and the plastoquinone pool wasin a highly oxidized state. An NDH-defective mutant consistentlyshowed a high level of P700 oxidation under FR before and afterthe dark starvation. Donation of electrons either from exogenousNADPH or from photoreduced NADPH⁺ to the intersystem chain viaplastoquinone was demonstrated using spheroplasts from wild-typecells, but not those from the NDH-defective mutant, as monitoredby following changes in the kinetics of Chi fluorescence andthe redox state of P700. The electron flow to PSI via plastoquinone,mediated by NADPH, was sensitive to rotenone, Hg²⁺ ions and2-thenoyltrifluoroacetone, inhibitors of mitochondrial NDH andsuccinate dehydrogenase, but not to antimycin A. The pool sizeof electrons that can be donated to P700⁺ from the cytosol throughthe intersystem chain increased with increasing duration ofillumination time by actinic light and was sensitive to rotenonein both wild-type cells and spheroplasts, but no such resultswere obtained in the NDH-defective mutant of Synechocystis 6803.The results support our previous conclusion that NDH is a mediatorof both respiratory electron flow and cyclic electron flow aroundPSI to the intersystem chain in the cyanobacterium Synechocystis. (Received August 20, 1993; Accepted November 22, 1993)     

Electron Donation from Cyclic and Respiratory Flows to the Photosynthetic Intersystem Chain is Mediated by Pyridine Nucleotide Dehydrogenase in the Cyanobacterium Synechocystis PCC 6803

The redox kinetics of P700 induced by far-red light and a pulseof strong white light in wild type cells were compared withthose in NAD(P)H dehydrogenase (NDH)-defective mutants of thecyanobacterium Synechocystis PCC 6803. The wild type cells showedthe electron donation from the respiratory donor and the photoreductantgenerated in PS I to P700⁺ through the plastoquinone, whichis mediated by a Hg²⁺-sensitive enzyme. The NDH-defective mutantcells, however, did not show the electron transfer to P700⁺through the plastoquinone from both the photoreductant in PSI and cytosolic electron donors using pyndine nucleotides asan intermediate. Thus, NDH appears to be the site of main entryof electrons into the plastoquinone pool in the NAD(P)H-mediatedcyclic electron flow and the respiratory electron flow in Synechocystis. (Received August 31, 1992; Accepted October 1, 1992)     

Oxidation and reduction of plastoquinone by photosynthetic and respiratory electron transport in a cyanobacterium Synechococcus sp.

The I-D dip, an early transient of the fluorescence induction, was examined as a means to monitor redox changes of plastoquinone in cells of a cyanobacterium, Synechococcus sp. That the occurrence of the dip depends upon the reduced state of the plastoquinone pool was indicated by observations that 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea did not affect the initial rise to I but abolished the subsequent decline from I to D and that illumination of the cells with light 1, prior to fluorescence measurements, eliminated the transient. The I-D dip was prominent in freshly harvested cells containing abundant endogenous substrates, disappeared slowly as the cells were starved by aeration but reappeared on addition of fructose to the starved cells in the dark. The dip that had been induced by a brief illumination of the starved cells with light 2 was rapidly diminished in the dark and KCN inhibited the dark decay of the transient. The results indicate that plastoquinone is reduced with endogenous as well as exogenous substrates and oxidized by a KCN-sensitive oxidase in the dark, thus providing strong support for the view that plastoquinone of photosynthetic electron transport also functions in respiration. In addition, the occurrence of a cyclic pathway of electrons from Photosystem I to plastoquinone, possibly via ferredoxin or NADP, was suggested. Several lines of evidence indicate that, under a strong light 2, Photosystem I-dependent oxidation of plastoquinone predominates over Photosystem II-dependent reduction of the quinone in the cyanobacterium which contains Photosystem I more abundantly than Photosystem II.     

Stromal over-reduction by high-light stress as measured by decreases in P700 oxidation by far-red light and its physiological relevance

The oxidation level of P700 induced by far-red light (DeltaA(FR)) in briefly dark-treated leaves of some sun plants decreased during the daytime and recovered at night. The dark recovery of decreased DeltaA(FR) proceeded slowly, with a half-time of about 5 h. We propose that stromal over-reduction induced by sunlight was the direct cause of the depression of DeltaA(FR). The depression of DeltaA(FR) found during the daytime was reproduced by controlled illumination with saturating light of fully dark-treated leaves. Simultaneous measurement of P700 redox and chlorophyll fluorescence showed that the depression of DeltaA(FR) was associated with dark reduction of the plastoquinone pool, which represented cyclic electron transport activity. The decrease of DeltaA(FR) in the light-stressed chloroplasts was partly reversed by treatment with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an inhibitor of electron transport at the cytochrome b6/f complex, and the subsequent addition of methyl viologen, an efficient electron acceptor from photosystem I (PSI), stimulated further recovery, showing that both cyclic electron flow around PSI and the charge recombination within PSI were responsible for the light-induced depression of DeltaA(FR). The dark level of blue-green fluorescence, an indicator of NAD(P)H concentration, from intact chloroplasts was increased by high-light stress, suggesting that NADPH accumulated in stroma as a result of the high-light treatment. Possible effects on photosynthetic activity of over-reduction and its physiological relevance are discussed.     

Kinetics of the photoreduction of cytochrome b-559 by photosystem II in chloroplasts.

The kinetics of the photoreduction of cytochrome b-559 and plastoquinone were measured using well-coupled spinach chloroplasts. High potential (i.e, hydroquinone reducible) cytochrome b-559 was oxidized with low intensity far-red light in the presence of N-methyl phenazonium methosulfate or after preillumination with high intensity light. Using long flashes of red light, the half-reduction time of cytochrome b-559 was found to be 100 +/- 10 ms, compared to 6-10 ms for the photoreduction of the plastoquinone pool. Light saturation of the photoreduction of cytochrome b-559 occurred at a light intensity less than one-third of the intensity necessary for the saturation of ferricyanide reduction under identical illumination conditions. The photoreduction of cytochrome b-559 was accelerated in the presence of dibromothymoquinone with a t 1/2 = 25-35 ms. The addition of uncouplers, which caused stimulatory effect on ferricyanide reduction under the same experimental conditions resulted in a decrease in the rate of cytochrome b-559 reduction. The relatively slow photoreduction rate of cytochrome b-559 compared to the plastoquinone pool implies that electrons can be transferred efficiently from Photosystem II to plastoquinone without the involvement of cytochrome b-559 as an intermediate. These results indicate that it is unlikely that high potential cytochrome b-559 functions as an obligatory redox component in the main electron transport chain joining the two photosystems.     

NAD(P)H Dehydrogenase-Dependent, Antimycin A-Sensitive Electron Donation to Plastoquinone in Tobacco Chloroplasts

Cyclic electron transport around PSI through the NAD(P)H dehydrogenasecomplex (NDH) in tobacco leaf disks, measured as an increasein the dark level of Chl fluorescence after the onset of darkness,was inhibited by antimycin A, an inhibitor of ferredoxin quinonereductase (FQR), suggesting that antimycin A inhibits not onlythe FQR-mediated cyclic flow but also the NDH-dependent flow.This electron flow was inhibited also by amytal, an inhibitorof mitochondrial NDH and by nigericin. The reduction of plastoquinonewas detected when NADPH and ferredoxin were added to the suspensionof the osmotically ruptured chloroplasts of the wild type andNDH-defective mutant. Because the addition of NADPH alone didnot induce the reduction, membrane-bound ferredoxin NADP⁺reductase(FNR) was supposed to reduce ferredoxin, which may be a moredirect electron donor for the plastoquinone reduction. The presenceof two types of reducing enzymes was suggested from the bi-phasicinhibition of plastoquinone reduction by antimycin A in thewild type. It is proposed that the reducing activity inhibitedby antimycin A at a low concentration is attributed to FQR andthe less sensitive activity to NDH. (Received June 29, 1998; Accepted September 7, 1998)     

Regulation of energy dissipation in photosystem I by the redox state of the plastoquinone pool

The effect of exogenous plastoquinone (PQ) on the different deexcitation pathways of photosystem I (PSI) was investigated. Addition of oxidized decyl-plastoquinone (dPQ) and PQ-2 strongly quenched the chlorophyll (Chl) emission spectra of PSI submembrane fractions over all wavelengths. This quenching increased with the concentration of exogenous PQ added and followed the modified Stern-Volmer law. The Stern-Volmer constants found for dPQ and PQ-2 were 1.25 x 10(6) M-1 and 0.55 x 10(6) M-1, respectively, and the fraction of fluorescence accessible to the quencher was 0.7 for both exogenous PQ. dPQ and PQ-2 also retarded the P700 photooxidation measured under limiting actinic light irradiances. Photoacoustic measurements showed that addition of dPQ increased the heat dissipation and decreased the photochemical capacity of PSI. From these results, exogenous oxidized PQ were shown to efficiently quench the Chl excited state in the PSI antenna and change the balance between Chl deexcitation pathways. Moreover, reduction of the endogenous PQ pool in whole thylakoid membranes by NADPH increased PSI fluorescence by 65%, indicating the importance of the redox state of the PQ pool on PSI energy dissipation.     

Voltammetric measurement of the plastoquinone redox state in isolated thylakoids

The oxidation of plastoquinol by the cytochrome bf complex is commonly believed to be the rate limiting step in photosynthetic electron transport. When input of electrons from PS II exceeds electron flow through the cytochrome bf complex the plastoquinone pool becomes reduced. A voltammetric technique previously used to measure the redox state of the ubiquinone pool in plant mitochondria, was modified to measure the redox state of the plastoquinone pool in thylakoids. The presence or absence of a proton gradient strongly influenced the relationship between the redox state of the plastoquinone pool and other photosynthetic parameters. A linear relationship between the rate of electron transport and the reduction of plastoquinone was found. The slope of this relationship was greater in coupled than in uncoupled thylakoids, indicating that under coupled conditions the plastoquinone pool is more reduced at any given rate of electron flow. A complex relationship was found between QA reduction, calculated as 1 – q_p, and the redox state of the plastoquinone pool. The extent of Q_A reduction was similar in coupled and uncoupled thylakoids, but at any given level of Q_A reduction, PQ was always more reduced in coupled thylakoids. These results suggest that the presence of a proton gradient changes the equilibrium constant between Q_A and PQ.     

Expression of the algal cytochrome c6 gene in Arabidopsis enhances photosynthesis and growth

Photosynthetic plants convert light energy into ATP and NADPH in photosynthetic electron transfer and photophosphorylation, and synthesize mainly carbohydrates in the Calvin-Benson cycle. Here we report the enhancement of photosynthesis and growth of plants by introducing the gene of an algal cytochrome c6, which has been evolutionarily eliminated from higher plant chloroplasts, into the model plant Arabidopsis thaliana. At 60 d after planting, the plant height, leaf length and root length of the transformants were 1.3-, 1.1- and 1.3-fold those in the wild-type plants, respectively. At the same time, in the transgenic plants, the amounts of chlorophyll, protein, ATP, NADPH and starch were 1.2-, 1.1-, 1.9-, 1.4- and 1.2-fold those in the wild-type plants, respectively. The CO2 assimilation capacity of the transgenic plants was 1.3-fold that of the wild type. Moreover, in transgenic Arabidopsis expressing algal cytochrome c6, the 1-qP, which reflects the reduced state of the plastoquinone pool, is 30% decreased compared with the wild type. These results show that the electron transfer of photosynthesis of Arabidopsis would be accelerated by the expression of algal cytochrome c6. Our results demonstrate that the growth and photosynthesis of Arabidopsis plants could be enhanced by the expression of the algal cytochrome c6 gene.     

Interplay between non-photochemical plastoquinone reduction and re-oxidation in pre-illuminated <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: a chlorophyll fluorescence study

In photosynthetic eukaryotes, the redox state of the plastoquinone (PQ) pool is an important sensor for mechanisms that regulate the photosynthetic electron transport. In higher plants, a multimeric nicotinamide adenine dinucleotide (phosphate) (NAD(P))H dehydrogenase (NDH) complex and a plastid terminal oxidase (PTOX) are involved in PQ redox homeostasis in the dark. We recently demonstrated that in the microalgae Chlamydomonas reinhardtii, which lacks the multimeric NDH complex of higher plants, non-photochemical PQ reduction is mediated by a monomeric type-II NDH (Nda2). In this study, we further explore the nature and the importance of non-photochemical PQ reduction and oxidation in relation to redox homeostasis in this alga by recording the ‘dark’ chlorophyll fluorescence transients of pre-illuminated algal samples. From the observation that this fluorescence transient is modified by addition of propyl gallate, a known inhibitor of PTOX, and in a Nda2-deficient strain we conclude that it reflects post-illumination changes in the redox state of PQ resulting from simultaneous PTOX and Nda2 activity. We show that the post-illumination fluorescence transient can be used to monitor changes in the relative rates of the non-photochemical PQ reduction and reoxidation in response to different physiological situations. We study this fluorescence transient in algae acclimated to high light and in a mutant deficient in mitochondrial respiration. Some of our observations indicate that the chlororespiratory pathway participates in redox homeostasis in C. reinhardtii.     

The relationship between changes in the redox state of plastoquinone and control of excitation energy distribution in photosynthesis

The hypothesis that excitation energy distribution between PS I and PS II is controlled by the redox state of the plastoquinone pool between the two photosystems was investigated using the green alga Chlorella vulgaris. Changes in the redox state of the pool were monitored by measurement of the area above the fluorescence induction curve on exposure to high-intensity light. In agreement with the hypothesis, exposure of state I adapted cells to light preferentially absorbed by PS II led to a reduction of the plastoquinone pool whilst exposure of State II adapted cells to light preferentially absorbed by PS I resulted in its oxidation. However, the limits within which these fluctuations occurred were much narrower than anticipated. The reasons for this are discussed in terms of the possible involvement of changes in the redox state of more specialised molecules associated with the main plastoquinone pool and the postulated role of plastoquinone as an electron shuttle between the two photosystems.     

The regulation of xanthophyll cycle activity and of non-photochemical fluorescence quenching by two alternative electron flows in the diatoms Phaeodactylum tricornutum and Cyclotella meneghiniana

Intact cells of diatoms are characterized by a rapid diatoxanthin epoxidation during low light periods following high light illumination while epoxidation is severely restricted in phases of complete darkness. The present study shows that rapid diatoxanthin epoxidation is dependent on the availability of the cofactor of diatoxanthin epoxidase, NADPH, which cannot be generated in darkness due to the inactivity of PSI. In the diatom Phaeodactylum tricornutum, NADPH production during low light is dependent on PSII activity, and addition of DCMU consequently abolishes diatoxanthin epoxidation. In contrast to P. tricornutum, DCMU does not affect diatoxanthin epoxidation in Cyclotella meneghiniana, which shows the same rapid epoxidation in low light both in the absence or presence of DCMU. Measurements of the reduction state of the PQ pool and PSI activity indicate that, in the presence of DCMU, NADPH production in C. meneghiniana occurs via alternative electron transport, which includes electron donation from the chloroplast stroma to the PQ pool and, in a second step, from PQ to PSI. Similar electron flow to PQ is also observed during high light illumination of DCMU-treated P. tricornutum cells. In contrast to C. meneghiniana, the electrons are not directed to PSI, but most likely to a plastoquinone oxidase. This chlororespiratory electron transport leads to the establishment of an uncoupler-sensitive proton gradient in the presence of DCMU, which induces diadinoxanthin de-epoxidation and NPQ. In C. meneghiniana, electron flow to the plastoquinone oxidase is restricted, and consequently, diadinoxanthin de-epoxidation and NPQ is not observed after addition of DCMU.     

Iron deficiency causes changes in chlorophyll fluorescence due to the reduction in the dark of the Photosystem II acceptor side

Iron deficiency was found to affect the redox state of the Photosystem II acceptor side in dark-adapted, attached leaves of sugar beet (Beta vulgaris L.). Dark-adapted iron-deficient leaves exhibited relatively high F_o and F_pl levels in the Kautsky chlorophyll fluorescence induction curve when compared to the iron-sufficient controls. However, far-red illumination led to marked decreases in the apparent F_o and F_pl levels. Modulated fluorescence showed that far-red light decreased the fluorescence yield to the true F_o levels by increasing photochemical quenching, without inducing changes in the level of non-photochemical quenching. In dark-adapted, iron-deficient leaves, far-red illumination induced a faster fluorescence decay in the µs-ms time domain, indicating an improvement in the electron transport after the primary quinone acceptor in the reducing side of Photosystem II. All these data indicate that in iron-deficient leaves the plastoquinone pool was reduced in the dark. The extent of the plastoquinone reduction in sugar beet depended on the chlorophyll concentration of the leaf, on the time of preillumination and on the duration of dark adaptation. The dark reduction of plastoquinone was observed not only in sugar beet but also in other plant species affected by iron deficiency both in controlled conditions and in the field.     

Altered photosynthetic electron channelling into cyclic electron flow and nitrite assimilation in a mutant of ferredoxin:NADP(H) reductase

The mechanism by which plants regulate channelling of photosynthetically derived electrons into different areas of chloroplast metabolism remains obscure. Possible fates of such electrons include use in carbon assimilation, nitrogen assimilation and redox signalling pathways, or return to the plastoquinone pool through cyclic electron flow. In higher plants, these electrons are made accessible to stromal enzymes, or for cyclic electron flow, as reduced ferredoxin (Fd), or NADPH. We investigated how knockout of an Arabidopsis ( Arabidopsis thaliana ) ferredoxin:NADPH reductase (FNR) isoprotein and the loss of strong thylakoid binding by the remaining FNR in this mutant affected the channelling of photosynthetic electrons into NADPH- and Fd-dependent metabolism. Chlorophyll fluorescence data show that these mutants have complex variation in cyclic electron flow, dependent on light conditions. Measurements of electron transport in isolated thylakoid and chloroplast systems demonstrated perturbed channelling to NADPH-dependent carbon and Fd-dependent nitrogen assimilating metabolism, with greater competition in the mutant. Moreover, mutants accumulate greater biomass than the wild type under low nitrate growth conditions, indicating that such altered chloroplast electron channelling has profound physiological effects. Taken together, our results demonstrate the integral role played by FNR isoform and location in the partitioning of photosynthetic reducing power.