Photosynthetic Electron Flow Research Articles

A type II NAD(P)H dehydrogenase mediates light-independent plastoquinone reduction in the chloroplast of Chlamydomonas

In photosynthetic eukaryotes, nonphotochemical plastoquinone (PQ) reduction is important for the regulation of photosynthetic electron flow. In green microalgae where this process has been demonstrated, the chloroplastic enzyme that catalyses nonphotochemical PQ reduction has not been identified yet. Here, we show by an RNA interference (RNAi) approach that the NDA2 gene, belonging to a type II NAD(P)H dehydrogenases family in the green microalga Chlamydomonas reinhardtii, encodes a chloroplastic dehydrogenase that functions to reduce PQ nonphotochemically in this alga. Using a specific antibody, we show that the Nda2 protein is localized in chloroplasts of wild-type cells and is absent in two Nda2-RNAi cell lines. In both mutant cell lines, nonphotochemical PQ reduction is severely affected, as indicated by altered chlorophyll fluorescence transients after saturating illumination. Compared with wild type, change in light excitation distribution between photosystems ('state transition') upon inhibition of mitochondrial electron transport is strongly impaired in transformed cells because of inefficient PQ reduction. Furthermore, the amount of hydrogen produced by Nda2-RNAi cells under sulfur deprivation is substantially decreased compared with wild type, which supports previous assumptions that endogenous substrates serve as source of electrons for hydrogen formation. These results demonstrate the importance of Nda2 for nonphotochemical PQ reduction and associated processes in C. reinhardtii.

Measurement of Changes in Cytosolic Ca2+ in Arabidopsis Guard Cells and Mesophyll Cells in Response to Blue Light

Phototropins (phot1 and phot2) are blue light (BL) receptors that mediate responses including phototropism, chloroplast movement and stomatal opening, and increased cytosolic Ca(2+). BL absorbed by phototropins activates plasma membrane H(+)-ATPase in guard cells, resulting in membrane hyperpolarization, and drives K(+) uptake and stomatal opening. However, it is unclear whether the phototropin-mediated Ca(2+) increase activates the H(+)-ATPase. Here, we determined cytosolic Ca(2+) concentrations in guard cell protoplasts (GCPs) from Arabidopsis transformed with aequorin. Cytosolic Ca(2+) increased rapidly in response to BL in GCPs from both the wild type and phot1 phot2 double mutants, but was mostly suppressed by an inhibitor of photosynthetic electron flow (DCMU). With depleted external K(+), we observed another slower Ca(2+) increase, which was phototropin- dependent. Fusicoccin, a H(+)-ATPase activator, mimicked the effect of BL. The slow Ca(2+) increase thus appears to result from membrane hyperpolarization. The slow Ca(2+) increase was suppressed by external K(+) and was restored by blockers of inward-rectifying K(+) channels, CsCl and tetraethylammonium, suggesting the preferential uptake of K(+) over Ca(2+). Such efficient K(+) uptake in response to BL was not found in mesophyll cells. Both the fast and the slow Ca(2+) increases were inhibited by Ca(2+) channel blockers (CoCl(2) and LaCl(3)) and a chelating agent (EGTA). These results indicate that the phototropin-mediated Ca(2+) increase was not observed prior to H(+)-ATPase activation in guard cells and that Ca(2+) entered guard cells via Ca(2+) channels through photosynthesis and phototropin-mediated membrane hyperpolarization.

Photosynthetic electron flow affects H2O2 signaling by inactivation of catalase in Chlamydomonas reinhardtii

A specific signaling role for H2O2 in Chlamydomonas reinhardtii was demonstrated by the definition of a promoter that specifically responded to this ROS. Expression of a nuclear-encoded reporter gene driven by this promoter was shown to depend not only on the level of exogenously added H2O2 but also on light. In the dark, the induction of the reporter gene by H2O2 was much lower than in the light. This lower induction was correlated with an accelerated disappearance of H2O2 from the culture medium in the dark. Due to a light-induced reduction in catalase activity, H2O2 levels in the light remained higher. Photosynthetic electron transport mediated the light-controlled down-regulation of the catalase activity since it was prevented by 3-(3′4′-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. In the presence of light and DCMU, expression of the reporter gene was low while the addition of aminotriazole, a catalase inhibitor, led to a higher induction of the reporter gene by H2O2 in the dark. The role of photosynthetic electron transport and thioredoxin in this regulation was investigated by using mutants deficient in photosynthetic electron flow and by studying the correlation between NADP-malate dehydrogenase and catalase activities. It is proposed that, contrary to expectations, a controlled down-regulation of catalase activity occurs upon a shift of cells from dark to light. This down-regulation apparently is necessary to maintain a certain level of H2O2 required to activate H2O2-dependent signaling pathways.

The Thermodynamics and Kinetics of Electron Transfer between Cytochrome b6f and Photosystem I in the Chlorophyll d-dominated Cyanobacterium, Acaryochloris marina

We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.

Mesostructure and activity of photosynthetic apparatus for three crassulacean species grown in cold climate

A comparative study of leaf anatomy and morphology and of CO2 exchange was conducted with Rhodiola rosea L., Hylotelephium triphyllum (Haw.) Holub., and Sedum acre L. as representative Crassulacean species occurring in the northeast European Russia. The leaf mesophyll in R. rosea was clearly differentiated into the palisade and spongy tissues, whereas the mesophyll of stonecrops (H. triphyllum and S. acre) was composed of round-shaped cells. The leaves of S. acre featured the largest volume of mesophyll cells and possessed water-retaining cells located around conducting bundles. The chloroplast volume in S. acre (50 μm3) was three times smaller and the number of chloroplasts per cell (170 cell−1) was three times higher than in R. rosea and H. triphyllum (50–55 cell−1). The content of chlorophylls (5–7 mg/g dry wt) and carotenoids (1.5–2.0 mg/g dry wt) in R. rosea leaves was 2–3 times higher than in leaves of stonecrops. The rate of CO2 net uptake in Crassulacean species depended on mesostructure and correlated with the content of pigments and soluble carbohydrates. The photosynthetic rate in R. rosea under optimal irradiance and temperature attained the value of 40 mg/(g dry wt), which is 3 and 8 times higher than in H. triphyllum and S. acre, respectively. The temperature optimum for photosynthesis of R. rosea was observed at 8–18°C, while the optimum for stonecrops was shifted towards higher temperatures by 3–5°C. At chilling temperatures (5–7°C), the leaves of R. rosea retained 50% of their maximal photosynthetic rate, while photosynthetic rates in H. triphyllum and S. acre leaves lowered to 25–30% of the maximal rate. The increase in temperature to 25–30°C led to depression of CO2 net uptake in leaves of Crassulacean species. In R. rosea and H. triphyllum, the rate of photosynthetic electron flow was depressed at high irradiances and temperatures that were supraoptimal for net photosynthesis. It is concluded that the photosynthetic apparatus of Crassulacean species is well adapted to moderate and chilling temperatures, which adjusts the plant metabolism to “life strategies” under conditions of cold climate.

Function of plastoquinone in heat stress reactions of plants

The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m − 2 s − 1 ) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of Q A − reoxidation by Tyr z ox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following Q A − reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with Q A − reoxidized through recombination with the S 2/S 3 state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from Q A − to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.

Preservation of photosynthetic electron transport from senescence-induced inactivation in primary leaves after decapitation and defoliation of bean plants

The comparative effects of decapitation and defoliation on the senescence-induced inactivation of photosynthetic activity in primary leaves of bean plants were investigated. Decapitation was performed during different phases of bean plant ontogenesis, immediately after the appearance of the 1st, 2nd, 3rd and 4th composite leaf. In addition, we examined a variant with primary leaves and stem with an apical bud, but without composite leaves, i.e. defoliated plants. Analyses of chlorophyll fluorescence, millisecond delayed fluorescence and absorption at 830nm in primary leaves were undertaken to investigate the alterations in photosystems II and I electron transport during the decapitation-induced delayed senescence in the non-detached leaves. Analysis of the OKJIP transients using the JIP-test (see [Strasser R, Srivastava A, Tsimilli-Michael M. Analysis of the chlorophyll a fluorescence transient. In: Papageorgiou G, Govindjee, editors. Chlorophyll a fluorescence: a signature of photosynthesis. The Netherlands: Kluwer Academic Publishers, 2004; pp. 321-362]) showed an increase in several biophysical parameters of photosystem II in decapitated plants, specifically, the density of active reaction centers on a chlorophyll basis, the yields of trapping and electron transport, and the performance index. We also observed a decrease in the absorbed light energy per reaction center. Such a decrease in light absorption could be a result of the photosystem II down regulation that appeared as an increase in Q(B)-non-reducing photosystem II centers. The effect was identical when all leaves except the primary leaves were removed. The variant with a preserved apical bud, the defoliated plant, showed values similar to those of decapitated plants with primary leaves only. The changes in the induction curves of the delayed fluorescence also indicated an acceleration of electron transport beyond photosystem II in the decapitated and in defoliated plants. In these plants, the photosystem I-driven electron transport was accelerated, and the size of the plastoquinone pool was enhanced. It was established that decapitation can retard the senescence of primary leaves, can expand leaf life span and can cause activation of both photosystems I and II electron transport. The decapitation procedure shows similarities to the process of defoliation. The overcompensation effect that is developed after defoliation could initially be manifested as an acceleration of the linear photosynthetic electron flow in the rest of the leaves.

Knockout of major leaf ferredoxin reveals new redox‐regulatory adaptations in Arabidopsis thaliana

Ferredoxins are the major distributors for electrons to the various acceptor systems in plastids. In green tissues, ferredoxins are reduced by photosynthetic electron flow in the light, while in heterotrophic tissues, nicotinamide adenine dinucleotide (reduced) (NADPH) generated in the oxidative pentose-phosphate pathway (OPP) is the reductant. We have used a Ds-T-DNA insertion line of Arabidopsis thaliana for the gene encoding the major leaf ferredoxin (Fd2, At1g60950) to create a situation of high electron pressure in the thylakoids. Although these plants (Fd2-KO) possess only the minor fraction of leaf Fd1 (At1g10960), they grow photoautotrophically on soil, but with a lower growth rate and less chlorophyll. The more oxidized conditions in the stroma due to the formation of reactive oxygen species are causing a re-adjustment of the redox state in these plants that helps them to survive even under high light. Redox homeostasis is achieved by regulation at both, the post-translational and the transcriptional level. Over-reduction of the electron transport chain leads to increased transcription of the malate-valve enzyme NADP-malate dehydrogenase (MDH), and the oxidized stroma leads to an increased transcription of the OPP enzyme glucose-6-P dehydrogenase. In isolated spinach chloroplasts, oxidized conditions give rise to a decreased activation state of NADP-MDH and an activation of glucose-6-P dehydrogenase even in the light. In Fd2-KO plants, NADPH-requiring antioxidant systems are upregulated. These adjustments must be caused by plastid signals, and they prevent oxidative damage under rather severe conditions.

A photosynthetic strategy for coping in a high‐light, low‐nutrient environment

Phytoplankton in high‐light, low‐nutrient ocean environments are challenged with maintaining high photosynthetic efficiency and simultaneously preventing photodamage that results from low levels of electron acceptors downstream of photosystem II (PSII). Here, we identify a process in open ocean picophytoplankton that preserves PSII activity by diverting electrons from the photosystem I (PSI) complex‐mediated carbon assimilation to oxygen via a propyl gallate‐sensitive oxidase associated with the photosynthetic electron transport chain. This process stabilizes diel photochemical efficiency of PSII, despite midday photoinhibition, by maintaining oxidized PSII reaction centers. Although measurements of the maximum photochemical efficiency of PSII, Fv : Fm show midday photoinhibition, midday CO2 fixation is not depressed. Moreover, CO2 fixation saturates at low irradiances even though PSII electron flow is not saturated at irradiances of 1,985 mmol photons m−2 s−1. This disparity between PSII fluorescence and CO2 fixation is consistent with the activity of an oxidase that serves as a terminal electron acceptor, maintaining oxidized PSII reaction centers even when CO2 fixation has saturated and the total number of functional reaction centers decreases because of photoinhibition (reflected in lower midday Fv : Fm values). This phenomenon is less apparent in coastal phytoplankton populations, suggesting that it is a strategy particularly distinctive of phytoplankton in the oligotrophic ocean. Spatial variability in features of photosynthetic electron flow could explain biogeographical differences in productivity throughout the ocean and should be represented in models that use empirical photosynthesis and chlorophyll fluorescence measurements from a limited number of ocean sites to estimate the productivity of the entire ocean.

Action potential in Chara cells intensifies spatial patterns of photosynthetic electron flow and non-photochemical quenching in parallel with inhibition of pH banding

Characean cells exposed to illumination arrange plasma-membrane H(+) fluxes and photosynthesis in coordinated spatial patterns. The limited availability of CO(2) in alkaline bands accounts for the lower effective quantum yield of photosystem II (DeltaF/F(m)') in chloroplasts of these bands compared to acidic zones. The effect of electrically triggered action potential on the spatial distribution of photosynthetic parameters (DeltaF/F(m)' and non-photochemical quenching, NPQ) and extracellular pH was studied with fluorescence imaging and pH microelectrodes. In the resting cell at a range of light intensities, the periodic profile of extracellular pH is parallel to the profile of NPQ and antiparallel to that of DeltaF/F(m)'. After triggering the action potential, the pH banding temporarily disappeared, but in contrast, the differences in effective quantum yield and NPQ patterns became more apparent. The transient changes in pH-banding, effective quantum yield and non-photochemical quenching are discussed in relation to alterations in intracellular Ca(2+) and H(+) concentrations during and after the action potential.

Partitioning of photosynthetic electron flow between CO<sub>2</sub> assimilation and O<sub>2</sub> reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P N) and stomatal conductance (g s) of attached leaves decreased as leaf water potential (Ψw) declined from -0.3 to -2.9 MPa. Although g s decreased over the whole range of Ψw, nearly constant values in the intercellular CO2 concentrations (C i) were observed as Ψw decreased to -1.8 MPa, but C i increased as Ψw decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (qNP) increased progressively. A highly significant negative relationship between qNP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 µmol m-2 s-1, the allocation of electrons from photosystem (PS) 2 to O2 reduction was increased by 51 %, while the allocation to CO2 assimilation was diminished by 32 %, as Ψw declined from -0.3 to -2.9 MPa. A significant linear relationship between mean P N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas qP declined only at very severe stress and the excess photon energy was dissipated by increasing qNP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of qNP against photoinhibition in sunflower.

Induction of cyclic electron flow around photosystem 1 and state transition are correlated with salt tolerance in soybean

We investigated the role of cyclic electron flow around photosystem 1 (CEF1) and state transition (ST) in two soybean cultivars that differed in salt tolerance. The CEF1 and maximum photochemical efficiency (Fv/Fm) were determined under control and NaCl (50 mM) stress and the NaCl-induced light-harvesting complex 2 (LHC2) phosphorylation in vitro was analysed in light and dark. NaCl induced the increase of CEF1 more greatly in wild soybean Glycine cyrtoloba (cv. ACC547) than in cultivated soybean Glycine max (cv. Melrose). The Fv/Fm was reduced less in G. cyrtoloba than in G. max after 10-d NaCl stress. In G. cyrtoloba, the increase of CEF1 was associated with enhancement of LHC2 phosphorylation in thylakoid membrane under both dark and light. However, in G. max the NaCl treatment decreased the LHC2 phosphorylation. Treatment with photosynthetic electron flow inhibitors (DCMU, DBMIB) inhibited LHC2 phosphorylation more in G. max than in G. cyrtoloba. Thus the NaCl-induced up-regulation in CEF1 and ST might contribute to salt resistance of G. cyrtoloba.

Alternative photosynthetic electron flow to oxygen in marine Synechococcus

Cyanobacteria dominate the world's oceans where iron is often barely detectable. One manifestation of low iron adaptation in the oligotrophic marine environment is a decrease in levels of iron-rich photosynthetic components, including the reaction center of photosystem I and the cytochrome b 6f complex [R.F. Strzepek and P.J. Harrison, Photosynthetic architecture differs in coastal and oceanic diatoms, Nature 431 (2004) 689–692.]. These thylakoid membrane components have well characterised roles in linear and cyclic photosynthetic electron transport and their low abundance creates potential impediments to photosynthetic function. Here we show that the marine cyanobacterium Synechococcus WH8102 exhibits significant alternative electron flow to O 2, a potential adaptation to the low iron environment in oligotrophic oceans. This alternative electron flow appears to extract electrons from the intersystem electron transport chain, prior to photosystem I. Inhibitor studies demonstrate that a propyl gallate-sensitive oxidase mediates this flow of electrons to oxygen, which in turn alleviates excessive photosystem II excitation pressure that can often occur even at relatively low irradiance. These findings are also discussed in the context of satisfying the energetic requirements of the cell when photosystem I abundance is low.

Regulation of Copper Homeostasis by Micro-RNA in Arabidopsis

Major copper proteins in the cytoplasm of plant cells are plastocyanin, copper/zinc superoxide dismutase, and cytochrome c oxidase. Under copper limited conditions, expression of copper/zinc superoxide dismutase is down-regulated and the protein is replaced by iron superoxide dismutase in chloroplasts. We present evidence that a micro-RNA, miR398, mediates this regulation in Arabidopsis thaliana, by directing the degradation of copper/zinc superoxide dismutase mRNA when copper is limited. Sequence analysis indicated that the transcripts encoding cytosolic copper/zinc superoxide dismutase and COX5b-1, a subunit of the mitochondrial cytochrome c oxidase, are also targeted by miR398. This regulation via miR398 takes place in response to changes in a low range of copper levels (0.2-0.5 microM), indicating that miR398 is involved in a response to copper limitation. On the other hand, another major copper protein, plastocyanin, which is involved in photosynthetic electron flow and is essential in higher plants, was not regulated via miR398. We propose that miR398 is a key factor in copper homeostasis in plants and regulates the stability of mRNAs of major copper proteins under copper-limited conditions.

Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution

Peroxydicarbonic acid (Podca), a proposed intermediate in photosynthetic oxygen evolution, was synthesized electrochemically. Consistent with literature descriptions of this compound, it was shown to be a highly reactive molecule, spontaneously hydrolyzed to H2O2, as well as susceptible to oxidative and reductive decomposition. In the presence of Mn2+ or Co2+, Podca was quickly broken down with release of O2. The liberation of O2, however, was partially suppressed at high O2 concentrations. In the presence of Ca-washed photosystem II-enriched membranes lacking extrinsic proteins, Podca was decomposed with the release of O2, but only under conditions favoring photosynthetic electron flow (light plus a Hill oxidant). A model is proposed that details how peroxydicarbonic acid could act as an oxygen-evolving intermediate. The hypothesis is consistent with the well-established Kok model and with recent findings related to the chemistry of oxygen evolution.

Macromolecular crowding and its influence on possible reaction mechanisms in photosynthetic electron flow

The diffusion of plastoquinol and its binding to the Qo site of the cyt bf complex in the course of photosynthetic electron transport was studied by following the sigmoidal flash-induced re-reduction kinetics of P700 after previous oxidation of the intersystem electron carriers. The data resulting from these experiments were matched with a simulation of electron transport using Monte Carlo techniques. The simulation was able to account for the experimental observations. Two different extreme cases of reaction mechanism at the Qo site were compared: a diffusion limited collisional mechanism and a non-diffusion limited tight binding mechanism. Assuming a tight binding mechanism led to best matches due to the high protein density in thylakoids. The varied parameters resulted in values well within the range of published data. The results emphasise the importance of structural characteristics of thylakoids in models of electron transport.

Photosynthetic electron transport in the cyanobacteriumSynechocystis sp. PCC 6803: High-Field W-band and X-band EPR study of electron flow through photosystem I

In this work, by using the respective advantages of W- and X-band electron paramagnetic resonance (EPR) spectroscopy techniques to investigate electron transport processes, we have studied the light-induced redox transients of the primary electron donor P700 and the secondary acceptor A1 in photosystem (PS) I complexes of intact cyanobacterial cellsSynechocystis sp. PCC 6803. We found that the kinetic behavior of the cation radical P700·+ generated by illumination with continuous light, and the EPR intensity of the radical pair P700·+A1·− generated upon laser pulse illumination strongly depend on the illumination prehistory (either the sample was frozen in the dark or during illumination). Both these processes were sensitive to the presence of electron transport inhibitors which block electron flow between the two photosystems. In line with our X-band EPR data on the kinetics of light-induced redox transients of P700, our high-field W-band EPR study of the radical-pair state P700·+A1·− shows that photosynthetic electron flow through the PS I reaction center is controlled both on the donor and on the acceptor side of PS I.

Deletion of PsbM in Tobacco Alters the QB Site Properties and the Electron Flow within Photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.

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.

Non-Invasive Monitoring of the Light-Induced Cyclic Photosynthetic Electron Flow during Cold Hardening in Wheat Leaves

The effect of irradiance during low temperature hardening was studied in a winter wheat variety. Ten-day-old winter wheat plants were cold-hardened at 5 degrees C for 11 days under light (250 micromol m(-2) S(-1)) or dark (20 micromol m(-2) s(-1)) conditions. The effectiveness of hardening was significantly lower in the dark, in spite of a slight decrease in the Fv/Fm chlorophyll fluorescence induction parameter, indicating the occurrence of photoinhibition during the hardening period in the light. Hardening in the light caused a downshift in the far-red induced AG (afterglow) thermoluminescence band. The faster dark re-reduction of P700+, monitored by 820-nm absorbance, could also be observed in these plants. These results suggest that the induction of cyclic photosynthetic electron flow may also contribute to the advantage of frost hardening under light conditions in wheat plants.