Quantification of cyclic electron flow around Photosystem I in spinach leaves during photosynthetic induction

The noninvasive monitoring of the redox status of photosynthetic electron transport chains in Hibiscus rosa-sinensis and Tradescantia leaves

We present an approach to the noninvasive determination of the electron capacity of the intersystem pool of electron carriers in chloroplasts in situ. As apt experimental models, we used the leaves of Hibiscus rosa-sinensis and Tradescantia species. Electron paramagnetic resonance and optical response of P₇₀₀ (the primary electron donor in Photosystem I) were applied to measuring electron transport in chloroplasts. Electron capacities of the intersystem electron transport chain (ETC) were determined from redox transients of P₇₀₀ upon chromatic transitions (white light → far-red light). During the induction period, we observed the nonmonotonic changes in the number of electron equivalents in the intersystem ETC per P₇₀₀ (parameter Q). In Hibiscus rosa-sinensis, the light-induced rise of Q from ≈2.5 (in the dark) to Q ≈ 12 was followed by its decrease to Q ≈ 6. The data obtained are discussed in the context of pH-dependent regulation of electron transport in chloroplasts, which provides the well-balanced operation of the intersystem ETC. The decay of Q is explained by the attenuation of Photosystem II activity due to the lumen acidification and the acceleration of plastoquinol re-oxidation as a result of the Calvin-Benson cycle activation. Our computer model of electron and proton transport coupled to ATP synthesis in chloroplasts was used to analyze the up and down feedbacks responsible for pH-dependent regulation of electron transport in chloroplasts. The procedures introduced here may be important for subsequent works aimed at defining the plastoquinone participation in regulation of photosynthetic processes in chloroplasts in situ.

Appropriate time interval of PPFD measurement to estimate daily photosynthetic gain

Photosynthetic models sometimes incorporate meteorological elements typically recorded at a time interval of 10 min or 1 h. Because these data are calculated by averaging instantaneous values over time, short-term environmental fluctuations are concealed, which may affect outputs of the model. To assess an appropriate time interval of photosynthetic photon flux density (PPFD) measurement for accurate estimation of photosynthetic gain under open field conditions, we simulated the daily integral net photosynthetic gain using photosynthetic models with or without considering induction kinetics in response to changes in PPFD. Compared with the daily gain calculated from 60-min-interval PPFD data using a steady-state model that ignored the induction kinetics (i.e. a baseline gain), the gains simulated using higher-resolution PPFD data (10-s, 1-min, and 10-min intervals) and using a dynamic model that considered slow induction kinetics were both smaller by ~2%. The gain estimated by the slow dynamic model with 10-s-interval PPFD data was smaller than the baseline gain by more than 5% with a probability of 66%. Thus, the use of low-resolution PPFD data causes overestimation of daily photosynthetic gain in open fields. An appropriate time interval for PPFD measurement is 1 min or shorter to ensure accuracy of the estimates.

Photosynthetic characterization of flavodoxin-expressing tobacco plants reveals a high light acclimation-like phenotype

Flavodoxins are electron carrier flavoproteins present in bacteria and photosynthetic microorganisms which duplicate the functional properties of iron-sulphur containing ferredoxins and replace them under adverse environmental situations that lead to ferredoxin decline. When expressed in plant chloroplasts, flavodoxin complemented ferredoxin deficiency and improved tolerance to multiple sources of biotic, abiotic and xenobiotic stress. Analysis of flavodoxin-expressing plants grown under normal conditions, in which the two carriers are present, revealed phenotypic effects unrelated to ferredoxin replacement. Flavodoxin thus provided a tool to alter the chloroplast redox poise in a customized way and to investigate its consequences on plant physiology and development. We describe herein the effects exerted by the flavoprotein on the function of the photosynthetic machinery. Pigment analysis revealed significant increases in chlorophyll a, carotenoids and chlorophyll a/b ratio in flavodoxin-expressing tobacco lines. Results suggest smaller antenna size in these plants, supported by lower relative contents of light-harvesting complex proteins. Chlorophyll a fluorescence and P700 spectroscopy measurements indicated that transgenic plants displayed higher quantum yields for both photosystems, a more oxidized plastoquinone pool under steady-state conditions and faster plastoquinone dark oxidation after a pulse of saturating light. Many of these effects resemble the phenotypes exhibited by leaves adapted to high irradiation, a most common environmental hardship faced by plants growing in the field. The results suggest that flavodoxin-expressing plants would be better prepared to cope with this adverse situation, and concur with earlier observations reporting that hundreds of stress-responsive genes were induced in the absence of stress in these lines.

Photosynthetic Mechanisms of Metaxenia Responsible for Enlargement of Carya cathayensis Fruits at Late Growth Stages

Fruits of hickory (Carya cathayensis) are larger and their peel is greener after interspecific pollination by pecan (Carya illinoinensis; later pp fruits) than after intraspecific pollination by hickory (later ph fruits). Previous studies have found little genetic differences between offspring and their maternal parent, indicating that the observed trait differences between pp and ph fruits are due to metaxenia. Fruit development depends on the amount of photosynthetic assimilate available. Since there is no difference in photosynthesis of the associated leaves between pp and ph fruits, the larger size of the pp fruits might be attributed to changes in fruit photosynthesis caused by the different pollen sources. To elucidate to the photosynthetic mechanisms behind the metaxenia effect on fruit development in hickory, the effects of intraspecific and interspecific pollination regimes were examined in the present study. We observed the photosynthetic capacity in the peel of fruits and the related ecophysiological and morphological traits of both ph and pp fruits over a period of 120 days after pollination. Significant differences in the appearance and dry weight between ph and pp fruits were observed at 50 days after pollination (DAP). More than 70% of dry matter accumulation of the fruits was completed during 60-120 DAP, while the true photosynthetic rate of the associated leaves significantly decreased by about 50% during the same period. In several cell layers of the peel, the number of chloroplasts per cell was significantly higher in pp than in ph fruits. Similarly, the ribulose 1, 5-bisphosphate carboxylase (Rubisco) activity, the total chlorophyll content, and the nitrogen content were all significantly higher in pp than in ph fruits during all growth stages; and all of these physiological quantities were positively correlated with the gross photosynthetic rate of the fruits. We conclude that the enhanced photosynthetic capacity of pp fruits contributes to their fast dry matter accumulation and oil formation. This result will provide a theoretical basis for improving hickory fruit yields in practical cultivation.

Partially Dissecting Electron Fluxes in Both Photosystems in Spinach Leaf Disks during Photosynthetic Induction

Photosynthetic induction, a gradual increase in photosynthetic rate on a transition from darkness or low light to high light, has ecological significance, impact on biomass accumulation in fluctuating light and relevance to photoprotection in strong light. However, the experimental quantification of the component electron fluxes in and around both photosystems during induction has been rare. Combining optimized chlorophyll fluorescence, the redox kinetics of P700 [primary electron donor in Photosystem I (PSI)] and membrane inlet mass spectrometry in the absence/presence of inhibitors/mediator, we partially estimated the components of electron fluxes in spinach leaf disks on transition from darkness to 1,000 �mol photons�m-2�s-1 for up to 10 min, obtaining the following findings: (i) the partitioning of energy between both photosystems did not change noticeably; (ii) in Photosystem II (PSII), the combined cyclic electron flow (CEF2) and charge recombination (CR2) to the ground state decreased gradually toward 0 in steady state; (iii) oxygen reduction by electrons from PSII, partly bypassing PSI, was small but measurable; (iv) cyclic electron flow around PSI (CEF1) peaked before becoming somewhat steady; (v) peak magnitudes of some of the electron fluxes, all probably photoprotective, were in the descending order: CEF1 > CEF2 + CR2 > chloroplast O2 uptake; and (vi) the chloroplast NADH dehydrogenase-like complex appeared to aid the antimycin A-sensitive CEF1. The results are important for fine-tuning in silico simulation of in vivo photosynthetic electron transport processes; such simulation is, in turn, necessary to probe partial processes in a complex network of interactions in response to environmental changes.

Regulation of cyclic electron flow by chloroplast NADPH-dependent thioredoxin system

Linear electron transport in the thylakoid membrane drives photosynthetic NADPH and ATP production, while cyclic electron flow (CEF) around photosystem I only promotes the translocation of protons from stroma to thylakoid lumen. The chloroplast NADH dehydrogenase-like complex (NDH) participates in one CEF route transferring electrons from ferredoxin back to the plastoquinone pool with concomitant proton pumping to the lumen. CEF has been proposed to balance the ratio of ATP/NADPH production and to control the redox poise particularly in fluctuating light conditions, but the mechanisms regulating the NDH complex remain unknown. We have investigated potential regulation of the CEF pathways by the chloroplast NADPH-thioredoxin reductase (NTRC) in vivo by using an Arabidopsis knockout line of NTRC as well as lines overexpressing NTRC. Here, we present biochemical and biophysical evidence showing that NTRC stimulates the activity of NDH-dependent CEF and is involved in the regulation of generation of proton motive force, thylakoid conductivity to protons, and redox balance between the thylakoid electron transfer chain and the stroma during changes in light conditions. Furthermore, protein-protein interaction assays suggest a putative thioredoxin-target site in close proximity to the ferredoxin-binding domain of NDH, thus providing a plausible mechanism for redox regulation of the NDH ferredoxin:plastoquinone oxidoreductase activity.

Transcriptome and phenotyping analyses support a role for chloroplast sigma factor 2 in red-light-dependent regulation of growth, stress, and photosynthesis

Sigma factor (SIG) proteins contribute to promoter specificity of the plastid-encoded RNA polymerase during chloroplast genome transcription. All six members of the SIG family, that is, SIG1-SIG6, are nuclear-encoded proteins targeted to chloroplasts. Sigma factor 2 (SIG2) is a phytochrome-regulated protein important for stoichiometric control of the expression of plastid- and nuclear-encoded genes that impact plastid development and plant growth and development. Among SIG factors, SIG2 is required not only for transcription of chloroplast genes (i.e., anterograde signaling), but also impacts nuclear-encoded, photosynthesis-related, and light signaling-related genes (i.e., retrograde signaling) in response to plastid functional status. Although SIG2 is involved in photomorphogenesis in Arabidopsis, the molecular bases for its role in light signaling that impacts photomorphogenesis and aspects of photosynthesis have only recently begun to be investigated. Previously, we reported that SIG2 is necessary for phytochrome-mediated photomorphogenesis specifically under red (R) and far-red light, thereby suggesting a link between phytochromes and nuclear-encoded SIG2 in light signaling. To explore transcriptional roles of SIG2 in R-dependent growth and development, we performed RNA sequencing analysis to compare gene expression in sig2-2 mutant and Col-0 wild-type seedlings at two developmental stages (1- and 7-day). We identified a subset of misregulated genes involved in growth, hormonal cross talk, stress responses, and photosynthesis. To investigate the functional relevance of these gene expression analyses, we performed several comparative phenotyping tests. In these analyses, strong sig2 mutants showed insensitivity to bioactive GA 3, high intracellular levels of hydrogen peroxide (H2O2) indicative of a stress response, and specific defects in photosynthesis, including elevated levels of cyclic electron flow (CEF) and nonphotochemical quenching (NPQ). We demonstrated that SIG2 regulates a broader range of physiological responses at the molecular level than previously reported, with specific roles in red-light-mediated photomorphogenesis.

Novel Insights into the Influence of Seed Sarcotesta Photosynthesis on Accumulation of Seed Dry Matter and Oil Content in Torreya grandis cv. "Merrillii"

Seed oil content is an important trait of nut seeds, and it is affected by the import of carbon from photosynthetic sources. Although green leaves are the main photosynthetic organs, seed sarcotesta photosynthesis also supplies assimilates to seed development. Understanding the relationship between seed photosynthesis and seed development has theoretical and practical significance in the cultivation of Torreya grandis cv. "Merrillii." To assess the role of seed sarcotesta photosynthesis on the seed development, anatomical and physiological traits of sarcotesta were measured during two growing seasons in the field. Compared with the attached current-year leaves, the sarcotesta had higher gross photosynthetic rate at the first stage of seed development. At the late second stage of seed development, sarcotesta showed down-regulation of PSII activity, as indicated by significant decrease in the following chlorophyll fluorescence parameters: the maximum PSII efficiency (Fv/Fm ), the PSII quantum yield (Φ PSII ), and the photosynthetic quenching coefficient (qP). The ribulose 1, 5-bisphosphate carboxylase (Rubisco) activity, the total chlorophyll content (Chl(a+b)) and nitrogen content in the sarcotesta were also significantly decreased during that period. Treatment with DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] preventing seed photosynthesis decreased the seed dry weight and the oil content by 25.4 and 25.5%, respectively. We conclude that seed photosynthesis plays an important role in the dry matter accumulation at the first growth stage. Our results also suggest that down-regulation of seed photosynthesis is a plant response to re-balance the source-sink ratio at the second growth stage. These results suggest that seed photosynthesis is important for biomass accumulation and oil synthesis of the Torreya seeds. The results will facilitate achieving higher yields and oil contents in nut trees by selection for higher seed photosynthesis cultivars.

Redox changes of ferredoxin, P700, and plastocyanin measured simultaneously in intact leaves

Properties and performance of the recently introduced Dual/KLAS-NIR spectrophotometer for simultaneous measurements of ferredoxin (Fd), P700, and plastocyanin (PC) redox changes, together with whole leaf chlorophyll a (Chl) fluorescence (emission >760, 540 nm excitation) are outlined. Spectral information on in vivo Fd, P700, and PC in the near-infrared region (NIR, 780-1000 nm) is presented, on which the new approach is based. Examples of application focus on dark-light and light-dark transitions, where maximal redox changes of Fd occur. After dark-adaptation, Fd reduction induced by moderate light parallels the Kautsky effect of Chl fluorescence induction. Both signals are affected analogously by removal of O₂. A rapid type of Fd reoxidation, observed after a short pulse of light before light activation of linear electron transport (LET), is more pronounced in C4 compared to C3 leaves and interpreted to reflect cyclic PS I (CET). Light activation of LET, as assessed via the rate of Fd reoxidation after short light pulses, occurs at very low intensities and is slowly reversed (half-time ca. 20 min). Illumination with strong far-red light (FR, 740 nm) reveals two fractions of PS I, PS I (LET), and PS I (CET), differing in the rates of Fd reoxidation upon FR-off and the apparent equilibrium constants between P700 and PC. Parallel information on oxidation of Fd and reduction of P700 plus PC proves essential for identification of CET. Comparison of maize (C4) with sunflower and ivy (C3) responses leads to the conclusion that segregation of two types of PS I may not only exist in C4 (mesophyll and bundle sheath cells), but also in C3 photosynthesis (grana margins plus end membranes and stroma lamellae).

Seasonal dynamics of photosynthetic activity in the representive brown macroalgae Sagrassum thunbergii (Sargassaceae Phaeophyta)

The present study evaluates the seasonal photosynthetic performances of Sargassum thunbergii via chlorophyll fluorescence technique. During summer and early winter, no significant change was observed in maximum photochemical efficiency (Fv/Fm), and performance index (PIabs). During late winter and early spring, Fv/Fm, and PIabs decreased significantly, implying that S. thunbergii photosystem II (PSII) suffered apparent photoinhibition. Subsequently, PSII gradually recovered during late spring and summer, as evidenced by an increase of both parameters. Throughout the year, the maximum decrease in the slope of MR/MR0 maintained low values indicated that photosystem I (PSI) was incative, the initial rate of P700+ re-reduction maintained low value indicated that cyclic electron transport (CET) were inactive; nevertheless, a seasonal down-regulation of both PSI and CET during late winter and early spring could be detected. The weak performance of PSI and CET can potentially limit the flexibility in response to winter stress and result in a delayed recovery of PSII. In conclusion, the seasonal variability of S. thunbergii photosynthetic activity was characterized by three periods: active state, down-regulation and restoration. The rapid growth during early spring was accompanied by weak photosynthetic performance, indicating that the carbohydrates consumed during this period were derived from previously stored starch.

The higher plant plastid NAD(P)H dehydrogenase-like complex (NDH) is a high efficiency proton pump that increases ATP production by cyclic electron flow

Cyclic electron flow around photosystem I (CEF) is critical for balancing the photosynthetic energy budget of the chloroplast by generating ATP without net production of NADPH. We demonstrate that the chloroplast NADPH dehydrogenase complex, a homolog to respiratory Complex I, pumps approximately two protons from the chloroplast stroma to the lumen per electron transferred from ferredoxin to plastoquinone, effectively increasing the efficiency of ATP production via CEF by 2-fold compared with CEF pathways involving non-proton-pumping plastoquinone reductases. By virtue of this proton-pumping stoichiometry, we hypothesize that NADPH dehydrogenase not only efficiently contributes to ATP production but operates near thermodynamic reversibility, with potentially important consequences for remediating mismatches in the thylakoid energy budget.

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants

Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5-PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved.

Photosynthetic response to fluctuating environments and photoprotective strategies under abiotic stress

Plants in natural environments must cope with diverse, highly dynamic, and unpredictable conditions. They have mechanisms to enhance the capture of light energy when light intensity is low, but they can also slow down photosynthetic electron transport to prevent the production of reactive oxygen species and consequent damage to the photosynthetic machinery under excess light. Plants need a highly responsive regulatory system to balance the photosynthetic light reactions with downstream metabolism. Various mechanisms of regulation of photosynthetic electron transport under stress have been proposed, however the data have been obtained mainly under environmentally stable and controlled conditions. Thus, our understanding of dynamic modulation of photosynthesis under dramatically fluctuating natural environments remains limited. In this review, first I describe the magnitude of environmental fluctuations under natural conditions. Next, I examine the effects of fluctuations in light intensity, CO2 concentration, leaf temperature, and relative humidity on dynamic photosynthesis. Finally, I summarize photoprotective strategies that allow plants to maintain the photosynthesis under stressful fluctuating environments. The present work clearly showed that fluctuation in various environmental factors resulted in reductions in photosynthetic rate in a stepwise manner at every environmental fluctuation, leading to the conclusion that fluctuating environments would have a large impact on photosynthesis.

Evidence for the role of cyclic electron flow in photoprotection for oxygen-evolving complex

Cyclic electron flow (CEF) alleviates PSII photo-inhibition under high light by at least two different mechanisms: one is liked to thermal energy dissipation (qE) and the other one is independent of qE. However, the latter mechanism is unclear. Because the photodamage to PSII primarily occurred at the oxygen-evolving complex (OEC), and the stability of OEC is dependent on proton gradient across thylakoid membrane (ΔpH), we hypothesize that the CEF-dependent generation of ΔpH can alleviate photodamage to OEC. To test this hypothesis, we determined the effects of antimycin A (AA), methyl viologen (MV), chloramphenicol (CM), nigericin (Nig) on PSII activity and the stability of OEC for leaves of a light-demanding tropical tree species Erythrophleum guineense by the analysis of OKJIP chlorophyll a fluorescence transient. After high light treatment, the stronger decrease in Fv/Fm in the AA-, CM-, MV-, and Nig-treated samples was accompanied with larger photo damage of OEC. The AA-treated samples significantly showed lower CEF activity than the H2O-treated samples. Although the AA-treated leaves significantly showed stronger PSII photo-inhibition and photo-damage of OEC compared to the H2O-treated leaves, the value of non-photochemical quenching did not differ between them. Therefore, CEF activity was partly inhibited in the AA-treated samples, and the stronger PSII photo-inhibition in the AA-treated leaves was independent of qE. Taking together, we propose a hypothesis that CEF-dependent generation of ΔpH under high light plays an important role in photoprotection for the OEC activity.

Contrasting response of biomass and grain yield to severe drought in Cappelle Desprez and Plainsman V wheat cultivars

We report a case study of natural variations and correlations of some photosynthetic parameters, green biomass and grain yield in Cappelle Desprez and Plainsman V winter wheat (Triticum aestivum L.) cultivars, which are classified as being drought sensitive and tolerant, respectively. We monitored biomass accumulation from secondary leaves in the vegetative phase and grain yield from flag leaves in the grain filling period. Interestingly, we observed higher biomass production, but lower grain yield stability in the sensitive Cappelle cultivar, as compared to the tolerant Plainsman cv. Higher biomass production in the sensitive variety was correlated with enhanced water-use efficiency. Increased cyclic electron flow around PSI was also observed in the Cappelle cv. under drought stress as shown by light intensity dependence of the ratio of maximal quantum yields of Photosystem I and Photosystem II, as well by the plot of the Photosystem I electron transport rate as a function of Photosystem II electron transport rate. Higher CO2 uptake rate in flag leaves of the drought-stressed Plainsman cv. during grain filling period correlates well with its higher grain yield and prolonged transpiration rate through spikes. The increase in drought factor (DFI) and performance (PI) indices calculated from variable chlorophyll fluorescence parameters of secondary leaves also showed correlation with higher biomass in the Cappelle cultivar during the biomass accumulation period. However, during the grain filling period, DFI and PI parameters of the flag leaves were higher in the tolerant Plainsman V cultivar and showed correlation with grain yield stability. Our results suggest that overall biomass and grain yield may respond differentially to drought stress in different wheat cultivars and therefore phenotyping for green biomass cannot be used as a general approach to predict grain yield. We also conclude that photosynthetic efficiency of flag and secondary leaves is correlated with grain yield and green biomass, respectively. In addition, secondary trait associated mechanisms like delayed senescence and higher water-use efficiency also contribute to biomass stability. Our studies further prove that photosynthetic parameters could be used to characterize environmental stress responses.

Overexpression of Glycolate Oxidase Confers Improved Photosynthesis under High Light and High Temperature in Rice

While glycolate oxidase (GLO) is well known as a key enzyme for the photorespiratory metabolism in plants, its physiological function and mechanism remains to be further clarified. Our previous studies have shown that suppression of GLO in rice leads to stunted growth and inhibited photosynthesis (Pn) which is positively and linearly correlated with decreased GLO activities. It is, therefore, of interest to further understand whether Pn can be improved when GLO is up-regulated? In this study, four independent overexpression rice lines, with gradient increases in GLO activity, were generated and functionally analyzed. Phenotypic observations showed that the growth could be improved when GLO activities were increased by 60 or 100%, whereas reduced growth was noticed when the activity was further increased by 150 or 210%. As compared with WT plants, all the overexpression plants exhibited significantly improved Pn under conditions of high light and high temperature, but not under normal conditions. In addition, the overexpression plants were more resistant to the MV-induced photooxidative stress. It was further demonstrated that the antioxidant enzymes, and the antioxidant metabolite glutathione was not significantly altered in the overexpression plants. In contrast, H2O2 and salicylic acid (SA) were correspondingly induced upon the GLO overexpression. Taken together, the results suggest that GLO may play an important role for plants to cope with high light and high temperature, and that H2O2 and SA may serve as signaling molecules to trigger stress defense responses but antioxidant reactions appear not to be involved in the defense.

Defects in the Expression of Chloroplast Proteins Leads to H2O2 Accumulation and Activation of Cyclic Electron Flow around Photosystem I

We describe a new member of the class of mutants in Arabidopsis exhibiting high rates of cyclic electron flow around photosystem I (CEF), a light-driven process that produces ATP but not NADPH. High cyclic electron flow 2 (hcef2) shows strongly increased CEF activity through the NADPH dehydrogenase complex (NDH), accompanied by increases in thylakoid proton motive force (pmf), activation of the photoprotective q_E response, and the accumulation of H₂O₂. Surprisingly, hcef2 was mapped to a non-sense mutation in the TADA1 (tRNA adenosine deaminase arginine) locus, coding for a plastid targeted tRNA editing enzyme required for efficient codon recognition. Comparison of protein content from representative thylakoid complexes, the cytochrome bf complex, and the ATP synthase, suggests that inefficient translation of hcef2 leads to compromised complex assembly or stability leading to alterations in stoichiometries of major thylakoid complexes as well as their constituent subunits. Altered subunit stoichiometries for photosystem I, ratios and properties of cytochrome bf hemes, and the decay kinetics of the flash-induced thylakoid electric field suggest that these defect lead to accumulation of H₂O₂ in hcef2, which we have previously shown leads to activation of NDH-related CEF. We observed similar increases in CEF, as well as increases in H₂O₂ accumulation, in other translation defective mutants. This suggests that loss of coordination in plastid protein levels lead to imbalances in photosynthetic energy balance that leads to an increase in CEF. These results taken together with a large body of previous observations, support a general model in which processes that lead to imbalances in chloroplast energetics result in the production of H₂O₂, which in turn activates CEF. This activation could be from either H₂O₂ acting as a redox signal, or by a secondary effect from H₂O₂ inducing a deficit in ATP.

Cyclic electron flow provides acclimatory plasticity for the photosynthetic machinery under various environmental conditions and developmental stages

Photosynthetic electron flow operates in two modes, linear and cyclic. In cyclic electron flow (CEF), electrons are recycled around photosystem I. As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast. At least two routes for CEF exist: a PROTON GRADIENT REGULATION5-PGRL1-and a chloroplast NDH-like complex mediated pathway. This review focuses on recent findings concerning the characteristics of both CEF routes in higher plants, with special emphasis paid on the crucial role of CEF in under challenging environmental conditions and developmental stages.

Acceleration of cyclic electron flow in rice plants (Oryza sativa L.) deficient in the PsbS protein of Photosystem II

When compared with Photosystem I (PSI) in wild-type (WT) rice plants, PSI in PsbS-knockout (KO) plants that lack the energy-dependent component of nonphotochemical quenching (NPQ) was less sensitive to photoinhibition. Therefore, we investigated the relationship between NPQ and cyclic electron flow (CEF) around PSI as a photoprotective mechanism. Activities of two CEF routes (PGR5-dependent or NDH-dependent) were compared between those genotypes by using both dark-adapted plants and pre-illuminated plants, i.e., those in which the Calvin-Benson cycle is de-activated and activated, respectively. In dark-adapted leaves activity of the PGR5-dependent route was determined as the rate of P700 photooxidation. Activity was higher in the mutants than in the WT. However, no difference was noted when plants of either genotype were pre-illuminated. When the electron transport pathway was switched to the cyclic mode by infiltrating leaf segments with 150 mM sorbitol, 40 μM DCMU, and 2 mM hydroxylamine, the rate of P700 oxidation was faster in the mutant. That difference disappeared when leaves were infiltrated with antimycin A to inhibit the PGR5-dependent route. Chlorophyll fluorescence (Fo) was also evaluated. To achieve an Fo level comparable to that of the WT, activation of the NDH-dependent route in the mutant required pre-illumination at a certain dose. Therefore, we propose that, as an alternate pathway for the photoprotection of photosystems in the absence of energy-dependent quenching, this PGR5-dependent route is more highly activated in the PsbS-KO mutants than in the WT. Moreover, that stronger activity is probably responsible for slower activation of the NDH-dependent route in the mutant.

Synechocystis: Not Just a Plug-Bug for CO2, but a Green E. coli

Following multiple reports warning for threats posed by raising levels of atmospheric CO2, it is of paramount importance that human society rapidly evolves to be sustainable. Processes relying on photosynthetic microorganisms, converting CO2 and water into compounds of interest, fueled by light, are very pertinent, particularly if not directly competing for arable land. Here, we identify specific research questions that remain to be targeted to exploit the full potential of cyanobacterial cell factories. We argue that this approach will be more likely to be successful if organisms such as Synechocystis are not perceived as mere chassis for CO2 fixation, but rather considered as the "green" E. coli.

The end of the line: can ferredoxin and ferredoxin NADP(H) oxidoreductase determine the fate of photosynthetic electrons?

At the end of the linear photosynthetic electron transfer (PET) chain, the small soluble protein ferredoxin (Fd) transfers electrons to Fd:NADP(H) oxidoreductase (FNR), which can then reduce NADP+ to support C assimilation. In addition to this linear electron flow (LEF), Fd is also thought to mediate electron flow back to the membrane complexes by different cyclic electron flow (CEF) pathways: either antimycin A sensitive, NAD(P)H complex dependent, or through FNR located at the cytochrome b6f complex. Both Fd and FNR are present in higher plant genomes as multiple gene copies, and it is now known that specific Fd iso-proteins can promote CEF. In addition, FNR iso-proteins vary in their ability to dynamically interact with thylakoid membrane complexes, and it has been suggested that this may also play a role in CEF. We will highlight work on the different Fd-isoproteins and FNR-membrane association found in the bundle sheath (BSC) and mesophyll (MC) cell chloroplasts of the C4 plant maize. These two cell types perform predominantly CEF and LEF, and the properties and activities of Fd and FNR in the BSC and MC are therefore specialized for CEF and LEF respectively. A diversity of Fd isoproteins and dynamic FNR location has also been recorded in C3 plants, algae and cyanobacteria. This indicates that the principles learned from the extreme electron transport situations in the BSC and MC of maize might be usefully applied to understanding the dynamic transition between these states in other systems.

Regulation of cyclic electron flow in Chlamydomonas reinhardtii under fluctuating carbon availability

The chloroplast must rapidly and precisely adjust photosynthetic ATP and NADPH output to meet changing metabolic demands imposed by fluctuating environmental conditions. Cyclic electron flow (CEF) around photosystem I is thought to contribute to this adjustment by providing ATP in excess of that supplied by linear electron low, balancing chloroplast energy budget when relative demand for ATP is high. We assessed the kinetics and energy production of CEF activation in Chlamydomonas reinhardtii under rapid changes of organic and inorganic carbon availability. Comparisons of transient electric field and chlorophyll fluorescence measurements indicated CEF was activated under conditions where ATP demand is expected to be high, consistent with a role in balancing the cellular ATP/NADPH budget under fluctuating environmental or metabolic conditions. CEF activation was not correlated with antenna state transitions, both in wild-type and the state transition mutant stt7-9, suggesting that CEF is rapidly regulated by allosteric or redox modulators. Comparing the CEF under ambient and high CO2 conditions suggests an increase in required energy output of approximately 1ATP/CO2 fixed, nearly sufficient to power proposed mechanistic models for the carbon-concentrating mechanism. Additionally, we see three-fold higher CEF rates in cells under steady-state conditions than cells under similar conditions with inhibited photosystem II, and up to five times higher in cells with severe depletion of inorganic carbon, implying that CEF has larger energetic capacity than predicted from some previous work.

Estimation of the steady-state cyclic electron flux around PSI in spinach leaf discs in white light, CO2-enriched air and other varied conditions

Cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis and aids photoprotection, especially in stressful conditions, but the difficulty in quantifying CEF is non-trivial. The total electron flux through PSI (ETR1) and the linear electron flux (LEFO2) through both photosystems in spinach leaf discs were estimated from the photochemical yield of PSI and the gross oxygen evolution rate, respectively, in CO2-enriched air. ΔFlux=ETR1 - LEFO2 is an upper estimate of CEF. Infiltration of leaf discs with 150μM antimycin A did not affect LEFO2, but decreased ΔFlux 10-fold. ΔFlux was practically negligible below 350μmolphotonsm-2s-1, but increased linearly above it. The following results were obtained at 980μmolphotonsm-2s-1. ΔFlux increased 3-fold as the temperature increased from 5°C to 40°C. It did not decline at high temperature, even when LEFO2 decreased. ΔFlux increased by 80% as the relative water content of leaf discs decreased from 100 to 40%, when LEFO2 decreased 2-fold. The method of using ΔFlux as a non-intrusive upper estimate of steady-state CEF in leaf tissue appears reasonable when photorespiration is suppressed.

Thioredoxin m4 controls photosynthetic alternative electron pathways in Arabidopsis

In addition to the linear electron flow, a cyclic electron flow (CEF) around photosystem I occurs in chloroplasts. In CEF, electrons flow back from the donor site of photosystem I to the plastoquinone pool via two main routes: one that involves the Proton Gradient Regulation5 (PGR5)/PGRL1 complex (PGR) and one that is dependent of the NADH dehydrogenase-like complex. While the importance of CEF in photosynthesis and photoprotection has been clearly established, little is known about its regulation. We worked on the assumption of a redox regulation and surveyed the putative role of chloroplastic thioredoxins (TRX). Using Arabidopsis (Arabidopsis thaliana) mutants lacking different TRX isoforms, we demonstrated in vivo that TRXm4 specifically plays a role in the down-regulation of the NADH dehydrogenase-like complex-dependent plastoquinone reduction pathway. This result was confirmed in tobacco (Nicotiana tabacum) plants overexpressing the TRXm4 orthologous gene. In vitro assays performed with isolated chloroplasts and purified TRXm4 indicated that TRXm4 negatively controls the PGR pathway as well. The physiological significance of this regulation was investigated under steady-state photosynthesis and in the pgr5 mutant background. Lack of TRXm4 reversed the growth phenotype of the pgr5 mutant, but it did not compensate for the impaired photosynthesis and photoinhibition sensitivity. This suggests that the physiological role of TRXm4 occurs in vivo via a mechanism distinct from direct up-regulation of CEF.

Cyclic electron flow plays an important role in photoprotection for the resurrection plant Paraboea rufescens under drought stress

Resurrection plants could survive severe drought stress, but the underlying mechanism for protecting their photosynthetic apparatus against drought stress is unclear. Cyclic electron flow (CEF) has been documented as a crucial mechanism for photoprotection in Arabidopsis and tobacco. We hypothesized that CEF plays an important role in protecting photosystem I (PSI) and photosystem II (PSII) against drought stress for resurrection plants. To address this hypothesis, the effects of mild drought stress on light energy distribution in PSII and P700 redox state were examined in a resurrection plant Paraboea rufescens. Cyclic electron flow was not activated below the photosynthetic photon flux density (PPFD) of 400 μmol m⁻² s⁻¹ in leaves without drought stress. However, CEF was activated under low light in leaves with mild drought stress, and the effective quantum yield of PSII significantly decreased. Meanwhile, non-photochemical quenching (NPQ) was significantly stimulated not only under high light but also under low light. Compared with the control, the fraction of overall P700 that cannot be oxidized in a given state (PSI acceptor side limitation) under high light was maintained at low level of 0.1 in leaves with water deficit, indicating that the over-reduction of the PSI acceptor side was prevented by the significant stimulation of CEF. Furthermore, methyl viologen could significantly increase the PSII photo-inhibition induced by high light compared with chloramphenicol. These results suggested that CEF is an important mechanism for protecting PSI and PSII from drought stress in resurrection plants.

Thermoluminescence and P700 redox kinetics as complementary tools to investigate the cyclic/chlororespiratory electron pathways in stress conditions in barley leaves

Cyclic electron flow around photosystem I drives additional proton pumping into the thylakoid lumen, which enhances the protective non-photochemical quenching and increases ATP synthesis. It involves several pathways activated independently. In whole barley leaves, P700 oxidation under far-red illumination and subsequent P700(+) dark reduction kinetics provide a major probe of the activation of cyclic pathways. Two 'intermediate' and 'slow' exponential reduction phases are always observed and they become faster after high light illumination, but dark inactivation of the Benson-Calvin cycle causes the emergence of both a transient in the P700 oxidation and a 'fast' phase in the P700(+) reduction. We investigate here the afterglow (AG) thermoluminescence emission as another tool to detect the activation of cyclic electron pathways from stroma reductants to the acceptor side of photosystem II. This transfer is activated by warming, yielding an AG band at about 45°C. However, treatments that accelerate the 'intermediate' and 'slow' P700(+) reduction phases (brief anoxia, hexose infiltration, fast dehydration of excised leaves) also produced a downshift of this AG band. This pathway ascribable to NADPH dehydrogenase (NDH) would be triggered by a deficit in ATP, while the 'fast' reduction phase corresponding to the ferredoxin plastoquinone reductase pathway is triggered by an overreduction of the photosystem I acceptor pool and is undetected in thermoluminescence. Contrastingly, slow dehydration of unwatered plants did not cause faster reduction of P700(+) nor temperature downshift of the AG band, that is no induction of the NDH pathway, whereas an increased intensity of the AG band indicated a strong NADPH + ATP assimilatory potential.

Kinetic analyses and mathematical modeling of primary photochemical and photoelectrochemical processes in plant photosystems

In this paper the model and simulation of primary photochemical and photo-electrochemical reactions in dark-adapted intact plant leaves is presented. A descriptive algorithm has been derived from analyses of variable chlorophyll a fluorescence and P700 oxidation kinetics upon excitation with multi-turnover pulses (MTFs) of variable intensity and duration. These analyses have led to definition and formulation of rate equations that describe the sequence of primary linear electron transfer (LET) steps in photosystem II (PSII) and of cyclic electron transport (CET) in PSI. The model considers heterogeneity in PSII reaction centers (RCs) associated with the S-states of the OEC and incorporates in a dark-adapted state the presence of a 15-35% fraction of Q(B)-nonreducing RCs that probably is identical with the S₀ fraction. The fluorescence induction algorithm (FIA) in the 10 μs-1s excitation time range considers a photochemical O-J-D, a photo-electrochemical J-I and an I-P phase reflecting the response of the variable fluorescence to the electric trans-thylakoid potential generated by the proton pump fuelled by CET in PSI. The photochemical phase incorporates the kinetics associated with the double reduction of the acceptor pair of pheophytin (Phe) and plastoquinone Q(A) [PheQ(A)] in Q(B) nonreducing RCs and the associated doubling of the variable fluorescence, in agreement with the three-state trapping model (TSTM) of PS II. The decline in fluorescence emission during the so called SMT in the 1-100s excitation time range, known as the Kautsky curve, is shown to be associated with a substantial decrease of CET-powered proton efflux from the stroma into the chloroplast lumen through the ATPsynthase of the photosynthetic machinery.

Drought stress-induced upregulation of components involved in ferredoxin-dependent cyclic electron transfer

Linear photosynthetic electron transfer, consisting of both Photosystem (PS) II and PSI, converts light energy into the chemical forms ATP and NADPH, whereas PSI cyclic electron transfer (CET) is exclusively involved in ATP synthesis. In the chloroplasts of higher plants, there are two partially redundant CET routes. The ferredoxin (FD) or ferredoxin-plastoquinone reductase (FQR)-dependent route cycles electrons from PSI to plastoquinone via ferredoxin (FD), while in the NDH-dependent route, NADPH donates electrons to the NDH-complex for reduction of the plastoquinone pool. In the present study, we show that drought stress induces transcriptional and translational upregulation of the PGR5 and PGRL1 genes, which are the only characterized components of the FQR-dependent CET thus far. In contrast, the expression of the NDH-H gene, a representative of the NDH-complex, did not differ between the drought-stressed and the control plants. The overall expression level of the ferredoxin-NADP(+)-oxidoreductase (FNR) genes increased upon drought stress, with a concomitant release of FNR from the thylakoid membrane. Moreover, drought stress accelerated the rate of P700(+) re-reduction, which may indicate induction of CET. Responses of the PSAE, FD and PSAD gene families upon drought stress are also described.

Photoelectrochemical control of the balance between cyclic- and linear electron transport in photosystem I. Algorithm for P700+ induction kinetics

Redox transients of chlorophyll P700, monitored as absorbance changes DeltaA810, were measured during and after exclusive PSI excitation with far-red (FR) light in pea (Pisum sativum, cv. Premium) leaves under various pre-excitation conditions. Prolonged adaptation in the dark terminated by a short PSII+PSI- exciting light pulse guarantees pre-conditions in which the initial photochemical events in PSI RCs are carried out by cyclic electron transfer (CET). Pre-excitation with one or more 10s FR pulses creates conditions for induction of linear electron transport (LET). These converse conditions give rise to totally different, but reproducible responses of P700- oxidation. System analyses of these responses were made based on quantitative solutions of the rate equations dictated by the associated reaction scheme for each of the relevant conditions. These provide the mathematical elements of the P700 induction algorithm (PIA) with which the distinguishable components of the P700+ response can be resolved and interpreted. It enables amongst others the interpretation and understanding of the characteristic kinetic profile of the P700+ response in intact leaves upon 10s illumination with far-red light under the promotive condition for CET. The system analysis provides evidence that this unique kinetic pattern with a non-responsive delay followed by a steep S-shaped signal increase is caused by a photoelectrochemically controlled suppression of the electron transport from Fd to the PQ-reducing Qr site of the cytb6f complex in the cyclic pathway. The photoelectrochemical control is exerted by the PSI-powered proton pump associated with CET. It shows strong similarities with the photoelectrochemical control of LET at the acceptor side of PSII which is reflected by release of photoelectrochemical quenching of chlorophyll a fluorescence.

Fast cyclic electron transport around photosystem I in leaves under far-red light: a proton-uncoupled pathway?

Fast cyclic electron transport (CET) around photosystem I (PS I) was observed in sunflower (Helianthus annuus L.) leaves under intense far-red light (FRL) of up to 200 mumol quanta m(-2) s(-1). The electron transport rate (ETR) through PS I was found from the FRL-dark transmittance change at 810 and 950 nm, which was deconvoluted into redox states and pool sizes of P700, plastocyanin (PC) and cytochrome f (Cyt f). PC and P700 were in redox equilibrium with K(e) = 35 (ΔE(m) = 90 mV). PS II ETR was based on O(2) evolution. CET [(PS I ETR) - (PS II ETR)] increased to 50-70 mumol e(-) m(-2) s(-1) when linear electron transport (LET) under FRL was limited to 5 mumol e(-) m(-2) s(-1) in a gas phase containing 20-40 mumol CO(2) mol(-1) and 20 mumol O(2) mol(-1). Under these conditions, pulse-saturated fluorescence yield F(m) was non-photochemically quenched; however, F(m) was similarly quenched when LET was driven by low green or white light, which energetically precluded the possibility for active CET. We suggest that under FRL, CET is rather not coupled to transmembrane proton translocation than the CET-coupled protons are short-circuited via proton channels regulated to open at high ΔpH. A kinetic analysis of CET electron donors and acceptors suggests the CET pathway is that of the reversed Q-cycle: Fd -> (FNR) -> Cyt c(n) -> Cyt b(h) -> Cyt b(l) -> Rieske FeS -> Cyt f -> PC -> P700 ->-> Fd. CET is activated when PQH(2) oxidation is opposed by high ΔpH, and ferredoxin (Fd) is reduced due to low availability of e(-) acceptors. The physiological significance of CET may be photoprotective, as CET may be regarded as a mechanism of energy dissipation under stress conditions.

Novel effects of methyl viologen on photosystem II function in spinach leaves

Methyl viologen (MV) is a well-known electron mediator that works on the acceptor side of photosystem I. We investigated the little-known, MV-induced inhibition of linear electron flow through photosystem II (PS II) in spinach-leaf discs. Even a low [MV] decreased the (1) average, light-adapted photochemical efficiency of PS II traps, (2) oxidation state of the primary quinone acceptor Q(A) in PS II during illumination, (3) photochemical efficiency of light-adapted open PS II traps, (4) fraction of absorbed light energy dissipated constitutively in a light-independent manner or as chlorophyll (Chl) a fluorescence emission, (5) Chl a fluorescence yield corresponding to dark-adapted open reaction-center traps (F (o)) and closed reaction-center traps (F (m)), and (6) half-time for re-oxidation of Q (A) (-) in PS II after a single-turnover flash. These effects suggest that the presence of MV accelerates various "downhill" electron-transfer steps in PS II. Therefore, when using the MV to quantify cyclic electron flow, the inhibitory effect of MV on PS II should be taken into account.

Differential effects of severe water stress on linear and cyclic electron fluxes through Photosystem I in spinach leaf discs in CO(2)-enriched air

Linear and cyclic electron fluxes through Photosystem I in 1% CO(2) were quantified in spinach leaf tissue under severe water stress. Using actinic light with a peak at 697 nm for preferential light absorption by Photosystem I while also stimulating Photosystem II to improve redox poising, the cyclic electron flux after 60 s of illumination was a substantial proportion (33-44%) of the total electron flux through PSI at irradiances up to ~1,070 micromol photons m(-2) s(-1). At the maximum irradiance, the cyclic electron flux changed little with the progressive water loss from leaf tissue up to ~60%; by contrast, the linear electron flux was approximately halved. A reason for this differential effect of water stress on the capacity for cyclic and linear electron flow could be the increased crowding of soluble proteins in the stroma due to chloroplast shrinkage. Indeed the confinement of soluble proteins to a smaller chloroplast volume was indicated by cryo-scanning electron microscopy. It is known that the diffusion coefficient of large proteins is decreased when the background concentration of small proteins is raised; by contrast, the diffusion coefficient of small proteins is not affected by increasing the concentration of a large protein (Muramatsu and Minton in Proc Natl Acad Sci USA 85:2984-2988, 1988). Therefore, we suggest that linear electron flow, being coupled to the Calvin-Benson cycle, is limited by the diffusion of large macromolecules, especially the ribulose 1, 5-bisphosphate carboxylase/oxygenase complex. By contrast, cyclic electron flow, involving relatively small macromolecules such as ferredoxin, is less susceptible to inhibition by crowding in the stroma.

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.