Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Ectopic expression of a cyanobacterial flavodoxin in creeping bentgrass impacts plant development and confers broad abiotic stress tolerance

Flavodoxin (Fld) plays a pivotal role in photosynthetic microorganisms as an alternative electron carrier flavoprotein under adverse environmental conditions. Cyanobacterial Fld has been demonstrated to be able to substitute ferredoxin of higher plants in most electron transfer processes under stressful conditions. We have explored the potential of Fld for use in improving plant stress response in creeping bentgrass (Agrostis stolonifera L.). Overexpression of Fld altered plant growth and development. Most significantly, transgenic plants exhibited drastically enhanced performance under oxidative, drought and heat stress as well as nitrogen (N) starvation, which was associated with higher water retention and cell membrane integrity than wild-type controls, modified expression of heat-shock protein genes, production of more reduced thioredoxin, elevated N accumulation and total chlorophyll content as well as up-regulated expression of nitrite reductase and N transporter genes. Further analysis revealed that the expression of other stress-related genes was also impacted in Fld-expressing transgenics. Our data establish a key role of Fld in modulating plant growth and development and plant response to multiple sources of adverse environmental conditions in crop species. This demonstrates the feasibility of manipulating Fld in crop species for genetic engineering of plant stress tolerance.

Contribution of Cyclic and Pseudo-cyclic Electron Transport to the Formation of Proton Motive Force in Chloroplasts

Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, resulting in the formation of a trans-thylakoid proton gradient (ΔpH) and membrane potential (Δψ). Ion transporters and channels localized to the thylakoid membrane regulate the contribution of each component to the proton motive force (pmf). Although both ΔpH and Δψ contribute to ATP synthesis as pmf, only ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen by inducing thermal dissipation of excessive absorbed light energy from photosystem II antennae and slowing down of the electron transport through the cytochrome b₆f complex. To optimize the tradeoff between efficient light energy utilization and protection of both photosystems against photodamage, plants have to regulate the pmf amplitude and its components, ΔpH and Δψ. Cyclic electron transport around photosystem I (PSI) is a major regulator of the pmf amplitude by generating pmf independently of the net production of NADPH by linear electron transport. Chloroplast ATP synthase relaxes pmf for ATP synthesis, and its activity should be finely tuned for maintaining the size of the pmf during steady-state photosynthesis. Pseudo-cyclic electron transport mediated by flavodiiron protein (Flv) forms a large electron sink, which is essential for PSI photoprotection in fluctuating light in cyanobacteria. Flv is conserved from cyanobacteria to gymnosperms but not in angiosperms. The Arabidopsis proton gradient regulation 5 (pgr5) mutant is defective in the main pathway of PSI cyclic electron transport. By introducing Physcomitrella patens genes encoding Flvs, the function of PSI cyclic electron transport was substituted by that of Flv-dependent pseudo-cyclic electron transport. In transgenic plants, the size of the pmf was complemented to the wild-type level but the contribution of ΔpH to the total pmf was lower than that in the wild type. In the pgr5 mutant, the size of the pmf was drastically lowered by the absence of PSI cyclic electron transport. In the mutant, ΔpH occupied the majority of pmf, suggesting the presence of a mechanism for the homeostasis of luminal pH in the light. To avoid damage to photosynthetic electron transport by periods of excess solar energy, plants employ an intricate regulatory network involving alternative electron transport pathways, ion transporters/channels, and pH-dependent mechanisms for downregulating photosynthetic electron transport.

Role of Ions in the Regulation of Light-Harvesting

Regulation of photosynthetic light harvesting in the thylakoids is one of the major key factors affecting the efficiency of photosynthesis. Thylakoid membrane is negatively charged and influences both the structure and the function of the primarily photosynthetic reactions through its electrical double layer (EDL). Further, there is a heterogeneous organization of soluble ions (K+, Mg2+, Cl-) attached to the thylakoid membrane that, together with fixed charges (negatively charged amino acids, lipids), provides an electrical field. The EDL is affected by the valence of the ions and interferes with the regulation of "state transitions," protein interactions, and excitation energy "spillover" from Photosystem II to Photosystem I. These effects are reflected in changes in the intensity of chlorophyll a fluorescence, which is also a measure of photoprotective non-photochemical quenching (NPQ) of the excited state of chlorophyll a. A triggering of NPQ proceeds via lumen acidification that is coupled to the export of positive counter-ions (Mg2+, K+) to the stroma or/and negative ions (e.g., Cl-) into the lumen. The effect of protons and anions in the lumen and of the cations (Mg2+, K+) in the stroma are, thus, functionally tightly interconnected. In this review, we discuss the consequences of the model of EDL, proposed by Barber (1980b) Biochim Biophys Acta 594:253-308) in light of light-harvesting regulation. Further, we explain differences between electrostatic screening and neutralization, and we emphasize the opposite effect of monovalent (K+) and divalent (Mg2+) ions on light-harvesting and on "screening" of the negative charges on the thylakoid membrane; this effect needs to be incorporated in all future models of photosynthetic regulation by ion channels and transporters.

Light acclimation of shade-tolerant and light-resistant Tradescantia species: induction of chlorophyll a fluorescence and P700 photooxidation, expression of PsbS and Lhcb1 proteins

In this work, we have compared photosynthetic performance and expression of the PsbS and Lhcb1 proteins in two contrast ecotypes of Tradescantia species, T. fluminensis (shade-tolerant) and T. sillamontana (light-resistant), grown at two intensities of light: 50-125 μmol photons m^-2 s^-1 (low light, LL) and 875-1000 μmol photons m^-2 s^-1 (high light, HL). Using the EPR method for measuring the P₇₀₀ content, we have found that LL-grown plants of both species have higher (by a factor of ≈1.7-1.8) contents of PSI per fresh weight unit as compared to HL-grown plants. Acclimation of plants to LL or HL irradiation also influences the Chl(a + b) level and expression of the PsbS and Lhcb1 proteins. Immunoblotting analysis showed that acclimation to HL stimulates (by a factor of ≈1.7-1.8) the level of PsbS related to the total number of P₇₀₀ centers. In light-resistant species T. sillamontana, the ratio PsbS/P₇₀₀ is about 2-times higher than in shade-tolerant species T. fluminensis grown under the same conditions. This should enhance the capacity of their leaves for protection against the light stress. In agreement with these observations, the capacity of leaves for NPQ induction was enhanced during plant acclimation to HL. Kinetic studies of P₇₀₀ photooxidation and light-induced changes in the yield of Chl a fluorescence also revealed that the short-term regulation of electron transport processes in chloroplasts, which manifested themselves in the kinetics of [Formula: see text] induction and the rate of Chl a fluorescence quenching, occurred more rapidly in HL-grown plants than in LL-grown plants. Thus, both factors, enhanced expression of PsbS and more rapid response of the photosynthetic electron transport chain to dark-to-light transitions should increase the capacity of HL-grown plants for their resistance to rapid fluctuations of solar light.

In silico metabolic network analysis of Arabidopsis leaves

Background

During the last decades, we face an increasing interest in superior plants to supply growing demands for human and animal nutrition and for the developing bio-based economy. Presently, our limited understanding of their metabolism and its regulation hampers the targeted development of desired plant phenotypes. In this regard, systems biology, in particular the integration of metabolic and regulatory networks, is promising to broaden our knowledge and to further explore the biotechnological potential of plants.

Results

The thale cress Arabidopsis thaliana provides an ideal model to understand plant primary metabolism. To obtain insight into its functional properties, we constructed a large-scale metabolic network of the leaf of A. thaliana. It represented 511 reactions with spatial separation into compartments. Systematic analysis of this network, utilizing elementary flux modes, investigates metabolic capabilities of the plant and predicts relevant properties on the systems level: optimum pathway use for maximum growth and flux re-arrangement in response to environmental perturbation. Our computational model indicates that the A. thaliana leaf operates near its theoretical optimum flux state in the light, however, only in a narrow range of photon usage. The simulations further demonstrate that the natural day-night shift requires substantial re-arrangement of pathway flux between compartments: 89 reactions, involving redox and energy metabolism, substantially change the extent of flux, whereas 19 reactions even invert flux direction. The optimum set of anabolic pathways differs between day and night and is partly shifted between compartments. The integration with experimental transcriptome data pinpoints selected transcriptional changes that mediate the diurnal adaptation of the plant and superimpose the flux response.

Conclusions

The successful application of predictive modelling in Arabidopsis thaliana can bring systems-biological interpretation of plant systems forward. Using the gained knowledge, metabolic engineering strategies to engage plants as biotechnological factories can be developed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0347-3) contains supplementary material, which is available to authorized users.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage

The thylakoid proton motive force (pmf) generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated pmf on photosynthesis in a library of Arabidopsis thaliana mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state pmf extents. We observed the expected pmf-dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage. Detailed analyses indicate this effect is related to an elevated electric field (Δψ) component of the pmf, rather than lumen acidification, which in vivo increased PSII charge recombination rates, producing singlet oxygen and subsequent photodamage. The effects are seen even in wild type plants, especially under fluctuating illumination, suggesting that Δψ-induced photodamage represents a previously unrecognized limiting factor for plant productivity under dynamic environmental conditions seen in the field.

DOI:http://dx.doi.org/10.7554/eLife.16921.001

The antimycin A-sensitive pathway of cyclic electron flow: from 1963 to 2015

Cyclic electron flow has puzzled and divided the field of photosynthesis researchers for decades. This mainly concerns the proportion of its overall contribution to photosynthesis, as well as its components and molecular mechanism. Yet, it is irrefutable that the absence of cyclic electron flow has severe effects on plant growth. One of the two pathways mediating cyclic electron flow can be inhibited by antimycin A, a chemical that has also widely been used to characterize the mitochondrial respiratory chain. For the characterization of cyclic electron flow, antimycin A has been used since 1963, when ferredoxin was found to be the electron donor of the pathway. In 2013, antimycin A was used to identify the PGRL1/PGR5 complex as the ferredoxin:plastoquinone reductase completing the last puzzle piece of this pathway. The controversy has not ended, and here, we review the history of research on this process using the perspective of antimycin A as a crucial chemical for its characterization.

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.

Protection of the photosynthetic apparatus against dehydration stress in the resurrection plant Craterostigma pumilum

The group of homoiochlorophyllous resurrection plants evolved the unique capability to survive severe drought stress without dismantling the photosynthetic machinery. This implies that they developed efficient strategies to protect the leaves from reactive oxygen species (ROS) generated by photosynthetic side reactions. These strategies, however, are poorly understood. Here, we performed a detailed study of the photosynthetic machinery in the homoiochlorophyllous resurrection plant Craterostigma pumilum during dehydration and upon recovery from desiccation. During dehydration and rehydration, C. pumilum deactivates and activates partial components of the photosynthetic machinery in a specific order, allowing for coordinated shutdown and subsequent reinstatement of photosynthesis. Early responses to dehydration are the closure of stomata and activation of electron transfer to oxygen accompanied by inactivation of the cytochrome b6 f complex leading to attenuation of the photosynthetic linear electron flux (LEF). The decline in LEF is paralleled by a gradual increase in cyclic electron transport to maintain ATP production. At low water contents, inactivation and supramolecular reorganization of photosystem II becomes apparent, accompanied by functional detachment of light-harvesting complexes and interrupted access to plastoquinone. This well-ordered sequence of alterations in the photosynthetic thylakoid membranes helps prepare the plant for the desiccated state and minimize ROS production.

Regulatory network of proton motive force: contribution of cyclic electron transport around photosystem I

Cyclic electron transport around photosystem I (PSI) generates ∆pH across the thylakoid membrane without net production of NADPH. In angiosperms, two pathways of PSI cyclic electron transport operate. The main pathway depends on PGR5/PGRL1 proteins and is likely identical to the historical Arnon's pathway. The minor pathway depends on chloroplast NADH dehydrogenase-like (NDH) complex. In assays of their rates in vivo, the two independent pathways are often mixed together. Theoretically, linear electron transport from water to NADP(+) cannot satisfy the ATP/NADPH production ratio required by the Calvin-Benson cycle and photorespiration. PGR5/PGRL1-dependent PSI cyclic electron transport contributes substantially to the supply of ATP for CO2 fixation, as does linear electron transport. Also, the contribution of chloroplast NDH cannot be ignored, especially at low light intensity, although the extent of the contribution depends on the plant species. An increase in proton conductivity of ATP synthase may compensate ATP synthesis to some extent in the pgr5 mutant. Combined with the decreased rate of ∆pH generation, however, this mechanism sacrifices homeostasis of the thylakoid lumen pH, seriously disturbing the pH-dependent regulation of photosynthetic electron transport, induction of qE, and downregulation of the cytochrome b 6 f complex. PGR5/PGRL1-dependent PSI cyclic electron transport produces sufficient proton motive force for ATP synthesis and the regulation of photosynthetic electron transport.

Live-cell visualization of excitation energy dynamics in chloroplast thylakoid structures

The intricate molecular processes underlying photosynthesis have long been studied using various analytic approaches. However, the three-dimensional (3D) dynamics of such photosynthetic processes remain unexplored due to technological limitations related to investigating intraorganellar mechanisms in vivo. By developing a system for high-speed 3D laser scanning confocal microscopy combined with high-sensitivity multiple-channel detection, we visualized excitation energy dynamics in thylakoid structures within chloroplasts of live Physcomitrella patens cells. Two distinct thylakoid structures in the chloroplast, namely the grana and stroma lamellae, were visualized three-dimensionally in live cells. The simultaneous detection of the shorter (than ~670 nm) and longer (than ~680 nm) wavelength regions of chlorophyll (Chl) fluorescence reveals different spatial characteristics-irregular and vertical structures, respectively. Spectroscopic analyses showed that the shorter and longer wavelength regions of Chl fluorescence are affected more by free light-harvesting antenna proteins and photosystem II supercomplexes, respectively. The high-speed 3D time-lapse imaging of the shorter and longer wavelength regions also reveals different structural dynamics-rapid and slow movements within 1.5 seconds, respectively. Such structural dynamics of the two wavelength regions of Chl fluorescence would indicate excitation energy dynamics between light-harvesting antenna proteins and photosystems, reflecting the energetically active nature of photosynthetic proteins in thylakoid membranes.

Promotion of Cyclic Electron Transport Around Photosystem I with the Development of C4 Photosynthesis

C4 photosynthesis is present in approximately 7,500 species classified into 19 families, including monocots and eudicots. In the majority of documented cases, a two-celled CO2-concentrating system that uses a metabolic cycle of four-carbon compounds is employed. C4 photosynthesis repeatedly evolved from C3 photosynthesis, possibly driven by the survival advantages it bestows in the hot, often dry, and nutrient-poor soils of the tropics and subtropics. The development of the C4 metabolic cycle greatly increased the ATP demand in chloroplasts during the evolution of malic enzyme-type C4 photosynthesis, and the additional ATP required for C4 metabolism may be produced by the cyclic electron transport around PSI. Recent studies have revealed the nature of cyclic electron transport and the elevation of its components during C4 evolution. In this review, we discuss the energy requirements of C3 and C4 photosynthesis, the current model of cyclic electron transport around PSI and how cyclic electron transport is promoted during C4 evolution using studies on the genus Flaveria, which contains a number of closely related C3, C4 and C3-C4 intermediate species.

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.

Physiological Functions of Cyclic Electron Transport Around Photosystem I in Sustaining Photosynthesis and Plant Growth

The light reactions in photosynthesis drive both linear and cyclic electron transport around photosystem I (PSI). Linear electron transport generates both ATP and NADPH, whereas PSI cyclic electron transport produces ATP without producing NADPH. PSI cyclic electron transport is thought to be essential for balancing the ATP/NADPH production ratio and for protecting both photosystems from damage caused by stromal overreduction. Two distinct pathways of cyclic electron transport have been proposed in angiosperms: a major pathway that depends on the PROTON GRADIENT REGULATION 5 (PGR5) and PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1) proteins, which are the target site of antimycin A, and a minor pathway mediated by the chloroplast NADH dehydrogenase-like (NDH) complex. Recently, the regulation of PSI cyclic electron transport has been recognized as essential for photosynthesis and plant growth. In this review, we summarize the possible functions and importance of the two pathways of PSI cyclic electron transport.

The sporulation of the green alga Ulva prolifera is controlled by changes in photosynthetic electron transport chain

Sporulation and spore release are essential phases of the life cycle in algae and land plants. Ulva prolifera, which is an ideal organism for studying sporulation and spore release, was used as the experimental material in the present study. The determination of photosynthetic parameters, combined with microscopic observation, treatment with photosynthetic inhibitors, limitation of carbon acquisition, and protein mass spectrometry, was employed in this experiment. Cycle electron transport (CEF) was found enhanced at the onset of sporangia formation. The inhibition effect of dibromothymoquinone (DBMIB) towards sporulation was always strong during the sporulation process whereas the inhibition effect of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) was continuously declined accompanied with the progress of sporulation. The changes of photosynthesis resulted from the limitation of CO₂ acquisition could stimulate sporulation onset. Quantitative protein analysis showed that enzymes involved in carbon fixation, including RUBISCO and pyruvate orthophosphate dikinase, declined during sporogenesis, while proteins involved in sporulation, including tubulin and centrin, increased. These results suggest that enhanced cyclic electron flow (CEF) and oxidation of the plastoquinone pool are essential for sporangia formation onset, and changes in photosynthetic electron transport chain have significant impacts on sporulation of the green algae.

The chloroplast NADPH thioredoxin reductase C, NTRC, controls non-photochemical quenching of light energy and photosynthetic electron transport in Arabidopsis

High irradiances may lead to photooxidative stress in plants, and non-photochemical quenching (NPQ) contributes to protection against excess excitation. One of the NPQ mechanisms, qE, involves thermal dissipation of the light energy captured. Importantly, plants need to tune down qE under light-limiting conditions for efficient utilization of the available quanta. Considering the possible redox control of responses to excess light implying enzymes, such as thioredoxins, we have studied the role of the NADPH thioredoxin reductase C (NTRC). Whereas Arabidopsis thaliana plants lacking NTRC tolerate high light intensities, these plants display drastically elevated qE, have larger trans-thylakoid ΔpH and have 10-fold higher zeaxanthin levels under low and medium light intensities, leading to extremely low linear electron transport rates. To test the impact of the high qE on plant growth, we generated an ntrc-psbs double-knockout mutant, which is devoid of qE. This double mutant grows faster than the ntrc mutant and has a higher chlorophyll content. The photosystem II activity is partially restored in the ntrc-psbs mutant, and linear electron transport rates under low and medium light intensities are twice as high as compared with plants lacking ntrc alone. These data uncover a new role for NTRC in the control of photosynthetic yield.

Response of Chloroplast NAD(P)H Dehydrogenase-Mediated Cyclic Electron Flow to a Shortage or Lack in Ferredoxin-Quinone Oxidoreductase-Dependent Pathway in Rice Following Short-Term Heat Stress

Cyclic electron flow (CEF) around photosystem I (PSI) can protect photosynthetic electron carriers under conditions of stromal over-reduction. The goal of the research reported in this paper was to investigate the responses of both PSI and photosystem II (PSII) to a short-term heat stress in two rice lines with different capacities of cyclic electron transfer, i.e., Q4149 with a high capacity (hcef) and C4023 with a low capacity (lcef). The absorbance change at 820 nm (ΔA820) was used here to assess the charge separation in the PSI reaction center (P700). The results obtained show that short-term heat stress abolishes the ferredoxin-quinone oxidoreductase (FQR)-dependent CEF in rice and accelerates the initial rate of P700 (+) re-reduction. The P700 (+) amplitude was slightly increased at a moderate heat-stress (35°C) because of a partial restriction of FQR but it was decreased following high heat-stress (42°C). Assessment of PSI and PSII activities shows that PSI is more susceptible to heat stress than PSII. Under high temperature, FQR-dependent CEF was completely removed and NDH-dependent CEF was up-regulated and strengthened to a higher extent in C4023 than in Q4149. Specifically, under normal growth temperature, hcef (Q4149) was characterized by higher FQR- and chloroplast NAD(P)H dehydrogenase (NDH)-dependent CEF rates than lcef (C4023). Following thermal stress, the activation of NDH-pathway was 130 and 10% for C4023 and Q4149, respectively. Thus, the NDH-dependent CEF may constitute the second layer of plant protection and defense against heat stress after the main route, i.e., FQR-dependent CEF, reaches its capacity. We discuss the possibility that under high heat stress, the NDH pathway serves as a safety valve to dissipate excess energy by cyclic photophosphorylation and overcome the stroma over-reduction following inhibition of CO2 assimilation and any shortage or lack in the FQR pathway. The potential role of the NDH-dependent pathway during the evolution of C4 photosynthesis is briefly discussed.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

G6PDH activity highlights the operation of the cyclic electron flow around PSI in Physcomitrella patens during salt stress

Photosynthetic performances and glucose-6-phosphate dehydrogenase (G6PDH) activity in Physcomitrella patens changed greatly during salt stress and recovery. In P. patens, the cyclic electron flow around photosystem (PS) I was much more tolerant to high salt stress than PSII. After high salt stress, the PSII activity recovered much more slowly than that of PSI, which was rapidly restored to pretreatment levels even as PSII was almost inactivate. This result suggested that after salt stress the recovery of the cyclic electron flow around PSI was independent of PSII activity. In addition, G6PDH activity and NADPH content increased under high salt stress. When G6PDH activity was inhibited by glucosamine (Glucm, a G6PDH inhibitor), the cyclic electron flow around PSI and the NADPH content decreased significantly. Additionally, after recovery in liquid medium containing Glucm, the PSI activity was much lower than in liquid medium without Glucm. These results suggested the PSI activity was affected significantly by G6PDH activity and the NADPH content. Based on the above results, we propose that G6PDH in P. patens has a close relationship with the photosynthetic process, possibly providing NADPH for the operation of the cyclic electron flow around PSI during salt stress and promoting the restoration of PSI.

The Coordination of Gene Expression within Photosynthesis Pathway for Acclimation of C4 Energy Crop Miscanthus lutarioriparius

As a promising candidate for the second-generation C4 energy crop, Miscanthus lutarioriparius has well acclimated to the water-limited and high-light Loess Plateau in China by improving photosynthesis rate and water use efficiency (WUE) compared to its native habitat along Yangtze River. Photosynthetic genes were demonstrated as one major category of the candidate genes underlying the physiological superiority. To further study how photosynthetic genes interact to improve the acclimation potential of M. lutarioriparius, population expression patterns within photosynthesis pathway were explored between one mild environment and one harsh environment. We found that 108 transcripts in assembled transcriptome of M. lutarioriparius were highly similar to genes in three Kyoto Encyclopedia of Genes and Genomes (KEGG) photosynthesis pathways of sorghum and maize. Phylogenetic analyses using sorghum, maize, rice, and Arabidopsis genes of dark reaction identified 23 orthologs and 30 paralogs of M. lutarioriparius photosynthetic genes. These genes were also clustered into two kinds of expression pattern. 87% of transcripts in dark reaction were up-regulated and all 14 chloroplast-encoded transcripts in light reaction increased degradation in the harsh environment compared to the mild environment. Moreover, 80.8% of photosynthetic transcripts were coordinated at transcription level under the two environments. Interestingly, LHCI and PSI were significantly correlated with F-ATPase and C4 cycle. Overall, this study indicates the coordinated expression between cyclic electron transport (consisting of LHCI, PSI, and ATPase) and CO2-concentrating mechanism (C4 cycle) could account for photosynthesis plasticity on M. lutarioriparius acclimation potential.

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.

Burkholderia phytofirmans PsJN Confers Grapevine Resistance against Botrytis cinerea via a Direct Antimicrobial Effect Combined with a Better Resource Mobilization

Plant innate immunity serves as a surveillance system by providing the first line of powerful weapons to fight against pathogen attacks. Beneficial microorganisms and Microbial-Associated Molecular Patterns might act as signals to trigger this immunity. Burkholderia phytofirmans PsJN, a highly efficient plant beneficial endophytic bacterium, promotes growth in a wide variety of plants including grapevine. Further, the bacterium induces plant resistance against abiotic and biotic stresses. However, no study has deciphered triggered-mechanisms during the tripartite interaction between grapevine, B. phytofirmans PsJN and Botrytis cinerea. Herein, we showed that in contrast with classical rhizobacteria, which are restricted in the root system and act through ISR, B. phytofirmans PsJN is able to migrate until aerial part and forms at leaves surface a biofilm around B. cinerea mycelium to restrict the pathogen. Nevertheless, considering the endophytic level of PsJN in leaves, the plant protection efficacy of B. phytofirmans PsJN could not be explained solely by its direct antifungal effect. Deeper investigations showed a callose deposition, H2O2 production and primed expression of PR1, PR2, PR5, and JAZ only in bacterized-plantlets after pathogen challenge. The presence of PsJN modulated changes in leaf carbohydrate metabolism including gene expression, sugar levels, and chlorophyll fluorescence imaging after Botrytis challenge. Our findings indicated that protection induced by B. phytofirmans PsJN was multifaceted and relied on a direct antifungal effect, priming of defense mechanisms as well as the mobilization of carbon sources in grapevine leaf tissues.

Burkholderia phytofirmans PsJN Confers Grapevine Resistance against Botrytis cinerea via a Direct Antimicrobial Effect Combined with a Better Resource Mobilization

Plant innate immunity serves as a surveillance system by providing the first line of powerful weapons to fight against pathogen attacks. Beneficial microorganisms and Microbial-Associated Molecular Patterns might act as signals to trigger this immunity. Burkholderia phytofirmans PsJN, a highly efficient plant beneficial endophytic bacterium, promotes growth in a wide variety of plants including grapevine. Further, the bacterium induces plant resistance against abiotic and biotic stresses. However, no study has deciphered triggered-mechanisms during the tripartite interaction between grapevine, B. phytofirmans PsJN and Botrytis cinerea. Herein, we showed that in contrast with classical rhizobacteria, which are restricted in the root system and act through ISR, B. phytofirmans PsJN is able to migrate until aerial part and forms at leaves surface a biofilm around B. cinerea mycelium to restrict the pathogen. Nevertheless, considering the endophytic level of PsJN in leaves, the plant protection efficacy of B. phytofirmans PsJN could not be explained solely by its direct antifungal effect. Deeper investigations showed a callose deposition, H2O2 production and primed expression of PR1, PR2, PR5, and JAZ only in bacterized-plantlets after pathogen challenge. The presence of PsJN modulated changes in leaf carbohydrate metabolism including gene expression, sugar levels, and chlorophyll fluorescence imaging after Botrytis challenge. Our findings indicated that protection induced by B. phytofirmans PsJN was multifaceted and relied on a direct antifungal effect, priming of defense mechanisms as well as the mobilization of carbon sources in grapevine leaf tissues.

Effect of Chlamydomonas plastid terminal oxidase 1 expressed in tobacco on photosynthetic electron transfer

The plastid terminal oxidase PTOX is a plastohydroquinone:oxygen oxidoreductase that is important for carotenoid biosynthesis and plastid development. Its role in photosynthesis is controversially discussed. Under a number of abiotic stress conditions, the protein level of PTOX increases. PTOX is thought to act as a safety valve under high light protecting the photosynthetic apparatus against photodamage. However, transformants with high PTOX level were reported to suffer from photoinhibition. To analyze the effect of PTOX on the photosynthetic electron transport, tobacco expressing PTOX-1 from Chlamydomonas reinhardtii (Cr-PTOX1) was studied by chlorophyll fluorescence, thermoluminescence, P700 absorption kinetics and CO2 assimilation. Cr-PTOX1 was shown to compete very efficiently with the photosynthetic electron transport for PQH2 . High pressure liquid chromatography (HPLC) analysis confirmed that the PQ pool was highly oxidized in the transformant. Immunoblots showed that, in the wild-type, PTOX was associated with the thylakoid membrane only at a relatively alkaline pH value while it was detached from the membrane at neutral pH. We present a model proposing that PTOX associates with the membrane and oxidizes PQH2 only when the oxidation of PQH2 by the cytochrome b6 f complex is limiting forward electron transport due to a high proton gradient across the thylakoid membrane.

Ion Channels in Native Chloroplast Membranes: Challenges and Potential for Direct Patch-Clamp Studies

Photosynthesis without any doubt depends on the activity of the chloroplast ion channels. The thylakoid ion channels participate in the fine partitioning of the light-generated proton-motive force (p.m.f.). By regulating, therefore, luminal pH, they affect the linear electron flow and non-photochemical quenching. Stromal ion homeostasis and signaling, on the other hand, depend on the activity of both thylakoid and envelope ion channels. Experimentally, intact chloroplasts and swollen thylakoids were proven to be suitable for direct measurements of the ion channels activity via conventional patch-clamp technique; yet, such studies became infrequent, although their potential is far from being exhausted. In this paper we wish to summarize existing challenges for direct patch-clamping of native chloroplast membranes as well as present available results on the activity of thylakoid Cl⁻ (ClC?) and divalent cation-permeable channels, along with their tentative roles in the p.m.f. partitioning, volume regulation, and stromal Ca²⁺ and Mg²⁺ dynamics. Patch-clamping of the intact envelope revealed both large-conductance porin-like channels, likely located in the outer envelope membrane and smaller conductance channels, more compatible with the inner envelope location. Possible equivalent model for the sandwich-like arrangement of the two envelope membranes within the patch electrode will be discussed, along with peculiar properties of the fast-activated cation channel in the context of the stromal pH control.

De novo assembly and functional annotation of Myrciaria dubia fruit transcriptome reveals multiple metabolic pathways for L-ascorbic acid biosynthesis

Background

Myrciaria dubia is an Amazonian fruit shrub that produces numerous bioactive phytochemicals, but is best known by its high L-ascorbic acid (AsA) content in fruits. Pronounced variation in AsA content has been observed both within and among individuals, but the genetic factors responsible for this variation are largely unknown. The goals of this research, therefore, were to assemble, characterize, and annotate the fruit transcriptome of M. dubia in order to reconstruct metabolic pathways and determine if multiple pathways contribute to AsA biosynthesis.

Results

In total 24,551,882 high-quality sequence reads were de novo assembled into 70,048 unigenes (mean length = 1150 bp, N50 = 1775 bp). Assembled sequences were annotated using BLASTX against public databases such as TAIR, GR-protein, FB, MGI, RGD, ZFIN, SGN, WB, TIGR_CMR, and JCVI-CMR with 75.2 % of unigenes having annotations. Of the three core GO annotation categories, biological processes comprised 53.6 % of the total assigned annotations, whereas cellular components and molecular functions comprised 23.3 and 23.1 %, respectively. Based on the KEGG pathway assignment of the functionally annotated transcripts, five metabolic pathways for AsA biosynthesis were identified: animal-like pathway, myo-inositol pathway, L-gulose pathway, D-mannose/L-galactose pathway, and uronic acid pathway. All transcripts coding enzymes involved in the ascorbate-glutathione cycle were also identified. Finally, we used the assembly to identified 6314 genic microsatellites and 23,481 high quality SNPs.

Conclusions

This study describes the first next-generation sequencing effort and transcriptome annotation of a non-model Amazonian plant that is relevant for AsA production and other bioactive phytochemicals. Genes encoding key enzymes were successfully identified and metabolic pathways involved in biosynthesis of AsA, anthocyanins, and other metabolic pathways have been reconstructed. The identification of these genes and pathways is in agreement with the empirically observed capability of M. dubia to synthesize and accumulate AsA and other important molecules, and adds to our current knowledge of the molecular biology and biochemistry of their production in plants. By providing insights into the mechanisms underpinning these metabolic processes, these results can be used to direct efforts to genetically manipulate this organism in order to enhance the production of these bioactive phytochemicals.

The accumulation of AsA precursor and discovery of genes associated with their biosynthesis and metabolism in M. dubia is intriguing and worthy of further investigation. The sequences and pathways produced here present the genetic framework required for further studies. Quantitative transcriptomics in concert with studies of the genome, proteome, and metabolome under conditions that stimulate production and accumulation of AsA and their precursors are needed to provide a more comprehensive view of how these pathways for AsA metabolism are regulated and linked in this species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2225-6) contains supplementary material, which is available to authorized users.

Increasing cyclic electron flow is related to Na+ sequestration into vacuoles for salt tolerance in soybean

In land plants, the NAD(P)H dehydrogenase (NDH) complex reduces plastoquinones and drives cyclic electron flow (CEF) around PSI. It also produces extra ATP for photosynthesis and improves plant fitness under conditions of abiotic environmental stress. To elucidate the role of CEF in salt tolerance of the photosynthetic apparatus, Na(+) concentration, chlorophyll fluorescence, and expression of NDH B and H subunits, as well as of genes related to cellular and vacuolar Na(+) transport, were monitored. The salt-tolerant Glycine max (soybean) variety S111-9 exhibited much higher CEF activity and ATP accumulation in light than did the salt-sensitive variety Melrose, but similar leaf Na(+) concentrations under salt stress. In S111-9 plants, ndhB and ndhH were highly up-regulated under salt stress and their corresponding proteins were maintained at high levels or increased significantly. Under salt stress, S111-9 plants accumulated Na(+) in the vacuole, but Melrose plants accumulated Na(+) in the chloroplast. Compared with Melrose, S111-9 plants also showed higher expression of some genes associated with Na(+) transport into the vacuole and/or cell, such as genes encoding components of the CBL10 (calcineurin B-like protein 10)-CIPK24 (CBL-interacting protein kinase 24)-NHX (Na(+)/H(+) antiporter) and CBL4 (calcineurin B-like protein 4)-CIPK24-SOS1 (salt overly sensitive 1) complexes. Based on the findings, it is proposed that enhanced NDH-dependent CEF supplies extra ATP used to sequester Na(+) in the vacuole. This reveals an important mechanism for salt tolerance in soybean and provides new insights into plant resistance to salt stress.

Inter-functional analysis of high-throughput phenotype data by non-parametric clustering and its application to photosynthesis

MOTIVATION

Phenomics is the study of the properties and behaviors of organisms (i.e. their phenotypes) on a high-throughput scale. New computational tools are needed to analyze complex phenomics data, which consists of multiple traits/behaviors that interact with each other and are dependent on external factors, such as genotype and environmental conditions, in a way that has not been well studied.

RESULTS

We deployed an efficient framework for partitioning complex and high dimensional phenotype data into distinct functional groups. To achieve this, we represented measured phenotype data from each genotype as a cloud-of-points, and developed a novel non-parametric clustering algorithm to cluster all the genotypes. When compared with conventional clustering approaches, the new method is advantageous in that it makes no assumption about the parametric form of the underlying data distribution and is thus particularly suitable for phenotype data analysis. We demonstrated the utility of the new clustering technique by distinguishing novel phenotypic patterns in both synthetic data and a high-throughput plant photosynthetic phenotype dataset. We biologically verified the clustering results using four Arabidopsis chloroplast mutant lines.

AVAILABILITY AND IMPLEMENTATION

Software is available at www.msu.edu/~jinchen/NPM.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

CONTACT

jinchen@msu.edu, kramerd8@cns.msu.edu or rongjin@cse.msu.edu.

Mitochondrial electron transport protects floating leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition: comparison with submerged leaves

Investigations were carried to unravel mechanism(s) for higher tolerance of floating over submerged leaves of long leaf pondweed (Potamogeton nodosus Poir) against photoinhibition. Chloroplasts from floating leaves showed ~5- and ~6.4-fold higher Photosystem (PS) I (reduced dichlorophenol-indophenol → methyl viologen → O2) and PS II (H2O → parabenzoquine) activities over those from submerged leaves. The saturating rate (V max) of PS II activity of chloroplasts from floating and submerged leaves reached at ~600 and ~230 µmol photons m(-2) s(-1), respectively. Photosynthetic electron transport rate in floating leaves was over 5-fold higher than in submerged leaves. Further, floating leaves, as compared to submerged leaves, showed higher F v/F m (variable to maximum chlorophyll fluorescence, a reflection of PS II efficiency), as well as a higher potential to withstand photoinhibitory damage by high light (1,200 µmol photons m(-2) s(-1)). Cells of floating leaves had not only higher mitochondria to chloroplast ratio, but also showed many mitochondria in close vicinity of chloroplasts. Electron transport (NADH → O2; succinate → O2) in isolated mitochondria of floating leaves was sensitive to both cyanide (CN(-)) and salicylhydroxamic acid (SHAM), whereas those in submerged leaves were sensitive to CN(-), but virtually insensitive to SHAM, revealing the presence of alternative oxidase in mitochondria of floating, but not of submerged, leaves. Further, the potential of floating leaves to withstand photoinhibitory damage was significantly reduced in the presence of CN(-) and SHAM, individually and in combination. Our experimental results establish that floating leaves possess better photosynthetic efficiency and capacity to withstand photoinhibition compared to submerged leaves; and mitochondria play a pivotal role in protecting photosynthetic machinery of floating leaves against photoinhibition, most likely by oxidation of NAD(P)H and reduction of O2.

Induction events and short-term regulation of electron transport in chloroplasts: an overview

Regulation of photosynthetic electron transport at different levels of structural and functional organization of photosynthetic apparatus provides efficient performance of oxygenic photosynthesis in plants. This review begins with a brief overview of the chloroplast electron transport chain. Then two noninvasive biophysical methods (measurements of slow induction of chlorophyll a fluorescence and EPR signals of oxidized P700 centers) are exemplified to illustrate the possibility of monitoring induction events in chloroplasts in vivo and in situ. Induction events in chloroplasts are considered and briefly discussed in the context of short-term mechanisms of the following regulatory processes: (i) pH-dependent control of the intersystem electron transport; (ii) the light-induced activation of the Calvin-Benson cycle; (iii) optimization of electron transport due to fitting alternative pathways of electron flow and partitioning light energy between photosystems I and II; and (iv) the light-induced remodeling of photosynthetic apparatus and thylakoid membranes.

Spectral effects of LEDs on chlorophyll fluorescence and pigmentation in Phalaenopsis 'Vivien' and 'Purple Star'

We examined the effect of light emitting diode (LED) lighting in greenhouse facilities on growth, chlorophyll fluorescence and pigmentation in Phalaenopsis 'Vivien' and 'Purple Star' under purpose-built LED arrays yielding c. 200 µmol m(-2)  s(-1) at plant height for 14 h per day and 24/18°C day/night temperature, respectively, from January to April 2013. The light treatments were (1) 40% blue in 60% red (40% B/R), (2) 0% blue in 100% red (0% B/R) and (3) white LEDs with 32% blue in white (32% B/W, control), with background daylight under shade screens. The plants were harvested twice for leaf growth and pigmentation. There was no clear pattern in the spectral effect on growth since the order of leaf size differed between harvests in March and April. Fv /Fm was in the range of 0.52-0.72, but overall slightly higher in the control, which indicated a permanent downregulation of PSII in the colored treatments. The fluorescence quenching showed no acclimation to color in 'Purple Star', while 'Vivien' had lower ETR and higher NPQ in the 40% B/R, resembling low light acclimation. The pigmentation showed corresponding spectral response with increasing concentration of lutein while increasing the fraction of blue light, which increased the light absorption in the green/yellow part of the spectrum. The permanent downregulation of PSII moved a substantial part of the thermal dissipation from the light regulated NPQ to non-regulated energy losses estimated by ΦNPQ and ΦNO and the difference found in the balance between ΦPSII and ΦNPQ in 'Vivien' disappeared when ΦNO was included in the thermal dissipation.

Modeling chlorophyll a fluorescence transient: relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.

Predawn and high intensity application of supplemental blue light decreases the quantum yield of PSII and enhances the amount of phenolic acids, flavonoids, and pigments in Lactuca sativa

To evaluate the effect of blue light intensity and timing, two cultivars of lettuce [Lactuca sativa cv. "Batavia" (green) and cv. "Lollo Rossa" (red)] were grown in a greenhouse compartment in late winter under natural light and supplemental high pressure sodium (SON-T) lamps yielding 90 (±10) μmol m(-2) s(-1) for up to 20 h, but never between 17:00 and 21:00. The temperature in the greenhouse compartments was 22/11°C day/night, respectively. The five light-emitting diode (LED) light treatments were Control (no blue addition), 1B 06-08 (Blue light at 45 μmol m(-2) s(-1) from 06:00 to 08:00), 1B 21-08 (Blue light at 45 μmol m(-2) s(-1) from 21:00 to 08:00), 2B 17-19 (Blue at 80 μmol m(-2) s(-1) from 17:00 to 19:00), and 1B 17-19 (Blue at 45 μmol m(-2) s(-1) from 17:00 to 19:00). Total fresh and dry weight was not affected with additional blue light; however, plants treated with additional blue light were more compact. The stomatal conductance in the green lettuce cultivar was higher for all treatments with blue light compared to the Control. Photosynthetic yields measured with chlorophyll fluorescence showed different response between the cultivars; in red lettuce, the quantum yield of PSII decreased and the yield of non-photochemical quenching increased with increasing blue light, whereas in green lettuce no difference was observed. Quantification of secondary metabolites showed that all four treatments with additional blue light had higher amount of pigments, phenolic acids, and flavonoids compared to the Control. The effect was more prominent in red lettuce, highlighting that the results vary among treatments and compounds. Our results indicate that not only high light level triggers photoprotective heat dissipation in the plant, but also the specific spectral composition of the light itself at low intensities. However, these plant responses to light are cultivar dependent.

Ions channels/transporters and chloroplast regulation

Ions play fundamental roles in all living cells and their gradients are often essential to fuel transports, to regulate enzyme activities and to transduce energy within and between cells. Their homeostasis is therefore an essential component of the cell metabolism. Ions must be imported from the extracellular matrix to their final subcellular compartments. Among them, the chloroplast is a particularly interesting example because there, ions not only modulate enzyme activities, but also mediate ATP synthesis and actively participate in the building of the photosynthetic structures by promoting membrane-membrane interaction. In this review, we first provide a comprehensive view of the different machineries involved in ion trafficking and homeostasis in the chloroplast, and then discuss peculiar functions exerted by ions in the frame of photochemical conversion of absorbed light energy.

Expression of a nuclear-encoded psbH gene complements the plastidic RNA processing defect in the PSII mutant hcf107 in Arabidopsis thaliana

The helical-repeat RNA-binding protein HCF107 is required for processing, stabilization and translation of plastid-encoded psbH mRNA. The psbH gene encodes a small, hydrophilic subunit of the PSII complex and is part of the plastidic psbB-psbT-psbH-petB-petD transcription unit. In Arabidopsis hcf107 mutants, only trace amounts of PSII proteins can be detected. Beside drastically reduced synthesis of PsbH, the synthesis of CP47 was also reduced in these mutants, although the corresponding psbB transcripts accumulate to wild type levels. This situation raises the question, whether the reduction of CP47 is a direct consequence of the mutation, based on targeting of HCF107 to the psbB mRNA, or a secondary affect due to the absent PsbH. To clarify this issue we introduced a chimeric psbH construct comprising a sequence encoding a chloroplast transit peptide into the hcf107-2 background. We found that the nucleus-localized psbH was able to complement the mutant defect resulting in photoautotrophic plants. The PSII proteins CP47 and D1 accumulated to barely half of the wild type level. Further experiments showed that cytosolically synthesized PsbH was imported into chloroplasts and assembled into PSII complexes. Using this approach, we showed that the tetratricopeptide repeat protein HCF107 of Arabidopsis is only responsible for expression of PsbH and not for synthesis of CP47. In addition the data suggest the necessity of the small, one-helix membrane spanning protein PsbH for the accumulation of CP47 in higher plants.

Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

The cytochrome b6f complex at the crossroad of photosynthetic electron transport pathways

Regulation of photosynthetic electron transport at the level of the cytochrome b6f complex provides efficient performance of the chloroplast electron transport chain (ETC). In this review, after brief overview of the structural organization of the chloroplast ETC, the consideration of the problem of electron transport control is focused on the plastoquinone (PQ) turnover and its interaction with the b6f complex. The data available show that the rates of plastoquinol (PQH2) formation in PSII and its diffusion to the b6f complex do not limit the overall rate of electron transfer between photosystem II (PSII) and photosystem I (PSI). Analysis of experimental and theoretical data demonstrates that the rate-limiting step in the intersystem chain of electron transport is determined by PQH2 oxidation at the Qo-site of the b6f complex, which is accompanied by the proton release into the thylakoid lumen. The acidification of the lumen causes deceleration of PQH2 oxidation, thus impeding the intersystem electron transport. Two other mechanisms of regulation of the intersystem electron transport have been considered: (i) "state transitions" associated with the light-induced redistribution of solar energy between PSI and PSII, and (ii) redistribution of electron fluxes between alternative pathways (noncyclic electron transport and cyclic electron flow around PSI).

Computer modeling of electron and proton transport in chloroplasts

Photosynthesis is one of the most important biological processes in biosphere, which provides production of organic substances from atmospheric CO2 and water at expense of solar energy. In this review, we contemplate computer models of oxygenic photosynthesis in the context of feedback regulation of photosynthetic electron transport in chloroplasts, the energy-transducing organelles of the plant cell. We start with a brief overview of electron and proton transport processes in chloroplasts coupled to ATP synthesis and consider basic regulatory mechanisms of oxygenic photosynthesis. General approaches to computer simulation of photosynthetic processes are considered, including the random walk models of plastoquinone diffusion in thylakoid membranes and deterministic approach to modeling electron transport in chloroplasts based on the mass action law. Then we focus on a kinetic model of oxygenic photosynthesis that includes key stages of the linear electron transport, alternative pathways of electron transfer around photosystem I (PSI), transmembrane proton transport and ATP synthesis in chloroplasts. This model includes different regulatory processes: pH-dependent control of the intersystem electron transport, down-regulation of photosystem II (PSII) activity (non-photochemical quenching), the light-induced activation of the Bassham-Benson-Calvin (BBC) cycle. The model correctly describes pH-dependent feedback control of electron transport in chloroplasts and adequately reproduces a variety of experimental data on induction events observed under different experimental conditions in intact chloroplasts (variations of CO2 and O2 concentrations in atmosphere), including a complex kinetics of P700 (primary electron donor in PSI) photooxidation, CO2 consumption in the BBC cycle, and photorespiration. Finally, we describe diffusion-controlled photosynthetic processes in chloroplasts within the framework of the model that takes into account complex architecture of chloroplasts and lateral heterogeneity of lamellar system of thylakoids. The lateral profiles of pH in the thylakoid lumen and in the narrow gap between grana thylakoids have been calculated under different metabolic conditions. Analyzing topological aspects of diffusion-controlled stages of electron and proton transport in chloroplasts, we conclude that along with the NPQ mechanism of attenuation of PSII activity and deceleration of PQH2 oxidation by the cytochrome b6f complex caused by the lumen acidification, the intersystem electron transport may be down-regulated due to the light-induced alkalization of the narrow partition between adjacent thylakoids of grana. The computer models of electron and proton transport described in this article may be integrated as appropriate modules into a comprehensive model of oxygenic photosynthesis.

PGR5 and NDH pathways in photosynthetic cyclic electron transfer respond differently to sublethal treatment with photosystem-interfering herbicides

Effects of sublethal levels of the photosystem-interfering herbicides atrazine (Atr) and methyl viologen (MV) on photosynthetic electron transport were investigated in Arabidopsis thaliana mutants with defects in cyclic electron transfer (CET) activity. Analysis based on chlorophyll fluorescence parameters showed that pgr5 mutant (a defect in the PGR5 pathway) was more sensitive to both Atr and MV than wild type (Wt) and pnsB3 mutant (a defect in the NDH pathway). Real-time PCR (polymerase chain reaction) analysis of transcripts indicated that Wt plants showed marked increases in transcripts in the PRG5 and NDH pathways under treatment with either Atr or MV. In contrast, Atr increased the gene transcripts in CET, but MV decreased them in pnsB3 mutant plants. Atr did not increase the transcripts, while MV down-regulated them in pgr5 mutant. Immunoblot analysis partially supported the changes in the transcripts; that is, the protein levels of PGRL1 and PGR5 were increased in pnsB3 mutant, while no protein level was increased in pgr5 mutant after the herbicide treatment. The present results suggest that cyclic electron transport is very sensitive to photosystem-interference induced by chemicals and that the PGR5 pathway is very critical for regulation. Thus, pgr5 mutants may be useful plants for monitoring photosystem-interfering herbicides.

Proton gradient regulation 5-mediated cyclic electron flow under ATP- or redox-limited conditions: a study of ΔATpase pgr5 and ΔrbcL pgr5 mutants in the green alga Chlamydomonas reinhardtii

The Chlamydomonas reinhardtii proton gradient regulation5 (Crpgr5) mutant shows phenotypic and functional traits similar to mutants in the Arabidopsis (Arabidopsis thaliana) ortholog, Atpgr5, providing strong evidence for conservation of PGR5-mediated cyclic electron flow (CEF). Comparing the Crpgr5 mutant with the wild type, we discriminate two pathways for CEF and determine their maximum electron flow rates. The PGR5/proton gradient regulation-like1 (PGRL1) ferredoxin (Fd) pathway, involved in recycling excess reductant to increase ATP synthesis, may be controlled by extreme photosystem I acceptor side limitation or ATP depletion. Here, we show that PGR5/PGRL1-Fd CEF functions in accordance with an ATP/redox control model. In the absence of Rubisco and PGR5, a sustained electron flow is maintained with molecular oxygen instead of carbon dioxide serving as the terminal electron acceptor. When photosynthetic control is decreased, compensatory alternative pathways can take the full load of linear electron flow. In the case of the ATP synthase pgr5 double mutant, a decrease in photosensitivity is observed compared with the single ATPase-less mutant that we assign to a decreased proton motive force. Altogether, our results suggest that PGR5/PGRL1-Fd CEF is most required under conditions when Fd becomes overreduced and photosystem I is subjected to photoinhibition. CEF is not a valve; it only recycles electrons, but in doing so, it generates a proton motive force that controls the rate of photosynthesis. The conditions where the PGR5 pathway is most required may vary in photosynthetic organisms like C. reinhardtii from anoxia to high light to limitations imposed at the level of carbon dioxide fixation.

AtFKBP16-1, a chloroplast lumenal immunophilin, mediates response to photosynthetic stress by regulating PsaL stability

Arabidopsis contains 16 putative chloroplast lumen-targeted immunophilins (IMMs). Proteomic analysis has enabled the subcellular localization of IMMs experimentally, but the exact biological and physiological roles of most luminal IMMs remain to be discovered. FK506-binding protein (FKBP) 16-1, one of the lumenal IMMs containing poorly conserved amino acid residues for peptidyl-prolyl isomerase (PPIase) activity, was shown to play a possible role in chloroplast biogenesis in Arabidopsis, and was also found to interact with PsaL in wheat. In this study, further evidence is provided for the notion that Arabidopsis FKBP16-1 (AtFKBP16-1) is transcriptionally and post-transcriptionally regulated by environmental stresses including high light (HL) intensity, and that overexpression of AtFKBP16-1 plants exhibited increased photosynthetic stress tolerance. A blue native-polyacrylamide gel electrophoresis/two-dimensional (BN-PAGE/2-D) analysis revealed that the increase of AtFKBP16-1 affected the levels of photosystem I (PSI)-light harvesting complex I (LHCI) and PSI-LHCI-light harvesting complex II (LHCII) supercomplex, and consequently enhanced tolerance under conditions of HL stress. In addition, plants overexpressing AtFKBP16-1 showed increased accumulation of PsaL protein and enhanced drought tolerance. Using a protease protection assay, AtFKBP16-1 protein was found to have a role in PsaL stability. The AtPsaL levels also responded to abiotic stresses derived from drought, and from methyl viologen stresses in wild-type plants. Taken together, these results suggest that AtFKBP16-1 plays a role in the acclimation of plants under photosynthetic stress conditions, probably by regulating PsaL stability.

Structural characterization of a plant photosystem I and NAD(P)H dehydrogenase supercomplex

Cyclic electron transport (CET) around photosystem I (PSI) plays an important role in balancing the ATP/NADPH ratio and the photoprotection of plants. The NAD(P)H dehydrogenase complex (NDH) has a key function in one of the CET pathways. Current knowledge indicates that, in order to fulfill its role in CET, the NDH complex needs to be associated with PSI; however, until now there has been no direct structural information about such a supercomplex. Here we present structural data obtained for a plant PSI-NDH supercomplex. Electron microscopy analysis revealed that in this supercomplex two copies of PSI are attached to one NDH complex. A constructed pseudo-atomic model indicates asymmetric binding of two PSI complexes to NDH and suggests that the low-abundant Lhca5 and Lhca6 subunits mediate the binding of one of the PSI complexes to NDH. On the basis of our structural data, we propose a model of electron transport in the PSI-NDH supercomplex in which the association of PSI to NDH seems to be important for efficient trapping of reduced ferredoxin by NDH.

Photosynthetic proton and electron transport in wheat leaves under prolonged moderate drought stress

In conditions of long-lasting moderate drought stress, we have studied the photoprotective responses in leaves of wheat (Triticum aestivum L., cv. Katya) related to the photosynthetic electron and proton transport. The dark-interval relaxation kinetics of electrochromic bandshift (ECS) indicated a decrease of electric and an increase of osmotic component of the proton motive force in drought stressed leaves, but neither the total proton motive force (pmf) nor the thylakoid proton conductance (gH(+)) were affected. We observed the enhanced protection against overreduction of PSI acceptor side in leaves of drought stressed plants. This was obviously achieved by the rapid buildup of transthylakoid pH gradient at relatively low light intensities, directly associated to the steep increase of NPQ and the down-regulation of linear electron transport. It was further accompanied by the steep increase of redox poise at PSII acceptor side and PSI donor side. The early responses related to thylakoid lumen acidification in drought-stressed leaves could be associated with the activity of an enhanced fraction of PSI not involved in linear electron flow, which may have led to enhanced cyclic electron pathway even in relatively low light intensities, as well as to the drought-induced decrease of IP-amplitude in fast chlorophyll fluorescence kinetics.