A new concept for ferredoxin-NADP(H) oxidoreductase binding to plant thylakoids

Thylakoid Rhodanese-like Protein-Ferredoxin:NADP+ Oxidoreductase Interaction Is Integrated into Plant Redox Homeostasis System

In vascular plants, the final photosynthetic electron transfer from ferredoxin (Fd) to NADP+ is catalyzed by the flavoenzyme ferredoxin:NADP+ oxidoreductase (FNR). FNR is recruited to thylakoid membranes via an integral membrane protein TROL (thylakoid rhodanese-like protein) and the membrane associated protein Tic62. We have previously demonstrated that the absence of TROL triggers a very efficient superoxide (O2•-) removal mechanism. The dynamic TROL-FNR interaction has been shown to be an apparently overlooked mechanism that maintains linear electron flow before alternative pathway(s) is(are) activated. In this work, we aimed to further test our hypothesis that the FNR-TROL pair could be the source element that triggers various downstream networks of chloroplast ROS scavenging. Tandem affinity purification followed by the MS analysis confirmed the TROL-FNR interaction and revealed possible interaction of TROL with the thylakoid form of the enzyme ascorbate peroxidase (tAPX), which catalyzes the H2O2-dependent oxidation of ascorbate and is, therefore, the crucial component of the redox homeostasis system in plants. Further, EPR analyses using superoxide spin trap DMPO showed that, in comparison with the wild type, plants overexpressing TROL (TROL OX) propagate more O2•- when exposed to high light stress. This indicates an increased sensitivity to oxidative stress in conditions when there is an excess of membrane-bound FNR and less free FNR is found in the stroma. Finally, immunohistochemical analyses of glutathione in different Arabidopsis leaf cell compartments showed highly elevated glutathione levels in TROL OX, indicating an increased demand for this ROS scavenger in these plants, likely needed to prevent the damage of important cellular components caused by reactive oxygen species.

Photosynthetic modification of plants through recent technologies: a valuable way to ensure crop fortification

The 2030 Sustainable Development Goals of the United Nations include a strong emphasis on ending hunger worldwide. According to the 2019 Global Food Security Index, while 88% of countries claim there is sufficient food supply in their country, the sad reality is that 1 in 3 countries is facing insufficient availability of food supply, which means that in those countries, more than 10% of the population is malnourished. Since nutrition is crucial to leading a healthy life and satisfying food security needs, several governments have turned to national nutrition surveys to gauge the extent of malnutrition in their populations. Plants are able to grow, develop, and store nutrients by photosynthesis, which convert light into chemical energy through cell redox regulatory networks. A photosynthesis system's electron flow may be adjusted to accommodate varying light and environmental circumstances. Many techniques exist for controlling the flow of electrons emitted during light processes in order to save or waste energy. The two protein molecules TROL and flavoenzyme ferredoxin (oxidoreductase+NADP) (FNR) interact dynamically to form an excellent molecular switch capable of splitting electrons from the photosystem. The TROL-FNR bifurcation may be limited by either generating NADPH or preventing reactive oxygen species from propagating. TROL-based genome editing is an experimental method for enhancing plant stress and defensive responses, efficiency, and ultimately agricultural production.

Characterizing membrane anchoring of leaf-form ferredoxin-NADP+ oxidoreductase in rice

Leaf-form ferredoxin-NADP⁺ oxidoreductases (LFNRs) function in the last step of the photosynthetic electron transport chain, exist as soluble proteins in the chloroplast stroma and are weakly associated with thylakoids or tightly anchored to chloroplast membranes. Arabidopsis thaliana has two LFNRs, and the chloroplast proteins AtTROL and AtTIC62 participate in anchoring AtLFNRs to the thylakoid membrane. By contrast, the membrane anchoring mechanism of rice (Oryza sativa) LFNRs has not been elucidated. Here, we investigated the membrane-anchoring mechanism of LFNRs and its physiological roles in rice. We characterized the rice protein OsTROL1 based on its homology to AtTROL. We determined that OsTROL1 is also a thylakoid membrane anchor and its loss leads to a compensatory increase in OsTIC62. OsLFNR1 attachment through a membrane anchor depends on OsLFNR2, unlike the Arabidopsis counterparts. In addition, OsTIC62 was more highly expressed in the dark than under light conditions, consistent with the increased membrane binding of OsLFNR in the dark. Moreover, we observed reciprocal stabilization between OsLFNRs and their membrane anchors. In addition, unlike in Arabidopsis, the loss of LFNR membrane anchor affects photosynthesis in rice. Overall, our study sheds light on the mechanisms anchoring LFNRs to membranes in rice and highlights differences with Arabidopsis.

Methylation of a MITE insertion in the MdRFNR1-1 promoter is positively associated with its allelic expression in apple in response to drought stress

Miniature inverted-repeat transposable elements (MITEs) are widely distributed in the plant genome and can be methylated. However, whether DNA methylation of MITEs is associated with induced allelic expression and drought tolerance is unclear. Here, we identified the drought-inducible MdRFNR1 (root-type ferredoxin-NADP+ oxidoreductase) gene in apple (Malus domestica). MdRFNR1 plays a positive role in drought tolerance by regulating the redox system, including increasing NADP+ accumulation and catalase and peroxidase activities and decreasing NADPH levels. Sequence analysis identified a MITE insertion (MITE-MdRF1) in the promoter of MdRFNR1-1 but not the MdRFNR1-2 allele. MdRFNR1-1 but not MdRFNR1-2 expression was significantly induced by drought stress, which was positively associated with the MITE-MdRF1 insertion and its DNA methylation. The methylated MITE-MdRF1 is recognized by the transcriptional anti-silencing factors MdSUVH1 and MdSUVH3, which recruit the DNAJ domain-containing proteins MdDNAJ1, MdDNAJ2, and MdDNAJ5, thereby activating MdRFNR1-1 expression under drought stress. Finally, we showed that MdSUVH1 and MdDNAJ1 are positive regulators of drought tolerance. These findings illustrate the molecular roles of methylated MITE-MdRF1 (which is recognized by the MdSUVH-MdDNAJ complex) in induced MdRFNR1-1 expression as well as the drought response of apple and shed light on the molecular mechanisms of natural variation in perennial trees.

Role of Histidine 78 of leaf ferredoxin in the interaction with ferredoxin-NADP+ reductase: regulation of pH dependency and negative cooperativity with NADP(H)

In chloroplast stroma, dynamic pH change occurs in response to fluctuating light conditions. We investigated the pH-dependent electron transfer activity between ferredoxin-NADP+ reductase (FNR) and ferredoxin (Fd) isoproteins from maize leaves. By increasing pH (from 5.5 to 8.5), the electron transfer activity from FNR to photosynthetic-type Fd (Fd1) significantly increased while the activity to nonphotosynthetic type Fd (Fd3) decreased, which was mainly due to their differences in the pH dependency of Km for Fd. Mutation of His78 of Fd1 to Val, corresponding amino acid residue in Fd3, lost the pH dependency, indicating a regulatory role of the His78 in the interaction with FNR. We previously showed that the interaction between FNR and Fd was weakened by the allosteric binding of NADP(H) on FNR. His78Val Fd1 mutant largely suppressed this negative cooperativity. These results indicate the involvement of Fd1 His78 in pH dependency and negative cooperativity in the interaction with FNR.

Protection of photosystem I during sudden light stress depends on ferredoxin:NADP(H) reductase abundance and interactions

Plant tolerance to high light and oxidative stress is increased by overexpression of the photosynthetic enzyme Ferredoxin:NADP(H) reductase (FNR), but the specific mechanism of FNR-mediated protection remains enigmatic. It has also been reported that the localization of this enzyme within the chloroplast is related to its role in stress tolerance. Here, we dissected the impact of FNR content and location on photoinactivation of photosystem I (PSI) and photosystem II (PSII) during high light stress of Arabidopsis (Arabidopsis thaliana). The reaction center of PSII is efficiently turned over during light stress, while damage to PSI takes much longer to repair. Our results indicate a PSI sepcific effect, where efficient oxidation of the PSI primary donor (P700) upon transition from darkness to light, depends on FNR recruitment to the thylakoid membrane tether proteins: thylakoid rhodanase-like protein (TROL) and translocon at the inner envelope of chloroplasts 62 (Tic62). When these interactions were disrupted, PSI photoinactivation occurred. In contrast, there was a moderate delay in the onset of PSII damage. Based on measurements of ΔpH formation and cyclic electron flow, we propose that FNR location influences the speed at which photosynthetic control is induced, resulting in specific impact on PSI damage. Membrane tethering of FNR therefore plays a role in alleviating high light stress, by regulating electron distribution during short-term responses to light.

A plant-derived natural photosynthetic system for improving cell anabolism

Insufficient intracellular anabolism is a crucial factor involved in many pathological processes in the body^1,2. The anabolism of intracellular substances requires the consumption of sufficient intracellular energy and the production of reducing equivalents. ATP acts as an 'energy currency' for biological processes in cells^3,4, and the reduced form of NADPH is a key electron donor that provides reducing power for anabolism⁵. Under pathological conditions, it is difficult to correct impaired anabolism and to increase insufficient levels of ATP and NADPH to optimum concentrations^1,4,6-8. Here we develop an independent and controllable nanosized plant-derived photosynthetic system based on nanothylakoid units (NTUs). To enable cross-species applications, we use a specific mature cell membrane (the chondrocyte membrane (CM)) for camouflage encapsulation. As proof of concept, we demonstrate that these CM-NTUs enter chondrocytes through membrane fusion, avoid lysosome degradation and achieve rapid penetration. Moreover, the CM-NTUs increase intracellular ATP and NADPH levels in situ following exposure to light and improve anabolism in degenerated chondrocytes. They can also systemically correct energy imbalance and restore cellular metabolism to improve cartilage homeostasis and protect against pathological progression of osteoarthritis. Our therapeutic strategy for degenerative diseases is based on a natural photosynthetic system that can controllably enhance cell anabolism by independently providing key energy and metabolic carriers. This study also provides an enhanced understanding of the preparation and application of bioorganisms and composite biomaterials for the treatment of disease.

Redox metabolism for improving whole-cell P450-catalysed terpenoid biosynthesis

The growing preference for producing cytochrome P450-mediated natural products in microbial systems stems from the challenging nature of the organic chemistry approaches. The P450 enzymes are redox-dependent proteins, through which they source electrons from reducing cofactors to drive their activities. Widely researched in biochemistry, most of the previous studies have extensively utilised expensive cell-free assays to reveal mechanistic insights into P450 functionalities in presence of commercial redox partners. However, in the context of microbial bioproduction, the synergic activity of P450- reductase proteins in microbial systems have not been largely investigated. This is mainly due to limited knowledge about their mutual interactions in the context of complex systems. Hence, manipulating the redox potential for natural product synthesis in microbial chassis has been limited. As the potential of redox state as crucial regulator of P450 biocatalysis has been greatly underestimated by the scientific community, in this review, we re-emphasize their pivotal role in modulating the in vivo P450 activity through affecting the product profile and yield. Particularly, we discuss the applications of widely used in vivo redox engineering methodologies for natural product synthesis to provide further suggestions for patterning on P450-based terpenoids production in microbial platforms.

Effect of Magnetopriming on Photosynthetic Performance of Plants

Magnetopriming has emerged as a promising seed-priming method, improving seed vigor, plant performance and productivity under both normal and stressed conditions. Various recent reports have demonstrated that improved photosynthesis can lead to higher biomass accumulation and overall crop yield. The major focus of the present review is magnetopriming-based, improved growth parameters, which ultimately favor increased photosynthetic performance. The plants originating from magnetoprimed seeds showed increased plant height, leaf area, fresh weight, thick midrib and minor veins. Similarly, chlorophyll and carotenoid contents, efficiency of PSII, quantum yield of electron transport, stomatal conductance, and activities of carbonic anhydrase (CA), Rubisco and PEP-carboxylase enzymes are enhanced with magnetopriming of the seeds. In addition, a higher fluorescence yield at the J-I-P phase in polyphasic chlorophyll a fluorescence (OJIP) transient curves was observed in plants originating from magnetoprimed seeds. Here, we have presented an overview of available studies supporting the magnetopriming-based improvement of various parameters determining the photosynthetic performance of crop plants, which consequently increases crop yield. Additionally, we suggest the need for more in-depth molecular analysis in the future to shed light upon hidden regulatory mechanisms involved in magnetopriming-based, improved photosynthetic performance.

Regulation of photosynthetic electron flow on dark to light transition by ferredoxin:NADP(H) oxidoreductase interactions

During photosynthesis, electron transport is necessary for carbon assimilation and must be regulated to minimize free radical damage. There is a longstanding controversy over the role of a critical enzyme in this process (ferredoxin:NADP(H) oxidoreductase, or FNR), and in particular its location within chloroplasts. Here we use immunogold labelling to prove that FNR previously assigned as soluble is in fact membrane associated. We combined this technique with a genetic approach in the model plant Arabidopsis to show that the distribution of this enzyme between different membrane regions depends on its interaction with specific tether proteins. We further demonstrate a correlation between the interaction of FNR with different proteins and the activity of alternative photosynthetic electron transport pathways. This supports a role for FNR location in regulating photosynthetic electron flow during the transition from dark to light.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

Relation of Photochemical Reflectance Indices Based on Different Wavelengths to the Parameters of Light Reactions in Photosystems I and II in Pea Plants

Measurement and analysis of the numerous reflectance indices of plants is an effective approach for the remote sensing of plant physiological processes in agriculture and ecological monitoring. A photochemical reflectance index (PRI) plays an important role in this kind of remote sensing because it can be related to early changes in photosynthetic processes under the action of stressors (excess light, changes in temperature, drought, etc.). In particular, we previously showed that light-induced changes in PRIs could be strongly related to the energy-dependent component of the non-photochemical quenching in photosystem II. The aim of the present work was to undertake comparative analysis of the efficiency of using light-induced changes in PRIs (ΔPRIs) based on different wavelengths for the estimation of the parameters of photosynthetic light reactions (including the parameters of photosystem I). Pea plants were used in the investigation; the photosynthetic parameters were measured using the pulse-amplitude-modulated (PAM) fluorometer Dual-PAM-100 and the intensities of the reflected light were measured using the spectrometer S100. The ΔPRIs were calculated as ΔPRI(band,570), where the band was 531 nm for the typical PRI and 515, 525, 535, 545, or 555 nm for modified PRIs; 570 nm was the reference wavelength for all PRIs. There were several important results: (1) ∆PRI(525,570), ∆PRI(531,570), ∆PRI(535,570), and ∆PRI(545,570) could be used for estimation of most of the photosynthetic parameters under light only or under dark only conditions. (2) The combination of dark and light conditions decreased the efficiency of ∆PRIs for the estimation of the photosynthetic parameters; ∆PRI(535,570) and ∆PRI(545,570) had maximal efficiency under these conditions. (3) ∆PRI(515,570) and ∆PRI(525,570) mainly included the slow-relaxing component of PRI; in contrast, ∆PRI(531,570), ∆PRI(535,570), ∆PRI(545,570), and ∆PRI(555,570) mainly included the fast-relaxing component of PRI. These components were probably caused by different mechanisms.

En route into chloroplasts: preproteins' way home

Chloroplasts are the characteristic endosymbiotic organelles of plant cells which during the course of evolution lost most of their genetic information to the nucleus. Thus, they critically depend on the host cell for allocation of nearly their complete protein supply. This includes gene expression, translation, protein targeting, and transport-all of which need to be tightly regulated and perfectly coordinated to accommodate the cells' needs. To this end, multiple signaling pathways have been implemented that interchange information between the different cellular compartments. One of the most complex and energy consuming processes is the translocation of chloroplast-destined proteins into their target organelle. It is a concerted effort from chaperones, receptor proteins, channels, and regulatory elements to ensure correct targeting, efficient transport, and subsequent folding. Although we have discovered and learned a lot about protein import into chloroplasts in the last decades, there are still many open questions and debates about the roles of individual proteins as well as the mechanistic details. In this review, I will summarize and discuss the published data with a focus on the translocation complex in the chloroplast inner envelope membrane.

Potential roles of YCF54 and ferredoxin-NADPH reductase for magnesium protoporphyrin monomethylester cyclase

Chlorophyll is synthesized from activated glutamate in the tetrapyrrole biosynthesis pathway through at least 20 different enzymatic reactions. Among these, the MgProto monomethylester (MgProtoME) cyclase catalyzes the formation of a fifth isocyclic ring to tetrapyrroles to form protochlorophyllide. The enzyme consists of two proteins. The CHL27 protein is proposed to be the catalytic component, while LCAA/YCF54 likely acts as a scaffolding factor. In comparison to other reactions of chlorophyll biosynthesis, this enzymatic step lacks clear elucidation and it is hardly understood, how electrons are delivered for the NADPH-dependent cyclization reaction. The present study intends to elucidate more precisely the role of LCAA/YCF54. Transgenic Arabidopsis lines with inactivated and overexpressed YCF54 reveal the mutual stability of YCF54 and CHL27. Among the YCF54-interacting proteins, the plastidal ferredoxin-NADPH reductase (FNR) was identified. We showed in N. tabacum and A. thaliana that a deficit of FNR1 or YCF54 caused MgProtoME accumulation, the substrate of the cyclase, and destabilization of the cyclase subunits. It is proposed that FNR serves as a potential donor for electrons required in the cyclase reaction and connects chlorophyll synthesis with photosynthetic activity.

Effects of TROL Presequence Mutagenesis on Its Import and Dual Localization in Chloroplasts

Thylakoid rhodanase-like protein (TROL) is involved in the final step of photosynthetic electron transport from ferredoxin to ferredoxin: NADP⁺ oxidoreductase (FNR). TROL is located in two distinct chloroplast compartments—in the inner envelope of chloroplasts, in its precursor form; and in the thylakoid membranes, in its fully processed form. Its role in the inner envelope, as well as the determinants for its differential localization, have not been resolved yet. In this work we created six N-terminal amino acid substitutions surrounding the predicted processing site in the presequence of TROL in order to obtain a construct whose import is affected or localization limited to a single intrachloroplastic site. By using in vitro transcription and translation and subsequent protein import methods, we found that a single amino acid exchange in the presequence, Ala67 to Ile67 interferes with processing in the stroma and directs the whole pool of in vitro translated TROL to the inner envelope of chloroplasts. This result opens up the possibility of studying the role of TROL in the chloroplast inner envelope as well as possible consequence/s of its absence from the thylakoids.

Electron Transfer through the Acceptor Side of Photosystem I: Interaction with Exogenous Acceptors and Molecular Oxygen

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin-Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A_1A/A_1B in the symmetric redox cofactor branches of PS I and iron-sulfur clusters F_X, F_A, and F_B have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A₁ site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.

The Cytochrome b 6 f Complex: Biophysical Aspects of Its Functioning in Chloroplasts

This chapter presents an overview of structural properties of the cytochrome (Cyt) b ₆ f complex and its functioning in chloroplasts. The Cyt b ₆ f complex stands at the crossroad of photosynthetic electron transport pathways, providing connectivity between Photosystem (PSI) and Photosysten II (PSII) and pumping protons across the membrane into the thylakoid lumen. After a brief review of the chloroplast electron transport chain, the consideration is focused on the structural organization of the Cyt b ₆ f complex and its interaction with plastoquinol (PQH₂, reduced form of plastoquinone), a mediator of electron transfer from PSII to the Cyt b ₆ f complex. The processes of PQH₂ oxidation by the Cyt b ₆ f complex have been considered within the framework of the Mitchell's Q-cycle. The overall rate of the intersystem electron transport is determined by PQH₂ turnover at the quinone-binding site Q_o of the Cyt b ₆ f complex. The rate of PQH₂ oxidation is controlled by the intrathylakoid pH_in, which value determines the protonation/deprotonation events in the Q_o-center. Two other regulatory mechanisms associated with the Cyt b ₆ f complex are briefly overviewed: (i) redistribution of electron fluxes between alternative (linear and cyclic) pathways, and (ii) "state transitions" related to redistribution of solar energy between PSI and PSII.

Interaction and electron transfer between ferredoxin-NADP+ oxidoreductase and its partners: structural, functional, and physiological implications

Ferredoxin-NADP⁺ reductase (FNR) catalyzes the last step of linear electron transfer in photosynthetic light reactions. The FAD cofactor of FNR accepts two electrons from two independent reduced ferredoxin molecules (Fd) in two sequential steps, first producing neutral semiquinone and then the fully anionic reduced, or hydroquinone, form of the enzyme (FNR_hq). FNR_hq transfers then both electrons in a single hydride transfer step to NADP⁺. We are presenting the recent progress in studies focusing on Fd:FNR interaction and subsequent electron transfer processes as well as on interaction of FNR with NADP⁺/H followed by hydride transfer, both from the structural and functional point of views. We also present the current knowledge about the physiological role(s) of various FNR isoforms present in the chloroplasts of higher plants and the functional impact of subchloroplastic location of FNR. Moreover, open questions and current challenges about the structure, function, and physiology of FNR are discussed.

Detailed characterization of Synechocystis PCC 6803 ferredoxin:NADP+ oxidoreductase interaction with model membranes

Direct interaction of ferredoxin:NADP⁺ oxidoreductase (FNR) with thylakoid membranes was postulated as a part of the cyclic electron flow mechanism. In vitro binding of FNR to digalactosyldiacylglycerol and monogalactosyldiacylglycerol membranes was also shown. In this paper we deal with the latter interaction in more detail describing the effect for two FNR forms of Synechocystis PCC 6803. The so-called short FNR (sFNR) is homologous to FNR from higher plant chloroplasts. The long FNR (lFNR) form contains an additional domain, responsible for the interaction with phycobilisomes. We compare the binding of both sFNR and lFNR forms to native and non-native lipids. We also include factors which could modulate this process: pH change, temperature change, presence of ferredoxin, NADP⁺ and NADPH and heavy metals. For the lFNR, we also include phycobilisomes as a modulating factor. The membrane binding is generally faster at lower pH. The sFNR was binding faster than lFNR. Ferredoxin isoforms with higher midpoint potential, as well as NADPH and NADP⁺, weakened the binding. Charged lipids and high phosphate promoted the binding. Heavy metal ions decreased the rate of membrane binding only when FNR was preincubated with them before injection beneath the monolayer. FNR binding was limited to surface lipid groups and did not influence hydrophobic chain packing. Taken together, FNR interaction with lipids appears to be non-specific, with an electrostatic component. This suggests that the direct FNR interaction with lipids is most likely not a factor in directing electron transfer, but should be taken into account during in vitro studies.

Influence of differing nitrate and nitrogen availability on flowering control in Arabidopsis

Nitrogen, an essential macronutrient for plants, regulates many aspects of plant growth and development. Nitrate is one of the major forms of nitrogen taken up by plants from the soil. Nitrate and nitrogen have been reported to regulate flowering; while some studies have shown that lower nitrate/nitrogen promoted flowering, others have reported the opposite trend. To elucidate how nitrate/nitrogen affects flowering, we reviewed the existing literature and conducted experiments to examine flowering time under a wide range of nitrate concentrations using two growth systems. From the literature review and our experiments, we established that differing nitrate availability results in a U-shaped flowering curve, with an optimal concentration of nitrate facilitating flowering and concentrations above or below this optimal concentration delaying flowering. The role of nitrate and nitrogen in regulating flowering has been elucidated by several transcriptomic and mutant studies, which have suggested close interactions between nitrate/nitrogen, phosphate, the circadian clock, photosynthesis, and, potentially, hormones. We discuss several possible molecular mechanisms underlying the U-shaped flowering response.

Mutation of FdC2 gene encoding a ferredoxin-like protein with C-terminal extension causes yellow-green leaf phenotype in rice

Ferredoxins (Fds) are small iron-sulfur proteins that mediate electron transfer in a wide range of metabolic reactions. Besides Fds, there is a type of Fd-like proteins designated as FdC, which have conserved elements of Fds, but contain a significant C-terminal extension. So far, only two FdC genes of Arabidopsis (Arabidopsis thaliana) have been identified in higher plants and thus the functions of FdC proteins remain largely unknown. In this study, we isolated a yellow-green leaf mutant, 501ys, in rice (Oryza sativa). The mutant exhibited yellow-green leaf phenotype and reduced chlorophyll level. The phenotype of 501ys was caused by mutation of a gene on rice chromosome 3. Map-based cloning of this mutant resulted in identification of OsFdC2 gene (LOC_Os03g48040) showing high identity with Arabidopsis FdC2 gene (AT1G32550). OsFdC2 was expressed most abundantly in leaves and its encoded protein was targeted to the chloroplast. In 501ys mutant, a missense mutation was detected in DNA sequence of the gene, resulting in an amino acid change in the encoded protein. The mutant phenotype was rescued by introduction of the wild-type gene. Therefore, we successfully identified FdC2 gene via map-based cloning approach, and demonstrated that mutation of this gene caused yellow-green leaf phenotype in rice.

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.

TROL-FNR interaction reveals alternative pathways of electron partitioning in photosynthesis

In photosynthesis, final electron transfer from ferredoxin to NADP(+) is accomplished by the flavo enzyme ferredoxin:NADP(+) oxidoreductase (FNR). FNR is recruited to thylakoid membranes via integral membrane thylakoid rhodanase-like protein TROL. We address the fate of electrons downstream of photosystem I when TROL is absent. We have employed electron paramagnetic resonance (EPR) spectroscopy to study free radical formation and electron partitioning in TROL-depleted chloroplasts. DMPO was used to detect superoxide anion (O2(.-)) formation, while the generation of other free radicals was monitored by Tiron. Chloroplasts from trol plants pre-acclimated to different light conditions consistently exhibited diminished O2(.-) accumulation. Generation of other radical forms was elevated in trol chloroplasts in all tested conditions, except for the plants pre-acclimated to high-light. Remarkably, dark- and growth light-acclimated trol chloroplasts were resilient to O2(.-) generation induced by methyl-viologen. We propose that the dynamic binding and release of FNR from TROL can control the flow of photosynthetic electrons prior to activation of the pseudo-cyclic electron transfer pathway.

Posttranslational modifications of FERREDOXIN-NADP+ OXIDOREDUCTASE in Arabidopsis chloroplasts

Rapid responses of chloroplast metabolism and adjustments to photosynthetic machinery are of utmost importance for plants' survival in a fluctuating environment. These changes may be achieved through posttranslational modifications of proteins, which are known to affect the activity, interactions, and localization of proteins. Recent studies have accumulated evidence about the crucial role of a multitude of modifications, including acetylation, methylation, and glycosylation, in the regulation of chloroplast proteins. Both of the Arabidopsis (Arabidopsis thaliana) leaf-type FERREDOXIN-NADP(+) OXIDOREDUCTASE (FNR) isoforms, the key enzymes linking the light reactions of photosynthesis to carbon assimilation, exist as two distinct forms with different isoelectric points. We show that both AtFNR isoforms contain multiple alternative amino termini and undergo light-responsive addition of an acetyl group to the α-amino group of the amino-terminal amino acid of proteins, which causes the change in isoelectric point. Both isoforms were also found to contain acetylation of a conserved lysine residue near the active site, while no evidence for in vivo phosphorylation or glycosylation was detected. The dynamic, multilayer regulation of AtFNR exemplifies the complex regulatory network systems controlling chloroplast proteins by a range of posttranslational modifications, which continues to emerge as a novel area within photosynthesis research.

Multiple complexes of nitrogen assimilatory enzymes in spinach chloroplasts: possible mechanisms for the regulation of enzyme function

Assimilation of nitrogen is an essential biological process for plant growth and productivity. Here we show that three chloroplast enzymes involved in nitrogen assimilation, glutamate synthase (GOGAT), nitrite reductase (NiR) and glutamine synthetase (GS), separately assemble into distinct protein complexes in spinach chloroplasts, as analyzed by western blots under blue native electrophoresis (BN-PAGE). GOGAT and NiR were present not only as monomers, but also as novel complexes with a discrete size (730 kDa) and multiple sizes (>120 kDa), respectively, in the stromal fraction of chloroplasts. These complexes showed the same mobility as each monomer on two-dimensional (2D) SDS-PAGE after BN-PAGE. The 730 kDa complex containing GOGAT dissociated into monomers, and multiple complexes of NiR reversibly converted into monomers, in response to the changes in the pH of the stromal solvent. On the other hand, the bands detected by anti-GS antibody were present not only in stroma as a conventional decameric holoenzyme complex of 420 kDa, but also in thylakoids as a novel complex of 560 kDa. The polypeptide in the 560 kDa complex showed slower mobility than that of the 420 kDa complex on the 2D SDS-PAGE, implying the assembly of distinct GS isoforms or a post-translational modification of the same GS protein. The function of these multiple complexes was evaluated by in-gel GS activity under native conditions and by the binding ability of NiR and GOGAT with their physiological electron donor, ferredoxin. The results indicate that these multiplicities in size and localization of the three nitrogen assimilatory enzymes may be involved in the physiological regulation of their enzyme function, in a similar way as recently described cases of carbon assimilatory enzymes.

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.

Cytosolic APX knockdown rice plants sustain photosynthesis by regulation of protein expression related to photochemistry, Calvin cycle and photorespiration

The biochemical mechanisms underlying the involvement of cytosolic ascorbate peroxidases (cAPXs) in photosynthesis are still unknown. In this study, rice plants doubly silenced in these genes (APX1/2) were exposed to moderate light (ML) and high light (HL) to assess the role of cAPXs in photosynthetic efficiency. APX1/2 mutants that were exposed to ML overexpressed seven and five proteins involved in photochemical activity and photorespiration, respectively. These plants also increased the pheophytin and chlorophyll levels, but the amount of five proteins that are important for Calvin cycle did not change. These responses in mutants were associated with Rubisco carboxylation rate, photosystem II (PSII) activity and potential photosynthesis, which were similar to non-transformed plants. The upregulation of photochemical proteins may be part of a compensatory mechanism for APX1/2 deficiency but apparently the finer-control for photosynthesis efficiency is dependent on Calvin cycle proteins. Conversely, under HL the mutants employed a different strategy, triggering downregulation of proteins related to photochemical activity, Calvin cycle and decreasing the levels of photosynthetic pigments. These changes were associated to strong impairment in PSII activity and Rubisco carboxylation. The upregulation of some photorespiratory proteins was maintained under that stressful condition and this response may have contributed to photoprotection in rice plants deficient in cAPXs. The data reveal that the two cAPXs are not essential for photosynthesis in rice or, alternatively, the deficient plants are able to trigger compensatory mechanisms to photosynthetic acclimation under ML and HL conditions. These mechanisms involve differential regulation in protein expression related to photochemistry, Calvin cycle and photorespiration.

Redirecting photosynthetic electron flow into light-driven synthesis of alternative products including high-value bioactive natural compounds

Photosynthesis in plants, green algae, and cyanobacteria converts solar energy into chemical energy in the form of ATP and NADPH, both of which are used in primary metabolism. However, often more reducing power is generated by the photosystems than what is needed for primary metabolism. In this review, we discuss the development in the research field, focusing on how the photosystems can be used as synthetic biology building blocks to channel excess reducing power into light-driven production of alternative products. Plants synthesize a large number of high-value bioactive natural compounds. Some of the key enzymes catalyzing their biosynthesis are the cytochrome P450s situated in the endoplasmic reticulum. However, bioactive compounds are often synthesized in low quantities in the plants and are difficult to produce by chemical synthesis due to their often complex structures. Through a synthetic biology approach, enzymes with a requirement for reducing equivalents as cofactors, such as the cytochrome P450s, can be coupled directly to the photosynthetic energy output to obtain environmentally friendly production of complex chemical compounds. By relocating cytochrome P450s to the chloroplasts, reducing power can be diverted toward the reactions catalyzed by the cytochrome P450s. This provides a sustainable production method for high-value compounds that potentially can solve the problem of NADPH regeneration, which currently limits the biotechnological uses of cytochrome P450s. We describe the approaches that have been taken to couple enzymes to photosynthesis in vivo and to photosystem I in vitro and the challenges associated with this approach to develop new green production platforms.

Arabidopsis tic62 trol mutant lacking thylakoid-bound ferredoxin-NADP+ oxidoreductase shows distinct metabolic phenotype

Ferredoxin-NADP+ oxidoreductase (FNR), functioning in the last step of the photosynthetic electron transfer chain, exists both as a soluble protein in the chloroplast stroma and tightly attached to chloroplast membranes. Surface plasmon resonance assays showed that the two FNR isoforms, LFNR1 and LFNR2, are bound to the thylakoid membrane via the C-terminal domains of Tic62 and TROL proteins in a pH-dependent manner. The tic62 trol double mutants contained a reduced level of FNR, exclusively found in the soluble stroma. Although the mutant plants showed no visual phenotype or defects in the function of photosystems under any conditions studied, a low ratio of NADPH/NADP+ was detected. Since the CO₂ fixation capacity did not differ between the tic62 trol plants and wild-type, it seems that the plants are able to funnel reducing power to most crucial reactions to ensure survival and fitness of the plants. However, the activity of malate dehydrogenase was down-regulated in the mutant plants. Apparently, the plastid metabolism is able to cope with substantial changes in directing the electrons from the light reactions to stromal metabolism and thus only few differences are visible in steady-state metabolite pool sizes of the tic62 trol plants.

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.

pH-dependent regulation of electron transport and ATP synthesis in chloroplasts

This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham-Benson-Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP(+). The mechanisms of the light-induced activation of ATP synthase are briefly considered.

Fusarium oxysporum f.sp. ciceri race 1 induced redox state alterations are coupled to downstream defense signaling in root tissues of chickpea (Cicer arietinum L.)

Reactive oxygen species are known to play pivotal roles in pathogen perception, recognition and downstream defense signaling. But, how these redox alarms coordinate in planta into a defensive network is still intangible. Present study illustrates the role of Fusarium oxysporum f.sp ciceri Race1 (Foc1) induced redox responsive transcripts in regulating downstream defense signaling in chickpea. Confocal microscopic studies highlighted pathogen invasion and colonization accompanied by tissue damage and deposition of callose degraded products at the xylem vessels of infected roots of chickpea plants. Such depositions led to the clogging of xylem vessels in compatible hosts while the resistant plants were devoid of such obstructions. Lipid peroxidation assays also indicated fungal induced membrane injury. Cell shrinkage and gradual nuclear adpression appeared as interesting features marking fungal ingress. Quantitative real time polymerase chain reaction exhibited differential expression patterns of redox regulators, cellular transporters and transcription factors during Foc1 progression. Network analysis showed redox regulators, cellular transporters and transcription factors to coordinate into a well orchestrated defensive network with sugars acting as internal signal modulators. Respiratory burst oxidase homologue, cationic peroxidase, vacuolar sorting receptor, polyol transporter, sucrose synthase, and zinc finger domain containing transcription factor appeared as key molecular candidates controlling important hubs of the defense network. Functional characterization of these hub controllers may prove to be promising in understanding chickpea-Foc1 interaction and developing the case study as a model for looking into the complexities of wilt diseases of other important crop legumes.

New insights into the reduction systems of plastidial thioredoxins point out the unique properties of thioredoxin z from Arabidopsis

In plants, thioredoxins (TRX) constitute a large protein disulphide oxidoreductase family comprising 10 plastidial members in Arabidopsis thaliana and subdivided in five types. The f- and m-types regulate enzymes involved mainly in carbon metabolism whereas the x, y, and z types have an antioxidant function. The reduction of TRXm and f in chloroplasts is performed in the light by ferredoxin:thioredoxin reductase (FTR) that uses photosynthetically reduced ferredoxin (Fd) as a reductant. The reduction system of Arabidopsis TRXx, y, and z has never been demonstrated. Recently, a gene encoding an atypical plastidial NADPH-dependent TRX reductase (NTRC) was found. In the present study, gene expression analysis revealed that both reductases are expressed in all organs of Arabidopsis and could potentially serve as electron donors to plastidial TRX. This ability was tested in vitro either with purified NTRC in presence of NADPH or with a light-driven reconstituted system comprising thylakoids and purified Fd and FTR. The results demonstrate that FTR reduces the x and y TRX isoforms but not the recently identified TRXz. Moreover, the results show that NTRC cannot be an efficient alternative reducing system, neither for TRXz nor for the other plastidial TRX. The data reveal that TRXf, m, x, and y, known as redox regulators in the chloroplast, have also the ability to reduce TRXz in vitro. Overall, the present study points out the unique properties of TRXz among plastidial TRX.

The importance of flavodoxin for environmental stress tolerance in photosynthetic microorganisms and transgenic plants. Mechanism, evolution and biotechnological potential

Ferredoxins are electron shuttles harboring iron-sulfur clusters which participate in oxido-reductive pathways in organisms displaying very different lifestyles. Ferredoxin levels decline in plants and cyanobacteria exposed to environmental stress and iron starvation. Flavodoxin is an isofunctional flavoprotein present in cyanobacteria and algae (not plants) which is induced and replaces ferredoxin under stress. Expression of a chloroplast-targeted flavodoxin in plants confers tolerance to multiple stresses and iron deficit. We discuss herein the bases for functional equivalence between the two proteins, the reasons for ferredoxin conservation despite its susceptibility to aerobic stress and for the loss of flavodoxin as an adaptive trait in higher eukaryotes. We also propose a mechanism to explain the tolerance conferred by flavodoxin when expressed in plants.

Cadmium inhibitory action leads to changes in structure of ferredoxin:NADP(+) oxidoreductase

This study deals with the influence of cadmium on the structure and function of ferredoxin:NADP(+) oxidoreductase (FNR), one of the key photosynthetic enzymes. We describe changes in the secondary and tertiary structure of the enzyme upon the action of metal ions using circular dichroism measurements, Fourier transform infrared spectroscopy and fluorometry, both steady-state and time resolved. The decrease in FNR activity corresponds to a gentle unfolding of the protein, caused mostly by a nonspecific binding of metal ions to multiple sites all over the enzyme molecule. The final inhibition event is most probably related to a bond created between cadmium and cysteine in close proximity to the FNR active center. As a result, the flavin cofactor is released. The cadmium effect is compared to changes related to ionic strength and other ions known to interact with cysteine. The complete molecular mechanism of FNR inhibition by heavy metals is discussed.Electronic supplementary material The online version of this article (doi:10.1007/s10867-012-9262-z) contains supplementary material, which is available to authorized users.

An in vivo system involving co-expression of cyanobacterial flavodoxin and ferredoxin-NADP(+) reductase confers increased tolerance to oxidative stress in plants

Oxidative stress in plants causes ferredoxin down-regulation and NADP⁺ shortage, over-reduction of the photosynthetic electron transport chain, electron leakage to oxygen and generation of reactive oxygen species (ROS). Expression of cyanobacterial flavodoxin in tobacco chloroplasts compensates for ferredoxin decline and restores electron delivery to productive routes, resulting in enhanced stress tolerance. We have designed an in vivo system to optimize flavodoxin reduction and NADP⁺ regeneration under stress using a version of cyanobacterial ferredoxin–NADP⁺ reductase without the thylakoid-binding domain. Co-expression of the two soluble flavoproteins in the chloroplast stroma resulted in lines displaying maximal tolerance to redox-cycling oxidants, lower damage and decreased ROS accumulation. The results underscore the importance of chloroplast redox homeostasis in plants exposed to adverse conditions, and provide a tool to improve crop tolerance toward environmental hardships.

A chloroplast-targeted DnaJ protein AtJ8 is negatively regulated by light and has rapid turnover in darkness

The DnaJ proteins (also called as J proteins, J domain proteins or HSP40 proteins) function as molecular co-chaperones for the HSP70 proteins. We assessed the expression of the small chloroplast-targeted DnaJ protein, the AtJ8 protein, by subjecting the wild type Arabidopsis plants to different illumination conditions. It is shown that the expression of the transcripts and proteins of the ATJ8 gene is primarily regulated at the level of transcription. When plants were incubated under high light for 3h, both the transcripts and proteins were completely abolished. Upon transfer of plants to darkness, the transcripts started rapidly accumulating, and subsequently, the AtJ8 protein became visible after 2h in darkness. Conversely, incubation of plants in darkness or under low light intensities induced expression of the ATJ8 transcripts and proteins. Feeding plants with sugars clearly decreased the transcript and protein levels, and incubation with cycloheximide revealed a rapid turnover for AtJ8 in darkness. Moreover, the AtJ8 protein was found to be nearly missing from the var1 mutant, which lacks the FTSH5 protease. It is concluded that AtJ8 is expressed mainly in darkness, is prone to a rapid turnover but is partially stabilized by the FTSH proteases.

Functional analysis of two isoforms of leaf-type ferredoxin-NADP(+)-oxidoreductase in rice using the heterologous expression system of Arabidopsis

Ferredoxin-NADP(+)-oxidoreductase (FNR) mediates electron transfer between ferredoxin (Fd) and NADP(+); therefore, it is a key enzyme that provides the reducing power used in the Calvin cycle. Other than FNR, nitrite reductase, sulfite reductase, glutamate synthase, and Fd-thioredoxin reductase also accept electrons from Fd, an electron carrier protein in the stroma. Therefore, the regulation of electron partitioning in the chloroplast is important for photosynthesis and other metabolic pathways. The regulatory mechanism of electron partitioning, however, remains to be elucidated. We found, by taking advantage of a gain-of-function approach, that expression of two rice (Oryza sativa) full-length cDNAs of leaf-type FNRs (OsLFNR1 and OsLFNR2) led to altered chlorophyll fluorescence and growth in Arabidopsis (Arabidopsis thaliana) and rice. We revealed that overexpression of the OsLFNR1 and OsLFNR2 full-length cDNAs resulted in distinct phenotypes despite the high sequence similarity between them. Expression of OsLFNR1 affected the nitrogen assimilation pathway without inhibition of photosynthesis under normal conditions. On the other hand, OsLFNR2 expression led to the impairment of photosynthetic linear electron transport as well as Fd-dependent cyclic electron flow around photosystem I. The endogenous protein level of OsLFNR was found to be suppressed in both OsLFNR1- and OsLFNR2-overexpressing rice plants, leading to changes in the stoichiometry of the two LFNR isoforms within the thylakoid and soluble fractions. Thus, we propose that the stoichiometry of two LFNR isoforms plays an important role in electron partitioning between carbon fixation and nitrogen assimilation.

A larger transcript is required for the synthesis of the smaller isoform of ferredoxin:NADP oxidoreductase

Ferredoxin:NADP oxidoreductases (FNRs) constitute a family of flavoenzymes that catalyse the exchange of electrons between ferredoxin and NADP(H). In cyanobacteria FNR provides NADPH for photoautotrophic metabolism, but the enzyme is also capable of oxidizing NADPH providing reduced ferredoxin. In the cyanobacterium Synechocystis sp. strain PCC6803, the unique petH gene has two translation products depending on growth conditions. As a consequence two isoforms of the FNR accumulate - FNR(L) and FNR(S) . In the present work, analysis of petH expression reveals that different transcriptional start points (tsp) are responsible for this differential translation initiation. Under standard conditions (where FNR(L) accumulates), two tsps were found at -52 and -34 relative to the first translation start site. Under nitrogen-starvation conditions (where FNR(S) accumulates) a tsp was mapped at -126 relative to the first translation start site. Therefore, the transcript responsible for FNR(S) translation is longer than that producing FNR(L) . In addition, expression of the short or long transcript in E. coli resulted in the accumulation of FNR(L) or FNR(S) respectively. This result demonstrates that translation can initiate at two different sites, 336-bases apart (ATG-1 to ATG-113), depending only on the 5'UTR structure.