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Flavodiiron proteins (FLVs) catalyze the reduction of oxygen to water by using electrons from Photosystem I (PSI). In several photosynthetic organisms such as cyanobacteria, green algae, mosses and gymnosperms, FLV-dependent electron flow protects PSI from over-reduction and consequent damage especially under fluctuating light conditions. In this work we investigated biochemical and structural properties of FLVA and FLVB from the model moss Physcomitrium patens. The two proteins, expressed and purified from Escherichia coli, bind both iron and flavin cofactors and show NAD(P)H oxidase activity as well as oxygen reductase capacities. Moreover, the co-expression of both FLVA and FLVB, coupled to a tandem affinity purification procedure with two different affinity tags, enabled the isolation of the stable and catalytically active FLVA/B hetero tetrameric protein complex with cooperative nature. The multimeric organization was shown to be stabilized by inter-subunit disulfide bonds. This investigation provides valuable new information on the biochemical properties of FLVs, with new insights into their in vivo activity.
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Plants rely on solar energy to synthesize ATP and NADPH for photosynthetic carbon fixation and all cellular need. Mitochondrial respiration is essential in plants, but this may be due to heterotrophic bottlenecks during plant development or because it is also necessary in photosynthetically active cells. In this study, we examined in vivo changes of cytosolic ATP concentration in response to light, employing a biosensing strategy in the moss Physcomitrium patens and revealing increased cytosolic ATP concentration caused by photosynthetic activity. Plants depleted of respiratory Complex I showed decreased cytosolic ATP accumulation, highlighting a critical role of mitochondrial respiration in light-dependent ATP supply of the cytosol. Consistently, targeting mitochondrial ATP production directly, through the construction of mutants deficient in mitochondrial ATPase (complex V), led to drastic growth reduction, despite only minor alterations in photosynthetic electron transport activity. Since P. patens is photoautotrophic throughout its development, we conclude that heterotrophic bottlenecks cannot account for the indispensable role of mitochondrial respiration in plants. Instead, our results support that mitochondrial respiration is essential for ATP provision to the cytosol in photosynthesizing cells. Mitochondrial respiration provides metabolic integration, ensuring supply of cytosolic ATP essential for supporting plant growth and development.
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Trifosfato de Adenosina , Bryopsida , Respiração Celular , Citosol , Mitocôndrias , Fotossíntese , Trifosfato de Adenosina/metabolismo , Citosol/metabolismo , Mitocôndrias/metabolismo , Bryopsida/metabolismo , Bryopsida/genética , Bryopsida/crescimento & desenvolvimento , LuzRESUMO
Introduction: Chloroplast calcium homeostasis plays an important role in modulating the response of plants to abiotic and biotic stresses. One of the greatest challenges is to understand how chloroplast calcium-permeable pathways and sensors are regulated in a concerted manner to translate specific information into a calcium signature and to elucidate the downstream effects of specific chloroplast calcium dynamics. One of the six homologs of the mitochondrial calcium uniporter (MCU) was found to be located in chloroplasts in the leaves and to crucially contribute to drought- and oxidative stress-triggered uptake of calcium into this organelle. Methods: In the present study we integrated comparative proteomic analysis with biochemical, genetic, cellular, ionomic and hormone analysis in order to gain an insight into how chloroplast calcium channels are integrated into signaling circuits under watered condition and under drought stress. Results: Altogether, our results indicate for the first time a link between chloroplast calcium channels and hormone levels, showing an enhanced ABA level in the cmcu mutant already in well-watered condition. Furthermore, we show that the lack of cMCU results in an upregulation of the calcium sensor CAS and of enzymes of chlorophyll synthesis, which are also involved in retrograde signaling upon drought stress, in two independent KO lines generated in Col-0 and Col-4 ecotypes. Conclusions: These observations point to chloroplasts as important signaling hubs linked to their calcium dynamics. Our results obtained in the model plant Arabidopsis thaliana are discussed also in light of our limited knowledge regarding organellar calcium signaling in crops and raise the possibility of an involvement of such signaling in response to drought stress also in crops.
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Photosynthesis in crops and natural vegetation allows light energy to be converted into chemical energy and thus forms the foundation for almost all terrestrial trophic networks on Earth. The efficiency of photosynthetic energy conversion plays a crucial role in determining the portion of incident solar radiation that can be used to generate plant biomass throughout a growth season. Consequently, alongside the factors such as resource availability, crop management, crop selection, maintenance costs, and intrinsic yield potential, photosynthetic energy use efficiency significantly influences crop yield. Photosynthetic efficiency is relevant to sustainability and food security because it affects water use efficiency, nutrient use efficiency, and land use efficiency. This review focuses specifically on the potential for improvements in photosynthetic efficiency to drive a sustainable increase in crop yields. We discuss bypassing photorespiration, enhancing light use efficiency, harnessing natural variation in photosynthetic parameters for breeding purposes, and adopting new-to-nature approaches that show promise for achieving unprecedented gains in photosynthetic efficiency.
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Fotossíntese , Melhoramento Vegetal , Produtos Agrícolas , Nutrientes , Segurança AlimentarRESUMO
Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.
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Fraxinus , Árvores , Árvores/metabolismo , Madeira/metabolismo , Clorofila A/metabolismo , Secas , Casca de Planta/metabolismo , Cloroplastos/metabolismo , Fotossíntese , Clorofila/metabolismo , Luz , Folhas de Planta/metabolismo , Complexo de Proteína do Fotossistema IIRESUMO
Plants exposed to light fluctuations are protected from photodamage by non-photochemical quenching (NPQ), a reversible mechanism that enables dissipation of excess absorbed energy as heat, which is essential for plant fitness and crop productivity. In plants NPQ requires the presence of the membrane protein PsbS, which upon activation interacts with antenna proteins, inducing their dissipative conformation. Here, we exploited base editing (BE) in the moss Physcomitrium patens to introduce specific amino acid changes in vivo and assess their impact on PsbS activity, targeting transmembrane regions to investigate their role in essential protein-protein interactions. This approach enabled the recognition of residues essential for protein stability and the identification of a hydrophobic cluster of amino acids impacting PsbS activity. This work provides new information on the molecular mechanism of PsbS while also demonstrating the potential of BE approaches for in planta gene function analysis.
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Luz , Fotossíntese , Complexo de Proteína do Fotossistema II/metabolismo , Edição de Genes , Complexos de Proteínas Captadores de Luz/metabolismoRESUMO
Light is the ultimate source of energy for photosynthetic organisms, but respiration is fundamental for supporting metabolism during the night or in heterotrophic tissues. In this work, we isolated Physcomitrella (Physcomitrium patens) plants with altered respiration by inactivating Complex I (CI) of the mitochondrial electron transport chain by independently targeting on two essential subunits. Inactivation of CI caused a strong growth impairment even in fully autotrophic conditions in tissues where all cells are photosynthetically active, demonstrating that respiration is essential for photosynthesis. CI mutants showed alterations in the stoichiometry of respiratory complexes while the composition of photosynthetic apparatus was substantially unaffected. CI mutants showed altered photosynthesis with high activity of both Photosystems I and II, likely the result of high chloroplast ATPase activity that led to smaller ΔpH formation across thylakoid membranes, decreasing photosynthetic control on cytochrome b6f in CI mutants. These results demonstrate that alteration of respiratory activity directly impacts photosynthesis in P. patens and that metabolic interaction between organelles is essential in their ability to use light energy for growth.
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Adenosina Trifosfatases/genética , Bryopsida/genética , Proteínas de Cloroplastos/genética , Proteínas de Plantas/genética , Adenosina Trifosfatases/metabolismo , Bryopsida/enzimologia , Proteínas de Cloroplastos/metabolismo , Proteínas de Plantas/metabolismoRESUMO
Diverse algae of the red lineage possess chlorophyll a-binding proteins termed LHCR, comprising the PSI light-harvesting system, which represent an ancient antenna form that evolved in red algae and was acquired through secondary endosymbiosis. However, the function and regulation of LHCR complexes remain obscure. Here we describe isolation of a Nannochloropsis oceanica LHCR mutant, named hlr1, which exhibits a greater tolerance to high-light (HL) stress compared to the wild type. We show that increased tolerance to HL of the mutant can be attributed to alterations in PSI, making it less prone to ROS production, thereby limiting oxidative damage and favoring growth in HL. HLR1 deficiency attenuates PSI light-harvesting capacity and growth of the mutant under light-limiting conditions. We conclude that HLR1, a member of a conserved and broadly distributed clade of LHCR proteins, plays a pivotal role in a dynamic balancing act between photoprotection and efficient light harvesting for photosynthesis.
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Adaptação Fisiológica/genética , Proteínas de Ligação à Clorofila/metabolismo , Luz/efeitos adversos , Complexo de Proteína do Fotossistema I/metabolismo , Estramenópilas/fisiologia , Adaptação Fisiológica/efeitos da radiação , Clorofila A/metabolismo , Proteínas de Ligação à Clorofila/genética , Proteínas de Ligação à Clorofila/isolamento & purificação , Mutação , Fotossíntese/genética , Fotossíntese/efeitos da radiação , Complexo de Proteína do Fotossistema I/genética , Estramenópilas/efeitos da radiaçãoRESUMO
CRISPR-Cas9 has proven to be highly valuable for genome editing in plants, including the model plant Physcomitrium patens. However, the fact that most of the editing events produced using the native Cas9 nuclease correspond to small insertions and deletions is a limitation. CRISPR-Cas9 base editors enable targeted mutation of single nucleotides in eukaryotic genomes and therefore overcome this limitation. Here, we report two programmable base-editing systems to induce precise cytosine or adenine conversions in P. patens. Using cytosine or adenine base editors, site-specific single-base mutations can be achieved with an efficiency up to 55%, without off-target mutations. Using the APT gene as a reporter of editing, we could show that both base editors can be used in simplex or multiplex, allowing for the production of protein variants with multiple amino-acid changes. Finally, we set up a co-editing selection system, named selecting modification of APRT to report gene targeting (SMART), allowing up to 90% efficiency site-specific base editing in P. patens. These two base editors will facilitate gene functional analysis in P. patens, allowing for site-specific editing of a given base through single sgRNA base editing or for in planta evolution of a given gene through the production of randomly mutagenised variants using multiple sgRNA base editing.
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Bryopsida , Bryopsida/genética , Sistemas CRISPR-Cas/genética , Repetições Palindrômicas Curtas Agrupadas e Regularmente Espaçadas , Edição de Genes , Mutagênese Sítio-DirigidaRESUMO
Alternative electron pathways contribute to regulation of photosynthetic light reactions to adjust to metabolic demands in dynamic environments. The chloroplast NADH dehydrogenase-like (NDH) complex mediates the cyclic electron transport pathway around PSI in different cyanobacteria, algae, and plant species, but it is not fully conserved in all photosynthetic organisms. In order to assess how the physiological role of this complex changed during plant evolution, we isolated Physcomitrella patens lines knocked out for the NDHM gene that encodes a subunit fundamental for the activity of the complex. ndhm knockout mosses indicated high PSI acceptor side limitation upon abrupt changes in illumination. In P. patens, pseudo-cyclic electron transport mediated by flavodiiron proteins (FLVs) was also shown to prevent PSI over-reduction in plants exposed to light fluctuations. flva ndhm double knockout mosses had altered photosynthetic performance and growth defects under fluctuating light compared with the wild type and single knockout mutants. The results showed that while the contribution of NDH to electron transport is minor compared with FLV, NDH still participates in modulating photosynthetic activity, and it is critical to avoid PSI photoinhibition, especially when FLVs are inactive. The functional overlap between NDH- and FLV-dependent electron transport supports PSI activity and prevents its photoinhibition under light variations.
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Bryopsida , Bryopsida/genética , Bryopsida/metabolismo , Cloroplastos/metabolismo , Transporte de Elétrons , Luz , NADH Desidrogenase/genética , NADH Desidrogenase/metabolismo , Fotossíntese , Complexo de Proteína do Fotossistema I/metabolismoRESUMO
Photosynthetic electron transport is regulated by cyclic and pseudocyclic electron flow (CEF and PCEF) to maintain the balance between light availability and metabolic demands. CEF transfers electrons from photosystem I to the plastoquinone pool with two mechanisms, dependent either on PGR5/PGRL1 or on the type I NADH dehydrogenase-like (NDH) complex. PCEF uses electrons from photosystem I to reduce oxygen and in many groups of photosynthetic organisms, but remarkably not in angiosperms, it is catalyzed by flavodiiron proteins (FLVs). In this study, Physcomitrella patens plants depleted in PGRL1, NDH and FLVs in different combinations were generated and characterized, showing that all these mechanisms are active in this moss. Surprisingly, in contrast to flowering plants, Physcomitrella patens can cope with the simultaneous inactivation of PGR5- and NDH-dependent CEF but, when FLVs are also depleted, plants show strong growth reduction and photosynthetic activity is drastically reduced. The results demonstrate that mechanisms for modulation of photosynthetic electron transport have large functional overlap but are together indispensable to protect photosystem I from damage and they are an essential component for photosynthesis in any light regime.
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Bryopsida , Complexo de Proteína do Fotossistema I , Bryopsida/metabolismo , Transporte de Elétrons , Luz , Fotossíntese , Complexo de Proteína do Fotossistema I/metabolismo , Desenvolvimento VegetalRESUMO
Although light is essential for photosynthesis, when in excess, it may damage the photosynthetic apparatus, leading to a phenomenon known as photoinhibition. Photoinhibition was thought as a light-induced damage to photosystem II; however, it is now clear that even photosystem I may become very vulnerable to light. One main characteristic of light induced damage to photosystem II (PSII) is the increased turnover of the reaction center protein, D1: when rate of degradation exceeds the rate of synthesis, loss of PSII activity is observed. With respect to photosystem I (PSI), an excess of electrons, instead of an excess of light, may be very dangerous. Plants possess a number of mechanisms able to prevent, or limit, such damages by safe thermal dissipation of light energy (non-photochemical quenching, NPQ), slowing-down of electron transfer through the intersystem transport chain (photosynthesis-control, PSC) in co-operation with the Proton Gradient Regulation (PGR) proteins, PGR5 and PGRL1, collectively called as short-term photoprotection mechanisms, and the redistribution of light between photosystems, called state transitions (responsible of fluorescence quenching at PSII, qT), is superimposed to these short term photoprotective mechanisms. In this manuscript we have generated a number of higher order mutants by crossing genotypes carrying defects in each of the short-term photoprotection mechanisms, with the final aim to obtain a direct comparison of their role and efficiency in photoprotection. We found that mutants carrying a defect in the ΔpH-dependent photosynthesis-control are characterized by photoinhibition of both photosystems, irrespectively of whether PSBS-dependent NPQ or state transitions defects were present or not in the same individual, demonstrating the primary role of PSC in photoprotection. Moreover, mutants with a limited capability to develop a strong PSBS-dependent NPQ, were characterized by a high turnover of the D1 protein and high values of Y(NO), which might reflect energy quenching processes occurring within the PSII reaction center.
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Proteínas de Arabidopsis/genética , Proteínas de Membrana/genética , Complexo de Proteínas do Centro de Reação Fotossintética/genética , Complexo de Proteína do Fotossistema I/genética , Complexo de Proteína do Fotossistema II/genética , Arabidopsis/genética , Arabidopsis/fisiologia , Genótipo , Concentração de Íons de Hidrogênio , Luz , Fotossíntese/genética , Fotossíntese/efeitos da radiação , Complexo de Proteína do Fotossistema I/efeitos da radiação , Complexo de Proteína do Fotossistema II/efeitos da radiaçãoRESUMO
In nature, photosynthetic organisms are exposed to highly dynamic environmental conditions where the excitation energy and electron flow in the photosynthetic apparatus need to be continuously modulated. Fluctuations in incident light are particularly challenging because they drive oversaturation of photosynthesis with consequent oxidative stress and photoinhibition. Plants and algae have evolved several mechanisms to modulate their photosynthetic machinery to cope with light dynamics, such as thermal dissipation of excited chlorophyll states (non-photochemical quenching, NPQ) and regulation of electron transport. The regulatory mechanisms involved in the response to light dynamics have adapted during evolution, and exploring biodiversity is a valuable strategy for expanding our understanding of their biological roles. In this work, we investigated the response to fluctuating light in Nannochloropsis gaditana, a eukaryotic microalga of the phylum Heterokonta originating from a secondary endosymbiotic event. Nannochloropsis gaditana is negatively affected by light fluctuations, leading to large reductions in growth and photosynthetic electron transport. Exposure to light fluctuations specifically damages photosystem I, likely because of the ineffective regulation of electron transport in this species. The role of NPQ, also assessed using a mutant strain specifically depleted of this response, was instead found to be minor, especially in responding to the fastest light fluctuations.
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Luz , Fotossíntese/fisiologia , Estramenópilas/metabolismo , Simbiose/fisiologia , Biodiversidade , Transporte de Elétrons/fisiologia , Estresse Oxidativo , Complexo de Proteína do Fotossistema I/metabolismo , Complexo de Proteína do Fotossistema I/efeitos da radiação , Plantas/metabolismo , Estramenópilas/crescimento & desenvolvimento , Estramenópilas/efeitos da radiaçãoRESUMO
The regulation of photosynthesis is crucial to efficiently support the assimilation of carbon dioxide and to prevent photodamage. One key regulatory mechanism is the pseudo-cyclic electron flow (PCEF) mediated by class-C flavodiiron proteins (FLVs). These enzymes use electrons coming from Photosystem I (PSI) to reduce oxygen to water, preventing over-reduction in the acceptor side of PSI. FLVs are widely distributed among organisms performing oxygenic photosynthesis and they have been shown to be fundamental in many different conditions such as fluctuating light, sulfur deprivation and plant submersion. Moreover, since FLVs reduce oxygen they can help controlling the redox status of the cell and maintaining the microoxic environment essential for processes such as nitrogen fixation in cyanobacteria. Despite these important roles identified in various species, the genes encoding for FLV proteins have been lost in angiosperms where their activity could have been at least partially compensated by a more efficient cyclic electron flow (CEF). The present work reviews the information emerged on FLV function, analyzing recent structural data that suggest FLV could be regulated through a conformational change.
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Proteínas de Bactérias/metabolismo , Cianobactérias/metabolismo , Fixação de Nitrogênio/fisiologia , Oxigênio/metabolismo , Fotossíntese/fisiologia , Complexo de Proteína do Fotossistema I/metabolismo , Proteínas de Bactérias/genética , Cianobactérias/genética , Oxirredução , Complexo de Proteína do Fotossistema I/genéticaRESUMO
Non-photochemical quenching, NPQ, of chlorophyll fluorescence regulates the heat dissipation of chlorophyll excited states and determines the efficiency of the oxygenic photosynthetic systems. NPQ is regulated by a pH-sensing protein, responding to the chloroplast lumen acidification induced by excess light, coupled to an actuator, a chlorophyll/xanthophyll subunit where quenching reactions are catalyzed. In plants, the sensor is PSBS, while the two pigment-binding proteins Lhcb4 (also known as CP29) and LHCII are the actuators. In algae and mosses, stress-related light-harvesting proteins (LHCSR) comprise both functions of sensor and actuator within a single subunit. Here, we report on expressing the lhcsr1 gene from the moss Physcomitrella patens into several Arabidopsis thaliana npq4 mutants lacking the pH sensing PSBS protein essential for NPQ activity. The heterologous protein LHCSR1 accumulates in thylakoids of A. thaliana and NPQ activity can be partially restored. Complementation of double mutants lacking, besides PSBS, specific xanthophylls, allowed analyzing chromophore requirement for LHCSR-dependent quenching activity. We show that the partial recovery of NPQ is mostly due to the lower levels of Zeaxanthin in A. thaliana in comparison to P. patens. Complemented npq2npq4 mutants, lacking besides PSBS, Zeaxanthin Epoxidase, showed an NPQ recovery of up to 70% in comparison to A. thaliana wild type. Furthermore, we show that Lutein is not essential for the folding nor for the quenching activity of LHCSR1. In short, we have developed a system to study the function of LHCSR proteins using heterologous expression in a variety of A. thaliana mutants.
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Arabidopsis/metabolismo , Bryopsida/genética , Complexos de Proteínas Captadores de Luz/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Arabidopsis/genética , Regulação da Expressão Gênica de Plantas , Luz , Complexos de Proteínas Captadores de Luz/genética , Mutação , Oxirredutases/genética , Oxirredutases/metabolismo , Processos Fotoquímicos , Fotossíntese , Plantas Geneticamente Modificadas , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Tilacoides/genética , Tilacoides/metabolismo , Xantofilas/metabolismo , Zeaxantinas/metabolismoRESUMO
MAIN CONCLUSION: Investigation of photosynthesis regulation in different plant groups exposed to variable conditions showed that all species have similar photosynthetic electron transport modulation while excess energy dissipation is species specific. Photosynthesis is regulated in response to dynamic environmental conditions to satisfy plant metabolic demands while also avoiding possible over-excitation of the electron transport chain and the generation of harmful reactive oxygen species. Photosynthetic organisms evolved several mechanisms to modulate light harvesting and electron transport efficiency to respond to conditions changing at different timescales, going from fast sun flecks to slow seasonal variations. These regulatory mechanisms changed during evolution of photosynthetic organisms, also adapting to various ecological niches, making the investigation of plant biodiversity highly valuable to uncover conserved traits and plasticity of photosynthetic regulation and complement studies on model species. In this work, a set of plants belonging to different genera of angiosperms, gymnosperms, ferns and lycophytes were investigated by monitoring their photosynthetic parameters in different seasons looking for common trends and differences. In all plants, analysed photosynthetic electron transport rate was found to be modulated by growth light intensity, ensuring a balance between available energy and photochemical capacity. Growth light also influenced the threshold where heat dissipation of excitation energy, a mechanism called non-photochemical quenching (NPQ), was activated. On the contrary, NPQ amplitude did not correlate with light intensity experienced by the plants but was a species-specific feature. The zeaxanthin-dependent component of NPQ, qZ, was found to be the most variable in different plants and its modulation influenced the intensity and the kinetic properties of the response.
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Biodiversidade , Fotossíntese/fisiologia , Plantas/metabolismo , Transporte de Elétrons , Meio Ambiente , Luz , Complexo de Proteína do Fotossistema II/metabolismoRESUMO
Photosynthetic organisms support cell metabolism by harvesting sunlight and driving the electron transport chain at the level of thylakoid membranes. Excitation energy and electron flow in the photosynthetic apparatus is continuously modulated in response to dynamic environmental conditions. Alternative electron flow around photosystem I plays a seminal role in this regulation contributing to photoprotection by mitigating overreduction of the electron carriers. Different pathways of alternative electron flow coexist in the moss Physcomitrella patens, including cyclic electron flow mediated by the PGRL1/PGR5 complex and pseudo-cyclic electron flow mediated by the flavodiiron proteins FLV. In this work, we generated P. patens plants carrying both pgrl1 and flva knock-out mutations. A comparative analysis of the WT, pgrl1, flva, and pgrl1 flva lines suggests that cyclic and pseudo-cyclic processes have a synergic role in the regulation of photosynthetic electron transport. However, although both contribute to photosystem I protection from overreduction by modulating electron flow following changes in environmental conditions, FLV activity is particularly relevant in the first seconds after a light change whereas PGRL1 has a major role upon sustained strong illumination.
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Bryopsida/fisiologia , Transporte de Elétrons/fisiologia , Complexo de Proteínas do Centro de Reação Fotossintética/genética , Complexo de Proteína do Fotossistema I/metabolismo , Bryopsida/genética , Cloroplastos/metabolismo , Transporte de Elétrons/genética , Luz , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Mutação , Fotossíntese/genética , Fotossíntese/fisiologia , Complexo de Proteínas do Centro de Reação Fotossintética/metabolismo , Complexo de Proteína do Fotossistema I/genética , Plantas Geneticamente Modificadas , Luz Solar , Tilacoides/metabolismoRESUMO
Contents Summary 105 I. Introduction 105 II. Diversity of molecular mechanisms for regulation of photosynthetic electron transport 106 III. Role of FLVs in the regulation of photosynthesis in eukaryotes 107 IV. Why were FLVs lost in angiosperms? 108 V. Conclusions 108 Acknowledgements 109 References 109 SUMMARY: Photosynthetic electron transport requires continuous modulation to maintain the balance between light availability and metabolic demands. Multiple mechanisms for the regulation of electron transport have been identified and are unevenly distributed among photosynthetic organisms. Flavodiiron proteins (FLVs) influence photosynthetic electron transport by accepting electrons downstream of photosystem I to reduce oxygen to water. FLV activity has been demonstrated in cyanobacteria, green algae and mosses to be important in avoiding photosystem I overreduction upon changes in light intensity. FLV-encoding sequences were nevertheless lost during evolution by angiosperms, suggesting that these plants increased the efficiency of other mechanisms capable of accepting electrons from photosystem I, making the FLV activity for protection from overreduction superfluous or even detrimental for photosynthetic efficiency.
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Evolução Biológica , Transporte de Elétrons , Fotossíntese/fisiologia , Fenômenos Fisiológicos Vegetais , Proteínas de Plantas/metabolismo , Magnoliopsida/fisiologia , Complexo de Proteína do Fotossistema I/metabolismoRESUMO
Photosystem I of the moss Physcomitrella patens has special properties, including the capacity to undergo non-photochemical fluorescence quenching. We studied the organization of photosystem I under different light and carbon supply conditions in wild-type moss and in moss with the lhcb9 (light-harvesting complex) knockout genotype, which lacks an antenna protein endowed with red-shifted absorption forms. Wild-type moss, when grown on sugars and in low light, accumulated LHCB9 proteins and a large form of the photosystem I supercomplex, which, besides the canonical four LHCI subunits, included a LHCII trimer and four additional LHC monomers. The lhcb9 knockout produced an angiosperm-like photosystem I supercomplex with four LHCI subunits irrespective of the growth conditions. Growth in the presence of sublethal concentrations of electron transport inhibitors that caused oxidation or reduction of the plastoquinone pool prevented or promoted, respectively, the accumulation of LHCB9 and the formation of the photosystem I megacomplex. We suggest that LHCB9 is a key subunit regulating the antenna size of photosystem I and the ability to avoid the over-reduction of plastoquinone: this condition is potentially dangerous in the shaded and sunfleck-rich environment typical of mosses, whose plastoquinone pool is reduced by both photosystem II and the oxidation of sugar substrates.