The complex architecture of oxygenic photosynthesis

Multienzyme System in Amorphous Metal-Organic Frameworks for Intracellular Lactate Detection

Lactate is an important downstream product of glycolysis in living cells, and its level is highly related with diseases. On the basis of amorphous metal-organic frameworks (aMOFs), a multienzyme system consisting of lactate oxidase (LOx) and horseradish peroxidase (HRP) was established for intracellular lactate detection. By coencapsulation in aMOFs with proximity, LOx and HRP were delivered into cells, serving as artificially constructed organelles, exhibiting high activity and selectivity for the intracellular detection of the important metabolite lactate, which improved the signal to noise ratio by ∼650-fold. As demonstrated by both experimental and simulation results, the high efficiency was attributed to the short distance between the two types of enzymes coencapsulated in aMOFs. The concept of constructing multienzyme systems in this study shows promise for the detection of various intracellular metabolites.

Light-induced electron transfer/phase migration of a redox mediator for photocatalytic C-C coupling in a biphasic solution

Photocatalytic molecular conversions that lead to value-added chemicals are of considerable interest. To achieve highly efficient photocatalytic reactions, it is equally important as it is challenging to construct systems that enable effective charge separation. Here, we demonstrate that the rational construction of a biphasic solution system with a ferrocenium/ferrocene (Fc⁺/Fc) redox couple enables efficient photocatalysis by spatial charge separation using the liquid-liquid interface. In a single-phase system, exposure of a 1,2-dichloroethane (DCE) solution containing a Ru(II)- or Ir(III)-based photosensitizer, Fc, and benzyl bromide (Bn-Br) to visible-light irradiation failed to generate any product. However, the photolysis in a H₂O/DCE biphasic solution, where the compounds are initially distributed in the DCE phase, facilitated the reductive coupling of Bn-Br to dibenzyl (Bn₂) using Fc as an electron donor. The key result of this study is that Fc⁺, generated by photooxidation of Fc in the DCE phase, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. This liquid-liquid phase migration of the mediator is essential for facilitating the reduction of Bn-Br in the DCE phase as it suppresses backward charge recombination. The co-existence of anions can further modify the driving force of phase migration of Fc⁺ depending on their hydrophilicity; the best photocatalytic activity was obtained with a turnover frequency of 79.5 h^-1 and a quantum efficiency of 0.2% for the formation of Bn₂ by adding NBu₄⁺Br^- to the biphasic solution. This study showcases a potential approach for rectifying electron transfer with suppressed charge recombination to achieve efficient photocatalysis.

Solution Equilibria Formation of Manganese(II) Complexes with Ethylenediamine, 1,3-Propanediamine and 1,4-ButanediaMine in Methanol

Manganese is an abundant element that plays critical roles and is at the reaction center of several enzymes. In order to promote an understanding of the behavior of manganese(II) ion with several aliphatic ligands, in this work, the stability and spectral behavior of the complexes with manganese(II) and ethylenediamine, 1,3-propanediamine or 1,4-butanediamine were explored. A spectrophotometric study of its speciation in methanol was performed at 293 K. The formation constants obtained for these systems were: manganese(II)-ethylenediamine log β110 = 3.98 and log β120 = 7.51; for the manganese(II)-1,3-propanediamine log β110 = 5.08 and log β120 = 8.66; and for manganese(II)-1,4-butanediamine log β110 = 4.36 and log β120 = 8.46. These results were obtained by fitting the experimental spectrophotometric data using the HypSpec software. The complexes reported in this study show a spectral pattern that could be related to a chelate effect in which the molar absorbance is not directly related to the increase in the carbon chain of the ligands.

The influence of electron utilization pathways on photosystem I photochemistry in Synechocystis sp. PCC 6803

The capacity of cyanobacteria to adapt to highly dynamic photon flux and nutrient availability conditions results from controlled management and use of reducing power, and is a major contributing factor to the efficiency of photosynthesis in aquatic environments. The response to changing conditions includes modulating gene expression and protein-protein interactions that serve to adjust the use of electron flux and mechanisms that control photosynthetic electron transport (PET). In this regard, the photochemical activity of photosystem I (PSI) reaction centers can support balancing of cyclic (CEF) and linear electron flow (LEF), and the coupling of redox carriers for use by electron utilization pathways. Therefore, changes in the utilization of reducing power might be expected to result in compensating changes at PSI as a means to support balance of electron flux. To understand this functional relationship, we investigated the properties of PSI and its photochemical activity in cells that lack flavodiiron 1 catalyzed oxygen reduction activity (ORR1). In the absence of ORR1, the oxygen evolution and consumption rates declined together with a shift in the oligomeric form of PSI towards monomers. The effect of these changes on PSI energy and electron transfer properties was examined in isolated trimer and monomer fractions of PSI reaction centers. Collectively, the results demonstrate that PSI photochemistry is modulated through coordination with the depletion of electron demand in the absence of ORR1.

Yang cycle enzyme DEP1: its moonlighting functions in PSI and ROS production during leaf senescence

Ethylene-mediated leaf senescence and the compromise of photosynthesis are closely associated but the underlying molecular mechanism is a mystery. Here we reported that apple DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1 (MdDEP1), initially characterized to its enzymatic function in the recycling of the ethylene precursor SAM, plays a role in the regulation of photosystem I (PSI) activity, activating reactive oxygen species (ROS) homeostasis, and negatively regulating the leaf senescence. A series of Y2H, Pull-down, CO-IP and Cell-free degradation biochemical assays showed that MdDEP1 directly interacts with and dephosphorylates the nucleus-encoded thylakoid protein MdY3IP1, leading to the destabilization of MdY3IP1, reduction of the PSI activity, and the overproduction of ROS in plant cells. These findings elucidate a novel mechanism that the two pathways intersect at MdDEP1 due to its moonlighting role in destabilizing MdY3IP1, and synchronize ethylene-mediated leaf senescence and the compromise of photosynthesis.

Quinone binding sites of cyt bc complexes analysed by X-ray crystallography and cryogenic electron microscopy

Cytochrome (cyt) bc1, bcc and b6f complexes, collectively referred to as cyt bc complexes, are homologous isoprenoid quinol oxidising enzymes present in diverse phylogenetic lineages. Cyt bc1 and bcc complexes are constituents of the electron transport chain (ETC) of cellular respiration, and cyt b6f complex is a component of the photosynthetic ETC. Cyt bc complexes share in general the same Mitchellian Q cycle mechanism, with which they accomplish proton translocation and thus contribute to the generation of proton motive force which drives ATP synthesis. They therefore require a quinol oxidation (Qo) and a quinone reduction (Qi) site. Yet, cyt bc complexes evolved to adapt to specific electrochemical properties of different quinone species and exhibit structural diversity. This review summarises structural information on native quinones and quinone-like inhibitors bound in cyt bc complexes resolved by X-ray crystallography and cryo-EM structures. Although the Qi site architecture of cyt bc1 complex and cyt bcc complex differs considerably, quinone molecules were resolved at the respective Qi sites in very similar distance to haem bH. In contrast, more diverse positions of native quinone molecules were resolved at Qo sites, suggesting multiple quinone binding positions or captured snapshots of trajectories toward the catalytic site. A wide spectrum of inhibitors resolved at Qo or Qi site covers fungicides, antimalarial and antituberculosis medications and drug candidates. The impact of these structures for characterising the Q cycle mechanism, as well as their relevance for the development of medications and agrochemicals are discussed.

Physiological and Molecular Changes in Cherry Red Tobacco in Response to Iron Deficiency Stress

The genotype CR60 is a spontaneous Cherry Red variant (containing granular red dapples on flue-cured leaves) of the Yunyan 87 (Y87) tobacco; it accumulates higher concentration of iron (Fe) in leaves than Y87, but the physiological differences between them remain largely unknown. We investigated the physiological and molecular mechanisms of CR60 in response to Fe deficiency under hydroponic conditions. Our results showed no significant phenotypic difference between Y87 and CR60 at optimal (40 μM) and high Fe (160 and 320 μM) concentrations. By contrast, CR60 exhibited higher tolerance to Fe deficiency (0 μM) than Y87, as shown by higher concentrations of chlorophyll in CR60 leaves after 21-day Fe-deficiency stress. Transcriptome profiling coupled with RT-PCR analyses found that the expression of IRT1 and several genes associated with chlorophyll biosynthesis and photosynthesis (e.g., PRO, GSA, FD1, PsbO, and PC) was higher in CR60 than Y87. These results indicated that CR60 maintains sufficient Fe uptake, chlorophyll biosynthesis and photosynthetic rate when subjected to Fe starvation.

The plastid-nucleus localized DNA-binding protein WHIRLY1 is required for acclimation of barley leaves to high light

In accordance with a key role of WHIRLY1 in light-acclimation mechanisms, typical features of acclimation to high light, including photosynthesis and leaf morphology, are compromised in WHIRLY1 deficient plants. Acclimation to the environment requires efficient communication between chloroplasts and the nucleus. Previous studies indicated that the plastid-nucleus located WHIRLY1 protein is required for the communication between plastids and the nucleus in situations of high light exposure. To investigate the consequences of WHIRLY1 deficiency on the light acclimation of photosynthesis and leaf anatomy, transgenic barley plants with an RNAi-mediated knockdown of HvWHIRLY1 were compared to wild-type plants when growing at low and high irradiance. While wild-type plants showed the typical light acclimation responses, i.e. higher photosynthetic capacity and thicker leaves, the WHIRLY1 deficient plants were not able to respond to differences in irradiance. The results revealed a systemic role of WHIRLY1 in light acclimation by coordinating responses at the level of the chloroplast and the level of leaf morphology.

The structure of the Physcomitrium patens photosystem I reveals a unique Lhca2 paralogue replacing Lhca4

The moss Physcomitrium patens diverged from green algae shortly after the colonization of land by ancient plants. This colonization posed new environmental challenges, which drove evolutionary processes. The photosynthetic machinery of modern flowering plants is adapted to the high light conditions on land. Red-shifted Lhca4 antennae are present in the photosystem I light-harvesting complex of many green-lineage plants but absent in P. patens. The cryo-EM structure of the P. patens photosystem I light-harvesting complex I supercomplex (PSI-LHCI) at 2.8 Å reveals that Lhca4 is replaced by a unique Lhca2 paralogue in moss. This PSI-LHCI supercomplex also retains the PsaM subunit, present in Cyanobacteria and several algal species but lost in vascular plants, and the PsaO subunit responsible for binding light-harvesting complex II. The blue-shifted Lhca2 paralogue and chlorophyll b enrichment relative to flowering plants make the P. patens PSI-LHCI spectroscopically unique among other green-lineage supercomplexes. Overall, the structure represents an evolutionary intermediate PSI with the crescent-shaped LHCI common in vascular plants, and contains a unique Lhca2 paralogue that facilitates the moss's adaptation to low-light niches.

Biological Approaches Integrating Algae and Bacteria for the Degradation of Wastewater Contaminants-A Review

The traditional approach for biodegradation of organic matter in sewage treatment used a consortium of bacterial spp. that produce untreated or partially treated inorganic contaminants resulting in large amounts of poor-quality sludge. The aeration process of activated sludge treatment requires high energy. So, a sustainable technique for sewage treatment that could produce less amount of sludge and less energy demanding is required for various developed and developing countries. This led to research into using microalgae for wastewater treatment as they reduce concentrations of nutrients like inorganic nitrates and phosphates from the sewage water, hence reducing the associated chemical oxygen demand (COD). The presence of microalgae removes nutrient concentration in water resulting in reduction of chemical oxygen demand (COD) and toxic heavy metals like Al, Ni, and Cu. Their growth also offers opportunity to produce biofuels and bioproducts from algal biomass. To optimize use of microalgae, technologies like high-rate algal ponds (HRAPs) have been developed, that typically use 22% of the electricity used in Sequencing Batch Reactors for activated sludge treatment with added economic and environmental benefits like reduced comparative operation cost per cubic meter, mitigate global warming, and eutrophication potentials. The addition of suitable bacterial species may further enhance the treatment potential in the wastewater medium as the inorganic nutrients are assimilated into the algal biomass, while the organic nutrients are utilized by bacteria. Further, the mutual exchange of CO2 and O2 between the algae and the bacteria helps in enhancing the photosynthetic activity of algae and oxidation by bacteria leading to a higher overall nutrient removal efficiency. Even negative interactions between algae and bacteria mediated by various secondary metabolites (phycotoxins) have proven beneficial as it controls the algal bloom in the eutrophic water bodies. Herein, we attempt to review various opportunities and limitations of using a combination of microalgae and bacteria in wastewater treatment method toward cost effective, eco-friendly, and sustainable method of sewage treatment.

Cryo-EM structure of a tetrameric photosystem I from Chroococcidiopsis TS-821, a thermophilic, unicellular, non-heterocyst-forming cyanobacterium

Photosystem I (PSI) is one of two photosystems involved in oxygenic photosynthesis. PSI of cyanobacteria exists in monomeric, trimeric, and tetrameric forms, in contrast to the strictly monomeric form of PSI in plants and algae. The tetrameric organization raises questions about its structural, physiological, and evolutionary significance. Here we report the ∼3.72 Å resolution cryo-electron microscopy structure of tetrameric PSI from the thermophilic, unicellular cyanobacterium Chroococcidiopsis sp. TS-821. The structure resolves 44 subunits and 448 cofactor molecules. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. The localization of chlorophyll molecules permits an excitation energy pathway within and between adjacent monomers. Bioinformatics analysis reveals conserved regions in the PsaL subunit that correlate with the oligomeric state. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms.

Photoresponsive DNA materials and their applications

Photoresponsive nucleic acids attract growing interest as functional constituents in materials science. Integration of photoisomerizable units into DNA strands provides an ideal handle for the reversible reconfiguration of nucleic acid architectures by light irradiation, triggering changes in the chemical and structural properties of the nanostructures that can be exploited in the development of photoresponsive functional devices such as machines, origami structures and ion channels, as well as environmentally adaptable 'smart' materials including nanoparticle aggregates and hydrogels. Moreover, photoresponsive DNA components allow control over the composition of dynamic supramolecular ensembles that mimic native networks. Beyond this, the modification of nucleic acids with photosensitizer functionality enables these biopolymers to act as scaffolds for spatial organization of electron transfer reactions mimicking natural photosynthesis. This review provides a comprehensive overview of these exciting developments in the design of photoresponsive DNA materials, and showcases a range of applications in catalysis, sensing and drug delivery/release. The key challenges facing the development of the field in the coming years are addressed, and exciting emergent research directions are identified.

Recent Advances in Understanding the Structural and Functional Evolution of FtsH Proteases

The FtsH family of proteases are membrane-anchored, ATP-dependent, zinc metalloproteases. They are universally present in prokaryotes and the mitochondria and chloroplasts of eukaryotic cells. Most bacteria bear a single ftsH gene that produces hexameric homocomplexes with diverse house-keeping roles. However, in mitochondria, chloroplasts and cyanobacteria, multiple FtsH homologs form homo- and heterocomplexes with specialized functions in maintaining photosynthesis and respiration. The diversification of FtsH homologs combined with selective pairing of FtsH isomers is a versatile strategy to enable functional adaptation. In this article we summarize recent progress in understanding the evolution, structure and function of FtsH proteases with a focus on the role of FtsH in photosynthesis and respiration.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)₃³⁺-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG⁰_ET and ΔG⁰_PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG⁰_ET and ΔG⁰_PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron–proton transfer (CEPT) to an electron transfer limited (ETPT_lim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT_lim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis.

Bioinspired Trinuclear Copper Catalyst for Water Oxidation with a Turnover Frequency up to 20000 s-1

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O-O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O₂ to water. This synthetic mimic exhibits a turnover frequency of 20000 s^-1 in sodium bicarbonate solution, which is about 150 and 15 times higher than that of the mononuclear Cu catalyst (F-N₂O₂Cu, 131.6 s^-1) and binuclear Cu₂ complex (HappCu₂, 1375 s^-1), respectively. This work shows that the cooperation between multiple metals is an effective strategy to regulate the formation of O-O bond in water oxidation catalysis.

Protein Assembly by Design

Proteins are nature's primary building blocks for the construction of sophisticated molecular machines and dynamic materials, ranging from protein complexes such as photosystem II and nitrogenase that drive biogeochemical cycles to cytoskeletal assemblies and muscle fibers for motion. Such natural systems have inspired extensive efforts in the rational design of artificial protein assemblies in the last two decades. As molecular building blocks, proteins are highly complex, in terms of both their three-dimensional structures and chemical compositions. To enable control over the self-assembly of such complex molecules, scientists have devised many creative strategies by combining tools and principles of experimental and computational biophysics, supramolecular chemistry, inorganic chemistry, materials science, and polymer chemistry, among others. Owing to these innovative strategies, what started as a purely structure-building exercise two decades ago has, in short order, led to artificial protein assemblies with unprecedented structures and functions and protein-based materials with unusual properties. Our goal in this review is to give an overview of this exciting and highly interdisciplinary area of research, first outlining the design strategies and tools that have been devised for controlling protein self-assembly, then describing the diverse structures of artificial protein assemblies, and finally highlighting the emergent properties and functions of these assemblies.

Plant and Algal PSII-LHCII Supercomplexes: Structure, Evolution and Energy Transfer

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy in chemical forms. The light-harvesting, excitation transfer, charge separation and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledge about the architectures and assemblies of plant and green algal PSII-light harvesting complex II (LHCII) supercomplexes, with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII-LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution and about the energy transfer pathways that define their light-harvesting and photoprotective properties.

The Plasticity of Photosystem I

Most of life's energy comes from sunlight, and thus, photosynthesis underpins the survival of virtually all life forms. The light-driven electron transfer at photosystem I (PSI) is certainly the most important generator of reducing power at the cellular level and thereby largely determines the global amount of enthalpy in living systems (Nelson 2011). The PSI is a light-driven plastocyanin:ferredoxin oxidoreductase, which is embedded into thylakoid membranes of cyanobacteria and chloroplasts of eukaryotic photosynthetic organism. Structural determination of complexes of the photosynthetic machinery is vital for the understanding of its mode of action. Here, we describe new structural and functional insights into PSI and associated light-harvesting proteins, with a focus on the plasticity of PSI.

Fullerenes Enhance Self-Assembly and Electron Injection of Photosystem I in Biophotovoltaic Devices

This paper describes the fabrication of microfluidic devices with a focus on controlling the orientation of photosystem I (PSI) complexes, which directly affects the performance of biophotovoltaic devices by maximizing the efficiency of the extraction of electron/hole pairs from the complexes. The surface chemistry of the electrode on which the complexes assemble plays a critical role in their orientation. We compared the degree of orientation on self-assembled monolayers of phenyl-C61-butyric acid and a custom peptide on nanostructured gold electrodes. Biophotovoltaic devices fabricated with the C61 fulleroid exhibit significantly improved performance and reproducibility compared to those utilizing the peptide, yielding a 1.6-fold increase in efficiency. In addition, the C61-based devices were more stable under continuous illumination. Our findings show that fulleroids, which are well-known acceptor materials in organic photovoltaic devices, facilitate the extraction of electrons from PSI complexes without sacrificing control over the orientation of the complexes, highlighting this combination of traditional organic semiconductors with biomolecules as a viable approach to coopting natural photosynthetic systems for use in solar cells.

Transcriptomic analysis unravels the modulating mechanisms of the biomass and value-added bioproducts accumulation by light spectrum in Eustigmatos cf. Polyphem (Eustigmatophyceae)

Light spectrum can influence microalgal growth and metabolites accumulation significantly. However, the related mechanism has not been fully elucidated. Here, an oleaginous microalga Eustigmatos cf. polyphem, which also featured with high content of palmitoleic acid (POA) and β-carotene, was cultured with LEDs-based red light (RL) and blue light (BL). The results showed that the biomass, total lipid content and POA content were much higher under RL than these under BL, regardless of nitrogen concentration. However, the β-carotene content under RL was significantly lower than that under BL. Transcriptomic analysis revealed that photosynthesis, central carbon metabolism, fatty acid and glycerolipid biosynthesis were elevated, supporting the fast cell growth and high lipid content with POA under RL. In contrast, upregulation of key enzymes in carotenoids biosynthesis and suppression of β-carotene conversion promoted β-carotene accumulation under BL. These findings provide a feasible strategy for promoting lipids, POA and β-carotene in E. cf. polyphem.

An Exon Skipping in CRS1 Is Associated with Perturbed Chloroplast Development in Maize

Chloroplasts of higher plants are semi-autonomous organelles that perform photosynthesis and produce hormones and metabolites. They play crucial roles in plant growth and development. Although many seedling-lethal nuclear genes or regulators required for chloroplast development have been characterized, the understanding of chloroplast development is still limited. Using a genetic screen, we isolated a mutant named ell1, with etiolated leaves and a seedling-lethal phenotype. Analysis by BN-PAGE and transmission electron microscopy revealed drastic morphological defects of chloroplasts in ell1 mutants. Genetic mapping of the mutant gene revealed a single mutation (G-to-A) at the 5′ splice site of intron 5 in CRS1, resulting in an exon skipping in CRS1, indicating that this mutation in CRS1 is responsible for the observed phenotype, which was further confirmed by genetic analysis. The incorrectly spliced CRS1 failed to mediate the splicing of atpF intron. Moreover, the quantitative analysis suggested that ZmCRS1 may participate in chloroplast transcription to regulate the development of chloroplast. Taken together, these findings improve our understanding of the ZmCRS1 protein and shed new light on the regulation of chloroplast development in maize.

Electrospinning for building 3D structured photoactive biohybrid electrodes

We describe the development of biohybrid electrodes constructed via combination of electrospun (e-spun) 3D indium tin oxide (ITO) with the trimeric supercomplex photosystem I and the small electrochemically active protein cytochrome c (cyt c). The developed 3D surface of ITO has been created by electrospinning of a mixture of polyelthylene oxide (PEO) and ITO nanoparticles onto ITO glass slides followed by a subsequent elimination of PEO by sintering the composite. Whereas the photosystem I alone shows only small photocurrents at these 3D electrodes, the co-immobilization of cyt c to the e-spun 3D ITO results in well-defined photoelectrochemical signals. The scaling of thickness of the 3D ITO layers by controlling the time (10 min and 60 min) of electrospinning results in enhancement of the photocurrent. Several performance parameters of the electrode have been analyzed for different illumination intensities.

ZmPPR26, a DYW-type pentatricopeptide repeat protein, is required for C-to-U RNA editing at atpA-1148 in maize chloroplasts

Pentatricopeptide repeat (PPR) proteins are involved in the C-to-U RNA editing of organellar transcripts. The maize genome contains over 600 PPR proteins and few have been found to function in the C-to-U RNA editing in chloroplasts. Here, we report the function of ZmPPR26 in the C-to-U RNA editing and chloroplast biogenesis in maize. ZmPPR26 encodes a DYW-type PPR protein targeted to chloroplasts. The zmppr26 mutant exhibits albino seedling-lethal phenotype. Loss of function of ZmPPR26 abolishes the editing at atpA-1148 site, and decreases the editing at ndhF-62, rpl20-308, rpl2-2, rpoC2-2774, petB-668, rps8-182, and ndhA-50 sites. Overexpression of ZmPPR26 in zmppr26 restores the editing efficiency and rescues the albino seedling-lethal phenotype. Abolished editing at atpA-1148 causes a Leu to Ser change at AtpA-383 that leads to a reduction in the abundance of chloroplast ATP synthase in zmppr26. The accumulation of photosynthetic complexes are also markedly reduced in zmppr26, providing an explanation for the albino seedling-lethal phenotype. These results indicate that ZmPPR26 is required for the editing at atpA-1148 and is important for editing at the other seven sites in maize chloroplasts. The editing at atpA-1148 is critical for AtpA function, assembly of ATP synthase complex, and chloroplast biogenesis in maize.

Dissipation of Light Energy Absorbed in Excess: The Molecular Mechanisms

Light is essential for photosynthesis. Nevertheless, its intensity widely changes depending on time of day, weather, season, and localization of individual leaves within canopies. This variability means that light collected by the light-harvesting system is often in excess with respect to photon fluence or spectral quality in the context of the capacity of photosynthetic metabolism to use ATP and reductants produced from the light reactions. Absorption of excess light can lead to increased production of excited, highly reactive intermediates, which expose photosynthetic organisms to serious risks of oxidative damage. Prevention and management of such stress are performed by photoprotective mechanisms, which operate by cutting down light absorption, limiting the generation of redox-active molecules, or scavenging reactive oxygen species that are released despite the operation of preventive mechanisms. Here, we describe the major physiological and molecular mechanisms of photoprotection involved in the harmless removal of the excess light energy absorbed by green algae and land plants. In vivo analyses of mutants targeting photosynthetic components and the enhanced resolution of spectroscopic techniques have highlighted specific mechanisms protecting the photosynthetic apparatus from overexcitation. Recent findings unveil a network of multiple interacting elements, the reaction times of which vary from a millisecond to weeks, that continuously maintain photosynthetic organisms within the narrow safety range between efficient light harvesting and photoprotection.

PAP90, a novel rice protein plays a critical role in regulation of D1 protein stability of PSII

Introduction

Photosystem II (PSII) protein complex plays an essential role in the entire photosynthesis process. Various known and unknown protein factors are involved in the dynamics of the PSII complex that need to be characterized in crop plants for enhancing photosynthesis efficiency and productivity.

Objectives

The experiments were conducted to decipher the regulatory proteins involved in PSII dynamics of rice crop.

Methods

A novel rice regulatory protein PAP90 (PSII auxiliary protein ~90 kDa) was characterized by generating a loss-of-function mutant pap90. The mutation was characterized at molecular level followed by various experiments to analyze the morphological, physiological and biochemical processes of mutant under control and abiotic stresses.

Results

The pap90 mutant showed reduced photosynthesis due to D1 protein instability that subsequently causes inadequate accumulation of thylakoid membrane complexes, especially PSII and decreases PSII functional efficiency. Expression of OsFtsH family genes and proteins were induced in the mutant, which are known to play a key role in D1 protein degradation and turnover. The reduced D1 protein accumulation in the mutant increased the production of reactive oxygen species (ROS). The accumulation of ROS along with the increased activity of antioxidant enzymes and induced expression of stress-associated genes and proteins in pap90 mutant contributed to its water-limited stress tolerance ability.

Conclusion

We propose that PAP90 is a key auxiliary protein that interacts with D1 protein and maintains its stability, thereby promoting subsequent assembly of the PSII and associated membrane complexes.

Trophic Transition Enhanced Biomass and Lipid Production of the Unicellular Green Alga Scenedesmus acuminatus

Microalgal heterotrophic cultivation is an emerging technology that can enable producing high cell-density algal cell cultures, which can be coupled with photoautotrophic cultivation for valuable chemicals such as lipids manufacturing. However, how the heterotrophically grown algal cells respond to the lipid-inducing conditions has not been fully elucidated so far. In this study, when the heterotrophically grown Scenedesmus acuminatus cells were subjected to the high light (HL) and nitrogen-limited (NL) conditions, both the biomass and lipid productivity were enhanced as compared to that of the photoautotrophically grown counterparts. The chlorophyll a fluorometry analysis showed that the Fv/Fm and Y(II) of the heterotrophically grown cells subjected to the HL and NL conditions was recovered to the maximum value of 0.75 and 0.43, respectively, much higher than those of the photoautotrophically grown cells under the same stress conditions. Transcriptomic analysis revealed that heterotrophically grown cells fully expressed the genes coding for the photosystems proteins, including the key photoprotective proteins D1, PsbS, light-harvesting-complex (LHC) I and LHC II. Meanwhile, downregulation of the carotenoid biosynthesis and upregulation of the glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle and oxidative phosphorylation pathways were observed when the heterotrophically grown cells were subjected to the HL and N-limited conditions for lipid production. It was deduced that regulation of these pathways not only enhanced the light utilization but also provided the reducing power and ATP by which the biomass accumulation was significantly elevated. Besides, upregulation of the acetyl-CoA carboxylase/biotin carboxylase, digalactosyl diacylglycerol synthase and diacylglycerol acyltransferase 2 encoding genes may be attributable to the enhanced lipid production. Understanding the cellular responses during the trophic transition process could guide improvement of the strength of trophic transition enhancing microalgal biomass and lipid production.

PSII Activity Was Inhibited at Flowering Stage with Developing Black Bracts of Oat

The color of bracts generally turns yellow or black from green during cereal grain development. However, the impact of these phenotypic changes on photosynthetic physiology during black bract formation remains unclear. Two oat cultivars (Avena sativa L.), ‘Triple Crown’ and ‘Qinghai 444’, with yellow and black bracts, respectively, were found to both have green bracts at the heading stage, but started to turn black at the flowering stage and become blackened at the milk stage for ‘Qinghai 444’. Their photosynthetic characteristics were analyzed and compared, and the key genes, proteins and regulatory pathways affecting photosynthetic physiology were determined in ‘Triple Crown’ and ‘Qinghai 444’ bracts. The results show that the actual PSII photochemical efficiency and PSII electron transfer rate of ‘Qinghai 444’ bracts had no significant changes at the heading and milk stages but decreased significantly (p < 0.05) at the flowering stage compared with ‘Triple Crown’. The chlorophyll content decreased, the LHCII involved in the assembly of supercomplexes in the thylakoid membrane was inhibited, and the expression of Lhcb1 and Lhcb5 was downregulated at the flowering stage. During this critical stage, the expression of Bh4 and C4H was upregulated, and the biosynthetic pathway of p-coumaric acid using tyrosine and phenylalanine as precursors was also enhanced. Moreover, the key upregulated genes (CHS, CHI and F3H) of anthocyanin biosynthesis might complement the impaired PSII activity until recovered at the milk stage. These findings provide a new insight into how photosynthesis alters during the process of oat bract color transition to black.

A type dependent effect of treated wastewater matrix on seed germination and food production

Municipal wastewater treated by membrane bioreactor, either aerobically (AeMBR) or anaerobically (AnMBR), can be reused to irrigate crops. However, post-AeMBR and post-AnMBR effluent have different water quality that may impact crop growth and yield. This study aims to assess for differences in water quality from both AeMBR and AnMBR, and determine if the type of treated wastewater matrix would impact seed germination and crop yield. Compared to post-AeMBR and control, post-AnMBR effluent had a negative impact on seed germination for both tomatoes and lettuces. The use of post-AnMBR but not post-AeMBR effluent also resulted in a higher number of unripe tomato fruits at the time of harvesting. However, when post-AnMBR effluent was diluted to 25% and 75% v/v with tap water, higher lettuce biomass was harvested compared to the same concentrations of post-AeMBR effluent and control. The observed differences in germination and yield were likely due to differences in the concentrations of heavy metals (e.g. Zn) and steroids or phytohormones (e.g. testosterone, gibberellic acid) present in both post-MBR effluents. This study demonstrated that the type of treated wastewater generated from different upstream treatment technologies can potentially impact crop yield based on the crop type. By understanding how the type of treated wastewater affect downstream agricultural activities, changes in management practices can be made accordingly.

GUN control in retrograde signaling: How GENOMES UNCOUPLED proteins adjust nuclear gene expression to plastid biogenesis

Communication between cellular compartments is vital for development and environmental adaptation. Signals emanating from organelles, so-called retrograde signals, coordinate nuclear gene expression with the developmental stage and/or the functional status of the organelle. Plastids (best known in their green photosynthesizing differentiated form, the chloroplasts) are the primary energy-producing compartment of plant cells, and the site for the biosynthesis of many metabolites, including fatty acids, amino acids, nucleotides, isoprenoids, tetrapyrroles, vitamins, and phytohormone precursors. Signals derived from plastids regulate the accumulation of a large set of nucleus-encoded proteins, many of which localize to plastids. A set of mutants defective in retrograde signaling (genomes uncoupled, or gun) was isolated over 25 years ago. While most GUN genes act in tetrapyrrole biosynthesis, resolving the molecular function of GUN1, the proposed integrator of multiple retrograde signals, has turned out to be particularly challenging. Based on its amino acid sequence, GUN1 was initially predicted to be a plastid-localized nucleic acid-binding protein. Only recently, mechanistic information on the function of GUN1 has been obtained, pointing to a role in plastid protein homeostasis. This review article summarizes our current understanding of GUN-related retrograde signaling and provides a critical appraisal of the various proposed roles for GUNs and their respective pathways.

High Carotenoid Mutants of Chlorella vulgaris Show Enhanced Biomass Yield under High Irradiance

Microalgae represent a carbon-neutral source of bulk biomass, for extraction of high-value compounds and production of renewable fuels. Due to their high metabolic activity and reproduction rates, species of the genus Chlorella are highly productive when cultivated in photobioreactors. However, wild-type strains show biological limitations making algal bioproducts expensive compared to those extracted from other feedstocks. Such constraints include inhomogeneous light distribution due to high optical density of the culture, and photoinhibition of the surface-exposed cells. Thus, the domestication of algal strains for industry makes it increasingly important to select traits aimed at enhancing light-use efficiency while withstanding excess light stress. Carotenoids have a crucial role in protecting against photooxidative damage and, thus, represent a promising target for algal domestication. We applied chemical mutagenesis to Chlorella vulgaris and selected for enhanced tolerance to the carotenoid biosynthesis inhibitor norflurazon. The NFR (norflurazon-resistant) strains showed an increased carotenoid pool size and enhanced tolerance towards photooxidative stress. Growth under excess light revealed an improved carbon assimilation rate of NFR strains with respect to WT. We conclude that domestication of Chlorella vulgaris, by optimizing both carotenoid/chlorophyll ratio and resistance to photooxidative stress, boosted light-to-biomass conversion efficiency under high light conditions typical of photobioreactors. Comparison with strains previously reported for enhanced tolerance to singlet oxygen, reveals that ROS resistance in Chlorella is promoted by at least two independent mechanisms, only one of which is carotenoid-dependent.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Understanding photosynthesis in a spatial-temporal multiscale: The need for a systemic view

In October 2020, at the peak of the COVID-19 pandemic, a group of young Brazilian photosynthesis researchers organized the 1st Brazilian Symposium on Photosynthesis. The event was free and online, with the presence of important guest speakers from all over the world, who discussed their recent works on topics related to the future and perspectives of photosynthesis research. Summarizing the expectations of this symposium we highlighted the importance of adopting a systemic perspective for a better understanding of photosynthesis as a complex and dynamic process. Plants are modular and self-regulating presenting metabolic redundancy and functional degeneration. Among the various biological processes, photosynthesis plays a crucial role in promoting the direct conversion of light energy into carbon skeletons for support growth and productivity. In the past decades, significant advances have been made in photosynthesis at the biophysical, biochemical, and molecular levels. However, this myriad of knowledge has been insufficient to answer crucial questions, such as: how can we understand and eventually increase photosynthetic efficiency and yield in crops subjected to adverse environment related to climate-changing? We believe that a crucial limitation to the whole comprehension of photosynthesis is associated with a vastly widespread classic reductionist view. Moreover, this perspective is commonly accompanied by non-integrative, simplistic, and descriptive approaches to investigate a complex and dynamic process as photosynthesis. Herein, we propose the use of new approaches, mostly based on the Systems Theory, which certainly comes closer to the real world, such as the complex systems that the plants represent.

Functional studies of CpSRP54 in diatoms show that the mechanism of thylakoid protein insertion differs from that in plants and green algae

The chloroplast signal recognition particle 54 kDa (CpSRP54) protein is a member of the CpSRP pathway known to target proteins to thylakoid membranes in plants and green algae. Loss of CpSRP54 in the marine diatom Phaeodactylum tricornutum lowers the accumulation of a selection of chloroplast-encoded subunits of photosynthetic complexes, indicating a role in the co-translational part of the CpSRP pathway. In contrast to plants and green algae, absence of CpSRP54 does not have a negative effect on the content of light-harvesting antenna complex proteins and pigments in P. tricornutum, indicating that the diatom CpSRP54 protein has not evolved to function in the post-translational part of the CpSRP pathway. Cpsrp54 KO mutants display altered photophysiological responses, with a stronger induction of photoprotective mechanisms and lower growth rates compared to wild type when exposed to increased light intensities. Nonetheless, their phenotype is relatively mild, thanks to the activation of mechanisms alleviating the loss of CpSRP54, involving upregulation of chaperones. We conclude that plants, green algae, and diatoms have evolved differences in the pathways for co-translational and post-translational insertion of proteins into the thylakoid membranes.

Biohydrogen production from microalgae-Major bottlenecks and future research perspectives

The imprudent use of fossil fuels has resulted in high greenhouse gas (GHG) emissions, leading to climate change and global warming. Reduction in GHG emissions and energy insecurity imposed by the depleting fossil fuel reserves led to the search for alternative sustainable fuels. Hydrogen is a potential alternative energy carrier and is of particular interest because hydrogen combustion releases only water. Hydrogen is also an important industrial feedstock. As an alternative energy carrier, hydrogen can be used in fuel cells for power generation. Current hydrogen production mainly relies on fossil fuels and is usually energy and CO₂ -emission intensive, thus the use of fossil fuel-derived hydrogen as a carbon-free fuel source is fallacious. Biohydrogen production can be achieved via microbial methods, and the use of microalgae for hydrogen production is outstanding due to the carbon mitigating effects and the utilization of solar energy as an energy source by microalgae. This review provides comprehensive information on the mechanisms of hydrogen production by microalgae and the enzymes involved. The major challenges in the commercialization of microalgae-based photobiological hydrogen production are critically analyzed and future research perspectives are discussed. Life cycle analysis and economic assessment of hydrogen production by microalgae are also presented.

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.

Mixed Models as a Tool for Comparing Groups of Time Series in Plant Sciences

Plants adapt to continual changes in environmental conditions throughout their life spans. High-throughput phenotyping methods have been developed to noninvasively monitor the physiological responses to abiotic/biotic stresses on a scale spanning a long time, covering most of the vegetative and reproductive stages. However, some of the physiological events comprise almost immediate and very fast responses towards the changing environment which might be overlooked in long-term observations. Additionally, there are certain technical difficulties and restrictions in analyzing phenotyping data, especially when dealing with repeated measurements. In this study, a method for comparing means at different time points using generalized linear mixed models combined with classical time series models is presented. As an example, we use multiple chlorophyll time series measurements from different genotypes. The use of additional time series models as random effects is essential as the residuals of the initial mixed model may contain autocorrelations that bias the result. The nature of mixed models offers a viable solution as these can incorporate time series models for residuals as random effects. The results from analyzing chlorophyll content time series show that the autocorrelation is successfully eliminated from the residuals and incorporated into the final model. This allows the use of statistical inference.

Label-Free Quantitative Proteomics Analysis in Susceptible and Resistant Brassica napus Cultivars Infected with Xanthomonas campestris pv. campestris

Black rot, caused by Xanthomonas campestris pv. campestris (Xcc), is the main disease of cruciferous vegetables. To characterize the resistance mechanism in the Brassica napus–Xcc pathosystem, Xcc-responsive proteins in susceptible (cv. Mosa) and resistant (cv. Capitol) cultivars were investigated using gel-free quantitative proteomics and analysis of gene expression. This allowed us to identify 158 and 163 differentially expressed proteins following Xcc infection in cv. Mosa and cv. Capitol, respectively, and to classify them into five major categories including antioxidative systems, proteolysis, photosynthesis, redox, and innate immunity. All proteins involved in protein degradation such as the protease complex, proteasome subunits, and ATP-dependent Clp protease proteolytic subunits, were upregulated only in cv. Mosa, in which higher hydrogen peroxide accumulation concurred with upregulated superoxide dismutase. In cv. Capitol, photosystem II (PS II)-related proteins were downregulated (excepting PS II 22 kDa), whereas the PS I proteins, ATP synthase, and ferredoxin-NADP⁺ reductase, were upregulated. For redox-related proteins, upregulation of thioredoxin, 2-cys peroxiredoxin, and glutathione S-transferase occurred in cv. Capitol, consistent with higher NADH-, ascorbate-, and glutathione-based reducing potential, whereas the proteins involved in the C₂ oxidative cycle and glycolysis were highly activated in cv. Mosa. Most innate immunity-related proteins, including zinc finger domain (ZFD)-containing protein, glycine-rich RNA-binding protein (GRP) and mitochondrial outer membrane porin, were highly enhanced in cv. Capitol, concomitant with enhanced expression of ZFD and GRP genes. Distinguishable differences in the protein profile between the two cultivars deserves higher importance for breeding programs and understanding of disease resistance in the B. napus–Xcc pathosystem.

Expression of Flavodiiron Proteins Flv2-Flv4 in Chloroplasts of Arabidopsis and Tobacco Plants Provides Multiple Stress Tolerance

With the notable exception of angiosperms, all phototrophs contain different sets of flavodiiron proteins that help to relieve the excess of excitation energy on the photosynthetic electron transport chain during adverse environmental conditions, presumably by reducing oxygen directly to water. Among them, the Flv2-Flv4 dimer is only found in β-cyanobacteria and induced by high light, supporting a role in stress protection. The possibility of a similar protective function in plants was assayed by expressing Synechocystis Flv2-Flv4 in chloroplasts of tobacco and Arabidopsis. Flv-expressing plants exhibited increased tolerance toward high irradiation, salinity, oxidants, and drought. Stress tolerance was reflected by better growth, preservation of photosynthetic activity, and membrane integrity. Metabolic profiling under drought showed enhanced accumulation of soluble sugars and amino acids in transgenic Arabidopsis and a remarkable shift of sucrose into starch, in line with metabolic responses of drought-tolerant genotypes. Our results indicate that the Flv2-Flv4 complex retains its stress protection activities when expressed in chloroplasts of angiosperm species by acting as an additional electron sink. The flv2-flv4 genes constitute a novel biotechnological tool to generate plants with increased tolerance to agronomically relevant stress conditions that represent a significant productivity constraint.

Protein Nanobarrel for Integrating Chlorophyll a Molecules and Its Photochemical Performance

Taking inspiration from biology's effectiveness in nanoscale organization of chlorophylls for photosynthesis, we describe here a design for chlorophyll-protein conjugates that exploits the central hydrophobic cavity of GroEL protein nanobarrel as a binding pocket for chlorophyll. We found water-soluble conjugates of chlorophyll with GroEL could be easily generated via detergent dialysis. The number of chlorophyll units bound to GroEL is tunable by varying the equilibrium concentration of chlorophyll during dialysis. Meanwhile, it is shown that an increase in the entrapped chlorophyll amount leads to an improvement of chlorophyll-GroEL photostability. Using methyl viologen as an electron acceptor, we demonstrate that chlorophyll-GroEL has photoreduction activity, which is also switchable in on/off illumination mode. Finally, it is shown that chlorophyll-GroEL-sensitized solar cells have good photoelectric properties, yielding a high photoelectric conversion efficiency of ∼0.9%. The current strategy may be adopted for integrating other photosensitizing dyes or for other photocatalytic reactions.

Effect of lhcsr gene dosage on oxidative stress and light use efficiency by Chlamydomonas reinhardtii cultures

Unicellular green algae, a promising source for renewable biofuels, produce lipid-rich biomass from light and CO₂. Productivity in photo-bioreactors is affected by inhomogeneous light distribution from high cell pigment causing heat dissipation of light energy absorbed in excess and shading of the deep layers. Contrasting reports have been published on the relation between photoprotective energy dissipation and productivity. Here, we have re-investigated the relation between energy quenching (qE) activity, photodamage and light use efficiency by comparing WT and two Chlamydomonas reinhardtii strains differing for their complement in LHCSR proteins, which catalyse dissipation of excitation energy in excess (qE). Strains were analysed for ROS production, protein composition, rate of photodamage and productivity assessed under wide light and CO₂ conditions. The strain lacking LHCSR1 and knocked down in LHCSR3, thus depleted in qE, produced O₂ at significantly higher rate under high light, accompanied by enhanced singlet oxygen release and PSII photodamage. However, biomass productivity of WT was delayed in respect for mutant strains under intermittent light conditions only, implying that PSII activity was not the limiting factor under excess light. Contrary to previous proposals, domestication of Chlamydomonas for carbon assimilation rate in photo-bioreactors by down-regulation of photoprotective energy dissipation was ineffective in increasing algal biomass productivity.

Membrane Protein Modified Electrodes in Bioelectrocatalysis

Transmembrane proteins involved in metabolic redox reactions and photosynthesis catalyse a plethora of key energy-conversion processes and are thus of great interest for bioelectrocatalysis-based applications. The development of membrane protein modified electrodes has made it possible to efficiently exchange electrons between proteins and electrodes, allowing mechanistic studies and potentially applications in biofuels generation and energy conversion. Here, we summarise the most common electrode modification and their characterisation techniques for membrane proteins involved in biofuels conversion and semi-artificial photosynthesis. We discuss the challenges of applications of membrane protein modified electrodes for bioelectrocatalysis and comment on emerging methods and future directions, including recent advances in membrane protein reconstitution strategies and the development of microbial electrosynthesis and whole-cell semi-artificial photosynthesis.

Comparative analysis of the metabolome and transcriptome between green and albino zones of variegated leaves from Hydrangea macrophylla 'Maculata' infected by hydrangea ringspot virus

In nature, many different factors cause plants to present variegated leaves. The purpose of this study was to reveal the changes in the green and albino leaves of Hydrangea macrophylla 'Maculata'. It was found that in the albino zone, the leaves became thinner, the chloroplast structure disappeared, and a large number of leucoplasts replaced chloroplasts. In addition, the albino zone of the leaves contained almost no chlorophyll and showed no function related to transforming and utilizing light energy, and more intense oxidative stress was observed in the albino zone of the leaves than in the green zone. RNA-seq analysis showed that the chlorophyll synthesis pathway of the albino zone of leaves was blocked. Upregulated expression of the hydrangea ringspot virus (HdRSV) coat protein (CP) gene was detected in albino tissue by RT-qPCR. Finally, combined UPLC-MS/MS and RNA-seq analyses revealed metabolic changes involving multiple pathways in albino leaf tissue, centered on the TCA cycle. We hypothesize that HdRSV may alter energy metabolism in the albino zone of leaves, including increased lipid metabolism, reduced sugar metabolism, and increased synthesis of amino acids and the viral capsid protein from ribosomes.

SsPsaH, a H subunit of the photosystem I reaction center of Suaeda salsa, confers the capacity of osmotic adjustment in tobacco

BACKGROUND

Abiotic stress effects agricultural production, so research on improving stress tolerance of crop is important. Suaeda salsa is a halophyte with high salt and drought tolerance and ability to desalinate saline soil and improve soil quality.

OBJECTIVE

To discover and utilize of salt and drought tolerance-related genes, we further investigated the mechanisms of salt and drought tolerance.

METHODS

Through screening a salt treated Suaeda salsa cDNA library and further cloning a H subunit of the photosystem I reaction center SsPsaH cDNA, and then the protein domain and phylogenetic analyses of PSI genes was conducted with the NCBI Blast, DNAMAN, and MotifScan programs. The S. salsa seedlings were subjected to various stress treatments and analyze expression of SsPsaH under these treatments by real-time RT-PCR. SsPsaH expression construct was introduced into S. pombe cells by electroporation and transformed into N. tabacum plants by the leaf disc transformation method.

RESULTS

A member of the H subunit of the Photosystem I reaction center (defined as SsPsaH) was obtained. The expression of SsPsaH was up-regulated by abscisic acid (ABA), salt, and drought stress treatments. Over-expressing SsPsaH in recombinant yeasts enhanced high salinity tolerance and increased tolerance to sorbitol during seed germination and seedling root development in tobacco, respectively. Some stress-related mark genes such as a LEA family gene of NtLEA, a binding protein of a drought response element of NtDREB, the ascorbate peroxidase gene (NtAPX) were also up-regulated in SsPsaH overexpressing transgenic tobacco lines.

CONCLUSIONS

These results show that SsPsaH may contribute to the salt and osmotic stress response of plants.

On the spectral properties and excitation dynamics of long-wavelength chlorophylls in higher-plant photosystem I

In higher-plant Photosystem I (PSI), the majority of "red" chlorophylls (absorbing at longer wavelengths than the reaction centre P700) are located in the peripheral antenna, but contradicting reports are given about red forms in the core complex. Here we attempt to clarify the spectroscopic characteristics and quantify the red forms in the PSI core complex, which have profound implication on understanding the energy transfer and charge separation dynamics. To this end we compare the steady-state absorption and fluorescence spectra and picosecond time-resolved fluorescence kinetics of isolated PSI core complex and PSI-LHCI supercomplex from Pisum sativum recorded at 77 K. Gaussian decomposition of the absorption spectra revealed a broad band at 705 nm in the core complex with an oscillator strength of three chlorophylls. Additional absorption at 703 nm and 711 nm in PSI-LHCI indicated up to five red chlorophylls in the peripheral antenna. Analysis of fluorescence emission spectra resolved states emitting at 705, 715 and 722 nm in the core and additional states around 705-710 nm and 733 nm in PSI-LHCI. The red states compete with P700 in trapping excitations in the bulk antenna, which occurs on a timescale of ~20 ps. The three red forms in the core have distinct decay kinetics, probably in part determined by the rate of quenching by the oxidized P700. These results affirm that the red chlorophylls in the core complex must not be neglected when interpreting kinetic experimental results of PSI.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (1^2-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (2^2- and 3^2-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 1^2- that is absent in 2^2- and 3^2-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 1^2- compared to 2^2- and 3^2- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a k_obs of 140 s^-1 at an overpotential of only 200 mV. In contrast, 2^2- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 3^2- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.

Structural basis of redox modulation on chloroplast ATP synthase

In higher plants, chloroplast ATP synthase has a unique redox switch on its γ subunit that modulates enzyme activity to limit ATP hydrolysis at night. To understand the molecular details of the redox modulation, we used single-particle cryo-EM to determine the structures of spinach chloroplast ATP synthase in both reduced and oxidized states. The disulfide linkage of the oxidized γ subunit introduces a torsional constraint to stabilize the two β hairpin structures. Once reduced, free cysteines alleviate this constraint, resulting in a concerted motion of the enzyme complex and a smooth transition between rotary states to facilitate the ATP synthesis. We added an uncompetitive inhibitor, tentoxin, in the reduced sample to limit the flexibility of the enzyme and obtained high-resolution details. Our cryo-EM structures provide mechanistic insight into the redox modulation of the energy regulation activity of chloroplast ATP synthase.

Jay-How Yang et al. use single-particle cryo-EM to determine the structures of spinach chloroplast ATP synthase in reduced and oxidized states. They report a torsional constraint in the oxidized γ subunit that is alleviated by free cysteines in the reduced state. Their work provides mechanistic insights into the redox modulation of the ATP synthesis by the chloroplast ATP synthase.