Chlorophyll triplet quenching and photoprotection in the higher plant monomeric antenna protein Lhcb5

Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves

Accumulation of carotenoid (Car) triplet states was investigated by singlet-triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4 μs length at half-height and 525-565 or 410-490 nm spectral band, maximum intensity 2 mol quanta m-2 s-1, flash photon dose up to 10 μmol m-2 or 4-10 PSII excitations. Superimposed upon the non-photochemically unquenched Fmd state, fluorescence was strongly quenched near the flash maximum (minimum yield Fe), but returned to the Fmd level after 30-50 μs. The fraction of PSII containing a 3Car in equilibrium with singlet excitation was calculated as Te = (Fmd-Fe)/Fmd. Light dependence of Te was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9 μs, but it was strongly shortened by the presence of O2. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 Fmd. The results show that in the Fmd state a light-adapted charge-separated PSIIL state is formed (Sipka et al., The Plant Cell 33:1286-1302, 2021) in which Pheo-P680+ radical pair formation is hindered, and excitation is terminated in the antenna by 3Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSIIL state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without 3Car formation.

Co-cultures from Plants and Cyanobacteria: A New Way for Production Systems in Agriculture and Bioprocess Engineering

Due to the global increase in the world population, it is not possible to ensure a sufficient food supply without additional nitrogen input into the soil. About 30-50% of agricultural yields are due to the use of chemical fertilizers in modern times. However, overfertilization threatens biodiversity, such as nitrogen-loving, fast-growing species overgrow others. The production of artificial fertilizers produces nitrogen oxides, which act as greenhouse gases. In addition, overfertilization of fields also releases ammonia, which damages surface waters through acidification and eutrophication. Diazotrophic cyanobacteria, which usually form a natural, stable biofilm, can fix nitrogen from the atmosphere and release it into the environment. Thus, they could provide an alternative to artificial fertilizers. In addition to this, biofilms stabilize soils and thus protect against soil erosion and desiccation. This chapter deals with the potential of cyanobacteria as the use of natural fertilizer is described. Possible partners such as plants and callus cells and the advantages of artificial co-cultivation will be discussed later. In addition, different cultivation systems for studying artificial co-cultures will be presented. Finally, the potential of artificial co-cultures in the agar industry will be discussed.

Direct observation of triplet energy transfer between chlorophylls and carotenoids in the core antenna of photosystem I from Thermosynechococcus elongatus

Quenching of chlorophyll triplet states by carotenoids is an essential photoprotective process, which prevents formation of reactive singlet oxygen in photosynthetic light-harvesting complexes. The process is usually very efficient in oxygenic organisms under physiological conditions, thus preventing any observable accumulation of chlorophyll triplets. However, it subsequently prevents also the determination of the triplet transfer rate. Here we report results of nanosecond transient absorption spectroscopy on photosystem I core complexes, where a major part of chlorophyll a triplet states (~60 %) accumulates on a nanosecond time scale at ambient temperature. As a consequence, the triplet energy transfer could be resolved and the transfer time was determined to be about 24 ns. A smaller fraction of chlorophyll a triplet states (~40 %) is quenched with a faster rate, which could not be determined. Our analysis indicates that these chlorophylls are in direct contact with carotenoids. The overall chlorophyll triplet yield in the core antenna was estimated to be ~0.3 %, which is a value two orders of magnitude smaller than in most other photosynthetic light-harvesting complexes. This explains why slower quenching of chlorophyll triplet states is sufficient for photoprotection of photosystem I. Nevertheless, the core antenna of photosystem I represents one of only few photosynthetic complexes of oxygenic organisms in which the quenching rate of the majority of chlorophyll triplets can be directly monitored under physiological temperature.

Nature-Inspired Thylakoid-Based Photosynthetic Nanoarchitectures for Biomedical Applications

"Drawing inspiration from nature" offers a wealth of creative possibilities for designing cutting-edge materials with improved properties and performance. Nature-inspired thylakoid-based nanoarchitectures, seamlessly integrate the inherent structures and functions of natural components with the diverse and controllable characteristics of nanotechnology. These innovative biomaterials have garnered significant attention for their potential in various biomedical applications. Thylakoids possess fundamental traits such as light harvesting, oxygen evolution, and photosynthesis. Through the integration of artificially fabricated nanostructures with distinct physical and chemical properties, novel photosynthetic nanoarchitectures can be catalytically generated, offering versatile functionalities for diverse biomedical applications. In this article, an overview of the properties and extraction methods of thylakoids are provided. Additionally, the recent advancements in the design, preparation, functions, and biomedical applications of a range of thylakoid-based photosynthetic nanoarchitectures are reviewed. Finally, the foreseeable challenges and future prospects in this field is discussed.

The Adaptive Role of Carotenoids and Anthocyanins in Solanum lycopersicum Pigment Mutants under High Irradiance

The effects of high-intensity light on the pigment content, photosynthetic rate, and fluorescence parameters of photosystem II in high-pigment tomato mutants (hp 3005) and low-pigment mutants (lp 3617) were investigated. This study also evaluated the dry weight percentage of low molecular weight antioxidant capacity, expression patterns of some photoreceptor-regulated genes, and structural aspects of leaf mesophyll cells. The 3005 mutant displayed increased levels of photosynthetic pigments and anthocyanins, whereas the 3617 mutant demonstrated a heightened content of ultraviolet-absorbing pigments. The photosynthetic rate, photosystem II activity, antioxidant capacity, and carotenoid content were most pronounced in the high-pigment mutant after 72 h exposure to intense light. This mutant also exhibited an increase in leaf thickness and water content when exposed to high-intensity light, suggesting superior physiological adaptability and reduced photoinhibition. Our findings indicate that the enhanced adaptability of the high-pigment mutant might be attributed to increased flavonoid and carotenoid contents, leading to augmented expression of key genes associated with pigment synthesis and light regulation.

Role of secondary metabolites in distressed microalgae

Proficient photosynthetic microalgae/cyanobacteria produce a remarkable amount of various biomolecules. Secondary metabolites (SM) represent high value products for global biotrend application. Production improvement can be achieved by nutritional, environmental, and physiological stress as a first line tools for their stimulation. In recent decade, an increasing interest in algal stress biology and omics techniques have deepened knowledge in this area. However, deep understanding and connection of specific stress elucidator are missing. Hence, the present review summarizes recent evidence with an emphasis on the carotenoids, phenolic, and less-discussed compounds (glycerol, proline, mycosporins-like amino acids). Even when they are synthesized at very low concentrations, it highlights the need to expand knowledge in this area using genome-editing tools and omics approaches.

Enhanced function of non-photoinhibited photosystem II complexes upon PSII photoinhibition

Light induced photosystem (PS)II photoinhibition inactivates and irreversibly damages the reaction center protein(s) but the light harvesting complexes continue the collection of light energy. Here we addressed the consequences of such a situation on thylakoid light harvesting and electron transfer reactions. For this purpose, Arabidopsis thaliana leaves were subjected to investigation of the function and regulation of the photosynthetic machinery after a distinct portion of PSII centers had experienced photoinhibition in the presence and absence of Lincomycin (Lin), a commonly used agent to block the repair of damaged PSII centers. In the absence of Lin, photoinhibition increased the relative excitation of PSII and decreased NPQ, together enhancing the electron transfer from still functional PSII centers to PSI. In contrast, in the presence of Lin, PSII photoinhibition increased the relative excitation of PSI and led to strong oxidation of the electron transfer chain. We hypothesize that plants are able to minimize the detrimental effects of high-light illumination on PSII by modulating the energy and electron transfer, but lose such a capability if the repair cycle is arrested. It is further hypothesized that dynamic regulation of the LHCII system has a pivotal role in the control of excitation energy transfer upon PSII damage and repair cycle to maintain the photosynthesis safe and efficient.

Image-Guided Precision Treatments

Chemotherapy, radiotherapy, and surgery are traditional cancer treatments, which usually produce unpredictable side effects and potential risks to normal healthy organs/tissues. Thus, safe and reliable treatment strategies are urgently required for maximized therapeutic efficiency to lesions and minimized risks to healthy regions. To this end, molecular imaging is responsible to undertake a specific targeting therapy. Besides that, the image guidance as a precision visualized approach for real-time in situ evaluations as well as an intraoperational navigation approach has earned attractive attention in the past decade. Along with the rapid development of multifunctional micro-/nanobiomaterials, versatile cutting-edge and advanced therapy strategies (e.g., thermal therapy, dynamic therapy, gas therapy, etc.) have been achieved and greatly contributed to the image-guided precision treatments in every aspect. Therefore, this chapter aims to discuss about both traditional and advanced cancer treatments and especially to elucidate the important roles that visualized medicine has been playing in the image-guided precision treatments.

Modulated excited state geometry and Electron-Phonon coupling of lutein by temperature and solvent polarizability

Resonance Raman spectroscopy is one of the spectroscopic methods often chosen for studying linear polyene molecules because the Raman intensities of their υ₁ (C = C) and υ₂ (C-C) stretching vibrations are sensitive to electron-phonon coupling and the π-electron energy gap. Here, the resonance Raman and absorption spectra of lutein were studied as a function of solvent polarizabilities and of temperature in the CS₂ solvent. For lutein in CS₂, as the temperature decreased and CS solidified, the Raman scattering cross-section (RSCS) and the electron-phonon coupling constant had opposite dependence trends on temperature. The wavenumber of the lutein 0-0 electronic transition showed a marked shift to lower wavenumbers when the polarizability of the solvents decreased, and the Huang-Rhys (HR) factors and electron-phonon coupling also decreased. This work helps explore the influence of the external environment (e.g., temperature and solvent) on the excited state geometry of linear polyene molecules.

Ultrahigh-Performance Liquid Chromatography-Mass Spectrometry Analysis of Carotenoid-Derived Hormones and Apocarotenoids in Plants

Carotenoid oxidative cleavage products, apocarotenoids (APOs), are a class of important plant secondary metabolites, which include phytohormones abscisic acid (ABA) and strigolactones (SLs), and growth regulators and signaling molecules such as β-cyclocitral, zaxinone, anchorene, β-apo-11-carotenoids, and retinal. Qualitative and quantitative analysis of these bioactive compounds is crucial for understanding their metabolism and may also enable discovering further regulatory APOs. The state-of-the-art mass spectrometry (MS) technology has advanced the detection of plant APOs; however, it is still challenging to perform an accurate analysis of the low-level phytohormones ABA and SL and the structurally diverse APOs from complex plant matrices. Here, we describe ultrahigh-performance liquid chromatography-MS (UHPLC-MS) methods to determine carotenoid-derived hormones and APOs from plants by integrating ultrasound-assisted extraction and solid-phase extraction. These assays enable an accurate quantification of carotenoid-derived hormones and APOs from plant tissues by using an UHPLC hybrid quadrupole-Orbitrap mass spectrometer. © 2022 Wiley Periodicals LLC. Basic Protocol 1: UHPLC-MS analysis of APOs from rice roots Support Protocol: Preparation of dried plant root powder Basic Protocol 2: UHPLC-MS analysis of SLs from rice roots Basic Protocol 3: UHPLC-MS analysis of ABA from rice roots.

Lutein and Zeaxanthin in the Lipid Bilayer-Similarities and Differences Revealed by Computational Studies

Lutein and zeaxanthin are two similar carotenoids of the xanthophyll subgroup. Carotenoids are synthesized almost entirely by plants but are also present in significant amounts in animals. They are essential components of the lipid matrix of biomembranes, and one of their functions is to protect cells from light radiation, free radicals and oxidative stress. Carotenoids, depending on their chemical structure, can locate at various positions and in different orientations in the bilayer. Xanthophylls (XAN) are polar and in the bilayer are positionally restricted. In the case of lutein and zeaxanthin, whose both ionone rings are hydroxy-substituted and as such are anchored in the lipid bilayer interfaces, the position is generally transmembrane. However, both experimental and computer modelling studies indicate that lutein can also locate horizontally below the bilayer interface. This location has never been observed for zeaxanthin. To find a molecular-level explanation for the difference in the orientations of the XAN molecules in the bilayer, a number of phosphatidylcholine-XAN bilayers were constructed and molecular dynamics (MD) simulated for 1.1 µs each. The all-trans XAN molecules were initially placed either parallel or perpendicular to the bilayer surface. With the exception of one lutein, the horizontally placed molecules adopted the transmembrane orientation within 100-600 ns. On the basis of detailed analyses of the XAN orientations and the numbers and lifetimes of their interactions in the bilayer, a plausible explanation is offered as to why a lutein molecule may remain in the horizontal orientation while zeaxanthin does not. Contrary to common believe, lutein horizontal orientation is not related to the ε-ring rotation around the C6'-C7' bond.

A theoretical framework of the hybrid mechanism of photosystem II photodamage

Photodamage of photosystem II is a significant physiological process that is prevalent in the fields of photobiology, photosynthesis research and plant/algal stress. Since its discovery, numerous efforts have been devoted to determine the causes and mechanisms of action of photosystem II photodamage. There are two contrasting hypotheses to explain photodamage: (1) the excitation pressure induced by light absorption by the photosynthetic pigments and (2) direct photodamage of the Mn cluster located at the water-splitting site, which is independent of excitation pressure. While these two hypotheses seemed mutually exclusive, during the last decade, several independent works have proposed an alternative approach indicating that both hypotheses are valid. This was termed the dual hypothesis of photosystem II photodamage, and it postulates that both excess excitation and direct Mn photodamage operate at the same time, independently or in a synergic manner, depending on the type of sample, temperature, light spectrum, or other environmental stressors. In this mini-review, a brief summary of the contrasting hypotheses is presented, followed by recapitulation of key discoveries in the field of photosystem II photodamage of the last decade, and a synthesis of how these works support a full hybrid framework (operation of several mechanisms and their permutations) to explain PSII photodamage. All these are in recognition of Prof. Wah Soon Chow (the Australian National University), one of the key proposers of the dual hypothesis.

Photodegradation of cyanotoxins in surface waters

Cyanotoxin-producing harmful algal blooms (HABs) are a global occurrence and pose ecotoxicological threats to humans and animals alike. The presence of cyanotoxins can seriously harm or kill nearby wildlife and restrict a body of water's use as a drinking water supply and recreational site, making it imperative to fully understand their fate and transport in natural waters. Photodegradation contributes to the overall degradation of cyanotoxins in environmental systems, especially for those present in the photic zone of surface waters. This makes photochemical transformation mechanisms important factors to account for when assessing the persistence of cyanotoxins in environmental systems. This paper reviews current knowledge on the photodegradation rates and pathways of cyanotoxins that can occur over the course of HABs. Sensitized, or indirect, photolysis contributes to the degradation of all cyanotoxins addressed in this paper (anatoxins, cylindrospermopsins, domoic acids, microcystins, and nodularins), with hydroxyl radicals (•OH), excited triplet states formed from the absorption of light by dissolved organic matter (³DOM*), and photosynthetic pigment sensitized pathways being of primary interest. Direct photolysis pathways play a less significant role, but are still relevant for most of the cyanotoxins discussed in this paper.

Thylakoid Membranes with Unique Photosystems Used to Simultaneously Produce Self-Supplying Oxygen and Singlet Oxygen for Hypoxic Tumor Therapy

Photodynamic therapy (PDT) efficacy has been dramatically limited by the insufficient oxygen (O₂ ) level in hypoxic tumors. Although various PDT nanosystems have been designed to deliver or produce O₂ in support of reactive oxygen species (ROS) formation, the feature of asynchronous O₂ generation and ROS formation still results in the low PDT efficacy. Herein, thylakoid membranes (TM) of chloroplasts is decorated on upconversion nanoparticles (UCNPs) to form UCTM NPs, aiming at realizing spatiotemporally synchronous O₂ self-supply and ROS production. Upon 980 nm laser irradiation, UC NPs can emit the red light to activate both photosystem-I and photosystem-II of TM, the Z-scheme electronic structure of which facilitates water to produce O₂ and further to singlet oxygen (¹ O₂ ). UCTM NPs showed excellent biocompatibility, and can effectively remove the hypoxic tumor of mice upon 980 nm laser irradiation. This study develops a new PDT strategy for hypoxic tumor therapy based on photosynthesis.

Photoprotective Role of Neoxanthin in Plants and Algae

Light is a paramount parameter driving photosynthesis. However, excessive irradiance leads to the formation of reactive oxygen species that cause cell damage and hamper the growth of photosynthetic organisms. Xanthophylls are key pigments involved in the photoprotective response of plants and algae to excessive light. Of particular relevance is the operation of xanthophyll cycles (XC) leading to the formation of de-epoxidized molecules with energy dissipating capacities. Neoxanthin, found in plants and algae in two different isomeric forms, is involved in the light stress response at different levels. This xanthophyll is not directly involved in XCs and the molecular mechanisms behind its photoprotective activity are yet to be fully resolved. This review comprehensively addresses the photoprotective role of 9′-cis-neoxanthin, the most abundant neoxanthin isomer, and one of the major xanthophyll components in plants’ photosystems. The light-dependent accumulation of all-trans-neoxanthin in photosynthetic cells was identified exclusively in algae of the order Bryopsidales (Chlorophyta), that lack a functional XC. A putative photoprotective model involving all-trans-neoxanthin is discussed.

Effects of high light and temperature on Microcystis aeruginosa cell growth and β-cyclocitral emission

Cyanobacteria always massively grow and even occur blooms in summer, with releasing amount of β-cyclocitral. To uncover the effects of summer high irradiance and temperature on cyanobacterial growth and β-cyclocitral emission, the cell growth, reactive oxygen species (ROS) levels, photosynthetic pigment content, chlorophyll fluorescence and β-cyclocitral emission were investigated in Microcystis aeruginosa under high light and temperature. Compared to the control under 50 μmol m^-2·s^-1, the cell growth was promoted under 100 μmol m^-2·s^-1, but inhibited under 500 and 1000 μmol m^-2·s^-1. The inhibition was also detected under high temperature at 30 and 35 °C in contrast to the control at 25 °C. Under high light and high temperature, M. aeruginosa increased ROS levels and reduced photosynthetic pigment content and photosystem II (PSII) efficiency, which resulted in the inhibition on cell growth. With increasing the light intensity and temperature, ¹O₂ levels gradually increased, while β-carotene content gradually decreased by quenching ¹O₂, with increasing β-cyclocitral emission. In summer, high irradiance and temperature not benefited the growth of cyanobacteria, but the emission of β-cyclocitral derived from β-carotene quenching ¹O₂ may offset the disadvantages by poisoning other algae.

A Method for Extraction and LC-MS-Based Identification of Carotenoid-Derived Dialdehydes in Plants

We developed a chemical derivatization based ultra-high performance liquid chromatography-hybrid quadrupole-Orbitrap mass spectrometer (UHPLC-Q-Orbitrap MS) analytical method to identify low-abundant and instable carotenoid-derived dialdehydes (DIALs, diapocarotenoids) from plants. Application of this method enhances the MS response signal of DIALs, enabling the detection of diapocarotenoids, which is crucial for understanding the function of these compounds and for elucidating the carotenoid oxidative metabolic pathway in plants.

Successes & challenges in the atomistic modeling of light-harvesting and its photoregulation

Light-harvesting is a crucial step of photosynthesis. Its mechanisms and related energetics have been revealed by a combination of experimental investigations and theoretical modeling. The success of theoretical modeling is largely due to the application of atomistic descriptions combining quantum chemistry, classical models and molecular dynamics techniques. Besides the important achievements obtained so far, a complete and quantitative understanding of how the many different light-harvesting complexes exploit their structural specificity is still missing. Moreover, many questions remain unanswered regarding the mechanisms through which light-harvesting is regulated in response to variable light conditions. Here we show that, in both fields, a major role will be played once more by atomistic descriptions, possibly generalized to tackle the numerous time and space scales on which the regulation takes place: going from the ultrafast electronic excitation of the multichromophoric aggregate, through the subsequent conformational changes in the embedding protein, up to the interaction between proteins.

In vitro and in vivo investigation of chlorophyll binding sites involved in non-photochemical quenching in Chlamydomonas reinhardtii

Non-photochemical quenching (NPQ) of the light energy absorbed is one of the main photoprotective mechanisms evolved by oxygenic photosynthetic organisms to avoid photodamage, at a cost of reduced photosynthetic efficiency. Tuning of NPQ has been reported as a promising biotechnological strategy to increase productivity in both higher plants and unicellular microalgae. Engineering of NPQ induction requires the comprehension of its molecular mechanism(s), strongly debated in the last three decades with several different models proposed. In this work, the molecular details of NPQ induction was investigated at intramolecular level by in vitro and in vitro site-specific mutagenesis on chlorophyll binding sites of the Light-Harvesting Complex Stress-Related 3 (LHCSR3) protein, the pigment binding complexes identified as the quencher during NPQ induction in the model organism for green algae Chlamydomonas reinhardtii. The results obtained demonstrate a correlation between the quenching activity of LHCSR3 variants in vitro and the NPQ phenotypes observed in vivo. In particular, multiple quenching sites in LHCSR3 cooperatively dissipating the excitation energy were revealed with a peculiar role of Chl 613, a chromophore located a close distance to carotenoid binding site L1.

Phyllosilicates as protective habitats of filamentous cyanobacteria Leptolyngbya against ultraviolet radiation

Phototrophic cyanobacteria are limited in growth locations by their need for visible light and must also cope with intermittent ultraviolet radiation (UVR), especially in extreme environments such as deserts and on early Earth. One survival method for cyanobacteria is growing endolithically within minerals such as micas, gypsum, and quartz minerals. However, the capability of different mica minerals to protect cyanobacteria from UVR, while at the same time allowing transmission of photosynthetically active radiation (PAR), has only been minimally examined. In this study, we performed laboratory incubation experiments to demonstrate that a model filamentous cyanobacterium, Leptolyngbya sp., can colonize micas, such as muscovite, phlogopite, and biotite. After inoculation experiments confirmed that these cyanobacteria grew between the sheets of mica, Leptolyngbya sp. colonies were exposed to UVB and UVC for up to 24 hrs, and the level of survival was determined using chlorophyll a and carotenoid assays. Of the three micas investigated, muscovite, being an Fe-poor and Al-rich mica, provided the least attenuation of UVR, however it transmitted the most visible light. Fe-rich biotite provided the best UVR shielding. Phlogopite, apparently because of its intermediate amount of Fe, showed the greatest ability to shield UVR while still transmitting an adequate amount of visible light, making it the ideal habitat for the cyanobacterium. Upon exposure to UVR, significant shifts in several important fatty acids of the cyanobacterium were detected such as linolenic acid and oleic acid, 18:3ω3 and 18:1ω9c, respectively. These cellular changes are interpreted to be a consequence of UVR and other accessory stress (such as O3).

Asymmetric Spontaneous Intercalation of Lutein into a Phospholipid Bilayer, a Computational Study

Lutein, a hydroxylated carotenoid, is a pigment synthesised by plants and bacteria. Animals are unable to synthesise lutein, nevertheless, it is present in animal tissues, where its only source is dietary intake. Both in plants and animals, carotenoids are associated mainly with membranes where they carry out important physiological functions. Their trafficking to and insertion into membranes are not well recognised due to experimental difficulties. In this paper, a computational approach is used to elucidate details of the dynamics and energetics of lutein intercalation from the water to the phospholipid bilayer phase. The dynamics is studied using molecular dynamics simulation, and the energetics using umbrella sampling. Lutein spontaneous insertion into the bilayer and translocation across it proceed via formation of hydrogen bonds between its hydroxyl groups and water and/or phospholipid oxygen atoms as well as desolvation of its polyene chain. As lutein molecule is asymmetric, its bilayer intercalation is also asymmetric. The course of events and timescale of the intercalation are different from those of helical peptides. The time of full lutein intercalation ranges from 20 to 100 ns and its final orientation is predominately vertical. Nevertheless, some lutein molecules are in the final horizontal position and some aggregate in the water phase and remain there for the whole simulation time. The highest energy barrier for the intercalation process is ~2.2 kcal/mol and the energy gain is ~18 kcal/mol. The results obtained for lutein can be applied to other xanthophylls and molecules of a similar structure.

•

Lutein as an amphiphilic molecule readily intercalates into a lipid bilayer.

•

MD simulations with dense sampling revealed details of the intercalation process.

•

Time of full lutein membrane intercalation ranges from 20 to 100 ns.

•

Lutein intercalation from the ε ring end is less probable than from the β ring end.

•

Horizontal position of lutein in the bilayer is less probable than vertical.

Rapid regulation of excitation energy in two pennate diatoms from contrasting light climates

Non-photochemical quenching (NPQ) is a fast acting photoprotective response to high light stress triggered by over excitation of photosystem II. The mechanism for NPQ in the globally important diatom algae has been principally attributed to a xanthophyll cycle, analogous to the well-described qE quenching of higher plants. This study compared the short-term NPQ responses in two pennate, benthic diatom species cultured under identical conditions but which originate from unique light climates. Variable chlorophyll fluorescence was used to monitor photochemical and non-photochemical excitation energy dissipation during high light transitions; whereas whole cell steady state 77 K absorption and emission were used to measure high light elicited changes in the excited state landscapes of the thylakoid. The marine shoreline species Nitzschia curvilineata was found to have an antenna system capable of entering a deeply quenched, yet reversible state in response to high light, with NPQ being highly sensitive to dithiothreitol (a known inhibitor of the xanthophyll cycle). Conversely, the salt flat species Navicula sp. 110-1 exhibited a less robust NPQ that remained largely locked-in after the light stress was removed; however, a lower amplitude, but now highly reversible NPQ persisted in cells treated with dithiothreitol. Furthermore, dithiothreitol inhibition of NPQ had no functional effect on the ability of Navicula cells to balance PSII excitation/de-excitation. These different approaches for non-photochemical excitation energy dissipation are discussed in the context of native light climate.

Coherent phenomena in photosynthetic light harvesting: part one-theory and spectroscopy

The role of non-trivial quantum mechanical effects in biology has been the subject of intense scrutiny over the past decade. Much of the focus on potential "quantum biology" has been on energy transfer processes in photosynthetic light harvesting systems. Ultrafast laser spectroscopy of several light harvesting proteins has uncovered coherent oscillations dubbed "quantum beats" that persist for hundreds of femtoseconds and are putative signatures for quantum transport phenomena. This review describes the language and basic quantum mechanical phenomena that underpin quantum transport in open systems such as light harvesting and photosynthetic proteins, including the photosystem reaction centre. Coherent effects are discussed in detail, separating various meanings of the term, from delocalized excitations, or excitons, to entangled states and coherent transport. In particular, we focus on the time, energy and length scales of energy transport processes, as these are critical in understanding whether or not coherent processes are important. The role played by the protein in maintaining chromophore systems is analysed. Finally, the spectroscopic techniques that are used to probe energy transfer dynamics and that have uncovered the quantum beats are described with reference to coherent phenomena in light harvesting.

Singlet oxygen imaging using fluorescent probe Singlet Oxygen Sensor Green in photosynthetic organisms

Formation of singlet oxygen (¹O₂) was reported to accompany light stress in plants, contributing to cell signaling or oxidative damage. So far, Singlet Oxygen Sensor Green (SOSG) has been the only commercialized fluorescent probe for ¹O₂ imaging though it suffers from several limitations (unequal penetration and photosensitization) that need to be carefully considered to avoid misinterpretation of the analysed data. Herein, we present results of a comprehensive study focused on the appropriateness of SOSG for ¹O₂ imaging in three model photosynthetic organisms, unicellular cyanobacteria Synechocystis sp. PCC 6803, unicellular green alga Chlamydomonas reinhardtii and higher plant Arabidopsis thaliana. Penetration of SOSG differs in both unicellular organisms; while it is rather convenient for Chlamydomonas it is restricted by the presence of mucoid sheath of Synechocystis, which penetrability might be improved by mild heating. In Arabidopsis, SOSG penetration is limited due to tissue complexity which can be increased by pressure infiltration using a shut syringe. Photosensitization of SOSG and SOSG endoperoxide formed by its interaction with ¹O₂ might be prevented by illumination of samples by a red light. When measured under controlled conditions given above, SOSG might serve as specific probe for detection of intracellular ¹O₂ formation in photosynthetic organisms.

Effects of Foliar Redox Status on Leaf Vascular Organization Suggest Avenues for Cooptimization of Photosynthesis and Heat Tolerance

The interaction of heat stress with internal signaling networks was investigated through Arabidopsisthaliana mutants that were deficient in either tocopherols (vte1 mutant) or non-photochemical fluorescence quenching (NPQ; npq1, npq4, and npq1 npq4 mutants). Leaves of both vte1 and npq1 npq4 mutants that developed at a high temperature exhibited a significantly different leaf vascular organization compared to wild-type Col-0. Both mutants had significantly smaller water conduits (tracheary elements) of the xylem, but the total apparent foliar water-transport capacity and intrinsic photosynthetic capacity were similarly high in mutants and wild-type Col-0. This was accomplished through a combination of more numerous (albeit narrower) water conduits per vein, and a significantly greater vein density in both mutants relative to wild-type Col-0. The similarity of the phenotypes of tocopherol-deficient and NPQ-deficient mutants suggests that leaf vasculature organization is modulated by the foliar redox state. These results are evaluated in the context of interactions between redox-signaling pathways and other key regulators of plant acclimation to growth temperature, such as the C-repeat binding factor (CBF) transcription factors, several of which were upregulated in the antioxidant-deficient mutants. Possibilities for the future manipulation of the interaction between CBF and redox-signaling networks for the purpose of cooptimizing plant productivity and plant tolerance to extreme temperatures are discussed.

Biomimetic nanothylakoids for efficient imaging-guided photodynamic therapy for cancer

Biomimetic nanothylakoids are constructed for the first time and show high efficiency in tumor microenvironment modulation and imaging-guided photodynamic antitumor therapy.

Exciton states and optical properties of the CP26 photosynthetic protein

The photosynthetic complex CP26, one of the minor antennae of the photosystem II, plays an important role in regulation of the excitation energy transfer in the PSII. Due to instability during isolation and purification, it remained poorly studied from the viewpoint of theoretical chemistry because of the absence of X-ray crystallography data. In this work, using the recently determined three-dimensional structure of the complex we apply the quantum chemical approach to study the properties of exciton states in it. Spectral properties, structure of exciton states and roles of the pigments in the complex and photosystem II are discussed.

Functional analysis of photosynthetic pigment binding complexes in the green alga Haematococcus pluvialis reveals distribution of astaxanthin in Photosystems

Astaxanthin is a ketocarotenoid produced by photosynthetic microalgae. It is a pigment of high industrial interest in acquaculture, cosmetics, and nutraceutics due to its strong antioxidant power. Haematococcus pluvialis, a fresh-water microalga, accumulates high levels of astaxanthin upon oxidative stress, reaching values up to 5% per dry weight. H. pluvialis accumulates astaxanthin in oil droplets in the cytoplasm, while the chloroplast volume is reduced. In this work, we investigate the biochemical and spectroscopic properties of the H. pluvialis pigment binding complexes responsible for light harvesting and energy conversion. Our findings demonstrate that the main features of chlorophyll and carotenoid binding complexes previously reported for higher plants or Chlamydomonas reinhardtii are preserved under control conditions. Transition to astaxanthin rich cysts however leads to destabilization of the Photosystems. Surprisingly, astaxanthin was found to be bound to both Photosystem I and II, partially substituting β-carotene, and thus demonstrating possible astaxanthin biosynthesis in the plastids or transport from the cytoplasm to the chloroplast. Astaxanthin binding to Photosystems does not however improve their photoprotection, but rather reduces the efficiency of excitation energy transfer to the reaction centers. We thus propose that astaxanthin binding partially destabilizes Photosystem I and II.

Fine control of chlorophyll-carotenoid interactions defines the functionality of light-harvesting proteins in plants

Photosynthetic antenna proteins can be thought of as "programmed solvents", which bind pigments at specific mutual orientations, thus tuning the overall energetic landscape and ensuring highly efficient light-harvesting. While positioning of chlorophyll cofactors is well understood and rationalized by the principle of an "energy funnel", the carotenoids still pose many open questions. Particularly, their short excited state lifetime (<25 ps) renders them potential energy sinks able to compete with the reaction centers and drastically undermine light-harvesting efficiency. Exploration of the orientational phase-space revealed that the placement of central carotenoids minimizes their interaction with the nearest chlorophylls in the plant antenna complexes LHCII, CP26, CP29 and LHCI. At the same time we show that this interaction is highly sensitive to structural perturbations, which has a profound effect on the overall lifetime of the complex. This links the protein dynamics to the light-harvesting regulation in plants by the carotenoids.

Concentration Effect on Quenching of Chlorophyll a Fluorescence by All-Trans-β-Carotene in Photosynthesis

Absorption, fluorescence spectra of chlorophyll a (Chl-a) and all-trans-β-carotene (β-Car) mixing solution are investigated in different polarity and polarizability solvents. The carotenoids regulate the energy flow in photosynthesis by interaction with chlorophyll, leading to an observable reduction of Chl-a fluorescence. The fluorescence red shifts with the increasing solvent polarizability. The energy transfer in the Chl-a and β-Car system is proposed. The electron transfer should be dominant in quenching Chl-a fluorescence rather than the energy transfer in this system. Polar solvent with large polarizability shows high quenching efficiency. When dissolved in carbon tetrachloride, Chl-a presents red shift of absorption and blue shift of fluorescence spectra with increasing β-Car concentration, which implies a Chl-a conformational change.

Drought stress memory in the photosynthetic mechanisms of an invasive CAM species, Aptenia cordifolia

Plants are known for their high capacity to acclimatise to fluctuating environmental conditions. A wide range of environmental conditions can lead to suboptimal physiological efficiency. However, recent studies have shown that plants can withstand repeated periods of stress. To find out how they do it, we studied photosynthetic adjustments to repeated water stress in Aptenia cordifolia: a facultative, invasive CAM species. Plants were subjected to three cycles of water deficit, and photosynthetic parameters and chloroplast antioxidants were quantified to gain an understanding of the mechanisms by which they cope with repeated stress periods. Significant modification of the photosystems' antenna and reaction centres was observed in plants subjected to previous water stress cycles, and this led to higher PSII efficiency than in plants challenged with drought for the first time. These findings underline the biological significance of stress memory and show how plants can adjust their photosynthetic apparatus to fluctuating environmental conditions and thus optimise photosynthesis and photoprotection under drought conditions.

The function of LHCBM4/6/8 antenna proteins in Chlamydomonas reinhardtii

In eukaryotic autotrophs, photosystems are composed of a core moiety, hosting charge separation and electron transport reactions, and an antenna system, enhancing light harvesting and photoprotection. In Chlamydomonas reinhardtii, the major antenna of PSII is a heterogeneous trimeric complex made up of LHCBM1-LHCBM9 subunits. Despite high similarity, specific functions have been reported for several members including LHCBM1, 2, 7, and 9. In this work, we analyzed the function of LHCBM4 and LHCBM6 gene products in vitro by synthesizing recombinant apoproteins from individual sequences and refolding them with pigments. Additionally, we characterized knock-down strains in vivo for LHCBM4/6/8 genes. We show that LHCBM4/6/8 subunits could be found as a component of PSII supercomplexes with different sizes, although the largest pool was free in the membranes and poorly connected to PSII. Impaired accumulation of LHCBM4/6/8 caused a decreased LHCII content per PSII and a reduction in the amplitude of state 1-state 2 transitions. In addition, the reduction of LHCBM4/6/8 subunits caused a significant reduction of the Non-photochemical quenching activity and in the level of photoprotection.

Floating and submerged leaves of Potamogeton nodosus exhibit distinct variation in the antioxidant system as an ecophysiological adaptive strategy

During evaluations of the ecophysiological adaptations of floating and submerged leaves of Potamogeton nodosus Poir, investigations were carried to assess their antioxidant status. Floating leaves possessed a significantly higher level of C skeletons per unit of area compared with submerged leaves as they possessed greater PSI and PSII activity (hence had superior potential to harness absorbed light energy and generate assimilatory power) and carboxylase activity of Rubisco (hence superior potential to fix CO2) compared with the latter. Interestingly, submerged leaves possessed ~2 times higher H2O2 levels compared with floating leaves. In contrast, the activity of all antioxidant enzymes tested (catalase, guaiacol peroxidase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) were significantly higher in floating leaves than in submerged leaves. Amazingly, catalase activity (a H2O2 detoxifying enzyme) was over fourfold higher in floating leaves than in submerged leaves. Among the nonenzymatic antioxidants, although levels of phenolics, ascorbate and thiols did not vary significantly between floating and submerged leaves, the level of total carotenoids was significantly higher in the former than the latter. In summary, floating leaves possess superior and efficient photosynthetic machinery for light and dark reactions, and also possess strong and superior enzymatic antioxidant machinery for scavenging reactive oxygen species and maintenance of the NAD(P)H to NAD(P)+ ratio compared with submerged leaves. Accordingly, floating leaves possessed superior potential to withstand photodamage compared with submerged leaves. We believe that excess H2O2 provides an ideal defence tool for submerged leaves to counter predators, pests and pathogens.

Identification of pH-sensing Sites in the Light Harvesting Complex Stress-related 3 Protein Essential for Triggering Non-photochemical Quenching in Chlamydomonas reinhardtii

Light harvesting complex stress-related 3 (LHCSR3) is the protein essential for photoprotective excess energy dissipation (non-photochemical quenching, NPQ) in the model green alga Chlamydomonas reinhardtii. Activation of NPQ requires low pH in the thylakoid lumen, which is induced in excess light conditions and sensed by lumen-exposed acidic residues. In this work we have used site-specific mutagenesis in vivo and in vitro for identification of the residues in LHCSR3 that are responsible for sensing lumen pH. Lumen-exposed protonatable residues, aspartate and glutamate, were mutated to asparagine and glutamine, respectively. By expression in a mutant lacking all LHCSR isoforms, residues Asp¹¹⁷, Glu²²¹, and Glu²²⁴ were shown to be essential for LHCSR3-dependent NPQ induction in C. reinhardtii. Analysis of recombinant proteins carrying the same mutations refolded in vitro with pigments showed that the capacity of responding to low pH by decreasing the fluorescence lifetime, present in the wild-type protein, was lost. Consistent with a role in pH sensing, the mutations led to a substantial reduction in binding the NPQ inhibitor dicyclohexylcarbodiimide.

Photoprotection and triplet energy transfer in higher plants: the role of electronic and nuclear fluctuations

Photosynthetic organisms employ several photoprotection strategies to avoid damage due to the excess energy in high light conditions.

Production of Reactive Oxygen Species by Photosystem II as a Response to Light and Temperature Stress

The effect of various abiotic stresses on photosynthetic apparatus is inevitably associated with formation of harmful reactive oxygen species (ROS). In this review, recent progress on ROS production by photosystem II (PSII) as a response to high light and high temperature is overviewed. Under high light, ROS production is unavoidably associated with energy transfer and electron transport in PSII. Singlet oxygen is produced by the energy transfer form triplet chlorophyll to molecular oxygen formed by the intersystem crossing from singlet chlorophyll in the PSII antennae complex or the recombination of the charge separated radical pair in the PSII reaction center. Apart to triplet chlorophyll, triplet carbonyl formed by lipid peroxidation transfers energy to molecular oxygen forming singlet oxygen. On the PSII electron acceptor side, electron leakage to molecular oxygen forms superoxide anion radical which dismutes to hydrogen peroxide which is reduced by the non-heme iron to hydroxyl radical. On the PSII electron donor side, incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. Under high temperature, dark production of singlet oxygen results from lipid peroxidation initiated by lipoxygenase, whereas incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. The understanding of molecular basis for ROS production by PSII provides new insight into how plants survive under adverse environmental conditions.

Production of Reactive Oxygen Species by Photosystem II as a Response to Light and Temperature Stress

The effect of various abiotic stresses on photosynthetic apparatus is inevitably associated with formation of harmful reactive oxygen species (ROS). In this review, recent progress on ROS production by photosystem II (PSII) as a response to high light and high temperature is overviewed. Under high light, ROS production is unavoidably associated with energy transfer and electron transport in PSII. Singlet oxygen is produced by the energy transfer form triplet chlorophyll to molecular oxygen formed by the intersystem crossing from singlet chlorophyll in the PSII antennae complex or the recombination of the charge separated radical pair in the PSII reaction center. Apart to triplet chlorophyll, triplet carbonyl formed by lipid peroxidation transfers energy to molecular oxygen forming singlet oxygen. On the PSII electron acceptor side, electron leakage to molecular oxygen forms superoxide anion radical which dismutes to hydrogen peroxide which is reduced by the non-heme iron to hydroxyl radical. On the PSII electron donor side, incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. Under high temperature, dark production of singlet oxygen results from lipid peroxidation initiated by lipoxygenase, whereas incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. The understanding of molecular basis for ROS production by PSII provides new insight into how plants survive under adverse environmental conditions.

Multi-Level Light Capture Control in Plants and Green Algae

Life on Earth relies on photosynthesis, and the ongoing depletion of fossil carbon fuels has renewed interest in phototrophic light-energy conversion processes as a blueprint for the conversion of atmospheric CO2 into various organic compounds. Light-harvesting systems have evolved in plants and green algae, which are adapted to the light intensity and spectral composition encountered in their habitats. These organisms are constantly challenged by a fluctuating light supply and other environmental cues affecting photosynthetic performance. Excess light can be especially harmful, but plants and microalgae are equipped with different acclimation mechanisms to control the processing of sunlight absorbed at both photosystems. We summarize the current knowledge and discuss the potential for optimization of phototrophic light-energy conversion.

A reductionist approach to model photosynthetic self-regulation in eukaryotes in response to light

Along with the development of several large-scale methods such as mass spectrometry or micro arrays, genome wide models became not only a possibility but an obvious tool for theoretical biologists to integrate and analyse complex biological data. Nevertheless, incorporating the dynamics of photosynthesis remains one of the major challenges while reconstructing metabolic networks of plants and other photosynthetic organisms. In this review, we aim to provide arguments that small-scale models are still a suitable choice when it comes to discovering organisational principles governing the design of biological systems. We give a brief overview of recent modelling efforts in understanding the interplay between rapid, photoprotective mechanisms and the redox balance within the thylakoid membrane, discussing the applicability of a reductionist approach in modelling self-regulation in plants and outline possible directions for further research.

Chlorophyll b in angiosperms: Functions in photosynthesis, signaling and ontogenetic regulation

Chlorophyll b (Chlb) is an antenna chlorophyll. The binding of Chlb by antenna proteins is crucial for the correct assembly of the antenna complexes in thylakoid membranes. Since the levels of the proteins of major and minor antenna are affected to different extents by Chlb binding, the availability of Chlb influences the composition and the size of antenna complexes which in turn determine the supramolecular organization of the thylakoid membranes in grana. Therefore, Chlb synthesis levels have a major impact on lateral mobility and diffusion of membrane molecules, and thus affect not only light harvesting and thermal energy dissipation processes, but also linear electron transport and repair processes in grana. Furthermore, in angiosperms Chlb synthesis affects plant functions beyond chloroplasts. First, the stability of pigment-protein complexes in the antennae, which depends on Chlb, is an important factor in the regulation of plant ontogenesis, and Chlb levels were recently shown to influence plant ontogenetic signaling. Second, the amounts of minor antenna proteins in chloroplasts, which depend on the availability of Chlb, were recently shown to affect ABA levels and signaling in plants. These mechanisms can be examined in mutants where Chlb synthesis is reduced or abolished. The dramatic effects caused by the lack of Chlb on plant productivity are interpreted in this review in light of the pleiotropic effects on photosynthesis and signaling, and the potential to manipulate Chlb biosynthesis for the improvement of crop production is discussed.

The action spectrum of Photosystem II photoinactivation in visible light

Photosynthesis is always accompanied by light induced damage to the Photosystem II (PSII) which is compensated by its subsequent repair. Photoinhibition of PSII is a complex process, balancing between photoinactivation, protective and repair mechanisms. Current understanding of photoinactivation is limited with competing hypotheses where the photosensitiser is either photosynthetic pigments or the Mn4CaO5 cluster itself, with little consensus on the mechanisms and consequences of PSII photoinactivation. The mechanism of photoinactivation should be reflected in the action spectrum of PSII photoinactivation, but there is a great diversity of the action spectra reported thus far. The only consensus is that PSII photoinactivation is greatest in the UV region of the electromagnetic spectrum. In this review, the authors revisit the methods, technical constraints and the different action spectra of PSII photoinactivation reported to date and compare them against the diverse mechanisms proposed. Upon critical examination of the reported action spectra, a hybrid mechanism of photoinactivation, sensitised by both photosynthetic pigments and the Mn4CaO5 appears to be the most plausible rationalisation.