The xanthophyll cycle affects reversible interactions between PsbS and light-harvesting complex II to control non-photochemical quenching

Hydrophobic Mismatch in the Thylakoid Membrane Regulates Photosynthetic Light Harvesting

The ability to harvest light effectively in a changing environment is necessary to ensure efficient photosynthesis and crop growth. One mechanism, known as qE, protects photosystem II (PSII) and regulates electron transfer through the harmless dissipation of excess absorbed photons as heat. This process involves reversible clustering of the major light-harvesting complexes of PSII (LHCII) in the thylakoid membrane and relies upon the ΔpH gradient and the allosteric modulator protein PsbS. To date, the exact role of PsbS in the qE mechanism has remained elusive. Here, we show that PsbS induces hydrophobic mismatch in the thylakoid membrane through dynamic rearrangement of lipids around LHCII leading to observed membrane thinning. We found that upon illumination, the thylakoid membrane reversibly shrinks from around 4.3 to 3.2 nm, without PsbS, this response is eliminated. Furthermore, we show that the lipid digalactosyldiacylglycerol (DGDG) is repelled from the LHCII-PsbS complex due to an increase in both the pKa of lumenal residues and in the dipole moment of LHCII, which allows for further conformational change and clustering in the membrane. Our results suggest a mechanistic role for PsbS as a facilitator of a hydrophobic mismatch-mediated phase transition between LHCII-PsbS and its environment. This could act as the driving force to sort LHCII into photoprotective nanodomains in the thylakoid membrane. This work shows an example of the key role of the hydrophobic mismatch process in regulating membrane protein function in plants.

Photosystem II Subunit S (PsbS): A Nano Regulator of Plant Photosynthesis

Light is required for photosynthesis, but plants are often exposed to excess light, which can lead to photodamage and eventually cell death. To prevent this, they evolved photoprotective feedback mechanisms that regulate photosynthesis and trigger processes that dissipate light energy as heat, called non-photochemical quenching (NPQ). In excess light conditions, the light reaction and activity of Photosystem II (PSII) generates acidification of the thylakoid lumen, which is sensed by special pH-sensitive proteins called Photosystem II Subunit S (PsbS), actuating a photoprotective "switch" in the light-harvesting antenna. Despite its central role in regulating photosynthetic energy conversion, the molecular mechanism of PsbS as well as its interaction with partner proteins are not well understood. This review summarizes the current knowledge on the molecular structure and mechanistic aspects of the light-stress sensor PsbS and addresses open questions and challenges in the field regarding a full understanding of its functional mechanism and role in NPQ.

Desiccation tolerance in the resurrection plant Barbacenia graminifolia involves changes in redox metabolism and carotenoid oxidation

Desiccation tolerance in vegetative tissues enables resurrection plants to remain quiescent under severe drought and rapidly recover full metabolism once water becomes available. Barbacenia graminifolia is a resurrection plant that occurs at high altitudes, typically growing on rock slits, exposed to high irradiance and limited water availability. We analyzed the levels of reactive oxygen species (ROS) and antioxidants, carotenoids and its cleavage products, and stress-related phytohormones in fully hydrated, dehydrated, and rehydrated leaves of B. graminifolia. This species exhibited a precise adjustment of its antioxidant metabolism to desiccation. Our results indicate that this adjustment is associated with enhanced carotenoid and apocarotenoids, α-tocopherol and compounds of ascorbate-glutathione cycle. While α-carotene and lutein increased in dried-leaves suggesting effective protection of the light-harvesting complexes, the decrease in β-carotene was accompanied of 10.2-fold increase in the content of β-cyclocitral, an apocarotenoid implicated in the regulation of abiotic stresses, compared to hydrated plants. The principal component analysis showed that dehydrated plants at 30 days formed a separate cluster from both hydrated and dehydrated plants for up to 15 days. This regulation might be part of the protective metabolic strategies employed by this resurrection plant to survive water scarcity in its inhospitable habitat.

Inter-Organellar Effects of Defective ER-Localized Linolenic Acid Formation on Thylakoid Lipid Composition, Non-Photochemical Quenching of Chlorophyll Fluorescence and Xanthophyll Cycle Activity in the Arabidopsis fad3 Mutant

Monogalactosyldiacylglycerol (MGDG) is the main lipid constituent of thylakoids and a structural component of photosystems and photosynthesis-related proteo-lipid complexes in green tissues. Previously reported changes in MGDG abundance upon stress treatments are hypothesized to reflect mobilization of MGDG-based polyunsaturated lipid intermediates to maintain extraplastidial membrane integrity. While exchange of lipid intermediates between compartmental membranes is well documented, physiological consequences of mobilizing an essential thylakoid lipid, such as MGDG, for an alternative purpose are not well understood. Arabidopsis seedlings exposed to mild (50 mM) salt treatment displayed significantly increased abundance of both MGDG and the extraplastidial lipid, phosphatidylcholine (PC). Interestingly, similar increases in MGDG and PC were observed in Arabidopsis fad3 mutant seedlings defective in endoplasmic reticulum (ER)-localized linolenic acid formation, in which compensatory plastid-to-ER-directed mobilization of linolenic acid-containing intermediates takes place. The postulated (salt) or evident (fad3) plastid-ER exchange of intermediates concurred with altered thylakoid function according to parameters of photosynthetic performance. While salt treatment of wild-type seedlings inhibited photosynthetic parameters in a dose-dependent manner, interestingly, untreated fad3 mutants did not show overall reduced photosynthetic quantum yield. By contrast, we observed a reduction specifically of non-photochemical quenching (NPQ) under high light, representing only part of observed salt effects. The decreased NPQ in the fad3 mutant was accompanied by reduced activity of the xanthophyll cycle, leading to a reduced concentration of the NPQ-effective pigment zeaxanthin. The findings suggest that altered ER-located fatty acid unsaturation and ensuing inter-organellar compensation impacts on the function of specific thylakoid enzymes, rather than globally affecting thylakoid function.

J, Hase T, Jensen P, Hanke GT. Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Photosynthetic and transcriptome responses to fluctuating light in Arabidopsis thylakoid ion transport triple mutant

Fluctuating light intensity challenges fluent photosynthetic electron transport in plants, inducing photoprotection while diminishing carbon assimilation and growth, and also influencing photosynthetic signaling for regulation of gene expression. Here, we employed in vivo chlorophyll-a fluorescence and P700 difference absorption measurements to demonstrate the enhancement of photoprotective energy dissipation of both photosystems in wild-type Arabidopsis thaliana after 6 h exposure to fluctuating light as compared with constant light conditions. This acclimation response to fluctuating light was hampered in a triple mutant lacking the thylakoid ion transport proteins KEA3, VCCN1, and CLCe, leading to photoinhibition of photosystem I. Transcriptome analysis revealed upregulation of genes involved in biotic stress and defense responses in both genotypes after exposure to fluctuating as compared with constant light, yet these responses were demonstrated to be largely upregulated in triple mutant already under constant light conditions compared with wild type. The current study illustrates the rapid acclimation of plants to fluctuating light, including photosynthetic, transcriptomic, and metabolic adjustments, and highlights the connection among thylakoid ion transport, photosynthetic energy balance, and cell signaling.

Photosystem II monomeric antenna CP26 plays a key role in nonphotochemical quenching in Chlamydomonas

Thermal dissipation of excess excitation energy, called nonphotochemical quenching (NPQ), is 1 of the main photoprotective mechanisms in oxygenic photosynthetic organisms. Here, we investigated the function of the monomeric photosystem II (PSII) antenna protein CP26 in photoprotection and light harvesting in Chlamydomonas reinhardtii, a model organism for green algae. We used CRISPR/Cas9 genome editing and complementation to generate cp26 knockout mutants (named k6#) that did not negatively affect CP29 accumulation, which differed from previous cp26 mutants, allowing us to compare mutants specifically deprived of CP26, CP29, or both. The absence of CP26 partially affected PSII activity, causing reduced growth at low or medium light but not at high irradiances. However, the main phenotype observed in k6# mutants was a more than 70% reduction of NPQ compared to the wild type (Wt). This phenotype was fully rescued by genetic complementation and complemented strains accumulating different levels of CP26, demonstrating that ∼50% of CP26 content, compared to the Wt, was sufficient to restore the NPQ capacity. Our findings demonstrate a pivotal role for CP26 in NPQ induction, while CP29 is crucial for PSII activity. The genetic engineering of these 2 proteins could be a promising strategy to regulate the photosynthetic efficiency of microalgae under different light regimes.

Overexpression of LHCSR and PsbS enhance light tolerance in Chlamydomonas reinhardtii

Nonphotochemical quenching (NPQ) is a crucial mechanism for fine-tuning light harvesting and protecting the photosystem II (PSII) reaction centres from excess light energy in plants and algae. This process is regulated by photoprotective proteins LHCSR1, LHCSR3, and PsbS in green algae, such as Chlamydomonas reinhardtii. The det1-2 phot mutant, which overexpresses these photoprotective proteins, resulting in a significantly higher NPQ response, has been recently discovered in C. reinhardtii. Here, we analysed the physiological impact of this response on algal cells and found that det1-2 phot was capable of efficient growth under high light intensities, where wild-type (WT) cells were unable to survive. The mutant exhibited a smaller PSII cross-section in the dark and showed a detachment of the peripheral light-harvesting complex II (LHCII) antenna in the NPQ state, as suggested by a rise in the chlorophyll fluorescence parameter of photochemical quenching in the dark (qPd > 1). Furthermore, fluorescence decay-associated spectra demonstrated a decreased excitation pressure on PSII, with excess energy being directed toward PSI. The amount of LHCSR1, LHCSR3, and PsbS in the mutant correlated with the magnitude of the protective NPQ response. Overall, the study suggests the mechanism by which the overexpression of photoprotective proteins in det1-2 phot brings about an efficient and effective photoprotective response, enabling the mutant to grow and survive under high light intensities that would otherwise be lethal for WT cells.

Cheap, cost-effective, and quick stress biomarkers for drought stress detection and monitoring in plants

The detection and monitoring of drought stress in plants growing in their natural habitat are essential for the study of plant stress physiology. However, with the advent of plant phenotyping and new -omics technologies, the application of simple, cheap, cost-effective, quick, and practical methods to assess drought stress in plants seems more challenging than ever, particularly in low-income countries. Here, currently available methods that do not require specialized equipment, but reliably detect and monitor drought stress in plants at low cost will be discussed. This will not only boost research on plant stress physiology in low-income countries but will also help several laboratories with very limited resources around the globe to perform high-quality research.

A perspective on the major light-harvesting complex dynamics under the effect of pH, salts, and the photoprotective PsbS protein

The photosynthetic apparatus is a highly modular assembly of large pigment-binding proteins. Complexes called antennae can capture the sunlight and direct it from the periphery of two Photosystems (I, II) to the core reaction centers, where it is converted into chemical energy. The apparatus must cope with the natural light fluctuations that can become detrimental to the viability of the photosynthetic organism. Here we present an atomic scale view of the photoprotective mechanism that is activated on this line of defense by several photosynthetic organisms to avoid overexcitation upon excess illumination. We provide a complete macroscopic to microscopic picture with specific details on the conformations of the major antenna of Photosystem II that could be associated with the switch from the light-harvesting to the photoprotective state. This is achieved by combining insight from both experiments and all-atom simulations from our group and the literature in a perspective article.

pH-Dependent Conformational Switch Impacts Stability of the PsbS Dimer

The photosystem II PsbS protein triggers the photoprotective mechanism of plants by sensing the acidification of the thylakoid lumen. Despite the mechanism of action of PsbS would require a pH-dependent monomerization of the dimeric form, a clear connection between the pH-induced structural changes and the dimer stability is missing. Here, by applying constant pH coarse-grained and all-atom molecular dynamics simulations, we investigate the pH-dependent structural response of the PsbS dimer. We find that the pH variation leads to structural changes in the lumen-exposed helices, located at the dimeric interface, providing an effective switch between PsbS inactive and active form. Moreover, the monomerization free energies reveal that in the neutral pH conformation, where the network of H-bond interactions at the dimeric interface is destroyed, the protein-protein interaction is weaker. Our results show how the pH-dependent conformations of PsbS affect their dimerization propensity, which is at the basis of the photoprotective mechanism.

Crossroads in the evolution of plant specialized metabolism

The monophyletic group of embryophytes (land plants) stands out among photosynthetic eukaryotes: they are the sole constituents of the macroscopic flora on land. In their entirety, embryophytes account for the majority of the biomass on land and constitute an astounding biodiversity. What allowed for the massive radiation of this particular lineage? One of the defining features of all land plants is the production of an array of specialized metabolites. The compounds that the specialized metabolic pathways of embryophytes produce have diverse functions, ranging from superabundant structural polymers and compounds that ward off abiotic and biotic challenges, to signaling molecules whose abundance is measured at the nanomolar scale. These specialized metabolites govern the growth, development, and physiology of land plants-including their response to the environment. Hence, specialized metabolites define the biology of land plants as we know it. And they were likely a foundation for their success. It is thus intriguing to find that the closest algal relatives of land plants, freshwater organisms from the grade of streptophyte algae, possess homologs for key enzymes of specialized metabolic pathways known from land plants. Indeed, some studies suggest that signature metabolites emerging from these pathways can be found in streptophyte algae. Here we synthesize the current understanding of which routes of the specialized metabolism of embryophytes can be traced to a time before plants had conquered land.

The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae

Ascorbate (Asc) is a multifunctional metabolite essential for various cellular processes in plants and animals. The best-known property of Asc is to scavenge reactive oxygen species (ROS), in a highly regulated manner. Besides being an effective antioxidant, Asc also acts as a chaperone for 2-oxoglutarate-dependent dioxygenases that are involved in the hormone metabolism of plants and the synthesis of various secondary metabolites. Asc also essential for the epigenetic regulation of gene expression, signaling and iron transport. Thus, Asc affects plant growth, development, and stress resistance via various mechanisms. In this review, the intricate relationship between Asc and photosynthesis in plants and algae is summarized in the following major points: (i) regulation of Asc biosynthesis by light, (ii) interaction between photosynthetic and mitochondrial electron transport in relation to Asc biosynthesis, (iii) Asc acting as an alternative electron donor of photosystem II, (iv) Asc inactivating the oxygen-evolving complex, (v) the role of Asc in non-photochemical quenching, and (vi) the role of Asc in ROS management in the chloroplast. The review also discusses differences in the regulation of Asc biosynthesis and the effects of Asc on photosynthesis in algae and vascular plants.

Light acclimation interacts with thylakoid ion transport to govern the dynamics of photosynthesis in Arabidopsis

Understanding photosynthesis in natural, dynamic light environments requires knowledge of long-term acclimation, short-term responses, and their mechanistic interactions. To approach the latter, we systematically determined and characterized light-environmental effects on thylakoid ion transport-mediated short-term responses during light fluctuations. For this, Arabidopsis thaliana wild-type and mutants of the Cl^- channel VCCN1 and the K⁺ exchange antiporter KEA3 were grown under eight different light environments and characterized for photosynthesis-associated parameters and factors in steady state and during light fluctuations. For a detailed characterization of selected light conditions, we monitored ion flux dynamics at unprecedented high temporal resolution by a modified spectroscopy approach. Our analyses reveal that daily light intensity sculpts photosynthetic capacity as a main acclimatory driver with positive and negative effects on the function of KEA3 and VCCN1 during high-light phases, respectively. Fluctuations in light intensity boost the accumulation of the photoprotective pigment zeaxanthin (Zx). We show that KEA3 suppresses Zx accumulation during the day, which together with its direct proton transport activity accelerates photosynthetic transition to lower light intensities. In summary, both light-environment factors, intensity and variability, modulate the function of thylakoid ion transport in dynamic photosynthesis with distinct effects on lumen pH, Zx accumulation, photoprotection, and photosynthetic efficiency.

Light harvesting regulation: A versatile network of key components operating under various stress conditions in higher plants

Light harvesting is finetuned through two main strategies controlling energy transfer to the reaction centers of photosystems: i) regulating the amount of light energy at the absorption level, ii) regulating the amount of the absorbed energy at the utilization level. The first strategy is ensured by changes in the cross-section, i.e., the size of the photosynthetic antenna. These changes can occur in a short-term (state transitions) or long-term way (changes in antenna protein biosynthesis) depending on the light conditions. The interrelation of these two ways is still underexplored. Regulating light absorption through the long-term modulation of photosystem II antenna size has been mostly considered as an acclimatory mechanism to light conditions. The present review highlights that this mechanism represents one of the most versatile mechanisms of higher plant acclimation to various conditions including drought, salinity, temperature changes, and even biotic factors. We suggest that H₂O₂ is the universal signaling agent providing the switch from the short-term to long-term modulation of photosystem II antenna size under these factors. The second strategy of light harvesting is represented by redirecting energy to waste mainly via thermal energy dissipation in the photosystem II antenna in high light through PsbS protein and xanthophyll cycle. In the latter case, H₂O₂ also plays a considerable role. This circumstance may explain the maintenance of the appropriate level of zeaxanthin not only upon high light but also upon other stress factors. Thus, the review emphasizes the significance of both strategies for ensuring plant sustainability under various environmental conditions.

Temporal heterogeneity in photosystem II photochemistry in Artemisia ordosica under a fluctuating desert environment

Acclimation strategies in xerophytic plants to stressed environmental conditions vary with temporal scales. Our understanding of environmentally-induced variation in photosystem II (PSII) processes as a function of temporal scales is limited, as most studies have thus far been based on short-term, laboratory-controlled experiments. In a study of PSII processes, we acquired near-continuous, field-based measurements of PSII-energy partitioning in a dominant desert-shrub species, namely Artemisia ordosica, over a six-year period from 2012-2017. Continuous-wavelet transformation (CWT) and wavelet coherence analyses (WTC) were employed to examine the role of environmental variables in controlling the variation in the three main PSII-energy allocation pathways, i.e., photochemical efficiency and regulated and non-regulated thermal dissipation, i.e., Φ _PSII, Φ _NPQ, and Φ _NO, respectively, across a time-frequency domain from hours to years. Convergent cross mapping (CCM) was subsequently used to isolate cause-and-effect interactions in PSII-energy partitioning response. The CWT method revealed that the three PSII-energy allocation pathways all had distinct daily periodicities, oscillating abruptly at intermediate timescales from days to weeks. On a diurnal scale, WTC revealed that all three pathways were influenced by photosynthetically active radiation (PAR), air temperature (T _a), and vapor pressure deficit (VPD). By comparing associated time lags for the three forms of energy partitioning at diurnal scales, revealed that the sensitivity of response was more acutely influenced by PAR, declining thereafter with the other environmental variables, such that the order of influence was greatest for T _a, followed by VPD, and then soil water content (SWC). PSII-energy partitioning on a seasonal scale, in contrast, displayed greater variability among the different environmental variables, e.g., Φ _PSII and Φ _NO being more predisposed to changes in T _a, and Φ _NPQ to changes in VPD. CCM confirmed the causal relationship between pairings of PSII-energy allocation pathways, according to shrub phenology. A. ordosica is shown to have an innate ability to (i) repair damaged PSII-photochemical apparatus (maximum quantum yield of PSII photochemistry, with F _v/F _m > 0.78), and (ii) acclimatize to excessive PAR, dry-air conditions, and prolonged drought. A. ordosica is relatively sensitive to extreme temperature and exhibits photoinhibition.

Non-rolling flag leaves use an effective mechanism to reduce water loss and light-induced damage under drought stress

BACKGROUND AND AIMS

The study reports on four different types of flag leaf rolling under soil drought in relation to the level of cell wall-bound phenolics. The flag leaf colonization by aphids, as a possible bioindicator of the accumulation of cell wall-bound phenolics, was also estimated.

METHODS

The proteins of the photosynthetic apparatus that form its core and are crucial for maintaining its stability (D1/PsbA protein), limit destructive effects of light (PsbS, a protein binding carotenoids in the antennas) and participate in efficient electron transport between photosystems II (PSII) and PSI (Rieske iron-sulfur protein of the cytochrome b6f complex) were evaluated in two types of flag leaf rolling. Additionally, biochemical and physiological reactions to drought stress in rolling and non-rolling flag leaves were compared.

KEY RESULTS

The study identified four types of genome-related types of flag leaf rolling. The biochemical basis for these differences was a different number of phenolic molecules incorporated into polycarbohydrate structures of the cell wall. In an extreme case of non-rolling dehydrated flag leaves, they were found to accumulate high amounts of cell wall-bound phenolics that limited cell water loss and protected the photosynthetic apparatus against excessive light. PSII was also additionally protected against excess light by the accumulation of photosynthetic apparatus proteins that ensured stable and efficient transport of excitation energy beyond PSII and its dissipation as far-red fluorescence and heat. Our analysis revealed a new type of flag leaf rolling brought about by an interaction between wheat and rye genomes, and resulting in biochemical specialization of flexible, rolling and rigid, non-rolling parts of the flag leaf. The study confirmed limited aphid colonization of the flag leaves with enhanced content of cell wall-bound phenolics.

CONCLUSIONS

Non-rolling leaves developed effective adaptation mechanisms to reduce both water loss and photoinhibitory damage to the photosynthetic apparatus under drought stress.

Cia Zeaxanthin Biosynthesis, OsZEP and OsVDE Regulate Striped Leaves Occurring in Response to Deep Transplanting of Rice

The rice leaf color mutant B03S was previously generated from the photoperiod- and thermo-sensitive genic male sterile (PTGMS) rice line Efeng 1S, of which male sterility manifests by photoperiod and temperature but exhibits mainly temperature-sensitive characteristics. After these plants were deeply transplanted, the new leaves manifested typical zebra stripe patterns. Here, B03S was subjected to deep and shallow transplanting, shading with soil and aluminum foil, and control conditions in situ to determine the cause of the striped-leaf trait. The direct cause of striped leaves is the base of the leaf sheath being under darkness during deep transplanting, of which the critical shading range reached or exceeds 4 cm above the base. Moreover, typical striped leaves were analyzed based on the targeted metabolome method by ultra-performance liquid chromatography/tandem mass spectrometry (UPLC–MS/MS) combined with transcriptome and real-time quantitative PCR (qPCR)-based verification to clarify the metabolic pathways and transcriptional regulation involved. Carotenoids enter the xanthophyll cycle, and the metabolites that differentially accumulate in the striped leaves include zeaxanthin and its derivatives for photooxidative stress protection, driven by the upregulated expression of OsZEP. These findings improve the understanding of the physiological and metabolic mechanisms underlying the leaf color mutation in rice plants, enrich the theoretical foundation of the nonuniform leaf color phenomenon widely found in nature and highlight key advancements concerning rice production involving the transplanting of seedlings or direct broadcasting of seeds.

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

Transcriptional regulation of photoprotection in dark-to-light transition-More than just a matter of excess light energy

In nature, photosynthetic organisms are exposed to different light spectra and intensities depending on the time of day and atmospheric and environmental conditions. When photosynthetic cells absorb excess light, they induce nonphotochemical quenching to avoid photodamage and trigger expression of "photoprotective" genes. In this work, we used the green alga Chlamydomonas reinhardtii to assess the impact of light intensity, light quality, photosynthetic electron transport, and carbon dioxide on induction of the photoprotective genes (LHCSR1, LHCSR3, and PSBS) during dark-to-light transitions. Induction (mRNA accumulation) occurred at very low light intensity and was independently modulated by blue and ultraviolet B radiation through specific photoreceptors; only LHCSR3 was strongly controlled by carbon dioxide levels through a putative enhancer function of CIA5, a transcription factor that controls genes of the carbon concentrating mechanism. We propose a model that integrates inputs of independent signaling pathways and how they may help the cells anticipate diel conditions and survive in a dynamic light environment.

Loss of a single chlorophyll in CP29 triggers re-organization of the Photosystem II supramolecular assembly

In land plants, both efficient light capture and photoprotective dissipation of chlorophyll excited states in excess require proper assembly of Photosystem II supercomplexes PSII-LHCs. These include a dimeric core moiety and a peripheral antenna system made of trimeric LHCII proteins connected to the core through monomeric LHC subunits. Regulation of light harvesting involves re-organization of the PSII supercomplex, including dissociation of its LHCII-CP24-CP29 domain under excess light. The Chl a603-a609-a616 chromophore cluster within CP29 was recently identified as responsible for the fast component of Non-Photochemical Quenching of chlorophyll fluorescence. Here, we pinpointed a chlorophyll-protein domain of CP29 involved in the macro-organization of PSII-LHCs. By complementing an Arabidopsis knock-out mutant with CP29 sequences deleted in the residue binding chlorophyll b614/b3-binding, we found that the site is promiscuous for chlorophyll a and b. By plotting NPQ amplitude vs. CP29 content we observed that quenching activity was significantly reduced in mutants compared to the wild type. Analysis of pigment-binding supercomplexes showed that the missing Chl did hamper the assembly of PSII-LHCs supercomplexes, while observation by electron microscopy of grana membranes highlighted the PSII particles were organized in two-dimensional arrays in mutant grana partitions. As an effect of such array formation electron transport rate between Q_A and Q_B reduced, likely due to restricted plastoquinone diffusion. We conclude that chlorophyll b614, rather being part of pigment cluster responsible for quenching, is needed to maintain full rate of electron flow in the thylakoids by controlling protein-protein interactions between PSII units in grana partitions.

Towards improved dynamic photosynthesis in C3 crops by utilizing natural genetic variation

Under field environments, fluctuating light conditions induce dynamic photosynthesis, which affects carbon gain by crop plants. Elucidating the natural genetic variations among untapped germplasm resources and their underlying mechanisms can provide an effective strategy to improve dynamic photosynthesis and, ultimately, improve crop yields through molecular breeding approaches. In this review, we first overview two processes affecting dynamic photosynthesis, namely (i) biochemical processes associated with CO2 fixation and photoprotection and (ii) gas diffusion processes from the atmosphere to the chloroplast stroma. Next, we review the intra- and interspecific variations in dynamic photosynthesis in relation to each of these two processes. It is suggested that plant adaptations to different hydrological environments underlie natural genetic variation explained by gas diffusion through stomata. This emphasizes the importance of the coordination of photosynthetic and stomatal dynamics to optimize the balance between carbon gain and water use efficiency under field environments. Finally, we discuss future challenges in improving dynamic photosynthesis by utilizing natural genetic variation. The forward genetic approach supported by high-throughput phenotyping should be introduced to evaluate the effects of genetic and environmental factors and their interactions on the natural variation in dynamic photosynthesis.

Photoprotective energy quenching in the red alga Porphyridium purpureum occurs at the core antenna of the photosystem II but not at its reaction center

Photosynthetic organisms have evolved light-harvesting antennae over time. In cyanobacteria, external phycobilisomes (PBSs) are the dominant antennae, whereas in green algae and higher plants, PBSs have been replaced by proteins of the Lhc family that are integrated in the membrane. Red algae represent an evolutionary intermediate between these two systems, as they employ both PBSs and membrane LHCR proteins as light-harvesting units. Understanding how red algae cope with light is not only interesting for biotechnological applications, but is also of evolutionary interest. For example, energy-dependent quenching (qE) is an essential photoprotective mechanism widely used by species from cyanobacteria to higher plants to avoid light damage; however, the quenching mechanism in red algae remains largely unexplored. Here, we used both pulse amplitude-modulated (PAM) and time-resolved chlorophyll fluorescence to characterize qE kinetics in the red alga Porphyridium purpureum. PAM traces confirmed that qE in P. purpureum is activated by a decrease in the thylakoid lumen pH, whereas time-resolved fluorescence results further revealed the quenching site and ultrafast quenching kinetics. We found that quenching exclusively takes place in the photosystem II (PSII) complexes and preferentially occurs at PSII’s core antenna rather than at its reaction center, with an overall quenching rate of 17.6 ± 3.0 ns⁻¹. In conclusion, we propose that qE in red algae is not a reaction center type of quenching, and that there might be a membrane-bound protein that resembles PsbS of higher plants or LHCSR of green algae that senses low luminal pH and triggers qE in red algae.

The role of photosynthesis related pigments in light harvesting, photoprotection and enhancement of photosynthetic yield in planta

Photosynthetic pigments are an integral and vital part of all photosynthetic machinery and are present in different types and abundances throughout the photosynthetic apparatus. Chlorophyll, carotenoids and phycobilins are the prime photosynthetic pigments which facilitate efficient light absorption in plants, algae, and cyanobacteria. The chlorophyll family plays a vital role in light harvesting by absorbing light at different wavelengths and allowing photosynthetic organisms to adapt to different environments, either in the long-term or during transient changes in light. Carotenoids play diverse roles in photosynthesis, including light capture and as crucial antioxidants to reduce photodamage and photoinhibition. In the marine habitat, phycobilins capture a wide spectrum of light and have allowed cyanobacteria and red algae to colonise deep waters where other frequencies of light are attenuated by the water column. In this review, we discuss the potential strategies that photosynthetic pigments provide, coupled with development of molecular biological techniques, to improve crop yields through enhanced light harvesting, increased photoprotection and improved photosynthetic efficiency.

A foliar pigment-based bioassay for interrogating chloroplast signalling revealed that carotenoid isomerisation regulates chlorophyll abundance

BACKGROUND

Some plastid-derived metabolites can control nuclear gene expression, chloroplast biogenesis, and chlorophyll biosynthesis. For example, norflurazon (NFZ) induced inhibition of carotenoid biosynthesis in leaves elicits a protoporphyrin IX (Mg-ProtoIX) retrograde signal that controls chlorophyll biosynthesis and chloroplast development. Carotenoid cleavage products, known as apocarotenoids, also regulate plastid development. The key steps in carotenoid biosynthesis or catabolism that can regulate chlorophyll biosynthesis in leaf tissues remain unclear. Here, we established a foliar pigment-based bioassay using Arabidopsis rosette leaves to investigate plastid signalling processes in young expanding leaves comprising rapidly dividing and expanding cells containing active chloroplast biogenesis.

RESULTS

We demonstrate that environmental treatments (extended darkness and cold exposure) as well as chemical (norflurazon; NFZ) inhibition of carotenoid biosynthesis, reduce chlorophyll levels in young, but not older leaves of Arabidopsis. Mutants with disrupted xanthophyll accumulation, apocarotenoid phytohormone biosynthesis (abscisic acid and strigolactone), or enzymatic carotenoid cleavage, did not alter chlorophyll levels in young or old leaves. However, perturbations in acyclic cis-carotene biosynthesis revealed that disruption of CAROTENOID ISOMERASE (CRTISO), but not ZETA-CAROTENE ISOMERASE (Z-ISO) activity, reduced chlorophyll levels in young leaves of Arabidopsis plants. NFZ-induced inhibition of PHYTOENE DESATURASE (PDS) activity caused higher phytoene accumulation in younger crtiso leaves compared to WT indicating a continued substrate supply from the methylerythritol 4-phosphate (MEP) pathway.

CONCLUSION

The Arabidopsis foliar pigment-based bioassay can be used to differentiate signalling events elicited by environmental change, chemical treatment, and/or genetic perturbation, and determine how they control chloroplast biogenesis and chlorophyll biosynthesis. Genetic perturbations that impaired xanthophyll biosynthesis and/or carotenoid catabolism did not affect chlorophyll biosynthesis. The lack of CAROTENOID ISOMERISATION reduced chlorophyll accumulation, but not phytoene biosynthesis in young leaves of Arabidopsis plants growing under a long photoperiod. Findings generated using the newly customised foliar pigment-based bioassay implicate that carotenoid isomerase activity and NFZ-induced inhibition of PDS activity elicit different signalling pathways to control chlorophyll homeostasis in young leaves of Arabidopsis.

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

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

Plant carotenoids: recent advances and future perspectives

Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

Biochemical and molecular properties of LHCX1, the essential regulator of dynamic photoprotection in diatoms

Light harvesting is regulated by a process triggered by the acidification of the thylakoid lumen, known as nonphotochemical "energy-dependent quenching" (qE). In diatoms, qE is controlled by the light-harvesting complex (LHC) protein LHCX1, while the LHC stress-related (LHCSR) and photosystem II subunit S proteins are essential for green algae and plants, respectively. Here, we report a biochemical and molecular characterization of LHCX1 to investigate its role in qE. We found that, when grown under intermittent light, Phaeodactylum tricornutum forms very large qE, due to LHCX1 constitutive upregulation. This "super qE" is abolished in LHCX1 knockout mutants. Biochemical and spectroscopic analyses of LHCX1 reveal that this protein might differ in the character of binding pigments relative to the major pool of light-harvesting antenna proteins. The possibility of transient pigment binding or not binding pigments at all is discussed. Targeted mutagenesis of putative protonatable residues (D95 and E205) in transgenic P. tricornutum lines does not alter qE capacity, showing that they are not involved in sensing lumen pH, differently from residues conserved in LHCSR3. Our results suggest functional divergence between LHCX1 and LHCSR3 in qE modulation. We propose that LHCX1 evolved independently to facilitate dynamic tracking of light fluctuations in turbulent waters. The evolution of LHCX(-like) proteins in organisms with secondary red plastids, such as diatoms, might have conferred a selective advantage in the control of dynamic photoprotection, ultimately resulting in their ecological success.

Chloroplast pH Homeostasis for the Regulation of Photosynthesis

The pH of various chloroplast compartments, such as the thylakoid lumen and stroma, is light-dependent. Light illumination induces electron transfer in the photosynthetic apparatus, coupled with proton translocation across the thylakoid membranes, resulting in acidification and alkalization of the thylakoid lumen and stroma, respectively. Luminal acidification is crucial for inducing regulatory mechanisms that protect photosystems against photodamage caused by the overproduction of reactive oxygen species (ROS). Stromal alkalization activates enzymes involved in the Calvin-Benson-Bassham (CBB) cycle. Moreover, proton translocation across the thylakoid membranes generates a proton gradient (ΔpH) and an electric potential (ΔΨ), both of which comprise the proton motive force (pmf) that drives ATP synthase. Then, the synthesized ATP is consumed in the CBB cycle and other chloroplast metabolic pathways. In the dark, the pH of both the chloroplast stroma and thylakoid lumen becomes neutral. Despite extensive studies of the above-mentioned processes, the molecular mechanisms of how chloroplast pH can be maintained at proper levels during the light phase for efficient activation of photosynthesis and other metabolic pathways and return to neutral levels during the dark phase remain largely unclear, especially in terms of the precise control of stromal pH. The transient increase and decrease in chloroplast pH upon dark-to-light and light-to-dark transitions have been considered as signals for controlling other biological processes in plant cells. Forward and reverse genetic screening approaches recently identified new plastid proteins involved in controlling ΔpH and ΔΨ across the thylakoid membranes and chloroplast proton/ion homeostasis. These proteins have been conserved during the evolution of oxygenic phototrophs and include putative photosynthetic protein complexes, proton transporters, and/or their regulators. Herein, we summarize the recently identified protein players that control chloroplast pH and influence photosynthetic efficiency in plants.

Functional characterization of proton antiport regulation in the thylakoid membrane

During photosynthesis, energy is transiently stored as an electrochemical proton gradient across the thylakoid membrane. The resulting proton motive force (pmf) is composed of a membrane potential (ΔΨ) and a proton concentration gradient (ΔpH) and powers the synthesis of ATP. Light energy availability for photosynthesis can change very rapidly and frequently in nature. Thylakoid ion transport proteins buffer the effects that light fluctuations have on photosynthesis by adjusting pmf and its composition. Ion channel activities dissipate ΔΨ, thereby reducing charge recombinations within photosystem II. The dissipation of ΔΨ allows for increased accumulation of protons in the thylakoid lumen, generating the signal that activates feedback downregulation of photosynthesis. Proton export from the lumen via the thylakoid K+ exchange antiporter 3 (KEA3), instead, decreases the ΔpH fraction of the pmf and thereby reduces the regulatory feedback signal. Here, we reveal that the Arabidopsis (Arabidopsis thaliana) KEA3 protein homo-dimerizes via its C-terminal domain. This C-terminus has a regulatory function, which responds to light intensity transients. Plants carrying a C-terminus-less KEA3 variant show reduced feed-back downregulation of photosynthesis and suffer from increased photosystem damage under long-term high light stress. However, during photosynthetic induction in high light, KEA3 deregulation leads to an increase in carbon fixation rates. Together, the data reveal a trade-off between long-term photoprotection and a short-term boost in carbon fixation rates, which is under the control of the KEA3 C-terminus.

Lack of plastid-encoded Ycf10, a homolog of the nuclear-encoded DLDG1 and the cyanobacterial PxcA, enhances the induction of non-photochemical quenching in tobacco

pH homeostasis in the chloroplast is crucial for the control of photosynthesis and other metabolic processes in plants. Recently, nuclear-encoded Day-Length-dependent Delayed Greening1 (DLDG1) and Fluctuating-Light Acclimation Protein1 (FLAP1) that are required for the light-inducible optimization of plastidial pH in Arabidopsis thaliana were identified. DLDG1 and FLAP1 homologs are specifically conserved in oxygenic phototrophs, and a DLDG1 homolog, Ycf10, is encoded in the chloroplast genome in plant cells. However, the function of Ycf10 and its physiological significance are unknown. To address this, we constructed ycf10 tobacco Nicotiana tabacum mutants and characterized their phenotypes. The ycf10 tobacco mutants grown under continuous-light conditions showed a pale-green phenotype only in developing leaves, and it was suppressed in short-day conditions. The ycf10 mutants also induced excessive non-photochemical quenching (NPQ) compared with those in the wild-type at the induction stage of photosynthesis. These phenotypes resemble those of Arabidopsis dldg1 mutants, suggesting that they have similar functions. However, there are distinct differences between the two mutant phenotypes: The highly induced NPQ in tobacco ycf10 and the Arabidopsis dldg1 mutants are diminished and enhanced, respectively, with increasing duration of the fluctuating actinic-light illumination. Ycf10 and DLDG1 were previously shown to localize in chloroplast envelope-membranes, suggesting that Ycf10 and DLDG1 differentially control H+ exchange across these membranes in a light-dependent manner to control photosynthesis.

Identification of sequence motifs in Lhcx proteins that confer qE-based photoprotection in the diatom Phaeodactylum tricornutum

Photosynthetic organisms in nature often experience light fluctuations. While low light conditions limit the energy uptake by algae, light absorption exceeding the maximal rate of photosynthesis may go along with enhanced formation of potentially toxic reactive oxygen species. To preempt high light-induced photodamage, photosynthetic organisms evolved numerous photoprotective mechanisms. Among these, energy-dependent fluorescence quenching (qE) provides a rapid mechanism to dissipate thermally the excessively absorbed energy. Diatoms thrive in all aquatic environments and thus belong to the most important primary producers on earth. qE in diatoms is provided by a concerted action of Lhcx proteins and the xanthophyll cycle pigment diatoxanthin. While the exact Lhcx activation mechanism of diatom qE is unknown, two lumen-exposed acidic amino acids within Lhcx proteins were proposed to function as regulatory switches upon light-induced lumenal acidification. By introducing a modified Lhcx1 lacking these amino acids into a Phaeodactylum tricornutum Lhcx1-null qE knockout line, we demonstrate that qE is unaffected by these two amino acids. Based on sequence comparisons with Lhcx4, being incapable of providing qE, we perform domain swap experiments of Lhcx4 with Lhcx1 and identify two peptide motifs involved in conferring qE. Within one of these motifs, we identify a tryptophan residue with a major influence on qE establishment. This tryptophan residue is located in close proximity to the diadinoxanthin/diatoxanthin-binding site based on the recently revealed diatom Lhc crystal structure. Our findings provide a structural explanation for the intimate link of Lhcx and diatoxanthin in providing qE in diatoms.

The Mechanism of Non-Photochemical Quenching in Plants: Localization and Driving Forces

Non-photochemical chlorophyll fluorescence quenching (NPQ) remains one of the most studied topics of the 21st century in photosynthesis research. Over the past 30 years, profound knowledge has been obtained on the molecular mechanism of NPQ in higher plants. First, the largely overlooked significance of NPQ in protecting the reaction center of photosystem II (RCII) against damage, and the ways to assess its effectiveness are highlighted. Then, the key in vivo signals that can monitor the life of the major NPQ component, qE, are presented. Finally, recent knowledge on the site of qE and the possible molecular events that transmit ΔpH into the conformational change in the major LHCII [the major trimeric light harvesting complex of photosystem II (PSII)] antenna complex are discussed. Recently, number of reports on Arabidopsis mutants lacking various antenna components of PSII confirmed that the in vivo site of qE rests within the major trimeric LHCII complex. Experiments on biochemistry, spectroscopy, microscopy and molecular modeling suggest an interplay between thylakoid membrane geometry and the dynamics of LHCII, the PsbS (PSII subunit S) protein and thylakoid lipids. The molecular basis for the qE-related conformational change in the thylakoid membrane, including the possible onset of a hydrophobic mismatch between LHCII and lipids, potentiated by PsbS protein, begins to unfold.

Curvature thylakoid 1 proteins modulate prolamellar body morphology and promote organized thylakoid biogenesis in Arabidopsis thaliana

The term "de-etiolation" refers to the light-dependent differentiation of etioplasts to chloroplasts in angiosperms. The underlying process involves reorganization of prolamellar bodies (PLBs) and prothylakoids into thylakoids, with concurrent changes in protein, lipid, and pigment composition, which together lead to the assembly of active photosynthetic complexes. Despite the highly conserved structure of PLBs among land plants, the processes that mediate PLB maintenance and their disassembly during de-etiolation are poorly understood. Among chloroplast thylakoid membrane-localized proteins, to date, only Curvature thylakoid 1 (CURT1) proteins were shown to exhibit intrinsic membrane-bending capacity. Here, we show that CURT1 proteins, which play a critical role in grana margin architecture and thylakoid plasticity, also participate in de-etiolation and modulate PLB geometry and density. Lack of CURT1 proteins severely perturbs PLB organization and vesicle fusion, leading to reduced accumulation of the light-dependent enzyme protochlorophyllide oxidoreductase (LPOR) and a delay in the onset of photosynthesis. In contrast, overexpression of CURT1A induces excessive bending of PLB membranes, which upon illumination show retarded disassembly and concomitant overaccumulation of LPOR, though without affecting greening or the establishment of photosynthesis. We conclude that CURT1 proteins contribute to the maintenance of the paracrystalline PLB morphology and are necessary for efficient and organized thylakoid membrane maturation during de-etiolation.

Robust Strategy for Photoprotection in the Light-Harvesting Antenna of Diatoms: A Molecular Dynamics Study

Diatoms generate a large portion of the oxygen produced on earth due to their exceptional light-harvesting properties involving fucoxanthin and chlorophyll-binding proteins (FCP). At the same time, an efficient adaptation of these complexes to fluctuating light conditions is necessary to protect the diatoms against photodamage. So far, structural and dynamic data for the interaction between FCP and the photoprotective LHCX family of proteins in diatoms are lacking. In this computational study, we provide a structural basis for a remarkable pH-dependent adaptation at the molecular level. Upon binding of the LHCX1 protein to the FCP complex together with a change in pH, conformational changes within the FCP protein result in a variation of the electronic coupling in a specific chlorophyll-fucoxanthin pair, leading to a change in the exciton transfer rate by almost an order of magnitude. A common strategy for photoprotection between diatoms and higher plants is identified and discussed.

Proton motive force in plant photosynthesis dominated by ΔpH in both low and high light

The proton motive force (pmf) across the thylakoid membrane couples photosynthetic electron transport and ATP synthesis. In recent years, the electrochromic carotenoid and chlorophyll absorption band shift (ECS), peaking ∼515 nm, has become a widely used probe to measure pmf in leaves. However, the use of this technique to calculate the parsing of the pmf between the proton gradient (ΔpH) and electric potential (Δψ) components remains controversial. Interpretation of the ECS signal is complicated by overlapping absorption changes associated with violaxanthin de-epoxidation to zeaxanthin (ΔA505) and energy-dependent nonphotochemical quenching (qE; ΔA535). In this study, we used Arabidopsis (Arabidopsis thaliana) plants with altered xanthophyll cycle activity and photosystem II subunit S (PsbS) content to disentangle these overlapping contributions. In plants where overlap among ΔA505, ΔA535, and ECS is diminished, such as npq4 (lacking ΔA535) and npq1npq4 (also lacking ΔA505), the parsing method implies the Δψ contribution is virtually absent and pmf is solely composed of ΔpH. Conversely, in plants where ΔA535 and ECS overlap is enhanced, such as L17 (a PsbS overexpressor) and npq1 (where ΔA535 is blue-shifted to 525 nm) the parsing method implies a dominant contribution of Δψ to the total pmf. These results demonstrate the vast majority of the pmf attributed by the ECS parsing method to Δψ is caused by ΔA505 and ΔA535 overlap, confirming pmf is dominated by ΔpH following the first 60 s of continuous illumination under both low and high light conditions. Further implications of these findings for the regulation of photosynthesis are discussed.

Growth-phase dependent morphological alteration in higher plant thylakoid is accompanied by changes in both photodamage and repair rates

Thylakoid membranes of young leaves consist of grana and stroma lamellae (stroma-grana [SG] structure). The SG thylakoid is gradually converted into isolated grana (IG), almost lacking the stroma lamellae during growth. This morphological alteration was found to cause a reduction in maximum photosynthetic rate and an enhancement of photoinhibition in photosystem II (PSII). In situ microspectrometric measurements of chlorophyll fluorescence in individual chloroplasts suggested an increase of the PSII/PSI ratio in IG thylakoids of mature leaves. Western blot analysis of isolated IG thylakoids showed relative increases in some PSII components, including the core protein (D1) and light-harvesting components CP24 and Lhcb2. Notably, a nonphotochemical quenching-related factor in the PSII supercomplex, PsbS, decreased by 40%. Changes in the high light response of PSII were detected through parameters of pulse-amplitude modulation fluorometry. Chlorophyll fluorescence lifetime indicated an increase of fluorescence quantum yield in IG. A minimal photodamage-repair rate analysis on a lincomycin treatment of the leaves indicated that repair rate constant of IG is slower than that of SG, while photodamage rate of IG is higher than that of SG. These results suggest that IG thylakoids are relatively sensitive to high light, which is not only due to a higher photodamage rate caused by some rearrangements of PS complexes, but also to the retarded PSII repair that may result from the lack of stroma lamellae. The IG thylakoids found among many plant species thus seem to be an adaptive form to low light environments, although their physiological roles still remain unclear.

Role of Thylakoid Protein Phosphorylation in Energy-Dependent Quenching of Chlorophyll Fluorescence in Rice Plants

Under natural environments, light quality and quantity are extremely varied. To respond and acclimate to such changes, plants have developed a multiplicity of molecular regulatory mechanisms. Non-photochemical quenching of chlorophyll fluorescence (NPQ) and thylakoid protein phosphorylation are two mechanisms that protect vascular plants. To clarify the role of thylakoid protein phosphorylation in energy-dependent quenching of chlorophyll fluorescence (qE) in rice plants, we used a direct Western blot assay after BN-PAGE to detect all phosphoproteins by P-Thr antibody as well as by P-Lhcb1 and P-Lhcb2 antibodies. Isolated thylakoids in either the dark- or the light-adapted state from wild type (WT) and PsbS-KO rice plants were used for this approach to detect light-dependent interactions between PsbS, PSII, and LHCII proteins. We observed that the bands corresponding to the phosphorylated Lhcb1 and Lhcb2 as well as the other phosphorylated proteins were enhanced in the PsbS-KO mutant after illumination. The qE relaxation became slower in WT plants after 10 min HL treatment, which correlated with Lhcb1 and Lhcb2 protein phosphorylation in the LHCII trimers under the same experimental conditions. Thus, we concluded that light-induced phosphorylation of PSII core and Lhcb1/Lhcb2 proteins is enhanced in rice PsbS-KO plants which might be due to more reactive-oxygen-species production in this mutant.

Downregulation of Light-Harvesting Complex II Induces ROS-Mediated Defense Against Turnip Mosaic Virus Infection in Nicotiana benthamiana

The light-harvesting chlorophyll a/b complex protein 3 (LHCB3) of photosystem II plays important roles distributing the excitation energy and modulating the rate of state transition and stomatal response to abscisic acid. However, the functions of LHCB3 in plant immunity have not been well investigated. Here, we show that the expression of LHCB3 in Nicotiana benthamiana (NbLHCB3) was down-regulated by turnip mosaic virus (TuMV) infection. When NbLHCB3 was silenced by tobacco rattle virus-induced gene silencing, systemic infection of TuMV was inhibited. H₂O₂ was over-accumulated in NbLHCB3-silenced plants. Chemical treatment to inhibit or eliminate reactive oxygen species (ROS) impaired the resistance of the NbLHCB3-silenced plants to TuMV infection. Co-silencing of NbLHCB3 with genes involved in ROS production compromised the resistance of plants to TuMV but co-silencing of NbLHCB3 with genes in the ROS scavenging pathway increased resistance to the virus. Transgenic plants overexpressing NbLHCB3 were more susceptible to TuMV. These results indicate that downregulation of NbLHCB3 is involved in defense against TuMV by inducing ROS production.

Genome-wide association study of cyanogenic glycosides, proline, sugars, and pigments in Eucalyptus cladocalyx after 18 consecutive dry summers

Natural variation of cyanogenic glycosides, soluble sugars, proline, and nondestructive optical sensing of pigments (chlorophyll, flavonols, and anthocyanins) was examined in ex situ natural populations of Eucalyptus cladocalyx F. Muell. grown under dry environmental conditions in the southern Atacama Desert, Chile. After 18 consecutive dry seasons, considerable plant-to-plant phenotypic variation for all the traits was observed in the field. For example, leaf hydrogen cyanide (HCN) concentrations varied from 0 (two acyanogenic individuals) to 1.54 mg cyanide g^-1 DW. Subsequent genome-wide association study revealed associations with several genes with a known function in plants. HCN content was associated robustly with genes encoding Cytochrome P450 proteins, and with genes involved in the detoxification mechanism of HCN in cells (β-cyanoalanine synthase and cyanoalanine nitrilase). Another important finding was that sugars, proline, and pigment content were linked to genes involved in transport, biosynthesis, and/or catabolism. Estimates of genomic heritability (based on haplotypes) ranged between 0.46 and 0.84 (HCN and proline content, respectively). Proline and soluble sugars had the highest predictive ability of genomic prediction models (PA = 0.65 and PA = 0.71, respectively). PA values for HCN content and flavonols were relatively moderate, with estimates ranging from 0.44 to 0.50. These findings provide new understanding on the genetic architecture of cyanogenic capacity, and other key complex traits in cyanogenic E. cladocalyx.

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Combined Proteomic and Physiological Analysis of Chloroplasts Reveals Drought and Recovery Response Mechanisms in Nicotiana benthamiana

Chloroplasts play essential roles in plant metabolic processes and stress responses by functioning as environmental sensors. Understanding chloroplast responses to drought stress and subsequent recovery will help the ability to improve stress tolerance in plants. Here, a combined proteomic and physiological approach was used to investigate the response mechanisms of Nicotiana benthamiana chloroplasts to drought stress and subsequent recovery. Early in the stress response, changes in stomatal movement were accompanied by immediate changes in protein synthesis to sustain the photosynthetic process. Thereafter, increasing drought stress seriously affected photosynthetic efficiency and led to altered expression of photosynthesis- and carbon-fixation-related proteins to protect the plants against photo-oxidative damage. Additional repair mechanisms were activated at the early stage of recovery to restore physiological functions and repair drought-induced damages, even while the negative effects of drought stress were still ongoing. Prolonging the re-watering period led to the gradual recovery of photosynthesis at both physiological and protein levels, indicating that a long repair process is required to restore plant function. Our findings provide a precise view of drought and recovery response mechanisms in N. benthamiana and serve as a reference for further investigation into the physiological and molecular mechanisms underlying plant drought tolerance.

An underlying mechanism of qE deficiency in marine angiosperm Zostera marina

Non-photochemical quenching (NPQ) of photosystem II (PSII) fluorescence is one of the most important protective mechanisms enabling the survival of phototropic organisms under high-light conditions. A low-efficiency NPQ, characterized by weak NPQ induction capacity and a low level of protective NPQ, was observed in the marine angiosperm Zostera marina, which inhabits the shallow water regions. Furthermore, chlorophyll fluorescence and Western blot analysis verified that the fast-inducted component of NPQ, i.e., the energy-dependent quenching (qE), was not present in this species. In contrast with the lack of PSII antenna quenching sites for qE induction in brown algae and the lack of functional XC in Ulvophyceae belonging to green algae, all the antenna proteins and the functional XC are present in Z. marina. A novel underlying mechanism was observed that the limited construction of the trans-thylakoid proton gradient (ΔpH) caused by photoinactivation of the oxygen evolving complex (OEC) did not induce protonation of PsbS, thus explaining the inability to form quenching sites for qE induction. Although the ΔpH established under light exposure activated violaxanthin (V) de-epoxidase enzyme to catalyze conversion of V via antheraxanthin (A) and then to zeaxanthin (Z), the quenching capacity of de-epoxidized pigment was weak in Z. marina. We suggest that the low-efficiency NPQ was conducive to efficiently utilize the limited electrons to perform photosynthesis, resisting the adverse effect of OEC photoinactivation on the photosynthetic rate.

Photosynthetic resistance and resilience under drought, flooding and rewatering in maize plants

Abnormally altered precipitation patterns induced by climate change have profound global effects on crop production. However, the plant functional responses to various precipitation regimes remain unclear. Here, greenhouse and field experiments were conducted to determine how maize plant functional traits respond to drought, flooding and rewatering. Drought and flooding hampered photosynthetic capacity, particularly when severe and/or prolonged. Most photosynthetic traits recovered after rewatering, with few compensatory responses. Rewatering often elicited high photosynthetic resilience in plants exposed to severe drought at the end of plant development, with the response strongly depending on the drought severity/duration. The associations of chlorophyll concentrations with photosynthetically functional activities were stronger during post-tasseling than pre-tasseling, implying an involvement of leaf age/senescence in responses to episodic drought and subsequent rewatering. Coordinated changes in chlorophyll content, gas exchange, fluorescence parameters (PSII quantum efficiency and photochemical/non-photochemical radiative energy dissipation) possibly contributed to the enhanced drought resistance and resilience and suggested a possible regulative trade-off. These findings provide fundamental insights into how plants regulate their functional traits to deal with sporadic alterations in precipitation. Breeding and management of plants with high resistance and resilience traits could help crop production under future climate change.

The molecular pH-response mechanism of the plant light-stress sensor PsbS

Plants need to protect themselves from excess light, which causes photo-oxidative damage and lowers the efficiency of photosynthesis. Photosystem II subunit S (PsbS) is a pH sensor protein that plays a crucial role in plant photoprotection by detecting thylakoid lumen acidification in excess light conditions via two lumen-faced glutamates. However, how PsbS is activated under low-pH conditions is unknown. To reveal the molecular response of PsbS to low pH, here we perform an NMR, FTIR and 2DIR spectroscopic analysis of Physcomitrella patens PsbS and of the E176Q mutant in which an active glutamate has been replaced. The PsbS response mechanism at low pH involves the concerted action of repositioning of a short amphipathic helix containing E176 facing the lumen and folding of the luminal loop fragment adjacent to E71 to a 310-helix, providing clear evidence of a conformational pH switch. We propose that this concerted mechanism is a shared motif of proteins of the light-harvesting family that may control thylakoid inter-protein interactions driving photoregulatory responses.

The PsbS protein and low pH are necessary and sufficient to induce quenching in the light-harvesting complex of plants LHCII

Photosynthesis is tightly regulated in order to withstand dynamic light environments. Under high light intensities, a mechanism known as non-photochemical quenching (NPQ) dissipates excess excitation energy, protecting the photosynthetic machinery from damage. An obstacle that lies in the way of understanding the molecular mechanism of NPQ is the large gap between in vitro and in vivo studies. On the one hand, the complexity of the photosynthetic membrane makes it challenging to obtain molecular information from in vivo experiments. On the other hand, a suitable in vitro system for the study of quenching is not available. Here we have developed a minimal NPQ system using proteoliposomes. With this, we demonstrate that the combination of low pH and PsbS is both necessary and sufficient to induce quenching in LHCII, the main antenna complex of plants. This proteoliposome system can be further exploited to gain more insight into how PsbS and other factors (e.g. zeaxanthin) influence the quenching mechanism observed in LHCII.

A novel method produces native light-harvesting complex II aggregates from the photosynthetic membrane revealing their role in nonphotochemical quenching

Nonphotochemical quenching (NPQ) is a mechanism of regulating light harvesting that protects the photosynthetic apparatus from photodamage by dissipating excess absorbed excitation energy as heat. In higher plants, the major light-harvesting antenna complex (LHCII) of photosystem (PS) II is directly involved in NPQ. The aggregation of LHCII is proposed to be involved in quenching. However, the lack of success in isolating native LHCII aggregates has limited the direct interrogation of this process. The isolation of LHCII in its native state from thylakoid membranes has been problematic because of the use of detergent, which tends to dissociate loosely bound proteins, and the abundance of pigment-protein complexes (e.g. PSI and PSII) embedded in the photosynthetic membrane, which hinders the preparation of aggregated LHCII. Here, we used a novel purification method employing detergent and amphipols to entrap LHCII in its natural states. To enrich the photosynthetic membrane with the major LHCII, we used Arabidopsis thaliana plants lacking the PSII minor antenna complexes (NoM), treated with lincomycin to inhibit the synthesis of PSI and PSII core proteins. Using sucrose density gradients, we succeeded in isolating the trimeric and aggregated forms of LHCII antenna. Violaxanthin- and zeaxanthin-enriched complexes were investigated in dark-adapted, NPQ, and dark recovery states. Zeaxanthin-enriched antenna complexes showed the greatest amount of aggregated LHCII. Notably, the amount of aggregated LHCII decreased upon relaxation of NPQ. Employing this novel preparative method, we obtained a direct evidence for the role of in vivo LHCII aggregation in NPQ.

Recent advances in understanding and improving photosynthesis

Since 1893, when the word "photosynthesis" was first coined by Charles Reid Barnes and Conway MacMillan, our understanding of the elements and regulation of this complex process is far from being entirely understood. We aim to review the most relevant advances in photosynthesis research from the last few years and to provide a perspective on the forthcoming research in this field. Recent discoveries related to light sensing, harvesting, and dissipation; kinetics of CO₂ fixation; components and regulators of CO₂ diffusion through stomata and mesophyll; and genetic engineering for improving photosynthetic and production capacities of crops are addressed.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

Identification of distinct pH- and zeaxanthin-dependent quenching in LHCSR3 from Chlamydomonas reinhardtii

Under high light, oxygenic photosynthetic organisms avoid photodamage by thermally dissipating absorbed energy, which is called nonphotochemical quenching. In green algae, a chlorophyll and carotenoid-binding protein, light-harvesting complex stress-related (LHCSR3), detects excess energy via a pH drop and serves as a quenching site. Using a combined in vivo and in vitro approach, we investigated quenching within LHCSR3 from Chlamydomonas reinhardtii. In vitro two distinct quenching processes, individually controlled by pH and zeaxanthin, were identified within LHCSR3. The pH-dependent quenching was removed within a mutant LHCSR3 that lacks the residues that are protonated to sense the pH drop. Observation of quenching in zeaxanthin-enriched LHCSR3 even at neutral pH demonstrated zeaxanthin-dependent quenching, which also occurs in other light-harvesting complexes. Either pH- or zeaxanthin-dependent quenching prevented the formation of damaging reactive oxygen species, and thus the two quenching processes may together provide different induction and recovery kinetics for photoprotection in a changing environment.

Green plants and algae rely on sunlight to transform light energy into chemical energy in a process known as photosynthesis. However, too much light can damage plants. Green plants prevent this by converting the extra absorbed light into heat. Both the absorption and the dissipation of sunlight into heat occur within so called light harvesting complexes. These are protein structures that contain pigments such as chlorophyll and carotenoids.

The process of photoprotection starts when the excess of absorbed light generates protons (elementary particles with a positive charge) faster than they can be used. This causes a change in the pH (a measure of the concentration of protons in a solution), which in turn, modifies the shape of proteins and the chemical identity of the carotenoids. However, it is still unclear what the exact mechanisms are.

To clarify this, Troiano, Perozeni et al. engineered the light harvesting complex LHCSR3 of the green algae Chlamydomonas reinhardtii to create mutants that either could not sense changes in the pH or contained the carotenoid zeaxanthin. Zeaxanthin is one of the main carotenoids accumulated by plants and algae upon high light stress. Measurements showed that both pH detection and zeaxanthin were able to provide photoprotection independently. Troiano, Perozeni et al. further found that pH and carotenoids controlled changes to the organisation of the pigment at two separate locations within the LHCSR3, which influenced whether the protein was able to prevent photodamage. When algae were unable to change pH or carotenoids, dissipation was less effective. Instead, specific molecules were produced that damage the cellular machinery.

The results shed light onto how green algae protect themselves from too much light exposure. These findings could pave the way for optimising dissipation, which could increase yields of green algae by up to 30%. This could lead to green algae becoming a viable alternative for food, biofuels and feedstock.