Ion antiport accelerates photosynthetic acclimation in fluctuating light environments

Redox regulation in chloroplast thylakoid lumen: The pmf changes everything, again

Photosynthesis is the foundation of life on Earth. However, if not well regulated, it can also generate excessive reactive oxygen species (ROS), which can cause photodamage. Regulation of photosynthesis is highly dynamic, responding to both environmental and metabolic cues, and occurs at many levels, from light capture to energy storage and metabolic processes. One general mechanism of regulation involves the reversible oxidation and reduction of protein thiol groups, which can affect the activity of enzymes and the stability of proteins. Such redox regulation has been well studied in stromal enzymes, but more recently, evidence has emerged of redox control of thylakoid lumenal enzymes. This review/hypothesis paper summarizes the latest research and discusses several open questions and challenges to achieving effective redox control in the lumen, focusing on the distinct environments and regulatory components of the thylakoid lumen, including the need to transport electrons across the thylakoid membrane, the effects of pH changes by the proton motive force (pmf) in the stromal and lumenal compartments, and the observed differences in redox states. These constraints suggest that activated oxygen species are likely to be major regulatory contributors to lumenal thiol redox regulation, with key components and processes yet to be discovered.

A rapid aureochrome opto-switch enables diatom acclimation to dynamic light

Diatoms often outnumber other eukaryotic algae in the oceans, especially in coastal environments characterized by frequent fluctuations in light intensity. The identities and operational mechanisms of regulatory factors governing diatom acclimation to high light stress remain largely elusive. Here, we identified the AUREO1c protein from the coastal diatom Phaeodactylum tricornutum as a crucial regulator of non-photochemical quenching (NPQ), a photoprotective mechanism that dissipates excess energy as heat. AUREO1c detects light stress using a light-oxygen-voltage (LOV) domain and directly activates the expression of target genes, including LI818 genes that encode NPQ effector proteins, via its bZIP DNA-binding domain. In comparison to a kinase-mediated pathway reported in the freshwater green alga Chlamydomonas reinhardtii, the AUREO1c pathway exhibits a faster response and enables accumulation of LI818 transcript and protein levels to comparable degrees between continuous high-light and fluctuating-light treatments. We propose that the AUREO1c-LI818 pathway contributes to the resilience of diatoms under dynamic light conditions.

Shining light on diurnal variation of non-photochemical quenching: Impact of gradual light intensity patterns on short-term NPQ over a day

Maximal sunlight intensity varies diurnally due to the earth's rotation. Whether this slow diurnal pattern influences the photoprotective capacity of plants throughout the day is unknown. We investigated diurnal variation in NPQ, along with NPQ capacity, induction, and relaxation kinetics after transitions to high light, in tomato plants grown under diurnal parabolic (DP) or constant (DC) light intensity regimes. DP light intensity peaked at midday (470 μmol m-2 s-1) while DC stayed constant at 300 μmol m-2 s-1 at a similar 12-hour photoperiod and daily light integral. NPQs were higher in the morning and afternoon at lower light intensities in DP compared to DC, except shortly after dawn. NPQ capacity increased from midday to the end of the day, with higher values in DP than in DC. At high light ΦPSII did not vary throughout the day, while ΦNPQ varied consistently with NPQ capacity. Reduced ΦNO suggested less susceptibility to photodamage at the end of the day. NPQ induction was faster at midday than at the start of the day and in DC than in DP, with overshoot occurring in the morning and midday but not at the end of the day. NPQ relaxation was faster in DP than in DC. The xanthophyll de-epoxidation state and reduced demand for photochemistry could not explain the observed diurnal variations in photoprotective capacity. In conclusion, this study showed diurnal variation in regulated photoprotective capacity at moderate growth light intensity, which was not explained by instantaneous light intensity or increasing photoinhibition over the day and was influenced by acclimation to constant light intensity.

Chloroplast ion homeostasis - what do we know and where should we go?

Plant yields heavily depend on proper macro- and micronutrient supply from the soil. In the leaf cells, nutrient ions fulfill specific roles in biochemical reactions, especially photosynthesis housed in the chloroplast. Here, a well-balanced ion homeostasis is maintained by a number of ion transport proteins embedded in the envelope and thylakoid membranes. Ten years ago, the first alkali metal transporters from the K+ EFFLUX ANTIPORTER family were discovered in the model plant Arabidopsis. Since then, our knowledge about the physiological importance of these carriers and their substrates has greatly expanded. New insights into the role of alkali ions in plastid gene expression and photoprotective mechanisms, both prerequisites for plant productivity in natural environments, were gained. The discovery of a Cl- channel in the thylakoid and several additional plastid alkali and alkali metal transport proteins have advanced the field further. Nevertheless, scientists still have long ways to go before a complete systemic understanding of the chloroplast's ion transportome will emerge. In this Tansley review, we highlight and discuss the achievements of the last decade. More importantly, we make recommendations on what areas to prioritize, so the field can reach the next milestones. One area, laid bare by our similarity-based comparisons among phototrophs is our lack of knowledge what ion transporters are used by cyanobacteria to buffer photosynthesis fluctuations.

Structure and mechanism of the K+/H+ exchanger KefC

Intracellular potassium (K+) homeostasis is fundamental to cell viability. In addition to channels, K+ levels are maintained by various ion transporters. One major family is the proton-driven K+ efflux transporters, which in gram-negative bacteria is important for detoxification and in plants is critical for efficient photosynthesis and growth. Despite their importance, the structure and molecular basis for K+-selectivity is poorly understood. Here, we report ~3.1 Å resolution cryo-EM structures of the Escherichia coli glutathione (GSH)-gated K+ efflux transporter KefC in complex with AMP, AMP/GSH and an ion-binding variant. KefC forms a homodimer similar to the inward-facing conformation of Na+/H+ antiporter NapA. By structural assignment of a coordinated K+ ion, MD simulations, and SSM-based electrophysiology, we demonstrate how ion-binding in KefC is adapted for binding a dehydrated K+ ion. KefC harbors C-terminal regulator of K+ conductance (RCK) domains, as present in some bacterial K+-ion channels. The domain-swapped helices in the RCK domains bind AMP and GSH and they inhibit transport by directly interacting with the ion-transporter module. Taken together, we propose that KefC is activated by detachment of the RCK domains and that ion selectivity exploits the biophysical properties likewise adapted by K+-ion-channels.

Low-CO2-inducible bestrophins outside the pyrenoid sustain high photosynthetic efficacy in diatoms

Anion transporters sustain a variety of physiological states in cells. Bestrophins (BSTs) belong to a Cl- and/or HCO3- transporter family conserved in bacteria, animals, algae, and plants. Recently, putative BSTs were found in the green alga Chlamydomonas reinhardtii, where they are upregulated under low CO2 (LC) conditions and play an essential role in the CO2-concentrating mechanism (CCM). The putative BST orthologs are also conserved in diatoms, secondary endosymbiotic algae harboring red-type plastids, but their physiological functions are unknown. Here, we characterized the subcellular localization and expression profile of BSTs in the marine diatoms Phaeodactylum tricornutum (PtBST1 to 4) and Thalassiosira pseudonana (TpBST1 and 2). PtBST1, PtBST2, and PtBST4 were localized at the stroma thylakoid membrane outside of the pyrenoid, and PtBST3 was localized in the pyrenoid. Contrarily, TpBST1 and TpBST2 were both localized in the pyrenoid. These BST proteins accumulated in cells grown in LC but not in 1% CO2 (high CO2 [HC]). To assess the physiological functions, we generated knockout mutants for the PtBST1 gene by genome editing. The lack of PtBST1 decreased photosynthetic affinity for dissolved inorganic carbon to the level comparable with the HC-grown wild type. Furthermore, non-photochemical quenching in LC-grown cells was 1.5 to 2.0 times higher in the mutants than in the wild type. These data suggest that HCO3- transport at the stroma thylakoid membranes by PtBST1 is a critical part of the CO2-evolving machinery of the pyrenoid in the fully induced CCM and that PtBST1 may modulate photoprotection under CO2-limited environments in P. tricornutum.

Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts

The proton motive force (pmf) generated across the thylakoid membrane rotates the Fo-ring of ATP synthase in chloroplasts. The pmf comprises two components: membrane potential (∆Ψ) and proton concentration gradient (∆pH). Acidification of the thylakoid lumen resulting from ∆pH downregulates electron transport in the cytochrome b6f complex. This process, known as photosynthetic control, is crucial for protecting photosystem I (PSI) from photodamage in response to fluctuating light. To optimize the balance between efficient photosynthesis and photoprotection, it is necessary to regulate pmf. Cyclic electron transport around PSI and pseudo-cyclic electron transport involving flavodiiron proteins contribute to the modulation of pmf magnitude. By manipulating the ratio between the two components of pmf, it is possible to modify the extent of photosynthetic control without affecting the pmf size. This adjustment can be achieved by regulating the movement of ions (such as K+ and Cl-) across the thylakoid membrane. Since ATP synthase is the primary consumer of pmf in chloroplasts, its activity must be precisely regulated to accommodate other mechanisms involved in pmf optimization. Although fragments of information about each regulatory process have been accumulated, a comprehensive understanding of their interactions is lacking. Here, I summarize current knowledge of the network for pmf regulation, mainly based on genetic studies.

Sensitivity and responses of chloroplasts to salt stress in plants

Chloroplast, the site for photosynthesis and various biochemical reactions, is subject to many environmental stresses including salt stress, which affects chloroplast structure, photosynthetic processes, osmotic balance, ROS homeostasis, and so on. The maintenance of normal chloroplast function is essential for the survival of plants. Plants have developed different mechanisms to cope with salt-induced toxicity on chloroplasts to ensure the normal function of chloroplasts. The salt tolerance mechanism is complex and varies with plant species, so many aspects of these mechanisms are not entirely clear yet. In this review, we explore the effect of salinity on chloroplast structure and function, and discuss the adaptive mechanisms by which chloroplasts respond to salt stress. Understanding the sensitivity and responses of chloroplasts to salt stress will help us understand the important role of chloroplasts in plant salt stress adaptation and lay the foundation for enhancing plant salt tolerance.

The thylakoid proton antiporter KEA3 regulates photosynthesis in response to the chloroplast energy status

Plant photosynthesis contains two functional modules, the light-driven reactions in the thylakoid membrane and the carbon-fixing reactions in the chloroplast stroma. In nature, light availability for photosynthesis often undergoes massive and rapid fluctuations. Efficient and productive use of such variable light supply requires an instant crosstalk and rapid synchronization of both functional modules. Here, we show that this communication involves the stromal exposed C-terminus of the thylakoid K+-exchange antiporter KEA3, which regulates the ΔpH across the thylakoid membrane and therefore pH-dependent photoprotection. By combining in silico, in vitro, and in vivo approaches, we demonstrate that the KEA3 C-terminus senses the energy state of the chloroplast in a pH-dependent manner and regulates transport activity in response. Together our data pinpoint a regulatory feedback loop by which the stromal energy state orchestrates light capture and photoprotection via multi-level regulation of KEA3.

Dynamics of mRNA fate during light stress and recovery: from transcription to stability and translation

Transcript stability is an important determinant of its abundance and, consequently, translational output. Transcript destabilisation can be rapid and is well suited for modulating the cellular response. However, it is unclear the extent to which RNA stability is altered under changing environmental conditions in plants. We previously hypothesised that recovery-induced transcript destabilisation facilitated a phenomenon of rapid recovery gene downregulation (RRGD) in Arabidopsis thaliana (Arabidopsis) following light stress, based on mathematical calculations to account for ongoing transcription. Here, we test this hypothesis and investigate processes regulating transcript abundance and fate by quantifying changes in transcription, stability and translation before, during and after light stress. We adapt syringe infiltration to apply a transcriptional inhibitor to soil-grown plants in combination with stress treatments. Compared with measurements in juvenile plants and cell culture, we find reduced stability across a range of transcripts encoding proteins involved in RNA binding and processing. We also observe light-induced destabilisation of transcripts, followed by their stabilisation during recovery. We propose that this destabilisation facilitates RRGD, possibly in combination with transcriptional shut-off that was confirmed for HSP101, ROF1 and GOLS1. We also show that translation remains highly dynamic over the course of light stress and recovery, with a bias towards transcript-specific increases in ribosome association, independent of changes in total transcript abundance, after 30 min of light stress. Taken together, we provide evidence for the combinatorial regulation of transcription and stability that occurs to coordinate translation during light stress and recovery in Arabidopsis.

Growth in fluctuating light buffers plants against photorespiratory perturbations

Photorespiration (PR) is the pathway that detoxifies the product of the oxygenation reaction of Rubisco. It has been hypothesized that in dynamic light environments, PR provides a photoprotective function. To test this hypothesis, we characterized plants with varying PR enzyme activities under fluctuating and non-fluctuating light conditions. Contrasting our expectations, growth of mutants with decreased PR enzyme levels was least affected in fluctuating light compared with wild type. Results for growth, photosynthesis and metabolites combined with thermodynamics-based flux analysis revealed two main causal factors for this unanticipated finding: reduced rates of photosynthesis in fluctuating light and complex re-routing of metabolic fluxes. Only in non-fluctuating light, mutants lacking the glutamate:glyoxylate aminotransferase 1 re-routed glycolate processing to the chloroplast, resulting in photooxidative damage through H2O2 production. Our results reveal that dynamic light environments buffer plant growth and metabolism against photorespiratory perturbations.

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Eukaryote-specific assembly factor DEAP2 mediates an early step of photosystem II assembly in Arabidopsis

The initial step of oxygenic photosynthesis is the thermodynamically challenging extraction of electrons from water and the release of molecular oxygen. This light-driven process, which is the basis for most life on Earth, is catalyzed by photosystem II (PSII) within the thylakoid membrane of photosynthetic organisms. The biogenesis of PSII requires a controlled step-wise assembly process of which the early steps are considered to be highly conserved between plants and their cyanobacterial progenitors. This assembly process involves auxiliary proteins, which are likewise conserved. In the present work, we used Arabidopsis (Arabidopsis thaliana) as a model to show that in plants, a eukaryote-exclusive assembly factor facilitates the early assembly step, during which the intrinsic antenna protein CP47 becomes associated with the PSII reaction center (RC) to form the RC47 intermediate. This factor, which we named DECREASED ELECTRON TRANSPORT AT PSII (DEAP2), works in concert with the conserved PHOTOSYNTHESIS AFFECTED MUTANT 68 (PAM68) assembly factor. The deap2 and pam68 mutants showed similar defects in PSII accumulation and assembly of the RC47 intermediate. The combined lack of both proteins resulted in a loss of functional PSII and the inability of plants to grow photoautotrophically on the soil. While overexpression of DEAP2 partially rescued the pam68 PSII accumulation phenotype, this effect was not reciprocal. DEAP2 accumulated at 20-fold higher levels than PAM68, together suggesting that both proteins have distinct functions. In summary, our results uncover eukaryotic adjustments to the PSII assembly process, which involve the addition of DEAP2 for the rapid progression from RC to RC47.

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.

FZL, a dynamin-like protein localized to curved grana edges, is required for efficient photosynthetic electron transfer in Arabidopsis

Photosynthetic electron transfer and its regulation processes take place on thylakoid membranes, and the thylakoid of vascular plants exhibits particularly intricate structure consisting of stacked grana and flat stroma lamellae. It is known that several membrane remodeling proteins contribute to maintain the thylakoid structure, and one putative example is FUZZY ONION LIKE (FZL). In this study, we re-evaluated the controversial function of FZL in thylakoid membrane remodeling and in photosynthesis. We investigated the sub-membrane localization of FZL and found that it is enriched on curved grana edges of thylakoid membranes, consistent with the previously proposed model that FZL mediates fusion of grana and stroma lamellae at the interfaces. The mature fzl thylakoid morphology characterized with the staggered and less connected grana seems to agree with this model as well. In the photosynthetic analysis, the fzl knockout mutants in Arabidopsis displayed reduced electron flow, likely resulting in higher oxidative levels of Photosystem I (PSI) and smaller proton motive force (pmf). However, nonphotochemical quenching (NPQ) of chlorophyll fluorescence was excessively enhanced considering the pmf levels in fzl, and we found that introducing kea3-1 mutation, lowering pH in thylakoid lumen, synergistically reinforced the photosynthetic disorder in the fzl mutant background. We also showed that state transitions normally occurred in fzl, and that they were not involved in the photosynthetic disorders in fzl. We discuss the possible mechanisms by which the altered thylakoid morphology in fzl leads to the photosynthetic modifications.

STN7 is not essential for developmental acclimation of Arabidopsis to light intensity

Plants respond to changing light intensity in the short term through regulation of light harvesting, electron transfer, and metabolism to mitigate redox stress. A sustained shift in light intensity leads to a long-term acclimation response (LTR). This involves adjustment in the stoichiometry of photosynthetic complexes through de novo synthesis and degradation of specific proteins associated with the thylakoid membrane. The light-harvesting complex II (LHCII) serine/threonine kinase STN7 plays a key role in short-term light harvesting regulation and was also suggested to be crucial to the LTR. Arabidopsis plants lacking STN7 (stn7) shifted to low light experience higher photosystem II (PSII) redox pressure than the wild type or those lacking the cognate phosphatase TAP38 (tap38), while the reverse is true at high light, where tap38 suffers more. In principle, the LTR should allow optimisation of the stoichiometry of photosynthetic complexes to mitigate these effects. We used quantitative label-free proteomics to assess how the relative abundance of photosynthetic proteins varied with growth light intensity in wild-type, stn7, and tap38 plants. All plants were able to adjust photosystem I, LHCII, cytochrome b6 f, and ATP synthase abundance with changing white light intensity, demonstrating neither STN7 nor TAP38 is crucial to the LTR per se. However, stn7 plants grown for several weeks at low light (LL) or moderate light (ML) still showed high PSII redox pressure and correspondingly lower PSII efficiency, CO2 assimilation, and leaf area compared to wild-type and tap38 plants, hence the LTR is unable to fully ameliorate these symptoms. In contrast, under high light growth conditions the mutants and wild type behaved similarly. These data are consistent with the paramount role of STN7-dependent LHCII phosphorylation in tuning PSII redox state for optimal growth in LL and ML conditions.

Impact of engineering the ATP synthase rotor ring on photosynthesis in tobacco chloroplasts

The chloroplast ATP synthase produces the ATP needed for photosynthesis and plant growth. The trans-membrane flow of protons through the ATP synthase rotates an oligomeric assembly of c subunits, the c-ring. The ion-to-ATP ratio in rotary F1F0-ATP synthases is defined by the number of c-subunits in the rotor c-ring. Engineering the c-ring stoichiometry is, therefore, a possible route to manipulate ATP synthesis by the ATP synthase and hence photosynthetic efficiency in plants. Here, we describe the construction of a tobacco (Nicotiana tabacum) chloroplast atpH (chloroplastic ATP synthase subunit c gene) mutant in which the c-ring stoichiometry was increased from 14 to 15 c-subunits. Although the abundance of the ATP synthase was decreased to 25% of wild-type (WT) levels, the mutant lines grew as well as WT plants and photosynthetic electron transport remained unaffected. To synthesize the necessary ATP for growth, we found that the contribution of the membrane potential to the proton motive force was enhanced to ensure a higher proton flux via the c15-ring without unwanted low pH-induced feedback inhibition of electron transport. Our work opens avenues to manipulate plant ion-to-ATP ratios with potentially beneficial consequences for photosynthesis.

The Potassium Efflux System Kef: Bacterial Protection against Toxic Electrophilic Compounds

Kef couples the potassium efflux with proton influx in gram-negative bacteria. The resulting acidification of the cytosol efficiently prevents the killing of the bacteria by reactive electrophilic compounds. While other degradation pathways for electrophiles exist, Kef is a short-term response that is crucial for survival. It requires tight regulation since its activation comes with the burden of disturbed homeostasis. Electrophiles, entering the cell, react spontaneously or catalytically with glutathione, which is present at high concentrations in the cytosol. The resulting glutathione conjugates bind to the cytosolic regulatory domain of Kef and trigger activation while the binding of glutathione keeps the system closed. Furthermore, nucleotides can bind to this domain for stabilization or inhibition. The binding of an additional ancillary subunit, called KefF or KefG, to the cytosolic domain is required for full activation. The regulatory domain is termed K⁺ transport-nucleotide binding (KTN) or regulator of potassium conductance (RCK) domain, and it is also found in potassium uptake systems or channels in other oligomeric arrangements. Bacterial RosB-like transporters and K⁺ efflux antiporters (KEA) of plants are homologs of Kef but fulfill different functions. In summary, Kef provides an interesting and well-studied example of a highly regulated bacterial transport system.

Comparative Transcriptome Profiling Reveals Key MicroRNAs and Regulatory Mechanisms for Aluminum Tolerance in Olive

Aluminum toxicity (Al) is one of the major constraints to crop production in acidic soils. MicroRNAs (miRNAs) have emerged as key regulatory molecules at post-transcriptional levels, playing crucial roles in modulating various stress responses in plants. However, miRNAs and their target genes conferring Al tolerance are poorly studied in olive (Olea europaea L.). Here, genome-wide expression changes in miRNAs of the roots from two contrasting olive genotypes Zhonglan (ZL, Al-tolerant) and Frantoio selezione (FS, Al-sensitive) were investigated by high-throughput sequencing approaches. A total of 352 miRNAs were discovered in our dataset, consisting of 196 conserved miRNAs and 156 novel miRNAs. Comparative analyses showed 11 miRNAs have significantly different expression patterns in response to Al stress between ZL and FS. In silico prediction identified 10 putative target gene of these miRNAs, including MYB transcription factors, homeobox-leucine zipper (HD-Zip) proteins, auxin response factors (ARF), ATP-binding cassette (ABC) transporters and potassium efflux antiporter. Further functional classification and enrichment analysis revealed these Al-tolerance associated miRNA-mRNA pairs are mainly involved in transcriptional regulation, hormone signaling, transportation and metabolism. These findings provide new information and perspectives into the regulatory roles of miRNAs and their target for enhancing Al tolerance in olives.

Temperature mapping of non-photochemical quenching in Chlorella vulgaris

Light intensity and temperature independently impact all parts of the photosynthetic machinery in plants and algae. Yet to date, the vast majority of pulse amplitude modulated (PAM) chlorophyll a fluorescence measurements have been performed at well-defined light intensities, but rarely at well-defined temperatures. In this work, we show that PAM measurements performed at various temperatures produce vastly different results in the chlorophyte Chlorella vulgaris. Using a recently developed Phenoplate technique to map quantum yield of Photosystem II (Y(II)) and non-photochemical quenching (NPQ) as a function of temperature, we show that the fast-relaxing NPQ follows an inverse normal distribution with respect to temperature and appears insensitive to previous temperature acclimation. The slow-relaxing or residual NPQ after 5 minutes of dark recovery follows a normal distribution similar to Y(II) but with a peak in the higher temperature range. Surprisingly, higher slow- and fast-relaxing NPQ values were observed in high-light relative to low-light acclimated cultures. Y(II) values peaked at the adaptation temperature regardless of temperature or light acclimation. Our novel findings show the complete temperature working spectrum of Y(II) and how excess energy quenching is managed across a wide range of temperatures in the model microalgal species C. vulgaris. Finally, we draw attention to the fact that the effect of the temperature component in PAM measurements has been wildly underestimated, and results from experiments at room temperature can be misleading.

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.

Year-round sustainable biomass production potential of Nannochloris sp. in outdoor raceway pond enabled through strategic photobiological screening

Microalgae cultivation utilizes the energy of sunlight to reduce carbon dioxide (CO₂) for producing renewable energy feedstock. The commercial success of the biological fixation of carbon in a consistent manner depends upon the availability of a robust microalgae strain. In the present work, we report the identification of a novel marine Nannochloris sp. through multiparametric photosynthetic evaluation. Detailed photobiological analysis of this strain has revealed a smaller functional antenna, faster relaxation kinetics of non-photochemical quenching, and a high photosynthetic rate with increasing light and temperatures. Furthermore, laboratory scale growth assessment demonstrated a broad range halotolerance of 10-70 parts per thousand (PPT) and high-temperature tolerance up to 45 °C. Such traits led to the translation of biomass productivity potential from the laboratory scale (0.2-3.0 L) to the outdoor 50,000 L raceway pond scale (500-m²) without any pond crashes. The current investigation revealed outdoor single-day peak areal biomass productivity of 43 g m^-2 d^-1 in summer with an annual (March 2019-February 2020) average productivity of 20 g m^-2 d^-1 in seawater. From a sustainability perspective, this is the first report of successful round-the-year (> 347 days) multi-season (summer, monsoon, and winter) outdoor cultivation of Nannochloris sp. in broad seawater salinity (1-57 PPT), wide temperature ranges (15-40 °C), and in fluctuating light conditions. Concurrently, outdoor cultivation of this strain demonstrated conducive fatty acid distribution, including increased unsaturated fatty acids in winter. This inherent characteristic might play a role in protecting photosynthesis machinery at low temperatures and in high light stress. Altogether, our marine Nannochloris sp. showed tremendous potential for commercial scale cultivation to produce biofuels, food ingredients, and a sustainable source for vegetarian protein.

Transcriptomic and metabolic signatures of diatom plasticity to light fluctuations

Unlike in terrestrial and freshwater ecosystems, light fields in oceans fluctuate due to both horizontal current and vertical mixing. Diatoms thrive and dominate the phytoplankton community in these fluctuating light fields. However, the molecular mechanisms that regulate diatom acclimation and adaptation to light fluctuations are poorly understood. Here, we performed transcriptome sequencing, metabolome profiling, and 13C-tracer labeling on the model diatom Phaeodactylum tricornutum. The diatom acclimated to constant light conditions was transferred to six different light conditions, including constant light (CL5d), short-term (1 h) high light (sHL1h), and short-term (1 h) and long-term (5 days) mild or severe light fluctuation conditions (mFL1h, sFL1h, mFL5d, and sFL5d) that mimicked land and ocean light levels. We identified 2,673 transcripts (25% of the total expressed genes) expressed differentially under different fluctuating light regimes. We also identified 497 transcription factors, 228 not reported previously, which exhibited higher expression under light fluctuations, including 7 with a light-sensitive PAS domain (Per-period circadian protein, Arnt-aryl hydrocarbon receptor nuclear translocator protein, Sim-single-minded protein) and 10 predicted to regulate genes related to light-harvesting complex proteins. Our data showed that prolonged preconditioning in severe light fluctuation enhanced photosynthesis in P. tricornutum under this condition, as evidenced by increased oxygen evolution accompanied by the upregulation of Rubisco and light-harvesting proteins. Furthermore, severe light fluctuation diverted the metabolic flux of assimilated carbon preferentially toward fatty acid storage over sugar and protein. Our results suggest that P. tricornutum use a series of complex and different responsive schemes in photosynthesis and carbon metabolism to optimize their growth under mild and severe light fluctuations. These insights underscore the importance of using more intense conditions when investigating the resilience of phytoplankton to light fluctuations.

Potassium in plant physiological adaptation to abiotic stresses

Potassium (K) is an integral part of plant nutrition, playing essential roles in plant growth and development. Despite its abundance in soils, the limitedly available form of K ion (K⁺) for plant uptake is a critical factor for agricultural production. Plants have evolved complex transport systems to maintain appropriate K⁺ levels in tissues under changing environmental conditions. Adequate stimulation and coordinated actions of multiple K⁺-channels and K⁺-transporters are required for nutrient homeostasis, reproductive growth, cellular signaling and stress adaptation responses in plants. Various contemporary studies revealed that K⁺-homeostasis plays a substantial role in plant responses and tolerance to abiotic stresses. The beneficial effects of K⁺ in plant responses to abiotic stresses include its roles in physiological and biochemical mechanisms involved in photosynthesis, osmoprotection, stomatal regulation, water-nutrient absorption, nutrient translocation and enzyme activation. Over the last decade, we have seen considerable breakthroughs in K research, owing to the advances in omics technologies. In this aspect, omics investigations (e.g., transcriptomics, metabolomics, and proteomics) in systems biology manner have broadened our understanding of how K⁺ signals are perceived, conveyed, and integrated for improving plant physiological resilience to abiotic stresses. Here, we update on how K⁺-uptake and K⁺-distribution are regulated under various types of abiotic stress. We discuss the effects of K⁺ on several physiological functions and the interaction of K⁺ with other nutrients to improve plant potential against abiotic stress-induced adverse consequences. Understanding of how K⁺ orchestrates physiological mechanisms and contributes to abiotic stress tolerance in plants is essential for practicing sustainable agriculture amidst the climate crisis in global agriculture.

Overdominant expression of related genes of ion homeostasis improves K+ content advantage in hybrid tobacco leaves

BACKGROUND

Potassium(K+) plays a vital role in improving the quality of tobacco leaves. However, how to improve the potassium content of tobacco leaves has always been a difficult problem in tobacco planting. K+ content in tobacco hybrid is characterized by heterosis, which can improve the quality of tobacco leaves, but its underlying molecular genetic mechanisms remain unclear.

RESULTS

Through a two-year field experiment, G70×GDH11 with strong heterosis and K326×GDH11 with weak heterosis were screened out. Transcriptome analyses revealed that 80.89% and 57.28% of the differentially expressed genes (DEGs) in the strong and weak heterosis combinations exhibited an overdominant expression pattern, respectively. The genes that up-regulated the overdominant expression in the strong heterosis hybrids were significantly enriched in the ion homeostasis. Genes involved in K+ transport (KAT1/2, GORK, AKT2, and KEA3), activity regulation complex (CBL-CIPK5/6), and vacuole (TPKs) genes were overdominant expressed in strong heterosis hybrids, which contributed to K+ homeostasis and heterosis in tobacco leaves.

CONCLUSIONS

K+ homeostasis and accumulation in tobacco hybrids were collectively improved. The overdominant expression of K+ transport and homeostasis-related genes conducted a crucial role in the heterosis of K+ content in tobacco leaves.

Loss of SALT OVERLY SENSITIVE 1 prevents virescence in chloroplast K+/H+ EFFLUX ANTIPORTER-deficient mutants

Defects in two plastid K⁺/H⁺ EFFLUX ANTIPORTERs in Arabidopsis can be relieved by loss of a plasma membrane Na⁺/H⁺ exchanger, presumably by altering plant K⁺ transport.

Genome Editing Targets for Improving Nutrient Use Efficiency and Nutrient Stress Adaptation

In recent years, the development of RNA-guided genome editing (CRISPR-Cas9 technology) has revolutionized plant genome editing. Under nutrient deficiency conditions, different transcription factors and regulatory gene networks work together to maintain nutrient homeostasis. Improvement in the use efficiency of nitrogen (N), phosphorus (P) and potassium (K) is essential to ensure sustainable yield with enhanced quality and tolerance to stresses. This review outlines potential targets suitable for genome editing for understanding and improving nutrient use (NtUE) efficiency and nutrient stress tolerance. The different genome editing strategies for employing crucial negative and positive regulators are also described. Negative regulators of nutrient signalling are the potential targets for genome editing, that may improve nutrient uptake and stress signalling under resource-poor conditions. The promoter engineering by CRISPR/dead (d) Cas9 (dCas9) cytosine and adenine base editing and prime editing is a successful strategy to generate precise changes. CRISPR/dCas9 system also offers the added advantage of exploiting transcriptional activators/repressors for overexpression of genes of interest in a targeted manner. CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) are variants of CRISPR in which a dCas9 dependent transcription activation or interference is achieved. dCas9-SunTag system can be employed to engineer targeted gene activation and DNA methylation in plants. The development of nutrient use efficient plants through CRISPR-Cas technology will enhance the pace of genetic improvement for nutrient stress tolerance of crops and improve the sustainability of agriculture.

Chloroplasts are key players to cope with light and temperature stress

Under natural environmental conditions, changes in light intensity and temperature are closely interwoven, and of all organelles, only chloroplasts react strongly upon alterations of these two parameters. We review increasing evidence indicating that changes in chloroplast metabolism are critical for the comprehensive cellular answer in a challenging environment. This cellular answer starts with rapid modifications of thylakoid-located processes, followed by modifications in the stroma and transport activities across the chloroplast envelope. We propose that the 'modulators' involved contribute to plant stress tolerance and that deciphering of their characteristics is essential to understand 'acclimation'. Especially in times of climatic changes, we must gain knowledge on physiological reactions that might become instrumental for directed breeding strategies aiming to develop stress-tolerant crop plants.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

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.

Into the Shadows and Back into Sunlight: Photosynthesis in Fluctuating Light

Photosynthesis is an important remaining opportunity for further improvement in the genetic yield potential of our major crops. Measurement, analysis, and improvement of leaf CO₂ assimilation (A) have focused largely on photosynthetic rates under light-saturated steady-state conditions. However, in modern crop canopies of several leaf layers, light is rarely constant, and the majority of leaves experience marked light fluctuations throughout the day. It takes several minutes for photosynthesis to regain efficiency in both sun-shade and shade-sun transitions, costing a calculated 10-40% of potential crop CO₂ assimilation. Transgenic manipulations to accelerate the adjustment in sun-shade transitions have already shown a substantial productivity increase in field trials. Here, we explore means to further accelerate these adjustments and minimize these losses through transgenic manipulation, gene editing, and exploitation of natural variation. Measurement andanalysis of photosynthesis in sun-shade and shade-sun transitions are explained. Factors limiting speeds of adjustment and how they could be modified to effect improved efficiency are reviewed, specifically nonphotochemical quenching (NPQ), Rubisco activation, and stomatal responses.

Cryo-EM structures of thylakoid-located voltage-dependent chloride channel VCCN1

In the light reaction of plant photosynthesis, modulation of electron transport chain reactions is important to maintain the efficiency of photosynthesis under a broad range of light intensities. VCCN1 was recently identified as a voltage-gated chloride channel residing in the thylakoid membrane, where it plays a key role in photoreaction tuning to avoid the generation of reactive oxygen species (ROS). Here, we present the cryo-EM structures of Malus domestica VCCN1 (MdVCCN1) in nanodiscs and detergent at 2.7 Å and 3.0 Å resolutions, respectively, and the structure-based electrophysiological analyses. VCCN1 structurally resembles its animal homolog, bestrophin, a Ca²⁺-gated anion channel. However, unlike bestrophin channels, VCCN1 lacks the Ca²⁺-binding motif but instead contains an N-terminal charged helix that is anchored to the lipid membrane through an additional amphipathic helix. Electrophysiological experiments demonstrate that these structural elements are essential for the channel activity, thus revealing the distinct activation mechanism of VCCN1.

Flavodiiron proteins enhance the rate of CO2 assimilation in Arabidopsis under fluctuating light intensity

The proton concentration gradient (ΔpH) and membrane potential (Δψ) formed across the thylakoid membrane contribute to ATP synthesis in chloroplasts. Additionally, ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen. K+ exchange antiporter 3 (KEA3) relaxes this downregulation by substituting ΔpH with Δψ in response to fluctuation of light intensity. In the Arabidopsis (Arabidopsis thaliana) line overexpressing KEA3 (KEA3ox), the rate of electron transport is elevated by accelerating the relaxation of ΔpH after a shift from high light (HL) to low light. However, the plant cannot control electron transport toward photosystem I (PSI), resulting in PSI photodamage. In this study, we crossed the KEA3ox line with the line (Flavodiiron [Flv]) expressing the Flv proteins of Physcomitrium patens. In the double transgenic line (Flv-KEA3ox), electrons overloading toward PSI were pumped out by Flv proteins. Consequently, photodamage of PSI was alleviated to the wild-type level. The rate of CO2 fixation was enhanced in Flv and Flv-KEA3ox lines during HL periods of fluctuating light, although CO2 fixation was unaffected in any transgenic lines in constant HL. Upregulation of CO2 fixation was accompanied by elevated stomatal conductance in fluctuating light. Consistent with the results of gas exchange experiments, the growth of Flv and Flv-KEA3ox plants was better than that of WT and KEA3ox plants under fluctuating light.

Does Fluctuating Light Affect Crop Yield? A Focus on the Dynamic Photosynthesis of Two Soybean Varieties

In natural environments, plants are exposed to variable light conditions, but photosynthesis has been mainly studied at steady state and this might overestimate carbon (C) uptake at the canopy scale. To better elucidate the role of light fluctuations on canopy photosynthesis, we investigated how the chlorophyll content, and therefore the different absorbance of light, would affect the quantum yield in fluctuating light conditions. For this purpose, we grew a commercial variety (Eiko) and a chlorophyll deficient mutant (MinnGold) either in fluctuating (F) or non-fluctuating (NF) light conditions with sinusoidal changes in irradiance. Two different light treatments were also applied: a low light treatment (LL; max 650 μmol m^-2 s^-1) and a high light treatment (HL; max 1,000 μmol m^-2 s^-1). Canopy gas exchanges were continuously measured throughout the experiment. We found no differences in C uptake in LL treatment, either under F or NF. Light fluctuations were instead detrimental for the chlorophyll deficient mutant in HL conditions only, while the green variety seemed to be well-adapted to them. Varieties adapted to fluctuating light might be identified to target the molecular mechanisms responsible for such adaptations.

Exogenous sucrose influences KEA1 and KEA2 to regulate abscisic acid-mediated primary root growth in Arabidopsis

Arabidopsis K⁺-efflux antiporter (KEA)1 and KEA2 are chloroplast inner envelope membrane K⁺/H⁺ antiporters that play an important role in plastid development and seedling growth. However, the function of KEA1 and KEA2 during early seedling development is poorly understood. In this work, we found that in Arabidopsis, KEA1 and KEA2 mediated primary root growth by regulating photosynthesis and the ABA signaling pathway. Phenotypic analyses revealed that in the absence of sucrose, the primary root length of the kea1kea2 mutant was significantly shorter than that of the wild-type Columbia-0 (Col-0) plant. However, this phenotype could be remedied by the external application of sucrose. Meanwhile, HPLC-MS/MS results showed that in sucrose-free medium, ABA accumulation in the kea1kea2 mutant was considerably lower than that in Col-0. Transcriptome analysis revealed that many key genes involved in ABA signals were repressed in the kea1kea2 mutant. We concluded that KEA1 and KEA2 deficiency not only affected photosynthesis but was also involved in primary root growth likely through an ABA-dependent manner. This study confirmed the new function of KEA1 and KEA2 in affecting primary root growth.

Impaired photoprotection in Phaeodactylum tricornutum KEA3 mutants reveals the proton regulatory circuit of diatoms light acclimation

Diatoms are successful phytoplankton clades able to acclimate to changing environmental conditions, including e.g. variable light intensity. Diatoms are outstanding at dissipating light energy exceeding the maximum photosynthetic electron transfer (PET) capacity via the nonphotochemical quenching (NPQ) process. While the molecular effectors of NPQ as well as the involvement of the proton motive force (PMF) in its regulation are known, the regulators of the PET/PMF relationship remain unidentified in diatoms. We generated mutants of the H⁺ /K⁺ antiporter KEA3 in the model diatom Phaeodactylum tricornutum. Loss of KEA3 activity affects the PET/PMF coupling and NPQ responses at the onset of illumination, during transients and in steady-state conditions. Thus, this antiporter is a main regulator of the PET/PMF coupling. Consistent with this conclusion, a parsimonious model including only two free components, KEA3 and the diadinoxanthin de-epoxidase, describes most of the feedback loops between PET and NPQ. This simple regulatory system allows for efficient responses to fast (minutes) or slow (e.g. diel) changes in light environment, thanks to the presence of a regulatory calcium ion (Ca²⁺ )-binding domain in KEA3 modulating its activity. This circuit is likely tuned by the NPQ-effector proteins, LHCXs, providing diatoms with the required flexibility to thrive in different ocean provinces.

Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

Photosynthesis started to evolve some 3.5 billion years ago CO₂ is the substrate for photosynthesis and in the past 200-250 years, atmospheric levels have approximately doubled due to human industrial activities. However, this time span is not sufficient for adaptation mechanisms of photosynthesis to be evolutionarily manifested. Steep increases in human population, shortage of arable land and food, and climate change call for actions, now. Thanks to substantial research efforts and advances in the last century, basic knowledge of photosynthetic and primary metabolic processes can now be translated into strategies to optimize photosynthesis to its full potential in order to improve crop yields and food supply for the future. Many different approaches have been proposed in recent years, some of which have already proven successful in different crop species. Here, we summarize recent advances on modifications of the complex network of photosynthetic light reactions. These are the starting point of all biomass production and supply the energy equivalents necessary for downstream processes as well as the oxygen we breathe.

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.

Light potentials of photosynthetic energy storage in the field: what limits the ability to use or dissipate rapidly increased light energy?

The responses of plant photosynthesis to rapid fluctuations in environmental conditions are critical for efficient conversion of light energy. These responses are not well-seen laboratory conditions and are difficult to probe in field environments. We demonstrate an open science approach to this problem that combines multifaceted measurements of photosynthesis and environmental conditions, and an unsupervised statistical clustering approach. In a selected set of data on mint (Mentha sp.), we show that 'light potentials' for linear electron flow and non-photochemical quenching (NPQ) upon rapid light increases are strongly suppressed in leaves previously exposed to low ambient photosynthetically active radiation (PAR) or low leaf temperatures, factors that can act both independently and cooperatively. Further analyses allowed us to test specific mechanisms. With decreasing leaf temperature or PAR, limitations to photosynthesis during high light fluctuations shifted from rapidly induced NPQ to photosynthetic control of electron flow at the cytochrome b6f complex. At low temperatures, high light induced lumen acidification, but did not induce NPQ, leading to accumulation of reduced electron transfer intermediates, probably inducing photodamage, revealing a potential target for improving the efficiency and robustness of photosynthesis. We discuss the implications of the approach for open science efforts to understand and improve crop productivity.

The different patterns of post-heat stress responses in wheat genotypes: the role of the transthylakoid proton gradient in efficient recovery of leaf photosynthetic capacity

The frequency and severity of heat waves are expected to increase in the near future, with a significant impact on physiological functions and yield of crop plants. In this study, we assessed the residual post-heat stress effects on photosynthetic responses of six diverse winter wheat (Triticum sp.) genotypes, differing in country of origin, taxonomy and ploidy (tetraploids vs. hexaploids). After 5 days of elevated temperatures (up to 38 °C), the photosynthetic parameters recorded on the first day of recovery (R1) as well as after the next 4-5 days of the recovery (R2) were compared to those of the control plants (C) grown under moderate temperatures. Based on the values of CO₂ assimilation rate (A) and the maximum rates of carboxylation (V_Cmax) in R1, we identified that the hexaploid (HEX) and tetraploid (TET) species clearly differed in the strength of their response to heat stress. Next, the analyses of gas exchange, simultaneous measurements of PSI and PSII photochemistry and the measurements of electrochromic bandshift (ECS) have consistently shown that photosynthetic and photoprotective functions in leaves of TET genotypes were almost fully recovered in R2, whereas the recovery of photosynthetic and photoprotective functions in the HEX group in R2 was still rather low. A poor recovery was associated with an overly reduced acceptor side of photosystem I as well as high values of the electric membrane potential (Δψ component of the proton motive force, pmf) in the chloroplast. On the other hand, a good recovery of photosynthetic capacity and photoprotective functions was clearly associated with an enhanced ΔpH component of the pmf, thus demonstrating a key role of efficient regulation of proton transport to ensure buildup of the transthylakoid proton gradient needed for photosynthesis restoration after high-temperature episodes.

Phenoplate: An innovative method for assessing interacting effects of temperature and light on non-photochemical quenching in microalgae under chemical stress

Rapid light curves are one of the most widely used methods for assessing the physiological state of photosynthetic organisms. While the method has been applied in a range of physiological studies over the last 20 years, little progress has been made in adapting it for the new age of multi-parametric phenotyping. In order to advance research that is aimed at evaluating the physiological impact of multiple factors, the Phenoplate was developed: a simultaneous assessment of temperature and light gradients. It was used to measure rapid light curves of three marine microalgae across a temperature gradient and altered phosphate availability. The results revealed that activation of photoprotective mechanisms occurred with high efficiency at lower temperatures, and relaxation of photoprotection was negatively impacted above a certain temperature threshold in Tetraselmis sp. It was observed that Thalassiosira pseudonana and Nannochloropsis oceanica exhibited two unique delayed non-photochemical quenching signatures: in combinations of low light with low temperature, and darkness with high temperature, respectively. These findings demonstrate that the Phenoplate approach can be used as a rapid and simple tool to gain insight into the photobiology of microalgae.

The Plasticity of Photosystem I

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

Recent Advances in Genome-wide Analyses of Plant Potassium Transporter Families

Plants require potassium (K⁺) as a macronutrient to support numerous physiological processes. Understanding how this nutrient is transported, stored, and utilized within plants is crucial for breeding crops with high K⁺ use efficiency. As K⁺ is not metabolized, cross-membrane transport becomes a rate-limiting step for efficient distribution and utilization in plants. Several K⁺ transporter families, such as KUP/HAK/KT and KEA transporters and Shaker-like and TPK channels, play dominant roles in plant K⁺ transport processes. In this review, we provide a comprehensive contemporary overview of our knowledge about these K⁺ transporter families in angiosperms, with a major focus on the genome-wide identification of K⁺ transporter families, subcellular localization, spatial expression, function and regulation. We also expanded the genome-wide search for the K⁺ transporter genes and examined their tissue-specific expression in Camelina sativa, a polyploid oil-seed crop with a potential to adapt to marginal lands for biofuel purposes and contribution to sustainable agriculture. In addition, we present new insights and emphasis on the study of K⁺ transporters in polyploids in an effort to generate crops with high K⁺ Utilization Efficiency (KUE).

Change in H+ Transport across Thylakoid Membrane as Potential Mechanism of 14.3 Hz Magnetic Field Impact on Photosynthetic Light Reactions in Seedlings of Wheat (Triticum aestivum L.)

Natural and artificial extremely low-frequency magnetic fields (ELFMFs) are important factors influencing physiological processes in living organisms including terrestrial plants. Earlier, it was experimentally shown that short-term and long-term treatments by ELFMFs with Schumann resonance frequencies (7.8, 14.3, and 20.8 Hz) influenced parameters of photosynthetic light reactions in wheat leaves. The current work is devoted to an analysis of potential ways of this ELFMF influence on the light reactions. Only a short-term wheat treatment by 14.3 Hz ELFMF was used in the analysis. First, it was experimentally shown that ELFMF-induced changes (an increase in the effective quantum yield of photosystem II, a decrease in the non-photochemical quenching of chlorophyll fluorescence, a decrease in time of changes in these parameters, etc.) were observed under the action of ELFMF with widely ranging magnitudes (from 3 to 180 µT). In contrast, the potential quantum yield of photosystem II and time of relaxation of the energy-dependent component of the non-photochemical quenching were not significantly influenced by ELFMF. Second, it was shown that the ELFMF treatment decreased the proton gradient across the thylakoid membrane. In contrast, the H⁺ conductivity increased under this treatment. Third, an analysis of the simplest mathematical model of an H⁺ transport across the thylakoid membrane, which was developed in this work, showed that changes in H⁺ fluxes related to activities of the photosynthetic electron transport chain and the H⁺-ATP synthase were not likely a mechanism of the ELFMF influence. In contrast, changes induced by an increase in an additional H⁺ flux (probably, through the proton leakage and/or through the H⁺/Ca²⁺ antiporter activity in the thylakoid membrane) were in good accordance with experimental results. Thus, we hypothesized that this increase is the mechanism of the 14.3 Hz ELFMF influence (and, maybe, influences of other low frequencies) on photosynthetic light reactions in wheat.

Comparative proteomics of thylakoids from Arabidopsis grown in laboratory and field conditions

Compared to controlled laboratory conditions, plant growth in the field is rarely optimal since it is frequently challenged by large fluctuations in light and temperature which lower the efficiency of photosynthesis and lead to photo-oxidative stress. Plants grown under natural conditions therefore place an increased onus on the regulatory mechanisms that protect and repair the delicate photosynthetic machinery. Yet, the exact changes in thylakoid proteome composition which allow plants to acclimate to the natural environment remain largely unexplored. Here, we use quantitative label-free proteomics to demonstrate that field-grown Arabidopsis plants incorporate aspects of both the low and high light acclimation strategies previously observed in laboratory-grown plants. Field plants showed increases in the relative abundance of ATP synthase, cytochrome b 6 f, ferredoxin-NADP+ reductases (FNR1 and FNR2) and their membrane tethers TIC62 and TROL, thylakoid architecture proteins CURT1A, CURT1B, RIQ1, and RIQ2, the minor monomeric antenna complex CP29.3, rapidly-relaxing non-photochemical quenching (qE)-related proteins PSBS and VDE, the photosystem II (PSII) repair machinery and the cyclic electron transfer complexes NDH, PGRL1B, and PGR5, in addition to decreases in the amounts of LHCII trimers composed of LHCB1.1, LHCB1.2, LHCB1.4, and LHCB2 proteins and CP29.2, all features typical of a laboratory high light acclimation response. Conversely, field plants also showed increases in the abundance of light harvesting proteins LHCB1.3 and CP29.1, zeaxanthin epoxidase (ZEP) and the slowly-relaxing non-photochemical quenching (qI)-related protein LCNP, changes previously associated with a laboratory low light acclimation response. Field plants also showed distinct changes to the proteome including the appearance of stress-related proteins ELIP1 and ELIP2 and changes to proteins that are largely invariant under laboratory conditions such as state transition related proteins STN7 and TAP38. We discuss the significance of these alterations in the thylakoid proteome considering the unique set of challenges faced by plants growing under natural conditions.

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.

Effects of Salinity Stress on Chloroplast Structure and Function

Salinity is a growing problem affecting soils and agriculture in many parts of the world. The presence of salt in plant cells disrupts many basic metabolic processes, contributing to severe negative effects on plant development and growth. This review focuses on the effects of salinity on chloroplasts, including the structures and function of these organelles. Chloroplasts house various important biochemical reactions, including photosynthesis, most of which are considered essential for plant survival. Salinity can affect these reactions in a number of ways, for example, by changing the chloroplast size, number, lamellar organization, lipid and starch accumulation, and interfering with cross-membrane transportation. Research has shown that maintenance of the normal chloroplast physiology is necessary for the survival of the entire plant. Many plant species have evolved different mechanisms to withstand the harmful effects of salt-induced toxicity on their chloroplasts and its machinery. The differences depend on the plant species and growth stage and can be quite different between salt-sensitive (glycophyte) and salt-tolerant (halophyte) plants. Salt stress tolerance is a complex trait, and many aspects of salt tolerance in plants are not entirely clear yet. In this review, we discuss the different mechanisms of salt stress tolerance in plants with a special focus on chloroplast structure and its functions, including the underlying differences between glycophytes and halophytes.