Measuring the dynamic response of the thylakoid architecture in plant leaves by electron microscopy

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.

Thylakoid ultrastructural variations in chlorophyll-deficient wheat: aberrations or structural acclimation?

MAIN CONCLUSION

A structural re-modeling of the thylakoid system, including granum size and regularity, occurs in chlorophyll-deficient wheat mutants affected by photosynthetic membrane over-reduction. In the chloroplast of land plants, the thylakoid system is defined by appressed grana stacks and unstacked stroma lamellae. This study focuses on the variations of the grana organization occurring in outdoor-grown wheat mutants characterized by low chlorophyll content and a tendency for photosynthetic membrane over-reduction. Triticum aestivum ANK-32A and Triticum durum ANDW-7B were compared to their corresponding WT lines, NS67 and LD222, respectively. Electron micrographs of chloroplasts were used to calculate grana ultrastructural parameters. Photosynthetic parameters were obtained by modulated chlorophyll fluorescence and applying Light Curves (LC) and Rapid Light Curves (RLC) protocols. For each photosynthetic parameter, the difference Δ(RLC-LC) was calculated to evaluate the flexible response to light in the examined lines. In the mutants, fewer and smaller disks formed grana stacks characterized by a marked increase in lateral and cross-sectional irregularity, both negatively correlated with the number of layers per granum. A relationship was found between membrane over-reduction and granum structural irregularity. The possible acclimative significance of a greater proportion of stroma-exposed grana domains in relieving the excess electron pressure on PSI is discussed.

Thylakoid membrane stacking controls electron transport mode during the dark-to-light transition by adjusting the distances between PSI and PSII

The balance between linear electron transport (LET) and cyclic electron transport (CET) plays an essential role in plant adaptation and protection against photo-induced damage. This balance is largely maintained by phosphorylation-driven alterations in the PSII-LHCII assembly and thylakoid membrane stacking. During the dark-to-light transition, plants shift this balance from CET, which prevails to prevent overreduction of the electron transport chain and consequent photo-induced damage, towards LET, which enables efficient CO2 assimilation and biomass production. Using freeze-fracture cryo-scanning electron microscopy and transmission electron microscopy of Arabidopsis leaves, we reveal unique membrane regions possessing characteristics of both stacked and unstacked regions of the thylakoid network that form during this transition. A notable consequence of the morphological attributes of these regions, which we refer to as 'stacked thylakoid doublets', is an overall increase in the proximity and connectivity of the two photosystems (PSI and PSII) that drive LET. This, in turn, reduces diffusion distances and barriers for the mobile carriers that transfer electrons between the two PSs, thereby maximizing LET and optimizing the plant's ability to utilize light energy. The mechanics described here for the shift between CET and LET during the dark-to-light transition are probably also used during chromatic adaptation mediated by state transitions.

Oldest thylakoids in fossil cells directly evidence oxygenic photosynthesis

Today oxygenic photosynthesis is unique to cyanobacteria and their plastid relatives within eukaryotes. Although its origin before the Great Oxidation Event is still debated1-4, the accumulation of O2 profoundly modified the redox chemistry of the Earth and the evolution of the biosphere, including complex life. Understanding the diversification of cyanobacteria is thus crucial to grasping the coevolution of our planet and life, but their early fossil record remains ambiguous5. Extant cyanobacteria include the thylakoid-less Gloeobacter-like group and the remainder of cyanobacteria that acquired thylakoid membranes6,7. The timing of this divergence is indirectly estimated at between 2.7 and 2.0 billion years ago (Ga) based on molecular clocks and phylogenies8-11 and inferred from the earliest undisputed fossil record of Eoentophysalis belcherensis, a 2.018-1.854 Ga pleurocapsalean cyanobacterium preserved in silicified stromatolites12,13. Here we report the oldest direct evidence of thylakoid membranes in a parallel-to-contorted arrangement within the enigmatic cylindrical microfossils Navifusa majensis from the McDermott Formation, Tawallah Group, Australia (1.78-1.73 Ga), and in a parietal arrangement in specimens from the Grassy Bay Formation, Shaler Supergroup, Canada (1.01-0.9 Ga). This discovery extends their fossil record by at least 1.2 Ga and provides a minimum age for the divergence of thylakoid-bearing cyanobacteria at roughly 1.75 Ga. It allows the unambiguous identification of early oxygenic photosynthesizers and a new redox proxy for probing early Earth ecosystems, highlighting the importance of examining the ultrastructure of fossil cells to decipher their palaeobiology and early evolution.

Expansion microscopy resolves the thylakoid structure of spinach

The light-harvesting reactions of photosynthesis take place on the thylakoid membrane inside chloroplasts. The thylakoid membrane is folded into appressed membranes, the grana, and nonappressed membranes that interconnect the grana, the stroma lamellae. This folding is essential for the correct functioning of photosynthesis. Electron microscopy and atomic force microscopy are commonly used to study the thylakoid membrane, but these techniques have limitations in visualizing a complete chloroplast and its organization. To overcome this limitation, we applied expansion microscopy (ExM) on isolated chloroplasts. ExM is a technique that involves physically expanding a sample in a swellable hydrogel to enhance the spatial resolution of fluorescence microscopy. Using all-protein staining, we visualized the 3D structure of spinach (Spinacia oleracea) thylakoids in detail. We were able to resolve stroma lamellae that were 60 nm apart and observe their helical wrapping around the grana. Furthermore, we accurately measured the dimensions of grana from top views of chloroplasts, which allow for precise determination of the granum diameter. Our results demonstrate that ExM is a fast and reliable technique for studying thylakoid organization in great detail.

Optimizing the electron transport chain to sustainably improve photosynthesis

Genetically improving photosynthesis is a key strategy to boosting crop production to meet the rising demand for food and fuel by a rapidly growing global population in a warming climate. Many components of the photosynthetic apparatus have been targeted for genetic modification for improving photosynthesis. Successful translation of these modifications into increased plant productivity in fluctuating environments will depend on whether the electron transport chain (ETC) can support the increased electron transport rate without risking overreduction and photodamage. At present atmospheric conditions, the ETC appears suboptimal and will likely need to be modified to support proposed photosynthetic improvements and to maintain energy balance. Here, I derive photochemical equations to quantify the transport capacity and the corresponding reduction level based on the kinetics of redox reactions along the ETC. Using these theoretical equations and measurements from diverse C3/C4 species across environments, I identify several strategies that can simultaneously increase the transport capacity and decrease the reduction level of the ETC. These strategies include increasing the abundances of reaction centers, cytochrome b6f complexes, and mobile electron carriers, improving their redox kinetics, and decreasing the fraction of secondary quinone-nonreducing photosystem II reaction centers. I also shed light on several previously unexplained experimental findings regarding the physiological impacts of the abundances of the cytochrome b6f complex and plastoquinone. The model developed, and the insights generated from it facilitate the development of sustainable photosynthetic systems for greater crop yields.

Plants cope with fluctuating light by frequency-dependent nonphotochemical quenching and cyclic electron transport

In natural environments, plants are exposed to rapidly changing light. Maintaining photosynthetic efficiency while avoiding photodamage requires equally rapid regulation of photoprotective mechanisms. We asked what the operation frequency range of regulation is in which plants can efficiently respond to varying light. Chlorophyll fluorescence, P700, plastocyanin, and ferredoxin responses of wild-types Arabidopsis thaliana were measured in oscillating light of various frequencies. We also investigated the npq1 mutant lacking violaxanthin de-epoxidase, the npq4 mutant lacking PsbS protein, and the mutants crr2-2, and pgrl1ab impaired in different pathways of the cyclic electron transport. The fastest was the PsbS-regulation responding to oscillation periods longer than 10 s. Processes involving violaxanthin de-epoxidase dampened changes in chlorophyll fluorescence in oscillation periods of 2 min or longer. Knocking out the PGR5/PGRL1 pathway strongly reduced variations of all monitored parameters, probably due to congestion in the electron transport. Incapacitating the NDH-like pathway only slightly changed the photosynthetic dynamics. Our observations are consistent with the hypothesis that nonphotochemical quenching in slow light oscillations involves violaxanthin de-epoxidase to produce, presumably, a largely stationary level of zeaxanthin. We interpret the observed dynamics of photosystem I components as being formed in slow light oscillations partially by thylakoid remodeling that modulates the redox rates.

From remotely sensed solar-induced chlorophyll fluorescence to ecosystem structure, function, and service: Part I-Harnessing theory

Solar-induced chlorophyll fluorescence (SIF) is a remotely sensed optical signal emitted during the light reactions of photosynthesis. The past two decades have witnessed an explosion in availability of SIF data at increasingly higher spatial and temporal resolutions, sparking applications in diverse research sectors (e.g., ecology, agriculture, hydrology, climate, and socioeconomics). These applications must deal with complexities caused by tremendous variations in scale and the impacts of interacting and superimposing plant physiology and three-dimensional vegetation structure on the emission and scattering of SIF. At present, these complexities have not been overcome. To advance future research, the two companion reviews aim to (1) develop an analytical framework for inferring terrestrial vegetation structures and function that are tied to SIF emission, (2) synthesize progress and identify challenges in SIF research via the lens of multi-sector applications, and (3) map out actionable solutions to tackle these challenges and offer our vision for research priorities over the next 5-10 years based on the proposed analytical framework. This paper is the first of the two companion reviews, and theory oriented. It introduces a theoretically rigorous yet practically applicable analytical framework. Guided by this framework, we offer theoretical perspectives on three overarching questions: (1) The forward (mechanism) question-How are the dynamics of SIF affected by terrestrial ecosystem structure and function? (2) The inference question: What aspects of terrestrial ecosystem structure, function, and service can be reliably inferred from remotely sensed SIF and how? (3) The innovation question: What innovations are needed to realize the full potential of SIF remote sensing for real-world applications under climate change? The analytical framework elucidates that process complexity must be appreciated in inferring ecosystem structure and function from the observed SIF; this framework can serve as a diagnosis and inference tool for versatile applications across diverse spatial and temporal scales.

Barley Cultivar Sarab 1 Has a Characteristic Region on the Thylakoid Membrane That Protects Photosystem I under Iron-Deficient Conditions

The barley cultivar Sarab 1 (SRB1) can continue photosynthesis despite its low Fe acquisition potential via roots and dramatically reduced amounts of photosystem I (PSI) reaction-center proteins under Fe-deficient conditions. We compared the characteristics of photosynthetic electron transfer (ET), thylakoid ultrastructure, and Fe and protein distribution on thylakoid membranes among barley cultivars. The Fe-deficient SRB1 had a large proportion of functional PSI proteins by avoiding P700 over-reduction. An analysis of the thylakoid ultrastructure clarified that SRB1 had a larger proportion of non-appressed thylakoid membranes than those in another Fe-tolerant cultivar, Ehimehadaka-1 (EHM1). Separating thylakoids by differential centrifugation further revealed that the Fe-deficient SRB1 had increased amounts of low/light-density thylakoids with increased Fe and light-harvesting complex II (LHCII) than did EHM1. LHCII with uncommon localization probably prevents excessive ET from PSII leading to elevated NPQ and lower PSI photodamage in SRB1 than in EHM1, as supported by increased Y(NPQ) and Y(ND) in the Fe-deficient SRB1. Unlike this strategy, EHM1 may preferentially supply Fe cofactors to PSI, thereby exploiting more surplus reaction center proteins than SRB1 under Fe-deficient conditions. In summary, SRB1 and EHM1 support PSI through different mechanisms during Fe deficiency, suggesting that barley species have multiple strategies for acclimating photosynthetic apparatus to Fe deficiency.

An exploratory steady-state redox model of photosynthetic linear electron transport for use in complete modelling of photosynthesis for broad applications

A photochemical model of photosynthetic electron transport (PET) is needed to integrate photophysics, photochemistry, and biochemistry to determine redox conditions of electron carriers and enzymes for plant stress assessment and mechanistically link sun-induced chlorophyll fluorescence to carbon assimilation for remotely sensing photosynthesis. Towards this goal, we derived photochemical equations governing the states and redox reactions of complexes and electron carriers along the PET chain. These equations allow the redox conditions of the mobile plastoquinone pool and the cytochrome b₆ f complex (Cyt) to be inferred with typical fluorometry. The equations agreed well with fluorometry measurements from diverse C₃ /C₄ species across environments in the relationship between the PET rate and fraction of open photosystem II reaction centres. We found the oxidation of plastoquinol by Cyt is the bottleneck of PET, and genetically improving the oxidation of plastoquinol by Cyt may enhance the efficiency of PET and photosynthesis across species. Redox reactions and photochemical and biochemical interactions are highly redundant in their complex controls of PET. Although individual reaction rate constants cannot be resolved, they appear in parameter groups which can be collectively inferred with fluorometry measurements for broad applications. The new photochemical model developed enables advances in different fronts of photosynthesis research.

Characterization of thylakoid division using chloroplast dividing mutants in Arabidopsis

Chloroplasts are double membrane bound organelles that are found in plants and algae. Their division requires a number of proteins to assemble into rings along the center of the organelle and to constrict in synchrony. Chloroplasts possess a third membrane system, the thylakoids, which house the majority of proteins responsible for the light-dependent reactions. The mechanism that allows chloroplasts to sort out and separate the intricate thylakoid membrane structures during organelle division remain unknown. By characterizing the sizes of thylakoids found in a number of different chloroplast division mutants in Arabidopsis, we show that thylakoids do not divide independently of the chloroplast division cycle. More specifically, we show that thylakoid division requires the formation of both the inner and the outer contractile rings of the chloroplast.

Single chloroplast in folio imaging sheds light on photosystem energy redistribution during state transitions

Oxygenic photosynthesis is driven by light absorption in photosystem I (PSI) and photosystem II (PSII). A balanced excitation pressure between PSI and PSII is required for optimal photosynthetic efficiency. State transitions serve to keep this balance. If PSII is overexcited in plants and green algae, a mobile pool of light-harvesting complex II (LHCII) associates with PSI, increasing its absorption cross-section and restoring the excitation balance. This is called state 2. Upon PSI overexcitation, this LHCII pool moves to PSII, leading to state 1. Whether the association/dissociation of LHCII with the photosystems occurs between thylakoid grana and thylakoid stroma lamellae during state transitions or within the same thylakoid region remains unclear. Furthermore, although state transitions are thought to be accompanied by changes in thylakoid macro-organization, this has never been observed directly in functional leaves. In this work, we used confocal fluorescence lifetime imaging to quantify state transitions in single Arabidopsis (Arabidopsis thaliana) chloroplasts in folio with sub-micrometer spatial resolution. The change in excitation-energy distribution between PSI and PSII was investigated at a range of excitation wavelengths between 475 and 665 nm. For all excitation wavelengths, the PSI/(PSI + PSII) excitation ratio was higher in state 2 than in state 1. We next imaged the local PSI/(PSI + PSII) excitation ratio for single chloroplasts in both states. The data indicated that LHCII indeed migrates between the grana and stroma lamellae during state transitions. Finally, fluorescence intensity images revealed that thylakoid macro-organization is largely unaffected by state transitions. This single chloroplast in folio imaging method will help in understanding how plants adjust their photosynthetic machinery to ever-changing light conditions.

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.

Granal thylakoid structure and function: explaining an enduring mystery of higher plants

In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi-media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome  b₆ f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon-reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well-known but seemingly unconnected phenomena of thylakoid structure and function in higher plants.

How to Measure Grana - Ultrastructural Features of Thylakoid Membranes of Plant Chloroplasts

Granum is a basic structural unit of the thylakoid membrane network of plant chloroplasts. It is composed of multiple flattened membranes forming a stacked arrangement of a cylindrical shape. Grana membranes are composed of lipids and tightly packed pigment-protein complexes whose primary role is the catalysis of photosynthetic light reactions. These membranes are highly dynamic structures capable of adapting to changing environmental conditions by fine-tuning photochemical efficiency, manifested by the structural reorganization of grana stacks. Due to a nanometer length scale of the structural granum features, the application of high-resolution electron microscopic techniques is essential for a detailed analysis of the granum architecture. This mini-review overviews recent approaches to quantitative grana structure analyses from electron microscopy data, highlighting the basic manual measurements and semi-automated workflows. We outline and define structural parameters used by different authors, for instance, granum height and diameter, thylakoid thickness, end-membrane length, Stacking Repeat Distance, and Granum Lateral Irregularity. This article also presents insights into efficient and effective measurements of grana stacks visualized on 2D micrographs. The information on how to correctly interpret obtained data, taking into account the 3D nature of grana stacks projected onto 2D space of electron micrograph, is also given. Grana ultrastructural observations reveal key features of this intriguing membrane arrangement, broadening our knowledge of the thylakoid network's remarkable plasticity.

Small-Angle X-Ray and Neutron Scattering on Photosynthetic Membranes

Ultrastructural membrane arrangements in living cells and their dynamic remodeling in response to environmental changes remain an area of active research but are also subject to large uncertainty. The use of noninvasive methods such as X-ray and neutron scattering provides an attractive complimentary source of information to direct imaging because in vivo systems can be probed in near-natural conditions. However, without solid underlying structural modeling to properly interpret the indirect information extracted, scattering provides at best qualitative information and at worst direct misinterpretations. Here we review the current state of small-angle scattering applied to photosynthetic membrane systems with particular focus on data interpretation and modeling.