From chloroplasts to photosystems: in situ scanning force microscopy on intact thylakoid membranes

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.

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.

A novel chloroplast super-complex consisting of the ATP synthase and photosystem I reaction center

Several 'super-complexes' of individual hetero-oligomeric membrane protein complexes, whose function is to facilitate intra-membrane electron and proton transfer and harvesting of light energy, have been previously characterized in the mitochondrial cristae and chloroplast thylakoid membranes. We report the presence of an intra-membrane super-complex dominated by the ATP-synthase, photosystem I (PSI) reaction-center complex and the ferredoxin-NADP+ Reductase (FNR) in the thylakoid membrane. The presence of the super-complex has been documented by mass spectrometry, clear-native PAGE and Western Blot analyses. This is the first documented presence of ATP synthase in a super-complex with the PSI reaction-center located in the non-appressed stromal domain of the thylakoid membrane.

Plastocyanin is the long-range electron carrier between photosystem II and photosystem I in plants

In photosynthetic electron transport, large multiprotein complexes are connected by small diffusible electron carriers, the mobility of which is challenged by macromolecular crowding. For thylakoid membranes of higher plants, a long-standing question has been which of the two mobile electron carriers, plastoquinone or plastocyanin, mediates electron transport from stacked grana thylakoids where photosystem II (PSII) is localized to distant unstacked regions of the thylakoids that harbor PSI. Here, we confirm that plastocyanin is the long-range electron carrier by employing mutants with different grana diameters. Furthermore, our results explain why higher plants have a narrow range of grana diameters since a larger diffusion distance for plastocyanin would jeopardize the efficiency of electron transport. In the light of recent findings that the lumen of thylakoids, which forms the diffusion space of plastocyanin, undergoes dynamic swelling/shrinkage, this study demonstrates that plastocyanin diffusion is a crucial regulatory element of plant photosynthetic electron transport.

Structural Adaptation and Physiological Mechanisms in the Leaves of Anthyllis vulneraria L. from Metallicolous and Non-Metallicolous Populations

Calamine wastes highly contaminated with trace metals (TMs) are spontaneously inhabited by a legume plant Anthyllis vulneraria L. This study determined an adaptation strategy of metallicolous (M) A. vulneraria and compared it with that of the non-metallicolous (NM) ecotype. We hypothesized that TMs may lead to (i) leaf apoplast modifications and (ii) changes in the antioxidant machinery efficiency that facilitate plant growth under severe contamination. To verify our hypothesis, we implemented immunolabelling, transmission electron microscopy and biochemical measurements. NM leaves were larger and thicker compared to the M ecotype. Microscopic analysis of M leaves showed a lack of dysfunctions in mesophyll cells exposed to TMs. However, changes in apoplast composition and thickening of the mesophyll and epidermal cell walls in these plants were observed. Thick walls were abundant in xyloglucan, pectins, arabinan, arabinogalactan protein and extensin. The tested ecotypes differed also in their physiological responses. The metallicolous ecotype featured greater accumulation of photosynthetic pigments, enhanced activity of superoxide dismutase and increased content of specific phenol groups in comparison with the NM one. Despite this, radical scavenging activity at the level of 20% was similar in M and NM ecotypes, which may implicate effective reduction of oxidative stress in M plants. In summary, our results confirmed hypotheses and suggest that TMs induced cell wall modifications of leaves, which may play a role in metal stress avoidance in Anthyllis species. However, when TMs reach the protoplast, activation of antioxidant machinery may significantly strengthen the status of plants naturally growing in TM-polluted environment.

Leaves of isoprene-emitting tobacco plants maintain PSII stability at high temperatures

At high temperatures, isoprene-emitting plants display a higher photosynthetic rate and a lower nonphotochemical quenching (NPQ) compared with nonemitting plants. The mechanism of this phenomenon, which may be very important under current climate warming, is still elusive. NPQ was dissected into its components, and chlorophyll fluorescence lifetime imaging microscopy (FLIM) was used to analyse the dynamics of excited chlorophyll relaxation in isoprene-emitting and nonemitting plants. Thylakoid membrane stiffness was also measured using atomic force microscope (AFM) to identify a possible mode of action of isoprene in improving photochemical efficiency and photosynthetic stability. We show that, when compared with nonemitters, isoprene-emitting tobacco plants exposed at high temperatures display a reduced increase of the NPQ energy-dependent component (qE) and stable (1) chlorophyll fluorescence lifetime; (2) amplitude of the fluorescence decay components; and (3) thylakoid membrane stiffness. Our study shows for the first time that isoprene maintains PSII stability at high temperatures by preventing the modifications of the surrounding environment, namely providing a more steady and homogeneous distribution of the light-absorbing centres and a stable thylakoid membrane stiffness. Isoprene photoprotects leaves with a mechanism alternative to NPQ, enabling plants to maintain a high photosynthetic rate at rising temperatures.

Pigment-protein complexes are organized into stable microdomains in cyanobacterial thylakoids

Thylakoids are the place of the light-photosynthetic reactions. To gain maximal efficiency, these reactions are conditional to proper pigment-pigment and protein-protein interactions. In higher plants thylakoids, the interactions lead to a lateral asymmetry in localization of protein complexes (i.e. granal/stromal thylakoids) that have been defined as a domain-like structures characteristic by different biochemical composition and function (Albertsson P-Å. 2001,Trends Plant Science 6: 349-354). We explored this complex organization of thylakoid pigment-proteins at single cell level in the cyanobacterium Synechocystis sp. PCC 6803. Our 3D confocal images captured heterogeneous distribution of all main photosynthetic pigment-protein complexes (PPCs), Photosystem I (fluorescently tagged by YFP), Photosystem II and Phycobilisomes. The acquired images depicted cyanobacterial thylakoid membrane as a stable, mosaic-like structure formed by microdomains (MDs). These microcompartments are of sub-micrometer in sizes (~0.5-1.5 μm), typical by particular PPCs ratios and importantly without full segregation of observed complexes. The most prevailing MD is represented by MD with high Photosystem I content which allows also partial separation of Photosystems like in higher plants thylakoids. We assume that MDs stability (in minutes) provides optimal conditions for efficient excitation/electron transfer. The cyanobacterial MDs thus define thylakoid membrane organization as a system controlled by co-localization of three main PPCs leading to formation of thylakoid membrane mosaic. This organization might represent evolutional and functional precursor for the granal/stromal spatial heterogeneity in photosystems that is typical for higher plant thylakoids.

Magnesium Deficiency Induced Global Transcriptome Change in Citrus sinensis Leaves Revealed by RNA-Seq

Magnesium (Mg) deficiency is one of the major constraining factors that limit the yield and quality of agricultural products. Uniform seedlings of the Citrus sinensis were irrigated with Mg deficient (0 mM MgSO₄) and Mg sufficient (1 mM MgSO₄) nutrient solutions for 16 weeks. CO₂ assimilation, starch, soluble carbohydrates, TBARS content and H₂O₂ production were measured. Transcriptomic analysis of C. sinensis leaves was performed by Illumina sequencing. Our results showed that Mg deficiency decreased CO₂ assimilation, but increased starch, sucrose, TBARS content and H₂O₂ production in C. sinensis leaves. A total of 4864 genes showed differential expression in response to Mg deficiency revealed by RNA-Seq and the transcriptomic data were further validated by real-time quantitative PCR (RT-qPCR). Gene ontology (GO) enrichment analysis indicated that the mechanisms underlying Mg deficiency tolerance in C. sinensis may be attributed to the following aspects: (a) enhanced microtubule-based movement and cell cycle regulation; (b) elevated signal transduction in response to biotic and abiotic stimuli; (c) alteration of biological processes by tightly controlling phosphorylation especially protein phosphorylation; (d) down-regulation of light harvesting and photosynthesis due to the accumulation of carbohydrates; (e) up-regulation of cell wall remodeling and antioxidant system. Our results provide a comprehensive insight into the transcriptomic profile of key components involved in the Mg deficiency tolerance in C. sinensis and enrich our understanding of the molecular mechanisms by which plants adapted to a Mg deficient condition.

Thylakoid membrane unstacking increases LHCII thermal stability and lipid phase fluidity

Thylakoids are highly protein-enriched membranes that harbor a number of multicomponent photosynthetic complexes. Similarly to other biological membranes the protein constituents are heterogeneously distributed laterally in the plane of the membrane, however the specific segregation into stacked (grana patches) and unstacked (stroma lamellae) membrane layers is a unique feature of the thylakoid. Both the lateral and the vertical arrangements of the integral membrane proteins within the three-dimensional thylakoid ultrastructure are thought to have important physiological function. In this work we explore the role of membrane stacking for the thermal stability of the photosynthetic complexes in thylakoid membranes. By means of circular dichroism and differential scanning calorimetry we demonstrate that the thermal stability of the monomeric and trimeric forms of the major light harvesting complex of photosystem II (LHCII) increases upon unstacking. This effect was suggested to be due to the detachment of LHCII from photosystem II and consequent attachment to photosystem I subunits and/or the fluidization of the lipid matrix upon unstacking. The changes in the physical properties of the protein and lipid membrane components upon unstacking result in strongly reduced photosystem II excitation energy utilization.

The acclimatization strategies of kidney vetch (Anthyllis vulneraria L.) to Pb toxicity

Kidney vetch (Anthyllis vulneraria L.) is a well-known Zn hyperaccumulator. Zn often occurs with Pb in one ore; thus, plants inhabiting waste dumps are exposed not only to Zn but also to Pb toxicity. While the response of kidney vetch to Zn toxicity is relatively well known, the Pb survival strategy of Anthyllis vulneraria has not been the subject of investigations. The aim of presented research was to determine the survival strategy of kidney vetch exposed to high lead concentrations. Shoot explants of a calamine kidney vetch ecotype were placed on agar media containing 0.0, 0.5, 1.0, and 1.5 mM Pb. Morphological, physiological, and biochemical responses, in particular photosynthetic apparatus of plantlets, were examined. The most pronounced changes were observed in plants grown on media supplemented with 1.5 mM Pb after 8 weeks of culture. Increased dry weight and high lead accumulation were observed in roots. Similarly, in shoots, increased dry weight and a decreased number of newly formed shoots were recorded. The accumulation of lead was many times lower in shoots than in roots. In leaf cells' ultra-structure, looser arrangement of chloroplast thylakoid grana was observed. Despite the decrease in chlorophyll a and carotenoid content, the photosynthetic apparatus remained efficient due to the lack of photoinhibition and increased electron transport rate beyond photosystem II (PSII). For the first time, an acclimatization mechanism based on maintaining the high efficiency of photosynthetic apparatus resulting from increasing of electron transport rate was described.

Sulla carnosa modulates root invertase activity in response to the inhibition of long-distance sucrose transport under magnesium deficiency

Being the principal product of photosynthesis, sucrose is involved in many metabolic processes in plants. As magnesium (Mg) is phloem mobile, an inverse relationship between Mg shortage and sugar accumulation in leaves is often observed. Mg deficiency effects on carbohydrate contents and invertase activities were determined in Sulla carnosa Desf. Plants were grown hydroponically at different Mg concentrations (0.00, 0.01, 0.05 and 1.50 mM Mg) for one month. Mineral analysis showed that Mg contents were drastically diminished in shoots and roots mainly at 0.01 and 0.00 mM Mg. This decline was adversely associated with a significant increase of sucrose, fructose and mainly glucose in shoots of plants exposed to severe deficiency. By contrast, sugar contents were severely reduced in roots of these plants indicating an alteration of carbohydrate partitioning between shoots and roots of Mg-deficient plants. Cell wall invertase activity was highly enhanced in roots of Mg-deficient plants, while the vacuolar invertase activity was reduced at 0.00 mM Mg. This decrease of vacuolar invertase activity may indicate the sensibility of roots to Mg starvation resulting from sucrose transport inhibition. ¹⁴ CO₂ labeling experiments were in accordance with these findings showing an inhibition of sucrose transport from source leaves to sink tissues (roots) under Mg depletion. The obtained results confirm previous findings about Mg involvement in photosynthate loading into phloem and add new insights into mechanisms evolved by S. carnosa to cope with Mg shortage in particular the increase of the activity of cell wall invertase.

Visualizing the dynamic structure of the plant photosynthetic membrane

The chloroplast thylakoid membrane is the site for the initial steps of photosynthesis that convert solar energy into chemical energy, ultimately powering almost all life on earth. The heterogeneous distribution of protein complexes within the membrane gives rise to an intricate three-dimensional structure that is nonetheless extremely dynamic on a timescale of seconds to minutes. These dynamics form the basis for the regulation of photosynthesis, and therefore the adaptability of plants to different environments. High-resolution microscopy has in recent years begun to provide new insights into the structural dynamics underlying a number of regulatory processes such as membrane stacking, photosystem II repair, photoprotective energy dissipation, state transitions and alternative electron transfer. Here we provide an overview of the essentials of thylakoid membrane structure in plants, and consider how recent advances, using a range of microscopies, have substantially increased our knowledge of the thylakoid dynamic structure. We discuss both the successes and limitations of the currently available techniques and highlight newly emerging microscopic methods that promise to move the field beyond the current 'static' view of membrane organization based on frozen snapshots to a 'live' view of functional membranes imaged under native aqueous conditions at ambient temperature and responding dynamically to external stimuli.

Light-harvesting II antenna trimers connect energetically the entire photosynthetic machinery - including both photosystems II and I

In plant chloroplasts, the two photosystems (PSII and PSI) are enriched in different thylakoid domains and, according to the established view, are regarded as energetically segregated from each other. A specific fraction of the light harvesting complex II (LHCII) has been postulated to get phosphorylated by the STN7 kinase and subsequently to migrate from PSII to PSI as part of a process called 'state transition'. Nevertheless, the thylakoid membrane incorporates a large excess of LHCII not present in the isolatable PSII-LHCII and PSI-LHCII complexes. Moreover, LHCII phosphorylation is not limited to a specific LHCII pool and "state 2" condition, but is found in all thylakoid domains in any constant light condition. Here, using a targeted solubilization of pigment-protein complexes from different thylakoid domains, we demonstrate that even a minor detachment of LHCII leads to markedly increased fluorescence emission from LHCII and PSII both in grana core and non-appressed thylakoid membranes and the effect of the detergent to detach LHCII is enhanced in the absence of LHCII phosphorylation. These findings provide evidence that PSII and PSI are energy traps embedded in the same energetically connected LHCII lake. In the lake, PSI and LHCII are energetically connected even in the absence of LHCII phosphorylation, yet the phosphorylation enhances the interaction required for efficient energy transfer to PSI in the grana margin regions.

Purification and photobiochemical profile of photosystem 1 from a high-salt tolerant, oleaginous Chlorella (Trebouxiophycaea, Chlorophyta)

The eukaryotic green alga Chlamydomonas reinhardtii has been studied extensively within the biofuel industry as a model organism, as researchers look towards algae to provide chemical feedstocks (i.e., lipids) for the production of liquid transportation fuels. C. reinhardtii, however, is unsuitable for high-level production of such precursors due to its relatively poor lipid accumulation and fresh-water demand. In this study we offer insight into the primary light harvesting and electron transfer reactions that occur during phototropic growth in a high-salt tolerant strain of Chlorella (a novel strain introduced here as NE1401), a single-celled eukaryotic algae also in the phylum Chlorophyta. Under nutrient starvation many eukaryotic algae increase dramatically the amount of lipids stored in lipid bodies within their cell interiors. Microscopy and lipid analyses indicate that Chlorella sp. NE1401 may become a superior candidate for algal biofuels production. We have purified highly active Photosystem 1 (PS1) complexes to study in vitro, so that we may understand further the photobiochemisty of this promising biofuel producer and how its characteristics compare and contrast with that of the better understood C. reinhardtii. Our findings suggest that the PS1 complex from Chlorella sp. NE1401 demonstrates similar characteristics to that of C. reinhardtii with respect to light-harvesting and electron transfer reactions. We also illustrate that the relative extent of the light state transition performed by Chlorella sp. NE1401 is smaller compared to C. reinhardtii, although they are triggered by the same dynamic light stresses.

Toxic effects of heavy metal terbium ion on the composition and functions of cell membrane in horseradish roots

The environmental safety of rare earth elements (REEs), especially the toxic effect of REEs on plants, has attracted increasing attention. However, the cellular mechanism of this toxic effect remains largely unknown. Here, the toxic effects of heavy REE terbium ion [Tb(III)] on the cell membrane of horseradish roots were investigated by using electron microscope autoradiography (EMARG) and histochemical methods. The results indicated that Tb(III) was distributed in the extracellular and intracellular spaces of the roots after horseradish was treated with Tb(III). Moreover, the percentage contents of the unsaturated fatty acids in the membrane lipids, the current of the outward K(+) channel and the average diameter of membrane proteins in the roots of horseradish treated with Tb(III) were decreased; on the contrary, the percentage contents of the saturated fatty acids and malondialdehyde in the roots of horseradish treated with Tb(III) were increased. Furthermore, the contents of intracellular N, P, Mg and Fe in the roots of horseradish treated with Tb(III) were decreased, while the contents of intracellular K and Ca in the roots of horseradish treated with Tb(III) were increased. Finally, the effects of Tb(III) on horseradish roots were increased with increasing concentration or duration of Tb(III) treatment. In conclusion, after horseradish was treated with Tb(III), Tb(III) could enter the cells of horseradish roots and lead to the toxic effects on horseradish, which caused the oxidation of the unsaturated fatty acids in the membrane lipids, the changes in the membrane proteins (including the outward K(+) channel), the decrease in the membrane fluidity, and then the inhibition of the intracellular/extracellular-ion exchange in horseradish roots.

Compartmentalization of the protein repair machinery in photosynthetic membranes

A crucial component of protein homeostasis in cells is the repair of damaged proteins. The repair of oxygen-evolving photosystem II (PS II) supercomplexes in plant chloroplasts is a prime example of a very efficient repair process that evolved in response to the high vulnerability of PS II to photooxidative damage, exacerbated by high-light (HL) stress. Significant progress in recent years has unraveled individual components and steps that constitute the PS II repair machinery, which is embedded in the thylakoid membrane system inside chloroplasts. However, an open question is how a certain order of these repair steps is established and how unwanted back-reactions that jeopardize the repair efficiency are avoided. Here, we report that spatial separation of key enzymes involved in PS II repair is realized by subcompartmentalization of the thylakoid membrane, accomplished by the formation of stacked grana membranes. The spatial segregation of kinases, phosphatases, proteases, and ribosomes ensures a certain order of events with minimal mutual interference. The margins of the grana turn out to be the site of protein degradation, well separated from active PS II in grana core and de novo protein synthesis in unstacked stroma lamellae. Furthermore, HL induces a partial conversion of stacked grana core to grana margin, which leads to a controlled access of proteases to PS II. Our study suggests that the origin of grana in evolution ensures high repair efficiency, which is essential for PS II homeostasis.

Static magnetic field (SMF) sensing of the P(723)/P(689) photosynthetic complex

Moderate intensity SMF have been shown to act as a controller of the protic potential in the coherent milieu of the thylakoid membranes. SMF of the order of 60-500 mT induces memory-like effect in photosystem I (PSI, P723) emission with a correlated oscillation of photosystem II (PSII, P689) fluorescence emission at a temperature of 77 K. The observed magnetic perturbation that affects the thylakoid photon capture circuitry was also found to be associated with the bio-energetic machinery of the thylakoid membranes. At normal pH, SMF causes an enhancement of PSI fluorescence emission intensity (P723/P689 > 1), followed by a slow relaxation on the removal of SMF. The enhancement of the PSI fluorescence intensity also occurs under no-field condition, if either the pH of the medium is lowered, or protonophores, such as carbonyl cyanide chlorophenylhydrazine or nigericin are added (P723/P689≥ 2). If SMF was applied under such a low pH condition or in the presence of protonophore, a reverse effect, particularly, a reduction of the enhanced PSI emission was observed. Because SMF is essentially equivalent to a spin perturbation, the observed effects can be explained in terms of spin re-organization, illustrating a memory effect via membrane re-alignment and assembly. The mimicry of conventional uncouplers by SMF is also notable; the essential difference being the reversibility and manoeuvrability of the latter (SMF). Finally, the effect implies numerous possibilities of externally regulating the photon capture and proton circulation in the thylakoid membranes using controlled SMF.

Dynamic mechanical responses of Arabidopsis thylakoid membranes during PSII-specific illumination

Remodeling of thylakoid membranes in response to illumination is an important process for the regulation of photosynthesis. We investigated the thylakoid network from Arabidopsis thaliana using atomic force microscopy to capture dynamic changes in height, elasticity, and viscosity of isolated thylakoid membranes caused by changes in illumination. We also correlated the mechanical response of the thylakoid network with membrane ultrastructure using electron microscopy. We find that the elasticity of the thylakoid membranes increases immediately upon PSII-specific illumination, followed by a delayed height change. Direct visualization by electron microscopy confirms that there is a significant change in the packing repeat distance of the membrane stacks in response to illumination. Although experiments with Gramicidin show that the change in elasticity depends primarily on the transmembrane pH gradient, the height change requires both the pH gradient and STN7-kinase-dependent phosphorylation of LHCII. Our studies indicate that lumen expansion in response to illumination is not simply a result of the influx of water, and we propose a dynamic model in which protein interactions within the lumen drive these changes.

The architecture of Rhodobacter sphaeroides chromatophores

The chromatophores of Rhodobacter (Rb.) sphaeroides represent a minimal bio-energetic system, which efficiently converts light energy into usable chemical energy. Despite extensive studies, several issues pertaining to the morphology and molecular architecture of this elemental energy conversion system remain controversial or unknown. To tackle these issues, we combined electron microscope tomography, immuno-electron microscopy and atomic force microscopy. We found that the intracellular Rb. sphaeroides chromatophores form a continuous reticulum rather than existing as discrete vesicles. We also found that the cytochrome bc1 complex localizes to fragile chromatophore regions, which most likely constitute the tubular structures that interconnect the vesicles in the reticulum. In contrast, the peripheral light-harvesting complex 2 (LH2) is preferentially hexagonally packed within the convex vesicular regions of the membrane network. Based on these observations, we propose that the bc1 complexes are in the inter-vesicular regions and surrounded by reaction center (RC) core complexes, which in turn are bounded by arrays of peripheral antenna complexes. This arrangement affords rapid cycling of electrons between the core and bc1 complexes while maintaining efficient excitation energy transfer from LH2 domains to the RCs.

Growing green electricity: progress and strategies for use of photosystem I for sustainable photovoltaic energy conversion

Oxygenic photosynthesis is driven via sequential action of Photosystem II (PSII) and (PSI)reaction centers via the Z-scheme. Both of these pigment-membrane protein complexes are found in cyanobacteria, algae, and plants. Unlike PSII, PSI is remarkably stable and does not undergo limiting photo-damage. This stability, as well as other fundamental structural differences, makes PSI the most attractive reaction centers for applied photosynthetic applications. These applied applications exploit the efficient light harvesting and high quantum yield of PSI where the isolated PSI particles are redeployed providing electrons directly as a photocurrent or, via a coupled catalyst to yield H₂. Recent advances in molecular genetics, synthetic biology, and nanotechnology have merged to allow PSI to be integrated into a myriad of biohybrid devices. In photocurrent producing devices, PSI has been immobilized onto various electrode substrates with a continuously evolving toolkit of strategies and novel reagents. However, these innovative yet highly variable designs make it difficult to identify the rate-limiting steps and/or components that function as bottlenecks in PSI-biohybrid devices. In this study we aim to highlight these recent advances with a focus on identifying the similarities and differences in electrode surfaces, immobilization/orientation strategies, and artificial redox mediators. Collectively this work has been able to maintain an annual increase in photocurrent density (Acm⁻²) of ~10-fold over the past decade. The potential drawbacks and attractive features of some of these schemes are also discussed with their feasibility on a large-scale. As an environmentally benign and renewable resource, PSI may provide a new sustainable source of bioenergy. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Quantitative local photosynthetic flux measurements at isolated chloroplasts and thylakoid membranes using scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) offers a fast and quantitative method to measure local fluxes within photosynthesis. In particular, we have measured the flux of oxygen and ferrocyanide (Fe(CN)6(4-)), from the artificial electron acceptor ferricyanide (Fe(CN)6(3-)), using a stationary ultramicroelectrode at chloroplasts and thylakoid membranes (sourced from chloroplasts). Oxygen generation at films of chloroplasts and thylakoid membranes was detected directly during photosynthesis, but in the case of thylakoid membranes, this switched to sustained oxygen consumption at longer illumination times. An initial oxygen concentration spike was detected over both chloroplast and thylakoid membrane films, and the kinetics of the oxygen generation were extracted by fitting the experimental data to a finite element method (FEM) simulation. In contrast to previous work, the oxygen generation spike was attributed to the limited size of the plastoquinone pool, a key component in the linear electron transport pathway and a contributing factor in photoinhibition. Finally, the mobile nature of the SECM probe, and its high spatial resolution, also allowed us to detect ferrocyanide produced from a single thylakoid membrane. These results further demonstrate the power of SECM for localized flux measurements in biological processes, in this case photosynthesis, and that the high time resolution, combined with FEM simulations, allows the elucidation of quantitative kinetic information.

Investigation of photosynthetic membrane structure using atomic force microscopy

Photosynthetic processes, including light capture, electron transfer, and energy conversion, are not only ensured by the activities of individual photosynthetic complexes but also substantially determined and regulated by the composition and assembly of the overall photosynthetic apparatus at the supramolecular level. In recent years, atomic force microscopy (AFM) has matured as a unique and powerful tool for directly assessing the supramolecular assembly of integral membrane protein complexes in their native membrane environment at submolecular resolution. This review highlights the major contributions and advances of AFM studies to our understanding of the structure of the bacterial photosynthetic machinery and its regulatory arrangement during chromatic adaptation. AFM topographs of other biological membrane systems and potential future applications of AFM are also discussed.

Increased imaging speed and force sensitivity for bio-applications with small cantilevers using a conventional AFM setup

In this study, we demonstrate the increased performance in speed and sensitivity achieved by the use of small AFM cantilevers on a standard AFM system. For this, small rectangular silicon oxynitride cantilevers were utilized to arrive at faster atomic force microscopy (AFM) imaging times and more sensitive molecular recognition force spectroscopy (MRFS) experiments. The cantilevers we used had lengths between 13 and 46 μm, a width of about 11 μm, and a thickness between 150 and 600 nm. They were coated with chromium and gold on the backside for a better laser reflection. We characterized these small cantilevers through their frequency spectrum and with electron microscopy. Due to their small size and high resonance frequency we were able to increase the imaging speed by a factor of 10 without any loss in resolution for images from several μm scansize down to the nanometer scale. This was shown on bacterial surface layers (s-layer) with tapping mode under aqueous, near physiological conditions and on nuclear membranes in contact mode in ambient environment. In addition, we showed that single molecular forces can be measured with an up to 5 times higher force sensitivity in comparison to conventional cantilevers with similar spring constants.

Post-genomic insight into thylakoid membrane lateral heterogeneity and redox balance

Photosynthetic machinery requires balanced distribution of excitation energy from the light-harvesting complexes to photosystems. The efficiency of light-harvesting is regulated by thermal dissipation of excess energy, while the distribution of energy between photosystems is dependent on STN7 kinase and phosphorylation of thylakoid proteins. The regulation of excitation energy transfer has been linked to the lateral segregation of photosynthetic complexes along the thylakoid membrane. The study of photosynthetic regulation mechanisms using Arabidopsis mutants, which have been available for the last ten years, has challenged traditional views on regulation of excitation energy distribution. Here, we discuss an urgent need to create a holistic view of the dynamics of the thylakoid membrane using systematic research of the mutants available today.

Composition, architecture and dynamics of the photosynthetic apparatus in higher plants

The process of oxygenic photosynthesis enabled and still sustains aerobic life on Earth. The most elaborate form of the apparatus that carries out the primary steps of this vital process is the one present in higher plants. Here, we review the overall composition and supramolecular organization of this apparatus, as well as the complex architecture of the lamellar system within which it is harbored. Along the way, we refer to the genetic, biochemical, spectroscopic and, in particular, microscopic studies that have been employed to elucidate the structure and working of this remarkable molecular energy conversion device. As an example of the highly dynamic nature of the apparatus, we discuss the molecular and structural events that enable it to maintain high photosynthetic yields under fluctuating light conditions. We conclude the review with a summary of the hypotheses made over the years about the driving forces that underlie the partition of the lamellar system of higher plants and certain green algae into appressed and non-appressed membrane domains and the segregation of the photosynthetic protein complexes within these domains.

Biogenesis of thylakoid networks in angiosperms: knowns and unknowns

Aerobic life on Earth depends on oxygenic photosynthesis. This fundamentally important process is carried out within an elaborate membranous system, called the thylakoid network. In angiosperms, thylakoid networks are constructed almost from scratch by an intricate, light-dependent process in which lipids, proteins, and small organic molecules are assembled into morphologically and functionally differentiated, three-dimensional lamellar structures. In this review, we summarize the major events that occur during this complex, largely elusive process, concentrating on those that are directly involved in network formation and potentiation and highlighting gaps in our knowledge, which, as hinted by the title, are substantial.

Supramolecular organization of photosystem II in green plants

Green plant photosystem II (PSII) is involved in the light reactions of photosynthesis, which take place in the thylakoid membrane of the chloroplast. PSII is organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. These supercomplexes are dimeric and contain usually 2-4 copies of trimeric LHCII complexes and have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. This review focuses on the overall composition and structure of the PSII supercomplex of green plants and its organization and interactions within the photosynthetic membrane. Further, we present the current knowledge how the thylakoid membrane is three-dimensionally organized within the chloroplast. We also discuss how the supramolecular organization in the thylakoid membrane and the PSII flexibility may play roles in various short-term regulatory mechanisms of green plant photosynthesis. This article is part of a Special Issue entitled: Photosystem II.

The plasma membrane of the cyanobacterium Gloeobacter violaceus contains segregated bioenergetic domains

The light reactions of oxygenic photosynthesis almost invariably take place in the thylakoid membranes, a highly specialized internal membrane system located in the stroma of chloroplasts and the cytoplasm of cyanobacteria. The only known exception is the primordial cyanobacterium Gloeobacter violaceus, which evolved before the appearance of thylakoids and harbors the photosynthetic complexes in the plasma membrane. Thus, studies on G. violaceus not only shed light on the evolutionary origin and the functional advantages of thylakoid membranes but also might include insights regarding thylakoid formation during chloroplast differentiation. Based on biochemical isolation and direct in vivo characterization, we report here structural and functional domains in the cytoplasmic membrane of a cyanobacterium. Although G. violaceus has no internal membranes, it does have localized domains with apparently specialized functions in its plasma membrane, in which both the photosynthetic and the respiratory complexes are concentrated. These bioenergetic domains can be visualized by confocal microscopy, and they can be isolated by a simple procedure. Proteomic analysis of these domains indicates their physiological function and suggests a protein sorting mechanism via interaction with membrane-intrinsic terpenoids. Based on these results, we propose specialized domains in the plasma membrane as evolutionary precursors of thylakoids.

3-D modelling of chloroplast structure under (Mg2+) magnesium ion treatment. Relationship between thylakoid membrane arrangement and stacking

We performed for the first time three-dimensional (3D) modelling of the entire chloroplast structure. Stacks of optical slices obtained by confocal laser scanning microscope (CLSM) provided a basis for construction of 3D images of individual chloroplasts. We selected pea (Pisum sativum) and bean (Phaseolus vulgaris) chloroplasts since we found that they differ in thylakoid organization. Pea chloroplasts contain large distinctly separated appressed domains while less distinguished appressed regions are present in bean chloroplasts. Different magnesium ion treatments were used to study thylakoid membrane stacking and arrangement. In pea chloroplasts, as demonstrated by 3D modelling, the increase of magnesium ion concentration changed the degree of membrane appression from wrinkled continuous surface to many distinguished stacked areas and significant increase of the inter-grana area. On the other hand 3D models of bean chloroplasts exhibited similar but less pronounced tendencies towards formation of appressed regions. Additionally, we studied arrangements of thylakoid membranes and chlorophyll-protein complexes by various spectroscopic methods, Fourier-transform infrared spectroscopy (FTIR) among others. Based on microscopic and spectroscopic data we suggested that the range of chloroplast structure alterations under magnesium ions treatment is a consequence of the arrangement of supercomplexes. Moreover, we showed that stacking processes always affect the structural changes of chloroplast as a whole.

Early transcriptomic changes induced by magnesium deficiency in Arabidopsis thaliana reveal the alteration of circadian clock gene expression in roots and the triggering of abscisic acid-responsive genes

*Plant growth and development ultimately depend on environmental variables such as the availability of essential minerals. Unravelling how nutrients affect gene expression will help to understand how they regulate plant growth. *This study reports the early transcriptomic response to magnesium (Mg) deprivation in Arabidopsis. Whole-genome transcriptome was studied in the roots and young mature leaves 4, 8 and 28 h after the removal of Mg from the nutrient solution. *The highest number of regulated genes was first observed in the roots. Contrary to other mineral deficiencies, Mg depletion did not induce a higher expression of annotated genes in Mg uptake. Remarkable responses include the perturbation of the central oscillator of the circadian clock in roots and the triggering of abscisic acid (ABA) signalling, with half of the up-regulated Mg genes in leaves being ABA-responsive. However, no change in ABA content was observed. *The specificity of the response of some Mg-regulated genes was challenged by studying their expression after other mineral deficiencies and environmental stresses. The possibility to develop markers for Mg incipient deficiency is discussed here.

Thylakoid protein phosphorylation in higher plant chloroplasts optimizes electron transfer under fluctuating light

Several proteins of photosystem II (PSII) and its light-harvesting antenna (LHCII) are reversibly phosphorylated according to light quantity and quality. Nevertheless, the interdependence of protein phosphorylation, nonphotochemical quenching, and efficiency of electron transfer in the thylakoid membrane has remained elusive. These questions were addressed by investigating in parallel the wild type and the stn7, stn8, and stn7 stn8 kinase mutants of Arabidopsis (Arabidopsis thaliana), using the stn7 npq4, npq4, npq1, and pgr5 mutants as controls. Phosphorylation of PSII-LHCII proteins is strongly and dynamically regulated according to white light intensity. Yet, the changes in phosphorylation do not notably modify the relative excitation energy distribution between PSII and PSI, as typically occurs when phosphorylation is induced by "state 2" light that selectively excites PSII and induces the phosphorylation of both the PSII core and LHCII proteins. On the contrary, under low-light conditions, when excitation energy transfer from LHCII to reaction centers is efficient, the STN7-dependent LHCII protein phosphorylation guarantees a balanced distribution of excitation energy to both photosystems. The importance of this regulation diminishes at high light upon induction of thermal dissipation of excitation energy. Lack of the STN7 kinase, and thus the capacity for equal distribution of excitation energy to PSII and PSI, causes relative overexcitation of PSII under low light but not under high light, leading to disturbed maintenance of fluent electron flow under fluctuating light intensities. The physiological relevance of the STN7-dependent regulation is evidenced by severely stunted phenotypes of the stn7 and stn7 stn8 mutants under strongly fluctuating light conditions.

Mapping of proteomic composition on the surfaces of bacillus spores by atomic force microscopy-based immunolabeling

Atomic force microscopy (AFM) provides a unique capability to image high-resolution architecture and structural dynamics of pathogens (e.g., viruses, bacteria, and bacterial spores) at near-molecular resolution in native conditions. Further development of atomic force microscopy to enable the correlation of pathogen protein surface structures with specific gene products is essential to understand the mechanisms of the pathogen life cycle. We applied an AFM-based immunolabeling technique for the proteomic mapping of macromolecular structures through the visualization of the binding of antibodies, conjugated with nanogold particles, to specific epitopes on Bacillus spore surfaces. This information is generated while simultaneously acquiring the surface morphology of the pathogen. The immunospecificity of this labeling method was established through the utilization of specific polyclonal and monoclonal antibodies that target spore coat and exosporium epitopes of Bacillus atrophaeus and Bacillus anthracis spores.

Watching the native supramolecular architecture of photosynthetic membrane in red algae: topography of phycobilisomes and their crowding, diverse distribution patterns

The architecture of the entire photosynthetic membrane network determines, at the supramolecular level, the physiological roles of the photosynthetic protein complexes involved. So far, a precise picture of the native configuration of red algal thylakoids is still lacking. In this work, we investigated the supramolecular architectures of phycobilisomes (PBsomes) and native thylakoid membranes from the unicellular red alga Porphyridium cruentum using atomic force microscopy (AFM) and transmission electron microscopy. The topography of single PBsomes was characterized by AFM imaging on both isolated and membrane-combined PBsomes complexes. The native organization of thylakoid membranes presented variable arrangements of PBsomes on the membrane surface. It indicates that different light illuminations during growth allow diverse distribution of PBsomes upon the isolated photosynthetic membranes from P. cruentum, random arrangement or rather ordered arrays, to be observed. Furthermore, the distributions of PBsomes on the membrane surfaces are mostly crowded. This is the first investigation using AFM to visualize the native architecture of PBsomes and their crowding distribution on the thylakoid membrane from P. cruentum. Various distribution patterns of PBsomes under different light conditions indicate the photoadaptation of thylakoid membranes, probably promoting the energy-harvesting efficiency. These results provide important clues on the supramolecular architecture of red algal PBsomes and the diverse organizations of thylakoid membranes in vivo.

Thylakoid membrane remodeling during state transitions in Arabidopsis

Adaptability of oxygenic photosynthetic organisms to fluctuations in light spectral composition and intensity is conferred by state transitions, short-term regulatory processes that enable the photosynthetic apparatus to rapidly adjust to variations in light quality. In green algae and higher plants, these processes are accompanied by reversible structural rearrangements in the thylakoid membranes. We studied these structural changes in the thylakoid membranes of Arabidopsis thaliana chloroplasts using atomic force microscopy, scanning and transmission electron microscopy, and confocal imaging. Based on our results and on the recently determined three-dimensional structure of higher-plant thylakoids trapped in one of the two major light-adapted states, we propose a model for the transitions in membrane architecture. The model suggests that reorganization of the membranes involves fission and fusion events that occur at the interface between the appressed (granal) and nonappressed (stroma lamellar) domains of the thylakoid membranes. Vertical and lateral displacements of the grana layers presumably follow these localized events, eventually leading to macroscopic rearrangements of the entire membrane network.

Phosphorylation-dependent regulation of excitation energy distribution between the two photosystems in higher plants

Phosphorylation-dependent movement of the light-harvesting complex II (LHCII) between photosystem II (PSII) and photosystem I (PSI) takes place in order to balance the function of the two photosystems. Traditionally, the phosphorylatable fraction of LHCII has been considered as the functional unit of this dynamic regulation. Here, a mechanical fractionation of the thylakoid membrane of Spinacia oleracea was performed from leaves both in the phosphorylated state (low light, LL) and in the dephosphorylated state (dark, D) in order to compare the phosphorylation-dependent protein movements with the excitation changes occurring in the two photosystems upon LHCII phosphorylation. Despite the fact that several LHCII proteins migrate to stroma lamellae when LHCII is phosphorylated, no increase occurs in the 77 K fluorescence emitted from PSI in this membrane fraction. On the contrary, such an increase in fluorescence occurs in the grana margin fraction, and the functionally important mobile unit is the PSI-LHCI complex. A new model for LHCII phosphorylation driven regulation of relative PSII/PSI excitation thus emphasises an increase in PSI absorption cross-section occurring in grana margins upon LHCII phosphorylation and resulting from the movement of PSI-LHCI complexes from stroma lamellae and subsequent co-operation with the P-LHCII antenna from the grana. The grana margins probably give a flexibility for regulation of linear and cyclic electron flow in plant chloroplasts.

Probing the organization of photosystem II in photosynthetic membranes by atomic force microscopy

Efficient photosynthetic energy transduction and its regulation depend on a precise supramolecular arrangement of the plant photosystem II (PSII) complex in grana membranes of chloroplasts. The topography of isolated photosystem II supercomplexes and the supramolecular organization of this complex in grana membrane preparations are visualized by high-resolution atomic force microscopy (AFM) in air in tapping mode with an active feedback control to minimize tip-sample interactions. Systematic comparison between topographic characteristics of the protrusions in atomic force microscopic images and well-established high-resolution and freeze-fracture electron microscopic data shows that the photosystem II organization can be properly imaged by AFM in air. Taking the protruding water-splitting apparatus as a topographic marker for PSII, its distribution and orientation in isolated grana membrane were analyzed. For the latter a new mathematical procedure was established, which revealed a preference for a parallel alignment of PSII that resembles the organization in highly ordered semicrystalline arrays. Furthermore, by analyzing the height of grana membrane stacks, we conclude that lumenal protrusions of adjacent photosystem II complexes in opposing membranes are displaced relative to each other. The functional consequences for lateral migration processes are discussed.

Contrasting effect of dark-chilling on chloroplast structure and arrangement of chlorophyll-protein complexes in pea and tomato: plants with a different susceptibility to non-freezing temperature

The effect of dark-chilling and subsequent photoactivation on chloroplast structure and arrangements of chlorophyll-protein complexes in thylakoid membranes was studied in chilling-tolerant (CT) pea and in chilling-sensitive (CS) tomato. Dark-chilling did not influence chlorophyll content and Chl a/b ratio in thylakoids of both species. A decline of Chl a fluorescence intensity and an increase of the ratio of fluorescence intensities of PSI and PSII at 120 K was observed after dark-chilling in thylakoids isolated from tomato, but not from pea leaves. Chilling of pea leaves induced an increase of the relative contribution of LHCII and PSII fluorescence. A substantial decrease of the LHCII/PSII fluorescence accompanied by an increase of that from LHCI/PSI was observed in thylakoids from chilled tomato leaves; both were attenuated by photoactivation. Chlorophyll fluorescence of bright grana discs in chloroplasts from dark-chilled leaves, detected by confocal laser scanning microscopy, was more condensed in pea but significantly dispersed in tomato, compared with control samples. The chloroplast images from transmission-electron microscopy revealed that dark-chilling induced an increase of the degree of grana stacking only in pea chloroplasts. Analyses of O-J-D-I-P fluorescence induction curves in leaves of CS tomato before and after recovery from chilling indicate changes in electron transport rates at acceptor- and donor side of PS II and an increase in antenna size. In CT pea leaves these effects were absent, except for a small but irreversible effect on PSII activity and antenna size. Thus, the differences in chloroplast structure between CS and CT plants, induced by dark-chilling are a consequence of different thylakoid supercomplexes rearrangements.

Evidences for interaction of PsbS with photosynthetic complexes in maize thylakoids

The PsbS subunit of Photosystem II (PSII) has received much attention in the past few years, given its crucial role in photoprotection of higher plants. The exact location of this small subunit in thylakoids is also debated. In this work possible interaction partners of PsbS have been identified by immunoaffinity and immunoprecipitation, performed with mildly solubilized whole thylakoid membrane. The interacting proteins, as identified by mass spectrometry analysis of the immunoaffinity eluate, include CP29, some LHCII components, but also components of Photosystem I, of the cytochrome b(6)f complex as well as of ATP synthase. These proteins can be co-immunoprecipitated by using highly specific anti-PsbS antibodies and, vice-versa, PsbS is co-immunoprecipitated by antisera against components of the interacting complexes. We also find that PsbS co-migrates with bands containing PSII, ATP synthase and cytochrome b(6)f as well as with LHCII-containing bands on non-denaturing Deriphat PAGE. These results suggest multiple location of PsbS in the thylakoid membrane and point to an unexpected lateral mobility of this PSII subunit. As revealed by immunogold labelling with antibody against PsbS, the protein is associated either with granal membranes or prevalently with stroma lamellae in low or high-intensity light-treated intact leaves, respectively. This finding is consistent with the capability of PsbS to interact with complexes located in stroma lamellae, even though the exact physiological condition(s) under which these interactions may take place remain to be clarified.

Structure and function of photosystems I and II

Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b(6)f complex, and F-ATPase. PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems. PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H(2)O, a substrate so abundant that it assures a practically unlimited electron source for life on earth. During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems. The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis.

Immunogold detection of types I and II chondrocyte collagen fibrils: An in situ atomic force microscopic investigation

Chondrocyte tissue engineering is a major challenge in the field of cartilage repair. The phenotype of chondrocytes consists of cartilage specific proteoglycan and type II collagen. During serial passages, chondrocytes dedifferentiate into cells, presenting a fibroblast-like phenotype consisting predominately of type I collagen synthesis. Observation of native collagen fibers could be visualized by atomic force microscope. Here, we developed an original and useful atomic force microscopy-based immunogold technique allowing biochemical distinction between types I and II collagen fibers. Imaging of 40-nm gold particles staining collagen fibers was performed in tapping mode. Rat 1 fibroblasts and human chondrosarcoma cells were used as positive models for types I and II collagen, respectively. As demonstrated by our data, primary rat chondrocytes adhering for 48 h on a glass substrate synthesize type II collagen native fibers. This technique allows analyses of local areas of the extracellular matrix of fixed cells, providing complementary data about cartilage phenotype. This simple approach could be of major interest for the biologist community in routine laboratory investigations, to localize in situ, macromolecules of the extracellular matrix.

A Circadian Timing Mechanism in the Cyanobacteria

Cyanobacteria such as Synechococcus elongatus PCC 7942, Thermosynechococcus elongatus BP-1, and Synechocystis species strain PCC 6803 have an endogenous timing mechanism that can generate and maintain a 24 h (circadian) periodicity to global (whole genome) gene expression patterns. This rhythmicity extends to many other physiological functions, including chromosome compaction. These rhythmic patterns seem to reflect the periodicity of availability of the primary energy source for these photoautotrophic organisms, the Sun. Presumably, eons of environmentally derived rhythmicity--light/dark cycles--have simply been mechanistically incorporated into the regulatory networks of these cyanobacteria. Genetic and biochemical experimentation over the last 15 years has identified many key components of the primary timing mechanism that generates rhythmicity, the input pathways that synchronize endogenous rhythms to exogenous rhythms, and the output pathways that transduce temporal information from the timekeeper to the regulators of gene expression and function. Amazingly, the primary timing mechanism has evidently been extracted from S. elongatus PCC 7942 and can also keep time in vitro. Mixing the circadian clock proteins KaiA, KaiB, and KaiC from S. elongatus PCC 7942 in vitro and adding ATP results in a circadian rhythm in the KaiC protein phosphorylation state. Nonetheless, many questions still loom regarding how this circadian clock mechanism works, how it communicates with the environment and how it regulates temporal patterns of gene expression. Many details regarding structure and function of the individual clock-related proteins are provided here as a basis to discuss these questions. A strong, data-intensive foundation has been developed to support the working model for the cyanobacterial circadian regulatory system. The eventual addition to that model of the metabolic parameters participating in the command and control of this circadian global regulatory system will ultimately allow a fascinating look into whole-cell physiology and metabolism and the consequential organization of global gene expression patterns.

Microscopy and single molecule detection in photosynthesis

Progress in various fields of microscopy techniques brought up enormous possibilities to study the photosynthesis down to the level of individual pigment-protein complexes. The aim of this review is to present recent developments in the photosynthesis research obtained using such highly advanced techniques. Three areas of microscopy techniques covering optical microscopy, electron microscopy and scanning probe microscopy are reviewed. Whereas the electron microscopy and scanning probe microscopy are used in photosynthesis mainly for structural studies of photosynthetic pigment-protein complexes, the optical microscopy is used also for functional studies.