Architecture of Thylakoid Membrane Networks

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.

A novel cyanobacterial geosmin producer, revising GeoA distribution and dispersion patterns in Bacteria

Cyanobacteria are ubiquitous organisms with a relevant contribution to primary production in all range of habitats. Cyanobacteria are well known for their part in worldwide occurrence of aquatic blooms while producing a myriad of natural compounds, some with toxic potential, but others of high economical impact, as geosmin. We performed an environmental survey of cyanobacterial soil colonies to identify interesting metabolic pathways and adaptation strategies used by these microorganisms and isolated, sequenced and assembled the genome of a cyanobacterium that displayed a distinctive earthy/musty smell, typical of geosmin, confirmed by GC-MS analysis of the culture's volatile extract. Morphological studies pointed to a new Oscillatoriales soil ecotype confirmed by phylogenetic analysis, which we named Microcoleus asticus sp. nov. Our studies of geosmin gene presence in Bacteria, revealed a scattered distribution among Cyanobacteria, Actinobacteria, Delta and Gammaproteobacteria, covering different niches. Careful analysis of the bacterial geosmin gene and gene tree suggests an ancient bacterial origin of the gene, that was probably successively lost in different time frames. The high sequence similarities in the cyanobacterial geosmin gene amidst freshwater and soil strains, reinforce the idea of an evolutionary history of geosmin, that is intimately connected to niche adaptation.

Fundamental helical geometry consolidates the plant photosynthetic membrane

Plant photosynthetic (thylakoid) membranes are organized into complex networks that are differentiated into 2 distinct morphological and functional domains called grana and stroma lamellae. How the 2 domains join to form a continuous lamellar system has been the subject of numerous studies since the mid-1950s. Using different electron tomography techniques, we found that the grana and stroma lamellae are connected by an array of pitch-balanced right- and left-handed helical membrane surfaces of different radii and pitch. Consistent with theoretical predictions, this arrangement is shown to minimize the surface and bending energies of the membranes. Related configurations were proposed to be present in the rough endoplasmic reticulum and in dense nuclear matter phases theorized to exist in neutron star crusts, where the right- and left-handed helical elements differ only in their handedness. Pitch-balanced helical elements of alternating handedness may thus constitute a fundamental geometry for the efficient packing of connected layers or sheets.

Transcriptome analysis highlights nuclear control of chloroplast development in the shoot apex

In dicots, the key developmental process by which immature plastids differentiate into photosynthetically competent chloroplasts commences in the shoot apical meristem (SAM), within the shoot apex. Using laser-capture microdissection and single-cell RNA sequencing methodology, we studied the changes in the transcriptome along the chloroplast developmental pathway in the shoot apex of tomato seedlings. The analysis revealed the presence of transcripts for different chloroplast functions already in the stem cell-containing region of the SAM. Thereafter, an en masse up-regulation of genes encoding for various proteins occurs, including chloroplast ribosomal proteins and proteins involved in photosynthesis, photoprotection and detoxification of reactive oxygen species. The results highlight transcriptional events that operate during chloroplast biogenesis, leading to the rapid establishment of photosynthetic competence.

Pea PSII-LHCII supercomplexes form pairs by making connections across the stromal gap

In higher plant thylakoids, the heterogeneous distribution of photosynthetic protein complexes is a determinant for the formation of grana, stacks of membrane discs that are densely populated with Photosystem II (PSII) and its light harvesting complex (LHCII). PSII associates with LHCII to form the PSII-LHCII supercomplex, a crucial component for solar energy conversion. Here, we report a biochemical, structural and functional characterization of pairs of PSII-LHCII supercomplexes, which were isolated under physiologically-relevant cation concentrations. Using single-particle cryo-electron microscopy, we determined the three-dimensional structure of paired C2S2M PSII-LHCII supercomplexes at 14 Å resolution. The two supercomplexes interact on their stromal sides through a specific overlap between apposing LHCII trimers and via physical connections that span the stromal gap, one of which is likely formed by interactions between the N-terminal loops of two Lhcb4 monomeric LHCII subunits. Fast chlorophyll fluorescence induction analysis showed that paired PSII-LHCII supercomplexes are energetically coupled. Molecular dynamics simulations revealed that additional flexible physical connections may form between the apposing LHCII trimers of paired PSII-LHCII supercomplexes in appressed thylakoid membranes. Our findings provide new insights into how interactions between pairs of PSII-LHCII supercomplexes can link adjacent thylakoids to mediate the stacking of grana membranes.

Low-pH induced reversible reorganizations of chloroplast thylakoid membranes - As revealed by small-angle neutron scattering

Energization of thylakoid membranes brings about the acidification of the lumenal aqueous phase, which activates important regulatory mechanisms. Earlier Jajoo and coworkers (2014 FEBS Lett. 588:970) have shown that low pH in isolated plant thylakoid membranes induces changes in the excitation energy distribution between the two photosystems. In order to elucidate the structural background of these changes, we used small-angle neutron scattering on thylakoid membranes exposed to low p2H (pD) and show that gradually lowering the p2H from 8.0 to 5.0 causes small but well discernible reversible diminishment of the periodic order and the lamellar repeat distance and an increased mosaicity - similar to the effects elicited by light-induced acidification of the lumen. Our data strongly suggest that thylakoids dynamically respond to the membrane energization and actively participate in different regulatory mechanisms.

Lipid and carotenoid cooperation-driven adaptation to light and temperature stress in Synechocystis sp. PCC6803

Polyunsaturated lipids are important components of photosynthetic membranes. Xanthophylls are the main photoprotective agents, can assist in protection against light stress, and are crucial in the recovery from photoinhibition. We generated the xanthophyll- and polyunsaturated lipid-deficient ROAD mutant of Synechocystis sp. PCC6803 (Synechocystis) in order to study the little-known cooperative effects of lipids and carotenoids (Cars). Electron microscopic investigations confirmed that in the absence of xanthophylls the S-layer of the cellular envelope is missing. In wild-type (WT) cells, as well as the xanthophyll-less (RO), polyunsaturated lipid-less (AD), and the newly constructed ROAD mutants the lipid and Car compositions were determined by MS and HPLC, respectively. We found that, relative to the WT, the lipid composition of the mutants was remodeled and the Car content changed accordingly. In the mutants the ratio of non-bilayer-forming (NBL) to bilayer-forming (BL) lipids was found considerably lower. Xanthophyll to β-carotene ratio increased in the AD mutant. In vitro and in vivo methods demonstrated that saturated, monounsaturated lipids and xanthophylls may stabilize the trimerization of Photosystem I (PSI). Fluorescence induction and oxygen-evolving activity measurements revealed increased light sensitivity of RO cells compared to those of the WT. ROAD showed a robust increase in light susceptibility and reduced recovery capability, especially at moderate low (ML) and moderate high (MH) temperatures, indicating a cooperative effect of xanthophylls and polyunsaturated lipids. We suggest that both lipid unsaturation and xanthophylls are required for providing the proper structure and functioning of the membrane environment that protects against light and temperature stress.

Isolation of novel PSII-LHCII megacomplexes from pea plants characterized by a combination of proteomics and electron microscopy

In higher plants, photosystem II (PSII) is a multi-subunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts, where it is present mostly in dimeric form within the grana. Its light-harvesting antenna system, LHCII, is composed of trimeric and monomeric complexes, which can associate in variable number with the dimeric PSII core complex in order to form different types of PSII-LHCII supercomplexes. Moreover, PSII-LHCII supercomplexes can laterally associate within the thylakoid membrane plane, thus forming higher molecular mass complexes, termed PSII-LHCII megacomplexes (Boekema et al. 1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452). In this study, pure PSII-LHCII megacomplexes were directly isolated from stacked pea thylakoid membranes by a rapid single-step solubilization, using the detergent n-dodecyl-α-D-maltoside, followed by sucrose gradient ultracentrifugation. The megacomplexes were subjected to biochemical and structural analyses. Transmission electron microscopy on negatively stained samples, followed by single-particle analyses, revealed a novel form of PSII-LHCII megacomplexes, as compared to previous studies (Boekema et al.1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452), consisting of two PSII-LHCII supercomplexes sitting side-by-side in the membrane plane, sandwiched together with a second copy. This second copy of the megacomplex is most likely derived from the opposite membrane of a granal stack. Two predominant forms of intact sandwiched megacomplexes were observed and termed, according to (Dekker and Boekema 2005 Biochim Biophys Acta 1706:12-39), as (C2S2)4 and (C2S2 + C2S2M2)2 megacomplexes. By applying a gel-based proteomic approach, the protein composition of the isolated megacomplexes was fully characterized. In summary, the new structural forms of isolated megacomplexes and the related modeling performed provide novel insights into how PSII-LHCII supercomplexes may bind to each other, not only in the membrane plane, but also between granal stacks within the chloroplast.

Robust Min-system oscillation in the presence of internal photosynthetic membranes in cyanobacteria

The oscillatory Min system of Escherichia coli defines the cell division plane by regulating the site of FtsZ-ring formation and represents one of the best-understood examples of emergent protein self-organization in nature. The oscillatory patterns of the Min-system proteins MinC, MinD and MinE (MinCDE) are strongly dependent on the geometry of membranes they bind. Complex internal membranes within cyanobacteria could disrupt this self-organization by sterically occluding or sequestering MinCDE from the plasma membrane. Here, it was shown that the Min system in the cyanobacterium Synechococcus elongatus PCC 7942 oscillates from pole-to-pole despite the potential spatial constraints imposed by their extensive thylakoid network. Moreover, reaction-diffusion simulations predict robust oscillations in modeled cyanobacterial cells provided that thylakoid network permeability is maintained to facilitate diffusion, and suggest that Min proteins require preferential affinity for the plasma membrane over thylakoids to correctly position the FtsZ ring. Interestingly, in addition to oscillating, MinC exhibits a midcell localization dependent on MinD and the DivIVA-like protein Cdv3, indicating that two distinct pools of MinC are coordinated in S. elongatus. Our results provide the first direct evidence for Min oscillation outside of E. coli and have broader implications for Min-system function in bacteria and organelles with internal membrane systems.

Dynamic regulation of photosynthesis in Chlamydomonas reinhardtii

Plants and algae have acquired the ability to acclimatize to ever-changing environments to survive. During photosynthesis, light energy is converted by several membrane protein supercomplexes into electrochemical energy, which is eventually used to assimilate CO2 . The efficiency of photosynthesis is modulated by many environmental factors, including temperature, drought, CO2 concentration, and the quality and quantity of light. Recently, our understanding of such regulators of photosynthesis and the underlying molecular mechanisms has increased considerably. The photosynthetic supercomplexes undergo supramolecular reorganizations within a short time after receiving environmental cues. These reorganizations include state transitions that balance the excitation of the two photosystems: qE quenching, which thermally dissipates excess energy at the level of the light-harvesting antenna, and cyclic electron flow, which supplies the increased ATP demanded by CO2 assimilation and the pH gradient to activate qE quenching. This review focuses on the recent findings regarding the environmental regulation of photosynthesis in model organisms, paying particular attention to the unicellular green alga Chlamydomonas reinhardtii, which offer a glimpse into the dynamic behavior of photosynthetic machinery in nature.

Structural changes of the thylakoid membrane network induced by high light stress in plant chloroplasts

Land plants live in a challenging environment dominated by unpredictable changes. A particular problem is fluctuation in sunlight intensity that can cause irreversible damage of components of the photosynthetic apparatus in thylakoid membranes under high light conditions. Although a battery of photoprotective mechanisms minimize damage, photoinhibition of the photosystem II (PSII) complex occurs. Plants have evolved a multi-step PSII repair cycle that allows efficient recovery from photooxidative PSII damage. An important feature of the repair cycle is its subcompartmentalization to stacked grana thylakoids and unstacked thylakoid regions. Thus, understanding the crosstalk between stacked and unstacked thylakoid membranes is essential to understand the PSII repair cycle. This review summarizes recent progress in our understanding of high-light-induced structural changes of the thylakoid membrane system and correlates these changes to the efficiency of the PSII repair cycle. The role of reversible protein phosphorylation for structural alterations is discussed. It turns out that dynamic changes in thylakoid membrane architecture triggered by high light exposure are central for efficient repair of PSII.

The ultrastructure and flexibility of thylakoid membranes in leaves and isolated chloroplasts as revealed by small-angle neutron scattering

We studied the periodicity of the multilamellar membrane system of granal chloroplasts in different isolated plant thylakoid membranes, using different suspension media, as well as on different detached leaves and isolated protoplasts-using small-angle neutron scattering. Freshly isolated thylakoid membranes suspended in isotonic or hypertonic media, containing sorbitol supplemented with cations, displayed Bragg peaks typically between 0.019 and 0.023Å(-1), corresponding to spatially and statistically averaged repeat distance values of about 275-330 Å⁻¹. Similar data obtained earlier led us in previous work to propose an origin from the periodicity of stroma thylakoid membranes. However, detached leaves, of eleven different species, infiltrated with or soaked in D2O in dim laboratory light or transpired with D2O prior to measurements, exhibited considerably smaller repeat distances, typically between 210 and 230 Å⁻¹, ruling out a stromal membrane origin. Similar values were obtained on isolated tobacco and spinach protoplasts. When NaCl was used as osmoticum, the Bragg peaks of isolated thylakoid membranes almost coincided with those in the same batch of leaves and the repeat distances were very close to the electron microscopically determined values in the grana. Although neutron scattering and electron microscopy yield somewhat different values, which is not fully understood, we can conclude that small-angle neutron scattering is a suitable technique to study the periodic organization of granal thylakoid membranes in intact leaves under physiological conditions and with a time resolution of minutes or shorter. We also show here, for the first time on leaves, that the periodicity of thylakoid membranes in situ responds dynamically to moderately strong illumination. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Architectural switch in plant photosynthetic membranes induced by light stress

Unavoidable side reactions of photosynthetic energy conversion can damage the water-splitting photosystem II (PSII) holocomplex embedded in the thylakoid membrane system inside chloroplasts. Plant survival is crucially dependent on an efficient molecular repair of damaged PSII realized by a multistep repair cycle. The PSII repair cycle requires a brisk lateral protein traffic between stacked grana thylakoids and unstacked stroma lamellae that is challenged by the tight stacking and low protein mobility in grana. We demonstrated that high light stress induced two main structural changes that work synergistically to improve the accessibility between damaged PSII in grana and its repair machinery in stroma lamellae: lateral shrinkage of grana diameter and increased protein mobility in grana thylakoids. It follows that high light stress triggers an architectural switch of the thylakoid network that is advantageous for swift protein repair. Studies of the thylakoid kinase mutant stn8 and the double mutant stn7/8 demonstrate the central role of protein phosphorylation for the structural alterations. These findings are based on the elaboration of mathematical tools for analyzing confocal laser-scanning microscopic images to study changes in the sophisticated thylakoid architecture in intact protoplasts.

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.

Dynamic control of protein diffusion within the granal thylakoid lumen

The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark- and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.

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.

Insights into the complex 3-D architecture of thylakoid membranes in unicellular cyanobacterium Cyanothece sp. ATCC 51142

In cyanobacteria and chloroplasts, thylakoids are the complex internal membrane system where the light reactions of oxygenic photosynthesis occur. In plant chloroplasts, thylakoids are differentiated into a highly interconnected system of stacked grana and unstacked stroma membranes. In contrast, in cyanobacteria, the evolutionary progenitors of chloroplasts, thylakoids do not routinely form stacked and unstacked regions, and the architecture of the thylakoid membrane systems is only now being described in detail in these organisms. We used electron tomography to examine the thylakoid membrane systems in one cyanobacterium, Cyanothece sp. ATCC 51142. Our data showed that thylakoids form a complicated branched network with a rudimentary quasi-helical architecture in this organism. A well accepted helical model of grana-stroma architecture of plant thylakoids describes an organization in which stroma thylakoids wind around stacked granum in right-handed spirals. Here we present data showing that the simplified helical architecture in Cyanothece 51142 is left-handed in nature. We propose a model comparing the thylakoid membranes in plants and this cyanobacterium in which the system in Cyanothece 51142 is composed of non-stacked membranes linked by fret-like connections to other membrane components of the system in a limited left-handed arrangement.