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Concept: Thylakoid


Despite our increasingly sophisticated understanding of mechanisms ensuring efficient photosynthesis under laboratory-controlled light conditions, less is known about the regulation of photosynthesis under fluctuating light. This is important because-in nature-photosynthetic organisms experience rapid and extreme changes in sunlight, potentially causing deleterious effects on photosynthetic efficiency and productivity. Here we report that the chloroplast thylakoid lumenal protein MAINTENANCE OF PHOTOSYSTEM II UNDER HIGH LIGHT 2 (MPH2; encoded by At4g02530) is required for growth acclimation of Arabidopsis thaliana plants under controlled photoinhibitory light and fluctuating light environments. Evidence is presented that mph2 mutant light stress susceptibility results from a defect in photosystem II (PSII) repair, and our results are consistent with the hypothesis that MPH2 is involved in disassembling monomeric complexes during regeneration of dimeric functional PSII supercomplexes. Moreover, mph2-and previously characterized PSII repair-defective mutants-exhibited reduced growth under fluctuating light conditions, while PSII photoprotection-impaired mutants did not. These findings suggest that repair is not only required for PSII maintenance under static high-irradiance light conditions but is also a regulatory mechanism facilitating photosynthetic adaptation under fluctuating light environments. This work has implications for improvement of agricultural plant productivity through engineering PSII repair.

Concepts: Cyanobacteria, Photosynthesis, Chloroplast, Plant, Photosystem, Chlorophyll, Thylakoid, Light-dependent reactions


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.

Concepts: Cyanobacteria, Photosynthesis, Chloroplast, Plastocyanin, Chlorophyll, Thylakoid, Light-dependent reactions, ATP synthase


Thylakoid membranes, the universal structure where photosynthesis takes place in all oxygenic photosynthetic organisms from cyanobacteria to higher plants, have a unique lipid composition. They contain a high fraction of 2 uncharged glycolipids, the galactoglycerolipids mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and an anionic sulfolipid, sulfoquinovosediacylglycerol (SQDG). A remarkable feature of the evolution from cyanobacteria to higher plants is the conservation of MGDG, DGDG, SQDG, and phosphatidylglycerol (PG), the major phospholipid of thylakoids. Using neutron diffraction on reconstituted thylakoid lipid extracts, we observed that the thylakoid lipid mixture self-organizes as a regular stack of bilayers. This natural lipid mixture was shown to switch from hexagonal II toward lamellar phase on hydration. This transition and the observed phase coexistence are modulated by the fine-tuning of the lipid profile, in particular the MGDG/DGDG ratio, and by the hydration. Our analysis highlights the critical role of DGDG as a contributing component to the membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. DGDG interactions balance the repulsive electrostatic contribution of the charged lipids PG and SQDG and allow the persistence of regularly stacked membranes at high hydration. In developmental contexts or in response to environmental variations, these properties can contribute to the highly dynamic flexibility of plastid structure.-Demé, B., Cataye, C., Block, M. A., Maréchal, E., Jouhet, J. Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids.

Concepts: Cyanobacteria, Photosynthesis, Chloroplast, Plant, Lipids, Thylakoid, Light-dependent reactions, ATP synthase


Chloroplasts are unique organelles that are responsible for photosynthesis. Although chloroplasts contain their own genome, the majority of chloroplast proteins are encoded by the nuclear genome. These proteins are transported to the chloroplasts after translation in the cytosol. Chloroplasts contain three membrane systems (outer/inner envelope and thylakoid membranes) that subdivide the interior into three soluble compartments known as the intermembrane space, stroma, and thylakoid lumen. Several targeting mechanisms are required to deliver proteins to the correct chloroplast membrane or soluble compartment. These mechanisms have been extensively studied using purified chloroplasts in vitro. Prior to targeting these proteins to the various compartments of the chloroplast, they must be correctly sorted in the cytosol. To date, it is not clear how these proteins are sorted in the cytosol and then targeted to the chloroplasts. Recently, the cytosolic carrier protein AKR2 and its associated cofactor Hsp17.8 for outer envelope membrane proteins of chloroplasts were identified. Additionally, a mechanism for controlling unimported plastid precursors in the cytosol has been discovered. This review will mainly focus on recent findings concerning the possible cytosolic events that occur prior to protein targeting to the chloroplasts. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Concepts: Cyanobacteria, Photosynthesis, Mitochondrion, Organelle, Ribosome, Chloroplast, Thylakoid, Inner membrane


The thylakoid-transfer signal is required for energy-dependent translocation of preproteins into the thylakoid lumen and is removed by the thylakoidal processing peptidase (TPP). PGRL1 is an essential component of antimycin A-sensitive photosynthetic cyclic electron flow in chloroplasts. Here we report that one of the TPP isoforms, Plsp1, forms a stable complex with PGRL1. Genetic data demonstrate that PGRL1 is not essential for Plsp1 activity in vivo, leading to a possibility that PGRL1 may act as a regulator of TPP. STRUCTURED SUMMARY OF PROTEIN INTERACTIONS:: PLSP1physically interactswithPGRL1Bbypull down(View interaction) PLSP1physically interactswithPGRL1B,PGRL1AandCASbypull down(View interaction).

Concepts: DNA, Cyanobacteria, Photosynthesis, Chloroplast, Emergence, Chlorophyll, Thylakoid, Light-dependent reactions


The embryos of some angiosperm taxa contain chlorophyll and this chlorophyllous stage is persisting until the embryo matures (further referred as chloroembryos). Besides being chlorophyllous, these embryos seem to have the ability to photosynthesize. This suggests that the chlorophyllous state of the embryo has an important role in seed development. The photosynthesis of chloroembryos is highly shade adaptive in nature as it is embedded within the supporting tissues (several layers of pod wall, seed coat and endosperm). Moreover, these chloroembryos are developing in a highly osmotic environment, and contain various components of the photosynthetic machinery. Detailed studies were performed in these chloroembryos in order to elucidate the structure of the chloroplasts, pigment composition, the photochemical activities, the rate of carbon assimilation and also the shade adaptive features. It has been shown that the respired CO2 within these chloroembryos is recycled by the efficient photosynthetic components of the chloroembryos and thus potentially influences the seed’s carbon economy. Thus, the major role of embryonic photosynthesis is to produce both energy-rich molecules and oxygen, of which the former can be directly used for biosynthesis. During embryogenesis oxygen production is especially important, in a situation wherein the oxygen is limited within the enclosed seed. As these chloroembryos grow in an environment of a sugar rich endosperm, it requires some adaptive mechanisms in this high osmotic environment. The additional polypeptides found in the thylakoids of chloroembryo chloroplasts in comparison to the thylakoids of leaf chloroplast have been suggested to have a role in protecting the photosynthetic components in the chloroembryos in an environment of high osmotic strength. An attempt to understand osmotic stress tolerance existing in these chloroembryos may lead to a better understanding of tolerance of photosynthesis to osmotic stress.

Concepts: Cyanobacteria, Photosynthesis, Oxygen, Carbon dioxide, Chloroplast, Plant, Chlorophyll, Thylakoid


In addition to ∆pH formed across the thylakoid membrane, membrane potential contributes to proton motive force (pmf) in chloroplasts. However, the regulation of photosynthetic electron transport is mediated solely by ∆pH. To assess the contribution of two cyclic electron transport pathways around photosystem I (one depending on PGR5/PGRL1 and one on NDH) to pmf formation, electrochromic shift (ECS) was analyzed in the Arabidopsis pgr5 mutant, NDH-defective mutants (ndhs and crr4-2), and their double mutants (ndhs pgr5 and crr4-2 pgr5). In pgr5, the size of the pmf, as represented by ECSt, was reduced by 30% to 47% compared with that in the wild type (WT). A gH(+) parameter, which is considered to represent the activity of ATP synthase, was enhanced at high light intensities. However, gH(+) recovered to its low-light levels after 20min in the dark, implying that the elevation in gH(+) is due to the disturbed regulation of ATP synthase rather than to photodamage. After long dark adaptation more than 2h, gH(+) was higher in pgr5 than in the WT. During induction of photosynthesis, gH(+) was more rapidly elevated in pgr5 than that in the WT. Both results suggest that ATP synthase is not fully inactivated in the dark in pgr5. In the NDH-deficient mutants, ECSt was slightly but significantly lower than in the WT, whereas gH(+) was not affected. In the double mutants, ECSt was even lower than in pgr5. These results suggest that both PGR5/PGRL1- and NDH-dependent pathways contribute to pmf formation, although to different extents. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Concepts: Cyanobacteria, Photosynthesis, Adenosine triphosphate, Organelle, Chloroplast, Cellular respiration, Thylakoid, ATP synthase


Plants are sessile organisms and need to acclimate to ever-changing light conditions in order to survive. These changes trigger a dynamic reorganization of the membrane protein complexes in the thylakoid membranes. Photosystem II (PSII) and its light harvesting system (LHCII) are the major target of this acclimation response, and accumulating evidences indicate that the amount and composition of PSII-LHCII supercomplexes in thylakoids are dynamically adjusted in response to changes in light intensity and quality. In this study, we characterized the PSII-LHCII supercomplexes in thylakoid membranes of pea plants in response to long-term acclimation to different light intensities. We provide evidence of a reorganization of the PSII-LHCII supercomplexes showing distinct changes in their antenna moiety. Mass spectrometry analysis revealed a specific reduction of Lhcb3, Lhcb6 and M-LHCII trimers bound to the PSII cores, while the Lhcb4.3 isoform increased in response to high light intensities. The modulation of Lhcb protein content correlates with the reduction of the functional PSII antenna size. These results suggest that the Lhcb3, Lhcb4.3 and Lhcb6 antenna subunits are major players in modulation of the PSII antenna size upon long-term acclimation to increased light levels. PsbS was not detected in the isolated PSII-LHCII supercomplexes at any light condition, despite an increased accumulation in thylakoids of high light acclimated plants, suggesting that PsbS is not a constitutive component of PSII-LHCII supercomplexes.

Concepts: Photosynthesis, Mass spectrometry, Photosystem, Chlorophyll, Plastoquinone, Thylakoid, Light-dependent reactions, ATP synthase


Enhanced light harvesting is an area of interest for optimizing both natural photosynthesis and artificial solar energy capture(1,2). Iridescence has been shown to exist widely and in diverse forms in plants and other photosynthetic organisms and symbioses(3,4), but there has yet to be any direct link demonstrated between iridescence and photosynthesis. Here we show that epidermal chloroplasts, also known as iridoplasts, in shade-dwelling species of Begonia(5), notable for their brilliant blue iridescence, have a photonic crystal structure formed from a periodic arrangement of the light-absorbing thylakoid tissue itself. This structure enhances photosynthesis in two ways: by increasing light capture at the predominantly green wavelengths available in shade conditions, and by directly enhancing quantum yield by 5-10% under low-light conditions. These findings together imply that the iridoplast is a highly modified chloroplast structure adapted to make best use of the extremely low-light conditions in the tropical forest understorey in which it is found(5,6). A phylogenetically diverse range of shade-dwelling plant species has been found to produce similarly structured chloroplasts(7-9), suggesting that the ability to produce chloroplasts whose membranes are organized as a multilayer with photonic properties may be widespread. In fact, given the well-established diversity and plasticity of chloroplasts(10,11), our results imply that photonic effects may be important even in plants that do not show any obvious signs of iridescence to the naked eye but where a highly ordered chloroplast structure may present a clear blue reflectance at the microscale. Chloroplasts are generally thought of as purely photochemical; we suggest that one should also think of them as a photonic structure with a complex interplay between control of light propagation, light capture and photochemistry.

Concepts: Cyanobacteria, Photosynthesis, Eukaryote, Chloroplast, Plant, Chlorophyll, Thylakoid, Light-dependent reactions


The thylakoid membranes of oxygenic photosynthetic organisms are dominated by the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). In cyanobacteria, MGDG is synthesized via monoglucosyldiacylglycerol (GlcDG). However, the putative epimerase involved in the conversion of GlcDG to MGDG has not been identified. Here we report the identification of the gene for the glucolipid epimerase (mgdE) by comparative genomic analysis. Knockout mutants of mgdE in Synechocystis sp. PCC 6803 lacked both MGDG and DGDG and accumulated GlcDG. The mutants did possess thylakoid membranes and showed normal maximal photosynthetic activity, albeit with reduced utilization of light energy. These results cast doubt on the long-standing belief that oxygenic photosynthesis is absolutely dependent on galactolipids.

Concepts: Cyanobacteria, Photosynthesis, Oxygen, Carbon dioxide, Bacteria, Chloroplast, Thylakoid, Light-dependent reactions