Thylakoid Membranes Of Higher Plants Research Articles

Overexpression of Rice Monogalactosyldiacylglycerol Synthase OsMGD Leads to Enhanced Salt Tolerance in Rice

Monogalactosyldiacylglycerol (MGDG) is the most abundant lipid on earth and constitutes about 50% of the polar lipids in the thylakoid membrane of higher plants. MGDG synthase (MGD) is the key enzyme responsible for MGDG synthesis in plants. However, so far, the function of the MGD gene in plant stress responses is still unknown, especially in crop plants. In this study, the performances of wild-type rice (Oryza sativa L. Nipponbare), OsMGD overexpression, and RNA interference lines were analyzed under salt stress to clarify the role of OsMGD in rice under salt stress. The results showed that, after 6 days of NaCl treatment, OsMGD-overexpressing rice seedlings had higher biomass than the wild-type rice, with significantly lower Na+ content and Na+ to K+ ratio in the shoots. Meanwhile, overexpression lines showed a significant increase in the contents of chlorophyll a and carotenoid and photosynthetic capabilities, including the net photosynthetic rate, maximal quantum yield (Fv/Fm), effective PSII quantum yield (ΦPSII), electron transport rate (ETR), and coefficient of photochemical quenching (qP), compared with the wild-type line. Also, OsMGD overexpression lines contained higher MGDG and 18:3 unsaturated fatty acid levels under salt stress. These results indicate that OsMGD participates in the regulation of improving salt tolerance through protecting photosynthetic capabilities and ion balance in rice.

Photosynthetic pigment-protein complexes optical response modeling optimized by Differential evolution: algorithm convergence study

Photosynthetic pigment-protein complexes are the essential parts of thylakoid membranes of higher plants and cyanobacteria. Besides many organic and inorganic molecules they contain pigments like chlorophyll, bacteriochlorophyll, and carotenoids, which absorb the incident light and transform it into the energy of the excited electronic states. The semiclassical theories such as molecular exciton theory and the multimode Brownian oscillator model allows us to simulate the linear and nonlinear optical response of any pigment-protein complex, however, the main disadvantage of those approaches is a significant amount of effective parameters needed to be found in order to reproduce the experimental data. To overcome these difficulties we used the Differential evolution method (DE) that belongs to the family of evolutionary optimization algorithms. Based on our preliminary studies of the linear optical properties of monomeric photosynthetic pigments using DE, we proceed to more complex systems like the reaction center of photosystem II isolated from higher plants (PSIIRC). PSIIRC contains only eight chlorophyll pigments, and therefore it is potentially a very promising subject to test DE as a powerful optimization procedure for simulation of the optical response of a system of interacting pigments. Using the theoretically simulated linear spectra of PSIIRC (absorption, circular dichroism, linear dichroism, and fluorescence), we investigated the dependence of the algorithm convergence on DE settings: strategies, crossover, weighting factor; eventually finding the optimal mode of operation of the optimization procedure.

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.

Lipid Dependence of Xanthophyll Cycling in Higher Plants and Algae.

The xanthophyll cycles of higher plants and algae represent an important photoprotection mechanism. Two main xanthophyll cycles are known, the violaxanthin cycle of higher plants, green and brown algae and the diadinoxanthin cycle of Bacillariophyceae, Xanthophyceae, Haptophyceae, and Dinophyceae. The forward reaction of the xanthophyll cycles consists of the enzymatic de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin or diadinoxanthin to diatoxanthin during periods of high light illumination. It is catalyzed by the enzymes violaxanthin or diadinoxanthin de-epoxidase. During low light or darkness the back reaction of the cycle, which is catalyzed by the enzymes zeaxanthin or diatoxanthin epoxidase, restores the epoxidized xanthophylls by a re-introduction of the epoxy groups. The de-epoxidation reaction takes place in the lipid phase of the thylakoid membrane and thus, depends on the nature, three dimensional structure and function of the thylakoid lipids. As the xanthophyll cycle pigments are usually associated with the photosynthetic light-harvesting proteins, structural re-arrangements of the proteins and changes in the protein-lipid interactions play an additional role for the operation of the xanthophyll cycles. In the present review we give an introduction to the lipid and fatty acid composition of thylakoid membranes of higher plants and algae. We introduce the readers to the reaction sequences, enzymes and function of the different xanthophyll cycles. The main focus of the review lies on the lipid dependence of xanthophyll cycling. We summarize the current knowledge about the role of lipids in the solubilization of xanthophyll cycle pigments. We address the importance of the three-dimensional lipid structures for the enzymatic xanthophyll conversion, with a special focus on non-bilayer lipid phases which are formed by the main thylakoid membrane lipid monogalactosyldiacylglycerol. We additionally describe how lipids and light-harvesting complexes interact in the thylakoid membrane and how these interactions can affect the structure of the thylakoids. In a dedicated chapter we offer a short overview of current membrane models, including the concept of membrane domains. We then use these concepts to present a model of the operative xanthophyll cycle as a transient thylakoid membrane domain which is formed during high light illumination of plants or algal cells.

Fine tuning of the photosystem II major antenna mobility within the thylakoid membrane of higher plants

Depending on the amount of light, the photosystem II (PSII) antennae or Light Harvesting Complexes (LHCII) switch between two states within the thylakoid membranes of higher plants, i.e., a light-harvesting and a photoprotective mode. This switch is co-regulated by a pH gradient (ΔpH) across the membrane and the interaction with the PSII subunit S (PsbS) that is proposed to induce LHCII aggregation. Herein we employ all-atom and coarse-grained molecular simulations of the major LHCII trimer at low and excess ΔpH, as well as in complexation with PsbS within a native thylakoid membrane model. Our results demonstrate the aggregation potential of LHCII and, consistent with the experimental literature, reveal the role of PsbS at atomic resolution. PsbS alters the LHCII-thylakoid lipid interactions and restores the LHCII mobility that is lost in the transition to photoprotective conditions (low lumenal pH). In agreement with this finding, diffusion of the integral membrane protein LHCII is dependent on both, electrostatic interactions and hydrophobic mismatch, while it does not obey the Saffman–Delbrück diffusion model.

The presence of the low molecular mass carbonic anhydrase in photosystem II of C3 higher plants

The carrier of carbonic anhydrase (CA) activity was detected in gel among low molecular mass proteins from pea, spinach and Arabidopsis, after nondenaturing electrophoresis in PAAG of the dodecyl-β-d-maltoside treated PSII membranes (the fragments of thylakoid membrane containing PSII complexes). The elimination of Mn-stabilizing protein PsbO by treatment of PSII membranes with salts, did not lead to a decrease in CA activity observed in the gel although it reduced the amount of this protein down to 25% compared to the original sample. The isolated protein PsbO did not demonstrated CA activity. The distinguished features of CA activity of PSII membranes were as follows: 1) resistance to heating, 2) high sensitivity to ethoxyzolamide, the specific inhibitor of CA, and 3) stimulation of this activity by acetazolamide, another specific inhibitor of CA at low concentration of the latter. CA activity was not stimulated by acetazolamide in the PSII membranes samples from Arabidopsis thaliana mutants with knocked out gene At4g20990 encoding αCA4 (according to the nomenclature by Fabre et al., 2007). Taking into account the above data and our previous findings that the energy-dependent part of nonphotochemical quenching of chlorophyll a fluorescence is highly suppressed in that mutant, we suppose that thylakoid membranes of higher plants contain in the vicinity of PSII complex a true CA belonging to the α family of CAs.

Metabolic regulation of photosynthetic membrane structure tunes electron transfer function.

The photosynthetic chloroplast thylakoid membrane of higher plants is a complex three-dimensional structure that is morphologically dynamic on a timescale of just a few minutes. The membrane dynamics are driven by the phosphorylation of light-harvesting complex II (LHCII) by the STN7 kinase, which controls the size of the stacked grana region relative to the unstacked stromal lamellae region. Here, I hypothesise that the functional significance of these membrane dynamics is in controlling the partition of electrons between photosynthetic linear and cyclic electron transfer (LET and CET), which determines the ratio of NADPH/ATP produced. The STN7 kinase responds to the metabolic state of the chloroplast by sensing the stromal redox state. A high NADPH/ATP ratio leads to reduction of thioredoxin f (TRXf), which reduces a CxxxC motif in the stromal domain of STN7 leading to its inactivation, whereas a low NADPH/ATP ratio leads to oxidation of TRXf and STN7 activation. Phosphorylation of LHCII leads to smaller grana, which favour LET by speeding up diffusion of electron carriers plastoquinone (PQ) and plastocyanin (PC) between the domains. In contrast, dephosphorylation of LHCII leads to larger grana that slow the diffusion of PQ and PC, leaving the PQ pool in the stroma more oxidised, thus enhancing the efficiency of CET. The feedback regulation of electron transfer by the downstream metabolism is crucial to plant fitness, since perturbations in the NADPH/ATP ratio can rapidly lead to the inhibition of photosynthesis and photo-oxidative stress.

Glycolipids from spinach suppress LPS-induced vascular inflammation through eNOS and NK-κB signaling

Glycolipids are the major constituent of the thylakoid membrane of higher plants and have a variety of biological and pharmacological activities. However, anti-inflammatory effects of glycolipids on vascular endothelial cells have not been elucidated. Here, we investigated the effect of glycolipids extracted from spinach on lipopolysaccharides (LPS)-induced endothelial inflammation and evaluated the underlying molecular mechanisms. Treatment with glycolipids from spinach had no cytotoxic effects on cultured human umbilical vein endothelial cells (HUVECs) and significantly blocked the expression of LPS-induced interleukin (IL)-6, monocyte chemoattractant protein-1 (MCP-1), vascular cell adhesion molecule-1 (VCAM-1), and intracellular adhesion molecule-1 (ICAM-1) in them. Glycolipids treatment also effectively suppressed monocyte adhesion to HUVECs. Treatment with glycolipids inhibited LPS-induced NF-κB phosphorylation and nuclear translocation. In addition, glycolipids treatment significantly promoted endothelial nitric oxide synthase (eNOS) activation and nitric oxide (NO) production in HUVECs. Furthermore, glycolipids treatment blocked LPS-induced inducible NOS (iNOS) expression in HUVECs. Pretreatment with a NOS inhibitor attenuated glycolipids-induced suppression of NF-κB activation and adhesion molecule expression, and abolished the glycolipids-mediated suppression of monocyte adhesion to HUVECs. These results indicate that glycolipids suppress LPS-induced vascular inflammation through attenuation of the NF-κB pathway by increasing NO production in endothelial cells. These findings suggest that glycolipids from spinach may have a potential therapeutic use for inflammatory vascular diseases.

Direct isolation of a functional violaxanthin cycle domain from thylakoid membranes of higher plants.

A special domain of the thylakoid membrane of higher plants has been isolated which carries out the de-epoxidation of the xanthophyll cycle pigment violaxanthin to zeaxanthin. Recent models indicate that in the chloroplast of higher plants, the violaxanthin (V) cycle takes place within specialized domains in the thylakoid membrane. Here, we describe a new procedure to directly isolate such a domain in functional state. The procedure consists of a thylakoid membrane isolation at a pH value of 5.2 which realizes the binding of the enzyme V de-epoxidase (VDE) to the membrane throughout the preparation process. Isolated thylakoid membranes are then solubilized with the very mild detergent n-dodecyl α-D-maltoside and the pigment-protein complexes are separated by sucrose gradient ultracentrifugation. The upper main fraction of the sucrose gradient represents a V cycle domain which consists of the major light-harvesting complex of photosystem II (LHCII), a special lipid composition with an enrichment of the galactolipid monogalactosyldiacylglycerol (MGDG) and the VDE. The domain is isolated in functional state as evidenced by the ability to convert the LHCII-associated V to zeaxanthin. The direct isolation of a V cycle domain proves the most important hypotheses concerning the de-epoxidation reaction in intact thylakoid membranes. It shows that the VDE binds to the thylakoid membrane at low pH values of the thylakoid lumen, that it binds to membrane regions enriched in LHCII, and that the domain contains high amounts of MGDG. The last point is in line with the importance of the galactolipid for V solubilisation and, by providing inverted hexagonal lipid structures, for VDE activity.

Molecular characterization of 5-chlorophyll a/b-binding protein genes from Panax ginseng Meyer and their expression analysis during abiotic stresses

The chlorophyll a/b-binding protein (CAB) serves in both photosystems (PS), I and II, as a coordinator of antenna pigments in the light-harvesting complex (LHC). The CABs constitute abundant and important proteins in the thylakoid membrane of higher plants. In our study, five CAB genes, which contained full-length cDNA sequences from the 4-year-old ginseng leaves (Panax ginseng Meyer), were isolated and named PgCAB. Phylogenetic comparison of the members of the subfamily between ginseng and higher plants, including Arabidopsis, revealed that the putative functions of these ginseng CAB proteins were clustered into the different family of Arabidopsis CABs; two PgCABs in LHCII family and three PgCABs in LHCI family. The expression analysis of PgCABs consistently showed dark-dependent inhibition in leaves. Expression analysis during abiotic stress identified that PgCAB genes responded to heavy metal, salinity, chilling, and UV stresses differently, suggesting their specific function during photosynthesis. This is the first comprehensive study of the CAB gene family in P. ginseng.

A computer model of a polyunsaturated monogalactolipid bilayer.

1,2-di-O-acyl-3-O-β-D-galactopyranosyl-sn-glycerol (MGDG) is the main lipid component of thylakoid membranes of higher plants and algae. This monogalactolipid is thought of as a non-bilayer lipid but actually it can form both lamellar and nonlamellar phases. In this study, molecular dynamics (MD) simulations of the fully hydrated di-18:3 MGDG bilayer in the lamellar phase were carried out at 310 and 295 K for 200 and 450 ns, respectively, using the GROMACS 4 software package and OPLS-AA force field. At both temperatures, the lamellar phase of the systems was stable. The pure di-18:3 MGDG bilayer is the first step towards creating a computer model of the lipid matrix of the thylakoid membrane and the main aim of this study was to validate the computer model of di-18:3 MGDG in the bilayer and also to assess the properties of the bilayer. However, only a few of the predicted properties could be compared with those derived experimentally and in other MD simulations because of insufficient amount of such data. Thus, direct validation of the MGDG bilayer proved difficult. Therefore, in the validation process also an indirect approach was used, in which a computer model of the 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) bilayer simulated at the same temperatures using the same force field as the MGDG bilayer was assessed. Successful validation of the DOPC bilayer parameterized in the OPLS-AA force field and similar properties of the MGDG molecules in the pure 18:3 MGDG and in binary 18:3 MGDG-PC bilayers indicate that the computer model of the MDGD molecule is faithful and the MGDG bilayer is representative on the time scales covered in these MD simulations.

Resonance assignment of PsbP: an extrinsic protein from photosystem II of Spinacia oleracea.

PsbP (23 kDa) is an extrinsic eukaryotic protein of photosystem II found in the thylakoid membrane of higher plants and green algae. It has been proven to be indispensable for proper functioning of the oxygen evolving complex. By interaction with other extrinsic proteins (PsbQ, PsbO and PsbR), it modulates the concentration of two cofactors of the water splitting reaction, Ca2+ and Cl−. The crystallographic structure of PsbP from Spinacia oleracea lacks the N-terminal part as well as two inner regions which were modelled as loops. Those unresolved parts are believed to be functionally crucial for the binding of PsbP to the thylakoid membrane. In this NMR study we report 1H, 15N and 13C resonance assignments of the backbone and side chain atoms of the PsbP protein. Based on these data, an estimate of the secondary structure has been made. The structural motifs found fit the resolved parts of the crystallographic structure very well. In addition, the complete assignment set provides preliminary insight into the dynamic regions.

Interaction of molecular oxygen with the donor side of photosystem II after destruction of the water-oxidizing complex.

Photosystem II (PSII) is a pigment-protein complex of thylakoid membrane of higher plants, algae, and cyanobacteria where light energy is used for oxidation of water and reduction of plastoquinone. Light-dependent reactions (generation of excited states of pigments, electron transfer, water oxidation) taking place in PSII can lead to the formation of reactive oxygen species. In this review attention is focused on the problem of interaction of molecular oxygen with the donor site of PSII, where after the removal of manganese from the water-oxidizing complex illumination induces formation of long-lived states (P680(+•) and TyrZ(•)) capable of oxidizing surrounding organic molecules to form radicals.

Transmembrane electric potential difference in the protein-pigment complex of photosystem 2

The protein-pigment complex of photosystem 2 (PS2) localized in the thylakoid membranes of higher plants, algae, and cyanobacteria is the main source of oxygen on Earth. The light-induced functioning of PS2 is directly linked to electron and proton transfer across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). The major contribution to ΔΨ of the PS2 reaction center is due to charge separation between the primary chlorophyll donor P(680) and the quinone acceptor Q(A), accompanied by re-reduction of P(680)(+) by the redox-active tyrosine residue Y(Z). The processes associated with the uptake and release of protons on the acceptor and donor sides of the enzyme, respectively, are also coupled with ΔΨ generation. The objective of this work was to describe the mechanisms of ΔΨ generation associated with the S-state transitions of the water-oxidizing complex in intact PS2 complex and in PS2 preparation depleted of Mn(4)Ca cluster in the presence of artificial electron donors. The findings elucidate the mechanisms of electrogenic reactions on the PS2 donor side and may be a basis for development of an effective solar energy conversion system.

Mechanism of action of anions on the electron transport chain in thylakoid membranes of higher plants

With an aim to improve our understanding of the mechanisms behind specific anion effects in biological membranes, we have studied the effects of sodium salts of anions of varying valency in thylakoid membranes. Rates of electron transport of PS II and PS I, 77K fluorescence emission and excitation spectra, cyclic electron flow around PS I and circular dichroism (CD) spectra were measured in thylakoid membranes in order to elucidate a general mechanism of action of inorganic anions on photosynthetic electron transport chain. Re-distribution of absorbed excitation energy has been observed as a signature effect of inorganic anions. In the presence of anions, such as nitrite, sulphate and phosphate, distribution of absorbed excitation energy was found to be more in favor of Photosystem I (PS I). The amount of energy distributed towards PS I depended on the valency of the anion. In this paper, we propose for the first time that energy re-distribution and its valence dependence may not be the effect of anions per se. The entry of negative charge (anion) is accompanied by influx of positive charge (protons) to maintain a balance of charge across the thylakoid membranes. As reflected by the CD spectra, the observed energy re-distribution could be a result of structural rearrangements of the protein complexes of PS II caused by changes in the ionic environment of the thylakoid lumen.

Visualizing the mobility and distribution of chlorophyll proteins in higher plant thylakoid membranes: effects of photoinhibition and protein phosphorylation

The diffusion of proteins in chloroplast thylakoid membranes is believed to be important for processes including the photosystem-II repair cycle and the regulation of light harvesting. However, to date there is very little direct information on the mobility of thylakoid proteins. We have used fluorescence recovery after photobleaching in a laser-scanning confocal microscope to visualize in real time the exchange of chlorophyll proteins between grana in intact spinach (Spinacia oleracea L.) and Arabidopsis chloroplasts. Most chlorophyll proteins in the grana appear immobile on the 10-min timescale of our measurements. However, a limited population of chlorophyll proteins (accounting for around 15% of chlorophyll fluorescence) can exchange between grana on this timescale. In intact, wild-type chloroplasts this mobile population increases significantly after photoinhibition, consistent with a role for protein diffusion in the photosystem-II repair cycle. No such increase in mobility is seen in isolated grana membranes, or in the Arabidopsis stn8 and stn7 stn8 mutants, which lack the protein kinases required for phosphorylation of photosystem II core proteins and light-harvesting complexes. Furthermore, mobility under low-light conditions is significantly lower in stn8 and stn7 stn8 plants than in wild-type Arabidopsis. The changes in protein mobility correlate with changes in the packing density and size of thylakoid protein complexes, as observed by freeze-fracture electron microscopy. We conclude that protein phosphorylation switches the membrane system to a more fluid state, thus facilitating the photosystem-II repair cycle.

Visualising the mobility and distribution of chlorophyll‐proteins in higher plant thylakoid membranes: effects of photoinhibition and protein phosphorylation

The diffusion of proteins in chloroplast thylakoid membranes is believed to be important for processes including the photosystem-II repair cycle and the regulation of light harvesting. However, to date there is very little direct information on the mobility of thylakoid proteins. We have used fluorescence recovery after photobleaching in a laser-scanning confocal microscope to visualize in real time the exchange of chlorophyll proteins between grana in intact spinach (Spinacia oleracea L.) and Arabidopsis chloroplasts. Most chlorophyll proteins in the grana appear immobile on the 10-min timescale of our measurements. However, a limited population of chlorophyll proteins (accounting for around 15% of chlorophyll fluorescence) can exchange between grana on this timescale. In intact, wild-type chloroplasts this mobile population increases significantly after photoinhibition, consistent with a role for protein diffusion in the photosystem-II repair cycle. No such increase in mobility is seen in isolated grana membranes, or in the Arabidopsis stn8 and stn7 stn8 mutants, which lack the protein kinases required for phosphorylation of photosystem II core proteins and light-harvesting complexes. Furthermore, mobility under low-light conditions is significantly lower in stn8 and stn7 stn8 plants than in wild-type Arabidopsis. The changes in protein mobility correlate with changes in the packing density and size of thylakoid protein complexes, as observed by freeze-fracture electron microscopy. We conclude that protein phosphorylation switches the membrane system to a more fluid state, thus facilitating the photosystem-II repair cycle.

A Femtosecond Visible/Visible and Visible/Mid-Infrared Transient Absorption Study of the Light Harvesting Complex II

Light harvesting complex II (LHCII) is the most abundant protein in the thylakoid membrane of higher plants and green algae. LHCII acts to collect solar radiation, transferring this energy mainly toward photosystem II, with a smaller amount going to photosystem I; it is then converted into a chemical, storable form. We performed time-resolved femtosecond visible pump/mid-infrared probe and visible pump/visible probe absorption difference spectroscopy on purified LHCII to gain insight into the energy transfer in this complex occurring in the femto-picosecond time regime. We find that information derived from mid-infrared spectra, together with structural and modeling information, provides a unique visualization of the flow of energy via the bottleneck pigment chlorophyll a604.

Effect of phosphorylation on the thermal and light stability of the thylakoid membranes.

Higher plant thylakoid membranes contain a protein kinase that phosphorylates certain threonine residues of light-harvesting complex II (LHCII), the main light-harvesting antenna complexes of photosystem II (PSII) and some other phosphoproteins (Allen, Biochim Biophys Acta 1098:275, 1992). While it has been established that phosphorylation induces a conformational change of LHCII and also brings about changes in the lateral organization of the thylakoid membrane, it is not clear how phosphorylation affects the dynamic architecture of the thylakoid membranes. In order to contribute to the elucidation of this complex question, we have investigated the effect of duroquinol-induced phosphorylation on the membrane ultrastructure and the thermal and light stability of the chiral macrodomains and of the trimeric organization of LHCII. As shown by small angle neutron scattering on thylakoid membranes, duroquinol treatment induced a moderate (~10%) increase in the repeat distance of stroma membranes, and phosphorylation caused an additional loss of the scattering intensity, which is probably associated with the partial unstacking of the granum membranes. Circular dichroism (CD) measurements also revealed only minor changes in the chiral macro-organization of the complexes and in the oligomerization state of LHCII. However, temperature dependences of characteristic CD bands showed that phosphorylation significantly decreased the thermal stability of the chiral macrodomains in phosphorylated compared to the non-phosphorylated samples (in leaves and isolated thylakoid membranes, from 48.3 degrees C to 42.6 degrees C and from 47.5 degrees C to 44.3 degrees C, respectively). As shown by non-denaturing PAGE of thylakoid membranes and CD spectroscopy on EDTA washed membranes, phosphorylation decreased by about 5 degrees C, the trimer-to-monomer transition temperature of LHCII. It also enhanced the light-induced disassembly of the chiral macrodomains and the monomerization of the LHCII trimers at 25 degrees C. These data strongly suggest that phosphorylation of the membranes considerably facilitates the heat- and light-inducible reorganizations in the thylakoid membranes and thus enhances the structural flexibility of the membrane architecture.

Dynamic flexibility in the structure and function of photosystem II in higher plant thylakoid membranes: the grana enigma

Grana are not essential for photosynthesis, yet they are ubiquitous in higher plants and in the recently evolved Charaphyta algae; hence grana role and its need is still an intriguing enigma. This article discusses how the grana provide integrated and multifaceted functional advantages, by facilitating mechanisms that fine-tune the dynamics of the photosynthetic apparatus, with particular implications for photosystem II (PSII). This dynamic flexibility of photosynthetic membranes is advantageous in plants responding to ever-changing environmental conditions, from darkness or limiting light to saturating light and sustained or intermittent high light. The thylakoid dynamics are brought about by structural and organizational changes at the level of the overall height and number of granal stacks per chloroplast, molecular dynamics within the membrane itself, the partition gap between appressed membranes within stacks, the aqueous lumen encased by the continuous thylakoid membrane network, and even the stroma bathing the thylakoids. The structural and organizational changes of grana stacks in turn are driven by physicochemical forces, including entropy, at work in the chloroplast. In response to light, attractive van der Waals interactions and screening of electrostatic repulsion between appressed grana thylakoids across the partition gap and most probably direct protein interactions across the granal lumen (PSII extrinsic proteins OEEp-OEEp, particularly PsbQ-PsbQ) contribute to the integrity of grana stacks. We propose that both the light-induced contraction of the partition gap and the granal lumen elicit maximisation of entropy in the chloroplast stroma, thereby enhancing carbon fixation and chloroplast protein synthesizing capacity. This spatiotemporal dynamic flexibility in the structure and function of active and inactive PSIIs within grana stacks in higher plant chloroplasts is vital for the optimization of photosynthesis under a wide range of environmental and developmental conditions.