Role of MGDG and Non-bilayer Lipid Phases in the Structure and Dynamics of Chloroplast Thylakoid Membranes

Role of isotropic lipid phase in the fusion of photosystem II membranes

It has been thoroughly documented, by using 31P-NMR spectroscopy, that plant thylakoid membranes (TMs), in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (HII) phase. However, our knowledge concerning the structural and functional roles of the non-bilayer phases is still rudimentary. The objective of the present study is to elucidate the origin of I phases which have been hypothesized to arise, in part, from the fusion of TMs (Garab et al. 2022 Progr Lipid Res 101,163). We take advantage of the selectivity of wheat germ lipase (WGL) in eliminating the I phases of TMs (Dlouhý et al. 2022 Cells 11: 2681), and the tendency of the so-called BBY particles, stacked photosystem II (PSII) enriched membrane pairs of 300-500 nm in diameter, to form large laterally fused sheets (Dunahay et al. 1984 BBA 764: 179). Our 31P-NMR spectroscopy data show that BBY membranes contain L and I phases. Similar to TMs, WGL selectively eliminated the I phases, which at the same time exerted no effect on the molecular organization and functional activity of PSII membranes. As revealed by sucrose-density centrifugation, magnetic linear dichroism spectroscopy and scanning electron microscopy, WGL disassembled the large laterally fused sheets. These data provide direct experimental evidence on the involvement of I phase(s) in the fusion of stacked PSII membrane pairs, and strongly suggest the role of non-bilayer lipids in the self-assembly of the TM system.

Inter-Organellar Effects of Defective ER-Localized Linolenic Acid Formation on Thylakoid Lipid Composition, Non-Photochemical Quenching of Chlorophyll Fluorescence and Xanthophyll Cycle Activity in the Arabidopsis fad3 Mutant

Monogalactosyldiacylglycerol (MGDG) is the main lipid constituent of thylakoids and a structural component of photosystems and photosynthesis-related proteo-lipid complexes in green tissues. Previously reported changes in MGDG abundance upon stress treatments are hypothesized to reflect mobilization of MGDG-based polyunsaturated lipid intermediates to maintain extraplastidial membrane integrity. While exchange of lipid intermediates between compartmental membranes is well documented, physiological consequences of mobilizing an essential thylakoid lipid, such as MGDG, for an alternative purpose are not well understood. Arabidopsis seedlings exposed to mild (50 mM) salt treatment displayed significantly increased abundance of both MGDG and the extraplastidial lipid, phosphatidylcholine (PC). Interestingly, similar increases in MGDG and PC were observed in Arabidopsis fad3 mutant seedlings defective in endoplasmic reticulum (ER)-localized linolenic acid formation, in which compensatory plastid-to-ER-directed mobilization of linolenic acid-containing intermediates takes place. The postulated (salt) or evident (fad3) plastid-ER exchange of intermediates concurred with altered thylakoid function according to parameters of photosynthetic performance. While salt treatment of wild-type seedlings inhibited photosynthetic parameters in a dose-dependent manner, interestingly, untreated fad3 mutants did not show overall reduced photosynthetic quantum yield. By contrast, we observed a reduction specifically of non-photochemical quenching (NPQ) under high light, representing only part of observed salt effects. The decreased NPQ in the fad3 mutant was accompanied by reduced activity of the xanthophyll cycle, leading to a reduced concentration of the NPQ-effective pigment zeaxanthin. The findings suggest that altered ER-located fatty acid unsaturation and ensuing inter-organellar compensation impacts on the function of specific thylakoid enzymes, rather than globally affecting thylakoid function.

Diversification of Plastid Structure and Function in Land Plants

Plastids represent a largely diverse group of organelles in plant and algal cells that have several common features but also a broad spectrum of morphological, ultrastructural, biochemical, and physiological differences. Plastids and their structural and metabolic diversity significantly contribute to the functionality and developmental flexibility of the plant body throughout its lifetime. In addition to the multiple roles of given plastid types, this diversity is accomplished in some cases by interconversions between different plastids as a consequence of developmental and environmental signals that regulate plastid differentiation and specialization. In addition to basic plastid structural features, the most important plastid types, the newly characterized peculiar plastids, and future perspectives in plastid biology are also provided in this chapter.

Evaluation of Lipids for the Study of Photosynthetic Membranes

The biological role of lipids goes far beyond the formation of a structural membrane bilayer platform for membrane proteins and controlling fluxes across the membranes. For example, in photosynthetic thylakoid membranes, lipids occupy well-defined binding niches within protein complexes and determine the structural organization of membrane proteins and their function by controlling generic physicochemical membrane properties. In this chapter, two-dimensional thin-layer chromatography (2D TLC) and gas chromatography (GC) techniques are presented for quantitative analysis of lipid classes and fatty acids in thylakoid membranes. In addition, lipid extraction methods from isolated thylakoid membranes and leaves are described together with a procedure for the derivatization of fatty acids to fatty acid methyl esters (FAME) that is required for GC analysis.

Lipidome of the Brown Macroalga Undaria pinnatifida: Influence of Season and Endophytic Infection

An analysis of the lipidome of the brown alga Undaria pinnatifida (Laminariales) was performed' more than 900 molecular species were identified in 12 polar lipids and 1 neutral lipid using HPLC/MS-MS. The seasonal changes of U. pinnatifida lipidome were determined. It was shown that acclimatization to winter and spring was accompanied by an increase in the unsaturation of both polar and neutral lipids. In autumn and summer, on the contrary, the contents of more saturated molecular species of all lipid classes increased. Based on the data obtained, a scheme for the polar and neutral lipid synthesis in brown algae was proposed. In addition, the influence of infection with the brown filamentous endophyte Laminariocolax aecidioides (Ectocarpales) on U. pinnatifida lipidome was studied. It was found that infection has the most noticeable effect on the molecular species composition of triacylglycerides, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylhydroxyethylglycine of the host macrophyte. In infected samples of algae, changes in the composition of triacylglycerides were revealed both in areas with the presence of an endophyte and in adjacent intact tissues, which may indicate the occurrence of a secondary infection.

Integrative analysis of different low-light-tolerant cucumber lines in response to low-light stress

INTRODUCTION

Low light stress inhibits plant growth due to a line of physiological disruptions in plants, and is one of the major barriers to protected cucumber cultivation in northern China.

METHODS

To comprehensively understand the responses of cucumber seedlings to low-light stress, the low-light-tolerant line (M67) and The low-light-sensitive line (M14) were conducted for the analysis of photosynthetic phenotype, RNA sequencing (RNA-seq) and the expression level of photosynthesis-related genes in leaves under low-light stress and normal light condition (control).

RESULTS

The results showed that there was a sharp decrease in the photosynthate accumulation in the leaves of the sensitive line, M14, resulting in a large decrease in the photosynthetic rate (Pn) (with 31.99%) of leaves compared to that of the control, which may have been caused by damage to chloroplast ultrastructure or a decrease in chlorophyll (Chl) content. However, under the same low-light treatment, there was no large drop in the photosynthate accumulation and even no decrease in Pn and Chl content for the tolerant line, M67. Moreover, results of gene expression analysis showed that the expression level of genes CsPsbQ (the photosystem II oxygen-evolving enhancer protein 3 gene) and Csgamma (ATPase, F1 complex gene) in the M14 leaves decreased sharply (by 35.04% and 30.58%, respectively) compared with the levels in the M67 leaves, which decreased by 14.78% and 23.61%, respectively. The expression levels of genes involved in Chl synthesis and carbohydrate biosynthesis in the leaves of M14 decreased markedly after low-light treatment; in contrast, there were no sharp decreases or changes in leaves of M67.

DISCUSSION

Over all, the ability of cucumber to respond to low-light stress, as determined on the basis of the degree of damage in leaf structure and chloroplast ultrastructure, which corresponded to decreased gene expression levels and ATP phosphorylase activity, significantly differed between different low-light-tolerant lines, which was manifested as significant differences in photosynthetic capacity between them. Results of this study will be a reference for comprehensive insight into the physiological mechanism involved in the low-light tolerance of cucumber.

Isolation of fucoxanthin chlorophyll protein complexes of the centric diatom Thalassiosira pseudonana associated with the xanthophyll cycle enzyme diadinoxanthin de-epoxidase

In the present study, low concentrations of the very mild detergent n-dodecyl-α-d-maltoside in conjunction with sucrose gradient ultracentrifugation were used to prepare fucoxanthin chlorophyll protein (FCP) complexes of the centric diatom Thalassiosira pseudonana. Two main FCP fractions were observed in the sucrose gradients, one in the upper part and one at high sucrose concentrations in the lower part of the gradient. The first fraction was dominated by the 18 kDa FCP protein band in SDS-gels. Since this fraction also contained other protein bands, it was designated as fraction enriched in FCP-A complex. The second fraction contained mainly the 21 kDa FCP band, which is typical for the FCP-B complex. Determination of the lipid composition showed that both FCP fractions contained monogalactosyl diacylglycerol as the main lipid followed by the second galactolipid of the thylakoid membrane, namely digalactosyl diacylglycerol. The negatively charged lipids sulfoquinovosyl diacylglycerol and phosphatidyl glycerol were also present in both fractions in pronounced concentrations. With respect to the pigment composition, the fraction enriched in FCP-A contained a higher amount of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (Dt), whereas the FCP-B fraction was characterized by a lower ratio of xanthophyll cycle pigments to the light-harvesting pigment fucoxanthin. Protein analysis by mass spectrometry revealed that in both FCP fractions the xanthophyll cycle enzyme diadinoxanthin de-epoxidase (DDE) was present. In addition, the analysis showed an enrichment of DDE in the fraction enriched in FCP-A but only a very low amount of DDE in the FCP-B fraction. In-vitro de-epoxidation assays, employing the isolated FCP complexes, were characterized by an inefficient conversion of DD to Dt. However, in line with the heterogeneous DDE distribution, the fraction enriched in FCP-A showed a more pronounced DD de-epoxidation compared with the FCP-B.

Role of Lipids of the Evergreen Shrub Ephedra monosperma in Adaptation to Low Temperature in the Cryolithozone

Lipids are the fundamental components of cell membranes and they play a significant role in their integrity and fluidity. The alteration in lipid composition of membranes has been reported to be a major response to abiotic environmental stresses. Seasonal dynamics of membrane lipids in the shoots of Ephedra monosperma J.G. Gmel. ex C.A. Mey. growing in natural conditions of permafrost ecosystems was studied using HPTLC, GC-MS and ESI-MS. An important role of lipid metabolism was established during the autumn-winter period when the shoots of the evergreen shrub were exposed to low positive (3.6 °C), negative (-8.3 °C) and extremely low temperatures (-38.4 °C). Maximum accumulation of phosphatidic acid (PA), the amount of which is times times greater than the sum of phosphatidylcholine and phosphatidylethanolamine (PC + PE) was noted in shoots of E. monosperma in the summer-autumn period. The autumn hardening period (3.6 °C) is accompanied by active biosynthesis and accumulation of membrane lipids, a decrease of saturated 34:1 PCs, 34:1 PEs and 34:1 PAs, and an increase in unsaturated long-chain 38:5 PEs, 38:6 PEs, indicating that the adaptation of E. monosperma occurs not at the level of lipid classes but at the level of molecular species. At a further decrease of average daily air temperature in October (-8.3 °C) a sharp decline of PA level was registered. At an extreme reduction of environmental temperature (-38.4 °C) the content of non-bilayer PE and PA increases, the level of unsaturated fatty acids (FA) rises due to the increase of C18:2(Δ9,12) and C18:3(Δ9,12,15) acids and the decrease of C16:0 acids. It is concluded that changes in lipid metabolism reflect structural and functional reorganization of cell membranes and are an integral component of the complex process of plant hardening to low temperatures, which contributes to the survival of E. monosperma monocotyledonous plants in the extreme conditions of the Yakutia cryolithozone.

Structural Entities Associated with Different Lipid Phases of Plant Thylakoid Membranes—Selective Susceptibilities to Different Lipases and Proteases

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (H_II) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the ³¹P-NMR spectra of isolated spinach plastoglobuli and TMs and tested their susceptibilities to lipases and proteases; the structural and functional characteristics of TMs were monitored using biophysical techniques and CN-PAGE. Phospholipase-A1 gradually destroyed all ³¹P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected. Parallel with the destabilization of their lamellar phase, TMs lost their impermeability; other effects, mainly on Photosystem-II, lagged behind the destruction of the original phases. Wheat-germ lipase selectively eliminated the isotropic phases but exerted little or no effect on the structural and functional parameters of TMs—indicating that the isotropic phases are located outside the protein-rich regions and might be involved in membrane fusion. Trypsin and Proteinase K selectively suppressed the H_II phase—suggesting that a large fraction of TM lipids encapsulate stroma-side proteins or polypeptides. We conclude that—in line with the Dynamic Exchange Model—the non-bilayer lipid phases of TMs are found in subdomains separated from but interconnected with the bilayer accommodating the main components of the photosynthetic machinery.

Chlorophyll fluorescence as a valuable multitool for microalgal biotechnology

Variable fluorescence of chlorophyll (CF) of the photosynthetic apparatus is an ample source of valuable information on physiological condition of photosynthetic organisms. Currently, the most widespread CF-based technique is represented by recording pulse-amplitude modulated (PAM) induction of CF by saturating light. The CF-based monitoring techniques are increasingly employed for characterization of performance and stress resilience of microalgae in microalgal biotechnology. Analysis of CF induction curves reveals the fate of light energy absorbed by photosynthetic apparatus, the proportions of the energy that have been utilized for photochemistry (culture growth), and heat dissipated by photoprotective mechanisms. Hence CF and its derived parameters are an accurate proxy of the metabolic activity of the photosynthetic cell and the engagement of photoprotective mechanisms. This information is a solid foundation for making decisions on the microalgal culture management during the lab-scale and industrial-scale cultivation. Applications of CF and PAM include the monitoring of stressor (high light, nutrient deprivation, extreme temperatures, etc.) effects for assessment of the culture robustness. It also serves as a non-invasive express test for gauging the effect of assorted toxicants in microalgae. This approach is becoming widespread in ecological toxicology and environmental biotechnology, particularly for bioprospecting strains capable of the destruction of dangerous pollutants such as pharmaceuticals. In the review, we discuss the advantages and drawbacks of using CF-based methods for assessment of the culture conditions. Special attention is paid to the potential caveats and applicability of different variations of CF and PAM measurements for solving problems of microalgal biotechnology.

Low light acclimation strategy of the brown macroalga Undaria pinnatifida: Significance of lipid and fatty acid remodeling for photosynthetic competence

Brown macroalgae, being important components of benthic communities in temperate regions, are frequently subjected to light limitation. To extend our understanding of their low light acclimation strategies to the regulation of membrane lipid environment, photosynthetic characteristics, lipid class, fatty acid profiles and chloroplast ultrastructure were compared in Undaria pinnatifida (Phaeophyceae, Ochrophyta) after long-term exposure to low and moderate light intensities (LL, 100 and ML, 280 µmol photons · m-2 · s-1 ). We show that light limitation significantly increased PSII quantum efficiency and photosynthetic electron transport rate, enhanced pigment contents and concentration of thylakoid membranes in chloroplasts but decreased the distance between the thylakoid stacks. These physiological alterations at LL were accompanied by a selective remodeling of thylakoid membrane lipids driven by increases in monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG) contents. Light limitation also induced active production of PG specific trans-Δ3 -hexadecenoic acid and accumulation of n-3 polyunsaturated fatty acids (PUFA) mostly in PG and MGDG at the expense of the rise in 18:3n-3 and 20:5n-3, 18:4n-3, respectively. These changes in lipid and FA profiles are apparently responsible for supporting thylakoid biogenesis and efficient photosynthesis at light limitation, thus contributing to photoacclimation strategies in brown algae. The content of triacylglycerols (TAG) and the level of their PUFA were decreased at LL, suggesting the consumption of TAG as a source of PUFA and energy reserves. Thus, U. pinnatifida is able to successfully overcome periods of low irradiance through the effective light harvesting and utilization that are provided by high flexibility of lipid biosynthesis.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. I. 31P-NMR Spectroscopy

Build-up of the energized state of thylakoid membranes and the synthesis of ATP are warranted by organizing their bulk lipids into a bilayer. However, the major lipid species of these membranes, monogalactosyldiacylglycerol, is a non-bilayer lipid. It has also been documented that fully functional thylakoid membranes, in addition to the bilayer, contain an inverted hexagonal (H_II) phase and two isotropic phases. To shed light on the origin of these non-lamellar phases, we performed ³¹P-NMR spectroscopy experiments on sub-chloroplast particles of spinach: stacked, granum and unstacked, stroma thylakoid membranes. These membranes exhibited similar lipid polymorphism as the whole thylakoids. Saturation transfer experiments, applying saturating pulses at characteristic frequencies at 5 °C, provided evidence for distinct lipid phases—with component spectra very similar to those derived from mathematical deconvolution of the ³¹P-NMR spectra. Wheat-germ lipase treatment of samples selectively eliminated the phases exhibiting sharp isotropic peaks, suggesting easier accessibility of these lipids compared to the bilayer and the H_II phases. Gradually increasing lipid exchanges were observed between the bilayer and the two isotropic phases upon gradually elevating the temperature from 5 to 35 °C, suggesting close connections between these lipid phases. Data concerning the identity and structural and functional roles of different lipid phases will be presented in the accompanying paper.

Influence of the compatible solute sucrose on thylakoid membrane organization and violaxanthin de-epoxidation

The compatible solute sucrose reduces the efficiency of the enzymatic de-epoxidation of violaxanthin, probably by a direct effect on the protein parts of violaxanthin de-epoxidase which protrude from the lipid phase of the thylakoid membrane. The present study investigates the influence of the compatible solute sucrose on the violaxanthin cycle of higher plants in intact thylakoids and in in vitro enzyme assays with the isolated enzyme violaxanthin de-epoxidase at temperatures of 30 and 10 °C, respectively. In addition, the influence of sucrose on the lipid organization of thylakoid membranes and the MGDG phase in the in vitro assays is determined. The results show that sucrose leads to a pronounced inhibition of violaxanthin de-epoxidation both in intact thylakoid membranes and the enzyme assays. In general, the inhibition is similar at 30 and 10 °C. With respect to the lipid organization only minor changes can be seen in thylakoid membranes at 30 °C in the presence of sucrose. However, sucrose seems to stabilize the thylakoid membranes at lower temperatures and at 10 °C a comparable membrane organization to that at 30 °C can be observed, whereas control thylakoids show a significantly different membrane organization at the lower temperature. The MGDG phase in the in vitro assays is not substantially affected by the presence of sucrose or by changes of the temperature. We conclude that the presence of sucrose and the increased viscosity of the reaction buffers stabilize the protein part of the enzyme violaxanthin de-epoxidase, thereby decreasing the dynamic interactions between the catalytic site and the substrate violaxanthin. This indicates that sucrose interacts with those parts of the enzyme which are accessible at the membrane surface of the lipid phase of the thylakoid membrane or the MGDG phase of the in vitro enzyme assays.

Cardiolipin, Non-Bilayer Structures and Mitochondrial Bioenergetics: Relevance to Cardiovascular Disease

The present review is an attempt to conceptualize a contemporary understanding about the roles that cardiolipin, a mitochondrial specific conical phospholipid, and non-bilayer structures, predominantly found in the inner mitochondrial membrane (IMM), play in mitochondrial bioenergetics. This review outlines the link between changes in mitochondrial cardiolipin concentration and changes in mitochondrial bioenergetics, including changes in the IMM curvature and surface area, cristae density and architecture, efficiency of electron transport chain (ETC), interaction of ETC proteins, oligomerization of respiratory complexes, and mitochondrial ATP production. A relationship between cardiolipin decline in IMM and mitochondrial dysfunction leading to various diseases, including cardiovascular diseases, is thoroughly presented. Particular attention is paid to the targeting of cardiolipin by Szeto–Schiller tetrapeptides, which leads to rejuvenation of important mitochondrial activities in dysfunctional and aging mitochondria. The role of cardiolipin in triggering non-bilayer structures and the functional roles of non-bilayer structures in energy-converting membranes are reviewed. The latest studies on non-bilayer structures induced by cobra venom peptides are examined in model and mitochondrial membranes, including studies on how non-bilayer structures modulate mitochondrial activities. A mechanism by which non-bilayer compartments are formed in the apex of cristae and by which non-bilayer compartments facilitate ATP synthase dimerization and ATP production is also presented.

Why Do Tethered-Bilayer Lipid Membranes Suit for Functional Membrane Protein Reincorporation?

Membrane proteins (MPs) are essential for cellular functions. Understanding the functions of MPs is crucial as they constitute an important class of drug targets. However, MPs are a challenging class of biomolecules to analyze because they cannot be studied outside their native environment. Their structure, function and activity are highly dependent on the local lipid environment, and these properties are compromised when the protein does not reside in the cell membrane. Mammalian cell membranes are complex and composed of different lipid species. Model membranes have been developed to provide an adequate environment to envisage MP reconstitution. Among them, tethered-Bilayer Lipid Membranes (tBLMs) appear as the best model because they allow the lipid bilayer to be decoupled from the support. Thus, they provide a sufficient aqueous space to envisage the proper accommodation of large extra-membranous domains of MPs, extending outside. Additionally, as the bilayer remains attached to tethers covalently fixed to the solid support, they can be investigated by a wide variety of surface-sensitive analytical techniques. This review provides an overview of the different approaches developed over the last two decades to achieve sophisticated tBLMs, with a more and more complex lipid composition and adapted for functional MP reconstitution.

Too rigid to fold: Carotenoid-dependent decrease in thylakoid fluidity hampers the formation of chloroplast grana

In chloroplasts of land plants, the thylakoid network is organized into appressed regions called grana stacks and loosely arranged parallel stroma thylakoids. Many factors determining such intricate structural arrangements have been identified so far, including various thylakoid-embedded proteins, and polar lipids that build the thylakoid matrix. Although carotenoids are important components of proteins and the lipid phase of chloroplast membranes, their role in determining the thylakoid network structure remains elusive. We studied 2D and 3D thylakoid network organization in carotenoid-deficient mutants (ccr1-1, lut5-1, szl1-1, and szl1-1npq1-2) of Arabidopsis (Arabidopsis thaliana) to reveal the structural role of carotenoids in the formation and dynamics of the internal chloroplast membrane system. The most significant structural aberrations took place in chloroplasts of the szl1-1 and szl1-1npq1-2 plants. Increased lutein/carotene ratio in these mutants impaired the formation of grana, resulting in a significant decrease in the number of thylakoids used to build a particular stack. Further, combined biochemical and biophysical analyses revealed that hampered grana folding was related to decreased thylakoid membrane fluidity and significant changes in the amount, organization, and phosphorylation status of photosystem (PS) II (PSII) supercomplexes in the szl1-1 and szl1-1npq1-2 plants. Such changes resulted from a synergistic effect of lutein overaccumulation in the lipid matrix and a decreased level of carotenes bound with PS core complexes. Moreover, more rigid membrane in the lutein overaccumulating plants led to binding of Rubisco to the thylakoid surface, additionally providing steric hindrance for the dynamic changes in the level of membrane folding.

The fluid-mosaic membrane theory in the context of photosynthetic membranes: Is the thylakoid membrane more like a mixed crystal or like a fluid?

Since the publication of the fluid-mosaic membrane theory by Singer and Nicolson in 1972 generations of scientists have adopted this fascinating concept for all biological membranes. Assuming the membrane as a fluid implies that the components embedded in the lipid bilayer can freely diffuse like swimmers in a water body. During the detailed biochemical analysis of the thylakoid protein components of chloroplasts from higher plants and algae, in the '80 s and '90 s it became clear that photosynthetic membranes are not homogeneous either in the vertical or the lateral directions. The lateral heterogeneity became obvious by the differentiation of grana and stroma thylakoids, but also the margins have been identified with a highly specific protein pattern. Further refinement of the fluid mosaic model was needed to take into account the presence of non-bilayer lipids, which are the most abundant lipids in all energy-converting membranes, and the polymorphism of lipid phases, which has also been documented in thylakoid membranes. These observations lead to the question, how mobile the components are in the lipid phase and how this ordering is made and maintained and how these features might be correlated with the non-bilayer propensity of the membrane lipids. Assuming instead of free diffusion, a "controlled neighborhood" replaced the model of fluidity by the model of a "mixed crystal structure". In this review we describe why basic photosynthetic regulation mechanisms depend on arrays of crystal-like lipid-protein macro-assemblies. The mechanisms which define the ordering in macrodomains are still not completely clear, but some recent experiments give an idea how this fascinating order is produced, adopted and maintained. We use the operation of the xanthophyll cycle as a rather well understood model challenging and complementing the standard Singer-Nicolson model via assigning special roles to non-bilayer lipids and non-lamellar lipid phases in the structure and function of thylakoid membranes.

Modulation of non-bilayer lipid phases and the structure and functions of thylakoid membranes: effects on the water-soluble enzyme violaxanthin de-epoxidase

The role of non-bilayer lipids and non-lamellar lipid phases in biological membranes is an enigmatic problem of membrane biology. Non-bilayer lipids are present in large amounts in all membranes; in energy-converting membranes they constitute about half of their total lipid content—yet their functional state is a bilayer. In vitro experiments revealed that the functioning of the water-soluble violaxanthin de-epoxidase (VDE) enzyme of plant thylakoids requires the presence of a non-bilayer lipid phase. ³¹P-NMR spectroscopy has provided evidence on lipid polymorphism in functional thylakoid membranes. Here we reveal reversible pH- and temperature-dependent changes of the lipid-phase behaviour, particularly the flexibility of isotropic non-lamellar phases, of isolated spinach thylakoids. These reorganizations are accompanied by changes in the permeability and thermodynamic parameters of the membranes and appear to control the activity of VDE and the photoprotective mechanism of non-photochemical quenching of chlorophyll-a fluorescence. The data demonstrate, for the first time in native membranes, the modulation of the activity of a water-soluble enzyme by a non-bilayer lipid phase.

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.

A proteoliposome-based system reveals how lipids control photosynthetic light harvesting

Integral membrane proteins are exposed to a complex and dynamic lipid environment modulated by nonbilayer lipids that can influence protein functions by lipid-protein interactions. The nonbilayer lipid monogalactosyldiacylglycerol (MGDG) is the most abundant lipid in plant photosynthetic thylakoid membranes, but its impact on the functionality of energy-converting membrane protein complexes is unknown. Here, we optimized a detergent-based reconstitution protocol to develop a proteoliposome technique that incorporates the major light-harvesting complex II (LHCII) into compositionally well-defined large unilamellar lipid bilayer vesicles to study the impact of MGDG on light harvesting by LHCII. Using steady-state fluorescence spectroscopy, CD spectroscopy, and time-correlated single-photon counting, we found that both chlorophyll fluorescence quantum yields and fluorescence lifetimes clearly indicate that the presence of MGDG in lipid bilayers switches LHCII from a light-harvesting to a more energy-quenching mode that dissipates harvested light into heat. It is hypothesized that in the in vitro system developed here, MGDG controls light harvesting of LHCII by modulating the hydrostatic lateral membrane pressure profile in the lipid bilayer sensed by LHCII-bound peripheral pigments.

An Advanced Lipid Metabolism System Revealed by Transcriptomic and Lipidomic Analyses Plays a Central Role in Peanut Cold Tolerance

Cold stress restricts peanut (Arachis hypogaea L.) growth, development, and yield. However, the specific mechanism of cold tolerance in peanut remains unknown. Here, the comparative physiological, transcriptomic, and lipidomic analyses of cold tolerant variety NH5 and cold sensitive variety FH18 at different time points of cold stress were conducted to fill this gap. Transcriptomic analysis revealed lipid metabolism including membrane lipid and fatty acid metabolism may be a significant contributor in peanut cold tolerance, and 59 cold-tolerant genes involved in lipid metabolism were identified. Lipidomic data corroborated the importance of membrane lipid remodeling and fatty acid unsaturation. It indicated that photosynthetic damage, resulted from the alteration in fluidity and integrity of photosynthetic membranes under cold stress, were mainly caused by markedly decreased monogalactosyldiacylglycerol (MGDG) levels and could be relieved by increased digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) levels. The upregulation of phosphatidate phosphatase (PAP1) and phosphatidate cytidylyltransferase (CDS1) inhibited the excessive accumulation of PA, thus may prevent the peroxidation of membrane lipids. In addition, fatty acid elongation and fatty acid β-oxidation were also worth further studied in peanut cold tolerance. Finally, we constructed a metabolic model for the regulatory mechanism of peanut cold tolerance, in which the advanced lipid metabolism system plays a central role. This study lays the foundation for deeply analyzing the molecular mechanism and realizing the genetic improvement of peanut cold tolerance.

Galactolipid deficiency disturbs spatial arrangement of the thylakoid network in Arabidopsis thaliana plants

The chloroplast thylakoid network is a dynamic structure which, through possible rearrangements, plays a crucial role in regulation of photosynthesis. Although the importance of the main components of the thylakoid membrane matrix, galactolipids, in the formation of the network of internal plastid membrane was found before, the structural role of monogalactosyldiacylglycerol (MGDG) and digalactosylidacylglycerol (DGDG) is still largely unknown. We elucidated detailed structural modifications of the thylakoid membrane system in Arabidopsis thaliana MGDG- and DGDG-deficient mutants. An altered MGDG/DGDG ratio was structurally reflected by formation of smaller grana, local changes in grana stacking repeat distance, and significant changes in the spatial organization of the thylakoid network compared with wild-type plants. The decrease of the MGDG level impaired the formation of the typical helical grana structure and resulted in a 'helical-dichotomic' arrangement. DGDG deficiency did not affect spatial grana organization but changed the shape of the thylakoid membrane network in situ from lens like into a flattened shape. Such structural disturbances were accompanied by altered composition of carotenoid and chlorophyll-protein complexes, which eventually led to the decreased photosynthetic efficiency of MGDG- and DGDG-deficient plants.

Chloroplast ultrastructure in plants

The chloroplast organelle in mesophyll cells of higher plants represents a sunlight-driven metabolic factory that eventually fuels life on our planet. Knowledge of the ultrastructure and the dynamics of this unique organelle is essential to understanding its function in an ever-changing and challenging environment. Recent technological developments promise unprecedented insights into chloroplast architecture and its functionality. The review highlights these new methodical approaches and provides structural models based on recent findings about the plasticity of the thylakoid membrane system in response to different light regimes. Furthermore, the potential role of the lipid droplets plastoglobuli is discussed. It is emphasized that detailed structural insights are necessary on different levels ranging from molecules to entire membrane systems for a holistic understanding of chloroplast function.

Lipid-polymorphism of plant thylakoid membranes. Enhanced non-bilayer lipid phases associated with increased membrane permeability

Earlier experiments, using ³¹ P-NMR and time-resolved merocyanine fluorescence spectroscopy, have shown that isolated intact, fully functional plant thylakoid membranes, in addition to the bilayer phase, contain three non-bilayer (or non-lamellar) lipid phases. It has also been shown that the lipid polymorphism of thylakoid membranes can be characterized by remarkable plasticity, i.e. by significant variations in ³¹ P-NMR signatures. However, changes in the lipid-phase behaviour of thylakoids could not be assigned to changes in the overall membrane organization and the photosynthetic activity, as tested by circular dichroism and 77 K fluorescence emission spectroscopy and the magnitude of the variable fluorescence of photosystem II, which all showed only marginal variations. In this work, we investigated in more detail the temporal stability of the different lipid phases by recording ³¹ P-NMR spectra on isolated thylakoid membranes that were suspended in sorbitol- or NaCl-based media. We observed, at 5°C during 8 h in the dark, substantial gradual enhancement of the isotropic lipid phases and diminishment of the bilayer phase in the sorbitol-based medium. These changes compared well with the gradually increasing membrane permeability, as testified by the gradual acceleration of the decay of flash-induced electrochromic absorption changes and characteristic changes in the kinetics of fast chlorophyll a-fluorescence transients; all variations were much less pronounced in the NaCl-based medium. These observations suggest that non-bilayer lipids and non-lamellar lipid phases play significant roles in the structural dynamics and functional plasticity of thylakoid membranes.

Evaluation of Lipids for the Study of Photosynthetic Membranes

The biological role of lipids goes far beyond the formation of a structural membrane bilayer platform for membrane proteins and controlling fluxes across the membranes. For example, in photosynthetic thylakoid membranes, lipids occupy well-defined binding niches within protein complexes and determine the structural organization of membrane proteins and their function by controlling generic physicochemical membrane properties. In this chapter, two-dimensional thin-layer chromatography (2D TLC) and gas chromatography (GC) techniques are presented for quantitative analysis of lipid classes and fatty acids in thylakoid membranes. In addition, lipid extraction methods from isolated thylakoid membranes and leaves are described together with a procedure for derivatization of fatty acids to fatty acid methyl esters (FAME) that is required for GC analysis.

Stress-induced changes in the ultrastructure of the photosynthetic apparatus of green microalgae

In photosynthetic organisms including unicellular algae, acclimation to and damage by environmental stresses are readily apparent at the level of the photosynthetic apparatus. Phenotypic manifestations of the stress responses include rapid and dramatic reduction of photosynthetic activity and pigment content aimed at mitigating the risk of photooxidative damage. Although the physiological and molecular mechanisms of these events are well known, the ultrastructural picture of the stress responses is often elusive and frequently controversial. We analyzed an extensive set of transmission electron microscopy images of the microalgal cells obtained across species of Chlorophyta and in a wide range of growth conditions. The results of the analysis allowed us to pinpoint distinct ultrastructural changes typical of normal functioning and emergency reduction of the chloroplast membrane system under high light exposure and/or mineral nutrient starvation. We demonstrate the patterns of the stress-related ultrastructural changes including peculiar thylakoid rearrangements and autophagy-like processes and provide an outlook on their significance for implementation of the stress responses.

Fatty Acid Composition and Thermotropic Behavior of Glycolipids and Other Membrane Lipids of Ulva lactuca (Chlorophyta) Inhabiting Different Climatic Zones

Increasing global temperatures are expected to increase the risk of extinction of various species due to acceleration in the pace of shifting climate zones. Nevertheless, there is no information on the physicochemical properties of membrane lipids that enable the adaptation of the algae to different climatic zones. The present work aimed to compare fatty acid composition and thermal transitions of membrane lipids from green macroalgae Ulva lactuca harvested in the Sea of Japan and the Adriatic Sea in summer. U. lactuca inhabiting the Adriatic Sea had bleached parts of thalli which were completely devoid of chloroplast glycolipids. The adaptation to a warmer climatic zone was also accompanied by a significant decrease in the ratio between unsaturated and saturated fatty acids (UFA/SFA) of membrane lipids, especially in bleached thalli. Hence, bleaching of algae is probably associated with the significant decrease of the UFA/SFA ratio in glycolipids. The decreasing ratio of n-3/n-6 polyunsaturated fatty acids (PUFAs) was observed in extra-plastidial lipids and only in the major glycolipid, non-lamellar monogalactosyldiacylglycerol. The opposite thermotropic behavior of non-lamellar and lamellar glycolipids can contribute to maintenance of the highly dynamic structure of thylakoid membranes of algae in response to the increasing temperatures of climatic zones.

Prediction of Thylakoid Lipid Binding Sites on Photosystem II

The thylakoid membrane has a unique lipid composition, consisting mostly of galactolipids. These thylakoid lipids have important roles in photosynthesis. Here, we investigate to what extent these lipids bind specifically to the Photosystem II complex. To this end, we performed coarse-grain MD simulations of the Photosystem II complex embedded in a thylakoid membrane with realistic composition. Based on >85 μs simulation time, we find that monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol lipids are enriched in the annular shell around the protein, and form distinct binding sites. From the analysis of residue contacts, we conclude that electrostatic interactions play an important role in stabilizing these binding sites. Furthermore, we find that chlorophyll a has a prevalent role in the coordination of the lipids. In addition, we observe lipids to diffuse in and out of the plastoquinone exchange cavities, allowing exchange of cocrystallized lipids with the bulk membrane and suggesting a more open nature of the plastoquinone exchange cavity. Together, our data provide a wealth of information on protein-lipid interactions for a key protein in photosynthesis.

Structure-function relationships in photosynthetic membranes: Challenges and emerging fields

Oxygenic photosynthesis is a fundamental biological process that shaped the earth's biosphere. The process of energy transformation is hosted in highly specialized thylakoid membranes that adjust their architecture in response to environmental cues at different structural levels leading to the adjustment of photosynthetic functions. This review presents structure-function dynamics ranging from the whole membrane system over the mesoscopic level (protein ensembles) down to interactions between lipids and protein complexes. On the whole membrane level, thylakoid membranes constantly change their overall shape (e.g. membranes swell and shrink or destack and stack) that controls vital functions of energy transformation. Furthermore, the physical connection and transition between stacked grana thylakoid and unstacked membrane regions that determines mass transport between these sub-compartments is a crucial open question. On the mesoscopic level, it turns out that reorganizations between disordered and ordered protein arrangements is central for light harvesting and lateral diffusion processes. It has to be unraveled how changes in mesoscopic protein organization are controlled. Finally, dynamic physicochemical properties of the lipid bilayer can determine the structure and organization of photosynthetic membrane proteins, a field that is highly neglected so far. This review focusses on open questions and challenging problems in photosynthesis research.

Lipid polymorphism in chloroplast thylakoid membranes - as revealed by 31P-NMR and time-resolved merocyanine fluorescence spectroscopy

Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosynthetic machinery. Their energized state, driving ATP synthesis, is enabled by the bilayer organization of the membrane. However, their most abundant lipid species is a non-bilayer-forming lipid, monogalactosyl-diacylglycerol; the role of lipid polymorphism in these membranes is poorly understood. Earlier ³¹P-NMR experiments revealed the coexistence of a bilayer and a non-bilayer, isotropic lipid phase in spinach thylakoids. Packing of lipid molecules, tested by fluorescence spectroscopy of the lipophilic dye, merocyanine-540 (MC540), also displayed heterogeneity. Now, our ³¹P-NMR experiments on spinach thylakoids uncover the presence of a bilayer and three non-bilayer lipid phases; time-resolved fluorescence spectroscopy of MC540 also reveals the presence of multiple lipidic environments. It is also shown by ³¹P-NMR that: (i) some lipid phases are sensitive to the osmolarity and ionic strength of the medium, (ii) a lipid phase can be modulated by catalytic hydrogenation of fatty acids and (iii) a marked increase of one of the non-bilayer phases upon lowering the pH of the medium is observed. These data provide additional experimental evidence for the polymorphism of lipid phases in thylakoids and suggest that non-bilayer phases play an active role in the structural dynamics of thylakoid membranes.

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.