Roles of Lipids in Photosynthesis

Arabidopsis seeds altered in the circadian clock protein TOC1 are characterized by higher level of linolenic acid

The circadian clock plays a critical role in regulating plant physiology and metabolism. However, the way in which the clock impacts the regulation of lipid biosynthesis in seeds is partially understood. In the present study, we characterized the seed fatty acid (FA) and glycerolipid (GL) compositions of pseudo-response regulator mutants. Among these mutants, toc1 (timing of cab expression 1) exhibited the most significant differences compared to control plants. These included an increase in total FA content, characterized by elevated levels of linolenic acid (18:3) along with a reduction in linoleic acid (18:2). Furthermore, our findings revealed that toc1 developing seeds showed increased expression of genes related to FA metabolism. Our results show a connection between TOC1 and lipid metabolism in Arabidopsis seeds.

Alteration in lipid metabolism is involved in nitrogen deficiency response in wheat seedlings

Changes of membrane lipid composition contribute to plant adaptation to various abiotic stresses. Here, a comparative study was undertaken to investigate the mechanisms of how lipid alteration affects plant growth and development under nitrogen (N) deficiency. Two wheat cultivars: the N deficiency-tolerant cultivar Xiaoyan 6 (XY) and the N deficiency-sensitive cultivar Aikang 58 (AK) were used to test if the high N-deficiency tolerance was related with lipid metabolism. The results showed that N deficiency inhibited the morpho-physiological parameters in both XY and AK cultivars, which showed a significant decrease in biomass, N content, photosynthetic efficiency, and lipid contents. However, these decreases were more pronounced in AK than XY. In addition, XY showed a notable increase in fatty acid unsaturation, relatively well-maintained chloroplast ultrastructure, and minimized damage of lipid peroxidation and enhanced PSII activity under N-deficient condition, as compared with AK. Transcription levels of many genes involved in lipid biosynthesis and fatty acid desaturation were up-regulated in response to N deficiency in two wheat cultivars, while the expressions were much higher in XY than AK under N deficiency. These results highlight the importance of alterations in lipid metabolism in N deficiency tolerance in wheat. High levels of lipid content and unsaturated fatty acids maintained the membrane structure and function, contributing to high photosynthesis and antioxidant capacities, thereby improved the tolerance to N deficiency.

The ZmNF-YC1-ZmAPRG pathway modulates low phosphorus tolerance in maize

Phosphorus (P) is an essential nutrient for plant growth and yield. Low phosphate use efficiency makes it important to clarify the molecular mechanism of low P stress. In our previous studies, a P efficiency gene ZmAPRG was identified. Here, we further screened the upstream regulator ZmNF-YC1 of ZmAPRG by yeast one hybrid (Y1H) assay, and found it was a low inorganic phosphorus (Pi)-inducible gene. The results of dual luciferase assays, expression analysis, and ChIP-qPCR assays showed that ZmNF-YC1 is a positive regulator of ZmAPRG. Overexpression of ZmNF-YC1 improved low P tolerance, whereas knockout of ZmNF-YC1 decreased low P tolerance in maize. Bimolecular fluorescence complementation (BiFC), yeast two hybrid (Y2H) assay, and yeast three hybrid (Y3H) assay further showed that ZmNF-YC1 can interact with ZmNF-YB14, and recruit ZmNF-YA4/10 to form NF-Y complexes. Transcriptional activation assay confirmed that the NF-Y complexes can activate the promoters of ZmAPRG. Meanwhile, transcriptome and metabolome analyses indicated that overexpression of ZmAPRG improves low P tolerance by regulating lipid composition and photosynthetic capacity, and chlorophyll fluorescence parameters provided evidence in support of this hypothesis. Furthermore, overexpression of ZmAPRG increased grain yield in inbred and hybrid maize under low P conditions. Taken together, our research revealed a low P tolerance mechanism of the ZmNF-YC1-ZmAPRG pathway.

Anionic lipids facilitate membrane development and protochlorophyllide biosynthesis in etioplasts

Dark-germinated angiosperm seedlings develop chloroplast precursors called etioplasts in cotyledon cells. Etioplasts develop lattice membrane structures called prolamellar bodies (PLBs), where the chlorophyll intermediate protochlorophyllide (Pchlide) forms a ternary complex with NADPH and light-dependent NADPH:Pchlide oxidoreductase (LPOR). The lipid bilayers of etioplast membranes are mainly composed of galactolipids, which play important roles in membrane-associated processes in etioplasts. Although etioplast membranes also contain 2 anionic lipids, phosphatidylglycerol (PG) and sulfoquinovosyldiacylglycerol (SQDG), their roles are unknown. To determine the roles of PG and SQDG in etioplast development, we characterized etiolated Arabidopsis (Arabidopsis thaliana) mutants deficient in PG and SQDG biosynthesis. A partial deficiency in PG biosynthesis loosened the lattice structure of PLBs and impaired the insertion of Mg2+ into protoporphyrin IX, leading to a substantial decrease in Pchlide content. Although a complete lack of SQDG biosynthesis did not notably affect PLB formation and Pchlide biosynthesis, lack of SQDG in addition to partial PG deficiency strongly impaired these processes. These results suggested that PG is required for PLB formation and Pchlide biosynthesis, whereas SQDG plays an auxiliary role in these processes. Notably, PG deficiency and lack of SQDG oppositely affected the dynamics of LPOR complexes after photoconversion, suggesting different involvements of PG and SQDG in LPOR complex organization. Our data demonstrate pleiotropic roles of anionic lipids in etioplast development.

Lipidome Plasticity Enables Unusual Photosynthetic Flexibility in Arctic vs. Temperate Diatoms

The diatom lipidome actively regulates photosynthesis and displays a high degree of plasticity in response to a light environment, either directly as structural modifications of thylakoid membranes and protein-pigment complexes, or indirectly via photoprotection mechanisms that dissipate excess light energy. This acclimation is crucial to maintaining primary production in marine systems, particularly in polar environments, due to the large temporal variations in both the intensity and wavelength distributions of downwelling solar irradiance. This study investigated the hypothesis that Arctic marine diatoms uniquely modify their lipidome, including their concentration and type of pigments, in response to wavelength-specific light quality in their environment. We postulate that Arctic-adapted diatoms can adapt to regulate their lipidome to maintain growth in response to the extreme variability in photosynthetically active radiation. This was tested by comparing the untargeted lipidomic profiles, pigmentation, specific growth rates and carbon assimilation of the Arctic diatom Porosira glacialis vs. the temperate species Coscinodiscus radiatus during exponential growth under red, blue and white light. Here, we found that the chromatic wavelength influenced lipidome remodeling and growth in each strain, with P. glacialis showing effective utilization of red light coupled with increased inclusion of primary light-harvesting pigments and polar lipid classes. These results indicate a unique photoadaptation strategy that enables Arctic diatoms like P. glacialis to capitalize on a wide chromatic growth range and demonstrates the importance of active lipid regulation in the Arctic light environment.

Loss of function mutations at the tomato SSI2 locus impair plant growth and development by altering the fatty acid desaturation pathway

The stearoyl-ACP desaturase (SACPD) is a key enzyme in the regulation of saturated to unsaturated fatty acid ratio, playing a crucial role in regulating membrane stability and fluidity, as well as photosynthesis efficiency, which makes it an important research focus in crop species. This study reports the characterization and molecular cloning of pale dwarf (pad), a new tomato (Solanum lycopersicum L.) T-DNA recessive mutant, which exhibits a dwarf and chlorotic phenotype. Functional studies of the T-DNA tagged gene were conducted, including phylogenetic analysis, expression and metabolomic analyses, and generation of CRISPR/Cas9 knockout lines. The cloning of T-DNA flanking genomic sequences and a co-segregation analysis found the pad phenotype was caused by a T-DNA insertion disrupting the tomato homologue of the Arabidopsis SUPPRESSOR OF SALICYLIC ACID INSENSITIVITY 2 (SlSSI2), encoding a plastid localized isoform of SACPD. The phenotype of CRISPR/Cas9 SlSSI2 knockout lines confirmed that the morphological abnormalities in pad plants were due to SlSSI2 loss of function. Functional, metabolomic and expression analyses proved that SlSSI2 disruption causes deficiencies in 18:1 fatty acid desaturation and leads to diminished jasmonic acid (JA) content and increased salicylic acid (SA) levels. Overall, these results proved that SSI2 plays a crucial role in the regulation of polyunsaturated fatty acid profiles in tomato, and revealed that SlSSI2 loss of function results in an inhibited JA-responsive signalling pathway and a constitutively activated SA-mediated defence signalling response. This study lays the foundation for further research on tomato SACPDs and their role in plant performance and fitness.

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Acquisition of Desiccation Tolerance Unveiled: Polar Lipid Profiles of Streptophyte Algae Offer Insights

Terrestrialization by photosynthetic eukaryotes took place in the two branches of green microalgae: Chlorophyta and Charophyta. Within the latter, the paraphyletic streptophytic algae divide into two clades. These are named Klebsormidiophyceae-Chlorokybophyceae-Mesostigmatophyceae (KCM), which is the oldest, and Zygnematophyceae-Coleochaetophyceae-Charophyceae (ZCC), which contains the closest relatives of vascular plants. Terrestrialization required the emergence of adaptations in response to new challenges, such as irradiance, temperature oscillations and water deprivation. In this study, we evaluated lipid composition in species representative of distinct phylogenetic clusters within Charophyta and Chlorophyta. We aim to study whether the inherent thylakoid lipid composition, as well as its adaptability in response to desiccation, were fundamental factors for the evolutionary history of terrestrial plants. The results showed that the lipid composition was similar to that found in flowering land plants, differing only in betaine lipids. Likewise, the largest constitutive pool of oligogalactolipids (OGL) was found only in the fully desiccation-tolerant species Klebsormidium nitens. After desiccation, the content of polar lipids decreased in all species. Conversely, the content of OGL increased, particularly trigalactosyldiacylglycerol and tetragalactosyldiacylglycerol in the ZCC clade. The analysis of the molecular species composition of the newly formed OGL may suggest a different biosynthetic route for the KCM and ZCC clades. We speculate that the appearance of a new OGL synthesis pathway, which eventually arose during the streptophyte evolutionary process, endowed algae with a much more dynamic regulation of thylakoid composition in response to stress, which ultimately contributed to the colonization of terrestrial habitats.

Curvature in Biological Systems: Its Quantification, Emergence, and Implications across the Scales

Surface curvature both emerges from, and influences the behavior of, living objects at length scales ranging from cell membranes to single cells to tissues and organs. The relevance of surface curvature in biology is supported by numerous experimental and theoretical investigations in recent years. In this review, first, a brief introduction to the key ideas of surface curvature in the context of biological systems is given and the challenges that arise when measuring surface curvature are discussed. Giving an overview of the emergence of curvature in biological systems, its significance at different length scales becomes apparent. On the other hand, summarizing current findings also shows that both single cells and entire cell sheets, tissues or organisms respond to curvature by modulating their shape and their migration behavior. Finally, the interplay between the distribution of morphogens or micro-organisms and the emergence of curvature across length scales is addressed with examples demonstrating these key mechanistic principles of morphogenesis. Overall, this review highlights that curved interfaces are not merely a passive by-product of the chemical, biological, and mechanical processes but that curvature acts also as a signal that co-determines these processes.

Cryo-electron microscopy structure of the intact photosynthetic light-harvesting antenna-reaction center complex from a green sulfur bacterium

The photosynthetic reaction center complex (RCC) of green sulfur bacteria (GSB) consists of the membrane-imbedded RC core and the peripheric energy transmitting proteins called Fenna-Matthews-Olson (FMO). Functionally, FMO transfers the absorbed energy from a huge peripheral light-harvesting antenna named chlorosome to the RC core where charge separation occurs. In vivo, one RC was found to bind two FMOs, however, the intact structure of RCC as well as the energy transfer mechanism within RCC remain to be clarified. Here we report a structure of intact RCC which contains a RC core and two FMO trimers from a thermophilic green sulfur bacterium Chlorobaculum tepidum at 2.9 Å resolution by cryo-electron microscopy. The second FMO trimer is attached at the cytoplasmic side asymmetrically relative to the first FMO trimer reported previously. We also observed two new subunits (PscE and PscF) and the N-terminal transmembrane domain of a cytochrome-containing subunit (PscC) in the structure. These two novel subunits possibly function to facilitate the binding of FMOs to RC core and to stabilize the whole complex. A new bacteriochlorophyll (numbered as 816) was identified at the interspace between PscF and PscA-1, causing an asymmetrical energy transfer from the two FMO trimers to RC core. Based on the structure, we propose an energy transfer network within this photosynthetic apparatus.

Impacts of phosphatidylglycerol on plastid gene expression and light induction of nuclear photosynthetic genes

Phosphatidylglycerol (PG) is the only major phospholipid in the thylakoid membrane of chloroplasts. PG is essential for photosynthesis, and loss of PG in Arabidopsis thaliana results in severe defects of growth and chloroplast development, with decreased chlorophyll accumulation, impaired thylakoid formation, and down-regulation of photosynthesis-associated genes encoded in nuclear and plastid genomes. However, how the absence of PG affects gene expression and plant growth remains unclear. To elucidate this mechanism, we investigated transcriptional profiles of a PG-deficient Arabidopsis mutant pgp1-2 under various light conditions. Microarray analysis demonstrated that reactive oxygen species (ROS)-responsive genes were up-regulated in pgp1-2. However, ROS production was not enhanced in the mutant even under strong light, indicating limited impacts of photooxidative stress on the defects of pgp1-2. Illumination to dark-adapted pgp1-2 triggered down-regulation of photosynthesis-associated nuclear-encoded genes (PhANGs), while plastid-encoded genes were constantly suppressed. Overexpression of GOLDEN2-LIKE1 (GLK1), a transcription factor gene regulating chloroplast development, in pgp1-2 up-regulated PhANGs but not plastid-encoded genes along with chlorophyll accumulation. Our data suggest a broad impact of PG biosynthesis on nuclear-encoded genes partially via GLK1 and a specific involvement of this lipid in plastid gene expression and plant development.

Lipids in photosynthetic protein complexes in the thylakoid membrane of plants, algae, and cyanobacteria

In the thylakoid membrane of cyanobacteria and chloroplasts, many proteins involved in photosynthesis are associated with or integrated into the fluid bilayer matrix formed by four unique glycerolipid classes, monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Biochemical and molecular genetic studies have revealed that these glycerolipids play essential roles not only in the formation of thylakoid lipid bilayers but also in the assembly and functions of photosynthetic complexes. Moreover, considerable advances in structural biology have identified a number of lipid molecules within the photosynthetic complexes such as PSI and PSII. These data have provided important insights into the association of lipids with protein subunits in photosynthetic complexes and the distribution of lipids in the thylakoid membrane. Here, we summarize recent high-resolution observations of lipid molecules in the structures of photosynthetic complexes from plants, algae, and cyanobacteria, and evaluate the distribution of lipids among photosynthetic protein complexes and thylakoid lipid bilayers. By integrating the structural information into the findings from biochemical and molecular genetic studies, we highlight the conserved and differentiated roles of lipids in the assembly and functions of photosynthetic complexes among plants, algae, and cyanobacteria.

Multiomic analyses of two sorghum cultivars reveals the change of membrane lipids in their responses to water deficit

Drought is one of the main abiotic stresses influencing crop production all over the world. Membranes are sensitive to drought stress and easy to be degraded and modified. Lipidome and transcriptome analyses were applied to analyze the responses of membrane lipids to drought stress in two sorghum (Sorghum bicolor (L.) Moench) cultivars, drought-sensitive cv. Hongyingzi and drought-tolerant cv. Kangsi. In total, 156 lipid compounds were identified and the contents of the predominant ones changed significantly under drought stress. Drought significantly decreased the unsaturation indices (UI) of digalactosyl-diacylglycerol (DGDG), monogalactosyl-diacylglycerol (MGDG), phosphatidylglycerol (PG) and phosphatidylcholine (PC) in both cultivars, except for insignificant changes of UI for DGDG in cv. Kangsi. Transcriptome sequencing analysis identified genes related to membrane lipid remodeling such as phospholipase D α1 (PLDα1), phospholipase D δ (PLDδ), and phospholipase A 2 (PLA2). By integrating transcriptome data and lipidome data, weighted gene co-expression network analysis (WGCNA) identified hub genes, transcription factors and the genes involved in lipid metabolism. Then, the protein and protein interaction (PPI) was analyzed using STRING and the possible candidate genes regulating membrane lipids under drought stress were obtained, including CCT2, CER1, DGK1, DGK5, EMB3174, KCS4, LCB2, PAH1, PLDP1, PKP-β1, and KCS11. The results from this study have the potential to accelerate the process to breed drought-tolerant sorghum lines.

ROS-induced dramatic lipid changes in Arabidopsis

Objectives: The beneficial role of ROS was probably in promoting intercellular communication by modifying membrane constituents [Liang D. A salutary role of reactive oxygen species in intercellular tunnel-mediated communication. Front Cell Dev Biol. 2018;6:2]. We investigated how the membrane lipids were responding to ROS and ROS inhibitors.

Methods: To examine how ROS affected the lipid profiles, we used thin-layer chromatography to characterize lipid profiles in Arabidopsis plants. Then, the confocal microscopy imaging was used to confirm the change of membrane lipid in a plasma membrane marker line exposed to ROS and ROS inhibitors.

Results: We found the relative contents of most lipids in H₂O₂-treated Arabidopsis plants were increased in roots, rather than in shoots. The increased fluorescent signal of membrane marker induced by H₂O₂ was mainly enriched in the conductive parts of roots. Several ROS inhibitors also strongly affected the lipid profiles. Among them, diethyldithiocarbamate (DDC) can progressively change the lipid profiles with treatment going on. Membrane marker signal was mainly accumulated in the root tips and epidermal cells after treatment by DDC.

Discussion: H₂O₂ may enhance intercellular communication by inducing different lipid species in the conductive parts of roots. The lipid profiles were widely responding to various ROS reagents and might play a role in intercellular signaling.

A critical role of PvFtsH2 in the degradation of photodamaged D1 protein in common bean

Light is required for initiating chloroplast biogenesis and photosynthesis; however, the photosystem II reaction center (PSII RC) can be photodamaged. In this study, we characterized pvsl1, a seedling-lethal mutant of Phaseolus vulgaris. This mutant showed lethality when exposed to sunlight irradiation and a yellow-green leaf phenotype when grown in a growth chamber under low-light conditions. We developed 124 insertion/deletion (INDEL) markers based on resequencing data of Dalong1 and PI60234, two local Chinese common bean cultivars, for genetic mapping. We identified Phvul.002G190900, which encodes the PvFtsH2 protein, as the candidate gene for this pvsl1 mutation through fine-mapping and functional analysis. A single-base deletion occurred in the coding region of Phvul.002G190900 in the pvsl1 mutant, resulting in a frameshift mutation and a truncated protein lacking the Zn²⁺ metalloprotease domain. Suppressed expression of Phvul.002G190900 at the transcriptional level was detected, while no change in the subcellular localization signal was observed. The seedlings of pvsl1 exhibited hypersensitivity to photoinhibition stress. In the pvsl1 mutant, abnormal accumulation of the D1 protein indicated a failure to rapidly degrade damaged D1 protein in the PSII RC. The results of this study demonstrated that PvFtsH2 is critically required for survival and maintaining photosynthetic activity by degrading photodamaged PSII RC D1 protein in common bean.

Architecture of the photosynthetic complex from a green sulfur bacterium

The photosynthetic apparatus of green sulfur bacteria (GSB) contains a peripheral antenna chlorosome, light-harvesting Fenna-Matthews-Olson proteins (FMO), and a reaction center (GsbRC). We used cryo-electron microscopy to determine a 2.7-angstrom structure of the FMO-GsbRC supercomplex from Chlorobaculum tepidum The GsbRC binds considerably fewer (bacterio)chlorophylls [(B)Chls] than other known type I RCs do, and the organization of (B)Chls is similar to that in photosystem II. Two BChl layers in GsbRC are not connected by Chls, as seen in other RCs, but associate with two carotenoid derivatives. Relatively long distances of 22 to 33 angstroms were observed between BChls of FMO and GsbRC, consistent with the inefficient energy transfer between these entities. The structure contains common features of both type I and type II RCs and provides insight into the evolution of photosynthetic RCs.

The roles of chloroplast membrane lipids in abiotic stress responses

Plant chloroplasts have complex membrane systems. Among these, thylakoids serve as the sites for photosynthesis and photosynthesis-related adaptation. In addition to the photosynthetic membrane complexes and associated molecules, lipids in the thylakoid membranes, are predominantly composed of MGDG (monogalactosyldiacylglycerol), DGDG (digalactosyldiacylglycerol), SQDG (sulfoquinovosyldiacylglycerol) and PG (phosphatidylglycerol), play essential roles in shaping the thylakoid architecture, electron transfer, and photoregulation. In this review, we discuss the effect of abiotic stress on chloroplast structure, the changes in membrane lipid composition, and the degree of unsaturation of fatty acids. Advanced understanding of the mechanisms regulating chloroplast membrane lipids and unsaturated fatty acids in response to abiotic stresses is indispensable for improving plant resistance and may inform the strategies of crop breeding.

Carbon Partitioning and Lipid Remodeling During Phosphorus and Nitrogen Starvation in the Marine Microalga Diacronema lutheri (Haptophyta)

The domesticated marine microalga Diacronema lutheri is of great interest for producing various highly valuable molecules like lipids, particularly long-chain polyunsaturated fatty acids (LC-PUFA). In this study, we investigated the impact of phosphorus (P) and nitrogen (N) starvation on growth, carbon fixation (photosynthetic activity) and partitioning, and membrane lipid remodeling in this alga during batch culture. Our results show that the photosynthetic machinery was similarly affected by P and N stress. Under N starvation, we observed a much lower photosynthetic rate and biomass productivity. The degradation and re-use of cellular N-containing compounds contributed to triacylglycerol (TAG) accumulation. On the other hand, P-starved cells maintained pigment content and a carbon partitioning pattern more similar to the control, ensuring a high biomass. Betaine lipids constitute the major compounds of non-plastidial membranes, which are rich in eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids. Under P and N starvations, EPA was transferred from the recycling of membrane polar lipids, most likely contributing to TAG accumulation.

Exploration of the Effects of Different Blue LED Light Intensities on Flavonoid and Lipid Metabolism in Tea Plants via Transcriptomics and Metabolomics

Blue light extensively regulates multiple physiological processes and secondary metabolism of plants. Although blue light quantity (fluence rate) is important for plant life, few studies have focused on the effects of different blue light intensity on plant secondary metabolism regulation, including tea plants. Here, we performed transcriptomic and metabolomic analyses of young tea shoots (one bud and two leaves) under three levels of supplemental blue light, including low-intensity blue light (LBL, 50 μmol m^–2 s^–1), medium-intensity blue light (MBL, 100 μmol m^–2 s^–1), and high-intensity blue light (HBL, 200 μmol m^–2 s^–1). The total number of differentially expressed genes (DEGs) in LBL, MBL and HBL was 1, 7 and 1097, respectively, indicating that high-intensity blue light comprehensively affects the transcription of tea plants. These DEGs were primarily annotated to the pathways of photosynthesis, lipid metabolism and flavonoid synthesis. In addition, the most abundant transcription factor (TF) families in DEGs were bHLH and MYB, which have been shown to be widely involved in the regulation of plant flavonoids. The significantly changed metabolites that we detected contained 15 lipids and 6 flavonoid components. Further weighted gene co-expression network analysis (WGCNA) indicated that CsMYB (TEA001045) may be a hub gene for the regulation of lipid and flavonoid metabolism by blue light. Our results may help to establish a foundation for future research investigating the regulation of woody plants by blue light.

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.

Relationship Between Glycerolipids and Photosynthetic Components During Recovery of Thylakoid Membranes From Nitrogen Starvation-Induced Attenuation in Synechocystis sp. PCC 6803

Thylakoid membranes, the site of photochemical and electron transport reactions of oxygenic photosynthesis, are composed of a myriad of proteins, cofactors including pigments, and glycerolipids. In the non-diazotrophic cyanobacterium Synechocystis sp. PCC 6803, the size and function of thylakoid membranes are reduced under nitrogen (N) starvation but are quickly recovered after N addition to the starved cells. To understand how the functionality of thylakoid membranes is adjusted in response to N status in Synechocystis sp. PCC 6803, we examined changes in thylakoid components and the photosynthetic activity during the N starvation and recovery processes. In N-starved cells, phycobilisome content, photosystem II protein levels and the photosynthetic activity substantially decreased as compared with those in N-sufficient cells. Although the content of chlorophyll (Chl) a, total protein and total glycerolipid also decreased under the N-starved condition based on OD₇₃₀ reflecting cell density, when based on culture volume, the Chl a and total protein content remained almost constant and total glycerolipid content even increased during N starvation, suggesting that cellular levels of these components decrease under the N-starved condition mainly through dilution due to cell growth. With N addition, the photosynthetic activity quickly recovered, followed by full restoration of photosynthetic pigment and protein levels. The content of phosphatidylglycerol (PG), an essential lipid constituent of both photosystems, increased faster than that of Chl a, whereas the content of glycolipids, the main constituents of the thylakoid lipid bilayer, gradually recovered after N addition. The data indicate differential regulation of PG and glycolipids during the construction of the photosynthetic machinery and regeneration of thylakoid membranes. Of note, addition of PG to the growth medium slightly accelerated the Chl a accumulation in wild-type cells during the recovery process. Because PG is required for the biosynthesis of Chl a and the formation of functional photosystem complexes, rapid PG biosynthesis in response to N acquisition may be required for the rapid formation of the photosynthetic machinery during thylakoid regeneration.

Galactolipid DGDG and Betaine Lipid DGTS Direct De Novo Synthesized Linolenate into Triacylglycerol in a Stress-Induced Starchless Mutant of Chlamydomonas reinhardtii

The increasing demand for triacylglycerol (TAG) enriching polyunsaturated fatty acids (PUFAs) has led to a surge of interest in microalgal TAG metabolism. Polar membrane lipids serve as the desaturation carrier for PUFA, and the functional group of PUFA can be incorporated into TAG. Monogalactoglycerolipid has been found to provide the de novo synthesized oleate acyl group or the nascent polyunsaturated diacylglycerol backbone for TAG biosynthesis in the model green alga, Chlamydomonas reinhardtii. However, whether other membrane lipids take part in the formation of PUFA-attached TAG has not been clearly discovered. A time course study of glycerolipidomics in the starchless mutant of C. reinhardtii, BAFJ5, which hyper-accumulates TAG, revealed that digalactosyldiacylglycerol (DGDG) and diacylglycerol-N,N,N-trimethylhomoserine (DGTS) turned into the main components of membrane lipids, accounting for 62% of the total polar lipids, under nitrogen deprivation combined with high light conditions. In addition, the membrane lipid molecules DGDG 18:3n3/16:0 and DGTS 16:0/18:3n6 were presumed to be involved in the consecutive integration of the de novo synthesized linolenates into TAG. Based on the stoichiometry calculation, DGDG and DGTS were demonstrated to provide a major contribution to the accumulation of linolenate-attached TAG. Our study gives insights into the potential PUFA-attached TAG formation pathway mediated by the turnover of de novo synthesized DGDG and DGTS in the starchless mutant of Chlamydomonas.

Dynamic changes in membrane lipid composition of leaves of winter wheat seedlings in response to PEG-induced water stress

BACKGROUND

Membrane lipid composition associates closely with membrane stability and fluidity under water stress. In this study, lipidomic analyses based on electrospray ionization mass spectrometry (ESI-MS/MS) were carried out to explore dynamic changes of membrane lipids in term of molecular species caused by PEG (Polyethylene glycol-6000)-induced water stress in wheat seedlings.

RESULTS

Among the main phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) are primary degradation targets, and PC was degraded in the largest degree. Membrane ion leakage dramatically increased later than the significant reduction of these phospholipids, indicating that the loss of membrane integrity lagged behind severe phospholipid degradation. Monogalactosyldiacylglycerol (MGDG) increased firstly and decreased later, while digalactosyldiacylglycerol (DGDG) ratcheted up with stress. DGDG/MGDG increased after stress for 3 days, and unsaturation of DGDG was promoted with stress. Variation trends of galactolipids differed among molecular species. The time when MGDG (34:3), DGDG (34:3) began to decline approached to the time when non-stomatal limitation impaired photosynthesis. While the two predominant molecular species MGDG (36:6) and DGDG (36:6) began to decline later. So we speculated that MGDG (34:3), DGDG (34:3) might be key components in photosynthesis apparatus and participate in photosynthesis directly. While the two predominant molecular species, MGDG (36:6) and DGDG (36:6) might locate in thylakoid lipid bilayer matrix and play roles in stabilizing the membrane. The research provides new insights into the dynamic response of lipid metabolism to PEG-induced water stress.

CONCLUSION

In wheat plants under water stress, the major molecular species of PC, PE and PG were degraded, MGDG and DGDG molecular species had differing degradation time courses.

Role of Galactolipids in Plastid Differentiation Before and After Light Exposure

Galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the predominant lipid classes in the thylakoid membrane of chloroplasts. These lipids are also major constituents of internal membrane structures called prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts, which develop in the cotyledon cells of dark-grown angiosperms. Analysis of Arabidopsis mutants defective in the major galactolipid biosynthesis pathway revealed that MGDG and DGDG are similarly and, in part, differently required for membrane-associated processes such as the organization of PLBs and PTs and the formation of pigment–protein complexes in etioplasts. After light exposure, PLBs and PTs in etioplasts are transformed into the thylakoid membrane, resulting in chloroplast biogenesis. During the etioplast-to-chloroplast differentiation, galactolipids facilitate thylakoid membrane biogenesis from PLBs and PTs and play crucial roles in chlorophyll biosynthesis and accumulation of light-harvesting proteins. These recent findings shed light on the roles of galactolipids as key facilitators of several membrane-associated processes during the development of the internal membrane systems in plant plastids.

OsDGD2β is the Sole Digalactosyldiacylglycerol Synthase Gene Highly Expressed in Anther, and its Mutation Confers Male Sterility in Rice

BACKGROUND

Digalactosyldiacylglycerol (DGDG) is one of the major lipids found predominantly in the photosynthetic membrane of cyanobacteria, eukaryotic algae and higher plants. DGDG, along with MGDG (Monogalactosyldiacylglycerol), forms the matrix in thylakoid membrane of chloroplast, providing the site for photochemical and electron transport reactions of oxygenic photosynthesis.

RESULTS

In silico analysis reveals that rice (Oryza sativa L.) genome has 5 genes encoding DGDG synthase, which are differentially expressed in different tissues, and OsDGD2β was identified to be the sole DGDG synthase gene expressed in anther. We then developed osdgd2β mutants by using the CRISPR/Cas9 system and elucidate its role, especially in the development of anther and pollen. The loss of function of OsDGD2β resulted in male sterility in rice characterized by pale yellow and shrunken anther, devoid of starch granules in pollen, and delayed degeneration of tapetal cells. The total fatty acid and DGDG content in the anther was reduced by 18.66% and 22.72% in osdgd2β, affirming the importance of DGDG in the development of anther. The mutants had no notable differences in the vegetative phenotype, as corroborated by relative gene expression of DGDG synthase genes in leaves, chlorophyll measurements, and analysis of photosynthetic parameters, implying the specificity of OsDGD2β in anther.

CONCLUSION

Overall, we showed the importance of DGDG in pollen development and loss of function of OsDGD2β results in male sterility. Here, we have also proposed the use of OsDGD2β in hybrid rice breeding using the nuclear male sterility system.

GhYGL1d, a pentatricopeptide repeat protein, is required for chloroplast development in cotton

BACKGROUND

The pentatricopeptide repeat (PPR) gene family, which contains multiple 35-amino acid repeats, constitutes one of the largest gene families in plants. PPR proteins function in organelles to target specific transcripts and are involved in plant development and growth. However, the function of PPR proteins in cotton is still unknown.

RESULTS

In this study, we characterized a PPR gene YELLOW-GREEN LEAF (GhYGL1d) that is required for cotton plastid development. The GhYGL1d gene has a DYW domain in C-terminal and is highly express in leaves, localized to the chloroplast fractions. GhYGL1d share high amino acid-sequence homology with AtECB2. In atecb2 mutant, overexpression of GhYGL1d rescued the seedling lethal phenotype and restored the editing of accD and ndhF transcripts. Silencing of GhYGL1d led to the reduction of chlorophyll and phenotypically yellow-green leaves in cotton. Compared with wild type, GhYGL1d-silenced cotton showed significant deformations of thylakoid structures. Furthermore, the transcription levels of plastid-encoded polymerase (PEP) and nuclear-encoded polymerase (NEP) dependent genes were decreased in GhYGL1d-silenced cotton.

CONCLUSIONS

Our data indicate that GhYGL1d not only contributes to the editing of accD and ndhF genes, but also affects the expression of NEP- and PEP-dependent genes to regulate the development of thylakoids, and therefore regulates leaf variegation in cotton.

Chloroplast Lipids and Their Biosynthesis

Chloroplasts contain high amounts of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and low levels of the anionic lipids sulfoquinovosyldiacylglycerol (SQDG), phosphatidylglycerol (PG), and glucuronosyldiacylglycerol (GlcADG). The mostly extraplastidial lipid phosphatidylcholine is found only in the outer envelope. Chloroplasts are the major site for fatty acid synthesis. In Arabidopsis, a certain proportion of glycerolipids is entirely synthesized in the chloroplast (prokaryotic lipids). Fatty acids are also exported to the endoplasmic reticulum and incorporated into lipids that are redistributed to the chloroplast (eukaryotic lipids). MGDG, DGDG, SQDG, and PG establish the thylakoid membranes and are integral constituents of the photosynthetic complexes. Phosphate deprivation induces phospholipid degradation accompanied by the increase in DGDG, SQDG, and GlcADG. During freezing and drought stress, envelope membranes are stabilized by the conversion of MGDG into oligogalactolipids. Senescence and chlorotic stress lead to lipid and chlorophyll degradation and the deposition of acyl and phytyl moieties as fatty acid phytyl esters.

How exposure to ALS-inhibiting gametocide tribenuron-methyl induces male sterility in rapeseed

BACKGROUND

Acetolactate synthase (ALS)-inhibiting herbicide tribenuron-methyl (TBM) is an efficient gametocide that can cause rapeseed (Brassica napus L.) to become male sterile and outcrossing. To find the reason the TBM treatment leads to male sterility, an integrated study using cytological, physiological, and transcriptomic methods was conducted.

RESULTS

Some temporary symptoms, including the discoloration of young leaves and a short halt of raceme elongation, were observed in the rapeseed plants exposed to TBM at an application rate of 1 μg per plant. Both chloroplasts in young leaves and plastids in anthers were deformed. TBM also reduced the leaf photosynthetic rate and the contents of chlorophyll, soluble sugar and pyruvate. Both the tapetal cells and uni-nucleate microspores in the treated plants showed large autophagic vacuoles, and the tissue degenerated quickly. A transcriptomic comparison with the control identified 200 upregulated and 163 downregulated differential expression genes in the small flower buds of the TBM treatment. The genes encoding functionally important proteins, including glucan endo-1,3-beta-glucosidase A6, QUARTET3 (QRT3), ARABIDOPSIS ANTHER 7 (ATA7), non-specific lipid-transfer protein LTP11 and LTP12, histone-lysine N-methyltransferase ATXR6, spermidine coumaroyl-CoA acyltransferase (SCT), and photosystem II reaction centre protein psbB, were downregulated by TBM exposure. Some important genes encoding autophagy-related protein ATG8a and metabolic detoxification related proteins, including DTX1, DTX6, DTX35, cytosolic sulfotransferase SOT12, and six members of glutathione S-transferase, were upregulated. In addition, several genes related to hormone stimulus, such as 1-aminocyclopropane-1-carboxylate synthase 8 (ACS8), ethylene-responsive factor ERF1A, ERF1, ERF71, CRF6, and RAP2-3, were also upregulated. The transcriptional regulation is in accordance with the functional abnormalities of pollen wall formation, lipid metabolism, chloroplast structure, ethylene generation, cell cycle, and tissue autophagy.

CONCLUSION

The results suggested that except for ALS, the metabolic pathways related to lipid metabolism, pollen exine formation, photosynthesis and hormone response are associated with male sterility induced by TBM. The results provide new insight into the molecular mechanisms of inducing male sterility by sulfonylurea.

Site-directed mutagenesis of two amino acid residues in cytochrome b559 α subunit that interact with a phosphatidylglycerol molecule (PG772) induces quinone-dependent inhibition of photosystem II activity

X-ray crystallographic analysis (1.9-Å resolution) of the cyanobacterial photosystem II (PSII) dimer showed the presence of five phosphatidylglycerol (PG) molecules per reaction center. One of the PG molecules, PG772, is located in the vicinity of the Q_B-binding site. To investigate the role of PG772 in PSII, we performed site-directed mutagenesis in the cytochrome (Cyt) b₅₅₉ α subunit of Synechocystis sp. PCC 6803 to change two amino acids, Thr-5 and Ser-11, which interact with PG772. The photosynthetic activity of intact cells was slightly lower in all mutants than that of cells in the control strain; however, the oxygen-evolving PSII activity was decreased markedly in cells of mutants, as measured using artificial quinones (such as p-benzoquinone). Furthermore, electron transport from Q_A to Q_B was inhibited in mutants incubated with quinones, particularly under high-intensity light conditions. Lipid analysis of purified PSII showed approximately one PG molecule per reaction center, presumably PG772, was lost in the PSII dimer from the T5A and S11A mutants compared with that in the PSII dimer from the control strain. In addition, protein analysis of monomer and dimer showed decreased levels of PsbV and PsbU extrinsic proteins in the PSII monomer purified from T5A and S11A mutants. These results suggest that site-directed mutagenesis of Thr-5 and Ser-11, which presumably causes the loss of PG772, induces quinone-dependent inhibition of PSII activity under high-intensity light conditions and destabilizes the binding of extrinsic proteins to PSII.

Structural roles of lipid molecules in the assembly of plant PSII-LHCII supercomplex

In plants, photosystem II (PSII) associates with light-harvesting complexes II (LHCII) to form PSII–LHCII supercomplexes. They are multi-subunit supramolecular systems embedded in the thylakoid membrane of chloroplast, functioning as energy-converting and water-splitting machinery powered by light energy. The high-resolution structure of a PSII–LHCII supercomplex, previously solved through cryo-electron microscopy, revealed 34 well-defined lipid molecules per monomer of the homodimeric system. Here we characterize the distribution of lipid-binding sites in plant PSII–LHCII supercomplex and summarize their arrangement pattern within and across the membrane. These lipid molecules have crucial roles in stabilizing the oligomerization interfaces of plant PSII dimer and LHCII trimer. Moreover, they also mediate the interactions among PSII core subunits and contribute to the assembly between peripheral antenna complexes and PSII core. The detailed information of lipid-binding sites within PSII–LHCII supercomplex may serve as a framework for future researches on the functional roles of lipids in plant photosynthesis.

Chloroplast Galactolipids: The Link Between Photosynthesis, Chloroplast Shape, Jasmonates, Phosphate Starvation and Freezing Tolerance

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) together constitute approximately 80% of chloroplast lipids. Apart from facilitating the photosynthesis light reaction in the thylakoid membrane, these two lipids are important for maintaining chloroplast morphology and for plant survival under abiotic stresses such as phosphate starvation and freezing. Recently it was shown that severe growth retardation phenotypes of the DGDG-deficient mutant dgd1 were due to jasmonate overproduction, linking MGDG and DGDG homeostasis with phytohormone production and suggesting MGDG as a major substrate for jasmonate biosynthesis. Induction of jasmonate synthesis and jasmonic acid (JA) signaling was also observed under conditions of phosphate starvation. We hypothesize that when DGDG is recruited to substitute for phospholipids in extraplastidic membranes during phosphate deficiency, the altered MGDG to DGDG ratio in the chloroplast envelope triggers the conversion of galactolipids into jasmonates. The conversion may contribute to rebalancing the MGDG to DGDG ratio rapidly to maintain chloroplast shape, and jasmonate production can reduce the growth rate and enhance predator deterrence. We also hypothesize that other conditions, such as suppression of dgd1 phenotypes by trigalactosyldiacylglycerol (tgd) mutations, may all be linked to altered jasmonate production, indicating that caution should be exercised when interpreting phenotypes caused by conditions that may alter the MGDG to DGDG ratio at the chloroplast envelope.

Membrane glycerolipid equilibrium under endoplasmic reticulum stress in Arabidopsis thaliana

Endoplasmic reticulum (ER) is an indispensable organelle for secretory protein synthesis as well as metabolism of phospholipids and their derivatives in eukaryotic cells. Various external and internal factors may cause an accumulation of aberrant proteins in the ER, which causes ER stress and activates cellular ER stress responses to cope with the stress. In animal research, molecular mechanisms for protein quality control upon ER stress are well documented; however, how cells maintain lipid homeostasis under ER stress is an emerging issue. The ratio of phosphatidylcholine (PC) to phosphatidylethanolamine (PE), two major phospholipid classes, is important under ER stress in animal cells. However, in seed plants, no study has reported on the changes in membrane lipid content under ER stress, although a number of physiologically important environmental stresses, such as heat and salinity, induce ER stress. Here, we investigated membrane glycerolipid metabolism under ER stress in Arabidopsis. ER stress transcriptionally affected PC and PE biosynthesis pathways differentially, with no significant changes in membrane glycerolipid content. Our results suggest that higher plants maintain membrane lipid equilibrium during active transcription of phospholipid biosynthetic genes under ER stress.

Alterations of Membrane Lipid Content Correlated With Chloroplast and Mitochondria Development in Euglena gracilis

Euglenoids are unique protists that can grow photoautotrophically, photomixotrophically, and heterotrophically. Here we grew Euglena gracilis under these different growth conditions and determined cellular contents of seven membrane lipids and one storage lipid (triacylglycerol), which account for more than 94 mol% of total membrane lipids. We also describe the relationship among chloroplast and mitochondria developments with lipid contents, protein contents, and oxygen evolution/consumption rates. In photoautotrophic growth conditions, E. gracilis cells accumulated chlorophyll, photosynthetic proteins, and glycolipids typical to thylakoid membranes. The same occurred for the cells grown under photomixotrophic conditions with higher respiration rates. In heterotrophic conditions, E. gracilis cells had higher respiration rates compared to cells grown in other conditions with the accumulation of pyruvate: NADP+ oxidoreductase, a mitochondrial protein and phospholipid common in mitochondria. Cells were also observed using a confocal laser scanning microscope and found to show more chlorophyll autofluorescence when grown photoautotrophically and photomixotrophycally, and fluorescence of MitoTracker when grown photomixotrophically and heterotrophically. These results suggest that under illumination, E. gracilis develops functional thylakoid membranes with membrane lipids and proteins for photosynthesis. In the medium with glucose, the cells develop mitochondria with phospholipids and proteins for respiration. Possible application based on lipid analysis for the enhancement of wax ester or alkene synthesis is discussed.

Membrane Lipid Oscillation: An Emerging System of Molecular Dynamics in the Plant Membrane

Biological rhythm represents a major biological process of living organisms. However, rhythmic oscillation of membrane lipid content is poorly described in plants. The development of lipidomic technology has led to the illustration of precise molecular profiles of membrane lipids under various growth conditions. Compared with conventional lipid signaling, which produces unpredictable lipid changes in response to ever-changing environmental conditions, lipid oscillation generates a fairly predictable lipid profile, adding a new layer of biological function to the membrane system and possible cross-talk with the other chronobiological processes. This mini review covers recent studies elucidating membrane lipid oscillation in plants.

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.

Specific Distribution of Phosphatidylglycerol to Photosystem Complexes in the Thylakoid Membrane

The thylakoid membrane is the site of photochemical and electron transport reactions of oxygenic photosynthesis. The lipid composition of the thylakoid membrane, with two galactolipids, one sulfolipid, and one phospholipid, is highly conserved among oxygenic photosynthetic organisms. Besides providing a lipid bilayer matrix, thylakoid lipids are integrated in photosynthetic complexes particularly in photosystems I and II and play important roles in electron transport processes. Thylakoid lipids are differentially allocated to photosynthetic complexes and the lipid bilayer fraction, but distribution of each lipid in the thylakoid membrane is unclear. In this study, based on published crystallographic and biochemical data, we estimated the proportions of photosynthetic complex-associated and bilayer-associated lipids in thylakoid membranes of cyanobacteria and plants. The data suggest that ∼30 mol% of phosphatidylglycerol (PG), the only major phospholipid in thylakoid membranes, is allocated to photosystem complexes, whereas glycolipids are mostly distributed to the lipid bilayer fraction and constitute the membrane lipid matrix. Because PG is essential for the structure and function of both photosystems, PG buried in these complexes might have been selectively conserved among oxygenic phototrophs. The specific and substantial allocation of PG to the deep sites of photosystems may need a unique mechanism to incorporate PG into the complexes possibly in coordination with the synthesis of photosynthetic proteins and pigments.

Species-specific roles of sulfolipid metabolism in acclimation of photosynthetic microbes to sulfur-starvation stress

Photosynthetic organisms utilize sulfate for the synthesis of sulfur-compounds including proteins and a sulfolipid, sulfoquinovosyl diacylglycerol. Upon ambient deficiency in sulfate, cells of a green alga, Chlamydomonas reinhardtii, degrade the chloroplast membrane sulfolipid to ensure an intracellular-sulfur source for necessary protein synthesis. Here, the effects of sulfate-starvation on the sulfolipid stability were investigated in another green alga, Chlorella kessleri, and two cyanobacteria, Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942. The results showed that sulfolipid degradation was induced only in C. kessleri, raising the possibility that this degradation ability was obtained not by cyanobacteria, but by eukaryotic algae during the evolution of photosynthetic organisms. Meanwhile, Synechococcus disruptants concerning sqdB and sqdX genes, which are involved in successive reactions in the sulfolipid synthesis pathway, were respectively characterized in cellular response to sulfate-starvation. Phycobilisome degradation intrinsic to Synechococcus, but not to Synechocystis, and cell growth under sulfate-starved conditions were repressed in the sqdB and sqdX disruptants, respectively, relative to in the wild type. Their distinct phenotypes, despite the common loss of the sulfolipid, inferred specific roles of sqdB and sqdX. This study demonstrated that sulfolipid metabolism might have been developed to enable species- or cyanobacterial-strain dependent processes for acclimation to sulfate-starvation.

Sulfoquinovose in the biosphere: occurrence, metabolism and functions

The sulfonated carbohydrate sulfoquinovose (SQ) is produced in quantities estimated at some 10 billion tonnes annually and is thus a major participant in the global sulfur biocycle. SQ is produced by most photosynthetic organisms and incorporated into the sulfolipid sulfoquinovosyl diacylglycerol (SQDG), as well as within some archaea for incorporation into glycoprotein N-glycans. SQDG is found mainly within the thylakoid membranes of the chloroplast, where it appears to be important for membrane structure and function and for optimal activity of photosynthetic protein complexes. SQDG metabolism within the sulfur cycle involves complex biosynthetic and catabolic processes. SQDG biosynthesis is largely conserved within plants, algae and bacteria. On the other hand, two major sulfoglycolytic pathways have been discovered for SQDG degradation, the sulfo-Embden–Meyerhof–Parnas (sulfo-EMP) and sulfo-Entner–Doudoroff (sulfo-ED) pathways, which mirror the major steps in the glycolytic EMP and ED pathways. Sulfoglycolysis produces C3-sulfonates, which undergo biomineralization to inorganic sulfur species, completing the sulfur cycle. This review discusses the discovery and structural elucidation of SQDG and archaeal N-glycans, the occurrence, distribution, and speciation of SQDG, and metabolic pathways leading to the biosynthesis of SQDG and its catabolism through sulfoglycolytic and biomineralization pathways to inorganic sulfur.

Arabidopsis phosphatidylglycerophosphate phosphatase 1 involved in phosphatidylglycerol biosynthesis and photosynthetic function

Phosphatidylglycerol (PG) is an indispensable lipid constituent of photosynthetic membranes, whose function is essential in photosynthetic activity. In higher plants, the biological function of the last step of PG biosynthesis remains elusive because an enzyme catalyzing this reaction step, namely phosphatidylglycerophosphate phosphatase (PGPP), has been a missing piece in the entire glycerolipid metabolic map. Here, we report the identification and characterization of AtPGPP1 encoding a PGPP in Arabidopsis thaliana. Heterologous expression of AtPGPP1 in yeast Δgep4 complemented growth phenotype and PG-producing activity, suggesting that AtPGPP1 encodes a functional PGPP. The GUS reporter assay showed that AtPGPP1 was preferentially expressed in hypocotyl, vasculatures, trichomes, guard cells, and stigmas. A subcellular localization study with GFP reporter indicated that AtPGPP1 is mainly localized at chloroplasts. A T-DNA-tagged knockout mutant of AtPGPP1, designated pgpp1-1, showed pale green phenotype with reduced PG and chlorophyll contents but no defect in embryo development. In the pgpp1-1 mutant, ultrastructure of plastids indicated defective development of chloroplasts and measurement of photosynthetic parameters showed impaired photosynthetic activity. These results suggest that AtPGPP1 is a primary plastidic PGPP required for PG biosynthesis and photosynthetic function in Arabidopsis.