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Thlaspi arvense (Pennycress) is an emerging feedstock for biofuel production because of its high seed oil content enriched in erucic acid. A transcriptomic and a lipidomic study were performed to analyze the dynamics of gene expression, glycerolipid content and acyl-group distribution during seed maturation. Genes involved in fatty acid biosynthesis were expressed at the early stages of seed maturation. Genes encoding enzymes of the Kennedy pathway like diacylglycerol acyltransferase1 ( TaDGAT1) , lysophosphatidic acid acyltransferase ( TaLPAT) or glycerol 3-phosphate acyltransferase ( TaGPAT) increased their expression with maturation, coinciding with the increase in triacylglycerol species containing 22:1. Positional analysis showed that the most abundant triacylglycerol species contained 18:2 at sn-2 position in all maturation stages, suggesting no specificity of the lysophosphatidic acid acyltransferase for very long chain fatty acids. Diacylglycerol acyltransferase2 ( TaDGAT2) mRNA was more abundant at the initial maturation stages, coincident with the rapid incorporation of 22:1 to triacylglycerol, suggesting a coordination between Diacylglycerol acyltransferase enzymes for triacylglycerol biosynthesis. Genes encoding the phospholipid-diacylglycerol acyltransferase ( Ta PDAT1), lysophosphatidylcholine acyltransferase ( Ta LPCAT) or phosphatidylcholine diacylglycerolcholine phosphotransferase ( Ta PDCT), involved in acyl-editing or phosphatidyl-choline (PC)-derived diacylglycerol (DAG) biosynthesis showed also higher expression at the early maturation stages, coinciding with a higher proportion of triacylglycerol containing C18 fatty acids. These results suggested a higher contribution of these two pathways at the early stages of seed maturation. Lipidomic analysis of the content and acyl-group distribution of diacylglycerol and phosphatidyl-choline pools was compatible with the acyl content in triacylglycerol at the different maturation stages. Our data point to a model in which a strong temporal coordination between pathways and isoforms in each pathway, both at the expression and acyl-group incorporation, contribute to high erucic triacylglycerol accumulation in Pennycress., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Claver, Luján, Escuín, Schilling, Jouhet, Savirón, López, Picorel, Jarne, Cebolla and Alfonso.)

Lysophosphatidic acid acyltransferases (LPAATs) catalyze the formation of phosphatidic acid (PA), a central metabolite in both prokaryotic and eukaryotic organisms for glycerolipid biosynthesis. Phaeodactylum tricornutum contains at least two plastid-localized LPAATs (ptATS2a and ptATS2b), but their roles in lipid synthesis remain unknown. Both ptATS2a and ptATS2b could complement the high temperature sensitivity of the bacterial plsC mutant deficient in LPAAT. In vitro enzyme assays showed that they prefer lysophosphatidic acid over other lysophospholipids. ptATS2a is localized in the plastid inner envelope membrane and CRISPR/Cas9-generated ptATS2a mutants showed compromised cell growth, significantly changed plastid and extra-plastidial membrane lipids at nitrogen-replete condition and reduced triacylglycerols (TAGs) under nitrogen-depleted condition. ptATS2b is localized in thylakoid membranes and its knockout led to reduced growth rate and TAG content but slightly altered molecular composition of membrane lipids. The changes in glycerolipid profiles are consistent with the role of both LPAATs in the sn-2 acylation of sn-1-acyl-glycerol-3-phosphate substrates harboring 20:5 at the sn-1 position. Our findings suggest that both LPAATs are important for membrane lipids and TAG biosynthesis in P. tricornutum and further highlight that 20:5-Lyso-PA is likely involved in the massive import of 20:5 back to the plastid to feed plastid glycerolipid syntheses., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

Background: Many organisms rely on mineral nutrients taken directly from the soil or aquatic environment, and therefore, developed mechanisms to cope with the limitation of a given essential nutrient. For example, photosynthetic cells have well-defined responses to phosphate limitation, including the replacement of cellular membrane phospholipids with non-phosphorous lipids. Under phosphate starvation, phospholipids in extraplastidial membranes are replaced by betaine lipids in microalgae. In higher plants, the synthesis of betaine lipid is lost, driving plants to other strategies to cope with phosphate starvation where they replace their phospholipids by glycolipids., Results: The aim of this work was to evaluate to what extent betaine lipids and PC lipids share physicochemical properties and could substitute for each other. By neutron diffraction experiments and dynamic molecular simulation of two synthetic lipids, the dipalmitoylphosphatidylcholine (DPPC) and the dipalmitoyl-diacylglyceryl-N,N,N-trimethylhomoserine (DP-DGTS), we found that DP-DGTS bilayers are thicker than DPPC bilayers and therefore are more rigid. Furthermore, DP-DGTS bilayers are more repulsive, especially at long range, maybe due to unexpected unscreened electrostatic contribution. Finally, DP-DGTS bilayers could coexist in the gel and fluid phases., Conclusion: The different properties and hydration responses of PC and DGTS provide an explanation for the diversity of betaine lipids observed in marine organisms and for their disappearance in seed plants., (© 2023. The Author(s).)

Sanguina nivaloides is the main alga forming red snowfields in high mountains and Polar Regions. It is non-cultivable. Analysis of environmental samples by X-ray tomography, focused-ion-beam scanning-electron-microscopy, physicochemical and physiological characterization reveal adaptive traits accounting for algal capacity to reside in snow. Cysts populate liquid water at the periphery of ice, are photosynthetically active, can survive for months, and are sensitive to freezing. They harbor a wrinkled plasma membrane expanding the interface with environment. Ionomic analysis supports a cell efflux of K + , and assimilation of phosphorus. Glycerolipidomic analysis confirms a phosphate limitation. The chloroplast contains thylakoids oriented in all directions, fixes carbon in a central pyrenoid and produces starch in peripheral protuberances. Analysis of cells kept in the dark shows that starch is a short-term carbon storage. The biogenesis of cytosolic droplets shows that they are loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress., (© 2023. The Author(s).)

Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Juliette Jouhet reports financial support was provided by French National Research Agency. Stephanie Bolik reports financial support was provided by Institut Laue-Langevin. Stephanie Bolik reports financial support was provided by French National Research Agency.

Enteric bacteria have to adapt to environmental stresses in the human gastrointestinal tract such as acid and nutrient stress, oxygen limitation and exposure to antibiotics. Membrane lipid composition has recently emerged as a key factor for stress adaptation. The E. coli ravA-viaA operon is essential for aminoglycoside bactericidal activity under anaerobiosis but its mechanism of action is unclear. Here we characterise the VWA domain-protein ViaA and its interaction with the AAA+ ATPase RavA, and find that both proteins localise at the inner cell membrane. We demonstrate that RavA and ViaA target specific phospholipids and subsequently identify their lipid-binding sites. We further show that mutations abolishing interaction with lipids restore induced changes in cell membrane morphology and lipid composition. Finally we reveal that these mutations render E. coli gentamicin-resistant under fumarate respiration conditions. Our work thus uncovers a ravA-viaA-based pathway which is mobilised in response to aminoglycosides under anaerobiosis and engaged in cell membrane regulation., (© 2022. The Author(s).)

Long-chain acyl-CoA synthetases (LACS) play diverse and fundamentally important roles in lipid metabolism. While their functions have been well established in bacteria, yeast and plants, the mechanisms by which LACS isozymes regulate lipid metabolism in unicellular oil-producing microalgae, including the diatom Phaeodactylum tricornutum, remain largely unknown. In P. tricornutum, a family of five genes (ptACSL1-ptACSL5) encodes LACS activities. We generated single lacs knockout/knockdown mutants using multiplexed CRISPR/Cas9 method, and determined their substrate specificities towards different fatty acids (FAs) and subcellular localisations. ptACSL3 is localised in the mitochondria and its disruption led to compromised growth and reduced triacylglycerol (TAG) content when cells were bubbled with air. The ptACSL3 mutants showed altered FA profiles in two galactoglycerolipids and phosphatidylcholine (PC) with significantly reduced distribution of 16:0 and 16:1. ptACSL5 is localised in the peroxisome and its knockdown resulted in reduced growth rate and altered molecular species of PC and TAG, indicating a role in controlling the composition of acyl-CoAs for lipid synthesis. Our work demonstrates the potential of generating gene knockout mutants with the mutation of large fragment deletion using multiplexed CRISPR/Cas9 and provides insight into the functions of LACS isozymes in lipid metabolism in the oleaginous microalgae., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Thraustochytrids are marine protists that naturally accumulate triacylglycerol with long chains of polyunsaturated fatty acids, such as ω3-docosahexaenoic acid (DHA). They represent a sustainable response to the increasing demand for these "essential" fatty acids (FAs). Following an attempt to transform a strain of Aurantiochytrium limacinum , we serendipitously isolated a clone that did not incorporate any recombinant DNA but contained two to three times more DHA than the original strain. Metabolic analyses indicated a deficit in FA catabolism. However, whole transcriptome analysis did not show down-regulation of genes involved in FA catabolism. Genome sequencing revealed extensive DNA deletion in one allele encoding a putative peroxisomal adenylate transporter. Phylogenetic analyses and yeast complementation experiments confirmed the gene as a peroxisomal adenylate nucleotide transporter ( AlANT1 ), homologous to yeast ScANT1 and plant peroxisomal adenylate nucleotide carrier AtPNC genes. In yeast and plants, a deletion of the peroxisomal adenylate transporter inhibits FA breakdown and induces FA accumulation, a phenotype similar to that described here. In response to this metabolic event, several compensatory mechanisms were observed. In particular, genes involved in FA biosynthesis were upregulated, also contributing to the high FA accumulation. These results support AlANT1 as a promising target for enhancing DHA production in Thraustochytrids.

The unicellular marine diatom Phaeodactylum tricornutum accumulates up to 35% eicosapentaenoic acid (EPA, 20:5n3) and has been used as a model organism to study long chain polyunsaturated fatty acids (LC-PUFA) biosynthesis due to an excellent annotated genome sequence and established transformation system. In P. tricornutum, the majority of EPA accumulates in polar lipids, particularly in galactolipids such as mono- and di-galactosyldiacylglycerol. LC-PUFA biosynthesis is considered to start from oleic acid (18:1n9). EPA can be synthesized via a series of desaturation and elongation steps occurring at the endoplasmic reticulum and newly synthesized EPA is then imported into the plastids for incorporation into galactolipids via an unknown route. The basis for the flux of EPA is fundamental to understanding LC-PUFA biosynthesis in diatoms. We used P. tricornutum to study acyl modifying activities, upstream of 18:1n9, on subsequent LC-PUFA biosynthesis. We identified the gene coding for the plastidial acyl carrier protein Δ9-desaturase, a key enzyme in fatty acid modification and analyzed the impact of overexpression and knock out of this gene on glycerolipid metabolism. This revealed a previously unknown role of this soluble desaturase in EPA synthesis and production of triacylglycerol. This study provides further insight into the distinctive nature of lipid metabolism in the marine diatom P. tricornutum and suggests additional approaches for tailoring oil composition in microalgae., (© 2021 The Authors. The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd.)

Algae belonging to the Microchloropsis genus are promising organisms for biotech purposes, being able to accumulate large amounts of lipid reserves. These organisms adapt to different trophic conditions, thriving in strict photoautotrophic conditions, as well as in the concomitant presence of light plus reduced external carbon as energy sources (mixotrophy). In this work, we investigated the mixotrophic responses of Microchloropsis gaditana (formerly Nannochloropsis gaditana ). Using the Biolog growth test, in which cells are loaded into multiwell plates coated with different organic compounds, we could not find a suitable substrate for Microchloropsis mixotrophy. By contrast, addition of the Lysogeny broth (LB) to the inorganic growth medium had a benefit on growth, enhancing respiratory activity at the expense of photosynthetic performances. To further dissect the role of respiration in Microchloropsis mixotrophy, we focused on the mitochondrial alternative oxidase (AOX), a protein involved in energy management in other algae prospering in mixotrophy. Knocking-out the AOX1 gene by transcription activator-like effector nuclease (TALE-N) led to the loss of capacity to implement growth upon addition of LB supporting the hypothesis that the effect of this medium was related to a provision of reduced carbon. We conclude that mixotrophic growth in Microchloropsis is dominated by respiratory rather than by photosynthetic energetic metabolism and discuss the possible reasons for this behavior in relationship with fatty acid breakdown via β-oxidation in this oleaginous alga., Competing Interests: EB, FL, LF, and SC are employed by the company Total Refining Chemicals. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Bo, Magneschi, Bedhomme, Billey, Deragon, Storti, Menneteau, Richard, Rak, Lapeyre, Lembrouk, Conte, Gros, Tourcier, Giustini, Falconet, Curien, Allorent, Petroutsos, Laeuffer, Fourage, Jouhet, Maréchal, Finazzi and Collin.)

The metabolic pathways of glycerolipids are well described in cells containing chloroplasts limited by a two-membrane envelope but not in cells containing plastids limited by four membranes, including heterokonts. Fatty acids (FAs) produced in the plastid, palmitic and palmitoleic acids (16:0 and 16:1), are used in the cytosol for the synthesis of glycerolipids via various routes, requiring multiple acyl-Coenzyme A (CoA) synthetases (ACS). Here, we characterized an ACS of the Bubblegum subfamily in the photosynthetic eukaryote Microchloropsis gaditana, an oleaginous heterokont used for the production of lipids for multiple applications. Genome engineering with TALE-N allowed the generation of MgACSBG point mutations, but no knockout was obtained. Point mutations triggered an overall decrease of 16:1 in lipids, a specific increase of unsaturated 18-carbon acyls in phosphatidylcholine and decrease of 20-carbon acyls in the betaine lipid diacylglyceryl-trimethyl-homoserine. The profile of acyl-CoAs highlighted a decrease in 16:1-CoA and 18:3-CoA. Structural modeling supported that mutations affect accessibility of FA to the MgACSBG reaction site. Expression in yeast defective in acyl-CoA biosynthesis further confirmed that point mutations affect ACSBG activity. Altogether, this study supports a critical role of heterokont MgACSBG in the production of 16:1-CoA and 18:3-CoA. In M. gaditana mutants, the excess saturated and monounsaturated FAs were diverted to triacylglycerol, thus suggesting strategies to improve the oil content in this microalga., (© American Society of Plant Biologists 2021. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Cholesterol is a major component of mammalian plasma membranes that not only affects the physical properties of the lipid bilayer but also is the function of many membrane proteins including G protein-coupled receptors. The oxytocin receptor (OXTR) is involved in parturition and lactation of mammals and in their emotional and social behaviors. Cholesterol acts on OXTR as an allosteric modulator inducing a high-affinity state for orthosteric ligands through a molecular mechanism that has yet to be determined. Using the ion channel-coupled receptor technology, we developed a functional assay of cholesterol modulation of G protein-coupled receptors that is independent of intracellular signaling pathways and operational in living cells. Using this assay, we discovered a stable binding of cholesterol molecules to the receptor when it adopts an orthosteric ligand-bound state. This stable interaction preserves the cholesterol-dependent activity of the receptor in cholesterol-depleted membranes. This mechanism was confirmed using time-resolved FRET experiments on WT OXTR expressed in CHO cells. Consequently, a positive cross-regulation sequentially occurs in OXTR between cholesterol and orthosteric ligands., Competing Interests: Conflict of interest The authors declare that they have no conflicts of interest with the contents of this article., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Eukaryotic Δ6-desaturases are microsomal enzymes that balance the synthesis of ω-3 and ω-6 C18-polyunsaturated fatty acids (C18-PUFAs) according to their specificity. In several microalgae, including Ostreococcus tauri , plastidic C18-PUFAs are strictly regulated by environmental cues suggesting an autonomous control of Δ6-desaturation of plastidic PUFAs. Here, we identified two putative front-end Δ6/Δ8-desaturases from O tauri that, together with putative homologs, cluster apart from other characterized Δ6-desaturases. Both were plastid-located and unambiguously displayed a Δ6-desaturation activity when overexpressed in the heterologous hosts Nicotiana benthamiana and Synechocystis sp. PCC6803, as in the native host. Detailed lipid analyses of overexpressing lines unveiled distinctive ω-class specificities, and most interestingly pointed to the importance of the lipid head-group and the nonsubstrate acyl-chain for the desaturase efficiency. One desaturase displayed a broad specificity for plastidic lipids and a preference for ω-3 substrates, while the other was more selective for ω-6 substrates and for lipid classes including phosphatidylglycerol as well as the peculiar 16:4-galactolipid species occurring in the native host. Overexpression of both Δ6-desaturases in O tauri prevented the regulation of C18-PUFA under phosphate deprivation and triggered glycerolipid fatty-acid remodeling, without causing any obvious alteration in growth or photosynthesis. Tracking fatty-acid modifications in eukaryotic hosts further suggested the export of plastidic lipids to extraplastidic compartments., (© 2020 American Society of Plant Biologists. All Rights Reserved.)

We have determined whether orange juice-derived nanovesicles (ONVs) could be used for the treatment of obesity-associated intestinal complications. ONVs were characterized by lipidomic, metabolomic, electron microscopy. In vitro , intestinal barriers (IBs = Caco-2+HT-29-MTX) were treated with ONVs and co-cultured with adipocytes to monitor IB fat release. In vivo , obesity was induced with a high-fat, high-sucrose diet (HFHSD mice) for 12 weeks. Then, half of HFHSD mice were gavaged with ONVs. One-month ONV treatment did not modify HFHSD-induced insulin resistance but reversed diet-induced gut modifications. In the jejunum, ONVs increased villi size, reduced triglyceride content, and modulated mRNA levels of genes involved in immune response (tumor necrosis factor [TNF]-α and interleukin [IL]-1β), barrier permeability (CLDN1, OCLN, ZO1), fat absorption, and chylomicron release. ONVs targeted microsomal triglyceride transfer protein (MTP) and angiopoietin-like protein-4 (ANGPTL4), two therapeutic targets to reduce plasma lipids and inflammation in gastrointestinal diseases. Interestingly, ONV treatment did not aggravate liver steatosis, as MTP mRNA was increased in the liver. Therefore, ONVs protected both intestine and the liver from fat overload associated with the HFHSD. As ONVs concentrated amino acids and bioactive lipids versus orange juice, which are deficient in obese patients, the use of ONVs as a dietary supplement could bring physiological relevant compounds in the jejunum to accelerate the restoration of intestinal functions during weight loss in obese patients., (© 2020 The Authors.)

Biogenesis of photosynthetic membranes depends on galactolipid synthesis, which relies on several cell compartments, notably the endoplasmic reticulum (ER) and the chloroplast envelope. Galactolipid synthesis involves lipid trafficking between both membrane compartments. In Arabidopsis , ALA10, a phospholipid flippase of the P 4 type-ATPase family, counteracts the limitation of monogalactosyldiacylglycerol (MGDG) production and has a positive effect on leaf development. ALA10 locates in distinct domains of the ER depending on the ALIS (ALA interacting subunit) subunit it interacts with: close to the plasma membrane with ALIS1, or next to chloroplasts with ALIS5. It interacts with FAD2 (Fatty acid desaturase 2) and prevents accumulation of linolenic (18:3) containing phosphatidylcholine (PC) stimulating an increase of MGDG synthesis. Here we report that ALA10 interacts with PUB11 (plant U-box type 11), an E3 protein ubiquitin ligase, in vitro and in vivo . ALA10 is however ubiquitinated and degraded by the 26S proteasome in a PUB11-independent process. In pub11 null mutant, the proteasome-dependent degradation of ALA10 is retained and ALA10 is still subject to ubiquitination although its ubiquitination profile appears different. In the absence of PUB11, ALA10 is constrained to the ER close to chloroplasts, which is the usual location when ALA10 is overexpressed. Additionally, in this condition, the decrease of 18:3 containing PC is no longer observed. Taken together these results suggest, that ALA10 contributes in chloroplast-distal ER interacting domains, to reduce the 18:3 desaturation of PC and that PUB11 is involved in reconditioning of ALA10 from chloroplast-proximal to chloroplast-distal ER interacting domains., (Copyright © 2020 Salvaing, Botella, Albrieux, Gros, Block and Jouhet.)

Phosphorus (P) is one of the limiting macronutrients for algal growth in marine environments. Microalgae have developed adaptation mechanisms to P limitation that involve remodelling of internal phosphate resources and accumulation of lipids. Here, we used in silico analyses to identify the P-stress regulator PtPSR (Phaeodactylum tricornutum phosphorus starvation response) in the diatom P. tricornutum. ptpsr mutant lines were generated using gene editing and characterised by various molecular, genetics and biochemical tools. PtPSR belongs to a clade of Myb transcription factors that are conserved in stramenopiles and distantly related to plant P-stress regulators. PtPSR bound specifically to a conserved cis-regulatory element found in the regulatory region of P-stress-induced genes. ptpsr knockout mutants showed reduction in cell growth under P limitation. P-stress responses were impaired in ptpsr mutants compared with wild-type, including reduced induction of P-stress response genes, near to complete loss of alkaline phosphatase activity and reduced phospholipid degradation. We conclude that PtPSR is a key transcription factor influencing P scavenging, phospholipid remodelling and cell growth in adaptation to P stress in diatoms., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

The wide latitudinal distribution of marine Synechococcus cyanobacteria partly relies on the differentiation of lineages adapted to distinct thermal environments. Membranes are highly thermosensitive cell components, and the ability to modulate their fluidity can be critical for the fitness of an ecotype in a particular thermal niche. We compared the thermophysiology of Synechococcus strains representative of major temperature ecotypes in the field. We measured growth, photosynthetic capacities and membrane lipidome variations. We carried out a metagenomic analysis of stations of the Tara Oceans expedition to describe the latitudinal distribution of the lipid desaturase genes in the oceans. All strains maintained efficient photosynthetic capacities over their different temperature growth ranges. Subpolar and cold temperate strains showed enhanced capacities for lipid monodesaturation at low temperature thanks to an additional, poorly regiospecific Δ9-desaturase. By contrast, tropical and warm temperate strains displayed moderate monodesaturation capacities but high proportions of double unsaturations in response to cold, thanks to regiospecific Δ12-desaturases. The desaturase genes displayed specific distributions directly related to latitudinal variations in ocean surface temperature. This study highlights the critical importance of membrane fluidity modulation by desaturases in the adaptive strategies of Synechococcus cyanobacteria during the colonization of novel thermal niches., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

Diatoms are unicellular heterokonts, living in oceans and freshwaters, exposed to frequent environmental variations. They have a sophisticated membrane compartmentalization and are bounded by a siliceous cell-wall. Formation of lipid droplets (LDs), filled with triacylglycerol (TAG), is a common response to stress. The proteome of mature-LDs from Phaeodactylum tricornutum highlighted the lack of proteins involved in early-LD formation, TAG biosynthesis or LD-to-LD connections. These features suggest that cytosolic LDs might reach a size limit. We analyzed the dynamics of LD formation in P. tricornutum (Pt1 8.6; CCAP 1055/1) during 7 days of nitrogen starvation, by monitoring TAG by mass spectrometry-based lipidomics, and LD radius using epifluorescence microscopy and pulse field gradient nuclear magnetic resonance. We confirmed that mature LDs reach a maximal size. Based on pulse field gradient nuclear magnetic resonance, we did not detect any LD-LD fusion. Three LD subpopulations were produced, each with a different maximal size, larger-sized LDs (radius 0.675 ± 0.125 µm) being generated first. Mathematical modeling showed how smaller LDs are produced once larger LDs have reached their maximum radius. In a mutant line having larger cells, the maximal size of the first LD subpopulation was higher (0.941 ± 0.169 µm), while the principle of stepwise formation of distinct LD populations was maintained. Results suggest that LD size is determined by available cytosolic space and sensing of an optimal size reached in the previous LD subpopulation. Future perspectives include the unraveling of LD-size control mechanisms upon nitrogen shortage. This study also provides novel prospects for the optimization of oleaginous microalgae for biotechnological applications., (Copyright © 2020 Jaussaud, Lupette, Salvaing, Jouhet, Bastien, Gromova and Maréchal.)

Jasmonic acid (JA) biosynthesis and signaling are activated in Arabidopsis cultivated in phosphate (Pi) deprived conditions. This activation occurs mainly in photosynthetic tissues and is less important in roots. In leaves, the enhanced biosynthesis of JA coincides with membrane glycerolipid remodeling triggered by the lack of Pi. We addressed the possible role of JA on the dynamics and magnitude of glycerolipid remodeling in response to Pi deprivation and resupply. Based on combined analyses of gene expression, JA biosynthesis and glycerolipid remodeling in wild-type Arabidopsis and in the coi1-16 mutant, JA signaling seems important in the determination of the basal levels of phosphatidylcholine, phosphatidic acid (PA), monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol. JA impact on MGDG steady state level and fluctuations seem contradictory. In the coi1-16 mutant, the steady state level of MGDG is higher, possibly due to a higher level of PA in the mutant, activating MGD1, and to an increased expression of MGD3. These results support a possible impact of JA in limiting the overall content of this lipid. Concerning lipid variations, upon Pi deprivation, JA seems rather associated with a specific MGDG increase. Following Pi resupply, whereas the expression of glycerolipid remodeling genes returns to basal level, JA biosynthesis and signaling genes are still upregulated, likely due to a JA-induced positive feedback remaining active. Distinct impacts on enzymes synthesizing MGDG, that is, downregulating MGD3, possibly activating MGD1 expression and limiting the activation of MGD1 via PA, might allow JA playing a role in a sophisticated fine tuning of galactolipid variations., (� The Author(s) 2019. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Photosymbiosis between single-celled hosts and microalgae is common in oceanic plankton, especially in oligotrophic surface waters. However, the functioning of this ecologically important cell-cell interaction and the subcellular mechanisms allowing the host to accommodate and benefit from its microalgae remain enigmatic. Here, using a combination of quantitative single-cell structural and chemical imaging techniques (FIB-SEM, nanoSIMS, Synchrotron X-ray fluorescence), we show that the structural organization, physiology, and trophic status of the algal symbionts (the haptophyte Phaeocystis) significantly change within their acantharian hosts compared to their free-living phase in culture. In symbiosis, algal cell division is blocked, photosynthesis is enhanced, and cell volume is increased by up to 10-fold with a higher number of plastids (from 2 to up to 30) and thylakoid membranes. The multiplication of plastids can lead to a 38-fold increase of the total plastid volume in a cell. Subcellular mapping of nutrients (nitrogen and phosphorous) and their stoichiometric ratios shows that symbiotic algae are impoverished in phosphorous and suggests a higher investment in energy-acquisition machinery rather than in growth. Nanoscale imaging also showed that the host supplies a substantial amount of trace metals (e.g., iron and cobalt), which are stored in algal vacuoles at high concentrations (up to 660 ppm). Sulfur mapping reveals a high concentration in algal vacuoles that may be a source of antioxidant molecules. Overall, this study unveils an unprecedented morphological and metabolic transformation of microalgae following their integration into a host, and it suggests that this widespread symbiosis is a farming strategy wherein the host engulfs and exploits microalgae., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The coexistence of different lipid phases is well-known in vitro , but evidence for their presence and function in cellular membranes remains scarce. Using a combination of fluorescent lipid probes, we observe segregation of domains that suggests the coexistence of liquid and gel phases in the membrane of Streptococcus pneumoniae , where they are localized to minimize bending stress in the ellipsoid geometry defined by the cell wall. Gel phase lipids with high bending rigidity would be spontaneously organized at the equator where curvature is minimal, thus marking the future division site, while liquid phase membrane maps onto the oblong hemispheres. In addition, the membrane-bound cell wall precursor with its particular dynamic acyl chain localizes at the division site where the membrane is highly curved. We propose a complete "chicken-and-egg" model where cell geometry determines the localization of lipid phases that positions the cell division machinery, which in turn alters the localization of lamellar phases by assembling the cell wall with a specific geometry.

The biogenesis of cellular membranes involves an important traffic of lipids from their site of synthesis to their final destination. Lipid transfer can be mediated by vesicular or non-vesicular pathways. The non-vesicular pathway requires the close apposition of two membranes to form a functional platform, called membrane contact sites (MCSs), where lipids are exchanged. These last decades, MCSs have been observed between virtually all organelles and a role in lipid transfer has been demonstrated for some of them. In plants, the lipid composition of membranes is highly dynamic and can be drastically modified in response to environmental changes. This highlights the importance of understanding the mechanisms involved in the regulation of membrane lipid homeostasis in plants. This review summarizes our current knowledge about the non-vesicular transport of lipids at MCSs in plants and its regulation during stress.

Lipids are key molecules in various biological processes, thus their quantification is a crucial point in a lot of studies and should be taken into account in lipidomics development. This family is complex and presents a very large diversity of structures, so analyzing and quantifying all this diversity is a real challenge. In this review, the different techniques to analyze lipids will be presented: from nuclear magnetic resonance (NMR) to mass spectrometry (with and without chromatography) including universal detectors. First of all, the state of the art of quantification, with the definitions of terms and protocol standardization, will be presented with quantitative lipidomics in mind, and then technical considerations and limitations of analytical chemistry's tools, such as NMR, mass spectrometry and universal detectors, will be discussed, particularly in terms of absolute quantification.

Background: In photosynthetic oleaginous microalgae, acyl-CoA molecules are used as substrates for the biosynthesis of membrane glycerolipids, triacylglycerol (TAG) and other acylated molecules. Acyl-CoA can also be directed to beta-oxidative catabolism. They can be utilized by a number of lipid metabolic enzymes including endogenous thioesterases, which catalyze their hydrolysis to release free fatty acids. Acyl-CoA availability thus plays fundamental roles in determining the quantity and composition of membrane lipids and storage lipids., Results: Here, we have engineered the model diatom Phaeodactylum tricornutum to produce significantly increased TAGs by disruption of the gene encoding a Hotdog-fold thioesterase involved in acyl-CoA hydrolysis (ptTES1). This plastidial thioesterase can hydrolyze both medium- and long-chain fatty acyl-CoAs, but has the highest activity toward long-chain saturated and monounsaturated fatty acyl-CoAs. The maximum rate was found with oleoyl-CoA, which is hydrolyzed at 50 nmol/min/mg protein. The stable and targeted interruption of acyl-CoA thioesterase gene was achieved using a genome editing technique, transcription activator-like effector nucleases (TALENs). Disruption of native ptTES1 gene resulted in a 1.7-fold increase in TAG content when algal strains were grown in nitrogen-replete media for 8 days, whereas the content of other lipid classes, including phosphoglycerolipids and galactoglycerolipids, remained almost unchanged. The engineered algal strain also exhibited a marked change in fatty acid profile, including a remarkable increase in 16:0 and 16:1 and a decrease in 20:5. Nitrogen deprivation for 72 h further increased TAG content and titer of the engineered strain, reaching 478 μg/10 9  cells and 4.8 mg/L, respectively. Quantitative determination of in vivo acyl-CoAs showed that the total acyl-CoA pool size was significantly higher in the engineered algal strain than that in the wild type., Conclusions: This study supports the role of ptTES1 in free fatty acid homeostasis in the plastid of Phaeodactylum and demonstrates the potential of TALEN-based genome editing technique to generate an enhanced lipid-producing algal strain through blocking acyl-CoA catabolism.

The ecological success of diatoms requires a remarkable ability to survive many types of stress, including variations in temperature, light, salinity, and nutrient availability. On exposure to these stresses, diatoms exhibit common responses, including growth arrest, impairment of photosynthesis, production of reactive oxygen species, and accumulation of triacylglycerol (TAG). We studied the production of cyclopentane oxylipins derived from fatty acids in the diatom Phaeodactylum tricornutum in response to oxidative stress. P. tricornutum lacks the enzymatic pathway for producing cyclopentane-oxylipins, such as jasmonate, prostaglandins, or thromboxanes. In cells subjected to increasing doses of hydrogen peroxide (H 2 O 2 ), we detected nonenzymatic production of isoprostanoids, including six phytoprostanes, three F 2t -isoprostanes, two F 3t -isoprostanes, and three F 4t -neuroprostanes, by radical peroxidation of α-linolenic, arachidonic, eicosapentaenoic, and docosahexanoic acids, respectively. H 2 O 2 also triggered photosynthesis impairment and TAG accumulation. F 1t -phytoprostanes constitute the major class detected (300 pmol per 1 million cells; intracellular concentration, ∼4 µm). Only two glycerolipids, phosphatidylcholine and diacylglycerylhydroxymethyl-trimethyl-alanine, could provide all substrates for these isoprostanoids. Treatment of P. tricornutum with nine synthetic isoprostanoids produced an effect in the micromolar range, marked by the accumulation of TAG and reduced growth, without affecting photosynthesis. Therefore, the emission of H 2 O 2 and free radicals upon exposure to stresses can lead to glycerolipid peroxidation and nonenzymatic synthesis of isoprostanoids, inhibiting growth and contributing to the induction of TAG accumulation via unknown processes. This characterization of nonenzymatic oxylipins in P. tricornutum opens a field of research on the study of processes controlled by isoprostanoid signaling in various physiological and environmental contexts in diatoms., (© 2018 American Society of Plant Biologists. All rights reserved.)

[This corrects the article DOI: 10.1371/journal.pone.0182423.].

Microalgae are a promising feedstock for the production of triacylglycerol (TAG) for a variety of potential applications, ranging from food and human health to biofuels and green chemistry. However, obtaining high TAG yields is challenging. A phenotypic assay for the accumulation of oil droplets was developed to screen a library of 1,200 drugs, annotated with pharmacology information, to select compounds that trigger TAG accumulation in the diatom Phaeodactylum tricornutum Using this screen, we identified 34 molecules acting in a dose-dependent manner. Previously characterized targets of these compounds include cell division and cell signaling effectors, membrane receptors and transporters, and sterol metabolism. Among the five compounds possibly acting on sterol metabolism, we focused our study on ethynylestradiol, a synthetic form of estrogen that is used in contraceptive pills and known for its ecological impact as an endocrine disruptor. Ethynylestradiol impaired the production of very-long-chain polyunsaturated fatty acids, destabilized the galactolipid versus phospholipid balance, and triggered the recycling of fatty acids from membrane lipids to TAG. The P. tricornutum transcriptomic response to treatment with ethynylestradiol was consistent with the reallocation of carbon from sterols to acetyl-coenzyme A and TAG. The mode of action and catabolism of ethynylestradiol are unknown but might involve several up-regulated cytochrome P450 proteins. A fatty acid elongase, Δ6-ELO-B1, might be involved in the impairment of very-long-chain polyunsaturated fatty acids and fatty acid turnover. This phenotypic screen opens new perspectives for the exploration of novel bioactive molecules, potential target genes, and pathways controlling TAG biosynthesis. It also unraveled the sensitivity of diatoms to endocrine disruptors, highlighting an impact of anthropogenic pollution on phytoplankton., (© 2018 American Society of Plant Biologists. All Rights Reserved.)

A unique feature of chloroplasts is their high content of the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), which constitute up to 80% of their lipids. These galactolipids are synthesized in the chloroplast envelope membrane through the concerted action of galactosyltransferases, the so-called 'MGDG synthases (MGDs)' and 'DGDG synthases (DGDs),' which use uridine diphosphate (UDP)-galactose as donor. In Arabidopsis leaves, under standard conditions, the enzymes MGD1 and DGD1 provide the bulk of galactolipids, necessary for the massive expansion of thylakoid membranes. Under phosphate limited conditions, plants activate another pathway involving MGD2/MGD3 and DGD2 to provide additional DGDG that is exported to extraplastidial membranes where they partly replace phospholipids, a phosphate-saving mechanism in plants. A third enzyme system, which relies on the UDP-Gal-independent GGGT (also called SFR2 for SENSITIVE TO FREEZING 2), can be activated in response to a freezing stress. The biosynthesis of galactolipids by these multiple enzyme sets must be tightly regulated to meet the cellular demand in response to changing environmental conditions. The cooperation between MGD and DGD enzymes with a possible substrate channeling from diacylglycerol to MGDG and DGDG is supported by biochemical and biophysical studies and mutant analyses reviewed herein. The fine-tuning of MGDG to DGDG ratio, which allows the reversible transition from the hexagonal II to lamellar α phase of the lipid bilayer, could be a key factor in thylakoid biogenesis.

Molecular mechanisms of phosphorus (P) limitation are of great interest for understanding algal production in aquatic ecosystems. Previous studies point to P limitation-induced changes in lipid composition. As, in microalgae, the molecular mechanisms of this specific P stress adaptation remain unresolved, we reveal a detailed phospholipid-recycling scheme in Nannochloropsis oceanica and describe important P acquisition genes based on highly corresponding transcriptome and lipidome data. Initial responses to P limitation showed increased expression of genes involved in P uptake and an expansion of the P substrate spectrum based on purple acid phosphatases. Increase in P trafficking displayed a rearrangement between compartments by supplying P to the chloroplast and carbon to the cytosol for lipid synthesis. We propose a novel phospholipid-recycling scheme for algae that leads to the rapid reduction of phospholipids and synthesis of the P-free lipid classes. P mobilization through membrane lipid degradation is mediated mainly by two glycerophosphoryldiester phosphodiesterases and three patatin-like phospholipases A on the transcriptome level. To compensate for low phospholipids in exponential growth, N. oceanica synthesized sulfoquinovosyldiacylglycerol and diacylglyceroltrimethylhomoserine. In this study, it was shown that an N. oceanica strain has a unique repertoire of genes that facilitate P acquisition and the degradation of phospholipids compared with other stramenopiles. The novel phospholipid-recycling scheme opens new avenues for metabolic engineering of lipid composition in algae., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

Nitric oxide (NO) is an intermediate of the nitrogen cycle, an industrial pollutant, and a marker of climate change. NO also acts as a gaseous transmitter in a variety of biological processes. The impact of environmental NO needs to be addressed. In diatoms, a dominant phylum in phytoplankton, NO was reported to mediate programmed cell death in response to diatom-derived polyunsaturated aldehydes. Here, using the Phaeodactylum Pt1 strain, 2E,4E-decadienal supplied in the micromolar concentration range led to a nonspecific cell toxicity. We reexamined NO biosynthesis and response in Phaeodactylum NO inhibits cell growth and triggers triacylglycerol (TAG) accumulation. Feeding experiments indicate that NO is not produced from Arg but via conversion of nitrite by the nitrate reductase. Genome-wide transcriptional analysis shows that NO up-regulates the expression of the plastid nitrite reductase and genes involved in the subsequent incorporation of ammonium into amino acids, via both Gln synthesis and Orn-urea pathway. The phospho enol pyruvate dehydrogenase complex is also up-regulated, leading to the production of acetyl-CoA, which can feed TAG accumulation upon exposure to NO. Transcriptional reprogramming leading to higher TAG content is balanced with a decrease of monogalactosyldiacylglycerol (MGDG) in the plastid via posttranslational inhibition of MGDG synthase enzymatic activity by NO. Intracellular and transient NO emission acts therefore at the basis of a nitrite-sensing and acclimating system, whereas a long exposure to NO can additionally induce a redirection of carbon to neutral lipids and a stress response., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

Diatoms are prominent marine microalgae, interesting not only from an ecological point of view, but also for their possible use in biotechnology applications. They can be cultivated in phototrophic conditions, using sunlight as the sole energy source. Some diatoms, however, can also grow in a mixotrophic mode, wherein both light and external reduced carbon contribute to biomass accumulation. In this study, we investigated the consequences of mixotrophy on the growth and metabolism of the pennate diatom Phaeodactylum tricornutum , using glycerol as the source of reduced carbon. Transcriptomics, metabolomics, metabolic modelling and physiological data combine to indicate that glycerol affects the central-carbon, carbon-storage and lipid metabolism of the diatom. In particular, provision of glycerol mimics typical responses of nitrogen limitation on lipid metabolism at the level of triacylglycerol accumulation and fatty acid composition. The presence of glycerol, despite provoking features reminiscent of nutrient limitation, neither diminishes photosynthetic activity nor cell growth, revealing essential aspects of the metabolic flexibility of these microalgae and suggesting possible biotechnological applications of mixotrophy.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'., (© 2017 The Author(s).)

Methods to analyze lipidomes have considerably evolved, more and more based on mass spectrometry technics (LC-MS/MS). However, accurate quantifications using these methods require 13C-labeled standards for each lipid, which is not feasible because of the very large number of molecules. Thus, quantifications rely on standard molecules representative of a whole class of lipids, which might lead to false estimations of some molecular species. Here, we determined and compared glycerolipid distributions from three different types of cells, two microalgae (Phaeodactylum tricornutum, Nannochloropsis gaditana) and one higher plant (Arabidopsis thaliana), using either LC-MS/MS or Thin Layer Chromatography coupled with Gas Chromatography (TLC-GC), this last approach relying on the precise quantification of the fatty acids present in each glycerolipid class. Our results showed that the glycerolipid distribution was significantly different depending on the method used. How can one reconcile these two analytical methods? Here we propose that the possible bias with MS data can be circumvented by systematically running in tandem with the sample to be analyzed a lipid extract from a qualified control (QC) of each type of cells, previously analyzed by TLC-GC, and used as an external standard to quantify the MS results. As a case study, we applied this method to compare the impact of a nitrogen deficiency on the three types of cells.

Membrane systems that naturally occur as densely packed membrane stacks contain high amounts of glycolipids whose saccharide headgroups display multiple small electric dipoles in the form of hydroxyl groups. Experimentally, the hydration repulsion between glycolipid membranes is of much shorter range than that between zwitterionic phospholipids whose headgroups are dominated by a single large dipole. Using solvent-explicit molecular dynamics simulations, here we reproduce the experimentally observed, different pressure-versus-distance curves of phospholipid and glycolipid membrane stacks and show that the water uptake into the latter is solely driven by the hydrogen bond balance involved in non-ideal water/sugar mixing. Water structuring effects and lipid configurational perturbations, responsible for the longer-range repulsion between phospholipid membranes, are inoperative for the glycolipids. Our results explain the tight cohesion between glycolipid membranes at their swelling limit, which we here determine by neutron diffraction, and their unique interaction characteristics, which are essential for the biogenesis of photosynthetic membranes.

The picoalga Ostreococcus tauri is a minimal photosynthetic eukaryote that has been used as a model system. O. tauri is known to efficiently produce docosahexaenoic acid (DHA). We provide a comprehensive study of the glycerolipidome of O. tauri and validate this species as model for related picoeukaryotes. O. tauri lipids displayed unique features that combined traits from the green and the chromalveolate lineages. The betaine lipid diacylglyceryl-hydroxymethyl-trimethyl-β-alanine and phosphatidyldimethylpropanethiol, both hallmarks of chromalveolates, were identified as presumed extraplastidial lipids. DHA was confined to these lipids, while plastidial lipids of prokaryotic type were characterized by the overwhelming presence of ω-3 C18 polyunsaturated fatty acids (FAs), 18:5 being restricted to galactolipids. C16:4, an FA typical of green microalgae galactolipids, also was a major component of O. tauri extraplastidial lipids, while the 16:4-coenzyme A (CoA) species was not detected. Triacylglycerols (TAGs) displayed the complete panel of FAs, and many species exhibited combinations of FAs diagnostic for plastidial and extraplastidial lipids. Importantly, under nutrient deprivation, 16:4 and ω-3 C18 polyunsaturated FAs accumulated into de novo synthesized TAGs while DHA-TAG species remained rather stable, indicating an increased contribution of FAs of plastidial origin to TAG synthesis. Nutrient deprivation further severely down-regulated the conversion of 18:3 to 18:4, resulting in obvious inversion of the 18:3/18:4 ratio in plastidial lipids, TAGs, as well as acyl-CoAs. The fine-tuned and dynamic regulation of the 18:3/18:4 ratio suggested an important physiological role of these FAs in photosynthetic membranes. Acyl position in structural and storage lipids together with acyl-CoA analysis further help to determine mechanisms possibly involved in glycerolipid synthesis., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

Lipid trafficking between mitochondria and other organelles is required for mitochondrial membrane biogenesis and signaling. This lipid exchange occurs by poorly understood nonvesicular mechanisms. In yeast and mammalian cells, this lipid exchange is thought to take place at contact sites between mitochondria and the ER or vacuolar membranes. Some proteins involved in the tethering between membranes or in the transfer of lipids in mitochondria have been identified. However, in plants, little is known about the synthesis of mitochondrial membranes. Mitochondrial membrane biogenesis is particularly important and noteworthy in plants as the lipid composition of mitochondrial membranes is dramatically changed during phosphate starvation and other stresses. This review focuses on the principal pathways involved in the synthesis of the most abundant mitochondrial glycerolipids in plants and the lipid trafficking that is required for plant mitochondria membrane biogenesis., (© 2016 Federation of European Biochemical Societies.)

Nannochloropsis species are oleaginous eukaryotes containing a plastid limited by four membranes, deriving from a secondary endosymbiosis. In Nannochloropsis, thylakoid lipids, including monogalactosyldiacylglycerol (MGDG), are enriched in eicosapentaenoic acid (EPA). The need for EPA in MGDG is not understood. Fatty acids are de novo synthesized in the stroma, then converted into very-long-chain polyunsaturated fatty acids (FAs) at the endoplasmic reticulum (ER). The production of MGDG relies therefore on an EPA supply from the ER to the plastid, following an unknown process. We identified seven elongases and five desaturases possibly involved in EPA production in Nannochloropsis gaditana Among the six heterokont-specific saturated FA elongases possibly acting upstream in this pathway, we characterized the highly expressed isoform Δ0-ELO1 Heterologous expression in yeast (Saccharomyces cerevisiae) showed that NgΔ0-ELO1 could elongate palmitic acid. Nannochloropsis Δ0-elo1 mutants exhibited a reduced EPA level and a specific decrease in MGDG In NgΔ0-elo1 lines, the impairment of photosynthesis is consistent with a role of EPA-rich MGDG in nonphotochemical quenching control, possibly providing an appropriate MGDG platform for the xanthophyll cycle. Concomitantly with MGDG decrease, the level of triacylglycerol (TAG) containing medium chain FAs increased. In Nannochloropsis, part of EPA used for MGDG production is therefore biosynthesized by a channeled process initiated at the elongation step of palmitic acid by Δ0-ELO1, thus acting as a committing enzyme for galactolipid production. Based on the MGDG/TAG balance controlled by Δ0-ELO1, this study also provides novel prospects for the engineering of oleaginous microalgae for biotechnological applications., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

The seawater microalga Nannochloropsis gaditana is capable of accumulating a large fraction of reduced carbon as lipids. To clarify the molecular bases of this metabolic feature, we investigated light-driven lipid biosynthesis in Nannochloropsis gaditana cultures combining the analysis of photosynthetic functionality with transcriptomic, lipidomic and metabolomic approaches. Light-dependent alterations are observed in amino acid, isoprenoid, nucleic acid, and vitamin biosynthesis, suggesting a deep remodeling in the microalgal metabolism triggered by photoadaptation. In particular, high light intensity is shown to affect lipid biosynthesis, inducing the accumulation of diacylglyceryl-N,N,N-trimethylhomo-Ser and triacylglycerols, together with the up-regulation of genes involved in their biosynthesis. Chloroplast polar lipids are instead decreased. This situation correlates with the induction of genes coding for a putative cytosolic fatty acid synthase of type 1 (FAS1) and polyketide synthase (PKS) and the down-regulation of the chloroplast fatty acid synthase of type 2 (FAS2). Lipid accumulation is accompanied by the regulation of triose phosphate/inorganic phosphate transport across the chloroplast membranes, tuning the carbon metabolic allocation between cell compartments, favoring the cytoplasm, mitochondrion, and endoplasmic reticulum at the expense of the chloroplast. These results highlight the high flexibility of lipid biosynthesis in N. gaditana and lay the foundations for a hypothetical mechanism of regulation of primary carbon partitioning by controlling metabolite allocation at the subcellular level., (© 2016 American Society of Plant Biologists. All Rights Reserved.)

The mitochondrion is an organelle originating from an endosymbiotic event and playing a role in several fundamental processes such as energy production, metabolite syntheses, and programmed cell death. This organelle is delineated by two membranes whose synthesis requires an extensive exchange of phospholipids with other cellular organelles such as endoplasmic reticulum (ER) and vacuolar membranes in yeast. These transfers of phospholipids are thought to occur by a non-vesicular pathway at contact sites between two closely apposed membranes. In plants, little is known about the biogenesis of mitochondrial membranes. Contact sites between ER and mitochondria are suspected to play a similar role in phospholipid trafficking as in yeast, but this has never been demonstrated. In contrast, it has been shown that plastids are able to transfer lipids to mitochondria during phosphate starvation. However, the proteins involved in such transfer are still unknown. Here, we identified in Arabidopsis thaliana a large lipid-enriched complex called the mitochondrial transmembrane lipoprotein (MTL) complex. The MTL complex contains proteins located in the two mitochondrial membranes and conserved in all eukaryotic cells, such as the TOM complex and AtMic60, a component of the MICOS complex. We demonstrate that AtMic60 contributes to the export of phosphatidylethanolamine from mitochondria and the import of galactoglycerolipids from plastids during phosphate starvation. Furthermore, AtMic60 promotes lipid desorption from membranes, likely as an initial step for lipid transfer, and binds to Tom40, suggesting that AtMic60 could regulate the tethering between the inner and outer membranes of mitochondria., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The biogenesis of photosynthetic membranes relies on galactoglycerolipids, which are synthesized via pathways that are dispatched over several cell compartments. This membrane biogenesis requires both trafficking of lipid intermediates and a tight homeostatic regulation. In this work, we address the role of ALA10 (for aminophospholipid ATPase), a P4-type ATPase, in a process counteracting the monogalactosyldiacylglycerol (MGDG) shortage in Arabidopsis (Arabidopsis thaliana) leaves. ALA10 can interact with protein partners, ALIS1 (for ALA-interacting subunit1) or ALIS5, leading to differential endomembrane localizations of the interacting proteins, close to the plasma membrane with ALIS1 or to chloroplasts with ALIS5. ALA10 interacts also with FATTY ACID DESATURASE2 (FAD2), and modification of ALA10 expression affects phosphatidylcholine (PC) fatty acyl desaturation by disturbing the balance between FAD2 and FAD3 activities. Modulation of ALA10 expression downstream impacts the fatty acyl composition of chloroplast PC. ALA10 expression also enhances leaf growth and improves the MGDG-PC ratio, possibly through MGDG SYNTHASE1 (MGD1) activation by phosphatidic acid. The positive effect of ALA10 on leaf development is significant in conditions such as upon treatment of plants with Galvestine-1, an inhibitor of MGDG synthases, or when plants are grown at chilling temperature., (© 2016 American Society of Plant Biologists. All Rights Reserved.)

In higher plants, fatty acids (FAs) with 18 carbons (18C) represent about 70% of total FAs, the most abundant species being 18:2 and 18:3. These two polyunsaturated FAs (PUFAs) represent about 55% of total FAs in Arabidopsis cell suspension cultures, whereas 18:1 represents about 10%. The level of PUFAs may vary, depending on ill-defined factors. Here, we compared various sets of plant cell cultures and noticed a correlation between the growth rate of a cell population and the level of unsaturation of 18C FAs. These observations suggest that the final level of PUFAs might depend in part on the rate of cell division, and that FAD2 and FAD3 desaturases, which are respectively responsible for the formation of 18:2 and 18:3 on phospholipids, have limiting activities in fast-growing cultures. In plant cell culture, phosphate (Pi) deprivation is known to impair cell division and to trigger lipid remodeling. We observed that Pi starvation had no effect on the expression of FAD genes, and that the level of PUFAs in this situation was also correlated with the growth rate. Thus, the level of PUFAs appears as a hallmark in determining cell maturity and aging.

In this work, we dissect the physiological role of the transient photosynthetic stage observed in developing seeds of Arabidopsis thaliana. By combining biochemical and biophysical approaches, we demonstrate that despite similar features of the photosynthetic apparatus, light absorption, chloroplast morphology and electron transport are modified in green developing seeds, as a possible response to the peculiar light environment experienced by them as a result of sunlight filtration by the pericarp. In particular, enhanced exposure to far-red light, which mainly excites photosystem I, largely enhances cyclic electron flow around this complex at the expenses of oxygen evolution. Using pharmacological, genetic and metabolic analyses, we show that both linear and cyclic electron flows are important during seed formation for proper germination timing. Linear flow provides specific metabolites related to oxygen and water stress responses. Cyclic electron flow possibly adjusts the ATP to NADPH ratio to cope with the specific energy demand of developing seeds. By providing a comprehensive scenario of the characteristics, function and consequences of embryonic photosynthesis on seed vigour, our data provide a rationale for the transient building up of a photosynthetic machinery in seeds., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)