. As shown in Extended Data Fig. 1a, an ECS signal is visible in P.tricornutum, the characteristics of which depend on the physiological conditions. Deconvolution of the ECS decay-associated spectra (DAS) (see Supplementary Information and Extended Data Fig. 1b,c) explains these observations by revealing the existence of two ECS components (Fig. 1a), respectively characterized by linear and quadratic responses to the ΔΨ (Fig. 1b). The existence of a quadratic ECS, predicted by theory 7 but observed to date only in green algae mutants (Fig. 1c), but was also suppressed by anaerobiosis or by pharmacological inhibition of mitochondrial activity (Fig. 1d, e). This suggests that the PMF is generated in the dark by the plastidial ATPase, which hydrolyses ATP of mitochondrial origin, as previously suggested in green algae 9 .In Viridiplantae (including green algae and higher plants), the AEPs generating additional ATP in the light comprise cyclic electron flow (CEF) around PS1 10 and the water-to-water cycles (WWC). uptake (U 0 ) increased with light, being ~2.5-fold higher at saturating light intensities than in the dark (Extended Data Fig. 2b, d). We further found that the light-stimulated consumption of oxygen was blocked by DCMU (Extended Data Fig. 2c, d), indicating that it was fed by electrons derived from PS2.Moreover, U 0 linearly increased with O 2 evolution, in agreement with earlier findings in the diatom Thalassiosira pseudonana 15, with a slope indicating that ~10% of the electron flow from PSII participate in WWC, regardless of light intensity (Fig 2b). These results indicate that WWC produces a constant extra ATP per photosynthetically-generated NADPH. This is expected for an AEP that contributes to optimizing CO 2 assimilation at any light intensity, and is not the case for CEF, which is completely insensitive to changes in the photosynthetic flux (LEF, Fig 2a).If this WWC is due to the export of photosynthetic products towards the mitochondrial oxidases, then any mitochondrial dysfunction should negatively affect photosynthetic electron transfer rates (ETR PSII ) and light-dependent growth. Mitochondrial respiration comprises a cyanidesensitive pathway (involving Complex III) and an insensitive pathway involving the alternative oxidase (AOX). We therefore modulated mitochondrial activity by adding increasing amounts of Antimycin A (AA) or myxothiazol (Mx), inhibitors of Complex III, in conditions where the AOX was inhibited by SHAM (see legend to Fig. 2d). Both the ΔΨ d and ETR PSII followed respiration linearly (Fig. 2c, d and Extended Data Fig. 3). The almost complete shut-down of respiration resulted in a decrease of photosynthesis which was independent of light intensity (Fig. 3b).Overall we found that in the dark a PMF is generated in the plastid by hydrolysis of ATP produced in mitochondria (Fig 1d,e and Fig. 2c). Conversely, in the light, respiration increases linearly with photosynthesis (Fig. 2b), and vice versa (Fig. 2d). This tight energetic coupling is likely instrumental for adjusting ...

Glycerolipids constituting the matrix of photosynthetic membranes, from cyanobacteria to chloroplasts of eukaryotic cells, comprise monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. This review covers our current knowledge on the structural and functional features of these lipids in various cellular models, from prokaryotes to eukaryotes. Their relative proportions in thylakoid membranes result from highly regulated and compartmentalized metabolic pathways, with a cooperation, in the case of eukaryotes, of non-plastidic compartments. This review also focuses on the role of each of these thylakoid glycerolipids in stabilizing protein complexes of the photosynthetic machinery, which might be one of the reasons for their fascinating conservation in the course of evolution. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

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Diatoms are a major phylum of phytoplankton biodiversity and a resource considered for biotechnological developments, as feedstock for biofuels and applications ranging from food, human health or green chemistry. They contain a secondary plastid limited by four membranes, the outermost one being connected with the endoplasmic reticulum (ER). Upon nitrogen stress, diatoms reallocate carbon to triacylglycerol storage inside lipid droplets (LDs). The comprehensive glycerolipid and sterol composition and the architecture of diatom LDs are unknown. In Phaeodactylum tricornutum, LDs are in contact with plastid, mitochondria and uncharacterized endomembranes. We purified LDs from nitrogen-starved P. tricornutum cells to high purity level (99 mol% triacylglycerol of total glycerolipids). We used the Stramenopile Lipid Droplet Protein (StLDP) as a previoulsy validated marker for the identity of P. tricornutum LD. Amphipathic lipids surrounding LDs consist of a betaine lipid, diacylglycerylhydroxymethyltrimethyl-β-alanine (0.4 mol%); sulfoquinovosyldiacylglycerol (0.35 mol%); phosphatidylcholine (0.15 mol%) and one sterol, brassicasterol. By contrast with whole cell extracts, the betaine lipid from LDs only contains eicosapentaenoic acid paired with palmitoleic or palmitolenic acids. This polar lipid composition suggests a budding of LDs from the cytosolic leaflet of the plastid outermost membrane. LD pigments reveal a specific accumulation of β-carotene. The LD proteome obtained from three independent biological replicates, based on stringent filtering of extracted data, and following subtraction of proteins downregulated by nitrogen starvation, highlights a core proteome of 86 proteins, including StLDP. LD-associated proteins suggest connections with vesicular trafficking (coatomer, clathrin), cytoskeleton, plastid and mitochondria. Unsuspected LD-associated function include protein synthesis (ribosomes), folding (chaperones), posttranslational modifications and quality control (ubiquitination and ERAD pathway), possibly preparing translation of specific mRNAs. The detection of histone proteins, as previously demonstrated in drosophila embryo LDs, also suggests the storage of nucleosome components, preparing cell division and chromatin packaging, when cells are not stressed anymore.

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.

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.

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the main lipids in photosynthetic membranes in plant cells. They are synthesized in the envelope surrounding plastids by MGD and DGD galactosyltransferases. These galactolipids are critical for the biogenesis of photosynthetic membranes, and they act as a source of polyunsaturated fatty acids for the whole cell and as phospholipid surrogates in phosphate shortage. Based on a high-throughput chemical screen, we have characterized a new compound, galvestine-1, that inhibits MGDs in vitro by competing with diacylglycerol binding. Consistent effects of galvestine-1 on Arabidopsis thaliana include root uptake, circulation in the xylem and mesophyll, inhibition of MGDs in vivo causing a reduction of MGDG content and impairment of chloroplast development. The effects on pollen germination shed light on the contribution of galactolipids to pollen-tube elongation. The whole-genome transcriptional response of Arabidopsis points to the potential benefits of galvestine-1 as a unique tool to study lipid homeostasis in plants.

Photosynthesis is a unique process that allows independent colonization of the land by plants and of the oceans by phytoplankton. Although the photosynthesis process is well understood in plants, we are still unlocking the mechanisms evolved by phytoplankton to achieve extremely efficient photosynthesis. Here, we combine biochemical, structural and in vivo physiological studies to unravel the structure of the plastid in diatoms, prominent marine eukaryotes. Biochemical and immunolocalization analyses reveal segregation of photosynthetic complexes in the loosely stacked thylakoid membranes typical of diatoms. Separation of photosystems within subdomains minimizes their physical contacts, as required for improved light utilization. Chloroplast 3D reconstruction and in vivo spectroscopy show that these subdomains are interconnected, ensuring fast equilibration of electron carriers for efficient optimum photosynthesis. Thus, diatoms and plants have converged towards a similar functional distribution of the photosystems although via different thylakoid architectures, which likely evolved independently in the land and the ocean.