Chloroplasts contain lipoprotein particles termed plastoglobules. Plastoglobules are generally believed to have little function beyond lipid storage. Here we report on the identification of plastoglobule proteins using mass spectrometry methods in Arabidopsis thaliana. We demonstrate specific plastoglobule association of members of the plastid lipid-associated proteins/fibrillin family as well as known metabolic enzymes, including the tocopherol cyclase (VTE1), a key enzyme of tocopherol (vitamin E) synthesis. Moreover, comparative analysis of chloroplast membrane fractions shows that plastoglobules are a site of vitamin E accumulation in chloroplasts. Thus, in addition to their lipid storage function, we propose that plastoglobules are metabolically active, taking part in tocopherol synthesis and likely other pathways.Chloroplasts are highly compartmentalized organelles. In addition to membrane-bound compartments, chloroplasts contain lipoprotein particles called plastoglobules. Although a role of plastoglobules in chromoplast differentiation has been defined (1, 2), the functions of plastoglobules in chloroplasts are largely unknown. Plastoglobules are associated with thylakoid membranes (3), suggesting that they play a role in thylakoid membrane function. Indeed, plastoglobules enlarge during thylakoid disassembly in senescing chloroplasts and during chromoplast differentiation (4 -10) and increasingly accumulate triacylglycerols and esterified isoprenoids derived from the disintegrating thylakoids (6, 11). Plastoglobules have been reported to contain prenylquinones tocopherol and plastoquinone (8,(11)(12)(13)(14)(15), but the relative abundance of these compounds with regard to other chloroplast compartments is unknown. Although tocopherols are thought to function as antioxidants mostly at the thylakoid membrane, most of the prenylquinone biosynthetic activities have been localized to the inner chloroplast envelope membrane (16 -18).Pea plastoglobules contain around a dozen different proteins (3) but so far only members of the plastid lipid-associated protein (PAP) 2 /fibrillin family have been identified (1, 3, 19 -28). At least two PAP/fibrillins were localized at the periphery of plastoglobules (3, 26), and purified fibrillin was able to promote carotenoid fibril assembly in vitro (1), suggesting a structural function (29). Upregulation of several PAP/fibrillins has been correlated with various treatments generating reactive oxygen species (20, 24, 30 -32), and enlarged plastoglobules have been described in chloroplasts under abiotic stress such as drought (33), nitrogen starvation (34), or hypersalinity (35). Taken together, these observations implicate plastoglobules in stress tolerance.In addition to a role in plastid lipid storage and a possible involvement in stress tolerance, plastoglobules may have other functions in chloroplasts. The presence of yet-unidentified proteins of plastoglobules (3) supports this idea.To identify candidate plastoglobule proteins we used mass spectrometry methods. We fou...

SummaryNitrogen is an essential nutrient for plants because it represents a major constituent of numerous cellular compounds, including proteins, amino acids, nucleic acids and lipids. While N deprivation is known to have severe consequences for primary carbon metabolism, the effect on chloroplast lipid metabolism has not been analysed in higher plants. Nitrogen limitation in Arabidopsis led to a decrease in the chloroplast galactolipid monogalactosyldiacylglycerol (MGDG) and a concomitant increase in digalactosyldiacylglycerol (DGDG), which correlated with an elevated expression of the DGDG synthase genes DGD1 and DGD2. The amounts of triacylglycerol and free fatty acids increased during N deprivation. Furthermore, phytyl esters accumulated containing medium-chain fatty acids (12:0, 14:0) and a large amount of hexadecatrienoic acid (16:3). Fatty acid phytyl esters were localized to chloroplasts, in particular to thylakoids and plastoglobules. Different polyunsaturated acyl groups were found in phytyl esters accumulating in Arabidopsis lipid mutants and in other plants, including 16:3 and 18:3 species. Therefore N deficiency in higher plants results in a co-ordinated breakdown of galactolipids and chlorophyll with deposition of specific fatty acid phytyl esters in thylakoids and plastoglobules of chloroplasts.

Plastoglobules are plastid-localized lipoprotein particles that contain tocopherols and other lipid isoprenoidderived metabolites, as well as structural proteins named plastoglobulins. Surprisingly, recent publications show that plastoglobules contain enzymes involved in the metabolism of these secondary metabolites, as well as enzymes of unknown function. The size and number of plastoglobules vary during plastid development and differentiation, and strongly increase during light stress, senescence and in mutants blocked in thylakoid formation. Given that plastoglobules are contiguous with the outer lipid leaflet of the thylakoid membrane, it is highly plausible that a function of plastoglobules is the active channeling of lipid molecules and lipid breakdown products. Understanding the function of plastoglobules should provide a foundation for improving the nutritional value and yield of plants.History of plastoglobule research and discovery Early electron microscopic studies revealed the presence of 'osmiophilic globuli' inside chloroplasts ( Figure 1) and chromoplasts, as well as other plastid types [1]. The diameter of these bodies, later termed plastoglobules, ranges from 30 nm to 5 mm. These plastoglobules could be conveniently isolated by flotation density centrifugation because of their relatively high lipid content [1,2]. The lipid composition of plastoglobules has been determined in several plant species -it consists mainly of prenyl-quinones and neutral lipids. Plastoglobules qualify as lipoprotein particles because they have been reported to associate with proteins. Members of the plastoglobulin family (also called fibrillin or PAP for plastid lipid-associated protein) [3], were the first known genuine plastoglobule protein components. In addition to vascular plants, plastoglobules are found in non-vascular species such as moss [4] and algae [5,6]. Interestingly, carotenoid-rich plastoglobule-like structures constitute the eyespot structure of Chlamydomonas reinhardtii, the proteome of which has been shown to contain members of the plastoglobulin family [6]. In cyanobacteria, the presence of 'lipid droplets' among the thylakoids has been reported [7]. The exact identity of these lipid droplets has not been defined; however, the presence of at least two plastoglobulin homologs in the genome of Synechocystis PCC6803 suggests that they could be plastoglobules.Although plastoglobules were largely viewed as passive lipid and carotenoid storage particles, their varying size in different species, plastid types and developmental stages suggested a more dynamic role. Moreover, correlative evidence suggested that plastoglobules are involved in thylakoid development as well as disassembly: (i) etioplasts with poorly developed thylakoids have more plastoglobules than are found in chloroplasts, but the plastoglobule abundance decreased during thylakoid biogenesis [8-10]; (ii) in senescent chloroplasts, during thylakoid disassembly, plastoglobules enlarge and accumulate [9,11,12]; (iii) several thylakoid biogenesis...

Plastoglobules, lipid-protein bodies in the stroma of plant chloroplasts, are enriched in non-polar lipids, in particular prenyl quinols. In the present study we show that, in addition to the thylakoids, plastoglobules also contain a considerable proportion of the plastidial PQ-9 (plastoquinol-9), the redox component of photosystem II, and of the cyclized product of PQ-9, PC-8 (plastochromanol-8), a tocochromanol with a structure similar to gamma-tocopherol and gamma-tocotrienol, but with a C-40 prenyl side chain. PC-8 formation was abolished in the Arabidopsis thaliana tocopherol cyclase mutant vte1, but accumulated in VTE1-overexpressing plants, in agreement with a role of tocopherol cyclase (VTE1) in PC-8 synthesis. VTE1 overexpression resulted in the proliferation of the number of plastoglobules which occurred in the form of clusters in the transgenic lines. Simultaneous overexpression of VTE1 and of the methyltransferase VTE4 resulted in the accumulation of a compound tentatively identified as 5-methyl-PC-8, the methylated form of PC-8. The results of the present study suggest that the existence of a plastoglobular pool of PQ-9, along with the partial conversion of PQ-9 into PC-8, might represent a mechanism for the regulation of the antioxidant content in thylakoids and of the PQ-9 pool that is available for photosynthesis.

Phylloquinone (vitamin K 1 ) is synthesized in cyanobacteria and in chloroplasts of plants, where it serves as electron carrier of photosystem I. The last step of phylloquinone synthesis in cyanobacteria is the methylation of 2-phytyl-1,4-naphthoquinone by the menG gene product. Here, we report that the uncharacterized Arabidopsis gene At1g23360, which shows sequence similarity to menG, functionally complements the Synechocystis menG mutant. An Arabidopsis mutant, AtmenG, carrying a T-DNA insertion in the gene At1g23360 is devoid of phylloquinone, but contains an increased amount of 2-phytyl-1,4-naphthoquinone. Phylloquinone and 2-phytyl-1,4-naphthoquinone in thylakoid membranes of wild type and AtmenG, respectively, predominantly localize to photosystem I, whereas excess amounts of prenyl quinones are stored in plastoglobules. Photosystem I reaction centers are decreased in AtmenG plants under high light, as revealed by immunoblot and spectroscopic measurements. Anthocyanin accumulation and chalcone synthase (CHS1) transcription are affected during high light exposure, indicating that alterations in photosynthesis in AtmenG affect gene expression in the nucleus. Photosystem II quantum yield is decreased under high light. Therefore, the loss of phylloquinone methylation affects photosystem I stability or turnover, and the limitation in functional photosystem I complexes results in overreduction of photosystem II under high light.

The Arabidopsis type II peroxiredoxin (PRXII) family is composed of six different genes, five of which are expressed. On the basis of the nucleotide and protein sequences, we were able to define three subgroups among the PRXII family. The first subgroup is composed of AtPRXII-B, -C, and -D, which are highly similar and localized in the cytosol. AtPRXII-B is ubiquitously expressed. More striking is the specific expression of AtPRXII-C and AtPRXII-D localized in pollen. The second subgroup comprises the mitochondrial AtPRXII-F, the corresponding gene of which is expressed constitutively. We show that AtPRXII-E, belonging to the last subgroup, is expressed mostly in reproductive tissues and that its product is addressed to the plastid. By in vitro enzymatic experiments, we demonstrate that glutaredoxin is the electron donor of recombinant AtPRXII-B for peroxidase reaction, but the donors of AtPRXII-E and AtPRXII-F have still to be identified.Plants generate reactive oxygen species (ROS) such as the superoxide anion, hydrogen peroxide, or the hydroxyl radical as by-products of electron transport chains in chloroplast and mitochondria, photorespiration in the peroxisome, and cell wall oxidases and peroxidases (Dat et al., 2000). ROS are necessary for plants, because they participate in signal transduction (Karpinski et al., 1999; Orozco-Cardenas et al., 2001; Mullineaux and Karpinski, 2002) and play a role in response to pathogen attack (Wojtaszek, 1997; Bolwell, 1999; Dat et al., 2000). However, biotic or abiotic stress may promote ROS generation and break the redox balance of the cell. To protect macromolecules such as lipids, proteins, or nucleic acids from damage caused by ROS, cells contain a large variety of antioxidant enzymes that include catalase, superoxide dismutase, ascorbate-and glutathionedependent peroxidases, and the more recently described peroxiredoxin (PRX) family. The PRX family was first described as alkyl hydroperoxide reductase C (Jacobson et al., 1989) and later as thiol-specific antioxidant in Brewer's yeast (Saccharomyces cerevisiae) and Escherichia coli (Chae et al., 1994). In contrast to other peroxidases, PRX enzymes do not have redox cofactors such as metal or prosthetic groups.They reduce hydrogen peroxides and alkyl peroxides to water and alcohols, respectively, by using reducing equivalents. These reducers are derived specifically from thiol-containing donor molecules such as thioredoxin (TRX; Chae et al., 1994; Kwon et al., 1994; Kang et al., 1998), glutaredoxin (GRX; Rouhier et al., 2001Rouhier et al., , 2002, and the flavin containing AhpF, a subunit of the Salmonella typhimurium alkyl hydroperoxide reductase highly similar to TRX reductase (Jacobson et al., 1989;Tartaglia et al., 1990). Members of the PRX family have now been identified in a wide variety of organisms ranging from archae and eubacteria to eukaryotes, including vertebrates and plants.All PRX proteins contain a conserved Cys in their N-terminal part and some of them possess a second conserved Cys residue. The ...

Lipid droplets (LDs) are cell compartments specialized for oil storage. Although their role and biogenesis are relatively well documented in seeds, little is known about their composition, structure and function in senescing leaves where they also accumulate. Here, we used a label free quantitative mass spectrometry approach to define the LD proteome of aging Arabidopsis leaves. We found that its composition is highly different from that of seed/cotyledon and identified 28 proteins including 9 enzymes of the secondary metabolism pathways involved in plant defense response. With the exception of the TRIGALACTOSYLDIACYLGLYCEROL2 protein, we did not identify enzymes implicated in lipid metabolism, suggesting that growth of leaf LDs does not occur by local lipid synthesis but rather through contact sites with the endoplasmic reticulum (ER) or other membranes. The two most abundant proteins of the leaf LDs are the CALEOSIN3 and the SMALL RUBBER PARTICLE1 (AtSRP1); both proteins have structural functions and participate in plant response to stress. CALEOSIN3 and AtSRP1 are part of larger protein families, yet no other members were enriched in the LD proteome suggesting a specific role of both proteins in aging leaves. We thus examined the function of AtSRP1 at this developmental stage and found that AtSRP1 modulates the expression of CALEOSIN3 in aging leaves. Furthermore, AtSRP1 overexpression induces the accumulation of triacylglycerol with an unusual composition compared to wild-type. We demonstrate that, although AtSRP1 expression is naturally increased in wild type senescing leaves, its overexpression in senescent transgenic lines induces an over-accumulation of LDs organized in clusters at restricted sites of the ER. Conversely, atsrp1 knock-down mutants displayed fewer but larger LDs. Together our results reveal that the abundancy of AtSRP1 regulates the neo-formation of LDs during senescence. Using electron tomography, we further provide evidence that LDs in leaves share tenuous physical continuity as well as numerous contact sites with the ER membrane. Thus, our data suggest that leaf LDs are functionally distinct from seed LDs and that their biogenesis is strictly controlled by AtSRP1 at restricted sites of the ER.

Plastoglobules are lipoprotein particles contained in chloroplasts and other plastids. They have long been regarded as lipid storage droplets. New results now indicate that plastoglobules actively participate in prenylquinone and other metabolic pathways. Structural work shows physical attachment of plastoglobules to the thylakoid membrane probably enabling the exchange of lipid molecules between the membrane compartments. This review will give a summary of research, past and present, attempting to elucidate the role of plastoglobules in the context of plastid function.