Glycerol-3-Phosphate Acyltransferase GPAT9 Enhanced Seed Oil Accumulation and Eukaryotic Galactolipid Synthesis in Brassica napus.

Glycerol-3-phosphate acyltransferase GPAT9 catalyzes the first acylation of glycerol-3-phosphate (G3P), a committed step of glycerolipid synthesis in Arabidopsis. The role of GPAT9 in Brassica napus remains to be elucidated. Here, we identified four orthologs of GPAT9 and found that BnaGPAT9 encoded by BnaC01T0014600WE is a predominant isoform and promotes seed oil accumulation and eukaryotic galactolipid synthesis in Brassica napus. BnaGPAT9 is highly expressed in developing seeds and is localized in the endoplasmic reticulum (ER). Ectopic expression of BnaGPAT9 in E. coli and siliques of Brassica napus enhanced phosphatidic acid (PA) production. Overexpression of BnaGPAT9 enhanced seed oil accumulation resulting from increased 18:2-fatty acid. Lipid profiling in developing seeds showed that overexpression of BnaGPAT9 led to decreased phosphatidylcholine (PC) and a corresponding increase in phosphatidylethanolamine (PE), implying that BnaGPAT9 promotes PC flux to storage triacylglycerol (TAG). Furthermore, overexpression of BnaGPAT9 also enhanced eukaryotic galactolipids including monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), with increased 36:6-MGDG and 36:6-DGDG, and decreased 34:6-MGDG in developing seeds. Collectively, these results suggest that ER-localized BnaGPAT9 promotes PA production, thereby enhancing seed oil accumulation and eukaryotic galactolipid biosynthesis in Brassica napus.

Phosphate starvation-inducible GLYCEROPHOSPHODIESTER PHOSPHODIESTERASE6 is involved in Arabidopsis root growth.

Plants that are starved of phosphate trigger membrane lipid remodeling, which hydrolyses phospholipids and presumably allows their phosphate to be utilized, whilst replacing them with galactolipids to maintain the integrity of the membrane system. In addition to the two concurrent pathways of phospholipid hydrolysis by phospholipases C and D that have already been established, an emerging third pathway has been proposed that includes a reaction step catalysed by glycerophosphodiester phosphodiesterases (GDPDs). However, its functional involvement in phosphate-starved plants remains elusive. Here, we show that Arabidopsis GDPD6 is a functional isoform responsible for glycerophosphocholine hydrolysis in vivo. Overexpression of GDPD6 promoted root growth whilst gdpd6 mutants showed impaired root growth under phosphate starvation, and this defect was rescued by supplementing with the reaction product glycerol 3-phosphate but not with choline. Since GDPD6 is induced by phosphate starvation, we suggest that GDPD6 might be involved in root growth via the production of glycerol 3-phosphate in phosphate-starved plants.

The phospho-base N-methyltransferases PMT1 and PMT2 produce phosphocholine for leaf growth in phosphorus-starved Arabidopsis.

Phosphorus (P) is an essential nutrient for plants. Membrane lipid remodeling is an adaptive mechanism for P-starved plants that replaces membrane phospholipids with non-P galactolipids, presumably to retrieve scarce P sources and maintain membrane integrity. Whereas metabolic pathways to convert phospholipids to galactolipids are well-established, the mechanism by which phospholipid biosynthesis is involved in this process remains elusive. Here, we report that phospho-base N-methyltransferases 1 and 2 (PMT1 and PMT2), which convert phosphoethanolamine to phosphocholine (PCho), are transcriptionally induced by P starvation. Shoots of seedlings of pmt1 pmt2 double mutant showed defective growth upon P starvation; however, membrane lipid profiles were unaffected. We found that P-starved pmt1 pmt2 with defective leaf growth had reduced PCho content, and the growth defect was rescued by exogenous supplementation of PCho. We propose that PMT1 and PMT2 are induced by P starvation to produce PCho mainly for leaf growth maintenance, rather than for phosphatidylcholine biosynthesis, in membrane lipid remodeling.

Decreased R:FR Ratio in Incident White Light Affects the Composition of Barley Leaf Lipidome and Freezing Tolerance in a Temperature-Dependent Manner.

In cereals, C-repeat binding factor genes have been defined as key components of the light quality-dependent regulation of frost tolerance by integrating phytochrome-mediated light and temperature signals. This study elucidates the differences in the lipid composition of barley leaves illuminated with white light or white light supplemented with far-red light at 5 or 15 °C. According to LC-MS analysis, far-red light supplementation increased the amount of monogalactosyldiacylglycerol species 36:6, 36:5, and 36:4 after 1 day at 5 °C, and 10 days at 15 °C resulted in a perturbed content of 38:6 species. Changes were observed in the levels of phosphatidylethanolamine, and phosphatidylserine under white light supplemented with far-red light illumination at 15 °C, whereas robust changes were observed in the amount of several phosphatidylserine species at 5 °C. At 15 °C, the amount of some phosphatidylglycerol species increased as a result of white light supplemented with far-red light illumination after 1 day. The ceramide (42:2)-3 content increased regardless of the temperature. The double-bond index of phosphatidylglycerol, phosphatidylserine, phosphatidylcholine ceramide together with total double-bond index changed when the plant was grown at 15 °C as a function of white light supplemented with far-red light. white light supplemented with far-red light increased the monogalactosyldiacylglycerol/diacylglycerol ratio as well. The gene expression changes are well correlated with the alterations in the lipidome.

Monoglucosyldiacylglycerol participates in phosphate stress adaptation in Synechococcus sp. PCC 7942.

Cyanobacterial monoglucosyldiacylglycerol (MGlcDG) not only serves as a precursor for monogalactosyldiacylglycerol (MGDG) synthesis, but also participates in stress acclimation. Two genes (mgdA and mgdE) related to MGDG synthesis of Synechococcus sp. PCC 7942 were identified. The mgdE-suppressed mutant (AE) accumulated MGlcDG (4.2%) and showed better growth and photosynthetic activities compared with WT and other mutants (mgdA/mgdE-overexpressed and mgdA-suppressed strains), which suggested that MGlcDG was involved in phosphate stress adaptation for Synechococcus sp. PCC 7942. A notable increase in contents of 18:1 fatty acid (FA) of MGDG (127%), DGDG (68%), and SQDG (105%) in AE were found under phosphate starvation. However, the expression of △9 desaturase (desC) was not higher in AE than that in WT during phosphate-starved period. These results suggested that MGlcDG might be involved in the process of FA desaturation, which contributed to membrane fluidity and cell basic metabolism for stress acclimation in cyanobacteria. In complementary experiments of E. coli, although the expression of mgdA and desC in the mgdA and desC coexpressed strain (OEAC) reduced by 22% and 35% compared with that of the strains only overexpressing mgdA (OEA) or desC (OEC), the content of unsaturated FA in OEAC was the highest. This further implied that the accumulation of MGlcDG could prompt FA desaturation in E. coli. Therefore, we propose that an overproduction of MGlcDG is responsible for FA desaturation and participates in phosphate stress adaptation in cyanobacteria.

In vitro digestion of galactolipids from chloroplast-rich fraction (CRF) of postharvest, pea vine field residue (haulm) and spinach leaves.

The removal of intact chloroplasts from their cell wall confinement offers a novel way to obtain lipophilic nutrients from green biomass, especially carotenoids and galactolipids. These latter are the main membrane lipids in plants and they represent a major source of the essential α-linolenic acid (18:3; ALA). Nevertheless, knowledge on their digestion is still limited. We have developed a physical method of recovering a chloroplast-rich fraction (CRF) from green biomass and tested its digestibility in vitro under simulated gastrointestinal conditions. Using a two-step static model, CRF from both spinach leaves and postharvest, pea vine field residue (haulm) were first exposed to enzymes from rabbit gastric extracts and then either to pancreatic enzymes from human pancreatic juice (HPJ) or to porcine pancreatic extracts (PPE). The lipolysis of monogalactosyldiacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG) was monitored by thin layer chromatography and gas chromatography of fatty acid methyl esters. For both CRF preparations, MGDG and DGDG were converted to monogalactosylmonoacylglycerol (MGMG) and digalactosylmonoacylglycerol (DGMG), respectively, during the intestinal phase and ALA was the main fatty acid released. Galactolipids were more effectively hydrolysed by HPJ than by PPE, and PPE showed a higher activity on MGDG than on DGDG. These findings may be explained by the higher levels of galactolipase activity in HPJ compared to PPE, which mainly results from pancreatic lipase-related protein 2. Thus, we showed that CRF galactolipids are well digested by pancreatic enzymes and represent an interesting vehicle for ALA supplementation in human diet.

Adaptation of Synechococcus sp. PCC 7942 to phosphate starvation by glycolipid accumulation and membrane lipid remodeling.

Organisms use various adaptive strategies against phosphate stress, including lipid remodeling. Here, the response of major membrane lipids to phosphate stress was analyzed in Synechococcus sp. PCC 7942. Unlike plants and eukaryotic microalgae, no significant increases in neutral lipids were found, whereas glycolipids content increased to as high as 6.13% (of dry cell weight, DCW) and phospholipids decreased to 0.34% (of DCW) after 16 days of cultivation without phosphate. Glycolipids accumulation were mainly attributed to the significant increase of digalactosyldiacylglycerol (DGDG) by 50% and sulfoquinovosyldiaclglycerol (SQDG) by 90%, both of which acted as complementary lipids for phosphatidylglycerol (PG) in the cyanobacterial membrane. Also, a notable increase in content (by 48%) of C18 fatty acids (especially C18:1) was observed in all glycolipids at the expense of C12 and C14 (72%). These changes may contribute to membrane fluidity and photosynthetic activity for basic cell metabolism and phosphate stress adaptation. Lipidomic analyses showed the reduction of PG 18:1/16: 0 (by 52%) with the increase of DGDG 18:1/16:0 (133%) and SQDG 18:1/16:0 (245%), strongly suggesting a direct conversion of PG to DGDG and SQDG. Moreover, the decreasing amount of monogalactosyldiacylglycerol (MGDG) 16:1/16:0 (22%) was consistent with the increase of free fatty acids (125%) on day 2 of phosphate absence, which suggested that MGDG is more likely to provide a pool of fatty acids for de novo synthesis of glycolipids. This study provides valuable insight into cyanobacteria adaptation strategies to phosphate stress by membrane lipid remodeling and unveils the underlying acyl chain fluxes into glycolipids.

FERONIA mutation induces high levels of chloroplast-localized Arabidopsides which are involved in root growth.

The FERONIA (FER) signaling pathway is known to have diverse roles in Arabidopsis thaliana, such as growth, reproduction, and defense, but how this receptor kinase is involved in various biological processes is not well established. In this work, we applied multiple mass spectrometry techniques to identify metabolites involved in the FER signaling pathway and to understand their biological roles. A direct infusion Fourier transform ion cyclotron resonance (FT-ICR)-MS approach was used for initial screening of wild-type and feronia (fer) mutant plant extracts, and Arabidopsides were found to be significantly enriched in the mutant. As Arabidopsides are known to be induced by wounding, further experiments on wounded and non-wounded leaf samples were carried out to investigate these oxylipins as well as related phytohormones using a quadrupole-time-of-flight (Q-TOF) MS by direct injection and LC-MS/MS. In a root growth bioassay with Arabidopside A isolated from fer mutants, the wild-type showed significant root growth inhibition compared with the fer mutant. Our results therefore implicated Arabidopsides, and Arabidopside A specifically, in FER functions and/or signaling. Finally, matrix-assisted laser desorption/ionization MS imaging (MALDI-MSI) was used to visualize the localization of Arabidopsides, and we confirmed that Arabidopsides are highly abundant at wounding sites in both wild-type and fer mutant leaves. More significantly, five micron high-spatial resolution MALDI-MSI revealed that Arabidopsides are localized to the chloroplasts where many stress signaling molecules are made.

Genetic enhancement of oil content in potato tuber (Solanum tuberosum L.) through an integrated metabolic engineering strategy.

Potato tuber is a high yielding food crop known for its high levels of starch accumulation but only negligible levels of triacylglycerol (TAG). In this study, we evaluated the potential for lipid production in potato tubers by simultaneously introducing three transgenes, including WRINKLED 1 (WRI1), DIACYLGLYCEROL ACYLTRANSFERASE 1 (DGAT1) and OLEOSIN under the transcriptional control of tuber-specific (patatin) and constitutive (CaMV-35S) promoters. This coordinated metabolic engineering approach resulted in over a 100-fold increase in TAG accumulation to levels up to 3.3% of tuber dry weight (DW). Phospholipids and galactolipids were also found to be significantly increased in the potato tuber. The increase of lipids in these transgenic tubers was accompanied by a significant reduction in starch content and an increase in soluble sugars. Microscopic examination revealed that starch granules in the transgenic tubers had more irregular shapes and surface indentations when compared with the relatively smooth surfaces of wild-type starch granules. Ultrastructural examination of lipid droplets showed their close proximity to endoplasmic reticulum and mitochondria, which may indicate a dynamic interaction with these organelles during the processes of lipid biosynthesis and turnover. Increases in lipid levels were also observed in the transgenic potato leaves, likely due to the constitutive expression of DGAT1 and incomplete tuber specificity of the patatin promoter. This study represents an important proof-of-concept demonstration of oil increase in tubers and provides a model system to further study carbon reallocation during development of nonphotosynthetic underground storage organs.

Membrane glycerolipidome of soybean root hairs and its response to nitrogen and phosphate availability.

Root hairs are tubular extensions of specific root epidermal cells important in plant nutrition and water absorption. To determine membrane glycerolipids in root hairs and roots may differ, as well as their respective response to nutrient availability, this study analyzed the membrane glycerolipid species in soybean root hairs and in roots stripped of root hairs, and their response to nitrogen (N) and phosphate (Pi) supplementation. The ratio of phospholipids to galactolipids was 1.5 fold higher in root hairs than in stripped roots. Under Pi deficiency, the ratio of phospholipids to galactolipids in stripped roots decreased with the greatest decrease found in the level of phosphatidylethanolamine (PE) in root hairs and stripped roots, and root hairs had an increased level of phosphatidic acid (PA). When seedlings were not supplied with N, the level of the N-containing lipids PE and phosphatidylserine in root hairs decreased whereas the level of non-N-containing lipids galactolipids and PA increased compared to N-supplied conditions. In stripped roots, the level of major membrane lipids was not different between N-sufficient and -deficient conditions. The results indicate that the membrane glycerolipidomes in root hairs are more responsive to nutrient availability than are the rest of roots.

Impact of UV-B on drought- or cadmium-induced changes in the fatty acid composition of membrane lipid fractions in wheat.

UV-B radiation may have either a positive or negative impact under the same conditions in wheat, depending on the type of secondary abiotic stressor: Cd or drought. Supplemental UV-B prevented the wilting and leaf rolling induced by PEG treatment. In contrast, combined UV-B and Cd treatment resulted in pronounced oxidative stress. The opposite effect of UV-B radiation in the case of drought or cadmium stress may be related to the alteration induced in the fatty acid composition. UV-B caused changes in the unsaturation of leaf phosphatidylglycerol fractions, and the accumulation of flavonoid in the leaves may prevent the stress induced by subsequent drought treatment. However it resulted in pronounced injury despite the increased flavonoid content in roots exposed to Cd. This was manifested in a drastic decrease in the unsaturation of the leaf monogalactosyldiacylglycerol and the root phosphatidylglycerol and digalactosyldiacylglycerol fractions. Data on the flavonoid content and fatty acid composition showed that oxidative stress was induced by drought in the leaves, by Cd in the roots, and interestingly, by UV-B radiation in both the leaves and roots. The additive effect of the combined stresses was also detected in the roots. The results presented here suggest a relationship between the capacity of the plant to remodel the fatty acid composition and its resistance to various stress factors.

Application of lipid extracts from Solidago canadensis to phytomonitoring of PCB126 in transgenic Arabidopsis plants.

The aim of this study is to elucidate the effect of lipid extracts from Solidago canadensis for phytomonitoring of polychlorinated biphenyl (PCB) 126 in the transgenic Arabidopsis plant XgD2V11-6 carrying the recombinant guinea pig (g) aryl hydrocarbon receptor (AhR)-mediated ß-glucuronidase (GUS) reporter gene expression system. A lipid extract was prepared from S. canadensis and separated into simple lipid, glycolipid, and phospholipid fractions by silica gel column chromatography. Sterylglucoside (SG), monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and glucosyl ceramide were found in the glycolipid fraction. When the transgenic Arabidopsis plants were treated with the glycolipid fraction together with PCB126, PCB126-induced GUS activity significantly increased in the whole plant. Moreover, S. canadensis-derived SG, MGDG, and DGDG also significantly increased PCB126-induced GUS activity. These results indicated that glycolipids in S. canadensis enhanced the sensitivity of this monitoring assay.

Dual role for phospholipid:diacylglycerol acyltransferase: enhancing fatty acid synthesis and diverting fatty acids from membrane lipids to triacylglycerol in Arabidopsis leaves.

There is growing interest in engineering green biomass to expand the production of plant oils as feed and biofuels. Here, we show that phospholipid:diacylglycerol acyltransferase1 (PDAT1) is a critical enzyme involved in triacylglycerol (TAG) synthesis in leaves. Overexpression of PDAT1 increases leaf TAG accumulation, leading to oil droplet overexpansion through fusion. Ectopic expression of oleosin promotes the clustering of small oil droplets. Coexpression of PDAT1 with oleosin boosts leaf TAG content by up to 6.4% of the dry weight without affecting membrane lipid composition and plant growth. PDAT1 overexpression stimulates fatty acid synthesis (FAS) and increases fatty acid flux toward the prokaryotic glycerolipid pathway. In the trigalactosyldiacylglycerol1-1 mutant, which is defective in eukaryotic thylakoid lipid synthesis, the combined overexpression of PDAT1 with oleosin increases leaf TAG content to 8.6% of the dry weight and total leaf lipid by fourfold. In the plastidic glycerol-3-phosphate acyltransferase1 mutant, which is defective in the prokaryotic glycerolipid pathway, PDAT1 overexpression enhances TAG content at the expense of thylakoid membrane lipids, leading to defects in chloroplast division and thylakoid biogenesis. Collectively, these results reveal a dual role for PDAT1 in enhancing fatty acid and TAG synthesis in leaves and suggest that increasing FAS is the key to engineering high levels of TAG accumulation in green biomass.

Proteaceae from severely phosphorus-impoverished soils extensively replace phospholipids with galactolipids and sulfolipids during leaf development to achieve a high photosynthetic phosphorus-use-efficiency.

Proteaceae species in south-western Australia occur on severely phosphorus (P)-impoverished soils. They have very low leaf P concentrations, but relatively fast rates of photosynthesis, thus exhibiting extremely high photosynthetic phosphorus-use-efficiency (PPUE). Although the mechanisms underpinning their high PPUE remain unknown, one possibility is that these species may be able to replace phospholipids with nonphospholipids during leaf development, without compromising photosynthesis. For six Proteaceae species, we measured soil and leaf P concentrations and rates of photosynthesis of both young expanding and mature leaves. We also assessed the investment in galactolipids, sulfolipids and phospholipids in young and mature leaves, and compared these results with those on Arabidopsis thaliana, grown under both P-sufficient and P-deficient conditions. In all Proteaceae species, phospholipid levels strongly decreased during leaf development, whereas those of galactolipids and sulfolipids strongly increased. Photosynthetic rates increased from young to mature leaves. This shows that these species extensively replace phospholipids with nonphospholipids during leaf development, without compromising photosynthesis. A considerably less pronounced shift was observed in A. thaliana. Our results clearly show that a low investment in phospholipids, relative to nonphospholipids, offers a partial explanation for a high photosynthetic rate per unit leaf P in Proteaceae adapted to P-impoverished soils.

Zeaxanthin epoxidation - an in vitro approach.

Zeaxanthin epoxidase (ZE) is an enzyme operating in the violaxanthin cycle, which is involved in photoprotective mechanisms. In this work model systems to study zeaxanthin (Zx) epoxidation were developed. Two assay systems are presented in which epoxidation of Zx was observed. In these assays two mutants of Arabidopsis thaliana which have active only one of the two xanthophyll cycle enzymes were used. The npq1 mutant possesses an active ZE and is thus able to convert Zx to violaxanthin (Vx) but the violaxanthin de-epoxidase (VDE) is inactive, so that Vx cannot be converted to Zx. The other mutant, npq2, possesses an active VDE and can convert exogenous Vx to Zx under strong light conditions but reverse reaction is not possible. The first assay containing thylakoids from npq1 and npq2 mutants of A. thaliana gave positive results and high efficiency of epoxidation reaction was observed. The amount of Zx was reduced by 25%. To optimize high efficiency of epoxidation reaction additional factors facilitating both fusion of the two types of thylakoids and incorporation of Zx to their membranes were also studied. The second kind of assay contained npq1 mutant thylakoids of A. thaliana supplemented with exogenous Zx and monogalactosyldiacylglycerol (MGDG). Experiments with different proportions of Zx and MGDG showed that their optimal ratio is 1:60. In such system, due to epoxidation, the amount of Zx was reduced by 38% of its initial level. The in vitro systems of Zx epoxidation described in this paper enable analysis some properties of the ZE without necessity of its isolation.

Galactolipids as potential health promoting compounds in vegetable foods.

Galactolipids are a class of compounds widely found in the plant kingdom, including edible plants, and are an important part of the cell membranes. Galactolipids in plants consists mainly of monogalactosyldiacylglycerols and digalactosyldiacylglycerols containing one or two saturated and/or unsaturated fatty acids linked to the glycerol moiety. Several galactolipids have been shown to possess in vitro and/or in vivo anti-tumor promoting activity and anti-inflammatory activity. Recently, it has been demonstrated that the galactolipid, 1,2-di-O-alpha-linolenoyl-3-O-beta-D-galactopyranosyl-sn-glycerol (1), may be important for the anti-inflammatory activity of dog rose (Rosa canina), a medicinal plant with documented effect on anti-inflammatory diseases such as arthritis. This galactolipid also occurs in relative high concentrations in certain legumes (e.g., common bean, pea), leaf vegetables (e.g., kale, leek, parsley, perilla and spinach), stem vegetables (e.g., asparagus, broccoli, brussels sprouts), and fruit vegetables (e.g., chilli, bell pepper, pumpkin). Furthermore, compound 1 has been isolated from spinach and several medicinal plants by bioassay-guided fractionation as a galactolipid with possible cancer preventive effects. In this review, the bioactivity of galactolipids is discussed and their potential role in human diet as important nutraceuticals. Moreover, recent patents on the bioactivity of specific galactolipids and inventions making use of this knowledge are presented and discussed.

cyclo-Oxylipin-galactolipids in plants: occurrence and dynamics.

cyclo-Oxylipin-galactolipids (cGL) are mono- or digalactosyldiglycerides carrying a cyclo-oxylipin in the sn1- and/or sn2-position or esterified to the galactose moiety. These compounds were recently identified in Arabidopsis thaliana. We provide evidence that cGL are mainly, if not exclusively, part of the thylakoid and can be hydrolysed by lipolytic activities associated with photosynthesis-related protein complexes in vitro. Using HPLC/ESI-mass spectrometry, cGL are shown to be restricted in occurrence to the genus Arabidopsis, they do not occur in other plants tested. A. thaliana cGL are rapidly and transiently formed upon wounding with characteristic changes in composition of the cGL-fraction. While the biological role of cGL is not understood, the genus Arabidopsis may present a model-case of chemical evolution of a novel class of regulatory molecules.

Quantitative profiling of Arabidopsis polar glycerolipids in response to phosphorus starvation. Roles of phospholipases D zeta1 and D zeta2 in phosphatidylcholine hydrolysis and digalactosyldiacylglycerol accumulation in phosphorus-starved plants.

Phosphorus is an essential macronutrient that often limits plant growth and development. Under phosphorus-limited conditions, plants undergo substantial alterations in membrane lipid composition to cope with phosphorus deficiency. To characterize the changes in lipid species and to identify enzymes involved in plant response to phosphorus starvation, 140 molecular species of polar glycerolipids were quantitatively profiled in rosettes and roots of wild-type Arabidopsis (Arabidopsis thaliana) and phospholipase D knockout mutants pld zeta1, pld zeta2, and pld zeta1 pld zeta2. In response to phosphorus starvation, the concentration of phospholipids was decreased and that of galactolipids was increased. Phospholipid lost in phosphorus-starved Arabidopsis rosettes was replaced by an equal amount of galactolipid. The concentration of phospholipid lost in roots was much greater than in rosettes. Disruption of both PLD zeta1 and PLD zeta2 function resulted in a smaller decrease in phosphatidylcholine and a smaller increase in digalactosyldiacylglycerol in phosphorus-starved roots. The results suggest that hydrolysis of phosphatidylcholine by PLD zetas during phosphorus starvation contributes to the supply of inorganic phosphorus for cell metabolism and diacylglycerol moieties for galactolipid synthesis.

Contribution of omega-3 fatty acid desaturase and 3-ketoacyl-ACP synthase II (KASII) genes in the modulation of glycerolipid fatty acid composition during cold acclimation in birch leaves.

Temperate and boreal tree species respond to low positive temperatures (LT) or a shortening of the photoperiod (SD) by inducing cold acclimation. One of the metabolic consequences of cold acclimation is an increase in fatty acid (FA) desaturation in membrane lipids, which allows functional membrane fluidity to be maintained at LT. The molecular mechanisms of FA desaturation were investigated in leaves of birch seedlings (Betula pendula) during cold acclimation. Four genes involved in FA biosynthesis were isolated: a 3-ketoacyl-ACP synthase II gene (BpKASII) involved in the elongation of palmitoyl-ACP to stearoyl-ACP, and three omega-3 FA desaturase genes (BpFAD3, BpFAD7, and BpFAD8) involved in the desaturation of linoleic acid (18:2) to alpha-linolenic acid (18:3). BpFAD7 was the main omega-3 FAD gene expressed in birch leaves, and it was down-regulated by LT under SD conditions. LT induced the expression of BpFAD3 and BpFAD8 and a synchronous increase in 18:3 occurred in glycerolipids. Changes in the photoperiod did not affect the LT-induced increase in 18:3 in chloroplast lipids (MGDG, DGDG, PG), but it modulated the LT response detected in extra-chloroplastic lipids (PC, PE, PI, PS). A decrease in the proportion of the 16-carbon FAs in lipids occurred at LT, possibly in relation to the regulation of BpKASII expression at LT. These results suggest that LT affects the whole FA biosynthesis pathway. They support a co-ordinated action of microsomal (BpFAD3) and chloroplast enzymes (BpFAD7, BpFAD8) in determining the level of 18:3 in extra-chloroplastic membranes, and they highlight the importance of dynamic lipid trafficking.

Mutation of the TGD1 chloroplast envelope protein affects phosphatidate metabolism in Arabidopsis.

Phosphatidate (PA) is a central metabolite of lipid metabolism and a signaling molecule in many eukaryotes, including plants. Mutations in a permease-like protein, TRIGALACTOSYLDIACYLGLYCEROL1 (TGD1), in Arabidopsis thaliana caused the accumulation of triacylglycerols, oligogalactolipids, and PA. Chloroplast lipids were altered in their fatty acid composition consistent with an impairment of lipid trafficking from the endoplasmic reticulum (ER) to the chloroplast and a disruption of thylakoid lipid biosynthesis from ER-derived precursors. The process mediated by TGD1 appears to be essential as mutation of the protein caused a high incidence of embryo abortion. Isolated tgd1 mutant chloroplasts showed a decreased ability to incorporate PA into galactolipids. The TGD1 protein was localized to the inner chloroplast envelope and appears to be a component of a lipid transporter. As even partial disruption of TGD1 function has drastic consequences on central lipid metabolism, the tgd1 mutant provides a tool to explore regulatory mechanisms governing lipid homeostasis and lipid trafficking in plants.