The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves

Gene silencing of stearoyl-ACP desaturase enhances the stearic acid content in Chlamydomonas reinhardtii

In this study, stearoyl-ACP desaturase (SAD), the enzyme that converts stearic acid into oleic acid, is silenced by artificial microRNA in the green microalga Chlamydomonas reinhardtii. Two different constructs, which target different positions on the mRNA of stearoyl-ACP desaturase, were tested. The mRNA levels for SAD were reduced after the silencing construct was induced. In one of the strains, the reduction in SAD mRNA resulted in a doubling of the stearic acid content in triacylglycerol molecules, which shows that stearic acid production in microalgae is possible.

Deciphering the roles of acyl-CoA-binding proteins in plant cells

Lipid trafficking is vital for metabolite exchange and signal communications between organelles and endomembranes. Acyl-CoA-binding proteins (ACBPs) are involved in the intracellular transport, protection, and pool formation of acyl-CoA esters, which are important intermediates and regulators in lipid metabolism and cellular signaling. In this review, we highlight recent advances in our understanding of plant ACBP families from a cellular and developmental perspective. Plant ACBPs have been extensively studied in Arabidopsis thaliana (a dicot) and to a lesser extent in Oryza sativa (a monocot). Thus far, they have been detected in the plasma membrane, vesicles, endoplasmic reticulum, Golgi apparatus, apoplast, cytosol, nuclear periphery, and peroxisomes. In combination with biochemical and molecular genetic tools, the widespread subcellular distribution of respective ACBP members has been explicitly linked to their functions in lipid metabolism during development and in response to stresses. At the cellular level, strong expression of specific ACBP homologs in specialized cells, such as embryos, stem epidermis, guard cells, male gametophytes, and phloem sap, is of relevance to their corresponding distinct roles in organ development and stress responses. Other interesting patterns in their subcellular localization and spatial expression that prompt new directions in future investigations are discussed.

Membranes as Structural Antioxidants: RECYCLING OF MALONDIALDEHYDE TO ITS SOURCE IN OXIDATION-SENSITIVE CHLOROPLAST FATTY ACIDS

Genetic evidence suggests that membranes rich in polyunsaturated fatty acids (PUFAs) act as supramolecular antioxidants that capture reactive oxygen species, thereby limiting damage to proteins. This process generates lipid fragmentation products including malondialdehyde (MDA), an archetypal marker of PUFA oxidation. We observed transient increases in levels of endogenous MDA in wounded Arabidopsis thaliana leaves, raising the possibility that MDA is metabolized. We developed a rigorous ion exchange method to purify enzymatically generated (13)C- and (14)C-MDA. Delivered as a volatile to intact plants, MDA was efficiently incorporated into lipids. Mass spectral and genetic analyses identified the major chloroplast galactolipid: α-linolenic acid (18:3)-7Z,10Z,13Z-hexadecatrienoic acid (16:3)-monogalactosyldiacylglycerol (18:3-16:3-MGDG) as an end-product of MDA incorporation. Consistent with this, the fad3-2 fad7-2 fad8 mutant that lacks tri-unsaturated fatty acids incorporated (14)C-MDA into 18:2-16:2-MGDG. Saponification of (14)C-labeled 18:3-16:3-MGDG revealed 84% of (14)C-label in the acyl groups with the remaining 16% in the head group. 18:3-16:3-MGDG is enriched proximal to photosystem II and is likely a major in vivo source of MDA in photosynthetic tissues. We propose that nonenzymatically generated lipid fragments such as MDA are recycled back into plastidic galactolipids that, in their role as cell protectants, can again be fragmented into MDA.

Acyl-CoA-Binding Proteins (ACBPs) in Plant Development

Acyl-CoA-binding proteins (ACBPs) play a pivotal role in fatty acid metabolism because they can transport medium- and long-chain acyl-CoA esters. In eukaryotic cells, ACBPs are involved in intracellular trafficking of acyl-CoA esters and formation of a cytosolic acyl-CoA pool. In addition to these ubiquitous functions, more specific non-redundant roles of plant ACBP subclasses are implicated by the existence of multigene families with variable molecular masses, ligand specificities, functional domains (e.g. protein-protein interaction domains), subcellular locations and gene expression patterns. In this chapter, recent progress in the characterization of ACBPs from the model dicot plant, Arabidopsis thaliana, and the model monocot, Oryza sativa, and their emerging roles in plant growth and development are discussed. The functional significance of respective members of the plant ACBP families in various developmental and physiological processes such as seed development and germination, stem cuticle formation, pollen development, leaf senescence, peroxisomal fatty acid β-oxidation and phloem-mediated lipid transport is highlighted.

Plant Cytosolic Acyl-CoA-Binding Proteins

A gene family encoding six members of acyl-CoA-binding proteins (ACBP) exists in Arabidopsis and they are designated as AtACBP1-AtACBP6. They have been observed to play pivotal roles in plant lipid metabolism, consistent to the abilities of recombinant AtACBP in binding different medium- and long-chain acyl-CoA esters in vitro. While AtACBP1 and AtACBP2 are membrane-associated proteins with ankyrin repeats and AtACBP3 contains a signaling peptide for targeting to the apoplast, AtACBP4, AtACBP5 and AtACBP6 represent the cytosolic forms in the AtACBP family. They were verified to be subcellularly localized in the cytosol using diverse experimental methods, including cell fractionation followed by western blot analysis, immunoelectron microscopy and confocal laser-scanning microscopy using autofluorescence-tagged fusions. AtACBP4 (73.2 kDa) and AtACBP5 (70.1 kDa) are the largest, while AtACBP6 (10.4 kDa) is the smallest. Their binding affinities to oleoyl-CoA esters suggested that they can potentially transfer oleoyl-CoA esters from the plastids to the endoplasmic reticulum, facilitating the subsequent biosynthesis of non-plastidial membrane lipids in Arabidopsis. Recent studies on ACBP, extended from a dicot (Arabidopsis) to a monocot, revealed that six ACBP are also encoded in rice (Oryza sativa). Interestingly, three small rice ACBP (OsACBP1, OsACBP2 and OsACBP3) are present in the cytosol in comparison to one (AtACBP6) in Arabidopsis. In this review, the combinatory and distinct roles of the cytosolic AtACBP are discussed, including their functions in pollen and seed development, light-dependent regulation and substrate affinities to acyl-CoA esters.

Tracking the metabolic pulse of plant lipid production with isotopic labeling and flux analyses: Past, present and future

Metabolism is comprised of networks of chemical transformations, organized into integrated biochemical pathways that are the basis of cellular operation, and function to sustain life. Metabolism, and thus life, is not static. The rate of metabolites transitioning through biochemical pathways (i.e., flux) determines cellular phenotypes, and is constantly changing in response to genetic or environmental perturbations. Each change evokes a response in metabolic pathway flow, and the quantification of fluxes under varied conditions helps to elucidate major and minor routes, and regulatory aspects of metabolism. To measure fluxes requires experimental methods that assess the movements and transformations of metabolites without creating artifacts. Isotopic labeling fills this role and is a long-standing experimental approach to identify pathways and quantify their metabolic relevance in different tissues or under different conditions. The application of labeling techniques to plant science is however far from reaching it potential. In light of advances in genetics and molecular biology that provide a means to alter metabolism, and given recent improvements in instrumentation, computational tools and available isotopes, the use of isotopic labeling to probe metabolism is becoming more and more powerful. We review the principal analytical methods for isotopic labeling with a focus on seminal studies of pathways and fluxes in lipid metabolism and carbon partitioning through central metabolism. Central carbon metabolic steps are directly linked to lipid production by serving to generate the precursors for fatty acid biosynthesis and lipid assembly. Additionally some of the ideas for labeling techniques that may be most applicable for lipid metabolism in the future were originally developed to investigate other aspects of central metabolism. We conclude by describing recent advances that will play an important future role in quantifying flux and metabolic operation in plant tissues.

Age-dependent variation in membrane lipid synthesis in leaves of garden pea (Pisum sativum L.)

To study membrane lipid synthesis during the life-span of a dicotyledon leaf, the second oldest leaf of 10-40-d-old plants of garden pea (Pisum sativum L.) was labelled with [1-(14)C]acetate and the distribution of radioactivity between the major membrane lipids was followed for 3 d. In the expanding second oldest leaf of 10-d-old plants, acetate was primarily allocated into phosphatidylcholine (PC) during the first 4 h of labelling. During the following 3 d, labelling of PC decreased and monogalactosyldiacylglycerol (MGDG) became the most radioactive lipid. In the fully expanded second oldest leaf of older plants, acetate was predominantly allocated into phosphatidylglycerol (PG), which remained the major radiolabelled lipid during the 3 d studied. The proportion of radioactivity recovered in MGDG decreased with increasing plant age up to 20 d, suggesting that, in expanded leaves, MGDG is more stable and requires renewal to a lower extent than PG. When the second oldest leaf approached senescence, labelling of MGDG again increased, indicating an increased need for thylakoid repair. The proportion of acetate allocated into phosphatidylethanolamine and free sterols was largest in leaves of 18-26-d-old plants and in the youngest leaves, respectively. Thus, these results demonstrate that the distribution of newly synthesized fatty acids between acyl lipid synthesis in the chloroplast and extraplastidial membranes strongly varies with leaf age, as do the proportion utilized for sterol synthesis. The findings emphasize the importance of defining the developmental stage of the leaf material used when performing studies on leaf lipid metabolism.

Testing models of fatty acid transfer and lipid synthesis in spinach leaf using in vivo oxygen-18 labeling

Oxygen-18 labeling has been applied to the study of plant lipid biosynthesis for the first time. [(13)C(2)(18)O(2)]Acetate was incubated with spinach (Spinacia oleracea) leaves and the (18)O content in fatty acid methyl esters isolated from different lipid classes measured by gas chromatography-mass spectometry. Fatty acids isolated from lipids synthesized within the plastid, such as monogalactosyldiacylglycerol, show an (18)O content consistent with the exogenous acetate undergoing a single activation step and with the direct utilization of acyl-acyl carrier protein by the acyl transferases of the chloroplast. In contrast, fatty acids isolated from lipids assembled in the cytosol, such as phosphatidylcholine, show a 50% reduction in the (18)O content. This is indicative of export of the fatty acyl groups from the plastid via a free carboxylate anion, and is consistent with the acyl-acyl carrier protein thioesterase:acyl-coenzyme A (CoA) synthetase mediated export mechanism. If this were not the case and the acyl group was transferred directly from acyl-acyl carrier protein to an acyl acceptor on the cytosolic side, there would be either complete retention of (18)O or, less likely, complete loss of (18)O, but not a 50% loss of (18)O. Thus, existing models for fatty acid transfer from the plastid and for spatially separate synthesis of "prokaryotic" and "eukaryotic" lipids have both been confirmed.

A Mutant of Arabidopsis with Increased Levels of Stearic Acid

A mutation at the fab2 locus of Arabidopsis caused increased levels of stearate in leaves. The increase in leaf stearate in fab2 varied developmentally, and the largest increase occurred in young leaves, where stearate accounted for almost 20% of total leaf fatty acids. The fatty acid composition of leaf lipids isolated from the fab2 mutant showed increased stearate in all the major glycerolipids of both the chloroplast and extrachloroplast membranes. Although the stearate content was increased, the fab2 mutant still contained abundant amounts of 18:1, 18:2, and 18:3 fatty acids. These results are consistent with the expectations for a mutation partially affecting the action of the stromal stearoyl-acyl carrier protein desaturase. Positional analysis indicated that the extra 18:0 is excluded with high specificity from the sn-2 position of both chloroplast and extrachloroplast glycerolipids. Although stearate content was increased in all the major leaf membrane lipids, the amount of increase varied considerably among the different lipids, from a high of 25% of fatty acids in phosphatidylcholine to a low of 2.9% of fatty acids in monogalactosyldiacylglycerol.

Pathway for the synthesis of triacylglycerols from monogalactosyldiacylglycerols in ozone-fumigated spinach leaves

When the upper leaf surface of spinach (Spinacia oleracea L.) plants was treated with [1-(14)C]acetate and grown for 2 days, (14)C was effectively incorporated into acyl moieties of leaf lipids in ratios approximately their composition by mass. Fumigation of the plants with ozone (0.5 microliter per liter) caused a redistribution of (14)C among lipid classes, i.e. a marked increase of (14)C content in triacylglycerol (TG) and 1,2-diacylglycerol (1,2-DG) and a decrease of label in monogalactosyldiacylglycerol (MGDG) without affecting (14)C distribution in leaf fatty acids. Label in both TG and 1,2-DG was found predominantly in their polyene molecular species. Since MGDG consists of similar polyene molecular species, the results indicate the synthesis of TG from MGDG via 1,2-DG. Label was also accumulated in tri- and tetragalactosyldiacylglycerol, products of galactolipid:galactolipid galactosyltransferase (GGGT). Moreover, there was a close relation between increases in the amounts of TG and the oligogalactolipids in ozonetreated leaves. These results indicate that MGDG was converted to 1,2-DG by GGGT and then to TG. In intact chloroplasts isolated from ozone-treated leaves, there was an enhanced production of free fatty acid (FFA), which was diminished by the addition of coenzyme A (CoA) and ATP, indicating that ozone stimulated the hydrolysis of MGDG to liberate FFA, which was in turn converted to acyl-CoA. The final step of TG synthesis, acylation of 1,2-DG with acyl-CoA, was confirmed by feeding with [1-(14)C]linolenic acid in leaf discs excised from ozone-fumigated leaves; (14)C was effectively incorporated into TG but not into 1,2-DG. These results demonstrate the synthesis of TG from 1,2-DG and FFA which were liberated from MGDG in ozone-fumigated spinach leaves.

Spinach Leaves Desaturate Exogenous [C]Palmitate to Hexadecatrienoate : Evidence that de Novo Glycerolipid Synthesis in Chloroplasts Can Utilize Free Fatty Acids Imported from Other Cellular Compartments

Long-chain (14)C-fatty acids applied to the surface of expanding spinach leaves were incorporated into all major lipid classes. When applied in diethyleneglycol monomethyl ether solution, as done by previous workers, [(14)C]palmitic acid uptake was much lower than that of [(14)C] oleic acid. However, when applied in a thin film of liquid paraffin the rate of [(14)C] palmitic acid metabolism was rapid and virtually complete. Considerable radioactivity from [(14)C]palmitate incorporated into lipids following either application method gradually appeared in polyunsaturated C(16) fatty acids esterified to those molecular species of galactolipids previously thought to be made using only fatty acids synthesized and retained within the chloroplast. Evidence for the incorporation of radioactivity from exogenous [(14)C]oleate into those same molecular species of galactolipids was less compelling. The unexpected availability of fatty acids bound to extrachloroplastidal lipids for incorporation into galactolipids characteristically assembled entirely within the chloroplast emphasizes the need to reassess interrelations between the "prokaryotic" and "eukaryotic" pathways of galactolipid formation.

Fluxes through the prokaryotic and eukaryotic pathways of lipid synthesis in the '16:3' plant Arabidopsis thaliana

The kinetics of [1-14C]acetate incorporation in Arabidopsis thaliana L. (Heyn) showed almost equal labelling of phosphatidylcholine (PC) and diacylgalactosylglycerol (DGG) at early times and the transfer of radioactivity from PC to DGG and diacyldigalactosylglycerol (DDG) at longer times. These kinetics demonstrated the parallel operation of the prokaryotic and eukaryotic pathways of lipid synthesis [Roughan & Slack (1982) Annu. Rev. Plant Physiol. 33, 97-132] in this tissue. At 2 h after the application of [1-14C]acetate, more than 85% of the radioactivity at the sn-2 position of each chloroplast lipid was in 16-carbon fatty acids. However, after 60 h, molecular species containing labelled C18 fatty acids at position sn-2 and presumably derived from microsomal PC made a large contribution (20-70%) to each chloroplast lipid except phosphatidylglycerol. These findings are consistent with the contention that the chain length of the fatty acid at the sn-2 position of glycerol is an accurate predictor of whether a particular lipid molecule has been synthesized by the prokaryotic or eukaryotic pathway. At 30 min after the start of [1-14C]acetate labelling, only 12.3% of the radioactivity in PC was in saturated fatty acids, but the proportion increased steadily to 24.3% after 142 h. It is suggested that steps involved in the conversion of PC to chloroplast lipids on the eukaryotic pathway discriminate against palmitate-containing species. The step involved does not appear to be transfer of PC to the chloroplast because extrachloroplastic and chloroplast membranes purified from Arabidopsis mesophyll protoplasts each contained PC with a fatty acid composition similar to that of the same lipid from leaves. Positional analysis of unlabelled lipids, together with the information summarized above, is used to construct a quantitative scheme of the fluxes through the prokaryotic and eukaryotic pathways during lipid synthesis in Arabidopsis. This scheme shows that 38% of the fatty acids synthesized de novo in the chloroplast enter the prokaryotic pathway in the chloroplast envelope. Of the 62% which are exported as acyl-CoA species to enter the eukaryotic pathway, 56% (34% of the total) are returned to complete synthesis of the chloroplast's complement of glycerolipids.

Regulation of lipogenesis and of non-saponifiable-lipid synthesis in vivo at birth and after prolonged starvation in the newborn rat

The rate of lipogenesis in the liver was increased by glucose injection at birth, mediated by the insulin secretion. In addition, glucagon decreased the rates of lipogenesis and non-saponifiable-lipid synthesis after birth. These rates decreased after prolonged starvation in the newborn rat. Tri-iodothyronine injection increased the rates of lipogenesis enhanced in response to glucose administration after prolonged starvation in liver and brown adipose tissue. Dexamethasone, however, increased the rates of lipogenesis enhanced in response to glucose in liver and prevented the increase in the rates of lipogenesis in brown adipose tissue.

Rates and products of long-chain Fatty Acid synthesis from [1-C]acetate in chloroplasts isolated from leaves of 16:3 and 18:3 plants

Chloroplasts highly active in the synthesis of long-chain fatty acids from [1-(14)C]acetate were prepared from leaves of Solanum nodiflorum, Chenopodium quinoa, Carthamus tinctorius, and Pisum sativum. These preparations were used to test whether the various additions to incubation media found to stimulate the synthesis of particular lipid classes in vitro by Spinacia oleracea chloroplasts were applicable generally. Chloroplasts from 18:3 plants incorporated a greater proportion of radioactivity into unesterified fatty acids under control conditions than did those from 16:3 plants. Supplying exogenous sn-glycerol 3-phosphate or Triton X-100 to chloroplasts increased the synthesis of glycerolipids in all cases and accentuated the capacity of chloroplasts from 18:3 plants to accumulate phosphatidic acid rather than the diacylglycerol accumulated by chloroplasts from 16:3 plants. The UDP-galactose-dependent synthesis of labeled diacylgalactosylglycerol was much less active in incubations of chloroplasts from 18:3 plants also containing sn-glycerol 3-phosphate and Triton X-100 compared with similar incubations from 16:3 plants. Exogenous CoA stimulated total fatty acid synthesis in all chloroplast preparations and the further addition of ATP diverted radioactivity from the unesterified fatty acid to acyl-CoA. The results have been discussed in terms of the two pathway hypothesis for lipid synthesis in leaves.

Similarities and differences in lipid metabolism of chloroplasts isolated from 18:3 and 16:3 plants

Photosynthetically active chloroplasts retaining high rates of fatty acid synthesis from [1-(14)C]acetate were purified from leaves of both 16:3 (Solanum nodiflorum, Chenopodium album) and 18:3 plants (Amaranthus lividus, Pisum sativum). A comparison of lipids into which newly synthesized fatty acids were incorporated revealed that, in 18:3 chloroplasts, enzymic activities catalyzing the conversion of phosphatidate to diacylglycerol and of diacylglycerol to monogalactosyl diacylglycerol (MGD) were significantly less active than in 16:3 chloroplasts. In contrast, labeling rates of MGD from UDP-[(14)C]gal were similar for both types of chloroplasts.The composition and positional distribution of labeled fatty acids within the glycerides synthesized by isolated 16:3 and 18:3 chloroplasts were similar and in each case only a C18/C16 diacylglycerol backbone was synthesized. In nodiflorum chloroplasts, C18:1/C16:0 MGD assembled de novo was completely desaturated to the C18:3/C16:3 stage.Whereas newly synthesized C18/C18 MGD could not be detected in any of these chloroplasts if incubated with [(14)C]acetate after isolation, chloroplasts isolated from acetate-labeled leaves contained MGD with labeled C18 fatty acids at both sn-1 and sn-2 positions. Taken together, these results provide further evidence on an organellar level for the operation of pro- and eucaryotic pathways in the biosynthesis of MGD in different groups of plants.

Relationship between fatty-acyl composition of diacylgalactosylglycerol and turnover of chloroplast phosphatidate

Chloroplasts from plants that contain different proportions of the 2-hexadecatrienoyl-1-linolenoyl molecular species in total diacylgalactosylglycerol were examined for their ability to dephosphorylate phosphatidate that had been labelled in situ with [1-14C]acetate. The turnover rate of chloroplast phosphatidate may be related to the accumulation of a prokaryotic diacylgalactosylglycerol, suggesting that chloroplast phosphatidate phosphatase controls the fatty acyl composition of the diacylgalactosylglycerol synthesized in the intact plant.

Manipulating the incorporation of [1-C]acetate into different leaf glycerolipids in several plant species

During short term labeling of expanding leaves of seven plant species with [1-(14)C]acetate, 35 to 64% of the label incorporated into lipids was found in phosphatidylcholine and 5 to 24% in phosphatidylglycerol. In pumpkin, sunflower, broad bean, and maize, only 4 to 12% of the label was found in diacylgalactosylglycerol, but in tomato, parsley, and spinach, the proportion was 17 to 31%. The latter group was further distinguished by having diacylgalactosylglycerol containing C16:3.The proportions of total incorporated [1-(14)C]acetate entering the lipids could be manipulated in a predictable manner. Phosphatidylcholine labeling was depressed by treating intact leaves with glycerol or ethylene glycol monomethyl ether or by incubating leaf discs in vitro. An associated increase in phosphatidylglycerol labeling occurred within the first group of plants, whereas an increase in labeling of either diacylgalactosylglycerol, phosphatidylglycerol, or sulfolipid occurred within the second group. Treating intact leaves with glycerol or incubating leaf discs in vitro was shown to elevate cellular concentrations of sn-glycerol 3-phosphate.These results have been interpreted in terms of the two-pathway hypothesis for glycerolipid biosynthesis, in which it is proposed that phosphatidylcholine is synthesized via a different pathway (eukaryotic) to that for synthesis of phosphatidylglycerol (prokaryotic). Both pathways may contribute toward the synthesis of diacylgalactosylglycerol, with the contribution of each being assessed from the proportion of hexadecatrienoic acid found in the particular plant.

Oleate desaturation in young winter wheat root tissue

[1,2-(14)C]Acetate was incorporated into the lipids of young wheat (Triticum aestivum L. cv Kharkov 22 MC) root tissue, but predominantly into sterols. [1-(14)C]Ammonium oleate was initially incorporated mainly into phosphatidylcholine (PC), and later into triglycerides (TGs). Diglycerides (DGs) contained 16% of the lipid (14)C after 5 minutes and 8% after 40 minutes. The proportion of the label of each lipid group incorporated into linoleate during an 80-minute incubation increased at similar rates for each group, and was always highest in PC. Radioactivity was detected in PC-linoleate earlier than in linoleate of the other groups. During a prolonged incubation after a 15-minute pulse labeling, the percentage of the lipid (14)C incorporated into PC and DGs was high at the end of the pulse but decreased later, while that in TGs increased to 64% after 4 hours. The proportion of the label of each group recovered in linoleic acid peaked in all groups after 4 hours, except for the TGs where it increased slowly throughout the experiment. Only traces of radioactivity were detected in linolenate. The data are compatible with a pathway in which oleate is incorporated into PC, is desaturated to linoleate on PC, and where the linoleate-enriched DGs are transferred from PC to TGs.

Effect of BASF 13-338, a Substituted Pyridazinone, on Lipid Metabolism in Leaf Tissue of Spinach, Pea, Linseed, and Wheat

A substituted pyridazinone (BASF 13-338) inhibited photosynthesis in spinach (Spinacia oleracea, Hybrid 102 Arthur Yates Ltd.) leaf discs and reduced the incorporation of [1-(14)C]acetate into trienoic acids of diacylgalactosylglycerol while causing radioactivity to accumulate in diacylgalac-tosylglycerol dienoic acids. Although BASF 13-338 inhibited photosynthesis in isolated spinach chloroplasts, it did not prevent dienoate desaturation. In discs, the labeling of fatty acids was affected by the inhibitor only in diacylgalactosylglycerol. Very little radioactivity was incorporated into trienes of phosphatidylcholine and the proportion of the label recovered in the fatty acids of phosphatidylcholine was not changed by BASF 13-338. The herbicides caused an increase in the proportion of the lipid (14)C incorporated into diacylgalactosylglycerol and a decrease in labeling of phosphatidylcholine, whereas the proportion of (14)C recovered in other lipids remained unchanged. Similar results were obtained with pea (Pisum sativum cv. Victory Freeze), linseed (Linum usitatissimum cv. Punjab), and wheat (Triticum aestivum cv. Karamu). With these species, a greater proportion of the label was incorporated into phosphatidylcholine and less into diacylgalactosylglycerol than with spinach. The data indicate that trienoate synthesis uses diacylgalactosylglycerol as substrate. BASF 13-338 appears to act at that step, and seems to cause in spinach a shift in polyenoate synthesis from the pathway involving microsomal phosphatidylcholine to the pathway operating inside the chloroplast.

Synthesis of acyl-CoAs by isolated spinach chloroplasts in relation to added CoA and ATP

The kinetics of incorporation of [2-(14)C] acetate into lipids and acyl-CoAs in relation to added CoA and ATP by isolated spinach chloroplasts have been examined. The effect of the concentration of these cofactors on lipid and acyl-CoA synthesis was also studied. In the absence of cofactors, or when only one was present, the incorporation was very low and went mainly into lipids. When both cofactors were present a strong stimulation of both activities occurred. After 25 min, acyl-CoAs were more strongly labeled than lipids and both activities continued linearly for at least 60 min.

Desaturation of Fatty acids associated with monogalactosyl diacylglycerol: the effects of san 6706 and san 9785

The effects of two substituted pyridazinone herbicides, San 6706 and San 9785, on photosynthesis, dark respiration, and fatty acid metabolism were studied in mature leaf tissue of Vicia faba.Both San 6706 and San 9785 inhibited photosynthesis within 2 hours after initial exposure of leaf tissue to the chemicals although San 9785 was more effective in inhibiting photosynthesis than San 6706. Neither San 6706 nor San 9785 had any marked effect on dark respiration.Kinetic studies using (14)CO(2) indicated that synthesis of 18:3 esterified to monogalactosyl diacylglycerol (MGDG) was not completely inhibited by San 9785 up to 48 hours after feeding. Radioactivity was observed to accumulate in MGDG (digalactosyl diacylglycerol [DGDG] and sulpholipid [SL]) 18:2 at the expense of 18:3 but the specific radioactivity of MGDG 18:3 continued to increase throughout the experiment indicating only partial inhibition of MGDG 18:3 synthesis. No significant differences were observed in the metabolism of other fatty acids. The metabolism of fatty acids from leaf tissue was not affected by treatment with San 6706.The data indicate that there are at least two sites for 18:2 desaturation to form 18:3, one associated with MGDG in the chloroplast, which is inhibited by San 9785, and one or more sites not inhibited by San 9785. The fatty acid specific radioactivity data support the hypothesis that there is vectorial transfer of fatty acids from phosphatidyl choline to MGDG to produce the large quantities of MGDG 18:3 found in higher plant tissue.

Photosynthesis of Lipids from CO(2) in Spinacia oleracea

Young expanding spinach leaves exposed to (14)CO(2) under physiological conditions for up to 20 minutes assimilated CO(2) into lipids at a mean rate of 7.6 micromoles per milligram chlorophyll per hour following a lag period of 5 minutes. Label entered into all parts of the lipid molecule and only 28% of the (14)C fixed into lipids was found in the fatty acid moieties, i.e. fatty acids were synthesized from CO(2)in vivo at a mean rate of 2.1 micromoles per milligram chlorophyll per hour. Intact spinach chloroplasts isolated from these leaves incorporated H(14)CO(3) into fatty acids at a maximal rate of 0.6 micromole per milligram chlorophyll per hour, but were unable to synthesize either the polar moieties of their lipids or polyunsaturated fatty acids. Since isolated chloroplasts will only synthesize fatty acids at rates similar to the one obtained with intact leaves in vivo if acetate is used as a precursor, it is suggested that acetate derived from leaf mitochondria is the physiological fatty acid precursor.

Light control of fatty acid synthesis and diurnal fluctuations of fatty acid composition in leaves

1. Although isolated spinach chloroplasts were almost entirely (greater than 99%) dependent on light for fatty acid synthesis, leaf discs were capable of fatty acid synthesis in the dark (up to 500nmol of 3H/h per mg of chlorophyll equivalent to approx. 400nmol of carbon/h per mg of chlorophyll), which represented 12-20% of the corresponding 'light rates'. 2. Net fatty acid accumulation by greening maize leaves occurred largely or entirely during the light period. 3. There was a diurnal fluctuation in the proportions of C18 unsaturated fatty acids in the lipids of developing spinach leaves, where an increase in the concentration of oleate during the day and a subsequent decline at night was observed; a complementary change occurred in the concentration of alpha-linolenate. The rhythm is interpreted as reflecting the continuation of oleate and linoleate desaturation at high rates when oleate synthesis is markedly decreased at night. 4. Changes in the fatty acid composition of 3-sn-phosphatidylcholine accounted for at least 60% of the total decrease in oleate over the dark period. This result is consistent with suggestions that this lipid is the substrate for the leaf microsomal oleate desaturase and an intermediate in leaf glycerolipid biosynthesis.

Fatty acid elongation by a particulate fraction from germinating pea

The synthesis of fatty acids from [14C]malonyl-CoA was studied with a high-speed particulate fraction from germinating pea (Pisum sativum). The variety used (Feltham First) produced mainly saturated fatty acids with palmitate (30--40%) and stearate (40--60%) predominating. Several palmitate-containing lipids stimulated overall synthesis and, in addition, increased the percentage of label in stearate. The production of stearate was severely inhibited by preincubation of the microsomal fraction with snake venom phospholipase A2 or by incubation with Rhizopus arrhizus lipase. Addition of a series of di-saturated phosphatidylcholines, with different acyl constituents, resulted in stimulation of overall fatty acid synthesis as well as an increase in the radiolabelling of the fatty acid two carbon atoms longer than the acyl chain added. This chain lengthening of fatty acids donated from phosphatidylcholine was due to the action of both fatty acid synthetase and palmitate elongase. The latter would utilize dipalmitoyl phosphatidylcholine and was sensitive to arsenite whereas fatty acid synthetase would use dilauroyl phosphatidylcholine and was sensitive to cerulenin. The results are discussed in relation to previous data obtained in vivo on plant fatty acid synthesis and current suggestions for the role of phosphatidylcholine in this process.

In Vivo Pathway of Oleate and Linoleate Desaturation in Developing Cotyledons of Cucumis sativus L. Seedlings

Exogenous [1-(14)C]oleic acid and [1-(14)C]linoleic acid were taken up and esterified to complex lipids by greening cucumber (Cucumis sativus L.) cotyledons. Both (14)C-labeled fatty acids were initially esterified to phosphatidylcholine prior to eventual accumulation in triacylglycerols and galactolipids. Kinetic data suggest that esterification occurs prior to desaturation and that phosphatidylcholine is the initial site of both [(14)C]-oleate and [1-(14)C]linoleate esterification and of [1-(14)C]oleate desaturation to [1-(14)C]linoleate. [1-(14)C]Linoleic acid was esterified more rapidly than [(14)C]oleic acid and its desaturation product, [1-(14)C]alpha-linolenate, occurred mainly on monogalactosyl diacylglycerol, although some was also observed on the other major acyl lipids, including phosphatidylcholine.

Some studies on the composition and surface properties of oil bodies from the seed cotyledons of safflower (Carthamus tinctorius) and linseed (Linum ustatissimum)

1. The average oil-body diameter in intact cells of developing linseed (Linum usitatissimum) and safflower (Carthamus tinctorius) cotyledons was similar (about 1.4 micrometer), and there was little change in size after oil bodies were isolated and repeatedly washed. 2. The glycerolipid composition of washed oil bodies from both developing and mature cotyledons of the two species was similar; oil bodies from ten different batches of cotyledons contained 4.3 +/- 0.16 mumol of 3-sn-phosphatidylcholine and 25.2 +/- 1.7 mumol of diacylglycerol per 1000 mumol of triacylglycerol. During four successive washings of a once-washed oil-body preparation, the proportion of diacylglycerol to triacylglycerol remained constant and that of 3-sn-phosphatidylcholine to triacylglycerol decreased by only 20%. 3. The protein content of thrice-washed oil bodies from the two species was similar, about 2.4% of the weight of glycerolipids, and appeared to be independent of the stage of cotyledon maturity. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis indicated that the protein of purified oil bodies from the two species consisted mainly of only four polypeptides and that two of the polypeptides from each species had apparent mol.wts. of 17500 and 15500. Similar patterns of polypeptides were obtained after the hydrolysis of the 15500-mol.wt. polypeptides from linseed and safflower oil bodies by Staphylococcus aureus V8 proteinase, whereas the proteolysis of the 17500-mol.wt. polypeptides from the two species produced different patterns of polypeptides. 4. The 3-sn-phosphatidylcholine in oil-body preparations was hydrolysed about 85% by bee-venom phospholipase A2 without any apparent coalescence of the oil bodies. Incubation with lipase from Rhizopus arrhizus caused rapid coalescence of the oil bodies, and this lipase appeared to initially hydrolyse diacylglycerols in preference to triacylglycerol. 5. Oil bodies from both species were almost completely dispersed in suspensions of pH between 7.1 and 8.3, but formed large aggregates at pH values between 6.7 and 3.9; pH-induced aggregation caused no coalescence. Aggregates formed under acidic conditions were dispersed by re-adjusting the pH of suspensions to 8.3. 6. A freeze-etch electron-microscopic examination of isolated oil bodies indicated that these organelles were bounded by some form of membrane with a particle-free outer surface.

Galactolipid synthesis in Vicia faba leaves. V. Redistribution of 14C-labelling in the polar moieties and the 14C-labelling kinetics of the fatty acids of the molecular species of mono- and digalactosyl diacylglycerols

The redistribution of 14C in the galactosyl glycerol and digalactosyl glycerol and the labelling kinetics of the fatty acids of mono- and digalactosyl diacylglycerol molecular species were examined over a period of 24 h following 14CO2-feeding of Vicia faba leaves. The results indicate that monogalactosyl diacylglycerol is formed from highly unsaturated diacylglycerols and that further desaturation of its C18 fatty acids occurs after formation of this galactolipid. There appear to be at least two pools of monogalactosyl diacylglycerol, differing in the degree of unsaturation of the fatty acids, from which digalactosyl diacyglycerol is formed. Digalactosyl diacylglycerol fatty acids did not appear to undergo further desaturation. A model for galactolipid biosynthesis and fatty acid desaturation is presented. There appear to be two distinct groups of diacyglycerol precursors, one consisting of C18/C18 molecular species derived from phosphatidylcholine, the other of C16/C18 molecular species derived from phosphatidylglycerol. The rapid turnover of monogalactosyl diacylglycerol to digalactosyl diacylglycerol from the C16/C18 group leads to one of the major differences in fatty acid composition between the two galactolipids observed in most plants.

Desaturation of Oleic and Linoleic Acids by Leaves of Dark- and Light-grown Maize Seedlings

Oleate and linoleate desaturation in leaves of maize seedlings was largely independent of previous light treatment of the seedlings; there was no evidence of light-induced desaturase activities. These results are in sharp contrast to those observed with developing cucumber cotyledons in which pronounced increase in desaturation occurs after exposure of tissue to light. The rates of desaturation of oleate were about four times those of linoleate in both etiolated and 16-hour greened maize leaves. In both etiolated and greened tissues, about two-thirds of the label from oleate was esterified after 4 hours, half of which was in phosphatidylcholine. Phosphatidylcholine and diglyceride contained large proportions of [(14)C]linoleate formed from [(14)C]oleate but not [(14)C]linolenate. In monogalactolipid, about two-thirds of the labeled fatty acids were linolenate. In vivo desaturase activity was present in tissue of widely different levels of differentiation and chlorophyll content obtained from light-grown maize seedlings.

The role of chloroplasts and microsomal fractions in polar-lipid synthesis from [1-14C]acetate by cell-free preparations from spinach (Spinacia oleracea) leaves

1. Isolated spinach (Spinacia oleracea) chloroplasts were incapable of accumulating polar lipids when incubated with [1-14C]acetate in a cofactor-free medium. When CoA, ATP and glycerol 3-phosphate were added to incubation media, the accumulated products were non-esterified fatty acids, acyl-CoA and 1,2-diacylglycerol, all intermediates of lipid metabolism. 2. Chloroplast acyl-CoA was used to synthesize phosphatidylcholine only when a microsomal fraction was added back to the incubation medium. 3. The 1,2-diacylglycerol synthesized by isolated chloroplasts was converted almost quantitatively into diacylgalactosylglycerol when exogenous UDP-galactose was available. 4. Stereospecific analyses of the isolated lipids suggested that the diacylglycerol synthesized by isolated chloroplasts may be an important precursor for the synthesis in vivo of diacylgalactosylglycerol and phosphatidylglycerol but was unlikely to be a precursor of phosphatidylcholine. 5. A scheme for plant-lipid biosynthesis is presented that integrates the functions of chloroplasts, the cytoplasm and the endoplasmic reticulum.

Linoleate and alpha-linolenate synthesis by isolated spinach (Spinacia oleracea) chloroplasts

Diacylgalactosylglycerol synthesis was a prerequisite for the incorporation of [1-14C]-acetate into linoleate and alpha-linolenate of isolated spinach (Spinacia oleracea) chloroplasts. Oleate at position 1 of diacylgalactosylglycerol was desaturated to linoleate and alpha-linolenate both in the light and in the dark. Some desaturation of palmitate was also observed after prolonged incubations.

On the control of long-chain-fatty acid synthesis in isolated intact spinach (Spinacia oleracea) chloroplasts

1. Chloroplasts isolated from spinach leaves by using the low-ionic-strength buffers of Nakatani & Barber [(1977) Biochim. Biophys. Acta.461, 510-512] had higher rates of HCO(3) (-)-dependent oxygen evolution (up to 369mumol/h per mg of chlorophyll) and higher rates of [1-(14)C]acetate incorporation into long-chain fatty acids (up to 1500nmol/h per mg of chlorophyll) than chloroplasts isolated by using alternative procedures. 2. Acetate appeared to be the preferred substrate for fatty acid synthesis by isolated chloroplasts, although high rates of synthesis were also measured from H(14)CO(3) (-) in assays permitting high rats of photosynthesis. Incorporation of H(14)CO(3) (-) into fatty acids was decreased by relatively low concentrations of unlabelled acetate. Acetyl-CoA synthetase activity was present 3-4 times in excess of that required to account for rates of [1-(14)C]acetate incorporation into fatty acids, but pyruvate dehydrogenase was either absent or present in very low activity in spinach chloroplasts. 3. Rates of long-chain-fatty acid synthesis from [1-(14)C]acetate in the highly active chloroplast preparations, compared with those used previously, were less dependent on added cofactors, but showed a greater response to light. The effects of added CoA plus ATP, Triton X-100 and sn-glycerol 3-phosphate on the products of [1-(14)C]acetate incorporation were similar to those reported for less active chloroplast preparations. 4. Endogenous [(14)C]acetyl-CoA plus [(14)C]malonyl-CoA was maintained at a constant low level even when fatty acid synthesis was limited by low HCO(3) (-) concentrations. Endogenous [(14)C]acyl-(acyl-carrier protein) concentrations increased with increasing HCO(3) (-) concentration and higher rates of fatty acid synthesis, but were slightly lower in the presence of Triton X-100. It is proposed that rates of long-chain-fatty acid synthesis in isolated chloroplasts at saturating [1-(14)C]acetate concentrations and optimal HCO(3) (-) concentrations may be primarily controlled by rates of removal of the products of the fatty acid synthetase.