Synthesis Of Linoleic Acid Research Articles

Differential Contribution of Endoplasmic Reticulum and Chloroplast ω-3 Fatty Acid Desaturase Genes to the Linolenic Acid Content of Olive (Olea europaea) Fruit.

Linolenic acid is a polyunsaturated fatty acid present in plant lipids, which plays key roles in plant metabolism as a structural component of storage and membrane lipids, and as a precursor of signaling molecules. The synthesis of linolenic acid is catalyzed by two different ω-3 fatty acid desaturases, which correspond to microsomal- (FAD3) and chloroplast- (FAD7 and FAD8) localized enzymes. We have investigated the specific contribution of each enzyme to the linolenic acid content in olive fruit. With that aim, we isolated two different cDNA clones encoding two ω-3 fatty acid desaturases from olive (Olea europaea cv. Picual). Sequence analysis indicates that they code for microsomal (OepFAD3B) and chloroplast (OepFAD7-2) ω-3 fatty acid desaturase enzymes, different from the previously characterized OekFAD3A and OekFAD7-1 genes. Functional expression in yeast of the corresponding OepFAD3A and OepFAD3B cDNAs confirmed that they encode microsomal ω-3 fatty acid desaturases. The linolenic acid content and transcript levels of olive FAD3 and FAD7 genes were measured in different tissues of Picual and Arbequina cultivars, including mesocarp and seed during development and ripening of olive fruit. Gene expression and lipid analysis indicate that FAD3A is the gene mainly responsible for the linolenic acid present in the seed, while FAD7-1 and FAD7-2 contribute mostly to the linolenic acid present in the mesocarp and, therefore, in the olive oil. These results also indicate the relevance of lipid trafficking between the endoplasmic reticulum and chloroplast in determining the linolenic acid content of membrane and storage lipids in oil-accumulating photosynthetic tissues.

Transcriptome Analysis Comparison of Lipid Biosynthesis in the Leaves and Developing Seeds of Brassica napus.

Brassica napus seed is a lipid storage organ containing approximately 40% oil, while its leaves contain many kinds of lipids for many biological roles, but the overall amounts are less than in seeds. Thus, lipid biosynthesis in the developing seeds and the leaves is strictly regulated which results the final difference of lipids. However, there are few reports about the molecular mechanism controlling the difference in lipid biosynthesis between developing seeds and leaves. In this study, we tried to uncover this mechanism by analyzing the transcriptome data for lipid biosynthesis. The transcriptome data were de novo assembled and a total of 47216 unigenes were obtained, which had an N50 length and median of 1271 and 755 bp, respectively. Among these unigenes, 36368 (about 77.02%) were annotated and there were 109 up-regulated unigenes and 72 down-regulated unigenes in the developing seeds lipid synthetic pathway after comparing with leaves. In the oleic acid pathway, 23 unigenes were up-regulated and four unigenes were down-regulated. During triacylglycerol (TAG) synthesis, the key unigenes were all up-regulated, such as phosphatidate phosphatase and diacylglycerol O-acyltransferase. During palmitic acid, palmitoleic acid, stearic acid, linoleic acid and linolenic acid synthesis in leaves, the unigenes were nearly all up-regulated, which indicated that the biosynthesis of these particular fatty acids were more important in leaves. In the developing seeds, almost all the unigenes in the ABI3VP1, RKD, CPP, E2F-DP, GRF, JUMONJI, MYB-related, PHD and REM transcript factorfamilies were up-regulated, which helped us to discern the regulation mechanism underlying lipid biosynthesis. The differential up/down-regulation of the genes and TFs involved in lipid biosynthesis in developing seeds and leaves provided direct evidence that allowed us to map the network that regulates lipid biosynthesis, and the identification of new TFs that are up-regulated in developing seeds will help us to further elucidate the lipids biosynthesis pathway in developing seeds and leaves.

Synthesis of linoleic acids combinatorially labeled at the vinylic positions as substrates for lipoxygenases

Mammalian lipoxygenases have been implicated in a number of inflammation-related human diseases. Soybean lipoxygenase-1 is the archetypical example of known lipoxygenases. Here we report the synthesis of linoleic acid and (11,11)-d2-linoleic acid which are combinatorially labeled at the vinylic positions (9, 10, 12, and 13). Combinatorial labeling schemes allow for the simultaneous determination of KIEs in enzymatic reactions using NMR. Substrates are, thus, available as probes of detailed mechanism in kinetic isotope effect (KIE) studies of lipoxygenases.

Genomic and biochemical analysis of lipid biosynthesis in the unicellular rhodophyte Cyanidioschyzon merolae: lack of a plastidic desaturation pathway results in the coupled pathway of galactolipid synthesis.

The acyl lipids making up the plastid membranes in plants and algae are highly enriched in polyunsaturated fatty acids and are synthesized by two distinct pathways, known as the prokaryotic and eukaryotic pathways, which are located within the plastids and the endoplasmic reticulum, respectively. Here we report the results of biochemical as well as genomic analyses of lipids and fatty acids in the unicellular rhodophyte Cyanidioschyzon merolae. All of the glycerolipids usually found in photosynthetic algae were found, such as mono- and digalactosyl diacylglycerol, sulfolipid, phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. However, the fatty acid composition was extremely simple. Only palmitic, stearic, oleic, and linoleic acids were found as major acids. In addition, 3-trans-hexadecanoic acid was found as a very minor component in phosphatidylglycerol. Unlike the case for most other photosynthetic eukaryotes, polyenoic fatty acids having three or more double bonds were not detected. These results suggest that polyunsaturated fatty acids are not necessary for photosynthesis in eukaryotes. Genomic analysis suggested that C. merolae lacks acyl lipid desaturases of cyanobacterial origin as well as stearoyl acyl carrier protein desaturase, both of which are major desaturases in plants and green algae. The results of labeling experiments with radioactive acetate showed that the desaturation leading to linoleic acid synthesis occurs on phosphatidylcholine located outside the plastids. Monogalactosyl diacylglycerol is therefore synthesized by the coupled pathway, using plastid-derived palmitic acid and endoplasmic reticulum-derived linoleic acid. These results highlight essential differences in lipid biosynthetic pathways between the red algae and the green lineage, which includes plants and green algae.

The predominance of polyunsaturated fatty acids in the butterfly Morpho peleides before and after metamorphosis

We hypothesized that the polyunsaturated fatty acids of the butterfly were probably derived from the diet and that there might be a great loss of body fat during metamorphosis. To substantiate these hypotheses, we analyzed the fatty acid composition and content of the diet, the larva, and the butterfly Morpho peleides. Both the diet and the tissues of the larva and butterfly had a high concentration of polyunsaturated fatty acids. In the diet, linolenic acid accounted for 19% and linoleic acid for 8% of total fatty acids. In the larva, almost 60% of the total fatty acids were polyunsaturated: linolenic acid predominated at 42% of total fatty acids, and linoleic acid was at 17%. In the butterfly, linolenic acid represented 36% and linoleic acid represented 11% of total fatty acids. The larva had a much higher total fatty acid content than the butterfly (20.2 vs. 6.9 mg). Our data indicate that the transformation from larva to butterfly during metamorphosis drastically decreased the total fatty acid content. There was bioenhancement of polyunsaturated fatty acids from the diet to the larva and butterfly. This polyunsaturation of membranes may have functional importance in providing membrane fluidity useful in flight.

Reduced levels of volatile emissions in jasmonate‐deficientspr2tomato mutants favour oviposition by insect herbivores

Tomato plants release volatile organic compounds (VOCs) following insect or mechanical damage. In this study, the constitutive and wound-induced emission levels of VOCs in suppressor of prosystemin-mediated responses2 (spr2) mutant plants, compromised in linolenic acid (LA) and jasmonic acid (JA) synthesis, and in 35S::prosystemin (35S::prosys) plants, having upregulated direct defence responses, were compared. The spr2 mutants produced constitutively lower levels of VOCs, which were nonetheless increased in response to (a)biotic damage, although at lower levels than wild-type (Wt) and 35S::prosys plants. No significant differences in VOC emissions were detected between the latter two genotypes, thereby suggesting that systemin does not regulate indirect defence responses, whereas differences in fatty acid composition in spr2 plants led to the predominant emission of saturated C6 volatiles in response to wounding. The expression of 1-deoxy-D-xylulose 5-phosphate synthase (DXS2), a key gene involved in VOC synthesis in the chloroplast, was only upregulated in Manduca sexta L.-damaged Wt and 35S::prosys plants. However, its expression was restored in spr2 plants by exogenous LA or JA, suggesting that abated VOC emissions in spr2 plants are correlated with lowered DXS2 expression. Bioassays with two different insects showed that adult females significantly preferred spr2 plants, indicating that lowered levels of VOCs in tomato influence plant selection by insects during oviposition.

Production of linolenic acid in yeast cells expressing an omega‐3 desaturase from tung (Aleurites fordii)

AbstractTung oil is an industrial drying oil containing ca. 90% PUFA. We previously reported on enzymes required for the synthesis of linoleic (6% of FA) and eleostearic (80%) acids and here describe the cloning and functional analysis of an omega‐3 FA desaturase (FAD3) required for the synthesis of linolenic acid (1%). The tung FAD3 cDNA was identified by screening a tung seed cDNA library using the polymerase chain reaction and degenerate primers encoding conserved regions of the FAD3 enzyme family. Expression of this cDNA in yeast cells, cultured in the presence of linoleic acid, resulted in the synthesis and accumulation of linolenic acid, which accounted for up to 18% w/w of total cellular FA. Tung FAD3 activity was significantly affected by cultivation temperature, with the greatest amount of linolenic acid accumulating in yeast cells grown at 15°C. The amount of linolenic acid synthesized in yeast cells by tung FAD3 is ca. 10‐fold higher than that observed by expression of a rapeseed (Brassica napus) FAD3 in yeast, suggesting that tung FAD3 might be useful for biotechnological production of omega‐3 FA in transgenic organisms.

Half-seed analysis for comparing linolenic acid synthesis between high and low oleic acid sunflower inbred lines

The modification of fatty acid composition of two sunflowers inbred lines HA89 and R978, low oleic acid (normal) and high oleic acid (mutant) respectively, in seeds and during the first stages of growth (A1-B2) was studied under controlled conditions. Enzymatic mechanisms have great effect on the catabolism of seed stored lipids. Temperature and oxygen regulation influence developing sunflower seeds. For simultaneous study of seed and developing seed, half-seed analysis technique was used. The behavior of the fatty acids during the germination in cotyledon of seed showed the increase of linolenic acid in both lines, demonstrating the activity of linoleic acid desaturase (Δ6-desaturase). But linoleic acid as a substrate for linoleate desaturase increased during all stages of developing only in mutant line that revealed higher activity of oleic acid desaturase (Δ12-desaturase) in transforming oleic acid to linoleic acid in this line, and lower activity of this enzyme in low oleic acid line, the reasons probably being the low availability of substrate of this enzyme in low oleic acid line and the complexity of enzymatic mechanisms. The modification of fatty acids in developing sunflower, depends not only on ambient conditions such as temperature and oxygen regulation as described by many authors, but also on the genotype.

Chilling-Sensitive, Post-Transcriptional Regulation of a Plant Fatty Acid Desaturase Expressed in Yeast

Plants respond to chilling exposure by increasing the relative proportion of polyunsaturated fatty acids in their lipids. However, unlike the response in many other organisms, plant fatty acid desaturase genes are typically not upregulated during this process. We expressed the Brassica napus FAD3 gene, which encodes an enzyme for synthesis of linolenic acid, in Saccharomyces cerevisiae and observed a temperature-dependent increase in linolenic acid production at cooler growth temperatures. Untransformed yeast cells, however, responded to cooler temperatures primarily by shortening fatty acid chains, even when polyunsaturated fatty acids were supplied in the growth media. Measurement of the steady-state levels of Fad3 protein in transformed yeast revealed an 8.5-fold increase in steady-state amount of desaturase enzyme when cells were cultivated at cooler temperatures. The increase was not due to changes in transcriptional activity, since Northern hybridization revealed no appreciable changes in abundance of FAD3 transcripts at cooler temperatures. Taken together, the results suggest that the increase in linolenic acid content in cells containing Fad3 was not due to enhanced physiological demand for polyunsaturated fatty acids by yeast, but rather a cold-inducible, post-transcriptional increase in steady-state amount of plant desaturase enzyme. Implications for plant adaptation to chilling are discussed.

Antisense-mediated depletion of potato leaf omega3 fatty acid desaturase lowers linolenic acid content and reduces gene activation in response to wounding.

Fatty acid omega3 desaturases act on membrane lipids to catalyse the formation of trienoic fatty acids, the most abundant in plant tissues being alpha-linolenic acid. This fatty acid is a precursor of jasmonic acid, a plant growth regulator involved in the control of wound-induced gene activation in plants and in the induction of tuberization in potato. We isolated a potato omega3 desaturase cDNA, possibly encoding a plastidial isoform, and used it to investigate its expression pattern throughout plant development and in response to wounding. Plastidial omega3 desaturase gene transcripts accumulate rapidly upon wounding, preceding the jasmonate-dependent induction of the wound-responsive proteinase inhibitor II gene. We generated transgenic potato plants constitutively expressing an antisense RNA to this plastidial omega3 desaturase. Selected transgenic lines in which the cognate omega3 desaturase mRNA is largely depleted show a marked reduction, of up to 60%, in trienoic acids in leaves and tubers. In these lines, a corresponding reduction in jasmonate content and proteinase inhibitor II expression is observed upon wounding. Our results indicate that a reduction in omega3 desaturase mRNA levels compromises the wound-induced activation of proteinase inhibitor II, suggesting that wound-induced synthesis of linolenic acid is required for jasmonic acid production. The antisense-mediated depletion of fatty acid omega3 desaturases is a viable alternative for reducing trienoic fatty acid content in plant species in which a mutant screening approach is not applicable.

Oil and fatty acid changes in Sinapis and Crambe seeds during germination and early development

Seeds of Sinapis alba L. and Crambe abyssinica L. contain oil, rich in erucic acid (50–60%), which is valuable for industrial purposes. During germination and early growth at 20°C changes in lipid content and fatty acid composition were followed for 15 days. Reduction in the oil content started only 8 days after the onset of germination and was more rapid in C. abyssinica. Oil reduction was accelerated by light and the content reached low levels of 6.7 and 13.7% in seeds of C. abyssinica and S. alba, respectively. No change in dry weight was observed during this period. In cotyledons of S. alba and C. abyssinica respectively, levels of 12 and 25% erucic acid were found after 15 days of development in darkness. Lower levels (4 and 18%) were found in cotyledons developed in the light. No more than 5% erucic acid was detected in the 15-day-old roots of both species. In cotyledons, the decline in erucic acid was accompanied by significant increases in linolenic acid (32% in S. alba and 27% in C. abyssinica) which was also stimulated by light (to 45 and 32%, respectively). No synthesis of linolenic acid was detected in the roots of either species. However, a novel synthesis of saturated palmitic and stearic acids was observed in these species, with a 10-fold increase in their contents. The present results provide an example of the selective utilization of the fatty acids in the reserve oil of seeds during germination and the subsequent growth of seedlings.

Synthesis of linoleic acid with chiral isotopic labelling at a flanking and a medial allylic methylene: the (8R,9Z,12Z)-[8-2H] and (11R,9Z,12Z)-[11-2H]-stereoisomers, and (Z)-[2,2-2H2]non-3-enal

[1-2H]Oct-2-ynal is converted by fermenting bakers' yeast into (1S)-[1-2H]oct-2-yn-1-ol with an enantiomeric purity of >96% as measured by Mosher's MTPA method. The alcohol, as its tosyl ester, was then converted by copper-catalysed coupling with the di-Grignard of dec-9-ynoic acid and catalytic semi-hydrogenation, into (11R)-[11-2H]linoleic acid having less than 2%E-material and >95 atom %D. Provision for (11S)-[11-2H]linoleic acid was made by configurational inversion of (1R)-[1-2H]oct-2-yn-1-ol using Mitsunobu chemistry.(8R)-[8-2H] Linoleic acid is made by a similar approach, the labelled chiral centre being formed on (8S)-8-hydroxy-[8-2H]octanoic acid (>96% ee). Reaction of the corresponding tosate with lithium acetylide–ethylenediamine complex gave, with configurational inversion, (8R)-[8-2H]dec-9-ynoic acid, built into (8R)-[8-2H]linoleic acid. A degradative circuit is applied to estimate the extent of configurational inversion in the displacement. Mitsunobu inversion of the (8S)-8-hydroxy compound provides access to (8S)-[8-2H] linoleic acid.(Z)-Non-3-enal, a labile aldehyde from the enzymic degradation of linoleic acid, is made in [2,2-2H2]-labelled form by a synthesis which uses, as a key step, the reaction between triheptynylborane and deuteriodiazoacetic ester.

ChemInform Abstract: Preparative, Enzymatic Synthesis of Linoleic Acid (13S)‐Hydroperoxide (I) Using Soybean Lipoxygenase‐1.

ChemInformVolume 21, Issue 31 Preparative Organic Chemistry ChemInform Abstract: Preparative, Enzymatic Synthesis of Linoleic Acid (13S)-Hydroperoxide (I) Using Soybean Lipoxygenase-1. G. IACAZIO, G. IACAZIO Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorG. LANGRAND, G. LANGRAND Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorJ. BARATTI, J. BARATTI Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorG. BUONO, G. BUONO Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorC. TRIANTAPHYLIDES, C. TRIANTAPHYLIDES Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this author G. IACAZIO, G. IACAZIO Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorG. LANGRAND, G. LANGRAND Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorJ. BARATTI, J. BARATTI Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorG. BUONO, G. BUONO Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this authorC. TRIANTAPHYLIDES, C. TRIANTAPHYLIDES Ecole Sup. Chim., CNRS, 13997 Marseille, FranceSearch for more papers by this author First published: July 31, 1990 https://doi.org/10.1002/chin.199031117AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue31July 31, 1990 RelatedInformation

Preparative, enzymic synthesis of linoleic acid (13S)-hydroperoxide using soybean lipoxygenase-1

Preparative, enzymic synthesis of linoleic acid (13S)-hydroperoxide using soybean lipoxygenase-1

Lipid biosynthesis in adult Acyrthosiphon pisum: Effect of age and symbiont population

AbstractThe incorporation of [1‐14C]acetate into total lipids in 1‐day‐old adult pea aphids is 3.3‐fold higher than in 20‐, 22‐, and 24‐day old adults. The polar lipid fraction was the main lipid class synthesized at any age and contained primarily eighteen carbon fatty acids. While the relative mass of 18:0, 18:1 and 18:2 decreased in older aphids, the relative amount of label incorporated into these fatty acids remained constant. Myristic acid was the main fatty acid of the triacylglycerol fraction, and the relative amount of radioactivity incorporated into this fatty acid decreased in older aphids. Twenty‐day‐old aphids had 60% fewer mycetocytes than did 1‐day‐old insects. We conclude that symbionts within the mycetocytes do not appear to be involved in the synthesis of linoleic acid, while their role in the synthesis of myristic acid is less clear.

Response of linseed (Linum usitatissimum L.) to fertilizer nitrogen, phosphorus and sulphur, and their effect on the removal of soil sulphur

Abstract. Between 1981 and 1987 field experiments were made on loam and sandy loam soils to test the effects of nitrogen, phosphorus and sulphur fertilizers on the yield and quality of linseed, and the uptake of S by the crop. In one experiment, all through the six years, the application of fertilizers up to 60 kg N, 40 kg P2O5 and 30 kg S ha‐1 increased yields. In the second experiment, the same combination of N, P and S maximized crop yield, uptake of P and S, % oil and oil production. These balanced and optimum rates of fertilizers decreased the percentage of stearic, oleic and linoleic acids but increased that of linolenic acid by accelerating the metabolic pathway of linolenic acid synthesis. Applying P decreased the removal of native S in the soil but when more S was applied, more native S was taken up. Large dressings of P diminished crop yield and quality as well as the availability of native soil S.

In vivo synthesis of [14C]Dilinoylgalactosylglycerol by gametophytes of the fern anemia phyllitidis

Blue light-grown gametophytes of Anemia phyllitidis were used for the in vivo synthesis of radioactively labeled dilinoylgalactosylglycerol, a potential intermediate in the synthesis of linolenic acid. Optimal yield was obtained by incubating gametophytes with [14C]linoleate first for 2 h under anaerobic conditions followed by 46 h under aerobic conditions. As product [14C]dilinoylgalactosylglycerol was identified

Genetic Regulation of Linolenic Acid Concentration in Rapeseed1

Rapeseed (Brassica napus L.) contains a relatively high level of linolenic acid. The flavor and nutritional quality of rapeseed oil could be improved by reducing linolenic acid content through plant breeding. Therefore, a study was conducted to show whether genetic regulation of linolenic acid concentration in two rapeseed mutants, M43 and M364, was determined by the genotype of the embryo, maternal influence, or cytoplasmic factors. Reciprocal crosses were made between the mutants. The reciprocal F1 plants and the mutants were mated in all possible two‐way combinations. The concentration of linolenic acid was measured in triacylglycerol and monogalactosyl diacylglycerol from F1 seed. These glycerolipids were synthesized by different pathways and accounted for 95% of the linolenic acid in the rapeseed mutants. It was found that the concentration of linolenic acid in triacylglycerol was determined by nuclear and cytoplasmic gene interaction, whereas linolenic acid in monogalactosyl diacylglyceroi was determined by cytoplasmic factors. Therefore, it appeared that at least two different genetic mechanisms governed linolenic acid synthesis in these seed. Although the control of linolenic acid synthesis may be complex, a more complete understanding of the genetics of this trait should aid the development of improved cultivars with low levels of linolenic acid.

Genetic control of polyunsaturated fatty acid biosynthesis in flax (Linum usitatissimum) seed oil

The inheritance of two mutants of flax (Linum usitatissimum), having altered proportions of the C18 polyunsaturated fatty acids, linoleic and linolenic, was examined. Both lines, 'M1589' and 'M1722', are homozygous for a single gene mutation which reduces linolenic acid content from 34% to 22% and raises linoleic acid from 15% to 27%. Genetic analysis of crosses involving 'M1589', 'M1722' and their parental cultivar 'Glenelg' revealed that these mutations are in different unlinked genes and exhibit additive (codominant) gene action. The symbolsLn1 andLn2 are proposed for the mutated genes in 'M1589' and 'M1722', respectively. Recombinant genotypes homozygous for the mutant alleles at both loci are very low in linolenic acid (2%) and high in linoleic acid (48%), with unaltered proportions of other fatty acids. The complete inverse correlation between linoleic and linolenic acids (r=-0.98) indicates that the mutations block the synthesis of linolenic acid at the linoleic desaturation step.

Metabolism of Unsaturated Monogalactosyldiacylglycerol Molecular Species in Arabidopsis thaliana Reveals Different Sites and Substrates for Linolenic Acid Synthesis

Synthesis of unsaturated monogalactosyldiacylglycerol (MGDG) was examined in a mutant of Arabidopsis thaliana (L.) Heynh. containing reduced levels of hexadecatrienoic (16:3) and linolenic (18:3) acids in leaf lipids. Molecular species composition and labeling kinetics following the incorporation of exogenous [(14)C]fatty acids suggest that at least two pathways and multiple substrates are involved in desaturation of linoleic acid (18:2) to 18:3 for production of unsaturated galactolipids. A reduction in 18:3/16:3 MGDG and an increase in 18:2/16:2 MGDG, together with labeling kinetics of these molecular species following the incorporation of exogenous [(14)C]12:0 fatty acids, suggests that a chloroplastic pathway for production of 18:3 at the sn-1 position of MGDG utilizes 18:2/16:2 MGDG as a substrate. This chloroplastic (prokaryotic) pathway is deficient in the mutant. When exogenous [(14)C]18:1 was supplied, a eukaryotic (cytoplasmic) pathway involving the desaturation of 18:2 to 18:3 on phosphatidylcholine serves as the source of 18:3 for the sn-2 position of MGDG. This eucaryotic pathway predominates in the mutant.