| Résumé | The seed oil of meadowfoam (Limnanthes alba) and other Limnanthes spp. is enriched in the unusual fatty acid Δ⁵-eicosenoic acid (20:1Δ⁵). This fatty acid has physical and chemical properties that make the seed oil of these plants useful for a number of industrial applications. An expressed sequence tag approach was used to identify cDNAs for enzymes involved in the biosynthesis of 20:1Δ⁵). By random sequencing of a library prepared from developing Limnanthes douglasii seeds, a class of cDNAs was identified that encode a homolog of acyl-coenzyme A (CoA) desaturases found in animals, fungi, and cyanobacteria. Expression of a cDNA for the L. douglasii acyl-CoA desaturase homolog in somatic soybean (Glycine max) embryos behind a strong seed-specific promoter resulted in the accumulation of Δ⁵-hexadecenoic acid to amounts of 2% to 3% (w/w) of the total fatty acids of single embryos. Δ⁵-Octadecenoic acid and 20:1Δ⁵ also composed <1% (w/w) each of the total fatty acids of these embryos. In addition, cDNAs were identified from the L. douglasii expressed sequence tags that encode a homolog of fatty acid elongase 1 (FAE1), a β-ketoacyl-CoA synthase that catalyzes the initial step of very long-chain fatty acid synthesis. Expression of the L. douglassi FAE1 homolog in somatic soybean embryos was accompanied by the accumulation of C₂₀ and C₂₂ fatty acids, principally as eicosanoic acid, to amounts of 18% (w/w) of the total fatty acids of single embryos. To partially reconstruct the biosynthetic pathway of 20:1Δ⁵ in transgenic plant tissues, cDNAs for the L. douglasii acyl-CoA desaturase and FAE1 were co-expressed in somatic soybean embryos. In the resulting transgenic embryos, 20:1Δ⁵ and Δ⁵-docosenoic acid composed up to 12% of the total fatty acids. |
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