Expression of meadowfoam Des5 and FAE1 genes in yeast and in transgenic soybean somatic embryos, and their roles in fatty acid modification

Meadowfoam (Limnanthes spp.) species are unique in that their seeds are rich in the unusual fatty acids Δ5-eicosenoic acid (C20:1Δ5) and the diene, C22:2Δ5, Δ13. Previously the cloning of Δ5 desaturase (Des5) and fatty acid elongase 1 (FAE1) meadowfoam genes and their expression in soybean were reported. Here, we present the first successful expression of the Limnanthes Des5 in yeast, resulting in the desaturation of C16:0, C18:0 and C20:0 to their corresponding cis Δ5 isomers. In soybean (Glycine max L.), Limnanthes Des5/FAE1 double transformant somatic embryos fed with radiolabeled C14:0 or C16:0 could elongate these substrates to C18:0, C20:0 and C22:0 and C24:0. However, radiolabeled C18:1Δ9 or C20:1Δ11 were not elongated to their respective monounsaturated very long-chain products, confirming that the cloned Limnanthes FAE1 homolog gene product was specific for elongating saturated fatty acids. To understand better the biosynthetic pathway for C22:2Δ5, Δ13, soybean somatic embryos transformed with the Des5 cDNA were fed in culture with 〚1-14C〛C 22:1Δ13 fatty acid, which resulted in the biosynthesis of 〚1-14C〛-labeled C22:2Δ5, Δ13. Cell-free preparations enriched with detergent-solubilized Δ5 desaturase activity extracted from both developing meadowfoam seeds and from Des5 transgenic soybean embryos, produced 14C-22:2Δ5, Δ13 when supplied with 〚1-14C〛 C22:1-CoA. Thus, both the in vivo and in vitro experiments showed that the biosynthesis of C22:2Δ5, Δ13 can occur in somatic soybean embryos transformed with the Limnanthes Des5 cDNA, and confirmed that the pathway for C22:2 biosynthesis in meadowfoam involves further desaturation of erucoyl-CoA by a Δ5-regiospecific desaturase.