The genetic modification of oilseed crops provides an opportunity to tailor the composition of seed oils for optimal dietary or processing characteristics. Until recently, modifications of oil composition could be achieved through traditional plant breeding, where natural diversity within closely related species could be exploited, or through mutagenesis. Transgenic technology widens the scope of modifications achievable in oil composition by allowing the introduction of a wider range of genetic elements than is otherwise possible. There has been considerable focus on the identification and analysis of enzymes and underlying genes involved in plant lipid biosynthesis (13). Coupled with efficient plant transformation systems for the key oilseed crops, canola and soybean, metabolic engineering of seed lipids in these two crops has become feasible. As a result there has been successful development of a variety of novel oils in which the fatty acid composition has been optimized for improved processing characteristics (14), healthfulness (15) or industrial applications (16).
Development of SDA-canola
In conventional canola oil, ~97% of the lipids are represented in three classes: SFA (5.25%, 16:0 and 18:0), monosaturated fatty acids (66%, 18:1) and PUFA (26%, 18:2 and 18:3) (Fig. 4), with the PUFA containing an ~2:1 ratio of LA to ALA. SDA, which is not normally present in canola oil, represents the Δ6 desaturation product of ALA. Because canola accumulates primarily OA in its oil, the production of significant quantities of SDA in canola required increased flux through the PUFA pathway from OA to ALA and then the addition of Δ6 desaturase activity (Fig. 1). Genes encoding the fatty acid desaturase enzymes that catalyze these reactions have been identified and characterized from diverse sources including higher plants and fungi (17). We generated transgenic canola lines that expressed in seeds the Δ6 and Δ12 fatty acid desaturases isolated from the commercially grown fungus, Mortierella alpina, and the Δ15 fatty acid desaturase from canola (Brassica napus). Seed oil from independent transformants accumulated SDA, as predicted (18). SDA accumulated up to 23% of the oil by weight, although the amount of SDA and the ratio of SDA to other seed lipids was influenced by the strategy utilized. For example, in one approach, the Δ6, Δ15 and Δ12 desaturases were combined on the same transformation vector. Resulting canola seeds were evaluated for seed fatty acid composition in the first transgenic generation and selected lines were self-pollinated and evaluated in the subsequent generation. The fatty acid composition of one line is shown in Figure 4. In this line, SDA accumulated to ~16% of the total fatty acids. The total omega-3 content in the seed lipids (ALA + SDA) was >60% of the fatty acids whereas the total omega-6 fatty acid content of the seed lipids (GLA + LA) was ~22%. OA was reduced from 60% of the seed lipids to ~12%. In an alternative approach, a crossing strategy was utilized wherein transgenic lines containing only the M. alpina Δ6 and Δ12 desaturase genes were hybridized with independently produced lines containing only the B. napus Δ15 desaturase gene. Seed lipids were evaluated in the F1 and F2 generations from several independent crosses. In these progeny, SDA accumulated up to 23% of the lipids F1 seed (Fig. 4). The total omega-3 content in the seed lipids (ALA + SDA) exceeded 55% of the seed lipids whereas the total omega-6 fatty acid content of the seed lipids (GLA + LA) was ~22% of the seed lipids.