Dynamic channelling during de novo fatty acid biosynthesis in Helianthus annuus seeds

We have obtained a simulation of the final intraplastidial steps of de novo fatty acid biosynthesis in sunflower (Helianthus annuus L.) seeds. For this simulation, we have used data from the fatty acid content of normal and high-saturated seed formation and from the enzymatic characterization of the stearoyl-ACP desaturase (SAD; EC 1.12.99.6), acyl-ACP thioesterase (TE; EC 3.1.2.14) and fatty acid synthase II complex (FAS II), and the program GEPASI (based on the metabolic control analysis theory). When developmental data from high-stearic acid mutant seeds were analysed and compared to those predicted with this model, the changes in SAD and TE actually found in the biochemical characterization of these mutants were very similar to the predictions. However, the model had to be modified when results from high-palmitic mutants, accumulating unusual fatty acids like palmitoleic, asclepic and palmitolinoleic acids, were used. The emerging model, consistent with all of our results, predicts the existence of a dynamic channelling between the FAS II complex and SAD, that channelling being responsible for an alternative pathway starting with the desaturation of palmitoyl-ACP by the SAD. For instance, the determination of SAD activity on crude extracts from sunflower seeds only rendered oleoyl-ACP when stearoyl-ACP used as a substrate was obtained from an FAS II assay but not when in vitro synthesized stearoyl-ACP was provided as a substrate. This theoretical approximation will be very useful in order to predict the evolution of the system when introducing new or modified activities; similar approximations in other oilseed crops could be of great interest.