Biosynthesis of triacylglycerol in Thunbergia alata: Additional evidence for involvement of phosphatidylcholine in unusual monoenoic oil production

The seed oil of Thunbergia alata has an unusual fatty acid composition which consists of more than 80 % 16:1Δ6. This fatty acid is produced in the plastid by the action of a Δ6 palmitoyl (16:0)-ACP desaturase. To examine the biosynthesis of triacylglycerol (TAG) containing high concentrations of this unusual monoenoic fatty acid, endosperm dissected from developing T. alata seeds was labeled with [1-14C]-acetate. At early time points (5–15 min), the predominant labeled lipid was PC whereas at later time points (greater than 30 min) TAG became the major labeled lipid. Analysis of the acyl group composition of each lipid revealed that radiolabeled 16:1Δ6 was highest at early time points in PC while at later time points, it was found to be highest in TAG. Further analysis of the distribution of labeled acyl groups within PC indicated that 16:1Δ6 at the sn-2 position comprised the majority (55–78 %) of total labeled acyl groups whereas 16:1Δ6 at the sn-1 position constituted only a small fraction (12–15 %) of the total labeled acyl groups. In contrast, unlabeled PC contained lower amounts of 16:1Δ6 (16 %) at the sn-2 position. These results are consistent with previous studies suggesting a flux of novel monoenoic acids through PC during TAG biosynthesis, and furthermore imply a stereospecific flux through the sn-2 position of PC.