Diacylglycerol acyltransferases from Vernonia and Stokesia prefer substrates with vernolic acid

Genetic engineering of common oil crops for industrially valuable epoxy FA production by expressing epoxygenase genes alone had limited success. Identifying other key genes responsible for the selective incorporation of epoxy FA into seed oil in natural high accumulators appears to be an important next step. We investigated the substrate preferences of acyl CoA: diacylglycerol acyltransferases (DGAT) of two natural high accumulators of vernolic acid, Vernonia galamensis and Stokesia laevis, as compared with a common oilseed crop soybean. Developing seed microsomes were fed with either [¹⁴C]oleoyl CoA or [¹⁴C]vernoloyl CoA in combinations with no exogenous DAG or with 1,2-dioleoyl-sn-glycerol, 1-palmitoyl-2-vernoloyl-sn-glycerol, 1,2-divernoloyl-sn-glycerol, 1,2-dioleoyl-rac-glycerol, or 1,2-divernoloyl-rac-glycerol to determine their relative incorporation into TAG. The results showed that in using sn-1,2-DAG, the highest DGAT activity was from the substrate combination of vernoloyl CoA with 1,2-divernoloyl-sn-glycerol, and the lowest was from vernoloyl CoA or oleoyl CoA with 1,2-dioleoyl-sn-glycerol in both V. galamensis and S. laevis. Soybean DGAT was more active with oleoyl CoA than vernoloyl CoA, and more active with 1,2-dioleoyl-sn-glycerol when oleoyl CoA was fed. DGAT assays without exogenous DAG, or with exogenous sn-1,2-DAG fed individually or simultaneously showed consistent results. In combinations with either oleoyl CoA or vernoloyl CoA, DGAT had much higher activity with rac-1,2-DAG than with their corresponding sn-1,2-DAG, and the substrate selectivity was diminished when rac-1,2-DAG were used instead of sn-1,2-DAG. These studies suggest that DGAT action might be an important step for selective incorporation of vernolic acid into TAG in V. galamensis and S. laevis.