BIOASSEMBLY OF ACYL LIPIDS IN MICROSPORE-DERIVED EMBRYOS OF BRASSICA-CAMPESTRIS L

The native lipid composition and the capacity of cell-free extracts to biosynthesize acyl lipids in vitro were determined for the first time using the recently reported microspore-derived (MD) embryo system fro m the Brassica campestris low erucic acid line BC-2 (Baillie et al. 19 92). The total lipid fraction isolated from mid-cotyledonary stage MD embryos (21 days in culture) was composed primarily of triacylglycerol (76 %) with an acyl composition quite similar to that of mature BC-2 seed. When incubated in the presence of glycerol-3-phosphate, C-14 18: 1-CoA, and reducing equivalents, homogenates prepared from 21-day cult ured MD embryos were able to biosynthesize glycerolipids via the Kenne dy pathway. The maximum in vitro rate of triacylglycerol biosynthesis could more than account for the known rate of lipid accumulation in vi vo. The homogenate catalyzed the desaturation of 18:1 to 18:2 and to a lesser extent, 18:3. The newly-synthesized polyunsaturated fatty acid s initially accumulated in the polar lipid fraction (primarily phospha tidic acid and phosphatidylcholine) but began to appear in the triacyl glycerol fraction after longer incubation periods. As expected for a l ow erucic acid cultivar, homogenates of MD embryos from the BC-2 line were incapable of biosynthesizing very long chain monounsaturated fatt y acyl moieties (20:1 and 22:1) from 18:1-CoA in vitro. Nonetheless, e mbryo extracts were still capable of incorporating these fatty acyl mo ieties into triacylglycerols when supplied with C-14 20:1-CoA or C-14 22:1-CoA. Collectively, the data suggest that developing BC-2 MD embry os constitute an excellent experimental system for studying pathways f or glycerolipid bioassembly and the manipulation of this process in B. campestris.