RADIOLABELING STUDIES OF ACYL LIPIDS IN DEVELOPING SEEDS OF BRASSICA-NAPUS - USE OF [1-C-14]ACETATE PRECURSOR

[1-C-14]Acetate has been used to label fatty acids and complex lipids in order to evaluate the capacity of embryos from developing oilseed rape seeds to synthesize lipid. Incorporation of radioactivity was linear for at least 4 hr. In terms of fresh weight, embryos about 20 days after pollination (DAP) were most active whilst activity expressed per embryo was high in the period 22-31 DAP, in agreement with the period of rapid accumulation of oil in vivo. Major labelled classes were diacylglycerols (DAGs), triacylglycerols (TAGs) and polar lipids (PLs). The proportion of label in these fractions remained constant for embryos from seeds of different ages up to 41 DAP when the proportion of radioactivity in DAGs declined from about 30 to 8% accompanied by a commensurate rise in that of TAGs. Phosphatidylcholine (PtdCho) was the best-labelled PL at all times of rapid lipid synthesis. After 41 DAP, the amount of label in this phospholipid declined and there was increased radioactivity in lysophosphatidylcholine (lysoPtdCho) and non-esterified fatty acids (NEFAs), possibly indicating increased turnover of PtdCho. The distribution of labelling within the fatty acids of individual lipid classes was studied and the results showed that the PLs contained significantly more [C-14]palmitate and significantly less [C-14]oleate than the TAGs for embryos at all stages of development. The data are discussed with regard to the possible influence of diacylglycerol acyltransferase (DAGAT) activity in exerting significant flux control during periods of rapid TAG accumulation.