Galactolipid synthesis in Vicia faba leaves. V. Redistribution of 14C-labelling in the polar moieties and the 14C-labelling kinetics of the fatty acids of the molecular species of mono- and digalactosyl diacylglycerols.

The redistribution of 14C in the galactosyl glycerol and digalactosyl glycerol and the labelling kinetics of the fatty acids of mono- and digalactosyl diacylglycerol molecular species were examined over a period of 24 h following 14CO2-feeding of Vicia faba leaves. The results indicate that monogalactosyl diacylglycerol is formed from highly unsaturated diacylglycerols and that further desaturation of its C18 fatty acids occurs after formation of this galactolipid. There appear to be at least two pools of monogalactosyl diacylglycerol, differing in the degree of unsaturation of the fatty acids, from which digalactosyl diacyglycerol is formed. Digalactosyl diacylglycerol fatty acids did not appear to undergo further desaturation. A model for galactolipid biosynthesis and fatty acid desaturation is presented. There appear to be two distinct groups of diacyglycerol precursors, one consisting of C18/C18 molecular species derived from phosphatidylcholine, the other of C16/C18 molecular species derived from phosphatidylglycerol. The rapid turnover of monogalactosyl diacylglycerol to digalactosyl diacylglycerol from the C16/C18 group leads to one of the major differences in fatty acid composition between the two galactolipids observed in most plants.