Acyl-exchange in diacylglycerol synthesis in the leaves of Brassica napus

The generally accepted hypothesis for phospholipid biosynthesis and desaturation in the extraplastidial membranes of plants is similar to monogalactosyldiacylglycerol synthesis in the chloroplast in that it involves acylation of fatty acids to a glycerol backbone followed by in situ desaturation. Alternatively, removal of acyl groups from phosphatidylcholine to an acylCoA pool in the extraplastidial membranes, followed by a reacylation of phosphatidylcholine from the acyl-CoA pool occurs. Fatty acids desaturated while esterified to phosphatidylcholine may enter the acyl-CoA pool to be used in de novo synthesis of phosphatidylcholine, providing a dynamic cycle to phosphatidylcholine synthesis. The objective of this thesis is to prove that acyl-exchange occurs between phosphatidylcholine and the acyl-CoA pool in the leaves of B. napus, and to propose a mechanism for the exchange.; The occurrence of acyl-exchange was confirmed by using a microsomal oleate desaturase mutant of B. napus that accumulates oleate in its extraplastidial diacylglycerols. By radiolabelling leaves with 32PO4 and 14CO2, the diacylglycerol precursors to phosphatidylcholine were shown to be polyunsaturated, and the fatty acids esterified to phosphatidylcholine were shown to have undergone at least one cycle of deacylation and reacylation. It was also shown that both sn-positions of phosphatidylcholine are involved in acyl-exchange, that exchange is non-selective for unsaturated acyl-CoAs, and that desaturation occurs on both sn-positions of phosphatidylcholine. By examining the level of 14C in the fatty acids on each sn-position in phosphatidylcholine and in the molecular species of phosphatidylcholine, it was apparent that there was a faster rate of acyl-transfer onto the sn-2 position. In contrast, the rate of desaturation was found to be similar on both sn-positions.; Acyl-exchange maintains a high level of unsaturation in the diacylglycerol pool, and it is possible that newly synthesized polyunsaturated diacylglycerol is used directly for the synthesis of other extrachloroplastic phospholipids. Newly synthesized polyunsaturated diacylglycerol may also be the precursor to eukaryotic galactolipid synthesis in the chloroplast.; Acyl-exchange may be a mechanism for remodelling the fatty acid composition of the membranes in higher plants in response to changing environmental conditions. Acyl-exchange operates in concert with in situ desaturation and lipid synthesis to provide a fluid response.