Lipid biosynthesis in plant cells is a highly coordinated process as a consequence of the compartmentalisation between the synthesis of fatty acids in the plastid and glycerolipids. In some species such as Arabidopsis, a significant proportion (> 35%) of the newly synthesised acyl chains are retained by the chloroplast and incorporated into membrane components such as galacto- and sulpho-lipids (Ohlrogge and Browse 1995). The remaining fatty acids (mainly the monounsaturated oleic acid—18 : 1) are exported from the chloroplast as acyl-CoAs and incorporated into phospholipids in the endoplasmic reticulum (ER). The 18 : 1 may be desaturated by the ER to form the polyunsaturated linoleic (18 : 2) and linolenic (18 : 3) fatty acids. A proportion of the lipids containing these fatty acids are returned from the ER to the chloroplast and incorporated into the thylakoids and envelope membranes. Many species retain little de novo synthesised fatty acid in the chloroplast and in these plants almost all of the acyl chains apparently pass through the ER lipid pathway before being returned to the chloroplast (Mongrand et al. 1998). The ubiquitous glycerol-3-phosphate pathway localised in the ER (the plant eukaryotic pathway) ensures the synthesis of phosphatidic acid (PA) for the production of phosphatidylinositol (PtdIns) and phosphatidylglyc-erol (PtdGro) and the synthesis of diacylglycerol (DAG) for the production of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn, reviewed in Browse and Somerville 1991; Ohlrogge and Browse 1995). The glyc-erolipid acyltransferases associated with the Kennedy pathway belong to gene families, raising the possibility that a functional specialisation exists among these acyltransferases, each isoform perhaps participating in reactions that are unique to particular tissues or contributing to additional pathways that are associated with the production of PA destined for the production of specialised lipids.
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