Fig. 2. (A) Diagram of the glycerolphosphate pathway and phosphatidylcholine synthesis as found in both animals and plants. The bold arrows indicate hypothesized pathways resulting in simultaneous accumulation of acylglycerols and trimethylamine oxide (TMAO) (only abbreviated pathways are drawn). Diacylglycerol (DAG), formed from the derivative glycerol-3-phosphate (DAG), can be shunted towards either acylglycerol (lipid) storage (e.g. triacylglycerol) or phosphatidylcholine (PtdCho) synthesis. The final step in the phosphatidylcholine pathway is reversible. The back reaction may occur to a significant extent so that diacylglycerol is formed from phosphatidylcholine and subsequently stored for seasonal or reproductive energy reserves (see Gur and Harwood, 1991). Choline, thus released, is oxidized to TMAO and either stored or excreted. (B) In some plants, PtdCho is produced by methylation of ethanolamine and may subsequently be hydrolysed for release of DAG and choline. DAG in plants is important during growth for the formation of photosynthetic membrane glycolipids. Choline, thus released, is oxidized to glycine betaine (Hitz et al. 1981). (C) In chenopods (e.g. spinach and sugarbeet), choline is synthesized directly from ethanolamine and, thus, appears to be a specific adaptation for glycine betaine accumulation during drought and salt-stress (Weretilnyke et al., 1995). The enzymes involved in the pathways illustrated are numbered: (1) glycerol-3-phosphate (G3P) is converted to phosphatidic acid by the successive actions of G3P acyltransferase and 1-acylglycerol-3-phosphate acyltransferase; (2) phosphatidic acid phosphatase; (3) 1,2-diacylglycerol:choline phosphotransferase; (4) diacylglycerol acyltransferase; (5,6) carnitine palmitoyl transferases I and II; (7) trimethylamine oxygenase; (8) choline monoxygenase; (9) betaine aldehyde dehydrogenase; (10) P-choline phosphatase. TMA, trimethylamine; P-choline, phosphocholine.
