Figure 1. Biosynthesis of storage lipids in plants: Import of carbon precursors. Sucrose is transported from photosynthetic tissues into developing seeds where it is converted in the cytosol of embryo and/or endosperm cells into precursors, such as glucose 6-phosphate and phosphoenolpyruvate, for onward transport into plastids for production of fatty acids. Import into plastids occurs via specific carriers such as GPT, glucose 6-phosphate transporter; TPT, triose phosphate transporter and PPT, phosphoenolpyruvate transporter. Fatty acid biosynthesis de novo: Acetyl–CoA and malonyl–CoA are the precursors for assembly of C8 to C18 saturated fatty acyl–ACPs on a plastidial multienzyme fatty acid synthetase complex. Plastids are also the site of the insertion of the first double bond by a soluble desaturase (SOL-DES) to produce fatty acid monounsaturates. Both unsaturates and monounsaturates are exported via an acyl–CoA transporter (ACT) from plastids to the endoplasmic reticulum for further processing. Fatty acid modification: Plastid-derived acyl–CoAs can be modified in the endoplasmic reticulum by a huge variety of enzymes to produce some of the hundreds of different fatty acids found in naturally occurring seed oils. However, since not all of these enzymes are present in any given plant species, nontransgenic oilseeds normally accumulate a relatively restricted range of fatty acids. Most fatty acid modification reactions occur via membrane-bound phosphatidylcholine (PC) specific ER desaturases or desaturase-like enzymes (ER-DES) such as hydroxylases or epoxidases. Acyl–CoAs are then assembled into complex lipids on the endoplasmic reticulum. Similar ER-located pathways produce the various membrane lipids, storage lipids and also some signalling lipids although recent evidence suggests that these pathways are spatially separated in discrete ER domains. Whereas storage oil bodies can accumulate virtually any type of fatty acid, the biological functions of membrane and signalling lipids require that they only contain a small range of C16 and C18 fatty acids. One of the challenges to producing oilseeds with novel acyl compositions is therefore to maintain the segregation of exotic fatty acids away from pools of membrane or signalling lipids. Assembly of triacylglycerols: Triacylglycerols are assembled via a complex process involving sequential acylation of a glycerol moiety (the traditional Kennedy pathway) plus extensive acyl editing via phosphatidylcholine-dependent desaturases or desaturase-like enzymes (see earlier). The final conversion of DAG into TAG can occur via at least three enzymes: DGAT, acyl–CoA dependent diacylglycerol acyltransferase; PDAT, phosphatidylcholine-dependent acyltransferase or DGTA, diacylglycerol transacylase. Nascent TAG droplets are coated with a phospholipid monolayer into which is embedded an annulus of specific proteins, such as oleosins and caleosins, hence forming the mature storage oil bodies that are finally released into the cytosol. DAG, diacylglycerol; G3P, glycerol 3-phosphate; MAG, monoacylglycerol; PA, phosphatidic acid and TAG, triacylglycerol.