TAG Synthesis and Oil Deposition

Acylation of the sn-3 position of DAG by acyl-CoA:diacylglycerol acyltranserase (DGAT) completes the synthesis of TAG. Plants, like mammals and fungi, appear to contain two very distinct families of DGAT genes. Members of the DGAT1 family are homologous to mammalian acyl CoA:cholesterol acyltransferase. However, inactivating TAG1, the single A. thaliana representative of this group, reduced DGAT activity up to 70% without an impact on sterol ester deposition (Zou et al., 1999). TAG synthesis catalyzed by an A. thaliana DGAT2 homologue, identified based on its similarity to a fungal DGAT2, was recently confirmed in transfected insect cells (Lardizabal et al., 2001).

At least one of two DGAT1 isoforms in Brassica napus cell suspensions was upregulated by sucrose (Nykiforuk et al., 2002). This could be related to the observation that low osmotic strength inhibits TAG synthesis in wheat embryos, but that abscisic acid overcomes this inhibition (Rodriguez-Sotres and Black, 1994). Overall levels of DGAT activity appear to have an impact on levels of oil deposition, since A. thaliana seeds that overexpress TAG1 displayed increased DGAT activity and seed oil (Jako et al., 2001).

In yeast, a proportion of TAG is produced not by DGAT, but by phospholipid:diacylglycerol acyltransferase (PDAT), an enzyme that transfers acyl units directly from the sn-2 position of PC or phosphatidylethanolamine to DAG (Oelkers et al., 2002). Dahlqvist et al. have implicated PDAT in TAG synthesis by both castor seeds and Crepis palaestina, plants with seed oils rich in hydroxy- and epoxy-fatty acids, respectively (Dahlqvist et al., 2000). PDAT from each plant is particularly active with its characteristic oxygenated fatty acid. Since polyunsaturated fatty acids, like the oxygenated fatty acids, are formed on phospholipid substrates, PDAT activity has been proposed to account for the flow of polyunsaturates from PC to TAG observed in numerous radiolabeling studies. PDAT activity has been observed in A. thaliana, and several genes related to the yeast PDAT gene have been identified, although not all encode proteins with PDAT activity (Banas´ et al., 2000; Stymne et al., 2003).

Alternative routes by which modified fatty acids could enter TAG include release of DAG from PC by the reverse reaction of CDP-choline phosphotransferase,or movement into the acyl-CoA pool via acyl-CoA:phospholipid acyltransferases or a combination of phospholipase and acyl-CoA synthase (Voelker and Kinney, 2001).

Completed TAGs are usually sequestered in 1–2 mm oil bodies bounded by a single layer of polar lipid (e.g., PC). These structures appear to arise at sites in the ER enriched in enzymes of TAG biosynthesis (Murphy, 2001). The half-unit membrane of the oil body is usually pictured as forming when TAG accumulates between the two leaflets of an ER bilayer (Huang, 1996). Oil body membranes are best known for their characteristic proteins, the oleosins and caleosins, although enzymes of TAG synthesis or catabolism have been identified in some lipid body preparations (Murphy, 2001).