The fatty acids released from plastids are rapidly converted to their respective acyl- CoAs by acyl-CoA synthetases, most likely those isozymes associated with the plastidial envelope (Schnurr et al., 2002). Phosphatidic acid synthesis may then be initiated by transfer of an acyl group to the sn-1 position of glycerol- 3-phosphate by membrane-bound acyl-CoA:glycerol-3-phosphate acyltransferase (GPAT) (Murata and Tasaka, 1997). Microsomal GPATs are typically capable of using a wide range of acyl-CoAs, but enzymes from some oil producing organs might bemore selective. For example, aGPATsolubilized fromoil palmmicrosomes was most active with palmitoyl (16:0)-CoA (Manaf and Harwood, 2000). Genes for ER-localized GPATs have been identified in Arabidopsis thaliana (Zheng et al., 2003). The identification ofGPATs specifically involved in the biosynthesis of TAGin seeds awaits further characterization of this seven-member gene family.
Acylation of the sn-2 position is subsequently catalyzed by an ER acyl-CoA: lysophosphatidic acid acyltransferases (LPAATs). In most edible oils, this position is dominated by unsaturated C18-fatty acids, reflecting LPAAT discrimination against 16:0-CoA and 18:0-CoA (Brown et al., 2002). Microsomal LPAAT cDNAs have been cloned from several species (Bourgis et al., 1999). As will be discussed later, some plants with oils enriched in unusual fatty acids also produce functionally divergent LPAATs that accept the corresponding acyl-CoAs (Voelker and Kinney, 2001).
Although most phosphatidic acid that is a precursor to TAG is produced by ER acyltransferases, it is important to note that plastids and mitochondria also assemble phosphatidic acid. Glycerolipid backbones formed in the plastids serve primarily as precursors of phosphatidylglycerol, sulfolipid, and galactolipid, while mitochondria are the sole site of cardiolipin production. However, studies of mutants have highlighted the ability of plants to move DAG units between compartments as needed (Kunst et al., 1988). In addition, genes for the acyltransferases native to any compartment have potential for seed oil modification. For example, A. thaliana transformed with a plastidial GPAT cDNA less its transit sequence produced about 20% more seed oil, even though the plastidial GPAT is a soluble enzyme that normally uses acyl-ACP rather than acyl-CoA (Jain et al., 2000). Plastidial LPAATs, envelope-localized proteins that likewise employ acyl-ACPs as substrates, are selective for 16:0 rather than 18:1Δ9 and 18:2Δ9,12 (Frentzen, 1998).
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