Metabolic Engineering of the Content and Fatty Acid Composition of Vegetable Oils

Abstract
This chapter discusses engineering of plants for yield and composition of edible and industrial triacylglycerols (TAGs). Total oil production has been increased moderately by overexpression of genes for the first and last steps of oil synthesis, acetyl-CoA carboxylase (ACCase), and diacylglycerol acyltransferase (DGAT), respectively. However, the single enzyme approach has proved less than satisfactory, and further progress may depend on identification of regulatory genes affecting overall expression of the lipid synthesis pathways and partitioning of carbon between oil and other plant products. The fatty acid composition of oilseeds has been more amenable to modification. Development of edible oils rich in monounsaturated fatty acids (18:1) has been achieved in several oilseeds normally dominated by polyunsaturated fatty acids such as 18:2. Approaches have included both chemical mutagenesis and transgenic alteration of the FAD2 genes responsible for desaturation of 18:1 to 18:2. Proportions of 16:0 have been reduced substantially by reduction of FatB, the gene for the thioesterase that releases 16:0 from the acyl carrier protein (ACP) on which it is assembled. The last major goal in edible oil modification, production of a temperate crop sufficiently rich in saturated fatty acids for use without hydrogenation and its associated trans-fatty acid production, remains elusive. Mechanisms for minimizing transfer of the upregulated saturated fatty acids to plant membranes are currently lacking. Excess saturated fatty acids in plant membranes are particularly damaging in colder temperature ranges.

Finally, a wide range of genes have been identified that encode enzymes for synthesis of unusual fatty acids with potential as food additives or industrial feedstocks. Genes for production of γ-linolenic acid (GLA) and polyunsaturated ω-3 fatty acids have been introduced into plants, as have genes permitting production of 10:0 and 12:0 for the detergents industry, longchain fatty acids for plastics and nylons, novel monounsaturated and conjugated fatty acids, and fatty acids with useful epoxy-, hydroxy-, and cyclic moieties. With the notable exception of the shorter-chain fatty acids, these efforts have been hampered by inadequate yields of the novel products. Given that plants from which many of the applicable genes were isolated do produce oils with high proportions of unusual fatty acids, increased yields in transgenic crops should be achievable. It is probable that introduction of the novel fatty acids must be coupled with appropriate modifications of the enzymes responsible for their flux into vegetable oils.

Key Words: Vegetable oil, Oilseed, Fatty acid, Triacylglycerol, Lipids, Fatty acid unsaturation, Polyunsaturated fatty acid, Saturated fatty acid, Fatty acid desaturase, Thioesterase, FAD2, Genetic engineering, Metabolic engineering.

Abbreviations: ACCase, acetyl coenzyme A carboxylase; ACP, acyl carrier protein; ARA, arachidonic acid; BCCP, biotin carboxyl carrier protein; DAG, diacylglycerol; DGAT, diacylglycerol acyltransferase; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; ER, endoplasmic reticulum; FAD, fatty acid desaturase; FAS, fatty acid synthase; Fat, fatty acid thioesterase; GLA, γ-linolenic acid; GPAT, acyl-CoA:glycerol-3-phosphate acyltransferase; KAS, 3-ketoacyl-ACP synthase; KCS, 3-ketoacyl-CoA synthase; LPAAT, acyl-CoA: lysophosphatidic acid acyltransferase; PC, phosphatidylcholine; PDAT, phospholipid:diacylglycerol acyltransferase; RNAi, RNA interference; TAG, triacylglycerol; VLCFA, very long-chain fatty acid.