How can we genetically engineer oilseed crops to produce high levels of medium-chain fatty acids?

Abstract

The end products of fatty acid synthase activities are usually 16- and 18-carbon fatty acids. There are however, several plant species that store 8- to 14-carbon (medium-chain) fatty acids in their oil seeds. Among the medium-chain fatty acids (MCFA), caprylic (8:0) and capric (10:0) are minor components of coconut oil, which are used in many industrial, nutritional and pharmaceutical products. Engineering crop plants such as Brassica could provide an economical source of these oils. During the last decade many laboratories have identified, cloned and characterized both the biosynthetic and catabolic enzymes regulating the composition and levels of these unusual fatty acids in seed oil. Among the biosynthetic enzymes thioesterases (TE), β-ketoacyl-ACP synthases (KAS) and acyltransferases are best characterized. In fact several independent investigators have shown that combined expression of the medium-chain specific enzymes, specifically, TE, KAS and lysophosphatidic acid acyltransferase (LPAAT) results in the production of significant levels of MCFA in seed that otherwise do not accumulate any medium-chain fatty acid. However, any additional increase in the levels of MCFA in transgenic seeds will require further detailed studies, such as possible induction of the medium-chain specific enzymes in β-oxidation and the glyoxylate pathways. To examine such a possibility, a number of genes involved in the β-oxidation cycle among them a novel enzyme now designated as ACX3, a medium-chain specific acyl-CoA-oxidase, has also been cloned. This article is an attempt to summarize our current knowledge and the present status of engineering oilseed crops for production of medium-chain fatty acids.