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. Les produits issus des activites de synthese d'acide gras sont habituellement les acides gras C16 et C18. Il existe cependant quelques especes de plantes qui renferment des acides gras de 8 a 14 atomes de carbone (chaine moyenne) dans leurs graines oleagineuses. Parmi ces acides gras, l'acide caprylique (C8) et l'acide caprique (C10) se retrouvent en faible quantite dans l'huile de coprah, qui est utilisee dans de nombreux produits industriels, alimentaires et pharmaceutiques. Pendant la derniere decennie, de nombreux laboratoires ont identifie, clone et caracterise les enzymes biosynthetiques et cataboliques regulant la composition et les taux de ces acides gras a chaine moyenne dans les oleagineux. Cet article resume les connaissances actuelles sur la transformation des plantes oleagineuses pour la production d'acide gras a chaine moyenne.