Biosynthetic origin of conjugated double bonds: Production of fatty acid components of high-value drying oils in transgenic soybean embryos

Vegetable oils that contain fatty acids with conjugated double bonds, such as tung oil, are valuable drying agents in paints, varnishes, and inks. Alt hough several reaction mechanisms have been proposed, little is known of th e biosynthetic origin of conjugated double bonds in plant fatty adds. An ex pressed sequence tag (EST) approach was undertaken to characterize the enzy matic basis for the formation of the conjugated double bonds of alpha-eleos tearic (18:3 Delta(9cis,11trans,13trans)) and alpha-parinaric (18:4 Delta(9 cis,11trans,13trans,15cis)) acids. Approximately 3,000 ESTs were generated from cDNA libraries prepared from developing seeds of Momordica charantia a nd Impatiens balsamina, tissues that accumulate large amounts of alpha-eleo stearic and alpha-parinaric acids, respectively. From ESTs of both species, a class of cDNAs encoding a diverged form of the Delta(12)-oleic acid desa turase was identified. Expression of full-length cDNAs for the Momordica (M omoFadX) and Impatiens (ImpFadX) enzymes in somatic soybean embryos resulte d in the accumulation of alpha-eleostearic and alpha-parinaric acids, neith er of which is present in untransformed soybean embryos, alpha-Eleostearic and alpha-parinaric acids together accounted for as much as 17% (wt/wt) of the total fatty acids of embryos expressing MomoFadX. These results demonst rate the ability to produce fatty acid components of high-value drying oils in transgenic plants. These findings also demonstrate a previously unchara cterized activity for Delta(12)-oleic acid desaturase-type enzymes that we have termed "conjugase.".