Submitted to: Phytochemical Society of North America Meeting and Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: July 30, 2005
Publication Date: July 30, 2005
Citation: Cahoon, E.B. 2005. Metabolic engineering of unusual fatty acid biosynthetic pathways for enhanced vegetable oil quality and plant pathogen resistance. Phytochemical Society of North America Meeting and Newsletter. Technical Abstract: Hundreds of unusual fatty acids occur in plants, and many of these fatty acids have chemical, physical, or nutritional properties that make them attractive targets for the biotechnological enhancement of vegetable oils. My lab has maintained a long research interest in the identification of genes associated with the biosynthesis of unusual fatty acids, with the eventual goal of transferring these genes to existing crop species to generate oils with improved food, feed, or industrial quality. We have recently identified a class of enzymes that catalyze the conversion of an existing double bond in linoleic acid (18:2delta9,12) into two conjugated double bonds. These enzymes, which we have termed “fatty acid conjugases,” are divergent forms of the ubiquitous delta12 oleic acid desaturase. One type of fatty acid conjugase converts the delta9 double bond of linoleic acid into delta8 and delta10 double bonds to form calendic acid (18:3delta8,10,12), and a second type of fatty acid conjugase converts the delta12 double bond into delta11 and delta13 double bonds to form eleosteraric acid (18:3delta9,11,13). Fatty acid conjugases appear to have evolved numerous, independent times in the plant kingdom, as these enzymes occur in a limited number of species from diverse families including Asteraceae, Cucurbitaceae, and Euphorbiaceae. Oils enriched in the products of fatty acid conjugases are prone to oxidation and therefore have significant value as drying oils for the coatings industry. Our attempts to engineer pathways for conjugated fatty acid synthesis in seeds of Arabidopsis and soybean have met with only moderate success. We have been able to produce transgenic seed oils that contain 15 to 20% of either calendic acid or eleostearic acid. However, these levels are far below the 60+% that are typically found in seed oils that naturally accumulate these fatty acids. A bottleneck in the accumulation of conjugated fatty acids in seeds of transgenic plants appears to be the inefficient flux of these fatty acids from their synthesis on phospholipids to their storage as components of triacylglycerols, which is a major focus of our current research. We have also recently reported the wide occurrence in the Asteraceae, Apiaceae, and Araliaceae families of a divergent form of the delta12 desaturase that catalyzes the synthesis of a triple bond in linoleic acid. This enzyme, which is known as a fatty acid acetylenase, appears to catalyze an early step in the biosynthesis of biologically active polyacetylenic compounds that are produced by members of these families. These compounds include panaxynol, which has demonstrated pest resistance properties. The discovery of this widely occurring fatty acid acetylenase suggests that some unusual fatty acid biosynthetic pathways have arisen in plants for defense against pathogens and herbivores.