ALTERATION OF SEED FATTY-ACID COMPOSITION BY AN ETHYL METHANESULFONATE-INDUCED MUTATION IN ARABIDOPSIS-THALIANA AFFECTING DIACYLGLYCEROL ACYLTRANSFERASE ACTIVITY

In characterizing the enzymes involved in the formation of very long-c hain fatty acids (VLCFAs) in the Brassicaceae, we have generated a ser ies of mutants of Arabidopsis thaliana that have reduced VLCFA content . Here we report the characterization of a seed lipid mutant, AS11 whi ch, in comparison to wild type (WT), has reduced levels of 20:1 and 18 :1 and accumulates 18:3 as the major fatty acid in triacylglycerols. P roportions of 18:2 remain similar to WT. Genetic analyses indicate tha t the fatty acid phenotype is caused by a semidominant mutation in a s ingle nuclear gene, designated TAG1, located on chromosome 2. Biochemi cal analyses have shown that the AS11 phenotype is not due to a defici ency in the capacity to elongate 18:1 or to an increase in the relativ e Delta 15 or Delta 12 desaturase activities. Indeed, the ratio of des aturase/elongase activities measured in vitro is virtually identical i n developing WT and AS11 seed homogenates. Rather, the fatty acid phen otype of AS11 is the result of reduced diacylglycerol acyltransferase activity throughout development, such that triacylglycerol biosynthesi s is reduced. This leads to a reduction in 20:1 biosynthesis during se ed development, leaving more 18:1 available for desaturation. Thus, we have demonstrated that changes to triacylglycerol biosynthesis can re sult in dramatic changes in fatty acid composition and, in particular, in the accumulation of VLCFAs in seed storage lipids.