The FATTY ACID ELONGATION1 (FAE1) gene of Arabidopsis is required for the synthesis of very long chain fatty acids in the seed. The product of the FAE1 gene is presumed to be a condensing enzyme that extends the chain length of fatty acids from C18 to C20 and C22. We report here the cloning of FAE1 by directed transposon tagging with the maize element Activator (Ac). An unstable fae1 mutant was isolated in a line carrying Ac linked to the FAE1 locus on chromosome 4. Cosegregation and reversion analyses established that the new mutant was tagged by Ac. A DNA fragment flanking Ac was cloned by inverse polymerase chain reaction and used to isolate FAE1 genomic clones and a cDNA clone from a library made from immature siliques. The predicted amino acid sequence of the FAE1 protein shares homology with those of other condensing enzymes (chalcone synthase, stilbene synthases, and beta-ketoacyl-acyl carrier protein synthase III), supporting the notion that FAE1 is the structural gene for a synthase or condensing enzyme. FAE1 is expressed in developing seed, but not in leaves, as expected from the effect of the fae1 mutation on the fatty acid compositions of those tissues.

Mutants of Arabidopsis that contain reduced levels of polyunsaturated fatty acids showed growth characteristics at 22°C that were very similar to wild type. By contrast, at 12C, the mutants failed to undergo stem elongation during reproductive growth although they produced normal flowers and fertile seeds.

Mutants of Arabidopsis thaliana were identified by screening pedigreed M3 seed collections from EMS-treated plants for changes in fatty acid (FA) composition. The FA phenotypes of the most dramatic mutants are as follows: G30 and 1E5 (allelic) lack linolenic acid (18∶3) and are elevated in linoleic acid (18∶2); 4A5 is deficient in 18∶2 and 18∶3 and fourfold increased in oleic acid (18∶1); 9A1 lacks all FAs > C18 and is twofold increased in 18∶1; 1A9 is twofold increased in palmitic acid (16∶0) and decreased by one-half in 18∶1; 2A11 is two-to threefold increased in stearic acid (18∶0) and decreased by one-half in 18∶1. Based on segregation of F2 selfed plants derived from crosses to wild type, all of these phenotypes are the result of single gene mutations.

The FATTYAClD ELONGATlONl (FAEl) gene of Arabidopsis is required for the synthesis of very long chain fatty acids in the seed. The product of the FAEl gene is presumed to be a condensing enzyme that extends the chain length of fatty acids from C18 to C20 and C22. We report here the cloning of FAE7 by directed transposon tagging with the maire element Activator (Ac). An unstable fael mutant was isolated in a line carrying Ac linked to the FAEl locus on chromosome 4. Cosegregation and reversion analyses established that the new mutant was tagged by Ac. A DNA fragment flanking Ac was cloned by inverse polymerase chain reaction and used to isolate FAEl genomic clones and a cDNA clone from a library made from immature siliques. The predicted amino acid sequence of the FAEl protein shares homology with those of other condensing enzymes (chalcone synthase, stilbene synthases, and p-ketoacyl-acyl carrier protein synthase lll), supporting the notion that FAE7 is the structural gene for a synthase or condensing enzyme. FAEl is expressed in developing seed, but not in leaves, as expected from the effect of the fael mutation on the fatty acid compositions of those tissues.

The overall fatty acid composition of leaf lipids in a mutant of Arabidopsis thaliana was characterized by an increased level of 160 and a concomitant decrease of 18-carbon fatty acids as a consequence of a single recessive nuclear mutation at the fabl In all plants studied, de novo fatty acid synthesis occurs in the chloroplasts or plastids of the cell (Ohlrogge et al., 1991) by the action of a dissociable type I1 fatty acid synthase (Browse and Somerville, 1991). The primary product of fatty acid synthesis, 16:O-ACP, can undergo one of three competing reactions: (a) hydrolysis by acyl-ACP thioesterase, (b) elongation to 18:O-ACP (which is followed by nearly quantitative desaturation to form 18:1-ACP), or (c) transfer to lysophosphatidic acid. Therefore, the ratio of 16-carbon to 18-carbon fatty acids in the membrane lipids could be determined by the relative fluxes through these three reactions. The significance of these reactions is related to the two pathways that are available for the synthesis of membrane glycerolipids in the leaf cells of higher plants (Browse and Somerville, 1991).In Arabidopsis thaliana, the "prokaryotic" pathway (Roughan and Slack, 1982), located in the chloroplast envelope, uses 18:l-ACP and 16:O-ACP for the sequential acylation of glycerol-3-P to form phosphatidic acid. The phosphatidic acid made by the prokaryotic pathway has 18:l at the sn-1 position and 16:O at the sn-2 position of the glycerol '