Seed-Specific Heterologous Expression of a Nasturtium FAE Gene in Arabidopsis Results in a Dramatic Increase in the Proportion of Erucic Acid

The fatty acid elongase [often designated FAE or beta-(or 3-) ketoacyl-CoA synthase] is a condensing enzyme and is the first component of the elongation complex involved in synthesis of erucic acid (22:1) in seeds of garden nasturtium (Tropaeolum majus). Using a degenerate primers approach, a cDNA of a putative embryo FAE was obtained showing high homology to known plant elongases. This cDNA contains a 1,512-bp open reading frame that encodes a protein of 504 amino acids. A genomic clone of the nasturtium FAE was isolated and sequence analyses indicated the absence of introns. Northern hybridization showed the expression of this nasturtium FAE gene to be restricted to the embryo. Southern hybridization revealed the nasturtium beta-ketoacyl-CoA synthase to be encoded by a small multigene family. To establish the function of the elongase homolog, the cDNA was introduced into two different heterologous chromosomal backgrounds (Arabidopsis and tobacco [Nicotiana tabacum]) under the control of a seed-specific (napin) promoter and the tandem 35S promoter, respectively. Seed-specific expression resulted in up to an 8-fold increase in erucic acid proportions in Arabidopsis seed oil, while constitutive expression in transgenic tobacco tissue resulted in increased proportions of very long chain saturated fatty acids. These results indicate that the nasturtium FAE gene encodes a condensing enzyme involved in the biosynthesis of very long chain fatty acids, utilizing monounsaturated and saturated acyl substrates. Given its strong and unique preference for elongating 20:1-CoA, the utility of the FAE gene product for directing or engineering increased synthesis of erucic acid is discussed.