Genetic engineering of a quantitative trait: Metabolic and genetic parameters influencing the accumulation of laurate in rapeseed

Laurate can be produced in the seed reserve oil of Brassica napus (rapeseed) by the expression of an heterologous lauroyl acyl-carrier protein thioesterase under the control of a napin seed-storage protein promoter. Analysis of a large number of transgenic events, and their progeny after self-pollination, shows that laurate can accumulate to nearly 60% of the triglyceride acyl groups. Up to 40 mol% laurate the phenotype is correlated positively with the number of thioesterase gene copies. The use of a tandem gene construct elevates the average laurate content. This effect correlates with an increased average number of T-DNA insertions per event; no cis-inactivation of tandem genes is apparent. Above 40 mol% laurate other factors apparently limit the phenotype. The expression timing conferred by the napin promoter is unlikely to be limiting, as it covers almost the entire period of oil deposition. A more significant limitation resides in the second acylation reaction of oil biosynthesis, as shown by the very low incorporation of laurate at the sn-2 acyl group. The novel, high-laurate oil is consequently rich in sn-1,3-dilauroyl triglycerides, but its unusual composition appears to pose no problems for mobilization during seed germination.