Increase of the tocochromanol content in transgenic Brassica napus seeds by overexpression of key enzymes involved in prenylquinone biosynthesis

Lipid soluble tocochromanols, only synthesised in photosynthetic organisms, are industrially interesting compounds because of their antioxidative properties and their essential function in nutrition. In order to increase the tocochromanol content in the seed oil of transgenic plants, approaches were undertaken to engineer the flux of substrates and intermediates through the pathway. To this end, we overexpressed genes encoding hydroxyphenylpyruvate dioxygenases, alone or in combination with chimeric homogentisate phytyltransferase and tocopherol cyclase genes, in seeds of transgenic Brassica napus plants and analysed total tocochromanol content and composition. Overexpression of chimeric hydroxyphenylpyruvate dioxygenase genes, both in the cytosol or in the plastids of developing seeds, yielded a slight although significant increase in total tocochromanol level. Coexpression of a hydroxyphenylpyruvate dioxygenase gene with both a homogentisate phytyltransferase gene and a tocopherol cyclase gene elevated this effect with maximum increases of up to two-fold in individual lines and this phenotype was found to be stably inherited. These data showed that the three enzymes are critical in determining the total tocochromanol content in the seed oil of Brassica napus plants, while the tocopherol cyclase, unlike hydroxyphenylpyruvate dioxygenase and homogentisate phytyltransferase, had additionally an effect on the relative abundance of individual tocochromanol species and resulted in an increase of δ-tocopherol and plastochromanol-8 in the seeds.Mirela Raclaru and Jens Gruber contributed equally