Increased α-tocotrienol content in seeds of transgenic rice overexpressing Arabidopsis γ-tocopherol methyltransferase

Vitamin E comprises a group of eight lipid soluble antioxidant compounds that are an essential part of the human diet. The alpha-isomers of both tocopherol and tocotrienol are generally considered to have the highest antioxidant activities. gamma-tocopherol methyltransferase (gamma-TMT) catalyzes the final step in vitamin E biosynthesis, the methylation of gamma- and delta-isomers to alpha- and beta-isomers. In present study, the Arabidopsis gamma-TMT (AtTMT) cDNA was overexpressed constitutively or in the endosperm of the elite japonica rice cultivar Wuyujing 3 (WY3) by Agrobacterium-mediated transformation. HPLC analysis showed that, in brown rice of the wild type or transgenic controls with empty vector, the alpha-/gamma-tocotrienol ratio was only 0.7, much lower than that for tocopherol ( 19.0). In transgenic rice overexpressing AtTMT driven by the constitutive Ubi promoter, most of the gamma-isomers were converted to alpha-isomers, especially the gamma- and delta-tocotrienol levels were dramatically decreased. As a result, the alpha-tocotrienol content was greatly increased in the transgenic seeds. Similarly, over-expression of AtTMT in the endosperm also resulted in an increase in the alpha-tocotrienol content. The results showed that the alpha-/gamma-tocopherol ratio also increased in the transgenic seeds, but there was no significant effect on alpha-tocopherol level, which may reflect the fact that gamma-tocopherol is present in very small amounts in wild type rice seeds. AtTMT overexpression had no effect on the absolute total content of either tocopherols or tocotrienols. Taken together, these results are the first demonstration that the overexpression of a foreign gamma-TMT significantly shift the tocotrienol synthesis in rice, which is one of the world's most important food crops.