Overexpression of the enzyme p-hydroxyphenolpyruvate dioxygenase in Arabidopsis and its relation to tocopherol biosynthesis

The enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD) catalyzes the conversion of p-hydroxyphenylpyruvate to homogentisic acid (HGA), the aromatic precursor for the biosynthesis of vitamin E (α-tocopherol) and plastoquinone. In order to determine if increased HPPD activity could positively impact tocopherol yields, transgenic plants were generated that overexpressed the gene encoding Arabidopsis HPPD. Transgenic plants exhibiting high levels of HPPD expression were identified by increased tolerance to a competitive inhibitor of HPPD, the herbicide sulcotrione. HPPD gene expression in these transgenic lines, as determined at the RNA, protein and activity levels, was at least 10-fold higher than that of wild-type plants. Subsequent tocopherol analysis of leaf and seed material revealed that the increased HPPD expression resulted in up to a 37% increase in leaf tocopherol levels and a 28% increase in seed tocopherol levels relative to control plants. These results demonstrate that HPPD activity, and likely HGA levels, are at least one factor limiting the production of tocopherols in photosynthetic and non-photosynthetic plant tissues.