Regulation of vitamin E and vitamin B(1) biosynthesis in plants

Vitamin E and Vitamin B₁ are essential nutrients in animal and human diets. Vitamin E (tocopherols) function as antioxidants by quenching various reactive oxygen species and they protect biological membranes from radical induced lipid peroxidation while Vitamin B₁ (thiamin pyrophosphate) plays a central role as an enzyme co-factor in several key enzymatic reactions involved in carbohydrate metabolism. Although Vitamin E deficiencies in human are rare, several studies continue to support the fact that increased intake of Vitamin E provides several health benefits. On the other hand, deficiency in thiamin results in beriberi, a disorder characterized by neurological and cardiac impairments. Because plants represent as major source of both Vitamin E and Vitamin B₁, understanding of the biosynthesis and the regulation of the biosynthetic pathway of the two vitamins in plants helps to genetically engineer major crops for increased level of tocopherol and thiamin, to positively impact the nutritional needs of the global population and to prevent several degenerative diseases.; In an effort to increase Vitamin E production in plants and to understand the regulation of Vitamin E pathway, transgenic Arabidopsis plants that overexpressed the gene encoding Arabidopsis hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme that catalyzes the first step in tocopherol and plastoquinone biosynthesis, were generated. The HPPD enzymatic activity in transgenic lines was at least 10 fold higher than wild type control plants. This increase in HPPD activity resulted in up to 37% and 28% increase in tocopherol levels in leaf and seed tocopherol levels respectively and shows that HPPD activity is one of the factors that limit the flux of tocopherol biosynthesis in plants.; To elucidate the biosynthesis and regulation of thiamin in plants, the Arabidopsis th1 mutant was characterized and the gene encoding the enzyme thiamin phosphate pyrophosphorylase was overexpressed in Arabidopsis plants. Genetic complementation and the determination of the molecular basis of th1 mutation show that the lesion occurs in the structural gene encoding the enzyme thiamin phosphate pyrophosphorylase. The overexpression of this enzyme in wild-type Arabidopsis plants shows, however, that the activity of this enzyme alone does not regulate the flux of thiamin biosynthesis in plants.