Genetic evidence for the role of GDP-mannose in plant ascorbic acid (vitamin C) biosynthesis

Vitamin C (L-ascorbic acid; AsA) acts as a potent antioxidant and cellular reductant in plants and animals. AsA has long been known to have many criti cal physiological roles in plants, yet its biosynthesis is only currently b eing defined. A pathway for AsA biosynthesis that features GDP-mannose and L-galactose has recently been proposed for plants. We have isolated a colle ction of AsA-deficient mutants of Arabidopsis thaliana that are valuable to ols for testing of an AsA biosynthetic pathway. The best-characterized of t hese mutants (vtc1) contains approximate to 25% of wildtype AsA and is defe ctive in AsA biosynthesis. By using a combination of biochemical, molecular , and genetic techniques, we have demonstrated that the VTC1 locus encodes a GDP-mannose pyrophosphorylase (mannose-1-P guanyltransferase). This enzym e provides GDP-mannose, which is used for cell wall carbohydrate biosynthes is and protein glycosylation as well as for Ash biosynthesis. In addition t o genetically defining the first locus involved in AsA biosynthesis, this w ork highlights the power of using traditional mutagenesis techniques couple d with the Arabidopsis Genome Initiative to rapidly clone physiologically i mportant genes.