i - Ascorbic Acid Metabolism in the Ascorbate - Deficient Arabidopsis Mutant vtc 7 ’

The biosynthesis of i-ascorbic acid (vitamin C) is not well understood in plants. The ozone-sensitive Arabidopsis thaliana mutant vifamin cl (vfcl; formerly known as sozl) is deficient in ascorbic acid, accumulating approximately 30% of wild-type levels. This deficiency could result from elevated catabolism or decreased biosynthesis. No differences that could account for the deficiency were found in the activities of enzymes that catalyze the oxidation or reduction of ascorbic acid. The absolute rate of ascorbic acid turnover is actually less in vtcl than in wild type; however, the turnover rate relative to the pool of ascorbic acid is not significantly different. The results from [U-’4C]Glc labeling experiments suggest that the deficiency is the result of a biosynthetic defect: less L[‘4C]ascorbic acid as a percentage of total soluble 14C accumulates in vfcl than in wild type. The feeding of two putative biosynthetic intermediates, o-glucosone and i-sorbosone, had no positive effect on ascorbic acid levels in either genotype. The vtcl defect does not appear to be the result of a deficiency in i-galactono-l,44actone dehydrogenase, an enzyme able to convert L-galactono-l,4-lactone to ascorbic acid.