L-ASCORBIC-ACID METABOLISM IN THE ASCORBATE-DEFICIENT ARABIDOPSIS MUTANT VTC1

The biosynthesis of L-ascorbic acid (vitamin C) is not well understood in plants. The ozone-sensitive Arabidopsis thaliana mutant vitamin c- 1 (vtc1; formerly known as soz1) is deficient in ascorbic acid, accumu lating approximately 30% of wild-type levels. This deficiency could re sult from elevated catabolism or decreased biosynthesis. No difference s that could account for the deficiency were found in the activities o f enzymes that catalyze the oxidation or reduction of ascorbic acid. T he absolute rate of ascorbic acid turnover is actually less in vtc1 th an in wild type; however, the turnover rate relative to the pool of as corbic acid is not significantly different. The results from [U-C-14]G lc labeling experiments suggest that the deficiency is the result of a biosynthetic defect: less L-[C-14]ascorbic acid as a percentage of to tal soluble C-14 accumulates in vtc1 than in wild type. The feeding of two putative biosynthetic intermediates, D-glucosone and L-sorbosone, had no positive effect on ascorbic acid levels in either genotype. Th e vtc1 defect does not appear to be the result of a deficiency in L-ga lactono-1,4-lactone dehydrogenase, an enzyme able to convert L-galacto no-1,4-lactone to ascorbic acid.