Ascorbic acid (AsA) biosynthesis in plants predominantly occurs via a pathway with dmannose and lgalactose as intermediates. One alternative pathway for AsA synthesis, which is similar to the biosynthesis route in mammals, is controversially discussed for plants. Here, myoinositol is cleaved to glucuronic acid and then converted via lgulonate to AsA. In contrast to animals, plants have an effective recycling pathway for glucuronic acid, being a competitor for the metabolic rate. Recycling involves a phosphorylation at C1 by the enzyme glucuronokinase.
Two previously described TDNA insertion lines in the gene coding for glucuronokinase1 show wild typelike expression levels of the mRNA in our experiments and do not accumulate glucuronic acid in labelling experiments disproving that these lines are true knockouts. As suitable TDNA insertion lines were not available, we generated frameshift mutations in the major expressed isoform glucuronokinase1 (At3g01640) to potentially redirect metabolites to AsA.
However, radiotracer experiments with 3Hmyoinositol revealed that the mutants in glucuronokinase1 accumulate only glucuronic acid and incorporate less metabolite into cell wall polymers. AsA was not labelled, suggesting that Arabidopsis cannot efficiently use glucuronic acid for AsA biosynthesis.
All four mutants in glucuronokinase as well as the wild type have the same level of AsA in leaves.