Myoinositol Oxygenase Controls the Level of Myoinositol in Arabidopsis, But Does Not Increase Ascorbic Acid

Ascorbic acid (AsA) is a major plant antioxidant. Mutants like vtc1 show a reduced AsA concentration, which confirmed by genetic evidence the previously proposed AsA pathway via GDP-Man. Here we investigate the role of an animal-like alternative biosynthesis route to AsA, starting from the metabolite D-GlcUA, which is produced in plants by myoinositol oxygenase (Miox). Miox-overexpressing lines have a more than 30-fold up-regulated transcript level and higher enzymatic activity as shown by increased incorporation of Miox-derived sugars into cell wall polymers. In addition, Miox overexpressors exhibit a lower steady-state level of myoinositol and accumulate less myoinositol in feeding experiments due to an enhanced turnover rate. The AsA concentration remains the same in wild-type and Miox overexpressor lines. Even challenging plants with stress, which increases AsA concentration 4-fold, reveals no difference in AsA biosynthesis between wild-type and Miox-overexpressing lines. We conclude that D-GlcUA derived from the Miox reaction plays a negligible role for AsA biosynthesis. However, Miox controls the metabolite level of myoinositol in plants.