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Published on December 17, 2008; 10.1104/pp.108.130948

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Received October 9, 2008
Accepted December 16, 2008

myo-Inositol oxygenase controls the level of myo-inositol in Arabidopsis but does not increase ascorbic acid

Stefanie Endres and Raimund Tenhaken *

University of Salzburg, Dep. Cell Biology, Plant Physiology, Hellbrunnerstr. 34, 5020 Salzburg, Austria

* Corresponding author; email: raimund.tenhaken{at}sbg.ac.at.

Ascorbic acid (AsA) is a major plant antioxidant. Mutants like vtc1 show a reduced AsA concentration which confirmed by genetic evidence the afore proposed AsA-pathway via GDP-mannose. Here we investigate the role of an animal-like alternative biosynthesis route to AsA, starting from the metabolite D-glucuronic acid which is produced in plants by myo-inositol oxygenase (Miox). Miox overexpressing lines have a more than 30-fold upregulated 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 myo-inositol and accumulate less myo-inositol in feeding experiments due to an enhanced turnover rate. The AsA concentration remains the same in wild type and the 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 overepxressing lines. We conclude that D-glucuronic acid derived from the Miox reaction plays a negligible role for AsA biosynthesis. However, the Miox controls the metabolite level of myo-inositol in plants.






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