Abstract
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.
Highlights
L-Ascorbate (AsA) is a very important antioxidant in plants protecting especially the chloroplast from oxidative damage (Noctor and Foyer, 1998)
We investigate the role of an animal-like alternative biosynthesis route to Ascorbic acid (AsA), starting from the metabolite D-glucuronic acid which is produced in plants by myoinositol oxygenase (Miox)
Transcript levels Two independent transgenic Arabidopsis lines produced by the Nessler Lab in Blacksburg, VA, overexpressing the Miox4 gene under the control of the cauliflower mosaic virus 35S promoter were analyzed for the expression level of Miox4 transcripts in mature leaves
Summary
L-Ascorbate (AsA) is a very important antioxidant in plants protecting especially the chloroplast from oxidative damage (Noctor and Foyer, 1998). The pathway starts from D-glucuronic acid followed by a reduction to L-gulonic acid and ring formation to gulonolactone (Smirnoff et al, 2001) The precursor mannose-1-phosphate is activated to GDP-mannose (vtc mutation) followed by an epimerization to GDP-L-galactose (Fig.1) After hydrolysis of this nucleotide sugar, the liberated L-Gal is reduced to L-galactono1,4-lactone which is eventually oxidized to L-AsA (Wheeler et al, 1998; Smirnoff et al, 2001). The overexpression of a gene for myo-inositol oxygenase (Miox) led to an increase of AsA in transgenic Arabidopsis lines by 2 to 3-fold. The increase in phosphatase transcripts could result in an increase of myoinositol which after ring cleavage to D-GlcUA would provide a precursor for an alternative AsA pathway in plants.
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