Light regulation of ascorbate biosynthesis is dependent on the photosynthetic electron transport chain but independent of sugars in Arabidopsis

It has been known that leaves exposed to high light contain more L-ascorbic acid (AsA) than those in the shade. However, the mechanism of the light regulation of the AsA pool size in plants is largely unknown. In this work, the relationship between gene expression levels related to AsA biosynthesis and photosynthesis have been studied. When 2-week-old Arabidopsis plants grown under a 16 h daily photoperiod were moved into the dark, the AsA level in the leaves was decreased by 91% in 72 h, whereas it increased by 171% in the leaves of plants exposed to continuous light during the same period. Among the several enzymes of the AsA biosynthesis pathway, the transcript levels of GDP-D-mannose pyrophosphorylase, L-galactose 1-P phosphatase, L-galactono-1,4-lactone dehydrogenase, and the VTC2 gene were down-regulated in the dark. Treatment with inhibitors of photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine, arrested a rise in the AsA pool size accompanying the decrease in the transcript levels of the genes of the above enzyme in the leaves. When the plants were transferred to a medium containing 0.5% (w/v) sucrose, the photosynthesis activities and the leaf AsA levels were lowered even under exposure to light compared with those in plants on the medium without sucrose. In contrast, the AsA level in leaves of the sugar-insensitive Arabidopsis mutant abi4/sun6 was unaffected by external sucrose. No significant difference in the expression profiles for AsA biosynthesis enzymes was observed between the wild-type and mutant plants by sucrose feeding. The results suggest that photosynthetic electron transport of chloroplasts is closely related to AsA pool size regulation in leaves.