抄録
Ascorbic acid (AsA) is commonly known as vitamin C. Plants are the main source of vitamin C for humans, and accumulate it in the order of several mM in their cells. Plants are always susceptible to the generation of reactive oxygen species (ROS) during light irradiation because of the coexistence of oxygen-evolving and photosynthetic electron transport systems through photosynthesis. Under harsh environmental conditions such as high light, the generation of ROS is further accelerated. Therefore, plants have developed an ability to synthesize such large amounts of AsA. Plants have also acquired ascorbate peroxidase (APX) as a plant-specific antioxidant enzyme, which enables efficient ROS metabolism while plants perform photosynthesis. The author has clarified the biological functions of APX isoforms from a molecular physiological perspective by identifying their genes. The expression of two spinach chloroplastic APX isoforms, which are distributed in both stroma and thylakoid-membrane, was found to be regulated by alternative splicing, and enhancement of their enzyme activities was found to be an efficient target for improvement of photooxidative stress tolerance in plants. In addition, the author has succeeded in identifying the pathway of AsA biosynthesis in land plants and algae by molecular genetics. Especially, land plants have developed the D-mannose (D-Man)/L-galactose (L-Gal) pathway as their dominant AsA supplying route, and GDP-L-Gal phosphorylase is the rate-limiting enzyme in the pathway on light-dependent AsA biosynthesis. A comprehensive analysis of the genome database was conducted to provide evolutionary considerations for the acquisition of AsA biosynthetic pathways between organisms. Focusing on the final enzyme in the D-Man/L-Gal pathway, L-galactono-1,4-lactone dehydrogenase, its distribution can be explained reasonably by considering that it was acquired and developed by eukaryotic cells through endosymbiotic gene transfer at the time of plastid acquisition, and functionally replaced by L-gulono-1,4-lactone oxidase, an orthologue commonly found in animal AsA biosynthesis pathway.
