Molecular analysis of ascorbate synthesis in plants
While ascorbic acid is a well-known molecule because of its dietary significance, most aspects of its metabolism and some aspects of its function in plants are poorly understood. The ascorbate biosynthetic pathway has not been firmly established even though it reaches millimolecular concentrations in most tissues. Humans and some other animals (including other primates and guinea pigs) depend on ascorbate in their diet because of loss of a functional form of the last enzyme (L-gulono-1,4-lactone dehydrogenase) of the biosynthesis pathway.
Thanks to recent advances and new approaches to the investigation of ascorbate biosynthesis and function, we can understand its role in photosynthesis and photoprotection, in defense against ozone and other oxidative stress and possibility about its role in cell division and cell expansion. Studies have shown that ascorbate is critical for scavenging of hydrogen peroxide (H2O2) in and by the finding of ascorbate peroxidase (APX) in plants and Euglena. These results indicate that ascorbate plays a central role in scavenging of H2O2 in plant tissues.
On the basis of this research, I attempted to compare the differences in ascorbate synthesis and oxidative stress tolerance in transgenic tobacco plants that express the putative ascorbate biosynthetic enzymes ; L-galactono-1,4- lactone dehydrogenase, L-gulono-1,4-lactone dehydrogenase, L-sorbose dehydrogenase and L-sorbosone dehydrogenase. Tobacco plants that express trans-genes for these enzymes were developed and tested. Plants that express GLDH and GLO showed increased activities of enzymes and increased amount of L-ascorbate. However, analysis of the stress tolerance characteristics of these plants indicates no remarkable increase in protection from oxidative stress.
These results, demonstrate that it is possible to manipulate ascorbate biosynthesis in plants, and the role of this increased ascorbate concentration in plants by genetic manipulation against environmental stress. Though the modifications in plants by genetic changes in ascorbate levels are small, it is possible that more optimal approaches could benefit for human nutrition, stress resistance in plants.