Study on generation of ascorbic acid-enriched transgenic rice

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L-ascorbic acid (AsA), Called vitamin Cm is accumulated up to millimolar concentrations in plant tissues. L-Galactono-1, 4-lactone dehydrogenase (GalLDH; EC 1.3.2.3) is the final enzyme in L-ascorbic acid (AsA) biosynthesis pathway of plant, and it catalyzes the oxidation of L-galactono-1, 4-lactone to AsA. AsA-overproducing transgenic tobacco BY-2 cell-lines were generated by over-expression of the GalLDH cDNA under the control of cauliflower mosaic virus 35S promoter. Over-expression of GalLDH mRNA let to increases in the GalLDH activity and fine protein. AsA levels of this transgenic tobacco were detected about 6-18 fold as much as wild type (W) cells. Interestingly in these AsA-enriched transgenic cell cultures, cell browning markedly restrained, suggesting that a higher level of AsA suppressed senescence of these transgenic cells. Furthermore, these AsA over-producing transgenic cells could acquire resistance against paraquat that produces some active oxygen species. Thus, AsA-overproducing transgenic tobacco cells were suggested to have a higher ability to scavenge active oxygen species. Rice GalLDH cDNA was also clones. The expression of GalLDH was positively correlated with ascorbic acid content of rice. In rice, the biosynthetic pathway of ascorbic acid was investigated by supplement experiment of 9 precursors of ascorbic acid. In rice root, an increase in ascorbic acid content stimulated root elongation. Furthermore, the increase in ascorbic acid inhibited degradation of leaf chlorophyll caused by UV.