Expressional characterization of dehydroascorbate reductase cDNA in transgenic potato plants

In plants, ascorbic acid (AsA) is a strong antioxidant or reductant that can be converted to dehydroascorbate (DHA) by oxidation. DHA, a very short-lived chemical, can either be hydrolyzed irreversibly to 2,3-diketogulonic acid or recycled to AsA by dehydroascorbate reductase (DHAR). DHAR cDNA, isolated from sesame hairy roots, was inserted into two plant expression vector systems with the CaMV35S promoter (CaMV35S::DHAR) and a potato tuber-specific promoter, Patatin (Patatin::DHAR). Southern and northern blot hybridization analyses indicated that DHAR cDNA was successfully integrated into the potato genome and actively transcribed. High levels of sesame DHAR transcript and DHAR enzyme activity were determined, by the Patatin promoter, in regenerated potato tubers, but their levels in leaves were very low. In contrast, much higher amounts of transcript were accumulated in the leaves of CaMV35S::DHAR regenerants than in the tubers while the activity of DHAR enzyme was higher in the latter. AsA content in the tubers of Patatin::DHAR transgenic lines was also increased (1.1- to 1.3-fold) compared with that of non-transgenic plants. However, this was not true for the transgenic leaves. in contrast, the CaMV35S promoter was associated with AsA accumulations in both the tubers (up to 1.6-fold) and the leaves (up to 1.5-fold). However, more detailed analyses indicated that this increased enzyme activity was not always accompanied by an elevation in AsA content from transgenic plants. This suggests that other factors may limit the accumulation of vitamin C via ascorbate-recycling in transgenic potato plants.