Differential Resistance to Methyl Viologen in Transgenic Tobacco Plants that Express Sweet Potato Peroxidases

To analyze the physiological role of each peroxidase (POD) isoenzyme in environmental stress adaptation, transgenic tobacco (Nicotiana tabacum) plants expressing either a sweet potato (Ipomoea batatas) anionic POD (swpa 1) or neutral POD (swpn 1) were tested for responses to oxidant methyl viologen (MV). Fully expanded leaves of both swpa 1- and swpn 1-transgenic plants had about two times higher POD activity than non transformed (NT) plants. When tobacco leaf discs were subjected to MV at 10 and 100 µmol/L, swpa 1-transgenic plants showed about a 25 % reduction in membrane damage relative to swpn 1-transgenic or NT plants. Leaves of swpn 1-transgenic and NT plants were bleached more than those of swpa 1-transgenics by 1 µmol/L MV treatment, whereas all plants were severely damaged at 3 µmol/L MV. These results indicate that the increased H2O2-scavenging capacity provided by the swpa 1 POD (a guaiacol-type POD) contributes to increased protection against MV-mediated oxidative damage. Furthermore, differences in response to oxidant treatments indicate unique functions for the two isoenzymes encoded by swpa 1 and swpn 1.