Overexpression of Mn-superoxide dismutase in maize leaves leads to increased monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase activities

The effect of increased Mn-superoxide dismutase (SOD) on antioxidant enzyme s and metabolites was studied using transformed maize, TG1+ and TG2+. The p rogeny of the backcross of each of the primary transformants with the paren tal line generated two populations denoted M6884 and M6885. These were grow n at optimal (25 degreesC) and sub-optimal (18, 14 and 10 degreesC) tempera tures to assess the impact of elevated SOD activity on cord tolerance and t he antioxidant defences in maize. The plants of the M6885 population had si milar foliar SOD activities to the untransformed maize plants. Within the s egregating M6884 population 50% of the plants had elevated SOD activity (up to four times the activity of the untransformed controls) and 50% of the p lants contained the product of the transgene. In untransformed plants grown at 25 degreesC and 18 degreesC, SOD activity was not detectable in mesophy ll extracts. Similarly, increased foliar SOD activity in the M6884 transfor med maize did not lead to detectable mesophyll SOD activity. Increased foli ar KCN-insensitive SOD activities were accompanied by enhancement of monode hydroascorbate reductase, dehydroascorbate reductase and glutathione reduct ase activities; enzymes which are localized exclusively in the leaf mesophy ll tissues. Increased foliar SOD activity had no effect on the hydrogen per oxide, glutathione or ascorbate contents of the leaves. This suggests that increased recycling of reduced ascorbate was required to compensate for enh anced hydrogen peroxide production in transformed plants.