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 enzymes and metabolites was studied using transformed maize, TG1+ and TG2+. The progeny of the backcross of each of the primary transformants with the parental line generated two populations denoted M6884 and M6885. These were grown at optimal (25 degrees C) and sub-optimal (18, 14 and 10 degrees C) temperatures to assess the impact of elevated SOD activity on cold tolerance and the antioxidant defences in maize. The plants of the M6885 population had similar foliar SOD activities to the untransformed maize plants. Within the segregating M6884 population 50% of the plants had elevated SOD activity (up to four times the activity of the untransformed controls) and 50% of the plants contained the product of the transgene. In untransformed plants grown at 25 degrees C and 18 degrees C, SOD activity was not detectable in mesophyll extracts. Similarly, increased foliar SOD activity in the M6884 transformed maize did not lead to detectable mesophyll SOD activity. Increased foliar KCN-insensitive SOD activities were accompanied by enhancement of monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase activities; enzymes which are localized exclusively in the leaf mesophyll tissues. Increased foliar SOD activity had no effect on the hydrogen peroxide, glutathione or ascorbate contents of the leaves. This suggests that increased recycling of reduced ascorbate was required to compensate for enhanced hydrogen peroxide production in transformed plants.