OVEREXPRESSION OF SUPEROXIDE-DISMUTASE PROTECTS PLANTS FROM OXIDATIVESTRESS - INDUCTION OF ASCORBATE PEROXIDASE IN SUPEROXIDE DISMUTASE-OVEREXPRESSING PLANTS

Photosynthesis of leaf discs from transgenic tobacco plants (Nicotiana tabacum) that express a chimeric gene that encodes chloroplast-locali zed Cu/Zn superoxide dismutase (SOD+) was protected from oxidative str ess caused by exposure to high light intensity and low temperature. Un der the same conditions, leaf discs of plants that did not express the pea SOD isoform (SOD-) had substantially lower photosynthetic rates. Young plants of both genotypes were more sensitive to oxidative stress than mature plants, but SOD+ plants retained higher photosynthetic ra tes than SOD- plants at all developmental stages tested. Not surprisin gly, SOD+ plants had approximately 3-fold higher SOD specific activity than SOD- plants. However, SOD+ plants also exhibited a 3- to 4-fold increase in ascorbate peroxidase (APX) specific activity and had a cor responding increase in levels of APX mRNA. Dehydroascorbate reductase and glutathione reductase specific activities were the same in both SO D+ and SOD- plants. These results indicate that transgenic tobacco pla nts that overexpress pea Cu/Zn SOD II can compensate for the increased levels of SOD with increased expression of the H2O2-scavenging enzyme APX. Therefore, the enhancement of the active oxygen-scavenging syste m that leads to increased oxidative stress protection in SOD' plants c ould result not only from increased SOD levels but from the combined i ncreases in SOD and APX activity.