DEVELOPMENTAL-CHANGES OF ANTIOXIDATIVE SYSTEMS IN TOBACCO-LEAVES AS AFFECTED BY LIMITED SUCROSE EXPORT IN TRANSGENIC PLANTS EXPRESSING YEAST-INVERTASE IN THE APOPLASTIC SPACE

It is generally believed that a restricted export of carbohydrates fro m source leaves causes oxidative stress because of an enhanced utilisa tion of O-2 instead of NADP(+) as electron acceptor in photosynthesis. To test this hypothesis, developmental changes of antioxidative syste ms were investigated in wild-type and transgenic tobacco (Nicotiana ta bacum L.) suffering from disturbed sink-source relations by expression of yeast invertase in the apoplastic space. Young expanding leaves of the wild type contained higher activities of superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), catalase (EC 1.11.1.6 ), dehydroascorbate reductase (EC 1.8.5.1), glutathione reductase (EC 1.6.4.2) and a higher glutathione content than mature source leaves. T he activity of monodehydroascorbate-radical reductase (EC 1.1.5.4) and the ascorbate content remained unaffected by the developmental stage in the wild type. In young expanding leaves of the transgenic plants t he capacity of the antioxidative systems was similar to or higher than in corresponding leaves from the wild type. Source leaves of transgen ic tobacco with an increased carbohydrate content showed a small chlor ophyll loss, an increased malondialdehyde content, a selective loss of the activities of Cu/Zn-superoxide dismutase isoenzymes and a fourfol d decrease in ascorbate compared with the wild type. There was no evid ence that the protection from H2O2 was insufficient since source leave s of transgenic tobacco contained increased activities of catalase, as corbate peroxidase, and monodehydroascorbate-radical reductase and an increased ascorbate-to-dehydroascorbate ratio compared with source lea ves of the wild type. In severely chlorotic leaf sections of the trans genic plants, most components of the antioxidative system were lower t han in green leaf sections, but the ascorbate-to-dehydroascorbate rati o was increased. These results suggest that carbohydrate-accumulating cells have an increased availability of reductant, which can increase the degree of reduction of the ascorbate system via glutathione-relate d systems or via the activity of monodehydroascorbate-radical reductas e. At the same time, transgenic tobacco leaves seem to suffer from an increased oxidative stress, presumably as a result of a decreased cons umption of O-2(.-) by Cu/Zn-superoxide dismutases in the chloroplasts. There was no evidence that carbohydrate-accumulating leaves acclimate d to enhanced O-2(.-) production rates in the chloroplasts.