PROTECTION AGAINST OXYGEN RADICALS - AN IMPORTANT DEFENSE-MECHANISM STUDIED IN TRANSGENIC PLANTS

Free radicals and other active derivatives of oxygen are inevitable by -products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radica ls, inactivate enzymes and damage important cellular components. In ad dition, singlet oxygen, produced via formation of triplet state chloro phyll, is highly destructive. This oxygen species initiates lipid pero xidation, and produces Lipid peroxy radicals and Lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stres s conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence syste m of the plant comprises a variety of antioxidant molecules and enzyme s. Considerable interest has been focused on the ascorbate-glutathione cycle because it has a central role in protecting the chloroplasts an d other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are imp ortant in limiting photo-oxidative damage and in destroying active oxy gen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could no t be obtained easily by either a purely physiological or purely bioche mical approach. Transgenic plants offered us a means by which to achie ve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man-made . The ability to engineer plants which express introduced genes at hig h levels provides an opportunity to manipulate the levels of these enz ymes, and hence metabolism in vivo. Studies on transformed plants expr essing increased activities of single enzymes of the antioxidative def ence system indicate that it is possible to confer a degree of toleren ce to stress by this means. However, attempts to increase stress resis tance by simply increasing the activity of one of the antioxidant enzy mes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these tran sformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stre ss has become a feasible objective through the application of molecula r genetic techniques in conjunction with a biochemical and physiologic al approach.