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Abstract

Free radicals and other active derivatives of oxygen are inevitable by-products of biological redox reac- tions. Reactive oxygen species inactivate enzymes and damage important cellular components. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plant and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. The effects of the action of free radi- cals on membranes include the induction of lipid peroxidation and fatty acid de-esterification. Both ethylene biosynthesis and membrane breakdown, which appear to be closely linked, seem to involve free radicals, although the sequence of events gener- ating these free radicals is still poorly understood. It is clear that the capacity and activity of the antioxi- dative defence system are important in limiting oxi- dative damage and in destroying active oxygen species that are produced in excess of those normally required for metabolism. Transgenic plants offered us a means by which to achieve complete under- standing of the roles of the enzymes involved in pro- tection against stress of many types, environmental and induced. Studies on transformed plants express- ing increased activities of single enzymes of the anti- oxidative defence system indicate that it is possible to confer a degree of tolerance to stress by these means. The advent of plant transformation has placed within our grasp the possibility of engineering greater stress tolerance in plants by enhancements of the antioxidative defence system.