USE OF TRANSGENIC PLANTS TO STUDY ANTIOXIDANT DEFENSES

The abundance of O-2 and the highly energetic electron transfer reacti ons associated with thylakoid membranes make chloroplasts a major sour ce of reactive oxygen intermediates (ROI) in photosynthetic tissues of plants. Attempts to reduce oxidative damage in chloroplasts have incl uded the manipulation of ROI-scavenging enzymes by gene transfer techn ology. Much of this work has focused on chloroplast-localized superoxi de dismutase (SOD), both chloroplast-targeted and cytosolic ascorbate peroxidase (APX) and glutathione reductase (GR). Increased activity of SOD in chloroplasts of transgenic tobacco plants generally leads to i ncreased protection from membrane damage caused by exposure to methyl viologen (MV). In addition, overexpression of chloroplastic Cu/Zn SOD can lead to increased protection from photooxidative damage caused by high light intensity and low temperatures. Transgenic tobacco plants t hat overexpress APX either in the cytosol or chloroplastic compartment s also show reduced damage following either MV exposure or photooxidat ive treatment and transgenic plants that express increased levels of G R have elevated pools of ascorbate and GSH. Though still preliminary, these results clearly indicate that alterations in the expression of e nzymes involved in ROI-scavenging can have significant metabolic effec ts and provide the hope that this strategy can be used to develop crop plants with increased stress tolerance. (C) 1997 Elsevier Science Inc .