10.5Transgenic Adaptation to Oxidat

10.5Transgenic Adaptation to Oxidative StressFortification of the antioxidative machinery using genetic engineering to ameliorateoxidative stress tolerance has been one of the many approaches for themaintenance of plant productivity under environmental stress conditions [79].Transformation of plants with a single transgene, encoding one of the antioxidantenzymes, has been carried out yielding contrasting results. Studies that addressedthe overproduction of Cu/Zn-SOD [80,81], Mn-SOD [82,83], or Fe-SOD [84] in thechloroplasts resulted in enhanced tolerance to oxidative stress. In addition,transgenic plants expressing gene constructs for either cytosolic or a chloroplast?targeted cytosolic APX have increased tolerance against various abiotic stresses,including water stress [85,86]. Faiz et al. [87] demonstrated the involvement ofcytosolic APX and Cu/Zn-SOD in transgenic tobacco showing improved toleranceagainst drought stress. However, several other reports indicated no improvementsof tolerance to oxidative or environmental stress [8891]. These contradictoryresults have usually been attributed to the complexity of the scavenging pathways,because modification of one enzyme may not affect the antioxidative system as awhole. To overcome this problem, pyramiding or stacking of antioxidant genes hasbeen attempted (for a review, see [92]). Therefore, instead of trying with any singlegene, a group of antioxidant genes are stacked to develop stress-tolerant transgenicplants [93,94]. While efforts are still ongoing to develop stress-tolerant transgeniccrops by cloning and overexpressing antioxidant enzymes involved in theascorbateglutathione cycle and other ROS detoxification mechanisms(Table 10.3), the success achieved so far has been rather limited. To date, onlyabout 10 different stress-tolerant transgenic plant species against drought, salinity,or temperature have been developed. Information on the performance of suchtransgenic stress-tolerant crops under abiotic stress under field conditions is,however, not readily available for assessment, which is essential for the assessmentof stress tolerance vis-
a-vis productivity or yield.