*4.1. Effects on Other Antioxidants and ROS-Detoxifying Enzymes*

As a major antioxidant in plants, changes in Asc content may well affect other antioxidant pools. Moreover, different approaches used for increasing Asc might be expected to affect specific antioxidants disproportionately. For example, increasing Asc by increasing biosynthetic activity would impact different antioxidant pools than would increasing Asc through improved Asc recycling which requires GSH and NADPH (or NADH) for the reduction of DHA or MDHA by DHAR or MDAR, respectively. Although most reports have observed increases in Asc content and/or in the Asc redox state following an increase in DHAR expression, the impact of this increase of DHAR activity on other antioxidants is less clear. Increases in GSH were observed in tobacco and maize expressing wheat DHAR with no change in glutathione reductase (GR), superoxide dismutase (SOD), APX, or CAT activities [5], suggesting coordinate regulation between DHAR and GSH. A similar increase in GSH content was reported for *Arabidopsis* overexpressing DHAR [62]. In contrast, expression of human DHAR in tobacco chloroplasts resulted in a reduction in GSH that was accompanied by a 1.43-fold increase in GR activity [59]. Whether this was a consequence of expression of DHAR in chloroplasts remains to be determined.
