Involvement of abscisic acid-dependent and - Independent pathways in the upregulation of antioxidant enzyme activity during NaCl stress in cotton callus tissue

The role of abscisic acid (ABA) in the signal transduction pathway associat ed with NaCl-induced up-regulation of antioxidant enzyme activity was exami ned in a NaCl-tolerant cotton callus cell line treated with NaCl, ABA, para quat, or H2O2 in the presence and absence or fluridone, an inhibitor of ter pene, and therefore, ABA synthesis. Treatment with NaCl resulted in a rapid increase (within 30 minutes) in the ABA levels of the callus tissue, and t he NaCl, ABA, and paraquat treatments induced rapid increases in the activi ties of superoxide dismutase, catalase, peroxidase, and glutathione reducta se. Pre-treatment with fluridone significantly suppressed the NaCl-induced increases, but only slightly delayed the increases in tissue subjected to e xogenous ABA treatment. This implies that ABA is involved in the signal tra nsduction pathway associated with the NaCl-induced up-regulation of these a ntioxidant enzymes. Pre-treatment with fluridone had no effect on the paraq uat-induced increases, suggesting that these enzymes can also be up-regulat ed by a pathway other than the one mediated by ABA. Both the NaCl and paraq uat treatments produced significant increases in the superoxide levels with in the callus, but the increase resulting from the paraquat treatment was s ignificantly higher than the increase resulting from the NaCl treatment. Th ese data suggest that NaCl stress results in the production of reactive oxy gen intermediates (ROI) which signals the induction of an ABA-dependent sig naling pathway. The production of very high levels of ROI, such as those th at occur with paraquat treatment or perhaps during periods of prolonged or extreme stress, may induce an ABA-independent signaling pathway.