ULTRAVIOLET-B-INDUCED AND OZONE-INDUCED BIOCHEMICAL-CHANGES IN ANTIOXIDANT ENZYMES OF ARABIDOPSIS-THALIANA

Earlier studies with Arabidopsis thaliana exposed to ultraviolet B (UV -B) and ozone (O-3) have indicated the differential responses of super oxide dismutase and glutathione reductase. In this study, we have inve stigated whether A. thaliana genotype Landsberg erecta and its flavono id-deficient mutant transparent testa (tt5) is capable of metabolizing UV-B- and O-3-induced activated oxygen species by invoking similar an tioxidant enzymes. UV-B exposure preferentially enhanced guaiacol-pero xidases, ascorbate peroxidase, and peroxidases specific to coniferyl a lcohol and modified the substrate affinity of ascorbate peroxidase. O- 3 exposure enhanced superoxide dismutase, peroxidases, glutathione red uctase, and ascorbate peroxidase to a similar degree and modified the substrate affinity of both glutathione reductase and ascorbate peroxid ase. Both UV-B and O-3 exposure enhanced similar Cu,Zn-superoxide dism utase isoforms,New isoforms of peroxidases and ascorbate peroxidase we re synthesized in tt5 plants irradiated with UV-B. UV-B radiation, in contrast to O-3, enhanced the activated oxygen species by increasing m embrane-localized NADPH-oxidase activity and decreasing catalase activ ities. These results collectively suggest that (a) UV-B exposure prefe rentially induces peroxidase-related enzymes, whereas O-3 exposure inv okes the enzymes of superoxide dismutase/ascorbate-glutathione cycle, and (b) in contrast to O-3, UV-B exposure generated activated oxygen s pecies by increasing NADPH-oxidase activity.