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Catalase (CAT) dismutates the reactive oxygen species H₂O₂ into water and dioxygen and in plants; it is located in peroxisomes and glyoxysomes. In the present study, we investigated the effect of cadmium (a well-known oxidative stress inducer) on catalase in roots and cotyledons of developing sunflower seedlings, at 10 μM and 100 μM. Although germination was unaltered after 48 h of exposure to 100 μM Cd²⁺, root length was significantly reduced. CAT activity was also significantly reduced, but this activity was completely restored (10 μM treatment) or even enhanced (100 μM treatment) 24 h later. Although CAT protein abundance remained similar to control in roots and cotyledons of Cd-treated seedlings, cadmium produced CAT protein oxidation, indicating that the mechanism of CAT inactivation by Cd²⁺ involves oxidation of the protein structure. The transcripts of the four genes described for sunflower …