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We have monitored the changes in antioxidative enzyme activities and their mRNA levels in tobacco (Nicotiana tabacum L. cv.) Bright Yellow 2 (BY-2) cell cultures subjected to salt stress, polyethylene glycol (PEG)-induced drought stress, and exogenous abscisic acid (ABA). Both NaCl and PEG treatments led to an increase in the total superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities, whereas glutathione reductase (GR) activity remained unchanged. Increase in total SOD and APX activities was observed in cells grown in the presence of ABA. CAT activity remained constant whereas GR activity was reduced after the ABA treatment. All osmotic stress and ABA treatments had a differential effect on the accumulation of transcripts of the tested antioxidant genes. Our results show that osmotic stress alters the enzymatic defense reactions in tobacco BY-2 cell suspensions and …