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Aim. In plants, the enzymatic and non-enzymatic environmental stress resistance mechanisms function in a concerted manner, but the role of specific isoforms of antioxidant enzymes and their relationship to low molecular weight protective compounds is poorly understood. To investigate this question free proline levels were compared under salt stress conditions in wild-type Arabidopsis thaliana and knockout mutants for the catalase genes Cat2 and Cat3. Methods. Free proline content was measured under various treatments of plants with sodium chloride. Results. It was shown that under salt stress conditions free proline increase is impaired in leaves of mutants lacking CAT2 and CAT3 activity. In addition, cat3 knockout line shows a reduced basal level of proline under non-stress conditions. Conclusions. Suppression of response to salt stress points to a possible link between the rearrangement of the antioxidant system in the Cat genes knockouts and the regulation of protective cellular response to salt stress.Key words: Arabidopsis thaliana, knockout mutants, catalase, proline, sodium chloride.

Aim. In plants, the defense response to abiotic stress includes induction of stress proteins and increase in content of protective metabolites. To date, the role of specific isoforms of antioxidant enzymes in stress responses and their relation to low-molecular weight protective compounds are still not clarified. To study this question the content of polyphenolic compounds (PPC) was evaluated under salt stress in Arabidopsis thaliana wild-type (WT) and in catalase 2 (Cat2) knockout mutant plants. Methods. PPC content in different variants of treatment with sodium chloride was measured. Results. It was shown that under optimal cultivation conditions the content of PPC in leaves of cat2 mutants is higher than in WT leaves. However, cultivation of isolated shoots in nutrient medium resulted in a faster depletion of the PPC pool in the cat2 line. Also, short-term salt stress results in equal depletion of the PPC pool in both, WT and cat2. Conclusions. The increase of PPC content in cat2 leaves is a manifestation of metabolic alterations that aim to compensate the reduced catalase activity.Keywords: multigenic family, knockout mutants, polyphenolic compounds, sodium chloride, Arabidopsis thaliana.