EN
Previous studies argue that salicylic acid (SA) plays an important role in the plant signal transduction pathway(s) leading to disease resistance. It has been proposed that one of its modes of action is inhibition of catalase and elevation of H₂O₂ level in the tissue. To verify the role of SA and H₂O₂ during pathogenesis, transgenic tobacco plants expressing Saccharomyces cerevisiae CTA1 gene coding for peroxisomal catalase were constructed. These plants possess 2-4-fold higher total catalase activity under normal growth conditions. No symptoms of chlorosis and/or necrosis were observed. Levels of pathogenesis-related proteins (PR) and their respective mRNAs were significantly reduced in the infected leaves of the transgenic plants. No change in PR expression was detected in uninfected leaves of both CTA1 and control plants challenged with TMV. These results suggest that elevation in catalase activity and resulting reduction of H₂O₂ level results in more severe local disease symptoms, apparently due to alteration of the hypersensitive response mechanism and does not influence systemic acquired resistance after viral infection.