Loss of stress-induced expression of catalase3 during leaf senescence in Arabidopsis thaliana is restricted to oxidative stress

Different stress conditions can induce changes in the activity of the antio xidant enzymes superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidas e (APX, EC 1.11.1.11) and catalase (CAT, EC 1.11.1.6). The enzyme activitie s of all SOD and APX isoforms detected in young Arabidopsis leaves remained unaffected or slightly decreased after moderate paraquat treatment. While CAT2 activity also remained unaffected under these conditions, CAT3 enzyme activity was enhanced. In contrast to the enzyme activities, mRNA levels of both cat2 and cat3 were enhanced under oxidative stress induced by either paraquat or the fungal toxin cercosporin. This indicates that, with respect to enzyme activity level, CAT3 is the enzyme which is most sensitive to ox idative stress in this developmental stage and that the enzyme activity of CAT2 is possibly regulated at the post-transcriptional level. Interestingly , cat3 mRNA level and CAT3 activity are not elevated by paraquat treatment in senescing leaves. In contrast, the response to other stress conditions, such as water stress induced by flooding of detached leaves and heat stress , is maintained in senescing leaves. Since changes in stress response are n ot a general phenomenon in leaf senescence but appear to be restricted to o xidative stress, this might be a specific mechanism to promote senescence i n Arabidopsis thaliana. (C) 2001 Elsevier Science Ireland Ltd. All rights r eserved.