Oxygen free radicals are thought to play an essential role in senescence, especially those derived from peroxisomes. Therefore, the activities of different isoforms of the peroxisomal hydrogen peroxide (H 2 O 2 )-scavenging enzyme catalase (CAT) were analysed during senescence of Arabidopsis . CAT2 activity decreased with bolting time parallel with cytosolic ascorbate peroxidase 1 (APX1) activity before loss of chlorophyll could be measured. At the same time point, the H 2 O 2 content increased. Subsequently, the stress-inducible CAT3 isoform was activated and APX1 activity was recovered, accompanied by a decline of the H 2 O 2 content. In very late stages, low activities of the seed-specific CAT1 became detectable in leaves, but H 2 O 2 increased again. Further analyses of CAT expression by promoter : β β β β -glucuronidase ( GUS ) fusions in transgenic plants revealed a vasculature-specific CAT3 expression, whereas CAT2 expression turned out to be specific for photosynthetic active tissues. CAT2 expression is down-regulated during leaf senescence, while CAT3 expression is induced with age and corresponds to an accumulation of H 2 O 2 in the vascular bundles. CAT2 downregulation on the transcriptional level appears as the initial step in creating the H 2 O 2 peak during bolting time, while the decrease in APX1 activity might only be a secondary and amplifying effect.

No abstract