Role of chloroplastidial proteases in leaf senescence

In this report the effect of hydrogen peroxide (H₂O₂) on peroxidase (POD) activity during leaf senescence was studied with and without phenylmethylsulfonyl fluoride (PMSF) pre-treatment in detached neem (Azadirachta indica A. juss) leaf chloroplasts. Increased POD activity was detected in natural and H₂O₂-promoted senescent leaf chloroplasts compared to untreated control mature green leaf chloroplasts. However, under H₂O₂ POD activity markedly increased at 1 day, and then significantly decreased until 4 days. In the presence of H₂O₂, PMSF, the induction of POD activity was alleviated at 1 day, whereas reduced after 4 days. In contrast, in the presence of H₂O₂, cycloheximide (CX), the induction of POD activity was reduced at 1 day, whereas alleviated after 4 days. The was a partial reduction in H₂O₂-induced POD activity with PMSF and CX, indicating the presence of pre-existing inactive PODs in chloroplasts. We also propose a new role for chloroplastidial proteases as activators of pre-existing inactive PODs during leaf senescence.Key words: chloroplast, leaf senescence, peroxidase, protease