The developmental transition to flowering represses ascorbate peroxidase activity and induces enzymatic lipid peroxidation in leaf tissue in Arabidopsis thaliana

Leaf senescence in many plant species is associated with increased oxidativ e damage to cellular macromolecules by reactive oxygen species (ROS). Since ROS levels and their damage products in many plants are known to increase during senescence; it is possible that these changes are due to a decline i n the levels of certain antioxidant enzymes. Using specific assays, we find that the developmental transition to bolting and flowering is associated w ith up to a 5-fold decline in ascorbate peroxidase activity and an increase in chloroplastid superoxide dismutase. As expected, these changes are asso ciated with a measured increase in lipid peroxidation products. By HPLC sep aration of the products, we identified the different positional isomers and find that stereospecific lipid peroxidation occurs after the bolting trans ition. The product distribution suggests that enzyme-mediated lipid peroxid ation. via a lipoxygenase, is responsible for the observed increase. Surpri singly, though consistent with the known induction of antioxidant defenses by hydrogen peroxide, the activity of APX rebounds with further development (reproduction and seed setting) and this increase (up to 5-fold) is associ ated with declines in lipid peroxidation and with the onset of visible sene scence symptoms. Thus, in Arabidopsis, ROS increases are associated with th e developmental transition to flowering, perhaps due to programmed declines in APX activity, and apparently lead to the oxidative activation of lipoxy genase and subsequent lipid peroxidation. The reactivation of APX at later stages appears to help reduce the Lipid peroxidation rate, although the sen escence program continues unabated. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.