Natural Senescence of Pea Leaves (An Activated Oxygen-Mediated Function for Peroxisomes)

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Abstract
We studied the activated oxygen metabolism of peroxisomes in naturally and dark-induced senescent leaves of pea (Pisum sativum L.). Peroxisomes were purified from three different types of senescent leaves and the activities of different peroxisomal and glyoxysomal enzymes were measured. The activities of the O2-- and H2O2-producing enzymes were enhanced by natural senescence. Senescence also produced an increase in the generation of active oxygen species (O2- and H2O2) in leaf peroxisomes and in the activities of two glyoxylate-cycle marker enzymes. A new fraction of peroxisomes was detected at an advanced stage of dark-induced senescence. Electron microscopy revealed that this new peroxisomal fraction varied in size and electron density. During senescence, the constitutive Mn-superoxide dismutase (SOD) activity of peroxisomes increased and two new CuZn-SODs were induced, one of which cross-reacted with an antibody against glyoxysomal CuZn- SOD. This fact and the presence of glyoxylate-cycle enzymes support the idea that foliar senescence is associated with the transition of peroxisomes into glyoxysomes. Our results indicate that natural senescence causes the same changes in peroxisome-activated oxygen metabolism as dark-induced senescence, and reinforce the hypothesis of an effective role of peroxisomes and their activated oxygen metabolism in this stage of the life cycle.