An activated-oxygen-mediated role for peroxisomes in the mechanism of senescence of Pisum sativum L. leaves

The possible involvement of peroxisomes and their activated-oxygen metabolism in the mechanism of leaf senescence was investigated in detached pea (Pisum sativum L.) leaves which were induced to senesce by incubation in complete darkness for up to 11 d. At days 0, 3, 8, and 11 of senescence, peroxisomes were purified from leaves and the activities of different peroxisomal and glyoxysomal enzymes were measured. Xanthine-oxidoreductase activity increased with senescence, especially the O₂^{. -}-producing xanthine oxidase (EC 1.1.3.22). The activities of H₂O₂-generating Mn-superoxide dismutase (EC 1.15.1.1) and urate oxidase (EC 1.7.3.3) were also enhanced by senescence, whereas catalase (EC 1.11.1.6) activity was severely depressed. Hydrogen peroxide concentrations increased significantly in senescent leaf peroxisomes. During the progress of senescence, glycollate oxidase (EC 1.1.3.1) and hydroxypyruvate reductase (EC 1.1.1.81), two marker enzymes of photorespiratory metabolism, gradually decreased in activity and disappeared. At the same time, the activities of malate synthase (EC 4.1.3.2) and isocitrate lyase (EC 4.1.3.1), key enzymes of the glyoxylate cycle, which were undetectable in presenescent leaves, increased dramatically upon induction of senescence. Ultrastructural studies of intact leaves showed that the population of peroxisomes and mitochondria increased with senescence. Results indicate that peroxisomes could play a role, mediated by activated oxygen species, in the oxidative mechanism of leaf senescence, and further support the idea, proposed by other authors, that foliar senescence is associated with the transition of leaf peroxisomes into glyoxysomes.Abbreviation Mn-SOD (manganese-containing) superoxide dismutaseThe authors thank Dr. A.J. Sánchez-Raya (Unidad de Fisiología Vegetal, Estación Experimental del Zaidín, Granada, Spain) for his valuable help in measuring ethylene production, and Dr. G. Barja de Quiroga (Departamento de Biología Animal II, Universidad Complutense, Madrid, Spain) for carrying out the malondialdehyde determinations by HPLC. This work was supported by grant PB87-0404-01 from the DGICYT and the Junta de Andaluc'ia (Research Group # 3315), Spain.