The Activated Oxygen Role of Peroxisomes in Senescence1

Abstract

Senescence is a genetically regulated oxidative process that involves a general degradation of the cellular structures and enzymes and the mobilization of the products of degradation to other parts of the plant. Senescence is mainly characterized by a cessation of photosynthesis, disintegration of organelle structures, intensive losses of chlorophyll and proteins, and dramatic increases in lipid peroxidation and membrane leakiness (Buchanan-Wollaston, 1997). These latter changes are mainly due to the strong enhancement in the generation of activated oxygen species that takes place in plant tissues during the senescence process (Thompson et al., 1987; Leshem, 1988). Chloroplasts are one of the earliest sites of catabolism in leaf senescence (Smart, 1994); chlorophyll is degraded and chlorophyll catabolites are transported to the cell vacuole for their disposal (Matile et al., 1996). Under the electron microscope, chloroplasts from senescent leaves show an increase in the number and diameter of osmiophilic plastoglobuli, loosening and disorientation of the grana, and dilation of the thylakoids (Smart, 1994). The formation of plastoglobuli is thought to be linked to the breakdown of thylakoids that accompanies senescence, and they have been reported to contain plastohydroquinones, carotenoids, glycerolipids, and proteins. Possible metabolic pathway of lipid breakdown in senescence (redrawn from Smart, 1994). Reactions involving oxygen free radicals are an intrinsic feature of plant senescence and they promote the process of oxidative deterioration that contributes to cell death (Thompson et al., 1987). Important factors in plant senescence are lipoxygenase activity, activated oxygen species, and several senescence-promoting compounds such as ethylene and jasmonic acid (Leshem, 1988). Lipoxygenase is an enzyme that catalyzes the direct reaction of polyunsaturated fatty acids with oxygen to give hydroperoxide products and has an important role in the breakdown of membrane lipids induced by senescence. Activated oxygen species are the primary mediators of the oxidative damage in plant senescence, and they mainly include superoxide radicals (O2·−), hydroxyl radicals (·OH), singlet oxygen (1O2), H2O2, alkoxyl radicals (RO·), peroxyl radicals (ROO·), polyunsaturated fatty acids, and semiquinone free radicals. Some of these activated oxygen species, particularly O2·−, are strong oxidizing species that can rapidly attack all types of biomolecules, including DNA, leading to irreparable metabolic dysfunction and cell death (Halliwell and Gutteridge, 1989). However, plants possess enzymatic and nonenzymatic antioxidative defense systems distributed in cell organelles. SODs, catalases, peroxidases, and the ascorbate-glutathione cycle enzymes are examples of antioxidative enzymes. The nonenzymatic antioxidants are mainly ascorbate (vitamin C), glutathione, α-tocopherol (vitamin E), β-carotene, and flavonoids, which are distributed chiefly in chloroplasts but also in other cellular compartments such as mitochondria and peroxisomes. Under normal conditions, the antioxidative defense system of plants provides adequate cellular protection against activated oxygen species, but when the generation of activated oxygen species overcomes the defense provided by the cellular antioxidant systems, as is the case in senescence, then oxidative stress is produced. The application of molecular biology techniques to the study of leaf senescence has in recent years permitted the characterization of genes that show increased expression in senescing leaves (Buchanan-Wollaston, 1997). Experiments with transgenic plants and mutants are supplying information about the role played by cytokinins and ethylene in regulating senescence in leaves (Buchanan-Wollaston, 1997). Once the regulatory mechanisms underlying leaf senescence are understood using molecular and genetic approaches, it might be possible to devise ways to manipulate leaf senescence for agricultural applications (Gan and Amasino, 1997). Peroxisomes can be broadly defined as ubiquitous subcellular organelles bounded by a single membrane that contain as basic enzymatic constituents catalase and H2O2-producing flavin oxidases (Huang et al., 1983). These cell organelles were first isolated and biochemically characterized by de Duve at the beginning of the 1960s (del Rı́o et al., 1992, and refs. therein). Initially, the only function described for peroxisomes was the removal by catalase of toxic H2O2 generated in the peroxisomal respiration pathway, but in recent years it has become increasingly clear that peroxisomes carry out essential functions in almost all eukaryotic cells (Huang et al., 1983; del Rı́o et al., 1992; van den Bosch et al., 1992). These organelles have an essentially oxidative type of metabolism and have the potential to carry out different metabolic pathways, depending on their source. Glyoxysomes are specialized peroxisomes that occur in the storage tissues of oilseeds