Catalase is a sink for H 2 O 2 and is indispensable for stress defence in C 3 plants arthritis , cataract , cancer

arthritis, cataract, cancer and Lou Gehrig’s disease, in Hilde Willekens, Sangpen Chamnongpol, developmental degenerative processes such as ageing Mark Davey, Martina Schraudner1,2, and apoptosis, in macrophage-mediated destruction of Christian Langebartels1, pathogens and in the activation of stress responses Marc Van Montagu3, Dirk Inzé4 (Halliwell and Gutteridge, 1989; Deng et al., 1993; and Wim Van Camp McConkey and Orrenius, 1994; Khan and Wilson, 1995; Spector, 1995; Sundaresan et al., 1995; Sohal and Laboratorium voor Genetica, Department of Genetics, Flanders Interuniversity Institute for Biotechnology (VIB), Universiteit Gent, Weindruch, 1996). In plants, the production of AOS has K.L. Ledeganckstraat 35, B-9000 Gent, Belgium, 1Institute of been evidenced during pathogen infection (Baker and Biochemical Plant Pathology, National Research for Environment and Orlandi, 1995) and in photooxidative processes induced Health (GSF), D-85764 Oberschleißheim, Germany and 4Laboratoire during abiotic stress conditions such as chilling, drought, Associé de l’Institut National de la Recherche Agronomique (France), Universiteit Gent, B-9000 Gent, Belgium salt and ozone stress (Foyer and Mullineaux, 1994). Recent evidence suggests that AOS formation in plant–pathogen 2Present address: Institut für Pflanzenwissenschaften, Eidgenössische Technische Hochschule, Zürich, CH-8092 Zürich, Switzerland interactions is mechanistically similar to the oxidative burst in macrophages (Groom et al., 1996). In contrast, 3Corresponding author processes leading to photooxidative damage during abiotic e-mail: mamon@gengenp.rug. ac.be stresses are mostly unique to plants and cyanobacteria. H.Willekens and S.Chamnongpol contributed equally to this work There is currently great interest in elucidating the nature of these photooxidative processes and in understanding Hydrogen peroxide (H2O2) has been implicated in how photooxidation can lead to cellular destruction. Biomany stress conditions. Control of H2O2 levels is chemical studies have identified over-reduction of photocomplex and dissection of mechanisms generating and system II (PSII), the Mehler reaction and photorespiration relieving H2O2 stress is difficult, particularly in intact as potentially important sources of AOS in photosynthetic plants. We have used transgenic tobacco with ~10% cells (Foyer, 1996). Over-reduction of PSII occurs when wild-type catalase activity to study the role of catalase carbon assimilation is repressed, e.g. due to environmental and effects of H2O2 stress in plants. Catalase-deficient stress and when light influx is high. In such a state of plants showed no visible disorders at low light, but in photon excess, PSII becomes progressively reduced, and elevated light rapidly developed white necrotic lesions this can lead to oxidative stress through the production of on the leaves. Lesion formation required photorespirasinglet oxygen and possibly superoxide. To prevent overtory activity since damage was prevented under elevreduction of PSII, reduction of oxygen through photoated CO2. Accumulation of H2O2 was not detected respiration and the Mehler reaction may function as during leaf necrosis. Alternative H2O2-scavenging alternate routes for de-energizing the photosystems mechanisms may have compensated for reduced cata(Krause, 1994; Osmond and Grace, 1995). In the Mehler lase activity, as shown by increased ascorbate peroxidreaction, oxygen is reduced monovalently first to superase and glutathione peroxidase levels. Leaf necrosis oxide (O2) and then to H2O2. This H2O2 is subsequently correlated with accumulation of oxidized glutathione converted to water by ascorbate peroxidase, thus generatand a 4-fold decrease in ascorbate, indicating that ing a pseudocyclic electron flow, in which electrons from catalase is critical for maintaining the redox balance the oxygen-splitting complex pass through the photosynduring oxidative stress. Such control may not be limited thetic electron carriers back to oxygen. Photorespiration to peroxisomal H2O2 production. Catalase functions as recycles carbon that is used by oxygenation of ribulosea cellular sink for H2O2, as evidenced by comple1,5-bisphosphate and produces H2O2 in the peroxisomes mentation of catalase deficiency by exogenous catalase, through the enzyme glycolate oxidase. The relative importand comparison of catalase-deficient and control leaf ance of these different AOS-forming pathways towards discs in removing external H2O2. Stress analysis photooxidation in natural stress conditions remains an revealed increased susceptibility of catalase-deficient issue of debate, due mainly to a lack of good experimental plants to paraquat, salt and ozone, but not to chilling. procedures to dissect mechanistically these photooxidative