INFLUENCE OF SALICYLIC-ACID ON H2O2 PRODUCTION, OXIDATIVE STRESS, ANDH2O2-METABOLIZING ENZYMES - SALICYLIC ACID-MEDIATED OXIDATIVE DAMAGE REQUIRES H2O2

We investigated how salicylic acid (SA) enhances H2O2 and the relative significance of SA-enhanced H2O2 in Arabidopsis thaliana. SA treatmen ts enhanced H2O2 production, lipid peroxidation, and oxidative damage to proteins, and resulted in the formation of chlorophyll and carotene isomers. SA-enhanced H2O2 levels were related to increased activities of Cu,Zn-superoxide dismutase and were independent of changes in cata lase and ascorbate peroxidase activities. Prolonging SA treatments ina ctivated catalase and ascorbate peroxidase and resulted in phytotoxic symptoms, suggesting that inactivation of H2O2-degrading enzymes serve s as an indicator of hypersensitive cell death. Treatment of leaves wi th H2O2 alone failed to invoke SA-mediated events. Although leaves tre ated with H2O2 accumulated in vivo H2O2 by 2-fold compared with leaves treated with SA, the damage to membranes and proteins was significant ly less, indicating that SA can cause greater damage than H2O2. Howeve r, pretreatment of leaves with dimethylthiourea, a trap for H2O2, redu ced SA-induced lipid peroxidation, indicating that SA requires H2O2 to initiate oxidative damage. The relative significance of the interacti on among SA, H2O2 and H2O2 metabolizing enzymes with oxidative damage and cell death is discussed.