Induction of disease resistance by the plant activator, acibenzolar-S-methyl (ASM), against bacterial canker (Clavibacter michiganensis subsp michiganensis) in tomato seedlings

The plant defence activator acibenzolar-S-methyl (benzo [1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester, ASM; Bion 50 WG) was assayed on tomato seedlings for its ability to induce resistance against Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial canker of tomato. Pre-treatment of plants with ASM reduced the severity of the disease as well as the growth of the bacteria in planta. In ASM-treated plants, reduction in disease severity (up to 75%) was correlated with suppression of bacterial growth (up to 68.2%) during the time course of infection. In plants treated with ASM, activities of peroxidase (POX) and glutathione peroxidase (GPX) were determined as markers of resistance. Applications of ASM induced a progressive and significant increase of both enzymes in locally treated tissues. Such responses were expressed earlier and with a much higher magnitude when ASM-treated seedlings were challenged with the pathogen, thus providing support to the concept that a signal produced by the pathogen is essential for triggering enhanced synthesis and accumulation of these enzymes. No such activities were observed in water-treated control plants. Therefore, the slower symptom development and reduction in bacterial growth in ASM-treated plants might be due to the increase in activity of both oxidative and antioxidative protection systems in planta. (C) 2003 Elsevier Ireland Ltd. All rights reserved.