Antioxidant Profile and Polyphenol Oxidase Activities in Apple Leaves after Erwinia amylovora Infection and Pretreatment with a Benzothiadiazole-type Resistance Inducer (BTH)

This study focused on the biochemical effects of benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), an active compound of the commercial preparation Bion, as an elicitor of resistance to fire blight (Erwinia amylovora) in apple. We determined activities of main antioxidant enzymes: ascorbate peroxidase (APX), catalase, glutathione peroxidase (GSH-Px) and glutathione transferase (GST), enzymes associated with phenolic metabolism: phenylalanine ammonia-lyase and polyphenol oxidases (PPOs), levels of low molecular antioxidants [ascorbate, glutathione, tocopherol (TOC)], phenolic acids and flavonoids as well as markers of oxidative processes: superoxide anion radical (O(2)(center dot-)) and thiobarbituric acid reactive substances in apple leaf tissues pretreated and non-pretreated with BTH before inoculation with E. amylovora. The results indicate that successful invasion of pathogen might be associated with a significant decrease in the ascorbate pool in an early phase of plant-pathogen interaction. Generation of O(2)(center dot-) and changes in some antioxidant parameters (APX, GSH-Px, GST, TOC) were less intensive and/or occurred earlier in the BTH-pretreated apple leaves than in non-pretreated ones. In the BTH-pretreated group, PPOs activities were higher than in the control throughout the experiment, whereas in the non-pretreated group, the increase started from the 7th day after inoculation. Infection strongly enhanced chlorogenic and o-coumaric acids accumulation, and BTH weakened this effect. The opposite was observed with respect to phloretin and phloridzin.