BTH-mediated antioxidant system responses in apple leaf tissues

BTH (S-methylbenzo-1,2,3-thiadiazole-7-carbothiate), an active compound of the commercial preparation Bion, has been studied as an elicitor of resistance to fire blight (Erwinia amylovora) in apple. However, the biochemical mechanisms of its action are not fully elucidated. Our study indicated that BTH at the best time of its protection activity (2–14 days after application) induced changes in prooxidant–antioxidant balance in the leaves of apple trees, but in different ways in the enzymatic and nonenzymatic antioxidants. Glutathione as low molecular antioxidant as well as superoxide anion radical and lipid peroxides as oxidants exhibited changes at the early phase of BTH action. Glutathione-dependent enzymes were strongly affected by the elicitor used. On the 2nd day glutathione transferase (GST) and glutathione peroxidase (GSH-Px) activities increased by about 70% and 30% above the control, respectively. GST activity normalized about the 14th day but GSH-Px at the same time showed 27% of the control value. Among enzymes utilising hydrogen peroxide only catalase showed increase (37%) at the early phase of experiment. Compared with the control, BTH-treated plants did not show changes in ascorbate peroxidase and phenylalanine ammonia-lyase activities. Tocopherol (TOC) level diminished starting from the 7th day after BTH treatment and on the 14th day it was only 28% of the control. It is proposed that extinguishing of BTH-mediated signal resulted from TOC and glutathione action. The diminished ascorbate level at all examined times may play a crucial role in BTH-mediated cell growth regulation. The direct influence of BTH on lipid metabolism should be also taken into consideration.