In this study, we investigated how 6.6 mM thiram induces to stress response in tomato and evaluated the possible protective role of different concentration of salicylic acid (0.01, 0.1 and 1 mM SA) against thiram toxicity by analyzing tomato leaf samples taken on the 1st, 5th, 11th day of the treatment. The thiram treatment resulted in oxidative stress through an increase in hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels in a time-dependent manner and led to a decline in the total chlorophyll and carotenoid levels. However, thiram-treated plants induced antioxidant enzyme activities, including catalase (CAT; EC 1.11.1.6), glutathione reductase (GR; EC 1.6.4.2), and ascorbate peroxidase (APX; EC 1.11.1.11), as well as pesticide detoxification enzymes such as peroxidase (POX; EC 1.11.1.7) and glutathione S-transferase (GST; EC.2.5.1.18). In addition, three genes (GST1, GST2, GST3) that encode for glutathione S-transferase and one gene (P450) that encodes for cytochrome P-450 monooxygenases were upregulated. SA showed a positive effect on the plants treated with thiram thanks to the decrease in the H2O2 and MDA levels, the enhancement of photosynthetic pigments, and the regulation in antioxidant enzyme activities in the tomato leaves. In addition, the SA-pretreatment triggered the activity and expression of pesticide detoxification enzymes in the thiram-treated leaves. Particularly the pretreatment with 1 mM SA significantly improved the activity of GST and led to the upregulation of GST1, GST2, GST3, and P450 expression levels. These results indicate that the application of thiram fungicide causes toxicity; however, the damaging effect could be mitigated through pretreatment with SA.
