Synthesis of Schiff bases of 4-amino-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole and their evaluation as SAR inducers

A series of twenty five Schiff bases 6a-y of 4-amino-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole having different substitution in the aryl ring attached to imine group designed incorporating the isonicotinic acid moiety present in INA, a known SAR inducer have been synthesized and characterized using H-1 and C-13 NMR, FT-IR spectroscopy and elemental analysis. All twenty five Schiff bases, 4-arylideneamino-3-mercapto-5-pyridin-4-yl-4H-1,2,4-triazoles have been screened for systemic acquired resistance (SAR) inducing activity against sheath blight of rice and five potential compounds viz. 6f, 6g, 6r, 6t and 6u analogues have been further evaluated. All the five compounds have considerably decreased the relative lesion height (RLH) as compared to control with maximum reduction in RLH shown by compound 6u (47.15%). These five potential compounds have been further studied for their effect on phenol content, PAL and peroxidase activity. The compound 6u has been identified as the most potent SAR inducer both based on phenotypic and biochemical study and also does not show direct fungicidal activity against R. solani. Its RI activity is found better than 2,6-dichloroisonicotinic acid (INA), a resistance inducing chemical used as standard.