Novel carbazole-triazole-thioether conjugates as multifunctional antimicrobial agents against phytopathogen

Carbazole and triazole derivatives exhibit diverse biological activities and pharmacological properties. Herein, we report a series of novel 1,2,4-triazole thioethers containing carbazole moiety and evaluate their biological activities. The results showed that some of the title compounds exhibited excellent antibacterial activities in vitro against Xanthomonas axonopodis pv. citri (Xac) in vitro. In particular, compound E36 exhibits the most excellent antibacterial effect against Xac, with an EC₅₀ value of 9.4 mg/L. This efficacy was significantly superior to those of the control drugs bismerthiazol (BMT, EC₅₀ values of 70.5 mg/L) and thiodiazole-copper (TDC, EC₅₀ values of 96.0 mg/L). Meanwhile, E36 also demonstrated a significant in vivo effect against Xac, with the therapeutic and protective efficacy of 48.57% and 51.96%, respectively, at a concentration of 200 mg/L, which was superior to TDC and equivalent to BMT. Additionally, E36 exhibited notable antifungal activity against Verticillium dahliae. Further mechanistic studies revealed that compound E36 attenuates the pathogenicity of Xac by suppressing bacterial motility and reducing extracellular polysaccharide (EPS) production. Concurrently, it enhances host disease resistance by upregulating the expression of the citrus rbcL protein, thereby promoting carbon fixation and improving photosynthetic efficiency. This work indicates that 1,2,4-triazole thioethers containing carbazole moiety has the potential to be developed as novel bactericidal agents.