Synthesis, characterization, and antimicrobial properties of 8-Hydroxyquinoline-based group 4 metal complexes for...

Rice bacterial leaf blight (BLB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is a destructive disease that seriously threatens rice production. Developing effective, low-dose, and low-risk bactericides is therefore important for sustainable agriculture. In this study, fifteen 8-hydroxyquinoline-stabilized group 4 metal complexes based on Ti, Zr, and Hf were synthesized and fully characterized. Among them, the 5-nitro-8-HQ zirconium complex [(L₅)₄Zr^IV] showed the best overall performance, including good aqueous stability, improved solubility, and potent antibacterial activity against Xoo, with a MIC value of 0.39 μg/mL, much lower than that of the commercial bactericide oxine copper. In vivo pot experiments further demonstrated that [(L₅)₄Zr^IV] effectively protected and cured rice leaves infected with Xoo, with protective and curative efficacies of 55.56% and 54.16%, respectively, at the MIC concentration. Mechanistic studies revealed that [(L₅)₄Zr^IV] acts through a multifaceted antibacterial mode involving suppression of Xoo virulence traits, disruption of membrane integrity, singlet oxygen accumulation, collapse of membrane potential, impaired respiration, disturbance of the tricarboxylic acid cycle, and inhibition of ATP synthase activity, ultimately leading to bacterial death. Preliminary uptake and soil incubation studies further suggested intracellular exposure and limited persistence under the tested conditions, although its environmental fate and ecotoxicological profile require further evaluation. Overall, this study identifies [(L₅)₄Zr^IV] as a promising low-dose lead candidate for controlling rice BLB and highlights 8-HQ-stabilized group 4 metal complexes as a potential platform for developing next-generation agricultural bactericides as alternatives to traditional copper-based pesticides.