Synthesis of organic-inorganic hybrid nanocomposites modified by catalase-like catalytic sites for the controlling of kiwifruit bacterial canker

Abstract

Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. Actinidiae (Psa), is one of the most important diseases in kiwifruit, creating huge economic losses to kiwifruit-growing countries around the world. Metal-based nanomaterials offer a promising alternative strategy to combat plant diseases induced by bacterial infection. However, it is still challenging to design highly active nanomaterials for controlling kiwifruit bacterial canker. Here, a novel multifunctional nanocomposite (ZnO@PDA-Mn) is designed that integrates the antibacterial activity of zinc oxide nanoparticles (ZnO NPs) with the plant reactive oxygen species scavenging ability of catalase (CAT) enzyme-like active sites through introducing manganese modified polydopamine (PDA) coating. The results reveal that ZnO@PDA-Mn nanocomposites can efficiently catalyze the conversion of H₂O₂ to O₂ and H₂O to achieve excellent CAT-like activity. In vitro experiments demonstrate that ZnO@PDA-Mn nanocomposites maintain the antibacterial activity of ZnO NPs and induce significant damage to bacterial cell membranes. Importantly, ZnO@PDA-Mn nanocomposites display outstanding curative and protective efficiencies of 47.7% and 53.8% at a dose of 200 μg mL^-1 against Psa in vivo, which are superior to those of zinc thiozole (20.6% and 8.8%) and ZnO (38.7% and 33.8%). The nanocomposites offer improved in vivo control efficacy through direct bactericidal effects and decreasing oxidative damage in plants induced by bacterial infection. Our research underscores the potential of nanocomposites containing CAT-like active sites in plant protection, offering a promising strategy for sustainable disease management in agriculture.

Scheme 1. (a) Illustration for the synthesis of ZnO@PDA-Mn nanocomposite. (b) Synergistic antibacterial mechanism of ZnO@PDA-Mn.

Fig. 1. TEM images of (a) ZnO nanoparticles, (b) ZnO@PDA nanocomposites and (c) ZnO@PDA-Mn nanocomposites. (d) Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) mapping of ZnO@PDA-Mn nanocomposites (dotted circles highlight the presence of the nanocomposites core).

Fig. 2. (a) Dynamic light scattering (DLS) analysis of ZnO, ZnO@PDA and ZnO@PDA-Mn. (b) Zeta-potentials of ZnO, ZnO@PDA and ZnO@PDA-Mn. (c) XPS full scan spectrum ZnO@PDA-Mn. XPS high-resolution scans of C 1s (d), O 1s (e), N 1s (f), and Mn 2p (g) of ZnO@PDA-Mn. (h) XRD patterns of ZnO@PDA-Mn. (i) FT-IR spectra of ZnO, ZnO@PDA and ZnO@PDA-Mn.

Fig. 3. (a) Steady-state kinetics assay of CAT-like activity of ZnO@PDA-Mn with varied H₂O₂. (b) Lineweaver–Burk plotting of CAT-like activity for ZnO@PDA-Mn with H₂O₂ as a substrate at room temperature (25 °C). (c) Evaluation of the steady-state kinetics assay for the POD-like activity of ZnO@PDA-Mn using varying concentrations of TMB. (d) Assessment of the steady-state kinetics assay for the POD-like activity of ZnO@PDA-Mn using varying concentrations of H₂O₂. (e) Analysis of electron spin resonance (ESR) spectra depicting hydroxyl radicals produced by ZnO@PDA-Mn in the presence of H₂O₂. (f) Detection of SOD-like activity of ZnO@PDA-Mn through WST-1 assay.

Fig. 4. (a) Antibacterial activity of H₂O₂, Zn–Th, ZnO, ZnO@PDA-Mn and ZnO@PDA-Mn + H₂O₂ against Psa. Concentrations of Zn–Th, ZnO, and ZnO@PDA-Mn were 1.57 μg mL⁻¹ H₂O₂ (1 mM). (b) Antibacterial activity of H₂O₂, Zn–Th, ZnO, ZnO@PDA-Mn and ZnO@PDA-Mn + H₂O₂ against Psa. Concentrations of Zn–Th, ZnO, and ZnO@PDA-Mn were 3.125 μg mL⁻¹ H₂O₂ (1 mM). (c) Visualization of representative bacterial colony formation across various treatment groups. Significance analysis was carried out by one-way ANOVA and t test. Single asterisks indicate p < 0.05, double asterisks indicate p < 0.01, triple asterisks indicate P < 0.001.

Fig. 5. (a) SEM images of Psa after treatment with Control (PBS), ZnO, ZnO@PDA-Mn. (b) TEM images of Psa after treatment with Control (PBS), ZnO, ZnO@PDA-Mn. (c) Live/dead bacterial viability assessment of Psa (the red arrows indicate that the bacteria were significantly deformed after treatment with ZnO@PDA-Mn).

Fig. 6. Under controlled greenhouse conditions, the efficacy of Zn–Th, ZnO nanoparticles, and ZnO@PDA-Mn nanocomposites against kiwifruit bacterial canker at a concentration of 200 μg mL⁻¹ was assessed. Curative activity (a and b), protective activities (c and d). (e) Schematic diagram of synergistic antibacterial activity of nanocomposites in vivo. Significance analysis was carried out by one-way ANOVA and t test. Single asterisks indicate p < 0.05, double asterisks indicate p < 0.01, triple asterisks indicate p < 0.001.

Fig. 7. SEM image of kiwifruit tissue section. (The dashed box denotes the zoom position, while the arrow signifies the presence of bacteria).