Root-knot nematode suppression through biogenic silver nanoparticles: a promising path for sustainable agriculture

Abstract

This study sought to identify and characterize Heterorhabditis indica, its symbiotic bacteria, and Meloidogyne incognita, while assessing the nematicidal efficacy of silver nanoparticles synthesized using Photorhabdus luminescens supernatant (PsAgNPs). Molecular and phylogenetic analyses verified the identity of H. indica and M. incognita, revealing no nucleotide discrepancies from previously characterized species. P. luminescens exhibited entomopathogenic properties, and its supernatant enabled the biosynthesis of PsAgNPs under optimal conditions (26 ± 2°C, pH 9). Characterization of PsAgNPs indicated a UV–visible absorption peak at 430 nm, a crystalline structure with an average particle size of 22.38 nm (XRD), and a zeta potential of -41.7 ± 0.74 mV, signifying high stability. FTIR analysis suggested that proteins and polysaccharides contributed to nanoparticle stabilization, while EDX confirmed 70.01% silver purity. SEM and TEM analyses demonstrated spherical nanoparticles with sizes ranging from 15.5 to 40 nm. In vitro bioassays revealed that PsAgNPs significantly suppressed M. incognita egg hatchability and juvenile mortality in a dose-dependent manner. At 200 µg/mL, PsAgNPs reduced egg hatchability to 24.6% and caused 100% juvenile mortality. In contrast, the bacterial supernatant alone exhibited a lower efficacy. The LC50 values for PsAgNPs were 13.1 µg/mL and 14 µg/mL at 12 and 24 h, respectively, indicating potent nematicidal activity. In vivo pot experiments on tomato plants demonstrated a pronounced reduction in gall formation (95.3%) and egg mass production (93.1%) at 100 µg/mL PsAgNPs. Soil nematode populations were significantly reduced, with the lowest density recorded in PsAgNP-treated plants (53.3 juveniles). Additionally, PsAgNPs substantially enhanced plant growth, increasing fresh and dry shoot and root biomass by 61.2% and 64.6%, respectively, compared to controls. Histopathological analysis corroborated reduced tissue damage in PsAgNP-treated plants. These results underscore the potential of PsAgNPs as a viable biocontrol agent for managing M. incognita, presenting an environmentally sustainable alternative to traditional nematicides.