Improved abamectin activity and tomato defense against Meloidogyne javanica using gamma-irradiated chitosan

Root-knot nematodes ( Meloidogyne javanica ) represent a primary constraint to tomato production worldwide, and the development of sustainable delivery systems for nematicides remains an unmet need. Chitosan has attracted considerable attention as a biodegradable biopolymer for nematode management due to its intrinsic nematicidal activity and its ability to enhance plant defense responses. However, its high molecular weight and limited solubility can restrict its practical application. This study aimed to determine whether gamma-irradiated low-molecular-weight chitosan could improve abamectin performance and simultaneously activate plant defense responses against M. javanica . Chitosan was irradiated at doses of 0, 50, 100, and 150 kGy and characterized by viscometry, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The optimized formulation 100L1% (low-molecular-weight chitosan irradiated at 100 kGy combined with 1% abamectin) reduced molecular weight from 8.6 kDa to 0.7 kDa, increased the degree of deacetylation from 63.9% to 75.3%, and enhanced solubility from 63.0% to 93.0%. In vitro, this formulation achieved 56% second-stage juvenile (J2) mortality, compared with 40% for abamectin alone, while maintaining comparable egg-hatching inhibition (47%). In greenhouse assays, the optimized treatment reduced the nematode reproduction factor to 0.6, versus 5.4 in infected controls, and increased tomato shoot fresh weight to 28.4 g, compared with 14.9 g in untreated infected plants. Quantitative real-time PCR analysis revealed upregulation of LOX1 (jasmonic acid pathway) at two days post-inoculation, indicating activation of systemic resistance. These findings demonstrate that gamma-irradiated chitosan has dual functionality as a promising biodegradable carrier for abamectin and as an elicitor of plant defense genes. The 100L1% formulation provides a scalable, eco-friendly strategy for integrated nematode management and supports the development of next-generation biopolymer-based crop protection products.