Thiamine enhances the resistance of tomato plants to infection by Meloidogyne incognita through modulation of the salicylic acid-related responses, antioxidant defenses, and histological changes

Root-knot nematodes ( Meloidogyne spp.) are economically important plant-parasitic nematodes, infecting a wide range of crops, including tomato. Induced resistance is considered an effective and environmentally safe strategy for managing plant-parasitic nematodes, including the root-knot nematode Meloidogyne incognita . In this study, the chemical inducer, thiamine, alongside three biological control agents ( Trichoderma virens strains T6 and T65, and Serendipita indica ), were evaluated for their efficacy in controlling M. incognita in tomato plants. Results showed that thiamine treatment effectively reduced gall formation and nematode reproduction, prolonged the nematode’s developmental period, and improved plant growth parameters. While biocontrol fungi caused an increase in the nematode’s developmental period, they did not show a significant effect on reducing nematode populations (except for T. virens strain T6). Moreover, they led to an increase in gall numbers and showed no impact on plant growth characteristics. Therefore, thiamine treatment was selected as the best-performing treatment under the specific conditions used in this study. Gene expression profiling of 5- and 10-day-old galls formed by nematodes revealed significant changes in thiamine-biosynthesis genes, suggesting that nematode infection may alter the transcription levels of the thiamine biosynthesis-related genes. Biochemical assays demonstrated that activities of catalase (CAT) and superoxide dismutase (SOD) were significantly increased in plants treated with thiamine, and then inoculated with nematode, compared to plants inoculated with nematode only. Molecular analyses further showed that, compared with plants that were only inoculated with nematode, the thiamine + nematode treatment enhanced the expression of the salicylic acid (SA)-related marker gene while reducing the expression of jasmonic acid (JA)- and ethylene (ET)-related marker genes. Moreover, SA-accumulation-deficient transgenic tomato plants ( NahG ) failed to show thiamine-induced resistance, pointing out that SA-related defenses may be involved in nematode suppression. The thiamine biosynthesis mutants ( tenuis ) of tomato plants were more susceptible to nematode infection. Histological observations showed that thiamine reduced the percentage of nuclear area in the feeding site without affecting giant cell or female size, suggesting that it may reduce the metabolic activity, thereby reducing the suitability of the feeding site, and probably affecting nematode development. Overall, it appears that thiamine acts as a potent inducer of systemic resistance against M. incognita in tomato, likely through activation of the SA marker gene and possibly SA-related responses and enhanced oxidative defenses. These changes may reduce feeding-site suitability, thereby providing a potential application against root-knot nematode infection.