Salicylic acid ameliorates Meloidogyne incognita suppression of growth and modulates defense gene expression in contrasting tomato cultivars

Abstract Root-knot nematodes ( Meloidogyne incognita ) are a major threat to tomato ( Solanum lycopersicum ) production. This study evaluated exogenous salicylic acid (SA) as a root-zone soil drench (0.1, 0.5, and 1.0 g/L) for reducing M. incognita infection and alleviating nematode-induced growth suppression in two tomato cultivars with contrasting infection outcomes, the highly susceptible cv. ‘380’ and the phenotypically less susceptible cv. ‘Thuria’, under greenhouse conditions over 45 days. SA suppressed nematode infection and reproduction in a clear, dose-dependent manner, with 1.0 g/L most effective in both cultivars; the reduction in J2 infection was proportionally greatest in the susceptible cv. ‘380’, while cv. ‘Thuria’ maintained consistently lower infection, egg-mass formation, and egg production throughout. This suppression is best explained by a dual mode of action rather than a single pathway: soil J2 populations declined in step with SA concentration, consistent with a direct rhizospheric effect, while time-course qRT-PCR of the defense genes PR2 (β-1,3-glucanase) and PR4 (hevein-like protein), normalized to time-matched non-inoculated and untreated controls, showed strong early upregulation at 1 day post-inoculation followed by suppression at 3 days, an early, transient host defense response consistent with a priming component that was subsequently weakened as nematode infection proceeded. These two mechanisms diverged in magnitude between cultivars: cv. ‘Thuria’ showed higher absolute PR2/PR4 induction and lower baseline nematode burden despite lacking the Mi-1 resistance gene, supporting its classification as phenotypically less susceptible rather than tolerant, whereas cv. ‘380’ derived comparatively greater relative benefit from exogenous SA. SA also alleviated nematode-induced growth suppression in both cultivars, particularly at 1.0 g/L, improving shoot and root length, fresh and dry biomass, and root morphology, with visibly reduced galling. Overall, SA mitigated M. incognita infection in tomato through a combination of direct rhizospheric suppression and host defense-gene priming, with the relative contribution of each shaped by cultivar background—supporting its potential as a dual-action, plant-compatible tool for integrated nematode management.