Cotton (Gossypium hirsutum L.) is a vital global crop but suffers heavy losses from insect pests that reduce yield and fiber quality. Conventional pesticide-based control causes environmental harm, underscoring the need for sustainable alternatives. Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales), an entomopathogenic fungus, offers promise as both a biocontrol agent and a plant endophyte that can enhance host defense responses. This study investigates how inoculation with live or heat-killed spores of B. bassiana GHA affects the expression of defense-related genes in cotton, focusing on key components of the ethylene, salicylic acid, and jasmonic acid signaling pathways. Four genes ERF1 and MPK2 (ethylene pathway), WRKY (salicylic acid pathway), and JAZ1 (jasmonic acid pathway) were analyzed at 2, 8, 24, and, 48 hours post-inoculation. Results showed that live spores markedly upregulated all target genes compared with controls, indicating strong systemic activation of defense signaling. ERF1 and MPK2 expression peaked at 48 hours, showing 46.55-fold and 29.75-fold increases, while WRKY and JAZ1 peaked at 24 hours with 26 and 21.98-fold increases, respectively. Heat-killed B. bassiana spores caused a significant increase in ERF1 and MPK2 expression (12.86- and 12.11-fold at 2–8 hours), while JAZ and WRKY genes showed no significant change. These findings demonstrate that B. bassiana can induce broad-spectrum systemic resistance in cotton through coordinated activation of multiple hormonal pathways. The results highlight its potential to strengthen plant immunity and reduce dependence on chemical pesticides, offering a promising strategy for sustainable pest management and environmentally resilient cotton production.

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