Enhanced Biochemical and Structural Defense in PGPR-Inoculated Sweet Basil Under Aphid Herbivory

Plants are naturally exposed to various biotic stresses, including pathogen attacks and insect herbivory, which activate distinct signaling pathways as part of their defense responses. Inoculation with beneficial microorganisms, such as plant growth-promoting rhizobacteria (PGPR), can trigger induced systemic resistance (ISR) in plants, a defense response that resembles the one activated by herbivore attack in terms of signaling pathways and physiological effects. However, these interactions have typically been studied independently, limiting our understanding of their combined effects. In this study, we examined the effects of aphid (<i>Acyrthosiphon pisum</i>) herbivory on <i>Ocimum basilicum</i> plants and assessed how these responses are modulated when the plants are inoculated with the PGPR strain <i>Bacillus amyloliquefaciens</i> GB03, with a particular focus on biochemical and structural defense mechanisms. Aphid herbivory significantly increased total essential oil (EO) content and volatile organic compound (VOC) emission and induced a greater density of glandular trichomes while also modifying the phytohormone profile. In contrast, total phenolic content remained unchanged. When aphid herbivory occurred on GB03-inoculated plants, the effects on defense-related parameters became more pronounced. EO and eugenol contents were further increased compared with inoculated controls, jasmonates remained comparable to levels induced by either factor alone, and SA levels nearly doubled relative to aphid-infested plants. Feeding assays revealed that aphids preferred inoculated plants over controls, a response that may be explained by the increased emission of eugenol in inoculated basil. These results demonstrate that GB03 inoculation modifies several defenses-related responses in <i>O. basilicum</i> upon aphid herbivory, including by hormonal signaling, specialized metabolites accumulation, and structural barriers such as glandular trichomes. These findings suggest that PGPR may contribute to modulating plant responses to herbivory under certain conditions, highlighting their context-dependent influence within plant-microbe-insect interactions.