Hormetic effects of TiO₂ nanoparticles on basil callus cultures: From growth elicitation to oxidative stress

• TiO₂ nanoparticles induce a typical biphasic hormetic response in basil callus cultures. • Low concentrations of TiO₂ nanoparticles promote biomass production and encourage the biosynthesis of phenolic metabolites. • High doses of TiO₂ nanoparticles cause oxidative stress, resulting in the activation of antioxidant enzymes and defense systems. • Distinct metabolic reprogramming links p-coumaric acid accumulation to growth promotion and benzoate derivatives to stress survival. Titanium dioxide nanoparticles (TiO₂ NPs) are emerging as potent modulators of plant metabolic processes, yet their dose-dependent effects on plant tissue culture systems remain underexplored. This study investigates the physiological and biochemical responses of Ocimum basilicum callus cultures to a range of TiO₂ NP concentrations (5–200 mg L⁻¹). The synthesized NPs were characterized using XRD, FTIR, and SEM to confirm their anatase crystalline structure and morphology. The results reveal a dose-dependent hormetic effect: lower concentrations (20–30 mg L⁻¹) significantly enhanced biomass accumulation, total phenolic content (by 60%), flavonoid content (by 27%), and peroxidase (POD) activity. The lowest dose (5 mg L⁻¹) induced an early metabolic priming response, characterized by the accumulation of rosmarinic and vanillic acids. Conversely, higher concentrations (≥80 mg L⁻¹) induced oxidative stress, as evidenced by increased activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). HPLC-MS/MS analyses highlighted a selective and dose-dependent metabolic reprogramming, shifting from early priming to the targeted production of hydroxybenzoic acids at toxic levels of TiO 2 NPs. These findings provide novel insights into the dual role of TiO₂ NPs, acting as growth promoters at low concentrations and stress inducers at higher levels. The study demonstrates that TiO 2 NPs exposure simultaneously modulates antioxidant mechanisms and specialized metabolism in basil callus culture, thereby broadening our insight into how NPs can be utilized in ecological and industrial applications.