Biogenic Fe₂O₃ nanoparticles enhance carotenoid pathway gene expression and suppress verticillium root rot in marigold (Tagetes erecta)

Verticillium dahliae is the most destructive root rot pathogen of marigold. For eco-friendly management of this disease, iron oxide nanoparticles (Fe 2 O 3 nanoparticles) were synthesized with the seed extract of Trachyspermum ammi . Synthesis of Fe 2 O 3 NPs was confirmed by ultraviolet-visible (UV) spectrum absorption peak at 275 nm. FTIR validated functional groups on the nanoparticle surface, XRD showed a crystalline structure with an average size of 42 nm, EDX proved elemental composition (Fe 71.6%, O 15.9%), and SEM showed a spherical morphology. Variable mycelial growth inhibition of V. dahliae was observed at different concentrations (0.25, 0.50, 0.75, 1.0, and 1.5 mg/mL) of synthesized Fe₂O₃ NPs, in vitro . The greatest mycelial growth inhibition (88.5%) was observed at 0.75 mg/mL concentration. This concentration was further used to control root rot disease of marigold. Root priming of NPs significantly improved plant growth parameters such as root and shoot lengths, fresh and dry weights, increasing root length by 32%, shoot length by 34%, fresh weight by 42%, and dry weight by 18%. NPs treatment also enhanced photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids), relative water content, and osmolytes (proline and sugars) accumulation. Furthermore, nano-priming significantly decreased H 2 O 2 and malondialdehyde levels, indicating less oxidative stress, as well as decreased relative electrolyte leakage. Histological analysis revealed a decreased mycelial colonization of V. dahliae in the root vascular tissues of marigold. Application of Fe 2 O 3 NPs upregulated the expression of carotenoid biosynthesis pathway genes including Phytoene synthase, Phytoene desaturase, Beta cyclase and Epsilon cyclase, indicating their role in plant defense. The biosynthesized Fe 2 O₃ nanoparticles improved physiological and biochemical parameters and activated defense-related gene expression, which successfully inhibited the V. dahliae growth and increased resistance in marigold plants. These results imply that Fe₂O₃ NPs produced from T. ammi seed extract could be a viable and environmentally friendly method of controlling Verticillium root rot in marigolds.