Essential oils have many functions in plant development; among them, they take part in plant–plant interactions, such as allelopathy. The phytotoxicity of the essential oil of Heterothalamus psiadioides was evaluated on Arabidopsis thaliana adventitious rooting. This development process has demonstrated great sensitivity to phytotoxic essential oils and has been primarily studied by our research group. The essential oil of H. psiadioides showed highly negative effects on A. thaliana wild type (WT) adventitious rooting in very small amounts. As the hormone auxin has an important role on adventitious rooting, A. thaliana mutant superroot (sur1) that overproduces indole-3-acetic acid was tested to verify if it could better respond to the oil than WT. The mutant had a WT-like rooting response and the expression of its phenotype was not evident in treatments that had oil application. The essential oil also induced the generation of H₂O₂, a reactive oxygen species in high levels and trying to recover these effects with the aid of the antioxidant Trolox® was unsuccessful. Increasing levels of H₂O₂ may affect leaf pigmentation of WT microcuttings and auxin levels of both WT and sur1 microcuttings. Since the expression of auxinresponsive genes is decreased by H₂O₂ treatment via mitogen-activated protein kinase activation, microcuttings growth and rooting are impaired causing altered development pattern.