Inhibition of root hair production upon NP exposure and effect of SA pre-treatment. (A) Accumulation of NPs in the root cap. Microscope photographs of the roots of plants exposed to COOH-MWCNTs 4 days upon addition of the NPs to the growth medium: black aggregates appear around the root cap. (B) Microphotographs of the lateral roots of plants exposed to Ag-NPs (20 nm); NP-exposed roots show a “hairless” phenotype. (C). Number of root hairs per cm of root in plants exposed to Ag-NPs, TiO2-NPs, and COOH-MWCNTs, compared to control (non NP-exposed) plants. A significant decrease is observed in all the plants treated with NPs. (D) Effect of the pre-treatment with ABA, Methyl Jasmonate (MJ) and SA phytohormones before exposure to 20 nm Ag-NPs; only the addition of SA can prevent the decrease in the number of root hairs/cm observed upon exposure to NPs. (E) Pre-treatment with SA prevents the reduction in root hair density upon treatment with TiO2-NPs and COOH-MWCNTs.