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The effect of copper oxide nanoparticles (CuONPs) on physiological and molecular level responses were studied in Arabidopsis thaliana. The seedlings were exposed to different concentrations of CuONPs (0, 0.5, 1, 2, 5, 10, 20, 50, and 100 mg/L) for 21 days in half strength Murashige and Skoog medium. The plant biomass significantly reduced under different concentrations (2, 5, 10, 20, 50, and 100 mg/L) of CuONPs stress. Exposure to 2, 5, 10, 20, 50, and 100 mg/L of CuONPs has resulted in significant reduction of total chlorophyll content. The anthocyanin content significantly increased upon exposure to 10, 20, 50, and 100 mg/L of CuONPs. Increased lipid peroxidation was observed upon exposure to 5, 10, and 20 mg/L of CuONPs and amino acid proline content was significantly high in plants exposed to 10 and 20 mg/L of CuONPs. Significant reduction in root elongation was observed upon exposure to 0.5-100 mg/L of CuONPs for 21 days. Exposure to CuONPs has resulted in retardation of primary root growth, enhanced lateral root formation, and also resulted in loss of root gravitropism. Staining with phloroglucionol detected the deposition of lignin in CuONPs-treated roots. Histochemical staining of leaves and roots of CuONPs-exposed plants with nitroblue tetrazolium and 3'3'-diaminobenzidine showed a concentration-dependant increase in superoxide and hydrogen peroxide formation in leaves and roots of CuONPs-exposed plants. Cytotoxicity was observed in root tips of CuONPs-exposed plants as evidenced by increased propidium iodide staining. Real-time PCR analysis showed significant induction of genes related to oxidative stress responses, sulfur assimilation, glutathione, and proline biosynthesis under CuONPs stress.

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In this study, the effect of silver nanoparticles and silver ions on Arabidopsis thaliana was investigated at physiological and molecular levels. The seedlings were grown in sublethal concentrations of silver nanoparticles and silver ions (0.2, 0.5, and 1 mg/L) in 1/4 Hoagland's medium for 14 days under submerged hydroponic conditions. Significantly higher reduction in the total chlorophyll and increase in anthocyanin content were observed after exposure to 0.5 and 1 mg/L silver nanoparticles as compared to similar concentrations of silver ions. Lipid peroxidation increased significantly after exposure to 0.2, 0.5, and 1 mg/L of silver nanoparticles and 0.5 and 1 mg/L of silver ions. Qualitative analysis with dichloro-dihydro-fluorescein diacetate and rhodamine 123 fluorescence showed a dose-dependent increase in reactive oxygen species production and changes in mitochondrial membrane potential in the roots of seedlings exposed to different concentrations of silver nanoparticles. Real-time PCR analysis showed significant upregulation in the expression of sulfur assimilation, glutathione biosynthesis, glutathione S-transferase, and glutathione reductase genes upon exposure to silver nanoparticles as compared with silver ions. Overall, based on the physiological and molecular level responses, it was observed that exposure to silver nanoparticles exerted more toxic response than silver ions in A. thaliana.

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