Particle-specific toxicity and bioavailability of cerium oxide (CeO₂) nanoparticles to Arabidopsis thaliana

The use of manufactured cerium oxide nanoparticles (CeO₂-NPs) in consumer products has increased markedly over the past decade, and their release into natural ecosystems is unavoidable. This study investigated the phytotoxicity and uptake of CeO₂-NPs in Arabidopsis thaliana grown in an agar medium. Although low concentrations of CeO₂-NPs had stimulatory effects on plant growth, at higher concentrations, CeO₂-NPs reduced growth and had adverse effects on the antioxidant systems and photosystem. Importantly, the toxicity resulted from the nanoparticles per se, rather than from the dissolved Ce ions. CeO₂-NPs were taken up and subsequently translocated to shoot tissues, and transmission electron microscopy (TEM) showed the presence of a large number of needle-like particle aggregations in the intercellular regions and the cytoplasm of leaf cells. The up-translocation factor to shoots was independent of the concentrations of Ce in the roots and the supplied forms of Ce (i.e. CeO₂-NPs, CeO₂-bulk, and ionic Ce), suggesting that endocytosis is likely to be a general mechanism responsible for the translocation of these Ce compounds. These findings provide important information regarding the toxicity and uptake of CeO₂-NPs in plants, which needs to be considered in environmental risk assessment for the safe use and disposal of CeO₂-NPs.