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The effects of cerium oxide (CeO 2 ) and indium oxide (In 2 O 3 ) nanoparticles (NPs) exposure on Arabidopsis thaliana (L.) Heynh. were investigated. After inoculation in half strength MS medium amended with 0−2000 ppm CeO 2 and In 2 O 3 NPs for 25 days, both physiological and molecular responses were evaluated. Exposure at 250 ppm CeO 2 NPs significantly increased plant biomass, but at 500−2000 ppm, plant growth was decreased by up to 85% in a dose-dependent fashion. At 1000 and 2000 ppm CeO 2 NPs, chlorophyll production was reduced by nearly 60% and 85%, respectively, and anthocyanin production was increased 3−5-fold. Malondialdehyde (MDA) production, a measure of lipid peroxidation, was unaffected by exposure to 250−500 ppm CeO 2 NPs, but at 1000 ppm, MDA formation was increased by 2.5-fold. Exposure to 25−2000 ppm In 2 O 3 NPs had no effect on A. thaliana biomass and only minor effects (15%) on root elongation. Total chlorophyll and MDA production were unaffected by In 2 O 3 NPs exposure. Molecular response to NP exposure as measured by qPCR showed that both types of elements altered the expression of genes central to the stress response such as the sulfur assimilation and glutathione (GSH) biosynthesis pathway, a series of genes known to be significant in the detoxification of metal toxicity in plants. Interestingly, In 2 O 3 NPs exposure resulted in a 3.8−4.6-fold increase in glutathione synthase (GS) transcript production, whereas CeO 2 NPs yielded only a 2-fold increase. It seems likely that the significantly greater gene regulation response upon In 2 O 3 NPs exposure was directly related to the decreased phytotoxicity relative to CeO 2 treatment. The use of NP rare earth oxide elements has increased dramatically, yet knowledge on fate and toxicity has lagged behind. To our knowledge, this is the first report evaluating both physiological and molecular plant response from exposure to these important nanoparticles. KEYWORDS: Arabidopsis thaliana, CeO 2 and In 2 O 3 NPs, Stress response, Gene expression, Anthocyanin, Lipid peroxidation, Sulfur assimilation pathway coli, Bacillus subtilis, and Pseudomonas f luorescens), 5,6 plants (Cucurbita pepo L., Solanum lycopersicum L., and Zea mays L.) 7−9 and animals (Zebrafish: Danio rerio). 10,11 Moreover, the potential hazards of nanoparticles to human health have been discussed in assays using human cells. 4,12 Given these findings, it is clear that a full and mechanistic understanding of the risks Special Issue: Sustainable Nanotechnology

The phytotoxicity of bulk and nanoparticle Cu and Ag was directly compared. NP Ag reduced biomass and transpiration by 66-84% when compared with bulk Ag. The Ag ion concentration was 4.4-10-times greater in NP than bulk particle solutions. The Cu ion concentration was 1.4-4.4-times greater in bulk than NP amended solutions. Humic acid (50 mg/L) decreased the ion content of bulk Cu solution by 38-42% but increased ion Cu content of NP solutions by 1.4-2.9 times. Bulk and NP Cu were highly phytotoxic; growth and transpiration were reduced by 60-70% relative to untreated controls. NP Cu phytotoxicity was unaffected by solution type, but humic acid (50 mg/L) completely alleviated phytotoxicity caused by bulk Cu. The data demonstrate differential toxicity of Ag NP relative to bulk Ag. The finding that humic acid and solution chemistry differentially impact bulk and NP behavior highlights the importance of evaluating nanoparticles under environmentally relevant conditions.

We investigate the defense mechanisms of Arabidopsis thaliana in response to cerium oxide and indium oxide nanoparticle exposure.