Co-exposure of imidacloprid and nanoparticle Ag or CeO2 to Cucurbita pepo (zucchini): Contaminant bioaccumulation and translocation

The use of engineered nanomaterial (ENMs) has increased dramatically and the possible interaction of these materials with soil-borne organic co-contaminants is largely unknown. Imidacloprid (IMDA) is a neonicotinoid insecticide and one of the most widely used pesticides in the United States and significant concerns have risen due to unknown role of these insecticides in pollinator decline. As such, understanding ENM interactions with these agrochemicals is important. In this study, the bioaccumulation, translocation, and toxicity of IMDA (10 mg/kg) to Cucurbita pepo L (zucchini) was evaluated upon simultaneous exposure to CeO2 or Ag in bulk (CeBulk or AgBulk) or nanoparticle (CeNP or AgNP) form at 100mg/kg under soil-grown conditions. Additionally, expression analysis of seven genes (related to stress, photosynthesis, and elemental transport) previously identified as putative biomarkers of nanoparticle (NPs) exposure in zucchini was also performed. Total IMDA and metabolites accumulation in plant root and aerial tissues (shoot-stem and leaf, flower, and stamen) was equivalent to controls (soil with IMDA minus NPs) in both CeO2 exposures. However, co-exposure to AgBulk and AgNP significantly suppressed IMDA accumulation in zucchini aerial tissues by 30% and 33%, respectively. The Ag and Ce concentration in aerial tissues exposed to NPs alone were 85.4% and 79.2%, re- spectively, higher than plants co-exposed to NPs with IMDA. The expression level of the seven genes studied shows that the response mechanisms of zucchini to IMDA and NPs are different. Moreover, no synergistic effects were observed in gene expression upon IMDA-NPs co-exposure. These findings show that ENMs may not only affect the bioavailability and translocation of currently used pesticides but that the reverse is true as well; these interactions should be considered when assessing the exposure and risk of these materials in the environment.