Metabolomics and transcriptomics reveal the toxic mechanism of Cd and nano TiO coexposure on rice (Oryza sativa L.).

Engineered nanoparticles (ENPs) can resist heavy metal toxicity in plants, but their coexposure still exhibits toxicity to plants compared to plants without exposure to ENPs and heavy metals. There have been few studies on the toxic mechanism of nano TiO-heavy metal coexposure and the effect mechanism of nano TiO in plants. Thus, transcriptomics and metabolomics were used to study the toxic mechanism of rutile nano TiO or TiO-Cd (rutile nano TiO and CdCl mixture) on rice (Oryza sativa L.). After 40 days of exposure, the plant height and root dry weight of rice were significantly decreased in the nano TiO-Cd group compared to the blank group (nano TiO and CdCl free). After Cd treatment, 423 differentially expressed genes (DEGs) and 16 differential metabolites were identified. Nano TiO exposure induced significant regulation of 299 DEGs and 6 metabolites. After nano TiO-Cd coexposure, 1660 DEGs and 181 differential metabolites were identified. Notably, the EDGs (e.g., chalcone isomerase and hydroxycinnamoyl transferase) and differential metabolites (e.g., chrysin and galangin) demonstrated the disruption of flavonoid biosynthesis in Cd-treated rice. After rice was exposed to nano TiO, the DEGs were related to ribosome, whereas the differential metabolites were associated with pyruvate metabolism and valine, leucine, and isoleucine biosynthesis. Furthermore, 14 DEGs (e.g., asparaginyl-tRNA synthetase and methionyl-tRNA formyltransferase) involved in aminoacyl-tRNA biosynthetic pathways were significantly upregulated in rice treated with nano TiO-Cd, in line with the changes in related metabolites (e.g., L-asparagine and 10-formyltetrahydrofolate). Our results show that it is necessary to pay close attention to the toxicity of nano TiO-Cd coexposure in paddy ecosystems and use ENPs with caution to combat the phytotoxicity of heavy metals.