Synergistic effect of foliar exposure to TiO2 nanoparticles and planting density modulates the metabolite profile and transcription to alleviate cadmium induced phytotoxicity to wheat (Triticum aestivum L.)

A reasonable planting density is vital for wheat resource efficiency and yield enhancement. However, systematic research on the impact of spraying TiO₂-NPs on wheat growth, metabolism, and stress tolerance cultivated in cadmium (Cd)-contaminated soil is limited, especially in integration with planting density, requiring a deeper understanding. Our study showed that spraying with 3.1 mg per plant TiO₂-NPs (in pots) and 21.6 mg m⁻² TiO₂-NPs combined with high planting densities (in the field) both significantly reduced the Cd content in wheat grains by 27.9 and 35.7%, respectively. Immobilization of subcellular water-soluble Cd and the conversion of Cd into inactive plant components in leaves were the primary reasons for this reduction. Metabolomics further revealed the up-regulation of metabolites related to antioxidant activity, plant stress resistance, growth promotion, and the tricarboxylic acid (TCA) cycle, which promotes plant growth, enhances wheat antioxidant enzyme activity, and alleviates oxidative stress. Transcriptomic analysis validated the association between these responses and improved plant stress resistance, with genes such as MYB, WRKY, P450, and Cd membrane transport-related genes like ABCG2 and ABCC3 contributing to the decrease in Cd levels in wheat. Importantly, the Cd-associated human health risk index was also reduced via foliar TiO₂-NPs application. Overall, foliar spraying of TiO₂-NPs combined with high plant density was beneficial in alleviating Cd levels in wheat grains, limiting the risk of Cd exposure to human health via the food chain.