In situ images of Cd²⁺ in rice reveal Cd²⁺ protective mechanism using DNAzyme fluorescent probe

As a common pollutant, cadmium (Cd) poses a serious threat to the growth and development of plants. Currently, there is no effective method to elucidate the protective mechanism of Cd²⁺ in plant cells. For the first time, we designed a Cd²⁺ fluorescent probe to observe the adsorption and sequestration of Cd²⁺ in rice cell walls and vacuoles. Specifically, Cd²⁺ is blocked by the Casparian strip and electrostatically attracted to hemicellulose, which is abundantly adsorbed and fixed to the cell walls of the endodermis. For Cd²⁺ that successfully entered the endodermis, one part entered the cells and was compartmentalised and fixed in the vacuoles, while the other part entered the vascular bundles and precipitated in the cell walls of the sclerenchyma through the ion exchange effect. Furthermore, with prolonged exposure to Cd²⁺, compartmentalised bodies that were strongly labelled by fluorescence gradually appeared in the vacuoles, which were assumed to be a new heavy metal protective mechanism activated by plants in response to continuous Cd²⁺ exposure. In conclusion, this study provides an innovative and effective method for the detection of adsorption, transportation, and accumulation of Cd²⁺ in plant tissues, which can be employed for the rapid identification of crops with low Cd accumulation.