Selenium-calcium-magnesium co-application mitigates cadmium toxicity in rice: Mechanisms of cell wall detoxification and transporter gene regulation

This study investigated the mechanisms by which combined selenium, calcium, and magnesium (SeCM) application mitigates cadmium (Cd) toxicity in rice (Oryza sativa L.). Hydroponic experiments were conducted to assess the effects of SeCM on Cd retention in root cell wall polysaccharides, functional group interactions, and the expression of Cd transporter genes. SeCM treatment significantly (p < 0.05) enhanced Cd binding to root cell wall fractions: pectin-bound Cd increased from 19.54 to 34.72 mu g g(-1), HC1 from 1.06 to 8.48 mu g g(-1), and HC2 from 0.39 to 2.20 mu g g(-1) respectively, compared to Cd treatment. Additionally, SeCM significantly (p < 0.05) increased the galacturonic acid content in pectin, total sugar contents in hemicellulose fractions, and lignin content. Adsorption isotherm and kinetic analyses revealed that SeCM improved the root cell wall's Cd binding capacity and affinity. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated the involvement of hydroxyl and carboxyl groups in Cd adsorption, with SeCM promoting the formation of Cd-hydroxyl and Cd-carboxyl complexes. Furthermore, SeCM significantly (p < 0.05) downregulated the expression of OsNRAMP5 and OsNRAMP1, involved in the uptake of Cd and other divalent metal ions; OsHMA2, which mediates Cd translocation from roots to shoots; and OsHMA3 and OsABCC9, which sequester Cd into vacuoles. Conversely, SeCM upregulated OsPCS2 and OsMT2b, which synthesize phytochelatins and metallothionein for Cd chelation. The combined effects of enhanced cell wall Cd binding, Cd transporter regulation, and improved Cd chelation capacity resulted in significantly (p < 0.05) reduced Cd concentrations in xylem and phloem sap by 42.4 % and 63.5 %, respectively, demonstrating SeCM's effectiveness in limiting Cd uptake and translocation in rice. These findings provide comprehensive insights into the multifaceted mechanisms by which SeCM enhances rice plant resilience to Cd stress, highlighting its potential for mitigating Cd contamination in agricultural systems.