Root iron plaque alleviates cadmium toxicity to rice (Oryza sativa) seedlings

Iron plaque (IP) on root surface can enhance the tolerance of plants to environmental stresses. However, it remains unclear the impact of Fe²⁺ on cadmium (Cd) toxicity to rice (Oryza sativa) seedlings. In this study, the effects of different Fe²⁺ and Cd²⁺ concentration combinations on rice growth were examined hydroponically. Results indicated that Fe²⁺ concentration up to 3.2 mM did not damage rice roots while induced IP formation obviously. Cd²⁺ of 10 μM repressed rice growth significantly, while the addition of 0.2 mM Fe²⁺ to 10 μM Cd²⁺ solution (Cd+Fe) did not damage rice roots, indicating that Fe²⁺ could ameliorate Cd toxicity to rice seedlings. Microstructure analysis showed Cd+Fe treatment induced the formation of IP with dense and intricate network structure, Cd adsorption on the root surface was reduced significantly. Cd concentration of rice roots and shoots and Cd translocation from roots to shoots with Fe+Cd treatment were reduced by 34.1%, 36.0% and 20.1%, respectively, in comparison to a single Cd treatment. Noteworthy, the removal of IP resulted in a larger loss of root biomass under Cd treatment. In addition, Cd+Fe treatment increased the activities of root superoxide dismutase and catalase by 105.5% and 177.4%, and decreased H₂O₂ and O₂·^- accumulation of rice roots by 56.9% and 35.9%, and recovered Cd-triggered electrolyte leakage obviously, when compared with a single Cd treatment. The results from this experiment indicated that the formed dense IP on rice roots decreased Cd absorption and reactive oxygen species accumulation, and Fe²⁺ supply alleviated Cd toxicity to rice seedlings.