Regulation of rhizosphere micro-characteristics in Paulownia for cadmium immobilization under cadmium stress

This study investigated the rhizosphere micro-characteristics of low-Cd-accumulation plants under Cd stress through a pot experiment. The effects of both low and high Cd treatments on Cd availability in the rhizosphere soil of Paulownia were evaluated. Additionally, the impacts of rhizospheric metabolites and bacterial consortia on the Cd2+ concentration in the nutrient solution were assessed through in vitro experiments. The results revealed that the Paulownia had a poor Cd accumulation capacity and mainly stored Cd in the fine roots. Under Cd treatments, the rhizosphere exhibited a lower available Cd concentration than the non-rhizosphere , indicating that the plant effectively immobilized Cd. In the nutrient solution, the rhizospheric metabolites under Cd treatment exhibited a higher Cd2+ removal rate than non-rhizospheric samples, and the removal rates increased with increasing soil Cd levels. This suggests that Cd stress induced Paulownia to regulate its rhizospheric metabolites for Cd immobilization. In liquid media, rhizospheric bacterial consortia achieved a higher Cd2+ removal rate than the non-rhizospheric samples, with removal rates increasing as the soil Cd levels increased. Both bacterial extracellular adsorption and intracellular bioaccumulation by the bacteria contributed to Cd2+ removal. The 16S rRNA gene sequencing analysis revealed that Paulownia enriched its rhizosphere with Cd-immobilizing bacteria (Arthrobacter and RB41) under Cd treatment. This indicates that Cd stress prompted Paulownia to regulate its rhizobacterial community for enhanced Cd immobilization. Our findings provide solid evidence that Cd stress can prompt Paulownia to modify its micro-characteristics, leading to enhanced Cd immobilization. This mechanism may contribute to the low levels of Cd accumulation in the plant.