Enhanced cadmium absorption and tolerance of rice epiphytic microbes by iron oxide nanoparticles

Nanoparticles present a promising approach for mitigating cadmium (Cd) uptake in crops. Nevertheless, limited research has been focused on the application nanoparticles to mitigate plant Cd stress through increasing Cd absorption and tolerance of epiphytic microbes. Herein, Methylobacterium currus DSD (Meb) was isolated and identified as the type strain. The application of iron oxide nanoparticles (200 mg L-1 IONPs, including Fe2O3NPs and Fe3O4NPs) resulted in considerably enhanced adsorption capacity for Cd by Meb, accompanied by increased surface functional groups, particularly carboxyl and hydroxyl groups. The extracellular polymeric substances (EPS) of Meb-Fe3O4NPs exhibited the higher contents of proteins and polysaccharides. The introduction of Fe3O4NPs increased the mole% of ribose, arabinose, and glucose, as well as the contents of most amino acids. Fe3O4NPs might drive the glycolytic process of converting glucose to amino acids. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analyses further confirmed the involvement of C--O and C-N groups in proteins, and C-O groups in polysaccharides. Moreover, the presence of IONPs enhanced Meb tolerance to Cd, which was a highlight. This study provides a novel perspective from epiphytic microbes for nanoparticles to reduce the toxicity and accumulation of Cd in crops.