Biogenic iron oxide nanoparticles mitigate cobalt toxicity in rice (Oryza sativa L.) under greenhouse conditions

Cobalt (Co) toxicity in agricultural soils significantly hampers crop productivity by disrupting iron (Fe) metabolism and inducing oxidative stress in plants. Few studies have explored eco-friendly nanoparticle (NP)-based interventions to mitigate heavy metal toxicity, particularly using biologically synthesized iron oxide nanoparticles (IONPs). This study investigated green-synthesized IONPs for alleviating Co toxicity in rice plants. IONPs were synthesized using Bacillus cereus and were comprehensively characterized. Characterization confirmed the successful synthesis of quasi-spherical IONPs (15-25 nm) with appropriate size distribution and elemental composition. Under Co stress, IONPs treatment (25, 50 and 100 ppm) significantly enhanced the plant growth. The optimal 100 ppm dose demonstrated substantial improvements of shoot length to 9.3-26.4 cm (Co stress) (100 ppm) (p < 0.001), root length to 6.8-29.4 cm (p < 0.001) and biomass to 0.48-1.25 g (p < 0.001). Similarly, chlorophyll content enhanced from 11.95 to 23.75 mg/g FW (p < 0.001), SOD activity from 42.67 to 52.50 U/mg FW (p < 0.05), soil dehydrogenase activity from 2.89 to 7.55 μg/g (p < 0.001) and decreased proline content from 26.69 to 16.02 μg/g (p < 0.001). In addition, the organic carbon and Fe contents were also enhanced; however, the Co uptake was reduced. This study suggests that biologically synthesized IONPs represent a promising sustainable strategy to mitigate Co-induced phytotoxicity in agricultural systems.