Role of silver nanoparticles and Bacillus cereus in modulating growth, photosynthetic activity, and antioxidant responses in sorghum under cobalt stress

In the current industrial scenario, cobalt (Co) as a metal is of great importance but poses a major threat to the ecosystem because of its toxicity, but fewer studies have been conducted on its effects and alleviation strategies by using plant growth-promoting rhizo-bacteria (PGPR) and nanoparticles (NPs). Taking into consideration the positive effects of silver nanoparticles (Ag−NPs) and Bacillus cereus in reducing Co toxicity in plants, the present study was conducted. A pot experiment was conducted to determine the effects of individual application of different levels (10 and 20 µL) of B. cereus and Ag−NPs (25 and 40 mg L ⁻1 ) on Co accumulation, morpho-physio-biochemical attributes of Sorghum bicolor L. exposed to severe Co stress 0 (without Co stress), 15 and 25 mg kg −1 in soil. The research outcomes indicated that elevated levels of Co stress in the soil significantly ( P ≤ 0.05) decreased plant growth and biomass, photosynthetic pigments, and gas exchange attributes. However, Co stress also induced oxidative stress in the plants by increasing malondialdehyde (MDA) and hydrogen peroxide (H 2 O 2 ), which also induced increased compounds of various enzymatic and non-enzymatic antioxidants, organic acids, and also the gene expression and sugar content. Furthermore, a significant ( P ≤ 0.05) increase in proline metabolism, was observed. Although, the application of B. cereus showed a significant ( P ≤ 0.05) increase in plant growth and biomass, gas exchange characteristics, enzymatic and non-enzymatic compounds, and their gene expression and also decreased oxidative stress and also organic acid exudation pattern. The application of B. cereus and Ag−NPs decreased the proline metabolism in S. bicolor plants. Research findings, therefore, suggest that the application of B. cereus and Ag−NPs can ameliorate Co toxicity in S. bicolor , resulting in improved plant growth and composition under metal stress, as depicted by balanced antioxidant defense mechanism. These findings highlight the potential of nanotechnology and beneficial microbes as sustainable strategies for mitigating heavy metal toxicity and improving crop performance in contaminated soils, thereby contributing to environmentally resilient agricultural systems. • Cobalt stress reduced growth, photosynthesis, and increased oxidative damage in S. bicolor . • Ag-NPs and Bacillus cereus enhanced antioxidant defense under Co stress. • Ag-NPs showed stronger effects than PGPR in improving plant responses. • Both treatments reduced Co accumulation and improved stress tolerance.