Mitigating Cerium Oxide Nanoparticle Stress in Oat Under Elevated CO ₂ Using <i>Micromonospora</i> sp. Treatment

The accumulation of cerium oxide nanoparticles (CeO 2 NPs) in the ecosystem adversely affects plant growth and development. Therefore, this study aimed to investigate the effects of Micromonospora sp. treatment, elevated CO 2 (eCO 2 ), and their combined application on the biomass, photosynthesis, oxidative stress markers, and anthocyanin metabolism in oat plants under CeO 2 NPs contamination. CeO 2 NPs contamination significantly reduced oat plant biomass (fresh weight by 49%, dry weight by 60%) and photosynthetic efficiency. However, Micromonospora sp. treatment substantially mitigated CeO 2 NPs‐induced growth inhibition and oxidative damage by enhancing photosynthesis, antioxidant activity, and anthocyanin metabolism, particularly under eCO 2 . CeO 2 NPs exposure induced oxidative stress, as evidenced by increased H 2 O 2 and MDA levels and impaired redox status. While CeO 2 NPs contamination suppressed antioxidant enzyme activities by up to 44%, treatment with Micromonospora sp. and CeO 2 NPs mitigated this damage by significantly boosting enzyme levels, particularly increasing POX and APX activities. In line with increased proline and anthocyanins, their metabolism was also improved. Micromonospora sp. treatment significantly enhanced anthocyanin metabolism in oat plants, increasing PAL activity by 57% and 171% under uncontaminated and contaminated conditions, respectively. Additionally, phenylalanine, cinnamic acid, coumaric acid, and naringenin levels were notably elevated. While eCO 2 also improved anthocyanin metabolism, the combined treatment with Micromonospora sp. and eCO 2 was less effective than individual treatments. The findings highlight the complexity of plant responses to environmental and contamination stressors, with bacterial treatment emerging as a particularly effective strategy.