Revealing the metabolomic response of rice seedlings roots to combined high CO2 and cadmium stress

Two CO2 concentrations (400 ± 20 and 800 ± 20 µmol mol− 1) and two CdCl2 concentrations (0 and 150 µmol L− 1) were applied to rice seedlings for a duration of 10 days to investigate the regulatory mechanisms underlying elevated CO2 on Cd stress resistance. Physiological parameters of rice roots, including electrical conductivity, malondialdehyde (MDA) content, and antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), were evaluated. Additionally, metabolomic analysis was conducted to elucidate the metabolic changes associated with the treatments. Our results demonstrated that Cd stress induced membrane damage, that was evident by a significant increase in electrical conductivity by 4.5% and MDA content by 82.24%. Moreover, SOD and POD activities were enhanced in response to Cd stress. Metabolomic analysis revealed significant enhancements in the metabolism of sugars, polyols, amino acids, and α-linolenic acid. These metabolic improvements contribute to osmotic regulation, membrane stability, energy provision, nitrogen assimilation, and antioxidative capacity, respectively. The presence of elevated concentrations of CO2 significantly impacts the activity of SOD in rice roots, the contents of glycine, serine and threonine were increased. Furthermore, the increase in CO2 concentration not only enhances CAT activity by 57.64% (compared to Cd stress), but also mitigates membrane damage under the combined treatment condition. Secondary metabolites (ferulic acid and phenylalanine) played crucial roles in both Cd stress response as well as combined treatment responses. Enhancement of tryptophan and tyrosine levels was also observed indicating their role in response to Cd stress or combined treatment responses respectively. Overall, our findings shed light on internal mechanisms underlying rice roots about the role of elevated CO2 in mitigating Cd stress, so as to provide valuable insights for future studies on plant adaptation towards environmental stresses within a changing climate.