Transcriptomics, metabolomics, antioxidant enzymes activities and respiration rate analysis reveal the molecular responses of rice to Cd stress and/or elevated CO2 concentration

Purpose To explore the regulatory mechanism of rice to Cd stress and/or elevated CO 2 concentration. Methods The rice seedlings ( Oryza sativa L.) were exposed to two CO 2 concentrations (400 ± 20 μmol mol −1 , AC; 800 ± 20 μmol mol −1 , EC) and CdCl 2 concentrations (0 µmol L −1 , 150 µmol L −1 ) for 10 days. Antioxidant enzymes activities, respiration rate, transcriptomics and metabolomics changes of leaves were studied. Results GR (glutathione reductase) activity, respiration rate, many sugars, polyols, amino acids and organic acids contents increased under Cd stress. DEGs (differentially expressed genes) annotated in photosynthesis-antenna proteins were down-regulated; When CO 2 increases, some antioxidant enzymes activities and respiration rate decreased. Genes and metabolites related to photosynthesis were enhanced; Under the composite treatment, the ascorbate–glutathione (ASA-GSH) cycle was regulated, some amino acids contents increased, respiration rate decreased. The DEGs mainly enriched in substances transmembrane movement and enzymes activities, etc. Conclusion Under Cd stress, GR played an important antioxidant role. Sugar, polyol and amino acid metabolisms were enhanced to provide energy, improve osmotic adjustments, maintain cell membrane stability, etc . Organic acids contents increased for regulating plant nutrition, the tricarboxylic acid (TCA) cycle and as the secondary metabolites. Photosynthesis was adversely affected; Under high CO 2 , photosynthesis increased, the decrease of partial O 2 pressure resulted in the decrease of some antioxidant enzymes activities and respiration rate; Under the composite treatment, Cd stress played a dominant role, elevated CO 2 alleviated the Cd stress damage by regulating ASA-GSH cycle and amino acids metabolism.