Responses of Brassica napus to soil cadmium under elevated CO2 concentration based on rhizosphere microbiome, root transcriptome and metabolome

Rising atmospheric carbon dioxide (CO₂) and soil heavy metal pollution affect crop safety and production. Exposure to elevated CO₂ (ECO₂) increases cadmium (Cd) uptake in some crops like wheat and rice, however, it remains unclear how ECO₂ affects Cd uptake by Brassica napus. Here, we investigated the responses of B. napus seedlings exposed to ECO₂ and Cd through analyses of physiology, transcriptome, metabolome, and rhizosphere microbes. Compared with Cd-stress alone (Cd50_ACO₂), ECO₂ boosted the uptake of Cd by B. napus roots by 38.78% under coupled stresses (Cd50_ECO₂). The biomass and leaf chlorophyll a content increased by 38.49% and 79.66% respectively in Cd50_ECO₂ relative to Cd50_ACO₂. Activities of superoxide dismutase (SOD) and peroxidase (POD) enhanced by 8.42% and 185.01%, respectively, while glutathione (GSH) and ascorbic acid (AsA) contents increased by 16.44% and 52.48%, and abundances of rhizosphere microbes changed significantly under coupled stresses (Cd50_ECO₂) relative to Cd-stress alone (Cd50_ACO₂). Also, the upregulation of glutathione, glutathione transferase genes, and heavy metal ATPase expression promoted the detoxification effect of rapeseed on Cd. Changes in the expression of transcription factors like MAPK, WRKY, BAK1 and PR1, as well as changes in metabolic pathways like β-alanine, may be involved in the regulatory mechanism of stress response. These findings provide new insights for studying the regulatory mechanism of rapeseed under ECO₂ on soil Cd stress, and also provide a basis for further research on Cd tolerant rapeseed varieties in the future climate context.