Rhizobia block Cd absorption in peanut via rhizosphere microbiome assembly and N-regulated host defense

Cadmium (Cd) contamination severely threatens peanut (Arachis hypogaea L.) production and symbiotic nitrogen fixation (SNF). Although rhizobia inoculation can alleviate heavy metal toxicity in plants, how nitrate (NO₃^-) regulates Cd translocation and the defense of the "rhizobia-root-nodule" system remains unclear. Here, we investigated the mechanisms by which the peanut rhizobium, Rhizobium sp. HM13, mitigates Cd toxicity and sustains SNF under varying NO₃^- levels (N- and N+) via field and pot trials. Field trials showed that HM13 reduced rhizosphere bioavailable Cd and decreased seed Cd accumulation by 49.9%. Rhizosphere sequencing revealed that HM13 enriched functional taxa, particularly Actinobacteriota, Bacillus, and Bradyrhizobium, enhancing network complexity and stability. Pot experiments confirmed that HM13's Cd-blocking effect was strongly modulated by NO₃^-. Under N+ conditions, Cd stress reduced nitrogenase activity; nitrate supply induced premature nodule senescence, disrupted the symbiotic Cd-exclusion barrier, resulting in increased Cd accumulation in pods. Conversely, under N- conditions, robust symbiotic nitrogen fixation strengthened the root-nodule Cd-exclusion barrier, sustaining nitrogenase activity and reducing pod Cd by 38.6-41.8%. Physiologically, HM13 established a synergistic root-nodule defense network. Roots intercepted Cd via the POD-CAT-GSH-PRO pathway, while nodules protected nitrogenase activity through GR-FLA-mediated redox regulation. Overall, Rhizobium sp. HM13 is a dual-functional strain sustaining SNF and blocking Cd. This study elucidates the nitrogen-regulated SNF-antioxidant defense mechanism, providing theoretical and technical support for safe peanut production in Cd-contaminated farmlands.