The Multifaceted Role of Exiguobacterium acetylicum in Reducing Cadmium Accumulation in Spinach

Cadmium (Cd) accumulation in edible plants presents a significant global challenge. This study investigates the potential of a manganese-oxidizing rhizobacterium, Exiguobacterium acetylicum 4-3-1, to enhance spinach growth while mitigating Cd uptake. The strain demonstrates the ability to produce indole-3-acetic acid (IAA) and siderophores, tolerates elevated Cd levels (minimum inhibitory concentration of 0.3 mM), and effectively removes 73.74% of Cd from aqueous solutions. Under Cd stress (10.5 mg/kg), E. acetylicum 4-3-1 significantly increased spinach biomass by 184.3% (dry weight) and chlorophyll content by 33.99%, while concurrently reducing stress markers and leaf Cd concentration by 53.07%. The strain decreases Cd levels in spinach leaves through both endogenous and exogenous mechanisms. It oxidizes Mn(II) to form manganese oxides that may immobilize Cd. Dual RNA-seq analysis indicated that inoculation with E. acetylicum 4-3-1 up-regulated pathways related to photosynthesis and energy metabolism in spinach, while down-regulating genes associated with heavy metal transport. Metagenomic analysis revealed that inoculation altered the rhizosphere microbiome and its functional capabilities, leading to an increased abundance of beneficial bacteria such as Bacillales. Furthermore, a synthetic community (SynCom) comprising E. acetylicum 4-3-1 and Bacillus subtilis exhibited synergistic effects on spinach growth under Cd stress, positioning it as a promising strategy for reducing Cd contamination in crops and supporting sustainable agricultural practices.