Dual mechanisms of Microbacterium algeriense C14 in alleviating cadmium toxicity during Zinnia elegans seed germination and early seedling growth

Cadmium (Cd) contamination, a critical type of heavy metal pollution, exerts severe adverse effects on plant growth and poses substantial risks to food safety. Although plant growth-promoting rhizobacteria (PGPR) have demonstrated great potential in mitigating metal-induced stress in plants, a critical limitation remains: most PGPR strains only provide protective effects to plants, failing to reduce the bioavailability of toxic metals in the environment. We herein report the isolation and characterization of Microbacterium algeriense strain C14, which exhibits dual functionality: indole-3-acetic acid (IAA) production to promote growth and Cd carboxylate precipitation to immobilize metals. Strain C14 demonstrated exceptional Cd tolerance (up to 120 mg·L−1) and maintained IAA production even under severe Cd stress (120 mg·L−1), with an IAA content of 5.87 ± 0.15 mg·L−1. Using Zinnia elegans as a model ornamental plant, we systematically evaluated strain C14’s protective effects during seed germination and early seedling development under gradient Cd concentrations (0-120 mg·L−1). Strain C14 inoculation significantly enhanced germination parameters, promoted seedling growth, elevated antioxidant enzyme activities, and reduced tissue Cd accumulation by 35.5–50.7% across Cd concentration gradients. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses confirmed that strain C14 secretes carboxyl-containing metabolites that coordinate with Cd2+ to form stable amorphous cadmium carboxylate precipitates, effectively reducing metal bioavailability. This study represents the first report of a Microbacterium species combining Cd biomineralization capability with growth-promoting traits during critical early plant developmental stages, providing new insights for developing integrated phytoremediation strategies for ornamental plants in contaminated urban environments.