Endophytic Enterobacter ludwigii S15 mitigates cadmium accumulation and promotes tobacco growth via polyamine biosynthesis and metal transporter regulation

Reducing cadmium (Cd) uptake in tobacco is critical for crop safety and human health. While heavy metal (HM)-immobilizing bacteria have been widely studied, the specific mechanisms by which exogenous endophytic bacteria regulate tobacco growth and Cd homeostasis remain unclear. Here, we isolated Enterobacter ludwigii S15 from maize roots in Cd-contaminated soil and identified its strong Cd-immobilizing capacity and plant growth-promoting traits. In pot experiments with 3 and 5 mg kg⁻¹ Cd, S15 colonization significantly enhanced tobacco growth by reducing Cd uptake and root-to-shoot translocation, modifying subcellular Cd compartmentalization, and increasing photosynthetic pigment content. Moreover, S15 downregulated the expression of key Cd transporter genes (NtZIP1, NtZIP2, NtZIP4, NtNRAMP3), enhanced Cd tolerance by modulating antioxidant enzyme activities, promoted soluble sugar accumulation, and upregulated polyamine biosynthesis genes (ADC, SAMDC, SPD synt). Collectively, these findings elucidate the mechanistic basis by which E. ludwigii S15 alleviates Cd stress and promotes plant growth, highlighting its potential as an effective biotechnological agent for mitigating Cd accumulation in tobacco cultivated on contaminated soils.