Cadmium Exposure Modulates Quality of the Medicinal Plant Dendrobium officinale via Regulation of Gene Expression, Antioxidant Systems, Bioactive Compound Accumulation, and Rhizosphere Microbial Community

With increasing industrialization, heavy metal contamination poses a growing threat to ecosystem stability and human health. Cadmium (Cd) is taken up by medicinal plants through the soil–plant continuum, altering rhizosphere microbial communities and thereby affecting plant metabolism and bioactive compound accumulation. In this study, Dendrobium officinale Kimura & Migo (D. officinale) was used as a model medicinal plant to investigate the effects of Cd exposure on plant quality and rhizosphere microbial community composition. The results show that different D. officinale strains exhibited significant differences in growth traits and Cd accumulation under Cd exposure. Under the 0.9 mg·kg−1 Cd treatment, mean Cd concentrations in the strain with the highest Cd accumulation (Strain 2) reached 6.42, 1.08, and 1.70 mg·kg−1 DW in roots, stems, and leaves, respectively, compared with 2.13, 0.73, and 0.66 mg·kg−1 DW in the strain with the lowest Cd accumulation (Strain 1), corresponding to approximately 3.0-, 1.5-, and 2.6-fold higher accumulation. Cd exposure was associated with strain-specific changes in metal transport-related gene expression, antioxidant responses, bioactive constituent accumulation, and rhizosphere microbial community composition. Together, these changes highlight the complex relationship between Cd accumulation, plant physiological responses, and medicinal quality. These findings provide a theoretical reference for the selection of elite D. officinale strains and for the safe cultivation of this medicinal plant.