Screening of Rhizobacteria Capable of Modulating Cd Accumulation and Enhancing Pharmacopoeial Constituents in Salvia miltiorrhiza

Cadmium (Cd) contamination poses a severe threat to the quality of cultivated land and the safe production of food crops and Chinese herbal medicines in China. The Cd accumulation characteristics of Salvia miltiorrhiza are closely associated with rhizosphere microorganisms; however, the underlying regulatory mechanisms remain elusive. In this study, two S. miltiorrhiza genotypes with high or low Cd accumulation (designated SD and SC, respectively) were employed to comparatively analyze the differences in their rhizosphere bacterial community structures. Combined with reinoculation experiments using bacterial strains tolerant to Cd, we aimed to elucidate the mechanistic roles of rhizosphere bacteria in modulating Cd accumulation in S. miltiorrhiza. 16S rRNA sequencing revealed significant differences in the rhizosphere bacterial community structures between SD and SC, with SC exhibiting higher bacterial diversity and richness, confirming a correlation between rhizosphere microbial community composition and Cd accumulation in S. miltiorrhiza. Functional validation demonstrated that strain Bacillus sp. C4, isolated from SC, significantly reduced Cd accumulation in S. miltiorrhiza roots by 20.35%, whereas strain Bacillus sp. D9, isolated from SD, exerted a contrasting effect, drastically increasing root Cd accumulation by 440.97%. Two distinct rhizobacterial strains were identified with contrasting strategies for modulating Cd partitioning in the soil root continuum of S. miltiorrhiza. Strain C4 acts as a Cd excluder: it elevates rhizosphere pH and adsorbs Cd externally, while decreasing pectin and hemicellulose contents in root cell walls to reduce Cd binding sites, collectively impeding Cd entry into roots. In contrast, strain D9 functions as a Cd accumulator through a dual mechanism. It acidifies the rhizosphere to mobilize sparingly soluble Cd, thereby increasing Cd influx. Concurrently, it upregulates hemicellulose abundance in root cell walls, which amplifies Cd binding capacity and promotes root sequestration. Furthermore, inoculation with strain C4 significantly elevated the contents of bioactive components in S. miltiorrhiza, including cryptotanshinone, tanshinone I, tanshinone IIA, and salvianolic acid B, demonstrating a dual beneficial functionality of concurrent Cd reduction and quality improvement. This study offers functional strains that reduce Cd accumulation and enhance pharmacopoeial constituents of S. miltiorrhiza, supporting the production of low-Cd, high-quality medicinal materials in Cd-contaminated farmland.