Monochromatic light reprograms transcription, metabolism, and rhizosphere microbial communities in <i>Salvia miltiorrhiza</i>

Salvia miltiorrhiza is a valuable medicinal plant with diverse pharmacological applications and high market demand. Light quality is a critical environmental factor regulating plant growth, secondary metabolism, and interactions with rhizosphere microorganisms. However, the effects of short-term, pure monochromatic light exposure on S. miltiorrhiza remain largely unexplored. In this study, we employed integrated transcriptomic, metabolomic, and rhizosphere metagenomic analyzes to investigate the responses of S. miltiorrhiza under different monochromatic light conditions: ultraviolet (UV), blue (B), red (R), and far-red (FR), with white light (WL) as the control. GO enrichment analysis indicated that all monochromatic light treatments activated defense responses, while specific pathways related to light stimulus, wounding, and reactive oxygen species were uniquely enriched under B, R, and FR light. Metabolomic analysis showed a general decrease in metabolite abundance under monochromatic light compared to WL, with the R treatment inducing the highest number of significantly upregulated metabolites. Integrated KEGG pathway analysis of differential transcripts and metabolites highlighted the enrichment of secondary metabolic pathways, including diterpenoid, monoterpenoid, and phenylpropanoid biosynthesis. Notably, quantitative HPLC analysis confirmed that UV, R, and FR light significantly promoted the accumulation of dihydrotanshinone I and tanshinone IIA, while decreasing salvianolic acid A content. Metagenomic analysis revealed that monochromatic light, especially B light, reduced rhizosphere microbial alpha diversity and altered the abundance of specific bacterial families and species. Functional gene annotation also showed treatment-specific shifts in microbial metabolic potential and virulence factors. In conclusion, short-term monochromatic light culture, particularly R and FR, effectively modulates the transcriptome and metabolome of S. miltiorrhiza, enhancing the accumulation of key bioactive tanshinones, while simultaneously reshaping its rhizosphere microbial community. These findings offer a potential light-based strategy for improving the quality of S. miltiorrhiza.