Metabolome and comparative genome provide insights into secondary metabolites generation of a rare karst‐growing <i>Rhododendron in vitro</i> culture

Rhododendron species have the potential to be rich in secondary metabolites with pharmaceutical or industrial value. However, there is a lack of comprehensive metabolome studies at the genome level, particularly for unique and rare species like Rhododendron bailiense, which exclusively grows in karst environments in Guizhou, southwest China. Recently, genome assembly data for this species was available. In this study, nontargeted metabolomics was employed to investigate the secondary metabolites profile of R. bailiense callus. The callus of R. bailiense was induced using 0.2 mg L^-1 TDZ (Thidiazuron) + 0.1 mg L^-1 IBA (3-Indole butyric acid). A comparison between light-treated calli and dark-cultured calli revealed differential accumulation of metabolites, particularly in flavonoids, terpenoids, coumarins, and hydroxycinnamic acids, known for their beneficial effects such as antioxidant, anticancer, and anti-inflammatory properties. Proanthocyanidins, with various health-promoting effects, were found to accumulate significantly in dark-cultured calli. Light conditions promoted diterpene and triterpene products, whereas darkness favored sesquiterpene products. Additionally, the study demonstrated the potential of utilizing Agrobacterium transformation technology on callus suspension cells to enhance secondary metabolite production. Comparison with the genome of Rhododendron molle revealed that the R. bailiense genome exhibited active 'glycosyltransferase activity,' possessed a higher number of copies of monoterpene and sesquiterpene terpene synthases, and contained high copies of specific cytochrome P450 members (CYP71, CYP76, CYP79, CYP82, CYP736). This study offers valuable insights and potential strategies for the biosynthesis and production of Rhododendron secondary metabolites with pharmaceutical or industrial significance.