UV-B enhances phenolic acid biosynthesis in Salvia miltiorrhiza through dual-level regulation of rosmarinic acid synthase

Rosmarinic acid (RA) and salvianolic acid B (SAB), two major phenolic acid constituents of the medicinal plant Salvia miltiorrhiza, are widely used in pharmaceuticals and healthcare products. The biosynthesis of these compounds is influenced by various environmental factors, including UV-B radiation. Although the transcriptional regulation of phenolic acid biosynthesis by UV-B is relatively well understood, the molecular mechanisms underlying its post-translational regulation remain largely unexplored. In this study, we identified a UV-B-repressed Kelch repeat F-box protein gene, SmKFB2, which negatively regulates the accumulation of phenolic acids in S. miltiorrhiza. Proteomic analyses combined with western blot assays demonstrated that SmKFB2 modulates the endogenous protein levels of rosmarinic acid synthase (RAS). Furthermore, protein-protein interaction assays and co-expression studies revealed that SmKFB2 interacts with SmRAS and effects its stability. Additionally, UV-B was found to upregulate the gene expression of SmRAS. These findings suggest a dual regulatory mechanism: UV-B suppresses SmKFB2 expression, thereby reducing SmRAS degradation, which increases SmRAS protein stability and promotes RA and SAB accumulation; simultaneously, UV-B directly enhances SmRAS gene expression, further stimulating phenolic acid biosynthesis. Collectively, this study provides novel insights into the post-translational regulation of phenolic acid biosynthesis by UV-B and identifies potential targets for agricultural scientists in the development of new S. miltiorrhiza cultivars with enhanced medicinal value.