Differential Biotransformation of Glycyrrhizin by Licorice-Derived Endophytic Fungi and Accumulation-Promoting Effects of Fungal Inoculation

Medicine food homology (MFH) plants are rich in nutrients and bioactive specialized metabolites, and their endophytic fungi mediate key biotransformation of host secondary metabolites. Licorice, a representative MFH herb, accumulates glycyrrhizin (GL) as its dominant bioactive triterpenoid saponin. Its hydrolysates glycyrrhetinic acid (GA) and glycyrrhetinic acid 3-O-mono-β-D-glucuronide (GAMG) show stronger bioactivity and bioavailability than GL. However, the enzymatic mechanisms of licorice-derived endophytic fungi-mediated GL biotransformation remain unclear. Here, nine licorice endophytic fungi were screened for significant GL-inducible β-glucuronidase activity. Four functional GH2 β-glucuronidase were obtained by prokaryotic and eukaryotic expression systems, and confirmed to catalyze glycyrrhizin biotransformation via two distinct hydrolytic pathways. Inoculation of these four strains into licorice markedly enhanced host glycyrrhizin accumulation. This study provides novel enzymatic resources for the efficient bioproduction of high-value glycyrrhizin derivatives, and proposes a green strategy to improve glycyrrhizin content in licorice, deepening the understanding of endophyte–host metabolic crosstalk in medicinal herbs.