Rhizome Diameter-Dependent Regulation of Glycyrrhizin Biosynthetic Genes and Glycyrrhizin Accumulation in Glycyrrhiza glabra

Glycyrrhiza glabra L. (licorice) is a valuable medicinal plant renowned for its triterpenoid saponin glycyrrhizin, which exhibits anti-inflammatory, antiviral, and hepatoprotective properties. Both developmental and environmental factors influence variations in glycyrrhizin accumulation; however, the role of rhizome morphology remains poorly understood. This study investigated the impact of rhizome diameter (0.5 cm, 1–1.5 cm, and 2 cm) on glycyrrhizin content and expression of key biosynthetic genes in wild G. glabra from Mansuriyeh, Khuzestan Province, Iran. Medium-diameter rhizomes (1–1.5 cm) accumulated the highest glycyrrhizin (21.46 mg/g DW), followed by large (17.83 mg/g DW) and small (14.59 mg/g DW) rhizomes, with leaves showing the lowest levels (12.75 mg/g DW). qRT-PCR analysis revealed upregulated expression of pathway genes (LUS, BAS, CYP88D6, CYP72A154, UGTs, and SQS) in medium- to large-diameter rhizomes, correlating strongly with glycyrrhizin content (Pearson's r up to 0.79). These findings demonstrate diameter-dependent transcriptional regulation of glycyrrhizin biosynthesis, highlighting rhizome size as a developmental proxy for metabolite optimization. This work provides insights for selective harvesting and cultivation strategies to enhance pharmaceutical-grade licorice production.