Coordinated Gene Regulation Drives Saponin Production

Triterpenoid saponins are widely valued for their diverse pharmacological activities and also play important defensive roles in plants. These compounds are synthesized through the cyclization of a common precursor, 2,3-oxidosqualene, a reaction catalyzed by the 2,3-oxidosqualene cyclase (OSC) enzyme family. Different OSCs can channel this precursor into either saponin or sterol biosynthetic pathways, but the regulatory logic governing this metabolic branching has remained poorly understood. Previous studies mainly focused on enzyme structure or downstream modifications, while gene-level regulation received less attention. Based on these challenges, it is necessary to conduct in-depth research on how specific OSC genes and their regulators coordinate saponin biosynthesis.

Researchers from North China University of Science and Technology reported (DOI: 10.1093/hr/uhaf133) on May 21, 2025, in Horticulture Research a comprehensive molecular analysis of saponin biosynthesis in Eleutherococcus senticosus. The study identified two key OSC genes that determine whether metabolic flux is directed toward triterpenoid saponins or sterols. By combining genome-wide screening, biochemical assays, promoter analysis, and transcription factor studies, the research clarifies how enzyme competition and gene regulation together shape the accumulation of medicinally important saponins.

The researchers first identified ten OSC genes in the E. senticosus genome and narrowed them down to two functionally dominant candidates through expression profiling and metabolite correlation analysis. Functional assays confirmed that one enzyme acts exclusively as a β-amyrin synthase, directing metabolism toward oleanane-type saponins, while the other functions as a cycloartenol synthase that feeds sterol biosynthesis. Both enzymes localize primarily to the cytoplasm and compete for the same substrate, creating a metabolic trade-off.

Detailed structural analyses revealed distinct conserved amino acid triplets that define the catalytic specificity of each enzyme. Site-directed mutagenesis demonstrated that even single amino acid changes could dramatically alter product profiles or abolish enzyme activity. Beyond enzyme function, the study showed that gene expression is finely regulated by light quality, DNA methylation, and multiple transcription factors. Importantly, several transcription factors were found to exert opposite regulatory effects on the two competing genes, simultaneously promoting saponin synthesis while repressing sterol formation, or vice versa. This coordinated regulation provides a molecular explanation for how plants optimize secondary metabolite production.

According to the researchers, the most significant insight of this work is the discovery of a coordinated regulatory system that controls metabolic direction at both enzymatic and transcriptional levels. They note that identifying transcription factors capable of oppositely regulating two competing biosynthetic genes is particularly striking, as such dual control has rarely been documented in plants. This mechanism allows the plant to fine-tune resource allocation between growth-related sterols and defense- or health-related saponins, offering a powerful strategy for metabolic optimization.

The findings have important implications for medicinal plant improvement and metabolic engineering. By targeting specific OSC genes or their regulatory transcription factors, it may be possible to enhance the accumulation of valuable saponins without compromising plant viability. This strategy could support the development of higher-quality herbal medicines and functional plant products. More broadly, the study provides a conceptual model for controlling metabolic branch points in plant secondary metabolism. Such insights may be applied to other medicinal or industrial crops, enabling more precise manipulation of bioactive compound synthesis through genetic and environmental regulation.