Degradation of Ginsenosides in Panax notoginseng Rhizosphere Soil: Dammarane-Type Transformation Products and Their Allelopathic Interference

Ginsenosides released into and subsequently degraded in the rhizosphere soil act as autotoxic allelochemicals contributing to the continuous-cropping obstacle (CCO) in Panax species. While soil microorganisms rapidly degrade prototype ginsenosides, the resulting transformation products (TPs) and their allelopathic roles remain poorly characterized. Using seed germination inhibition-guided fractionation, 27 dammarane-type triterpene aglycones (1-27), including 11 novel analogues (1-11), were isolated and identified from the most bioactive fraction (Fr. C) of ginsenoside-enriched Panax notoginseng rhizosphere soil extracts. Their proposed microbial transformation pathways involved deglycosylation, tetracyclic skeleton oxidation, and C17 side-chain modifications. Allelopathic assays revealed that dammarane-type TPs (DT-TPs) significantly inhibited seed germination and radicle elongation in both P. notoginseng and Arabidopsis thaliana, while also modulating the growth of dominant pathogenic and beneficial fungi isolated from P. notoginseng rhizosphere soil. This study establishes DT-TPs as potent allelochemicals that influence plant growth and soil microbial communities, thereby providing crucial mechanistic insights into CCO development and highlighting risks to sustainable agriculture posed by persistent phytotoxic TPs.