Transcriptional regulation modulates saponin biosynthesis of Panax notoginseng response to root knot nematodes

Root knot nematodes (RKNs) are highly destructive pests that pose a negative impact on the growth of Panax notoginseng and also affect its quality. P. notoginseng is widely known for its medicinal properties regarded to ginsenosides which are chemically triterpene saponins. However, the effect of RKNs infection on the saponin content in the roots and the transcriptional regulation mechanisms of their synthesis are not well understood. Our study aimed to assess the impact on saponin content and investigate the expression pattern of key genes related to saponin synthesis in RKN-infected P. notoginseng. Our results highlight that RKNs infection significantly affected the content of notoginseng R1 in the main root, with a major impact on the levels of ginsenoside Rg1 and Rb1 in lateral roots. Furthermore, the expression of saponin synthesis key genes in RKN-infected P. notoginseng varied as compared with the healthy plant. Specifically, various genes including HMGS2, DS1, and DS2, were up-regulated in response to RKNs infection of grade I and II. In contrast, except for DXS5, various genes were down-regulated when the infection degree of RKNs reached grade II and III. In addition, our study highlights the strong correlation between transcription factor families and triterpenoid saponin synthesis genes, such as AP2/ERF-ERF, bHLH, WRKY, GRAS and MYB. It was found that the expression pattern of the PnMYB61 transcription factor closely reflected that of squalene epoxidase (SE2), a key gene that controls the biosynthesis of 2,3-oxidosqualene, following RKNs infection. Molecular docking analysis also suggested that PnMYB61 may play a crucial regulatory role in expression of SE2 protein. Overall, our findings contribute to the quality evaluation of RKN-infected P. notoginseng roots and provide valuable insights into the identification of key regulators involved in ginsenoside biosynthesis.