Regulation of Steroidal Alkaloid Biosynthesis in Bulbs of Fritillaria thunbergii Miq. By Shading and Potassium Application: Integrating Transcriptomics and Metabolomics Analyses

<i>Fritillaria thunbergii</i> Miq., a medicinal plant rich in steroidal alkaloids, produces bulbs that clear heat, resolve phlegm, and detoxify. However, excessive yield-oriented cultivation has reduced the number of <i>F. thunbergii</i> plants that meet commercial standards. This study explored the effects of potassium application and shading on the bulb biomass and medicinal substance content of <i>F. thunbergii</i>. Shading increased the active ingredient content in bulbs by approximately 20.71% but reduced biomass by approximately 17.24%. Fertilization with different potassium concentrations under shading (K1S-K3S) alleviated shading-induced biomass reduction and increased active ingredient accumulation, with the K2S and K3S groups yielding significantly better results than the K1S group. Pharmacological experiments showed that the K2S group exerted the best antitussive, expectorant, and anti-inflammatory effects. Metabolome analysis showed that compared with those in the controls, peiminine, peimine, imperialine, solasodine, and cyclopamine were the most abundant steroidal alkaloids under K2S treatment. Transcriptome analysis identified key genes and biosynthetic pathways for major steroidal alkaloids, namely, farnesyl pyrophosphate synthase (<i>FtFPS</i>) involved in steroidal alkaloid biosynthesis. Transcription factor analysis revealed that nine transcription factors predominantly expressed under the K2S treatment might regulate steroidal alkaloid biosynthesis. Furthermore, FtFPS was identified as a hub gene in the co-expression network and was verified to catalyze the biosynthesis of farnesyl pyrophosphate. The interaction between FtFPS and FtAP2/ERF was verified through yeast two-hybrid experiments. These findings offer new insights into the steroidal alkaloid biosynthesis mechanism triggered in <i>F. thunbergii</i> by potassium application and shading, supporting ecological strategies to enhance steroidal alkaloid levels in this species.