The bulbs of the lily plant Fritillaria thunbergii Miq. possess substantial medicinal properties for relieving coughs and clearing the lungs. However, excessive pursuit of yield during cultivation has led to a decrease in medicinal ingredients. Therefore, we aimed to investigate the effects of two single-factor treatments, shading (SK0) and potassium application (S0K), and their coupling treatment (SK) on bulb biomass and medicinal substance content, along with the role of rhizosphere microorganisms. Shading increased the content of active ingredients in bulbs by approximately 11.7% while decreasing bulb biomass by approximately 11.3%. SK treatment mitigated the biomass reduction caused by SK0 treatment while enhancing the accumulation of active ingredients in F. thunbergii, up to 1.2 times higher than that of SK0 treatment. In rhizosphere soil, Allorhizobium–Neorhizobium–Pararhizobium–Rhizobium (ANPR), Chryseobacterium, Brevundimonas, and Phoma exhibited significant positive correlations with medicinal components, among which ANPR, Brevundimonas, Chryseobacterium, and Phoma were responsive to SK treatments. Also, Burkholderia–Caballeronia–Paraburkholderia (BCP) and Brevundimonas responded to changes at different growth stages of F. thunbergii. The relative abundance of these microorganisms was associated with the alterations of soil factors resulting from shading or K application. Our results indicate that these microorganisms are beneficial to the growth of bulbs and the synthesis of active components in F. thunbergii. The combination of shading and K application may regulate the accumulation of medicinal substances in F. thunbergii by modulating the structure of the soil microbial community. Our results serve as a reference for soil improvement for medicinal plant cultivation.

Fritillaria thunbergii 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 F. thunbergii plants that meet commercial standards. This study explored the effects of potassium application and shading on the bulb biomass and medicinal substance content of F. thunbergii. 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 (FtFPS) 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 F. thunbergii by potassium application and shading, supporting ecological strategies to enhance steroidal alkaloid levels in this species.

The APETALA 2/ethylene-responsive element binding factors (AP2/ERFs) are crucial modulators of plant development and stress responses. Fritillaria thunberg