Discovery, Isolation, and Characterization of 23-Spirocholestane Derivatives as Novel Plant Root Growth Inhibitors from Ypsilandra thibetica

In the present study, we first discovered that the 70% ethanol extract of Ypsilandra thibetica significantly inhibited Arabidopsis root growth. Through a bioactivity-guided strategy, 18 rare 23-spirocholestane derivatives (1–18), including 12 new ones (1–12), featuring a dioxane ring between C-24/C-23 of the aglycone moiety and C-1′/C-2′ of the d-fucose ligand, were obtained from the strongest active elution fraction. Their structures were elucidated via a comprehensive method of MS and NMR spectroscopy, single-crystal X-ray diffraction, and chemical methods. Evaluation of their inhibitory effects of all isolates on Arabidopsis root growth revealed that 5/6, 3/4, 1, and 2 exhibited pronounced activity, with EC₅₀ values of 6.89, 9.13, 18.01, and 26.83 μM, respectively. Then, a preliminary structure–activity relationship (SAR) analysis of these compounds was conducted. Furthermore, transcriptome data indicated that Arabidopsis responded to the stress caused by these bioactive compounds through the modulation of ethylene and auxin signaling pathways and the activation of various detoxification-related genes. Finally, the herbicidal evaluation showed that 3/4 and 5/6 significantly inhibited the root growth of Echinochloa crusgalli, while 3/4 remarkably suppressed the root growth of Lolium multiflorum. These findings highlight that 23-spirocholestane derivatives represent promising candidates for the development of new bioherbicides.