Spatial Mapping of Bioactive Metabolites in the Roots of Three Bupleurum Species by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Bupleurum is a kind of medicinal plant that has made a great contribution to human health because of the presence of bioactive metabolites: Bupleurum saikosaponins and flavonoids. Despite their importance, it has been a challenge to visually characterize the spatial distribution of these metabolites in situ within the plant tissue, which is essential for assessing the quality of Bupleurum. The development of a new technology to identify and evaluate the quality of medicinal plants is therefore necessary. Here, the spatial distribution and quality characteristics of metabolites of three Bupleurum species: Bupleurum smithii (BS), Bupleurum marginatum var. stenophyllum (BM), and Bupleurum chinense (BC) were characterized by Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Twenty-nine metabolites, including saikosaponins, non-saikosaponins, and compounds from the saikosaponin synthesis pathway, were characterized. Some of these were successfully localized and visualized in the transverse section of roots. In these Bupleurum species, twelve saikosaponins, five non-saikosaponins, and five saikosaponin synthesis pathway compounds were detected. Twenty-two major influencing components, which exhibit higher ion intensities in higher quality samples, were identified as potential quality markers of Bupleurum. The final outcome indicates that BC has superior quality compared to BS and BM. MALDI-MSI has effectively distinguished the quality of these Bupleurum species, providing an intuitive and effective marker for the quality control of medicinal plants.