Untargeted metabolomics approach reveals the tissue-specific markers of balloon flower root (Platycodi Radix) using UPLC-Q-TOF/MS

Platycodi Radix, the root of balloon flower, has been widely used as food and traditional oriental medicine in East Asian countries. Secondary metabolites are generally selected as quality markers to evaluate the quality of Chinese herbal medicines, and their distribution is tissue-specific. However, the similarities and differences of secondary metabolites in different tissues of Platycodi Radix have not been fully elucidated. In this study, an untargeted metabolomics approach based on ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) was developed to dissect the differential distribution of major secondary metabolites in three different tissues of periderm, phloem and xylem and explore the tissue-specific markers of Platycodi Radix. First, an optimized UPLC-Q-TOF/MS method was applied in the global metabolite profiling of three different tissues of Platycodi Radix. The results demonstrated that different tissues from the same Platycodi Radix sample have different profiles, and the periderm, phloem and xylem of Platycodi Radix are rich in triterpenoid saponins, especially in the phloem. Notably, the periderm of Platycodi Radix also contains significant secondary metabolites with pharmacological activities, thereby suggesting that the periderm should be kept on the root for medicinal use. Next, 23 robust tissue-specific markers of Platycodi Radix were selected for the first time using the partial least squares discriminant analysis (PLS-DA) loading plot and the variable importance for projection (VIP) plot. Finally, a hierarchical clustering analysis heat map was further used to visualize the relationships between three different tissues of Platycodi Radix and these tissue-specific markers. To sum up, our strategy provides systematic and comprehensive protocols for screening the tissue-specific markers of Platycodi Radix and chemical bases for the distinct usage of different tissues of Platycodi Radix, and is therefore helpful for the quality evaluation, processing method optimization and rational utilization of Platycodi Radix.