Abstract
Rheum emodi is a perennial herb and an important medicinal plant, with anthraquinones and flavonoids as its main bioactive compounds. However, there is little knowledge about the biosynthetic pathway of anthraquinones in rhubarbs. In this study, we qualitatively and quantitatively assessed 62 pharmacological metabolites in rhubarb using dynamic multiple reaction monitoring (dMRM) of triple-quadrupole mass spectrometry (QqQ-MS), including 21 anthraquinones, 17 flavonoids, 6 stilbenes, 12 gallate esters, 3 tannins, and 3 others. Besides, the metabolomics results showed significant differences among all the 60 metabolites, except for gallic acid and piceatannol-O-β-glucoside. The combined transcriptome data of R. palmatum L. (RPL) and R. officinale Baill. (ROB) showed that 21,691 unigenes were annotated in the metabolic pathways. Taken together, 17 differentially expressed genes (DEGs) were associated with the anthraquinone biosynthetic pathway. Additionally, a significant correlation between anthraquinone peak intensity and DEG expression level existed, validating that DEGs contribute to the anthraquinone biosynthetic pathway. RT-qPCR results showed that the cluster-14354.38156 gene may catalyze the O-methylation of emodin to produce physcion. This study provides a useful resource for further studies on secondary metabolism in rhubarb and the combination analysis of transcriptome and metabolome, which can help with the discovery of enzyme genes involved in metabolite biosynthesis.
Highlights
Rheum emodi is a perennial herb and an important medicinal plant, with anthraquinones and flavonoids as its main bioactive compounds
The metabolic analysis of R. officinale Baill. (ROB) and R. palmatum L. (RPL) leaves were optimized by comparing the performance of several candidate elution systems, based on previous reference data[25]
differentially expressed genes (DEGs) involved in metabolic pathways are possible genes functioning in the synthesis of metabolites
Summary
Rheum emodi is a perennial herb and an important medicinal plant, with anthraquinones and flavonoids as its main bioactive compounds. This study provides a useful resource for further studies on secondary metabolism in rhubarb and the combination analysis of transcriptome and metabolome, which can help with the discovery of enzyme genes involved in metabolite biosynthesis. There are two known distinct and major anthraquinone biosynthetic pathways in higher plant species, including the polyketide pathway and chorismate/o-succinylbenzoic acid pathway, and the former leads to the biosynthesis of emodin-type anthraquinones and flavonoids[7,8]. Since knowledge about the biosynthetic pathway of anthraquinones in rhubarb is insufficient, the combination of technologies, such as plant metabolomics, high-throughput transcriptome sequencing, and qRT-PCR, was applied to study the anthraquinone and flavonoid pathways in our study. Our study may serve as a resource for future research of rhubarb, which lays a solid foundation for the molecular mechanism of anthraquinone biosynthesis in rhubarb
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