1 Introduction
Traditional Chinese medicine has traditionally utilized Taxilli Herba (TH), which is composed of the dried stems and branches with leaves ofTaxillus chinensis (DC.) Danser [1]. It is distributed in Guangxi, Yunnan, Sichuan, Gansu, and other provinces in China [2]. Shennong Materia Medica classified TH as a top grade medication and listed it as being used to treat rheumatic arthralgia, waist and knee weakness, among other symptoms [3]. Meanwhile, multiple types of chemical constituents have been identified, including flavonoids, phenolic acids, and tannins, etc. These constituents act synergistically to exert multiple pharmacological effects such as anti-inflammatory and analgesic [4,5]. Additionally, TH tea has a number of health advantages as well as significant commercial growth potential [6].
The diversity of hosts is an influential biological feature of TH. The hosts of TH used are mainly Morus alba L. (SS) andLiquidambar formosana Hance (FXS) [7,8]. It is worth noting that the relationship between hosts and the quality of TH has attracted the attention of experts and scholars in recent years, and relevant studies have been carried out one after another in terms of chemical constituents and pharmacology [9–15]. Since TH takes water, inorganic salts, and nutrients from its hosts, the host has a significant influence on how it grows and develops [16]. To the best of our knowledge, molecular changes and elucidating the mechanisms underlying the accumulation of bioactive constituents in TH remain vacant. Therefore, it is necessary to analyze the differences at protein level and investigate the proteins associated with the biosynthesis of these constituents in TH from two hosts.
The proteomics of medicinal plants is a useful tool for understanding mechanisms, metabolic pathways, and functional gene mining [17,18]. Studies are gradually expanding to include non-model plants instead of only model plants such Arabidopsis, rice, and soybean [19–23]. In this study, mapping of proteins and related bioactive constituents in metabolic pathways was investigated using tandem mass tag (TMT)-based quantitative proteomics in TH from SS and FXS. First, a proteome database of TH was established by annotating protein sequences according to the related existing database. A total of 5681 proteins were detected, of which 533 were significantly differentially expressed proteins (DEPs). The DEPs were enriched in cellular process and secondary metabolite biosynthesis. Candidate proteins involved in flavonoid biosynthesis were selected for further validation. Meanwhile, ultrafast liquid chromatography simultaneously coupled with triple quadrupole/linear ion trap tandem mass spectrometry (UFLC-QTRAP-MS/MS) was applied to determine the content of sixteen bioactive constituents. Ultimately, parallel reaction monitoring (PRM) was carried out to verify the abundance of associated proteins. This work aimed to provide a theoretical foundation for the selection of TH on hosts by examining the protein differences between the two host-derived TH and the functions of proteins in the biosynthesis pathway of bioactive constituents.