3.1 Morphological Differences. Bupleurum chinense DC. and Bupleurum scorzonerifolium Willd. (Figure 2) grown in the same season and under the conditions. The morphology particularly the roots was obviously different. The roots of Bc have many branches, and the branches of Bs are few. The skin color of Bc is chocolate-brown, but Bs is reddish-brown.
3.1 Morphological Differences. Bupleurum chinense DC. and Bupleurum scorzonerifolium Willd. (Figure 2) grown in the same season and under the conditions. The morphology particularly the roots was obviously different. The roots of Bc have many branches, and the branches of Bs are few. The skin color of Bc is chocolate-brown, but Bs is reddish-brown.
3.3 Multivariate Analysis Revealed Differences among the Metabolite Profiles. Multivariate Statistical Analysis were performed to assess the differences among the metabolite profiles of 8 groups. The results of HCA showed that there were significant differences in different groups, which were divided into four clusters (Figure 4A). The metabolites in cluster 1 were highest in BsF and BsL, cluster 2 in BcF and BsF, cluster 3 in BsS and BcL, and cluster 4 in BcR and BsR. It also clustered between biological replicates, indicating good homogeneity between biological replicates and reliability of data.
PCA was used to uncover the internal structure of multiple variables through several principal components (Figure 4D). In the PCA diagram (Figure 4B-C), quality control (QC) samples is the mixture of bupleurum sample extracts, were projected to the same area and some even overlapped, so the analysis was stable and repeatable. This result showed that the tissue sites were separated on the first principal component and the varieties had obvious separation on the second principal component. There was obvious grouping tendency among different tissues of different varieties.
In general, HCA and PCA showed that there were significant differences in metabolites in different groups, with BcR and BsF showing the biggest differences.
3.4 Differential Metabolite Analysis. Pairwise comparisons were conducted among 8 groups (Figure 5D). In the OPLS-DA models (Figure 5A-B), significant segregation occurred in all comparison groups.
We further performed different metabolite screening based on the fold-change (FC ≥ 2 or ≤ 0.5) and variables identified as important in the projection scores (VIP > 1). The screening results are presented as volcano plots (Figure 5C) and Venn diagrams (Figure 6B).
In all comparison groups, BcR and BsR were down-regulated the most, which was consistent with the result of cluster analysis. We paid special attention to the differential metabolites in the medicinal part root group. There were 759 differential metabolites, among which 247 metabolites were up-regulated and 512 metabolites were down-regulated. There were 150 terpenoids in the differential metabolites, accounting for about 20% (Table 1).
Table 1 Classification and quantity of differential metabolites between BcR and BsR
Type
|
Number
|
Percentage(%)
|
Terpenoids
|
150
|
19.8
|
Phenolic acids
|
73
|
9.6
|
Flavonoids
|
72
|
9.5
|
Lipids
|
44
|
5.8
|
Lignans and Coumarins
|
43
|
5.7
|
Hydrocarbons
|
27
|
3.6
|
Alkaloids
|
15
|
2.0
|
In the group of BcR and BsR, seven of the top ten down-regulated saponins were saikosaponins (Figure 6A),and the specific of them were Saikosaponin A, Prosaikogenin G, Saikosapanin B1, Saikosaponin G, Saikosaponin C, Saikosaponin L, Saikosaponin BK1. Basically all saponins metabolites of Bs were down-regulated, indicating that the content of Saikosaponins in the root of Bc was relatively high. The top ten down-regulated metabolites had lignans, phenolic acids and the volatile organic chemicals. In BsR, 6-O-Galloyl-D-glucose which has antihypertensive activity more than BcR26. More saikosaponins (SSA,SSC,SSD,SSB4,SSF), which have been shown to have antidepressant and anti-inflammatory effects, were detected in BcR than in BsR. Compared with BcR, lignans, phenolic acids and the volatile organic chemicals were higher in BsR. In terms of isorhamnetin, a heart-protecting substance, we found 16 isorhamnosides were more accumulated in BsR than in BcR.
In the comparison between BcS and BsS, there were 729 differential metabolites in total, among which 423 metabolites were up-regulated and 306 metabolites were down-regulated, and flavonoids were the most different substances. In the comparison between BcL and BsL, there were 844 differential metabolites in total, among which 641 metabolites were up-regulated and 203 metabolites were down-regulated, and flavonoids and terpenoids were mainly different substances. In the comparison between BcF and BsF, there were 722 differential metabolites in total, among which 452 metabolites were up-regulated and 270 metabolites were down-regulated. In BcF, hispidulin-8-C-(2''-O-xylosyl)glucoside were found more than BsF. In BsF, kaempferol-3-O-glucuronide which had been found to treat non-alcoholic steatohepatitis27 were found more than BcF.
We also compared the differential metabolites between different tissue parts. In contrast group of BcR and BcF, 841 metabolites were up-regulated, including 149 flavonoids. In Bc, there are more triterpenoid saponins in roots, but more flavonoids and volatile components in flowers. In contrast group of BsR and BsF, 966 metabolites were up-regulated including 109 flavonoids and 78 phenolic acids, and the number of up-regulated metabolites was the largest in all contrast groups. So we can develop flowers of Bc and Bs to use active ingredients such as flavonoids in flowers. In addition, there were more flavonoids and phenolic acids in the aboveground part than in the root of Bs.
We identified 144 common differential metabolites from Venn diagrams. The common differential substance indicated that Bc and Bs had differences in roots, stems, leaves and flowers, belonging to interspecific differential substance. We focused on triterpenoid saponins in all down-regulated components in four contrast groups, such as Oleanolic acid-3-O-xylosyl(1→3)glucuronide, Oleanolic acid-3-O-glucosyl(1→2)glucoside, Bupleurumwenchuanense, saikosaponin K, Rotundifolioside A, Rotundifolioside H, Medicagenic acid-3-O-glucuronide-28-O-rhamnosyl(1,2)-arabinoside. More triterpenoid saponins were synthesized in Bc than Bs, which was closely related to the expression of genes related to triterpenoid saponins synthesis pathway. The other substances all down-regulated most were flavones, flavonoid carbonoside and flavonols, including 8 kinds of kaempferitrins and apiins. All up-regulated substances included 25 flavonoids, 11 phenolic acids, as well as some volatile components.
It is noteworthy that shikimic acid, as the prerequisite of phenylpropane pathway, was lower in all parts of Bs than that of Bc, which may be related to the metabolism of phenylpropane pathway in the two varieties. Isoscopoletin (6-Hydroxy-7-Methoxycoumarin), which had antiasthmic effect, was higher in Bc than Bs.
3.5 KEGG annotation and enrichment analysis. The differential metabolites among the eight accessions were mapped to the KEGG Pathway database to obtain detailed pathway information (Figure 6.A-F). Differential metabolites of BcR and BsR are mainly annotated and enriched in sesquiterpenoid and triterpenoid biosynthesis, flavonoid biosynthesis, and biosynthesis of secondary metabolites. Differential metabolites of BcS and BsS are mainly annotated and enriched in flavone and flavonol biosynthesis, limonene and pinene biosynthesis, monoterpenoid biosynthesis ,and folate biosynthesis. Differential metabolites of BcL and BsL are mainly annotated and enriched in monoterpenoid biosynthesis, limonene and pinene biosynthesis, and biosynthesis of secondary metabolites. Differential metabolites of BcF and BsF are mainly annotated and enriched in monoterpenoid biosynthesis, flavone and flavonol, and biosynthesis of secondary metabolites.
Some metabolic pathways overlap in these comparison groups, such as biosynthesis of secondary metabolites and monoterpenoid biosynthesis. The differential metabolites of the aboveground part of the two species mainly concentrated in the biosynthesis pathway of monoterpenoids, while the differential metabolites of the root mainly concentrated in the sesquiterpenoid and triterpenoid biosynthesis. The difference in function between the two species can be explained by pathway, which is mainly due to the difference in terpenoids.
Differential metabolites of BcR and BcF are mainly annotated and enriched in flavonoid biosynthesis, flavone and flavonol biosynthesis, pyrimidine metabolism, zeatin biosynthesis, esquiterpenoid and triterpenoid biosynthesis. Flavonoid biosynthesis pathway in flowers deserves attention.