The rhizome and daughter rhizome samples of Yujiang 1 were selected for metabolomic analysis (Fig. 1A). The LC-MS-based metabolomic analysis was performed to identify differentially accumulated metabolites between the rhizome and daughter rhizome samples. A PCA scores plot was generated to visualize the metabolic differences between the two rhizome types. The present data showed distinct discrimination of the metabolome of the rhizome and daughter rhizome samples (Fig. 1B).
Identification of differentially accumulated metabolites in ginger samples
A total of 1212 differentially accumulated metabolites with p < 0.05 and |fold change| ≥ 2 were identified, of which 399 metabolites were more highly accumulated in R samples than in DR samples, whereas 211 metabolites were more highly accumulated in DR samples (Fig. 2). Among the differentially accumulated metabolites, pathway analysis of key bioactive compounds (Table 1), including volatile oils, gingerols, diarylheptanoids, and flavonoids, is shown in Table 2 together with a pathway summary (Fig. 3). The pathways “flavonoid biosynthesis” and “stilbenoid, diarylheptanoid, and gingerol biosynthesis” are branches of the general “phenylpropanoid biosynthesis” pathway.
Key bioactive compounds in ginger
The KEGG metabolic pathways associated with volatile oils, gingerols, and diarylheptanoids, such as the “terpenoid backbone biosynthesis” and “stilbenoid, diarylheptanoid, and gingerol biosynthesis” pathways, were enriched between the two samples (Fig. 4). The terpenoid backbone biosynthesis pathway includes two biosynthetic pathways: the mevalonate pathway and the non-mevalonate pathway (MEP/DOXP). In the present study, pyruvate in MEP/DOXP, mevalonate in the mevalonate pathway, and (E,E)--farnesyl pyrophosphate were more highly accumulated in R samples compared with DR samples. In the “stilbenoid, diarylheptanoid, and gingerol biosynthesis” pathway, gingerols (especially 6-gingerol) showed significantly higher contents in R samples than those in DR samples; additional compounds in this pathway, such as caffeoylquinic acid and curcumin, shown similar accumulation patterns (Fig. 5).
Flavonoids show broad pharmacological activities, including antimicrobial, antioxidant, cytotoxic, chemoprevention, and anticancer activities. Kaempferol and quercetin are two intermediate products in the “flavonoid biosynthesis” pathway. In the present study, contents of flavonoid compounds, such as kaempferol and quercetin, were higher in R samples than in DR samples. Caffeoylquinic acid were also identified in the flavonoid pathway (Fig. 6).
LC-MS analysis of gingerols in R and DR samples
Gingerols are the main compounds responsible for the pungency of ginger, and are the main flavoring substances and core medicinal ingredients of ginger. To evaluate expression profiles of differentially expressed metabolites, the contents of four gingerols, namely 6-gingerol, 8-gingerol, 10-gingerol, and 6-zingiberene, were detected by LC-MS analysis. The contents of the four metabolites in R samples were 489.3 μg/g, 29.5 μg/g, 7.8 μg/g and 52.1 μg/g respectively, which were all considerably higher than those of DR samples (Fig. 7). The results were consistent with the metabolome data, which indicated that metabolomic analysis is an effective method to investigate the differences in accumulation of secondary metabolites among different tissues.