Gingerols and Flavonoid biosynthesis
Gingerols are reported to have a variety of pharmacological properties like anti-inflammatory, anticancer, and anti-oxidant activities (Lee and Seo et al., 2008; Dugasani and Pichika et al., 2010; Liao and Leu et al., 2012; Tiwari and Mishra, 2016; Liang and Sang et al., 2018). Three types of gingerols, 6-gingerol, 8-gingerol, and 10-gingerol, were detected in the rhizomes of the four ginger varieties according to the LC–MS-based metabolomics results (Supplemental File 2). 6-Gingerol was primarily enriched in the stilbenoid, diarylheptanoid, and gingerol biosynthesis pathway (map 00945) and was found in higher amounts in the SD rhizomes, and in decreasing amounts in the LP, YJ and SX varieties (SD > LP > YJ > SX, Figs. 2 and 3). Since the concentrations of 8-gingerol and 10-gingerol were much lower compared with that of 6-gingerol (Jiang and Huang et al., 2018), 6-gingerol could be used as an indicator to evaluate the quality of ginger drugs. The amounts of 8-gingerol and 10-gingerol in the four rhizome varieties were found in the order of SD > LP > SX > YJ and YJ > LP > SD > SX, respectively (Table 2, Supplemental File 2). According to our results, the concentrations of gingerols are generally proportional to the spicy taste of ginger. This is consistent with the results from a previous study that described that the pungent components of ginger, which mainly consists of gingerols, are the main contributors to the taste of ginger (Ramirez-Ahumada and Timmermann et al., 2006).
Other bioactive compounds were also identified among the four ginger varieties. Epicatechin is a flavonoid that is a polyphenolic antioxidant plant secondary metabolite. The concentration of epicatechin in YJ was found to be 21.27 and 47.61 times higher than that of LP and SX (Figs. 2 and 3). In fact, the stilbenoid, diarylheptanoid, gingerol, and flavonoid biosynthesis pathways are branches of the general phenylpropanoid biosynthesis pathway (Fig. 3). α-Zingiberene is a sesquiterpene that constitutes the main component of the volatile oil of ginger. The concentration of α-zingiberene was found in the order of SD > LP > SX > YJ, with SD having 8.06 times more α-zingiberene than YJ (Table 2, Supplemental File 2). According to the above results, gingerols and α-zingiberene were found in higher amounts in SD and LP than in SX and YJ, indicating that SD and LP could be used as medicinal source of ginger while SX and YJ could be used in food because of their anticipated lighter taste as a result of their lower amount of gingerols.
Biosynthesis Pathway of Bioactive Steroids
Steroid hormones are a class of widely relevant, biologically active compounds originating from cholesterol that are altered in a stepwise fashion while maintaining a basic 17-carbon, 4-ring structure (Payne and Freishtat, 2012). They were reported to act as signaling molecules important for normal growth, development and differentiation of plants (Friedrichsen and Chory, 2010), and were also revealed as bioactive compounds that help control metabolism, inflammation, immune functions, salt and water balance, the development of sexual characteristics, and withstand illness and injury in animals(Payne and Freishtat, 2012; Yadav and Yadav et al., 2014). In this study, the concentrations of five metabolites significantly enriched in the steroid hormone biosynthesis pathway were found to vary among the four ginger varieties (Table 2, Fig. 4). The amounts of the steroid hormones corticosterone and tetrahydrocortisone were 2.4 and 6.27 times higher in LP compared with YJ, while 2-methoxy-estradiol-17β-3-glucuronide, 2-methoxyestrone-3-glucuronide, and estrone glucuronide revealed a similar accumulation pattern (LP > SX > YJ > SD), with amounts 4.85, 6.19, and 6.07 times higher in LP than SD, respectively (Figs. 2 and 4). Thus, LP might be a valuable ginger variety for producing larger quantities of bioactive compounds for medical applications.
Gibberellin is an important diterpenoid phytohormone with extensive and complex biological functions that regulates the growth and development of plants throughout their life cycle (Richards and King et al., 2001; Ludmila and Thomas et al., 2004; Colebrook and Thomas et al., 2014; Tang and Liu et al., 2018). Two types of gibberellins, gibberellin A14 and gibberellin A51-catabolite, were detected among the four ginger varieties (Fig. 5). The concentration of gibberellin A14 was found in the order of LP > SX and SD > YJ, while no significant difference in gibberellin A14 was detected between SX and SD. The concentration of gibberellin A51-catabolite was significantly higher at 1.90-fold in LP compared with SD (Table 2, Supplemental File 2). Thus, terpenoids accumulated to a higher extent in the LP ginger compared with the other varieties.