![]() Subsequently, the oxidation, reduction, glycosylation, and methylation reactions occur under the catalysis of various functional enzymes ( Burlat et al., 2004 Geu-Flores et al., 2012 Wu and Liu, 2017). The production of GPP is the first step in the synthesis of monoterpene ( Wang et al., 2013 Xi et al., 2016). Geranyl diphosphate (GPP), the precursor of monoterpene, is synthesized from IPP and DMAPP under the catalysis of geranyl pyrophosphate synthase (GPPS). DXP is catalyzed to form MEP by 1-deoxy-D-xylulose-5-phosphate reductase (DXR) ( Lichtenthaler, 1999 Carretero-Paulet, 2002). 1-deoxyxylulose-5-phosphate synthetase (DXS) is the first rate-limiting enzyme in MEP pathway, which catalyzes the reaction of pyruvate and glycerol-3-phosphate to produce 1-deoxyxylulose-5-phosphate (DXP) ( Sprenger et al., 1997). The study of iridoid glycoside biosynthesis is recently focused on a series of key enzymes in MEP pathway and terpene synthetase. Iridoid glycoside belongs to monoterpene, and its biosynthetic precursors IPP and DMAPP mainly come from 2-C-methyl-D-erythritol-4-phosphoric acid pathway (MEP) ( Choi and Sano, 2007 Li et al., 2010 Liu et al., 2016). ![]() The abundant and widely distributed terpenes are synthesized from isoprene diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) ( Pu et al., 2021). There are diverse SMs in plants, such as nitrogen-containing compounds, phenolic compounds, and terpenes ( Fang et al., 2011). Plant secondary metabolites (SMs) not only play a pivotal role in plant growth and environmental adaptation, but also are used as medicines by humans because of their rich pharmacological effects ( Li Y. glutinosa, but also expand our current knowledge of the function of methylation in iridoid glycoside accumulation. ![]() These findings not only identify the key genes of iridoid glycoside synthesis from R. glutinosa, followed by triggering the iridoid glycoside accumulation. Taken together, the expression of iridoid glycoside synthesis gene was upregulated by the demethylation in R. Similar to the changes in 5-methyl cytosine (5mC) content, the DNA demethylation could be induced by 5-azaC and responded in a dose-dependent manner to 15, 50, and 100 μM 5-azaC. glutinosa, in which the hemi-methylation was the main reason for the change in DNA methylation levels. In addition, the changes in the spatiotemporal specificity of degree and levels of DNA methylation were observed in R. glutinosa, in which the application of the different concentrations of 5-azaC, especially 50 μM 5-azaC, could significantly upregulate the expression of five genes above and iridoid glycoside content. Further, our results showed that under normal and 5-azacytidine (5-azaC) treatment, the expression levels of DXS, DXR, GPPS, G10H, and 10HGO were consistent with the iridoid glycoside accumulation in R. The analysis of the above genes confirmed that the co-occurrence ratio of DXS, DXR, and GPPS was high in plants. A total of 357 unigenes were involved in iridoid glycoside synthesis, in which the highly conservative genes, such as DXS, DXR, GPPS, G10H, and 10HGO, in organisms were overexpressed. Herein, the analysis of RNA-seq data revealed that 3,394 unigenes related to the biosynthesis of secondary metabolites were identified in R. However, the mechanisms that underlie the regulation of iridoid glycoside biosynthesis remain poorly understood in R. Iridoid glycoside is the important secondary metabolite and the main active component in Rehmannia glutinosa.
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