Metabolic and transcriptomic analyses elucidate a novel insight into the network for biosynthesis of carbohydrate and secondary metabolites in the stems of a medicinal orchid Dendrobium nobile

Dendrobium nobile is an important medicinal and nutraceutical herb. Although the ingredients of D. nobile have been identified as polysaccharides, alkaloids, amino acids, flavonoids and bibenzyls, our understanding of the metabolic pathways that regulate the synthesis of these compounds is limited. Here, we used transcriptomic and metabolic analyses to elucidate the genes and metabolites involved in the biosynthesis of carbohydrate and several secondary metabolites in the stems of D. nobile. A total of 1005 metabolites and 31,745 genes were detected in the stems of D. nobile. The majority of these metabolites and genes were involved in the metabolism of carbohydrates (fructose, mannose, glucose, xylulose and starch), while some were involved in the metabolism of secondary metabolites (alkaloids, β-tyrosine, ferulic acid, 4-hydroxybenzoate and chrysin). Our predicted regulatory network indicated that five genes (AROG, PYK, DXS, ACEE and HMGCR) might play vital roles in the transition from carbohydrate to alkaloid synthesis. Correlation analysis identified that six genes (ALDO, PMM, BGLX, EGLC, XYLB and GLGA) were involved in carbohydrate metabolism, and two genes (ADT and CYP73A) were involved in secondary metabolite biosynthesis. Our analyses also indicated that phosphoenol-pyruvate (PEP) was a crucial bridge that connected carbohydrate to alkaloid biosynthesis. The regulatory network between carbohydrate and secondary metabolite biosynthesis established will provide important insights into the regulation of metabolites and biological systems in Dendrobium species.