## 1 Introduction  
_Dioscorea zingiberensis_, commonly known as \"yellow ginger\" in China, is extensively cultivated as a traditional medicinal crop in the southern region of Shaanxi province in China. Extracts from \(D\). _zingiberensis_ rhizomes have been suggested in Chinese medicine for the treatment of chest pain, coronary heart disease, and hypoglycemic problems [1]. These medicinal effects are mostly attributed to the presence of steroidal saponins in its rhizome tissue [2], and of which dioscin is the principle active component [3]. A variety of bioactivities, such as anti-tumor [4], anti-inflammatory [5], and bone-protecting [6] properties, have been shown for dioscin. More importantly, the aglycone of dioscin, namely diosgenin, serves as the most important starting material for the synthesis of many types of sterol drugs, such as progesterone, androgen, and contraceptives [7]. Despite of the medicinal importance of dioscin, there is not yet a complete list of genes identified for its biosynthesis, especially the genes coding for tailoring enzymes in the late steps to dioscin biosynthesis are missing at present. Previous labeling studies have suggested cholesterol as the precursor for dioscin biosynthesis [8; 9; 10]. In plants, cholesterol is synthesized via the isoprenoid biosynthetic pathway through 2,3-oxidase collagenase followed by cycloartenol synthase, and genes involved in the steps from cycloartenol to cholesterol have been discovered (Figure 1) [11]. Cholesterol can be transformed into dioscin through modifications which include oxidation at the C-22, C-26, and C-16 positions by enzymes such as P450-dependent monooxygenases, dioxygenases, and other catalysts, and subsequent addition of glucose and rhamn groups to the C-3 position of diosgenin by UDP- glycosyltransferases (UGTs) (Figure 1). Although the specific steps in dioscin biosynthesis starting from cholesterol were previously proposed by [PERSON] et al [12], the proposed steps have never been experimentally validated, and genes involved in these steps have never been isolated. Figure 1: Putative pathway for dioscin biosynthesis in _D. zingiherensis_. Abbreviations used are: AACT, acetyl CoA acetyltransferase; HMGS, hydroxymethyl glutaryl CoA synthase; HMGR, 3-hydroxy-3-methylglutaryl CoA reductase; MK, mevalonate kinase; PMK, phosphomevalonate kinase; MDC, mevalonate diphosphophate decarboxylase; IPI, IPP diphosphateisomerase; DXS, 1-deoxy-D-xylulose-5-phosphate synthase; DXR, 1-deoxy-D-xylulose-5-phosphate reductoisomerase; MCT, 2-C-methyl-D-erythnitol 4-phosphate cytidylyl transferase; CMK, 4-diphospho-cytidyl-2-C-methyl-D-erythnitol kinase; MECPS, 2-C-methyl-D-erythnitol-2, 4-cyclodiphosphate synthase; HDS, 4-hydroxy-3-methylbut-2-(\(E\))-enyl diphosphate synthase; HDR, 4-hydroxy-3-methylbut-2-enyldiphosphate reductase; GPPS, geranyl diphosphate synthase; FPPS, farnesyl diphosphate synthase; SS, squalene synthase; SE, squalene epoxidase; CAS, cycloartenol synthase; SSR, sterol side chain reductase; SMO1, sterol 4a-methyl oxidase 1; SMO2, sterol 4a-methyl oxidase 2; CPI, cycloencalenol cycloisomerase; CYP51, sterol C-14 demethylase; C14-R, sterol C-14 reductase; 8,7-SI, sterol 8,7-isomerase; C5-SD, sterol C-5(6) desaturase; 7-DR, 7-dihydrocholesterol reductase. Advances in RNA sequencing technology provide an efficient and economical platform for gene discovery in non-model plants without knowing their genome sequences [13]. Owing to the tissue-specificity for the synthesis of many types of plant metabolites, transcript profiling together with metabolite analysis from different tissues could prove valuable for the identification of genes in different metabolic pathways [14; 15; 16]. We have confirmed that dioscin is highly synthesized in the underground rhizome tissue of \(D\). _zingiberensis_ while in the aerial leaf tissue it is formed at very low concentrations (Figure 2).