# Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants
## 3 Molecular Regulation of Steroidal Saponin Biosynthesis in Plants  
At present, there have been few in-depth studies on the regulation mechanism of steroidal saponin biosynthesis in plants. Steroidal saponins may be induced by a variety of elicitors, including abiotic and biotic elicitors. Methyl jasmonate (MeJA) is used as an abiotic elicitor to induce the synthesis of steroidal saponins in fenugreek and _D. zingiherensis_[38; 39]. Ethylene, salt stress, and melatonin had the same effect as MeJA on increasing the production of steroidal saponins [40; 41]. Biotic elicitors usually include fungi or bacteria; for example, _Aspergillus niger_, _Fusarium oxysporum_ Dzf17, and _Saccharomyces cerevisiae_ are used as fungal elicitors to increase diosogenin production in _Helictere siona_, while _Escherichia coli_ and _Bacillus subtilis_ are used as bacterial elicitors to significantly increase diosogenin production [42]. However, the specific molecular regulation mechanism of steroidal saponin biosynthesis is still unknown. One of the main reasons is that it is difficult to establish an appropriate and efficient transgenic system in medicinal plants (such as _D. zingiherensis_, _P. polyphyllla_, and fenugreek), which hinders the verification of gene functions and makes it difficult to conduct in-depth research. [PERSON] et al. believed that the molecular regulation mechanism of steroidal saponin biosynthesis in plants is multipronged, including environmental factors and various regulatory proteins, such as transcription factors (TFs), protein kinases, and phosphatases [1]. Among them, transcription factors are considered to play an important role in the regulation of the biosynthesis of steroidal saponins, steroidal glycoside alkaloids, and triterpene saponins [1; 43; 44; 45]. Steroidal glycoside alkaloids (SGAs) and steroidal saponins have the same biosynthetic source; that is, they both use cholesterol as a synthetic precursor [1]. Therefore, the regulation of SGA biosynthesis can be used as a reference for research on the regulation of steroidal saponin biosynthesis. It has been reported that SGAs can also be regulated by multiple transcription factors. For example, _GLYCOALKALOID METABOLISM 9_ (_GAME9_) is an AP2/ERF transcription factor that can positively regulate the biosynthesis of SGAs in tomato and potato [46]. Specifically, _GAME9_ can change the expression levels of the upstream MVA pathway genes of SGA biosynthesis and cholesterol synthesis gene _SSR2_, and cholesterol synthesis gene _C5-SD_ is the direct target of _GAME9_. Moreover, _GAME9_ plays a regulatory role in SGA synthesis by cooperating with the _SIMYC2_ transcription factor [18; 46; 47]. The _GAME_ genes seem to be the target genes of the SGA biosynthetic pathway, and the silencing of the _GAME11_ gene leads to decreased levels of \(\alpha\)-lycoposogle in tomato [48]. In addition, MYB, AP2/EEBP, pHLH, WRKY, and other TFs may be involved in the biosynthesis of SGAs in _Fritillaria imperialis_ and _Fritillaria anhuiensis_[43; 44; 49]. The jasmonate-responsive transcription factors of the ETHYLENE RESPONSE FACTOR (ERF) family (JREs) can regulate SGA biosynthesis by binding to the GCC box-like promoter elements of SGA biosynthesis genes in tomato [47]. Phytosterols are important metabolites of steroids, which include cholesterol, \(\beta\)-sitosterol, campesterol, and so on. Campesterol is a precursor of brassinosteroid (BR), which is a competitive synthesis compound of steroidal saponins and cholesterol or \(\beta\)-sitosterol, as a precursor of steroidal saponins [20; 50]. The structure of BR is close to the molecular skeleton structure of steroidal saponins; moreover, we have found that BR is also regulated by drought and salt stress [25; 51; 52]. Therefore, the research results of BR biosynthesis regulation can also be used as a reference to explore the molecular regulation of steroidal saponins in plants.