# Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants
## 1 Introduction
Pennogenin saponins are also rare in plants (Figure 1H). If diosgenin is also hydroxylated at C-17, C-23, C-24, and C-27, it can form pennogenin or hydroxy-penogenin saponin, such as polyphyllin D, Paris VI, and Paris VII isolated from _Paris_ species [8]. The diversity of steroidal saponins in plants is related to the differences in the aglycone structures, especially the differences in the aglycone oxidation mode, such as C-16, C-22, and C-26 sites in the aglycone skeleton of spirostand saponins. They also occur rarely at sites such as C-1 and C-3, of spirostand saponins; C-6, C-7, C-12, and C27 of furostanol saponins; and C-2 and C-5 of cholestane saponins. In addition, it is also possible to introduce double bonds between C-25 and C-27 of spirostand's aglycone skeleton, and between C-20 and C-22 of the aglycone skeleton of furostanol saponins [5; 7; 9]. According to the number of sugar units connected to the aglycone of steroidal saponins, they are categorized as monoglycos, dioglycosides, trioglycosides, or tetraoglycosides, and the common sugar units are pyranose, rhannose, and arabinopyranose [5; 6; 7; 8; 9]. The glycosylation sites of different types of steroidal saponins are different. Spirostanol saponins are usually in 1-OH, 3-OH, and 24-OH, and furostanol saponins are usually in 1-OH, 3-OH, and 26-OH [5; 6; 7; 8; 9]. Steroidal saponins have a wide range of functions and pharmacological activities due to their connections with different aglycone skeletons and different numbers of sugar chains, and there are also many different types of steroidal saponins in the same plant. Therefore, the distribution and existing forms of steroidal saponins in plants are diverse, and may also be related to the different functions of steroidal saponins in different plants. For plants, steroidal saponins can help them to cope with biological and abiotic stresses such as pathogens, herbivores, and drought [14]. [PERSON] et al. believed that saponins in plants are produced in response to biological stress [1]. In stress reactions, such as when they are being attacked by herbivous insects or during pathogen infection, the increase in the saponin contents in plants is usually mediated by hormones, such as jasmonic acid and salicylic acid [14; 15; 16]. For human beings, steroidal saponins isolated from plants have a variety of pharmacological uses, including antibacterial activity, antitumor activity, anti-inflammatory activity, and hypoglycemic and lipid-lowering effects, so they are an important industrial raw material for the production of steroidal drugs [1; 3]. Due to the complexity of the chemical structure of steroidal saponins, their biosynthesis in plants and the formation and evolution of their synthetic pathways have not been thoroughly analyzed. Here, we introduce the biosynthesis, regulation, functions, and distribution of steroidal saponins in plants. At present, the research on steroidal saponin  
Figure 1: Structural diagrams of eight types of steroidal saponins isolated from plants. (**A**) Spirostanol saponins, (**B**) furostanol saponins, (**C**) cholestane saponins, (**D**) pregnane saponins, (**E**) isospirostand saponins, (**F**) polyhydroxylated saponins, (**G**) pseudospirostand saponins, and (**H**) pennogenin saponins. Furostanol and spirostand saponins are the two most common types of steroidal saponins in plants, while other types of steroidal saponins are unique to different plants. biosynthesis is focusing on the key enzymes involved in the side-chain hydroxylation, oxidation, and glycosylation of sterols (cholesterol or \(\beta\)-sitosterol). Although great progress has been made in the research of steroidal saponin biosynthesis, there may still be some unknown key steps, and their regulatory mechanisms are still unclear. The distribution, possible formation, and molecular evolution of steroidal saponins in plants are also reviewed here.