# Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants
## 6 Molecular Evolution of Steroidal Saponin Biosynthetic Pathway Genes
For example, sterol side-chain catalytic enzyme-encoding genes, such as _CYP90B_, _CYP94_, and _CYP72A_, play an important role in steroidal saponin biosynthesis, and the functions of these genes vary between different plants [132; 5; 18]. In plants without steroidal saponins, the _CYP90_ gene is involved in catalyzing the conversion of cholesterol into 22S hydroxycholesterol and then into brassinosteroids. However, in plants containing steroidal saponins, such as _D. zingiberensis_, _P. polyphyllla_, and fenugreek, the new _CYP90_ gene generated through gene duplication has evolved new functions, such as sterol 16,22-polyhydroxylation during molecular evolution, which can catalyze the synthesis of cholesterol into diosigenin [133; 19; 20]. This also indicates that diosigenin and steroidal saponins, as new defense compounds, coordinate plant growth and defense in the evolution of plants containing steroidal saponins. Similarly, the main function of the _CYP72A_ gene in plants is involved in the 13 hydroxylation of gibberellin, while in plants containing steroidal saponins, it is involved in the biosynthesis of steroidal saponins [134; 135; 19]. Duplicated genes usually have newly evolved functions that drive the evolution of the biosynthetic pathways of specific metabolites [79; 136]. It can be inferred that the new members of the CYP450 and UGT gene families (such as _CYP90_, _CYP94_, _CYP72A_, _UGT73_, and _UGT80_ genes) generated by the gene tandem duplication and WGD event in the _D. zingiberensis_ genome provide abundant genetic and evolutionary resources for the formation and evolution of the steroidal saponin biosynthetic pathway. [PERSON] et al. also found that the _CYP90_ gene from _P. polyphylla_ and fenugreek was mainly derived from gene duplication events in plants [19]. Phylogenetic analysis shows that members of the UGT80 family have been reported in algae, bryophytes, ferns, and angiosperms, indicating that UGT80 is the ancestor of the UGT family in plants [24; 26]. [PERSON] et al. believe that the UGT73 family appeared after the emergence of ferns; however, a large number of members appear in Arabidopsis, _O. sativa_, and _P. polyphylla_[24; 26; 137]. The UGT73 family members in plants contain a variety of functional enzymes involved in the glycosylation of plant secondary metabolites, including flavonoids, triterpenes, steroidal saponins, and alkaloids [138; 139]. In contrast, UGT80 has more diverse functions in plants, and is involved in the catalysis of sterols, steroidal glycoside alkaloids (SGAs), and steroidal saponins [25; 26; 140]. In conclusion, we speculate that the gene duplication events in the plant genome may provide the driving force for the species-specific evolution of steroidal saponin biosynthesis in plants; further, new defense substances, such as diosogenin and steroidal saponins, evolve to help plants coordinate their growth and defend against stress. In our opinion, it is necessary to verify the function of steroidal saponin biosynthesis genes in plants containing steroidal saponins using transgenic methods. It should also be noted that the current speculation needs further systematic exploration and verification in combination with phylogenetic analysis methods.