# Advances in the Biosynthesis and Molecular Evolution of Steroidal Saponins in Plants
## 1 Introduction  
Saponins are a kind of natural secondary metabolite with various structures and functions, and are widely distributed in plants [1]. They are a group of compounds with complex chemical structures composed of a triterpenoid or steroidal glycone molecular skeleton conjugated to a sugar moiety, classed as triterpen saponins and steroidal saponins, respectively [1]. Because of the combination of a hydrophobic aglycone skeleton and hydrophilic sugar chain, these compounds have water surface tension, amphiphilic (i.e., hydrophilic and hydrophobic), foaming, and emulsifying properties, which is why they are named saponins, derived from _sapo_, i.e., soap [1, 2, 3]. Saponins isolated from plants are considered to have a variety of properties, often acting to protect plants from pathogens and plant-feeding insects. For humans and animals, saponins can affect cell membrane permeability or the lysing of cells, have hypoglycemic activity, and can also affect cholesterol metabolism; they, therefore, have a wide range of applications in food, cosmetics, pharmaceuticals, and other fields [1, 4]. Although saponins are ubiquitous in plants, the steroidal saponins discussed in this paper are mainly distributed in monocots, such as the Dioscoreaceae (_Dioscorea zingibreensis_), Melanthiaceae (_Paris polyphyllla_), and Asparagaceae (_Asparagus officinalis_) [1, 3]. Steroidal saponins isolated from plants can be divided into the following five types according to their molecular skeleton structures:  
1. [label=(0)]
2. Spirosanol saponins: a hexacyclic ABCDEF-ring system characterized by an axial methyl or hydroxymethyl on the F ring (C-27) [5, 6, 7, 8, 9, 10] (Figure 1A). The core aglycone of spirostand saponins has cis- or trans-fusion between ring A and ring B, or has doublebonds between ring C-5 and C-6 [7]. The common spirostand saponins isolated from plants include dioscin, gracillin, and trillin [5; 9]. 2. Furostanol saponins: a pentacyclic ABCDE ring with a sixth open F ring [5; 6; 7; 8; 9; 10] (Figure 1B). The common furostanol saponins include parvifloside, protogracillin, and protodioscin [5]. Furostanol saponins usually have 25(R) and 25(S) structures, or are saturated at the C-20 (22) or C-22 (23) positions on the open F ring [5; 6]. The two sugar chains of furostanol saponins are usually connected at the C-3 and C-26 positions, while the \(\beta\)-glucose at the C-26 position will produce a closed ring reaction under the catalysis of glycosidase and convert it into spirostand saponin [8; 11]. 3. Cholesterane saponins: produced by the oxidative cracking of the C-22/C-23 bond of the aglycone skeleton [7; 8; 9] (Figure 1C). Cholesterane saponins (such as Anguivioside XV and Smilaxchinoside D) have been found in some _Smilax_ species [7]; a homodelastane saponin with aromatic ring E (such as Paris pseudoside A and B) has also been found in some _Paris_ species [8]. 4. Pregnane saponins: a tetracyclic ABCD-ring system [5; 7] (Figure 1D). They may be biosynthesized via the oxidative cleavage of the double bond between C-20 and C-22 in the structure of furostane [7; 9]. Pregnane saponins isolated from plants include Spongipregnnologide A/B/C/D/E, Trinervuloside A, Riparoside B, and Timosaponin J/K [5; 7]. 5. Isospiorstand saponins: monosaccharide chain saponins. Their unique feature is the equatorial methyl or hydroxymethyl on the F ring (C-27) [7] (Figure 1E). Isospiorstand saponins include the following types: dehydrogenation between C-5 and C-6, carbonylation at C-6, hydroxylation at C-17 or C-27, and cis-trans fusion between the A ring and B ring [7]. 6. Polyhydroxylated saponins: a six-ring system [8] (Figure 1F). They have hydroxyl groups at the C-1 position, and may also be hydroxylated at the C-1, C-3, C-21, C-23, and C-24 positions [8; 12; 13]. Moreover, the fructose at the C-24 position of the aglycone is unique in _Paris_ plants [8]. 7. Pseudospiorstand saponins, a kind of tetrahydropryan F ring, are rare in plants, and include nuatigenin and isonutigenin isolated from _Paris_ species [8] (Figure 1G). 8.