## 3 Steviol Glycos Chemical Diversity and Biosynthesis  
_Stevia rehaudiana_ is also commonly called the \"producer of diterpenoid steviol glycosides\". Steviol glycosides belong to the diterpenoid group, specifically tetracyclic diterpenoids of plant secondary metabolites. SvGIs are non-toxic, non-mutagenic, and highly sweet diterpenoids with commercially important uses in the pharmaceutical and food and beverage industry. By hydrolytic cleavage, these glycosides can be transformed into sugar and a non-sugar component. These glycosides comprise a carbohydrate sugar molecule (glycone) attached to a non-sugar component (aglycone). Glucosides, pentosides, fructose, and a few other sugar compounds are present as the sweet glycosides of stevia [16]. Different glycone lengths are attached to the core of steviol (ent-13-hydroxyur-16-ent-19-oic acid) at the R1 position at C19 and the R2 position at C13. This is the major reason for the diversity of SvGIs. The eleven most common diterpenoids that have been identified in the leaf tissues of stevia include steviobioside, stevioside, rebaudioside A (Reb A), rebaudioside B (Reb B), rebaudioside C (Reb C), rebaudioside D (Reb D), rebaudioside E (Reb E), rebaudioside F (Reb F), rebaudioside I (Reb I), rebaudioside M (Reb M), rubsusoide and duloside [17,18]. Stevioside was the first steviol to be extracted from the stevia leaves. The steviol glycosides extracted from the stevia plant consist of 80% stevioside, 8% Reb A, and 0.6% Reb C, depicting that stevioside is present in quantities two-three-fold that of rebaudioside. These SvGIs vary in the degree of sweetness, which depends on the type and number of sugar moiety attached. The concentration of SvGIs varies from one genotype to another and is influenced by the growth conditions. Stevioside are \(\sim\)200-300 times sweeter than sucrose. In the stevia leaves, the ratio of stevioside and rebaudioside governs the sweetness quality of the stevia genotype. The leaves with more rebaudioside than stvoicide give a more desirable taste profile, since stevioside is associated with a slight bitterness, astringency, and menthol taste [19]. Reb-M's significantly similar taste to sugar has increased its popularity in the food industry. It is very beneficial for making sweetness-based products such as candies, where the sweet taste is desirable. Reb M elicits a significantly less bitter taste than Reb A; however, the extremely low extraction rate (\(\leq\)1%) of Reb M makes it economically unfeasible for its mass production from _Stevia rebaudiana_ leaves [10]. Thus, increasing the production and extraction rate of Reb M through genetic engineering is very important for its future applications. The biosynthesis pathway of steviol glycosides is of immense interest to many researchers, since it is a very complex pathway and shares a common precursor compound ent-kaurenoic acid which is also involved in the gibberellic acid pathway. The SvGIs synthesis pathway has previously been described by several researchers; however, it still requires detailed research to better understand the regulation of this multi-step complex process. The whole process of SvGIs is divided into three stages. The initial stage is the methylerythitol-4-phosphate (MEP) pathway, which takes place in the plastid. The condensation reaction between pyruvate and glyceraldehyde 3-phosphate (G3P) is catalyzed by deoxy-D-xylulose 5-phosphate synthase (DXS) to give the product deoxyxylulose-5-phosphate (DXP). The DXS is the first-rate limiting enzyme in the MEP pathway. Further, DXP is converted to MEP by the DXR enzyme [20]. This is the second rate-limiting step in the MEP pathway and also the regulatory step for isoprene synthesis. Following this, the subsequent generation of the five-carbon building blocks dimethylallyl diphosphate (DMAP) and isopentyl diphosphate (IPP) takes place. The second stage of SvGls synthesis starts with the condensation of DMAP and IPP catalyzed by geranylgeranyl diphosphate synthase (GGDPS) to produce geranylgeranyl diphosphate (GGPP).