CHAPTER 2: Biosynthesis of Steviol Glycosides and Related Diterpenes in Leaves and Glandular Trichomes of Stevia rebaudiana Bertoni

Steviol glycosides, the sweet ent-kaurene diterpenoid glycosides extracted from the perennial shrub Stevia rebaudiana Bertoni, are widely appreciated for their intensive sweetness exceeding that of sucrose up to 300 times. More than 200 species of stevia have been described until today, but only Stevia rebaudiana Bertoni is attributed to an intense sweet taste. Whereas stevia plants have been used e.g. as medicine by indigenous people in South America for centuries, nowadays the use of purified steviol glycosides as a non-caloric and non-synthetic sweetener, e.g. in beverages and confectionary products, is increasing worldwide. Decades ago stevioside was the only steviol glycoside identified, but today more than 35 steviol glycosides have been described in the literature, differing in structure and taste. Biosynthetic pathways of these terpenoid secondary metabolites have already been elucidated. However, considering plant morphology and localisation of biosynthetic pathways, new cultivation and extraction techniques might evolve and improve the harvest yield, purification methods and the sweet tasting properties of steviol glycoside compositions. In this review we will set the focus onto the biosynthesis of steviol glycosides in Stevia rebaudiana Bertoni and its localisation, with regard to the plant's morphology and in particular the ultrastructure of foliar secreting trichomes.