Overexpression of SrDXS1 and SrKAH enhances steviol glycosides content in transgenic Stevia plants

Abstract

BACKGROUND: Stevia rebaudiana produces sweet-tasting steviol glycosides (SGs) in its leaves which can be used as natural sweeteners. Metabolic engineering of Stevia would offer an alternative approach to conventional breeding for enhanced production of SGs. However, an effective protocol for Stevia transformation is lacking. RESULTS: Here, we present an efficient and reproducible method for Agrobacterium-mediated transformation of Stevia. In our attempts to produce transgenic Stevia plants, we found that prolonged dark incubation is critical for increasing shoot regeneration. Etiolated shoots regenerated in the dark also facilitated subsequent visual selection of transformants by green fluorescent protein during Stevia transformation. Using this newly established transformation method, we overexpressed the Stevia 1-deoxy-d-xylulose-5-phosphate synthase 1 (SrDXS1) and kaurenoic acid hydroxylase (SrKAH), both of which are required for SGs biosynthesis. Compared to control plants, the total SGs content in SrDXS1- and SrKAH-overexpressing transgenic lines were enhanced by up to 42-54% and 67-88%, respectively, showing a positive correlation with the expression levels of SrDXS1 and SrKAH. Furthermore, their overexpression did not stunt the growth and development of the transgenic Stevia plants. CONCLUSION: This study represents a successful case of genetic manipulation of SGs biosynthetic pathway in Stevia and also demonstrates the potential of metabolic engineering towards producing Stevia with improved SGs yield.