Chapter Five Triterpenoid saponin biosynthesis in plants

Abstract

This chapter reviews the progress that has been made in the characterization of the enzymes and genes involved in the synthesis of triterpenoid saponins. The enzyme is unusual in that it is more closely related to lanosterol synthases from animals and fungi than to other oxidosqualene cyclases (triterpenoid synthases or cycloartenol synthases) from plants. β-amyrin synthase (AsbAS1) is required for the synthesis of triterpenoid avenacin saponins and for resistance to a variety of pathogens, indicating that avenacins contribute to disease resistance in oat. AsbAS1 and other as yet uncharacterized genes required for saponin biosynthesis have potential for the development of plants with altered saponin content through metabolite engineering. In some cases, enhanced levels of saponins or the synthesis of novel saponins may be beneficial (for example, for drug production or improved disease resistance), while in others the objective may be to reduce the content of undesirable saponins (such as those associated with antinutritional effects in legumes).