Unravelling triterpene glycoside synthesis in plants: phytochemistry and functional genomics join forces

Abstract

Plants produce a vast array of secondary metabolites, many of which have important ecological functions. The ability to perform in vivo combinatorial chemistry by mixing, matching and evolving the genes required for different secondary metabolite biosynthetic pathways is likely to have been critical for survival and diversifica tion of the members of the plant kingdom. The terpenes form one of the largest and most diverse groups of plant natural products. This group of compounds includes sterols and triterpenes, which can accumulate as glyco side conjugates (saponins) in substantial quantities in plants. Saponins have a diverse range of properties, includ ing antimicrobial, anti-insect and allelopathic activity (Hostettmann and Marston 1995; Oleszek and Marston 2000). They also have pharmacological applications, for example as anti-cancer agents (Hanausek et al. 2001). This review is concerned with recent developments in our understanding of the synthesis of the triterpene glycoside family of saponins (Fig. la). Triterpene sapo nins occur predominantly in dicotyledonous plants (Hostettmann and Marston 1995). Monocots appear to lack these compounds with the notable exception of oats, which produce defence-related triterpenoid sapo nins known as avenacins in the roots (Osbourn 2003). In higher plants, 2,3-oxidosqualene is a common precursor in the synthesis of both triterpenes and sterols. The first committed step in sterol biosynthesis in plants is the conversion of 2,3-oxidosqualene to cycloartenol, catalysed by the oxidosqualene cyclase cycloartenol synthase. Synthesis of triterpenes involves cyclisation of 2,3-oxidosqualene into various triterpene skeletons,