Evolution of Structural Diversity of Triterpenoids

Abstract

Plants have evolved to produce a blend of specialized metabolites that serve functional roles in plant adaptation. Among them, triterpenoids are one of the largest subclasses of such specialized metabolites, with more than 14,000 known structures. They play a role in plant defense and development and have potential applications within food and pharma. Triterpenoids are cyclized from oxidized squalene precursors by oxidosqualene cyclases, creating more than 100 different cyclical triterpene scaffolds. This limited number of scaffolds is the first step towards creating the vast structural diversity of triterpenoids followed by extensive diversification, in particular, by oxygenation and glycosylation. Gene duplication, divergence, and selection are major forces that drive triterpenoid structural diversification. The triterpenoid biosynthetic genes can be organized in non-homologous gene clusters, such as in Avena spp., Cucurbitaceae and Solanum spp., or scattered along plant chromosomes as in Barbarea vulgaris. Paralogous genes organized as tandem repeats reflect the extended gene duplication activities in the evolutionary history of the triterpenoid saponin pathways, as seen in B. vulgaris. We review and discuss examples of convergent and divergent evolution in triterpenoid biosynthesis, and the apparent mechanisms occurring in plants that drive their increasing structural diversity within and across species. Using B. vulgaris' saponins as examples, we discuss the impact a single structural modification can have on the structure of a triterpenoid and how this affect its biological properties. These examples provide insight into how plants continuously evolve their specialized metabolome, opening the way to study uncharacterized triterpenoid biosynthetic pathways.

Keywords: convergent evolution; plant specialized metabolism; structural diversity; triterpenoid saponins; unlinked versus clustered pathways.

Figure 1

Simplified representation of the biosynthesis of sterols and triterpenoids in plants. (A) OSC signature enzymes catalyze the cyclization of 2,3-oxidosqualene, and in more rare cases bis-oxidosqualene, into several triterpenoid scaffolds. These structures can be further modified by tailoring enzymes, including oxygenation by P450s, glycosylation by UGTs, acylation by ACT, and methylation by MT. Selected structures are depicted and discussed in more detail in the text. Dashed arrows represent multiple biosynthetic reactions whereas solid arrows represent a single step. (B) Biosynthesis of plant triterpenoids can be mediated by non-homologous clustered genes or through non-linked genes. In Avena spp., a cluster of five genes are involved in the biosynthesis of avenacin A-1. In Arabidopsis thaliana, two clusters have been reported: thalianol cluster with four genes (up) and marneral cluster with three genes (down). In Cucumis sativus, six genes associated with cucurbitacin biosynthesis are located in a cluster in chromosome 6, while four other genes are elsewhere in the genome. The core genes for biosynthesis of SGAs are clustered in chromosome 7 and 12 of S. tuberosum and S. lycopersicum, respectively. The key genes for biosynthesis of the insect-feeding deterrent hederagenin cellobioside are distributed along B. vulgaris genome in tandem repeats located at different pseudomolecules (PM). OSC, oxidosqualene cyclase; P450, cytochrome P450; UGT, UDP-glycosyltransferase; ACT, acyltransferase; MT, methyltransferase.

Figure 2

Structural changes may affect the biological activity of triterpenoids. (A) Structural formulae (up) and 3D models (down) of cholesterol and Δ7 sterol. In the Δ7 sterol the side chain is bent as compared to cholesterol. In sea cucumber cell membrane systems, cholesterol are replaced by Δ7 sterols to possible modulate the lytic action of saponins as the 3D structure is altered (Claereboudt et al., 2018). (B) Structural formulae (left) and 3D models (right) of B. vulgaris saponins. In the deterrent hederagenin monoglucoside, the glucose (C3) is twisted in respect to the triterpenoid backbone when a hydroxyl group is added at position C23 (Liu et al., 2019). 3D models are derived from Claereboudt et al. (2018) and Liu et al. (2019), respectively.