For example, many terpenoids exhibit potent toxicity and serve as core components of chemical defenses against herbivores, insect pests, and microbial pathogens ( Keeling and Bohlmann, 2006a Vaughan et al., 2013 Schmelz et al., 2014). Conversely, the vast majority of plant terpenoids represent specialized metabolites that are dedicated to mediating interorganismal interactions or environmental defense and adaptation ( Gershenzon and Dudareva, 2007 Tholl, 2015). This includes a few isoprenoid derivatives with essential roles in plant growth and development such as gibberellins, brassinosteroids, carotenoids, and chlorophylls ( Pallardy, 2008 Tripathy and Pattanayak, 2012). Plants are the champions of producing different terpenoid structures ( Tholl, 2015). From this origin, the staggering diversity of the terpenome has arisen, comprising more than 80,000 compounds ( Christianson, 2018) that are widespread across living organisms, including archaea ( Matsumi et al., 2011), bacteria ( Yamada et al., 2012), fungi ( Schmidt-Dannert, 2015), social amoeba ( Chen et al., 2016 Chen et al., 2019), marine organisms ( Gross and König, 2006), insects ( Beran et al., 2016 Lancaster et al., 2018), and plants ( Gershenzon and Dudareva, 2007 Tholl, 2015). Ubiquitous presence of terpenoids in membranes supports this hypothesis and suggests that ancient archaebacterial diphytanylglycerol ether membrane components, polyprenols, and derived steranes and sterols represent early terpenoid predecessors ( Ourisson and Nakatani, 1994 Rohmer and Bisseret, 1994 Van De Vossenberg et al., 1998 Matsumi et al., 2011). This article reviews current knowledge on the functional diversity and molecular evolution of the plant TPS family that underlies the chemical diversity of bioactive terpenoids across the plant kingdom.Īmong the wealth of small molecule natural products, terpenoids (also referred to as isoprenoids) form an especially diversified and evolutionary ancient superfamily, which likely emerged alongside the formation of primitive membranes at the very origins of cellular life ( Ourisson and Nakatani, 1994).
Accompanying gene family expansion, the TPS family shows a profound functional plasticity, where minor active site alterations can dramatically impact product outcome, thus enabling the emergence of new functions with minimal investment in evolving new enzymes. Lineage-specific expansion of these TPS clades led to variable family sizes that may range from a single TPS gene to families of more than 100 members that may further function as part of modular metabolic networks to maximize the number of possible products. The seven currently defined clades (a-h) forming the plant TPS family evolved from ancestral triterpene synthase- and prenyl transferase–type enzymes through repeated events of gene duplication and subsequent loss, gain, or fusion of protein domains and further functional diversification. Terpene synthase (TPS) enzymes are the gatekeepers in generating terpenoid diversity by catalyzing complex carbocation-driven cyclization, rearrangement, and elimination reactions that enable the transformation of a few acyclic prenyl diphosphate substrates into a vast chemical library of hydrocarbon and, for a few enzymes, oxygenated terpene scaffolds. Driven by selective pressure to adapt to their specific ecological niche, individual species form only a fraction of the myriad plant terpenoids, typically representing unique metabolite blends. Plants produce by far the largest array of terpenoids with various roles in development and chemical ecology. Terpenoids comprise tens of thousands of small molecule natural products that are widely distributed across all domains of life.
Department of Plant Biology, University of California Davis, Davis, CA, United States.
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