Diversity and Biosynthesis of Volatile Terpenoid Secondary Metabolites in the Chrysanthemum Genus

Chrysanthemum is a significant genus belonging to one of the largest plant families, the Asteraceae. Among the over 40 recognized species, C. morifolium is best-known for its long history of cultivation and countless varieties. Like some other genera of this family, many Chrysanthemum species are producers of high levels of secondary metabolites. This review focuses on the largest class of plant secondary metabolites, namely terpenoids produced by Chrysanthemum species. An extensive literature search revealed a total of 390 terpenoids from these species, which include 183 monoterpenoids and 207 sesquiterpenoids. While some terpenes are produced by almost all of the investigated varieties/species of Chrysanthemum, many terpenoids show specificity to certain varieties or species, indicating species-specific functions. The enzymes and genes participating in the pathways for terpene biosynthesis are also described. In addition to a general overview, in this review we focus on isoprenyl diphosphate synthases and terpene synthases, two enzyme classes pivotal for generating the chemical diversity of terpenes, as established by our analysis of two sequenced genomes of Chrysanthemum. While our knowledge about the biological functions of terpenoids in Chrysanthemum is limited, we discuss their functions in chemical defenses against insects and microbial pathogens. Finally, we present the prospects of understanding the molecular basis of terpene biosynthesis in Chrysanthemum and elucidating its biological significance, as well as the prospects of translational research on terpene-based applications.