High Terpene Production in Myrtaceae: Evolutionary Insights from Terpene Pathway Genes

Myrtaceae is one of the largest families of flowering plants and is well known for its prolific terpene production. To investigate the genetic basis underlying this high-level terpene output, we conducted comparative genomic analyses of genes of the entire terpene biosynthetic pathways in selected Myrtaceae species and representative species from three other families within the order Myrtales. Our analyses revealed that genes encoding enzymes in the upstream terpene biosynthetic pathways are generally conserved in copy number across Myrtales. Similarly, isoprenyl diphosphate synthases, which are positioned centrally and responsible for producing the direct precursors of major terpene classes, also exhibit conserved gene numbers among these species. In contrast, substantial differences were observed in the number of terpene synthase (TPS) genes, which function downstream and directly catalyze terpene formation. Myrtaceae species possess markedly more TPS genes than species from other Myrtales families. This expansion is primarily attributable to increased gene numbers in the TPS-a, TPS-b, TPS-g, and TPS-e/f subfamilies, with the first three subfamilies largely associated with sesquiterpene and monoterpene biosynthesis. Further analyses indicate that the enlarged TPS-a and TPS-g subfamilies resulted at the origination of Myrtaceae-specific groups, whereas TPS-b exhibited Myrtaceae-specific expansion. In Eucalyptus grandis, tandem duplication makes a larger contribution to the expansion of TPS-a, TPS-b and TPS-g subfamilies than interchromosomal duplication. The majority of these TPS genes exhibit moderate to high levels of expression in leaves, consistent with their role in elevated terpene production in leaves of E. grandis. Collectively, our findings are consistent with the hypothesis that the novel terpene-producing capacity of Myrtaceae is driven primarily by Myrtaceae-specific origination and/or expansion of downstream TPS genes rather than changes in upstream pathway gene copy numbers.