The terpene synthase genes of Melaleuca alternifolia (tea tree) and comparative gene family analysis among Myrtaceae essential oil crops

Abstract

Terpene synthases (TPS) are responsible for the terminal biosynthetic step of terpenoid production. They are encoded by a highly diverse gene family believed to evolve by tandem duplication in response to adaptive pressures. Taxa in the Myrtaceae family are renowned for their diversity of terpenoid-rich essential oils, and among them, the tribe Eucalypteae has the largest TPS gene family found in any plant (> 100 TPS). In this study, comparative analysis of Melaleuca alternifolia (tea tree), from the related tribe Melaleuceae, revealed some Myrtaceae have smaller TPS families, as a total of 58 putatively functional full-length TPS genes, and 21 pseudogenes were identified by manual annotation of a newly released long-read assembly of the genome. The TPS-a and TPS-b2 subfamilies that synthesise secondary compounds often mediating plant-environment interactions were more diminutive than those in eucalypts, probably reflecting key differences in the evolutionary histories of the two lineages. Of the putatively functional TPS-b1, 13 clustered into a region of around 400 kb on one scaffold. The organisation of these TPS suggested that tandem duplication was instrumental in the evolution and diversity of terpene chemistry in Melaleuca. Four TPS-b1 likely to catalyse the synthesis of the three monoterpenoid components that are used to classify tea tree chemotypes were encoded within a single small region of 87 kb in the larger cluster of TPS-b1, raising the possibility that coregulation and linkage may lead to their behaviour as a single locus, providing an explanation for the categorical inheritance of complex multiple-component chemotypes in the taxon.