Intensive sampling identifies previously unknown chemotypes, population divergence and biosynthetic connections among terpenoids in 'Eucalyptus tricarpa'

Abstract

Australian members of the Myrtaceae produce large quantities of ecologically and economically important terpenes and display abundant diversity in both yield and composition of their oils. In a survey of the concentrations of leaf terpenes in 'Eucalyptus tricarpa' (L.A.S. Johnson) L.A.S. Johnson & K.D. Hill, which were previously known from few samples, exceptional variability was found in composition. The aim was to characterize the patterns of variation and covariation among terpene components in this species and to use this information to enhance our understanding of their biosynthesis. There were marked discontinuities in the distributions of numerous compounds, including the overall proportions of mono- and sesquiterpenes, leading us to delineate three distinct chemotypes. Overall, positive covariation predominated, but negative covariation suggested competitive interactions involved in monoterpene synthesis. Two groups of covarying monoterpenes were found, each of which was positively correlated with a group of sesquiterpenes and negatively correlated with the alternate sesquiterpene group. These results imply substantial cross-talk between mono- and sesquiterpene biosynthesis pathways. However, only those compounds hypothesized to share final carbocation intermediates or post-processing steps were strongly positively correlated within chemotypes. This suggests that the broader patterns of covariation among groups of compounds may result from co-regulation of multiple biosynthetic genes, controlling the complex terpene profiles of the chemotypes of 'Eucalyptus'.