Mosaic eucalypts : chemical variation and differential gene expression within a eucalyptus melliodora and a eucalyptus sideroxylon tree

Abstract

Mosaic Eucalyptus trees provide unparalleled natural experiments to understand how plants control the synthesis of chemical defences against herbivorous insects and mammals. In natural eucalypt mosaics, different branches on a single tree show vastly different leaf chemical profiles (chemotypes) and thus vastly different responses to herbivores (resistant or susceptible). I have been working with two such mosaic eucalypts (E. melliodora and E. sideroxylon), where the leaves of the resistant chemotype have a different terpene profile compared with the leaves of the susceptible ecotype. Chemotypic variation of terpenes is under strong genetic control and the terpene biosynthetic pathway is well described, however little is known about the genetic control over genes within this pathway, especially the terpene synthase gene family (the last step in the pathway). The overall aim of my thesis is to explore the genetic variation, particularly in the terpene biosynthetic pathway that accompanies the chemical variation in these two mosaics and propose a mechanism for the development of mosaic eucalypts. The key outcomes of my thesis were: 1. Despite being found in a number of recognisable species of Myrtaceae, high foliar concentrations of the terpene 1,8 cineole is unlikely to have been present in the common ancestor to this family. 2. There is a chemotype associated with the leaves resistant to herbivory that is distinct to that of the leaves susceptible to herbivory. These chemotypes differ in monoterpenes, sesquiterpenes and formylated phloroglucinol compounds (FPCs). 3. The number of genes differentially regulated between the leaves of the two branches of the E. melliodora mosaic is very similar to the number of genes differentially regulated between functionally different tissues in the same organism (e.g. roots and flowers). 4. There are biosynthetic pathways with no genes differentially regulated between the leaves of the two branches, however we found that the entire pathway is differentially regulated. 5. There are many terpene synthases expressed in the leaves of the mosaics that produce the same terpenes, suggesting there may be another role for terpenes in eucalypts besides defence within the leaf. In summary, I have investigated the chemical differences between resistant and susceptible leaves within the mosaics, and found many genetic differences that could contribute to development and maintenance of mosaicism. The conclusions from this work can be applied more broadly to terpene chemotypes in all plants, which many industries rely heavily upon. Show more