Molecular and Chemical Mechanisms of Defence against Myrtle Rust in Australian Myrtaceae

Abstract

Increased human disturbance to forest ecosystems has exacerbated the spread of fungal pathogens to non-native environments. Rust pathogens (Pucciniales) can spread long distances by human activity and wind dispersal, and can cause severe disease outbreaks in cereal crops and in forest trees. The exotic fungus Austropuccinia psidii (myrtle rust) arrived in Australia in 2010 and most species of native Myrtaceae including Eucalyptus and Melaleuca are susceptible to infection to various degrees. Plants infected by A. psidii can suffer from crown loss and eventual mortality, which can be detrimental to ecosystems as well as to many rural industries that produce essential oils and flavourings from species of Myrtaceae. Within-species variation in resistance to A. psidii has been discovered in many native species. However, the molecular and chemical mechanisms of resistance to A. psidii infection in these species are largely unknown. Finding the molecular and chemical basis of resistance against A. psidii is therefore an essential part of ensuring that future plantations and re-afforestation programs are resistant to this pathogen.

This thesis therefore aims to elucidate the molecular and chemical mechanisms of resistance to A. psidii in Myrtaceae in Australia, with the goal of obtaining a comprehensive view of potential mechanisms involved in defence to identify candidate genes that may be implemented into resistance breeding. After first screening multiple species of Myrtaceae, I selected Melaleuca alternifolia (tea tree) and M. quinquenervia (broadleaf paperbark) for detailed molecular study because they showed varying disease symptoms from resistance to susceptibility among individuals, and were economically and ecologically important and amenable to molecular studies. I used a variety of experimental approaches, including RNA-Seq, qRT-PCR, GC-MS, and functional characterisation through heterologous gene expression in E. coli to apply an integrated analysis that examined both molecular and chemical aspects of plant defence. I constructed the transcriptomes of M. alternifolia and M. quinquenervia de novo and investigated differential gene expressions between resistant and susceptible plants. I showed that resistant M. alternifolia and M. quinquenervia over-express genes which may be contribute to defence against to A. psidii infection, and have found and functionally characterised new terpene synthase genes that showed induction in response to infection by A. psidii in M. quinquenervia.