Eucalyptus species maintain secondary metabolite production under water stress conditions at the expense of growth

Susceptibility to the fungal plant pathogen Austropuccinia psidii is related to monoterpene production in Australian Myrtaceae species

In 2010, the fungal plant pathogen that causes Myrtle rust, Austropuccinia psidii, which is native to South America, was first detected in Australia and has since had significant impacts on several Australian Myrtaceae species. Despite this, our understanding of the role secondary metabolites play in plant susceptibility to A. psidii is limited. This study aimed to determine: (1) whether secondary metabolite (phenolics, terpenes) production is induced after A. psidii inoculation and if so, (2) how their production relates to A. psidii susceptibility. To test these aims, we selected seven Myrtaceae species that have a wide range of within-species variability in their susceptibility to A. psidii. We found that five of the study species significantly increased either their phenolic or sesquiterpene production post-inoculation suggesting their pre-inoculation secondary metabolite levels were not sufficient to combat A. psidii infection. The two species (Angophora costata and Corymbia citriodora) that did not increase their secondary metabolite production post-inoculation tended to have the greatest pre-inoculation production levels amongst the species. Interestingly, across all species, monoterpenes were the only secondary metabolite found to reduce plant susceptibility to A. psidii. This study contributes to our limited understanding of the role that secondary metabolites play in plant susceptibility to A. psidii. In light of these findings, future research should aim to identify biomarkers (e.g. individual chemical compounds) that confer resistance to A. psidii, so that individuals with these biomarkers can be utilised in commercial and conservation projects.