Both Constitutive and Infection-Responsive Secondary Metabolites Linked to Resistance against Austropuccinia psidii (Myrtle Rust) in Melaleuca quinquenervia.

Michelle C Moffitt,Johanna Wong-Bajracharya,Geoff S Pegg,Robert F Park,Louise S Shuey,Jonathan M Plett

doi:10.3390/microorganisms10020383

Abstract

Austropuccinia psidii is a fungal plant pathogen that infects species within the Myrtaceae, causing the disease myrtle rust. Myrtle rust is causing declines in populations within natural and managed ecosystems and is expected to result in species extinctions. Despite this, variation in response to A. psidii exist within some species, from complete susceptibility to resistance that prevents or limits infection by the pathogen. Untargeted metabolomics using Ultra Performance Liquid Chromatography with Ion Mobility followed by analysis using MetaboAnalyst 3.0, was used to explore the chemical defence profiles of resistant, hypersensitive and susceptible phenotypes within Melaleuca quinquenervia during the early stages of A. psidii infection. We were able to identify three separate pools of secondary metabolites: (i) metabolites classified structurally as flavonoids that were naturally higher in the leaves of resistant individuals prior to infection, (ii) organoheterocyclic and carbohydrate-related metabolites that varied with the level of host resistance post-infection, and (iii) metabolites from the terpenoid pathways that were responsive to disease progression regardless of resistance phenotype suggesting that these play a minimal role in disease resistance during the early stages of colonization of this species. Based on the classes of these secondary metabolites, our results provide an improved understanding of key pathways that could be linked more generally to rust resistance with particular application within Melaleuca.

Highlights

The biotrophic plant pathogen Austropuccinia psidii, causal agent of the disease commonly known as myrtle rust, has a broad host range across many species within the Myrtaceae family
Analysis of the leaf metabolomes identified that the resistance phenotype of the individual, time post-inoculation and the interaction of these two factors were all significant as determined by permutational multivariate analysis of variance (PERMANOVA; Table 1)
Time post-inoculation, regardless of resistance phenotype, appears to be the most influential factor with respect to changes within the metabolome. While some of these responses may be associated with leaf aging, as we only used one timepoint for an uninfected control, these results show that A. psidii infection is able to trigger major metabolite responses in M. quinquenervia leaves

Summary

Introduction

The biotrophic plant pathogen Austropuccinia psidii, causal agent of the disease commonly known as myrtle rust, has a broad host range across many species within the Myrtaceae family. In Australasia, Myrtaceae predominate in most ecosystems, with more than 2250 species recorded [1]. They are found in biological hotspots around the globe [2,3]. As many species within this family are important globally due to their value in forestry, the essential oil industry as well as for cultural foods, this rust pathogen poses a significant threat economically [5,7]. 539 Myrtaceae species are known to be vulnerable to A. psidii infection worldwide, including

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