Project Leader, Organisation

Michelle Moffitt, University of Western Sydney

Status

COMPLETE

Background

Before disease symptoms appear, plant pathogens deploy chemical cues, including metabolies and proteins, that impact the speed and degree of pathogenesis. Some chemical cues act as molecular ‘fingerprints’ as they are pathogen-specific. Plants that are able to recognise these cues are more likely to mount an effective immune response and defend against the disease. We need to identify the specific chemical cues used by Austropuccinia psidii (myrtle rust) to develop novel screening tools to track early disease occurrence, identify disease tolerant germoplasm and to monitor changes in A. psidii populations.

Objectives and impact

Objectives: This project will identify A. psidii-specific chemical cues produced prior to visible disease and investigate how these cues influence pathogenesis and relate to plant susceptibility.

Methods: We will employ next-generation sequencing to identify novel fungal protein-coding genes correlated with disease and combine untargeted metabolomics with isotopic tracing to identify A. psidii-specific chemical cues expressed during pathogenesis. Specifically, we will consider small molecules and proteins released during the interaction between A. psidii and host plants (e.g. Syzygium sp.) exhibiting different disease susceptibility. This will be complemented with novel mass spectrometry imaging techniques to understand how the distribution of these molecules alters on resistant versus susceptible hosts.

Outputs:

Identification of A. psidii-specific chemicals enabling novel, sensitive screening/genetic tests
Data on plant traits associated with disease resistance
Scoping of new resistance pathways and control options for future research
Identification of fast-evolving proteins in A. psidii for use in future population and evolutionary studies

Outcomes/Impact: These data will make significant contributions toward the Myrtle Rust Action Plan including screening for A. psidii and informing breeding strategies (Action 4.3.1), specialist scoping new areas for research (Actions 4.3.3; 4.3.5) and contributing to the methodologies associated with monitoring changes in Australian A. psidii populations (Action 5.3.1).

Summary results

Using an untargeted metabolomics approach, we identified a unique molecular fingerprint in A. psidii infected Melaleuca quinquenervia leaves during the early stages of infection. Further analysis of the metabolome at 24 hours and 48 hours after infection identified a unique subset of 19 metabolites that are unique to the resistant phenotype which may be important in the resistance mechanism and could be used as a defined metabolite fingerprint to detect the resistant phenotype during early infection. Metabolomics also established that a different metabolite fingerprint also exists according to the plant susceptibility prior to infection. These metabolites may play an important role in the plant’s resistance mechanism. A. psidii expression of small secreted proteins (SSPs) could be detected 48 hours after inoculation in Eucalyptus grandis leaves. These encoded SSPs are important in the pathogenesis of A. psidii. In particular, three of these SSPs were highly expressed in the susceptible plant leaves, indicating their importance for host colonisation in the susceptible phenotype and showing that they could be used for detecting early infection in susceptible hosts. Gene expression in the host, E. grandis, was analysed prior to- and at 48 hours after A. psidii inoculation. The host response to infection differed depending on resistance profile. Differentially expressed genes included genes encoding known disease resistance proteins, as well as other pathways associated with disease resistance including secondary metabolic and phenylpropanoid metabolic pathways and protein phosphorylation. Gene expression markers for plant susceptibility in uninfected cells were also identified. Together, the results of this study highlight that both metabolomics and transcriptomics reveal chemical and genetic cues that can be used as molecular fingerprints to detect early infection and the corresponding phenotype of the host. In addition, molecular fingerprints were identified that characterise the phenotype prior to infection, which may be an important tool to identify resistant plants in the field or for plant breeding. Future work could further characterise the genes and molecules identified in this study to elucidate the molecular mechanisms of host resistance and A. psidii pathogenesis.

PUBLICATIONS

The Final Report can be downloaded here, and the journal paper accessed here.