Transcriptome analysis of Eucalyptus grandis implicates brassinosteroid signaling in defense against myrtle rust (Austropuccinia psidii)

Eucalyptus grandis, in its native Australian range, varies in resistance to Austropuccinia psidii (syn. Puccinia psidii). The biotrophic rust fungus, A. psidii is the causal agent of myrtle rust and poses a serious threat to Australian biodiversity. The pathogen produces yellow pustules of urediniospores on young leaves and shoots, resulting in shoot tip dieback, stunted growth, and death. Dissecting the underlying mechanisms of resistance against this pathogen will contribute to improved breeding and control strategies to mitigate its devastating effects. The aim of this study was to determine the molecular dialogue between E. grandis and A. psidii, using an RNA-sequencing approach. Resistant and susceptible E. grandis seedlings grown from seed collected across its natural range were inoculated with the pandemic biotype of A. psidii. The leaf tissue was harvested at 12-h post inoculation (hpi), 1-day post inoculation (dpi), 2-dpi and 5-dpi and subjected to RNA-sequencing using Illumina 50 bp PE reads to a depth of 40 million reads per sample. Differential gene expression and gene ontology enrichment indicated that the resistant seedlings showed controlled, coordinated responses with a hypersensitive response, while the susceptible seedlings showed no systemic response against myrtle rust. Brassinosteroid signaling was apparent as an enriched term in the resistant interaction at 2-dpi, suggesting an important role of this phytohormone in defense against the pathogen. Brassinosteroid mediated signaling genes were also among the candidate genes within two major disease resistance loci (Puccinia psidii resistance), Ppr3 and Ppr5. While brassinosteroids have been tagged as positive regulators in other plant disease resistance interactions, this is the first report in the Eucalyptus – Austropuccinia psidii interaction. Furthermore, several putative resistance genes, underlying known resistance loci and implicated in the interaction have been identified and highlighted for future functional studies. This study provided further insights into the molecular interactions between E. grandis and A. psidii, contributing to our understanding of this pathosystem.The South African National Research Foundation (NRF) and the Technology Innovation Agency of South Africa.https://www.frontiersin.org/journals/forests-and-global-change#am2022BiochemistryForestry and Agricultural Biotechnology Institute (FABI)GeneticsMicrobiology and Plant Patholog

The in planta gene expression of Austropuccinia psidii in resistant and susceptible Eucalyptus grandis

Austropuccinia psidii, commonly known as myrtle rust, is an obligate, biotrophic rust pathogen that causes rust disease on a broad host range of Myrtaceae species. Eucalyptus grandis, a widely cultivated hardwood Myrtaceae species, is susceptible to A. psidii infection, with this pathogen threatening both their natural range and various forest plantations across the world. This study aimed to investigate the A. psidii transcriptomic responses in resistant and susceptible E. grandis at four time points. RNA-seq reads were mapped to the A. psidii reference genome to quantify expressed genes at 12-hours post inoculation (hpi), 1-, 2- and 5-days post inoculation (dpi). A total of eight hundred and ninety expressed genes were found, of which forty-three were candidate effector proteins. These included a rust transferred protein (RTP1) gene, expressed in susceptible hosts at 5-dpi and a hydrolase protein gene expressed in both resistant and susceptible hosts over time. Functional categorisation of expressed genes revealed processes enriched in susceptible hosts, including malate metabolic and malate dehydrogenase activity, implicating oxalic acid in disease susceptibility. These results highlight putative virulence or pathogenicity mechanisms employed by A. psidii to cause disease and provides the first insight into the molecular responses of A. psidii in E. grandis over time

Transcriptome Analysis of Eucalyptus grandis Implicates Brassinosteroid Signaling in Defense Against Myrtle Rust (Austropuccinia psidii)

Eucalyptus grandis, in its native Australian range, varies in resistance to Austropuccinia psidii (syn. Puccinia psidii). The biotrophic rust fungus, A. psidii is the causal agent of myrtle rust and poses a serious threat to Australian biodiversity. The pathogen produces yellow pustules of urediniospores on young leaves and shoots, resulting in shoot tip dieback, stunted growth, and death. Dissecting the underlying mechanisms of resistance against this pathogen will contribute to improved breeding and control strategies to mitigate its devastating effects. The aim of this study was to determine the molecular dialogue between E. grandis and A. psidii, using an RNA-sequencing approach. Resistant and susceptible E. grandis seedlings grown from seed collected across its natural range were inoculated with the pandemic biotype of A. psidii. The leaf tissue was harvested at 12-h post inoculation (hpi), 1-day post inoculation (dpi), 2-dpi and 5-dpi and subjected to RNA-sequencing using Illumina 50 bp PE reads to a depth of 40 million reads per sample. Differential gene expression and gene ontology enrichment indicated that the resistant seedlings showed controlled, coordinated responses with a hypersensitive response, while the susceptible seedlings showed no systemic response against myrtle rust. Brassinosteroid signaling was apparent as an enriched term in the resistant interaction at 2-dpi, suggesting an important role of this phytohormone in defense against the pathogen. Brassinosteroid mediated signaling genes were also among the candidate genes within two major disease resistance loci (Puccinia psidii resistance), Ppr3 and Ppr5. While brassinosteroids have been tagged as positive regulators in other plant disease resistance interactions, this is the first report in the Eucalyptus – Austropuccinia psidii interaction. Furthermore, several putative resistance genes, underlying known resistance loci and implicated in the interaction have been identified and highlighted for future functional studies. This study provided further insights into the molecular interactions between E. grandis and A. psidii, contributing to our understanding of this pathosystem