Abstract
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.
Highlights
IntroductionMyrtle rust has an unusually broad host range on various members of the Myrtaceae family, affecting approximately 480 species within 86 genera, producing yellow pustules of urediniospores on growing leaves and shoots (Coutinho et al, 1998; Soewarto et al, 2019)
We identified pathways that may be involved in the E. grandis defense response against myrtle rust, such as the hypersensitive response, oxidative burst, and a range of phytohormones, with brassinosteroids signaling identified in resistant seedlings
BAK1 has been implicated in disease resistance against fungal pathogens including Sclerotinia sclerotiorum and Verticillium dahliae (Fradin et al, 2009; Chaparro-Garcia et al, 2011) and the identification of nine differentially expressed (DE) BAK1 genes in the present study suggests a putative role of BR and associated receptors in the defense response against A. psidii
Summary
Myrtle rust has an unusually broad host range on various members of the Myrtaceae family, affecting approximately 480 species within 86 genera, producing yellow pustules of urediniospores on growing leaves and shoots (Coutinho et al, 1998; Soewarto et al, 2019). This results in dieback of shoot tips, stunted growth and in severe cases plant death (Carnegie et al, 2016; Pegg et al, 2017, 2020; Fensham et al, 2020). Eucalyptus spp. are amongst the most susceptible members of this family to myrtle rust, posing a risk to their natural and introduced ranges (Coutinho et al, 1998; Junghans et al, 2003a), with Eucalyptus grandis showing variable responses ranging from highly susceptible to resistant
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