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

Abstract

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.