De novo transcriptome study identifies candidate genes involved in resistance to Austropuccinia psidii (myrtle rust) in Syzygium luehmannii (riberry)

Austropuccinia psidii, causal agent of myrtle rust, was discovered in Australia in 2010 and has since become established on a wide range of species within the family Myrtaceae. Syzygium luehmannii, endemic to Australia, is an increasingly valuable berry crop. Plants were screened for responses to A. psidii inoculation, and specific resistance, in the form of localized necrosis, was determined in 29% of individuals. To understand the molecular basis underlying this response, mRNA was sequenced from leaf samples taken preinoculation, and at 24 and 48 h postinoculation, from four resistant and four susceptible plants. Analyses, based on de novo transcriptome assemblies for all plants, identified significant expression changes in resistant plants (438 transcripts) 48 h after pathogen exposure compared with susceptible plants (three transcripts). Most significantly up-regulated in resistant plants were gene homologs for transcription factors, receptor-like kinases, and enzymes involved in secondary metabolite pathways. A putative G-type lectin receptor-like kinase was exclusively expressed in resistant individuals and two transcripts incorporating toll/ interleukin-1, nucleotide binding site, and leucine-rich repeat domains were up-regulated in resistant plants. The results of this study provide the first early gene expression profiles for a plant of the family Myrtaceae in response to the myrtle rust pathogen.