The Eucalyptus Defensome Reveals Insights into Tailored and Shared Defence Mechanisms Against Pest and Pathogen Challenges

Eucalyptus trees grown world-wide as plantation fibre crops are subject to pest and pathogen challenges throughout their life-times. Among these are the insect pest, Leptocybe invasa, the root rot pathogen, Phytophthora cinnamomi, and the stem canker pathogen, Chrysoporthe austroafricana. These and other emerging pests and pathogens pose a serious threat to plantation forestry, which may in future be exacerbated by climate change. A detailed understanding of Eucalyptus defence responses will be required to address these challenges. Using the recently completed E. grandis genome reference and deep RNAseq expression profiling (30 million PE50 per tree), we examined the Eucalyptus defensome (defence transcriptomes) of plants challenged with L. invasa, P. cinnamomi and C. austroafricana to identify shared and unique defense responses. Gene Ontology analysis of the three data sets suggest that phytoalexins and proteinase inhibitors are important for defence against L. invasa, while defence against P. cinnamomi is marked by the down-regulation of terms related to gibberellic acid. A signature feature of the defence response against C. austroafricana is the over-representation of terms related to ethylene production. The up-regulation of pathogenesis related (PR) genes, in particular, PR-5, were noted in the response against both pathogens. Interestingly, a shared set of 80 genes are up-regulated in response to all three challenges. Some of these genes may represent targets of pathogen/pest manipulation, or core defence genes necessary for broad-spectrum defence. Our results provide novel insight into the defence architecture of Eucalyptus trees and provide a platform for identifying targets to improve defence responses in Eucalyptus.