Uncovering the defence responses of Eucalyptus to pests and pathogens in the genomics age

Abstract

Long-lived tree species are subject to attack by various pests and pathogens during their lifetime. This problem is exacerbated by climate change which may increase the host range for pathogens and extend the period of infestation by pests. Plant defences may involve pre-formed barriers or induced resistance mechanisms based on recognition of the invader, complex signalling cascades, hormone signalling, the activation of transcription factors and the production of pathogenesis-related (PR) proteins with direct antimicrobial or anti-insect activity. Trees have evolved some unique defence mechanisms compared to well-studied model plants. The genome sequence of Eucalyptus grandis has recently become available and provides a resource to extend our understanding of defence in large woody perennials. This review synthesises existing knowledge of defence mechanisms in model systems and other tree species and features mechanisms that may be important for defence in Eucalyptus, such as anatomical variants and the role of chemicals and proteins. Based on the E. grandis genome sequence, we identified putative PR proteins based on sequence identity to the previously described plant PR proteins. Putative orthologs for PR-1, PR-2, PR-4, PR-5, PR-6, PR-7, PR-8, PR-9, PR-10, PR-12, PR-14, PR-15 and PR-17 were identified and compared to the number of PR genes in Populus trichocarpa and Arabidopsis thaliana. Genomic resources available for Eucalyptus are discussed and approaches for improving resistance in these hardwood trees, earmarked as a bioenergy source in future, are considered.