Investigating Eucalyptus – pathogen and pest interactions to dissect broad spectrum defense mechanisms

Background Eucalyptus species, hybrids and clones are attacked by various fungal and bacterial pathogens and pests during their life-time. Global climate changes are predicted to create favourable environments for such pathogens and pests and increase incidence of host jumping from other crops, resulting in increased losses to the forestry industry [1]. The use of tolerant or resistant plant varieties as part of an integrated disease management strategy is recognised as a desirable means to curb disease incidence. Vertical resistance mediated by resistance (R) genes, may be easily overcome by a pathogen and is thus not adequate on plantation species such as Eucalyptus, which would be exposed to various pathogens during its life-time. Broad spectrum resistance on the other hand, would be desirable to provide resistance against multiple challenges [2]. The aim of this study is to investigate mechanisms involved in host resistance with an emphasis on broadspectrum resistance. The availability of the complete genome sequence of Eucalyptus grandis (http://www. eucagen.org) and the transcriptome sequence of a E. grandis X E. urophylla (GU) hybrid [3] has provided resources to investigate defense responses in the natural host. When a pathogen attacks a plant, the plant launches a sophisticated defense response involving phytohormones such as salicylic acid (SA), methyl jasmonate (MeJA) and ethylene (ET). These responses are finely tuned and tailored to the invader [4]. Downstream of the signalling cascade is the production of pathogenesis related (PR) genes and antimicrobial genes which serve to limit the pathogen and afford protection. PR genes, such as PR-1 and PR-5, are known markers of the salicylic acid defense pathway, while PR-3, PR-4 and the lipoxygenase (LOX) genes are known markers of the MeJA and ET signalling pathways. The discovery of PR genes in Eucalyptus is desirable as these genes have previously been shown to afford broad spectrum resistance in other crops. We present our progress in exploiting the Eucalyptus genomic and transcriptomic data for the discovery of tree defense genes and explore the application thereof in determining which pathways are activated in response to various pathogens.