Abstract:
Induction of systemic defences to fungal pathogens is well described in model plant systems, but not extensively studied in
large woody perennials. Systemic induced defences in long-lived tree species, such as Eucalyptus grandis, could provide
resistance against subsequent biotic challenges. Terpenoids are a class of plant-specialized metabolites implicated in
defence against herbivores and pathogens. The aim of this study was to characterize the systemic induction of terpenoids
in E. grandis clones challenged with the fungal pathogen Chrysoporthe austroafricana. Ramets of E. grandis clones previously
classified as moderately resistant (TAG5) and susceptible (ZG14) were inoculated with C. austroafricana on the
main stem, with mock-inoculated ramets to serve as controls. Leaf tissue was harvested 3 days post-inoculation and terpenoid
levels were measured using gas chromatography coupled with mass spectrometry. Foliar mRNA abundance profiles
of four candidate terpenoid biosynthesis pathway genes were compared in inoculated and mock-inoculated ramets of each
genotype. Monoterpene levels differed between clonal genotypes and p-cymene was induced systemically in TAG5.
Expression profiling of geranyl pyrophosphate synthase (EgrGPPS), farnesyl pyrophosphate synthase (EgrFPPS2), myrcene
synthase (EgrTPS084) and b-caryophyllene synthase (EgrTPS038) show induction of transcripts involved in terpenoid
biosynthesis in leaves following pathogen challenge on the main stem. Together these results suggest that resistant
and susceptible Eucalyptus genotypes may have different constitutive and induced terpenoid profiles. The significance of
the systemic induction of terpenoid levels in TAG5 and terpenoid biosynthesis transcripts in both genotypes in response to
C. austroafricana challenge in the stem, and their role in defence against secondary biotic attack, remains to be determined.
