Foliar terpenoid levels and corresponding gene expression are systemically and differentially induced in Eucalyptus grandis clonal genotypes in response to Chrysoporthe austroafricana challenge

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 β‐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.

Characterization of terpene biosynthesis in Melaleuca quinquenervia and ecological consequences of terpene accumulation during myrtle rust infection

“…Induction of TPS genes or changes of terpene concentrations in response to biotic and abiotic stress has rarely been shown in Myrtaceae (Webb et al, 2014 ). Apart from a recent study where some TPS ‐encoding genes and terpenes were observed to be upregulated in E. grandis upon challenge by Leptocybe invasa (gall wasp; Oates et al, 2015 ) and another study which found induced expression of isoprene synthase‐ and β‐caryophyllene synthase‐encoding genes ( EgrTPS084 and EgrTPS038 , respectively) in E. grandis in response to C. austroafricana infection (Visser et al, 2015 ), there has been little to no evidence of induction of terpenes in Eucalyptus or Melaleuca in response to wounding, herbivory, or even application of methyl jasmonate (MeJA; Henery et al, 2008 ). Henery et al ( 2008 ) noted that it was possible that herbivory and MeJA could still induce TPS genes in Eucalyptus without increasing the amount of constitutive terpenes contained in the leaves due to volatile losses.…”

“…Other than studies that investigated the effects of terpenes to beetle‐vectored fungal defense in several conifer species (Keeling & Bohlmann, 2006 ; Zeneli et al, 2006 ), only few studies investigated induced terpenes with potential antifungal properties in plants in response to fungal infections alone. For example, Visser et al ( 2015 ) compared gene expression profiles between resistant and susceptible Eucalyptus grandis in response to the necrotrophic fungus Chrysoporthe austroafricana and identified inductions of transcripts encoding a putative isoprene synthase ( EgrTPS084 ) and a β‐caryophyllene synthase ( EgrTPS038 ) after infection in resistant plants. Maize ( Zea mays ) responds to infection by Fusarium graminearum by producing zealexins and kauralexins (both terpenoids) in response to Cercospora zeina (Huffaker et al, 2011 ; Meyer et al, 2017 ).…”

The Threat of the Combined Effect of Biotic and Abiotic Stress Factors in Forestry Under a Changing Climate

“…Thus, improving trees for resistance to combined biotic and abiotic stresses using these techniques may also become possible. Existing model systems that have been used to study biotic stresses such as those in Eucalyptus (Naidoo et al, 2013;Mangwanda et al, 2015;Visser et al, 2015), Pine (Visser et al, 2018), and Poplar (Feau et al, 2007;Hacquard et al, 2011) can be used to develop a workable approach in this regard. We propose the use of drought-pathogen interactions, which has been considered a model in annual crops (Pandey et al, 2017), to study combined biotic and abiotic stresses in forest trees.…”