Molecular markers for tolerance of European ash (Fraxinus excelsior) to dieback disease identified using Associative Transcriptomics

Abstract

Tree disease epidemics are a global problem, impacting food security, biodiversity and national economies. The potential for conservation and breeding in trees is hampered by complex genomes and long lifecycles, with most species lacking genomic resources. The European Ash tree Fraxinus excelsior is being devastated by the fungal pathogen Hymenoscyphus fraxineus, which causes ash dieback disease. Taking this system as an example and utilizing Associative Transcriptomics for the first time in a plant pathology study, we discovered gene sequence and gene expression variants across a genetic diversity panel scored for disease symptoms and identified markers strongly associated with canopy damage in infected trees. Using these markers we predicted phenotypes in a test panel of unrelated trees, successfully identifying individuals with a low level of susceptibility to the disease. Co-expression analysis suggested that pre-priming of defence responses may underlie reduced susceptibility to ash dieback.

The use of ascospores of the dieback fungus Hymenoscyphus fraxineus for infection assays reveals a significant period of biotrophic interaction in penetrated ash cells

“…The differences in development in relation to latent infection need to be explored in more detail. It will also be important to examine if the reported quantitative variation in susceptibility of ash accessions (Harper et al ., ) is related to differences in the establishment of intracellular hyphae and the ability to restrict biotrophic development. The internode infection system developed here should be suitable for the characterization of fungal transcription in relation to the biotroph–necrotroph transition.…”

“…Despite the wealth of molecular data now available (MacLean et al ., ; Harper et al ., ; Downie, ; Sollars et al ., ), there is only one detailed report on the progress of infection from ascospore inoculum (Cleary et al ., ). In their paper, Cleary et al .…”

The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history

“…Recently a set of genetic markers that is closely associated to variation in susceptibility among genotypes of F. excelsior has been identified (Harper et al 2016). A cDNA-based SNP polymorphism was associated with susceptibility, where highly susceptible F. excelsior trees only showed allele A, while trees with reduced susceptibility also showed allele G. The species F. mandschurica, F. ornus and F. americana were genotyped as G/G, which fits well with the observations from the present study that these species exhibit high levels of tolerance.…”

Anatomical and genetic aspects of ash dieback: a look at the wood structure

“…In this study, the two health iForest 10: 522-528 526 groups of ash trees affected by dieback were screened at the Femsatl loci, finding no differences in their allele frequencies. Harper et al (2016). The interaction between European ash genotype and environmental conditions was previously assessed using RAPD markers in a Lithuanian population trial by Pliura & Baliuckas (2007), who reported a high genetic variation within populations affecting the adaptive potential and competitive ability of the studied trees.…”