Transcriptional responses in developing lesions of European common ash (Fraxinus excelsior) reveal genes responding to infection by Hymenoscyphus fraxineus

Abstract

Background With the expanding ash dieback epidemic that has spread across the European continent, an improved functional understanding of the disease development in afflicted hosts is needed. The study investigated whether differences in necrosis extension between common ash (Fraxinus excelsior) trees with different levels of susceptibility to the fungus Hymenoscyphus fraxineus are associated with, and can be explained by, the differences in gene expression patterns. We inoculated seemingly healthy branches of each of two resistant and susceptible ash genotypes with H. fraxineus grown in a common garden. Results Ten months after the inoculation, the length of necrosis on the resistant genotypes were shorter than on the susceptible genotypes. RNA sequencing of bark samples collected at the border of necrotic lesions and from healthy tissues distal to the lesion revealed relatively limited differences in gene expression patterns between susceptible and resistant genotypes. At the necrosis front, only 138 transcripts were differentially expressed between the genotype categories while 1082 were differentially expressed in distal, non-symptomatic tissues. Among these differentially expressed genes, several genes in the mevalonate (MVA) and iridoid pathways were found to be co-regulated, possibly indicating increased fluxes through these pathways in response to H. fraxineus. Comparison of transcriptional responses of symptomatic and non-symptomatic ash in a controlled greenhouse experiment revealed a relatively small set of genes that were differentially and concordantly expressed in both studies. This gene-set included the rate-limiting enzyme in the MVA pathway and a number of transcription factors. Furthermore, several of the concordantly expressed candidate genes show significant similarity to genes encoding players in the abscisic acid- or Jasmonate-signalling pathways. Conclusions A set of candidate genes, concordantly expressed between field and greenhouse experiments, was identified. The candidates are associated with hormone signalling and specialized metabolite biosynthesis pathways indicating the involvement of these pathways in the response of the host to infection by H. fraxineus.

Priming of ash saplings with a low virulent Hymenoscyphus fraxineus strain as a possible disease control approach for reducing symptoms of ash dieback

“…(Whitehill et al 2014;Villari et al 2016). Genes encoding for abscisic acid-or jasmonate-signalling pathways have been identified (Sahraei et al 2020) and their potential role in induced resistance in ash trees is indicated. For example, Whitehill et al (2014) found that the application of methyl jasmonate (MeJA) on the bark of Fraxinus mandshuria and Fraxinus americana can induce phenolic compounds in the phloem of treated stems.…”

Genomic prediction of resistance to Hymenoscyphus fraxineus in common ash (Fraxinus excelsior L.) populations

“…Previous genetic studies of F. excelsior have demonstrated heritability values above 0.4 (Enderle et al., 2015; Kjær et al., 2012; Lobo et al., 2014, 2015; McKinney et al., 2011; Muñoz et al., 2016; Pliūra et al., 2011; Stener, 2013) and the polygenic nature of resistance to H. fraxineus (Muñoz et al., 2016; Stocks et al., 2019). Several studies have identified genetic markers that are associated with increased tolerance to ash dieback (Chaudhary et al., 2020; Harper et al., 2016; Havrdová et al., 2016; McKinney et al., 2014; Menkis et al., 2020; Sahraei et al., 2020; Sollars et al., 2017; Stocks et al., 2019). These markers can be potentially utilized to identify resistant trees that can serve as the foundation for a breeding program (Plumb et al., 2020).…”

Differential gene expression of Hymenoscyphus fraxineus grown on Fraxinus excelsior and F. mandshurica supplemented media

“…Also, numerous single nucleotide polymorphisms (SNPs) have been identified in the ash genome that are associated with tree susceptibility (Stocks et al, 2019). Another study focusing on transcriptional responses of F. excelsior trees to H. fraxineus infection showed limited differences in gene expression patterns between resistant and susceptible F. excelsior clones (Sahraei et al 2020). However, a set of candidate genes likely involved in the containment of the necrosis had been identified.…”