Development of expression markers for reduced susceptibility in Fraxinus excelsior

Abstract: Common ash (Fraxinus excelsior) extends throughout temperate Europe, and in southern and central Sweden it is one of the most common broad-leaved deciduous trees. It is economically valuable thanks to its elastic, hard and pressure-resistant wood, and pollarded ash trees provide a cultural heritage in many European landscapes. The species is also important for biodiversity, as many organisms depend on it. F. excelsior is currently suffering from ash dieback disease, an epidemic caused by the fungus Hymenoscyphus fraxineus. Ash dieback lethally affects trees of all ages. Common ash is listed as ‘endangered’ in the Red list of Sweden since 2015, due to trees being affected by dieback all over the F. excelsior distribution range in the country. Molecular markers for resistance or tolerance in plants have been studied mainly with the aim to improve plant breeding in agriculture. Using tolerance-predictive markers, trees with reduced susceptibility to pathogens can be identified. To identify markers, score them and correlate the variation in both gene sequence and expression with variation in traits associative transcriptomics are of use. Using this method in a study on F. excelsior, Harper et al. (2016) identified a SNP as a predictor of reduced susceptibility to ash dieback. In another study on common ash and dieback by Sahraei (2016), 1082 differentially expressed genes in susceptible and tolerant clones were revealed. In this study, the aim was to identify and evaluate molecular markers for reduced susceptibility in F. excelsior. Phenotypically classified tolerant and susceptible ash trees located in Gotland, Sweden, were used. To evaluate the SNP, Sanger sequencing was used to score the presence of the polymorphism. To identify molecular markers for reduced susceptibility, ten candidate genes out of the 1082 differentially expressed genes were tested as possible markers for tolerance using qPCR. The SNP was shown to be a moderately good predictor of lower susceptibility to dieback. One of the genes in the study showed a significant difference in relative expression between tolerant and susceptible individuals (p-value <0.05)