Identification of molecular markers associated with fungal resistance in Norway spruce and common ash

Abstract: Trees are giants of the forests, possessing nutrient-rich tissues that make them natural targets for pathogens. In Europe, the fungal pathogens Heterobasidion annosum s.l. and Hymenoscyphus fraxineus are associated with Norway spruce and common ash, respectively. H. annosum s.l. causes stem and root-rot and, in economic terms, is a major pathogen of Norway spruce. H. fraxineus, which causes ash dieback disease, results in severe mortality of common ash, which leads to adverse ecological losses. The aim of this thesis was to use modern molecular methods to identify markers for fungal resistance in Norway spruce and common ash for early selection of superior genotypes for resistance tree breeding. By combining genetic map-based information and transcriptome analyses, differentially expressed genes were identified that associate with resistance to Heterobasidion parviporum in Norway spruce. Among the candidate genes were PaNAC04 and two of its paralogues in subgroup III-3 in the NAC family of transcription factors, a transcription factor gene PgMYB11, and a number of genes encoding enzyme in the biosynthesis of phenylpropanoids. Eleven Norway spruce markers that correlated with variation in resistance to H. parviporum were identified in an association genetics study. Laccase PaLAC5 was associated with the control of lesion length development and is likely to be involved in induced defence in close proximity to the H. parviporum infection site. PaLAC5 may be associated with lignosuberized boundary zone formation in spruce inner bark. Finally, one non-synonymous SNP associated with disease severity in common ash was identified in a gene encoding a subtilisin-related peptidase S8/S53 domain. The Hi-Plex-PCR amplification method demonstrated an inexpensive, time-effective method for generating data with potential for use in future tree breeding programmes. Low population differentiation between genotypes selected for disease resistance and the wild population susceptible to ash dieback indicated opportunities for further selection without significantly losing genetic diversity in the ash population.

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