Fraxinus excelsior updated long-read genome reveals the importance of MADS-box genes in tolerance mechanisms against ash dieback

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Abstract
Ash dieback caused by the fungus Hymenoscyphus fraxineus has devastated the European ash tree population since it arrived in Europe in 1992. Great effort has been put into breeding programmes to increase the genetic diversity of ash trees and find heritable genetic markers associated with resistance, or tolerance mechanisms, to ash dieback. To facilitate identification of molecular markers, we used Oxford Nanopore Technologies combined with Illumina sequencing to obtain an accurate and contiguous ash genome. We used this genome to reanalyse transcriptome data from a Danish ash panel of 182 tree accessions. Using associative transcriptomics, we identified 175 gene expression markers (GEMs), including 11 genes annotated as dormancy MADS-box transcription factors which are associated with ash bud dormancy, flowering and senescence. We hypothesize that tolerant trees both break dormancy earlier in the year by increasing the expression of flowering-related SOC1 MADS-box and reducing the expression of SVP-like MADS-box, whilst also accelerating senescence by increasing the expression of JOINTLESS MADS-box genes. DNA methylation differences in the promoters of MADS-box genes between one tolerant and one susceptible tree indicate potential epigenetic regulation of these traits. Article Summary: Ash dieback has devastated European ash tree populations. To aid in breeding programmes focused on finding solutions against this pathogen, we have assembled a new ash genome. This new genome helped us to identify genes related to tree biological life cycles, expressed differently in tolerant and susceptible trees. For the first time, we have also discovered that susceptible and tolerant trees showed different DNA methylation frequencies in those genes, suggesting epigenetic regulation. DNA methylation can turn on/off gene expression without changing the DNA sequence. These genes, and their regulatory elements, are ideal targets during breeding programmes combating this pathogen.

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