Abstract
Malus ×arnoldiana accession MAL0004 has been found to be resistant to moderately and highly virulent strains of the fire blight causal pathogen – the Gram-negative bacterium, Erwinia amylovora. Genetic analyses with an F1 segregating population derived from crossing the highly susceptible apple cultivar ‘Idared’ and MAL0004 led to the detection and mapping of the fire blight resistance locus of M. ×arnoldiana to linkage group (LG)12 (FB_Mar12). FB_Mar12 mapped at the distal end of LG12 below the apple SSR Hi07f01 in an interval of approximately 6 cM (Centimorgan), where both the fire blight resistance loci of M. floribunda 821 and ‘Evereste’ were located. We fine mapped the region containing FB_Mar12 using 892 progenies. Mining of the region of interest (ROI) on the ‘Golden Delicious’ doubled haploid genome (GDDH13) identified the presence of 2.3 Mb (megabases) in the homologous region. Of 40 primer pairs designed within this region, 20 were polymorphic and nine were mapped, leading to the identification of 24 significant recombinant individuals whose phenotypes were informative in determining the precise position of the locus within a 0.57 cM interval. Analyses of tightly linked marker sequences on the M. baccata draft genome revealed scaffolds of interest putatively harboring the resistance loci of M. ×arnoldiana, a hybrid between M. baccata and M. floribunda. Open reading frame (ORF) analyses led to the prediction of first fire blight resistance candidate genes with serine/threonine kinase and leucine-rich repeat domains, including homologs of previously identified ‘Evereste’ candidate genes. We discuss the implications of these results on breeding for resistance to fire blight.
Highlights
Erwinia amylovora (Burrill; Winslow et al, 1920) is the causative pathogen of fire blight – the most destructive bacterial disease of the domesticated apple (Malus domestica Borkh.)
Since most commercial apple cultivars are susceptible to fire blight, there is a heavy dependence on antibiotics’ application especially in the United States to avoid fire blight epidemics (McManus, 2014; Beckerman and Sundin, 2016) even until recently – in organic apple orchards (Johnson and Temple, 2013)
Fire blight resistance loci of three wild Malus species, M. floribunda 821 (Mf821), ‘Evereste’ and M. ×arnoldiana MAL0004 are located at the distal end of LG12 (Durel et al, 2009; Emeriewen et al, 2017a)
Summary
Erwinia amylovora (Burrill; Winslow et al, 1920) is the causative pathogen of fire blight – the most destructive bacterial disease of the domesticated apple (Malus domestica Borkh.). The Gramnegative bacterium uses the type III secretion system (T3SS) to deposit effector proteins into host cells thereby causing fire blight in susceptible hosts (Oh and Beer, 2005; Khan et al, 2012; Malnoy et al, 2012; Emeriewen et al, 2019; Yuan et al, 2020). Fire blight symptoms, such as blossom blight, necrosis, shepherd’s crook, shoot and rootstock blights, begin to develop the following entry of the pathogen via flowers and wounds into host plants and the subsequent internal spread within the host (Peil et al, 2009). Breeding of elite fire blight resistant cultivars (Peil et al, 2020) remains the silver bullet
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