Abstract
BackgroundBreeding of fire blight resistant scions and rootstocks is a goal of several international apple breeding programs, as options are limited for management of this destructive disease caused by the bacterial pathogen Erwinia amylovora. A broad, large-effect quantitative trait locus (QTL) for fire blight resistance has been reported on linkage group 3 of Malus ‘Robusta 5’. In this study we identified markers derived from putative fire blight resistance genes associated with the QTL by integrating further genetic mapping studies with bioinformatics analysis of transcript profiling data and genome sequence databases.ResultsWhen several defined E.amylovora strains were used to inoculate three progenies from international breeding programs, all with ‘Robusta 5’ as a common parent, two distinct QTLs were detected on linkage group 3, where only one had previously been mapped. In the New Zealand ‘Malling 9’ X ‘Robusta 5’ population inoculated with E. amylovora ICMP11176, the proximal QTL co-located with SNP markers derived from a leucine-rich repeat, receptor-like protein ( MxdRLP1) and a closely linked class 3 peroxidase gene. While the QTL detected in the German ‘Idared’ X ‘Robusta 5’ population inoculated with E. amylovora strains Ea222_JKI or ICMP11176 was approximately 6 cM distal to this, directly below a SNP marker derived from a heat shock 90 family protein gene ( HSP90). In the US ‘Otawa3’ X ‘Robusta5’ population inoculated with E. amylovora strains Ea273 or E2002a, the position of the LOD score peak on linkage group 3 was dependent upon the pathogen strains used for inoculation. One of the five MxdRLP1 alleles identified in fire blight resistant and susceptible cultivars was genetically associated with resistance and used to develop a high resolution melting PCR marker. A resistance QTL detected on linkage group 7 of the US population co-located with another HSP90 gene-family member and a WRKY transcription factor previously associated with fire blight resistance. However, this QTL was not observed in the New Zealand or German populations.ConclusionsThe results suggest that the upper region of ‘Robusta 5’ linkage group 3 contains multiple genes contributing to fire blight resistance and that their contributions to resistance can vary depending upon pathogen virulence and other factors. Mapping markers derived from putative fire blight resistance genes has proved a useful aid in defining these QTLs and developing markers for marker-assisted breeding of fire blight resistance.
Highlights
Breeding of fire blight resistant scions and rootstocks is a goal of several international apple breeding programs, as options are limited for management of this destructive disease caused by the bacterial pathogen Erwinia amylovora
Genetic analysis suggests differences in Robusta 5’ (R5) accessions In order to verify the identity of the three R5 accessions used in the three mapping populations, the accessions were genotyped with six simple sequence repeat (SSR) markers distributed over the upper 30 Mb of LG3 (Table 1)
The results demonstrate that while R5-DE and R5-New Zealand (NZ) accessions are genotypically identical, there is a strong possibility that R5-United States of America (US) is not equivalent, as it differs in allelotype at marker NZmsCN943818
Summary
Breeding of fire blight resistant scions and rootstocks is a goal of several international apple breeding programs, as options are limited for management of this destructive disease caused by the bacterial pathogen Erwinia amylovora. Fire blight, caused by the bacterial pathogen E. amylovora (Ea), is a destructive disease of apple, pear and several other rosaceous species, infecting blossoms, fruit, vegetative shoots, woody tissues, and rootstock crowns. Genetic studies suggest that there have been as many as four introductions of Ea into Europe, and due to their limited diversity they most likely were not the result of repeated introductions from North America [9]. It is presumed that Ea evolved as a pathogen of North American rosaceous species, such as Crataegus and Sorbus, and first came into contact with its current economically important hosts, Malus x domestica (apple) and Pyrus communis (pear), approximately 350 years ago during European colonization of North America
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