Fire blight resistance of the wild apple species Malus fusca: mapping, gene isolation and functional characterization

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Fire blight, caused by the Gram-negative bacterium, Erwinia amylovora, is the most destructive bacterial disease of apple (Malus ×domestica). The economic impacts as a result of fire blight epidemics are usually huge, reaching millions of dollars. The application of antibiotics is the most effective control measure available to apple growers. However, this measure is unsustainable and forbidden in Germany. An alternative measure is therefore inevitable. Natural resistance is thought to be the most sustainable approach to manage fire blight. Genetic resistance has been investigated in Malus leading to the detection of several quantitative trait loci (QTLs) in apple cultivars and apple wild species accessions. Nevertheless, only one functionally characterized fire blight resistance gene, FB_MR5, underlying the Malus ×robusta 5 (Mr5) QTL, has been isolated till date. The situation is more critical since it has been proven that resistance of Mr5 is strain specific and completely broken down by a highly virulent strain of E. amylovora (Ea3049) and an avrRpt2EA effector deletion mutant (ZYRKD3-1). The exchange of only one amino acid in this effector protein or its loss is responsible for the breakdown of resistance of Mr5. It is therefore crucial to identify more resistant donors and to isolate the underlying resistance genes in order to functionally characterize them and to finally establish durable resistance through pyramiding of mechanisms acting differentially. Consequently, resistance to fire blight was investigated in the wild apple species Malus fusca, which resulted in the identification of a major QTL on linkage group 10 (LG10) which explained about 66 % of the phenotypic variation. In contrast to Mr5 the M. fusca QTL (Mfu10) was not overcome after inoculation with isolate Ea3049. The objective of this research proposal is to isolate and functionally characterize the gene(s) underlying Mfu10. A population of 1,336 progeny individuals was established for a fine mapping approach. Genotyping with closely linked SSR markers has allowed for the identification of individuals showing recombination within the QTL interval. Recombinant individuals will form the basis for the isolation of the gene. To study the interaction of Mfu10 with the avrRpt2EA effector, the mapping population will be phenotyped with ZYRKD3-1 and an avrRpt2EA deletion mutant of strain Ea3049 and the respective complemented strain. Additionally, preliminary results suggest that another resistance locus in M. fusca or minor QTLs are contributing to the resistance of M. fusca. Therefore, to precisely detect these loci, the completion and saturation of the M. fusca genetic map is necessary. Overall, it is expected that the underlying resistance genes of M. fusca will provide resistance against the highly virulent E. amylovora isolates responsible for the breakdown of the Mr5 QTL and that pyramiding of these factors together with FB_MR5 will provide more durable resistance.