A candidate gene for fire blight resistance in Malus × robusta 5 is coding for a CC–NBS–LRR

Fire blight is the most important bacterial disease in apple (Malus × domestica) and pear (Pyrus communis) production. Today, the causal bacterium Erwinia amylovora is present in many apple- and pear-growing areas. We investigated the natural resistance of the wild apple Malus × robusta 5 against E. amylovora, previously mapped to linkage group 3. With a fine-mapping approach on a population of 2,133 individuals followed by phenotyping of the recombinants from the region of interest, we developed flanking markers useful for marker-assisted selection. Open reading frames were predicted on the sequence of a BAC spanning the resistance locus. One open reading frame coded for a protein belonging to the NBS–LRR family. The in silico investigation of the structure of the candidate resistance gene against fire blight of M. × robusta 5, FB_MR5, led us hypothesize the presence of a coiled-coil region followed by an NBS and an LRR-like structure with the consensus ‘LxxLx[IL]xxCxxLxxL’. The function of FB_MR5 was predicted in agreement with the decoy/guard model, that FB_MR5 monitors the transcribed RIN4_MR5, a homolog of RIN4 of Arabidopsis thaliana that could interact with the previously described effector AvrRpt2EA of E. amylovora