Gene‐for‐gene relationship in the host–pathogen system M alus × robusta 5– E rwinia amylovora

Summary Fire blight is a destructive bacterial disease caused by E rwinia amylovora affecting plants in the family R osaceae, including apple. Host resistance to fire blight is present mainly in accessions of M alus spp. and is thought to be quantitative in this pathosystem. In this study we analyzed the importance of the E . amylovora effector avr R pt2 EA , a homolog of P seudomonas syringae avr R pt2 , for resistance of M alus  ×  robusta 5 ( M r5). The deletion mutant E . amylovora E a1189Δ avr R pt2 EA was able to overcome the fire blight resistance of M r5. One single nucleotide polymorphism ( SNP ), resulting in an exchange of cysteine to serine in the encoded protein, was detected in avr R pt2 EA of several E rwinia strains differing in virulence to M r5. E . amylovora strains encoding serine ( S ‐allele) were able to overcome resistance of M r5, whereas strains encoding cysteine ( C ‐allele) were not. Allele specificity was also observed in a coexpression assay with A rabidopsis thaliana RIN 4 in N icotiana benthamiana . A homolog of RIN 4 has been detected and isolated in M r5. These results suggest a system similar to the interaction of RPS 2 from A . thaliana and A vr R pt2 from P . syringae with RIN 4 as guard. Our data are suggestive of a gene‐for‐gene relationship for the host–pathogen system M r5 and E . amylovora .