Malus Hosts–Erwinia amylovora Interactions: Strain Pathogenicity and Resistance Mechanisms

Abstract

The bacterium, Erwinia amylovora , deposits effector proteins such as AvrRpt2 EA into hosts through the type III secretion pathogenicity island to cause fire blight in susceptible Malus genotypes. A single nucleotide polymorphism in the AvrRpt2 EA effector plays a key role in pathogen virulence on Malus hosts by exchanging one cysteine to serine in the effector protein sequence. Fire blight resistance quantitative trait loci (QTLs) were detected in a few apple cultivars and wild Malus genotypes with the resistance of wild apples generally found to be stronger than their domestic relatives. The only candidate and functionally analyzed fire blight resistance genes proposed are from wild apple genotypes. Nevertheless, the aforementioned AvrRpt2 EA SNP and a couple of effector mutants of E . amylovora are responsible for the breakdown of resistance from a few Malus donors including detected QTLs and underlying R -genes. This review summarizes a key finding related to the molecular basis underpinning an aspect of virulence of E . amylovora on Malus genotypes, as well as mechanisms of host recognition and specificity, and their implications on the results of genetic mapping and phenotypic studies within the last 5–6 years. Although the knowledge gained has improved our understanding of the Malus – E. amylovora system, more research is required to fully grasp the resistance mechanisms in this genus especially as they pertain to direct interactions with pathogen effectors.

Fire blight QTL analysis in a multi-family apple population identifies a reduced-susceptibility allele in ‘Honeycrisp’

“…Selection for haplotype 6E in ‘Honeycrisp’-derived parents (e.g., ‘WA 38’) or selection against offspring with haplotype 6R might be valuable approaches to developing breeding populations with low susceptibility to fire blight. Although reduced susceptibility is not complete resistance, a successful combination of multiple reduced-susceptibility alleles from different sources will contribute to achieving more durable resistance 8 . In the long-term, breeders should focus on selecting against increased-susceptibility alleles and pyramiding resistance alleles at major genes derived from wild germplasm with reduced-susceptibility alleles to achieve durable resistance to fire blight.…”

“…Several quantitative trait loci (QTLs) additive and/or epistatic associated with resistance/susceptibility to fire blight have been detected throughout the apple genome 8 – 20 . Multiple QTLs have been mapped in wild Malus germplasm (e.g., ‘Evereste’, M. floribunda 821, M. robusta 5) characterized by astringent, crabapple-type fruit, with non-immediate breeding utility for apple scion improvement 8 .…”

Mapping of fire blight resistance in Malus ×robusta 5 flowers following artificial inoculation

“…Furthermore, susceptibility of a given apple variety also depends on the strain of E. amylovora [22–24], as some have been identified that can overcome the fire blight resistance of a wild species, such as Mr5 [25, 26]. Vogt et al [27] proposed a gene-for-gene interaction in the host-pathogen system Mr5 – E. amylovora, whereby a single nucleotide polymorphism (SNP) in the avrRPT2 EA effector leading to a change from cysteine to serine determines the difference between a compatible and an incompatible interaction.…”

Candidate gene mapping identifies genomic variations in the fire blight susceptibility genes HIPM and DIPM across the Malus germplasm

“…This strain likely had higher co-evolutionary compatibility with the North American domesticated accessions in this study. The chances of host–pathogen compatibility are mainly determined by the profile of effector proteins in individual E. amylovora strains that are responsible for fire blight pathogenicity and triggering of the hypersensitive response in Malus 6 , 66 . However, the strains used in this study exhibit high genome similarity 49 and their considerably different phenotypic and genetic responses in host plants can lay out further investigations of Malus-Erwinia interactions.…”