Identification of candidate genes responsible for the susceptibility of apple ( Malus × domestica Borkh.) to Alternaria blotch

Background The mechanism underlying the interaction between host plant and host-selective toxin (HST)-producing Alternaria alternata during infection is of particular interest for sustainable crop production. Alternaria blotch of apple ( Malus × domestica Borkh.) caused by A. alternata apple pathotype is a major disease particularly in East Asia, which is the largest producer of apples globally. A single dominant gene, Alt , controls the susceptibility of the apple cultivar ‘Delicious’ to Alternaria blotch. In this study, we fine mapped the Alt locus and characterized three potential candidate genes. Results We used 797 F 1 individuals derived from 15 crosses between apple accessions susceptible ( Alt / alt ) and resistant ( alt / alt ) to Alternaria blotch to construct physical and genetic maps of the Alt locus located on the top of chromosome 11. Susceptible accessions were derived from ‘Delicious.’ To fine map the Alt locus, we constructed a BAC library of ‘Starking Delicious,’ a sport of ‘Delicious,’ and used graphical genotyping to delimit the Alt locus to a region of 43 kb. Three genes predicted within the candidate Alt region were potentially involved in plant defense response, among which the gene encoding a coiled coil-nucleotide binding-leucine rich repeat (CC-NB-LRR) type disease resistance protein was the most promising. Moreover, a 12-bp insertion was uniquely identified in the 5′ untranslated region of the Alt -associated allele of this gene, the presence or absence of which co-segregated with the susceptibility or resistance to A. alternata apple pathotype, respectively, among 43 tested cultivars including old ones and founders of modern apple breeding. Conclusion A disease resistance protein has been suggested as a determinant of susceptibility/resistance to HST-producing A. alternata for the first time. Our finding provides new insight into the mechanism of HST-mediated disease control used by A. alternata against host plants.