Identification and molecular analysis of candidate genes homologous to HcrVf genes for scab resistance in apple

The genetic locus for resistance to apple scab most frequently used in apple breeding is Vf, derived from Malus floribunda 821. For the Vf locus a cluster of four resistance gene paralogs (called as HcrVf genes) encoding receptor-like proteins (RLP) with similarity to the tomato Cf resistance genes is known. Based on published sequences for HcrVf1 and HcrVf2 PCR primers were designed from the domain B and the variable leucine-rich repeat (LRR) C1 subdomain. PCR products with high amino acid identity (85–100%) to HcrVf1 and HcrVf2 were obtained not only from M. floribunda 821 and Vf cultivars but also from other apple scab resistance sources, such as ‘Russian Seedling’ R12740-7A (Vr resistance) or ‘Antonovka polutorafuntovaya’ (VA resistance). A series of 13 HcrVf candidate genes have been partly cloned from the PCR fragments spanning N-terminal LRRs 20–30. A considerable number of amino acid exchanges within the solvent-exposed xxLxLxx structural motives were detected among the homologous sequences. Expression analyses and mapping focused on a selected Vf-homologous candidate gene (called Vf2ARD) identified in resistant Malus genotypes known for carrying other scab resistance genes than Vf. RT-PCR experiments showed that Vf2ARD is expressed under pathogen-free conditions. The results of a quantitative PCR-based transcription profiling suggest that this gene is scab-inducible in some resistant cultivars. Vf2ARD has been mapped on linkage group LG 1. It is separated from the Vf gene cluster with a genetic distance of about 2 cM and might be a member of a second Vf -like locus on apple linkage group LG 1.