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

Abstract

AbstractThe genetic locus for resistance to apple scab most frequently used in apple breeding isVf, derived fromMalus floribunda821. For theVflocus a cluster of four resistance gene paralogs (called asHcrVfgenes) encoding receptor‐like proteins (RLP) with similarity to the tomatoCfresistance genes is known. Based on published sequences forHcrVf1andHcrVf2PCR 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%) toHcrVf1andHcrVf2were obtained not only fromM. floribunda821 andVfcultivars but also from other apple scab resistance sources, such as ‘Russian Seedling’ R12740‐7A (Vrresistance) or ‘Antonovka polutorafuntovaya’ (VAresistance). A series of 13HcrVfcandidate 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 selectedVf‐homologous candidate gene (calledVf2ARD) identified in resistantMalusgenotypes known for carrying other scab resistance genes thanVf. RT‐PCR experiments showed thatVf2ARDis 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.Vf2ARDhas been mapped on linkage group LG 1. It is separated from theVfgene cluster with a genetic distance of about 2 cM and might be a member of a secondVf ‐like locus on apple linkage group LG 1.