TMV resistance gene N homologues are linked to Synchytrium endobioticum resistance in potato

The fungus Synchytrium endobioticum, the causal agent of potato wart diseas e, is subject to worldwide quarantine regulations due to the production of persistent resting spores and lack of effective chemical control measures. The selection of Synchytrium-resistant potato cultivars may be facilitated by using markers closely linked with a resistance gene or by transferring a cloned gene for resistance into susceptible cultivars. Sen1, a gene for re sistance to Synchytrium endobioticum race 1, was localized on potato chromo some XI in a genomic region which is related to the tobacco genome segment harbouring the N gene for resistance to TMV. Using N as probe, we isolated homologous cDNA clones from a Synchytrium-resistant potato line. The N-homo logous sequences of potato identified by RFLP mapping a family of resistanc e gene-like sequences closely linked with the Sen1 locus. Sequence analysis of two full-length N-homologous cDNA clones revealed the presence of struc tural domains associated with resistance gene function. One clone (N1-25) e ncodes a polypeptide of 61 kDa and harbours a Toll-interleukin like region (TIR) and a putative nucleotide binding site (NBS). The other clone (N1-27) encodes a polypeptide of 95 kDa and harbours besides the TIR and NBS domai ns five imperfect leucine-rich repeats (LRRs). Both clones have at their am ino terminus a conserved stretch of serine residues that was also found in the N gene, the RPP5 gene from Arabidopsis thaliana and several other resis tance gene homologues, suggesting a function in the resistance response. Cl oning of the disease resistance locus based on map position and the establi shment of PCR-based marker assays to assist selection of wart resistant pot ato genotypes are discussed.