Genenet Mapping Of The TMV Resistance Gene In Nicotiana Paniculata

Plant resistance (R) genes often trigger hypersensitive response at the site of pathogen infection, which limits the spread of the pathogen. Most cloned R genes encode nucleotide binding site-leucine rich repeat (NBS-LRR) domains. Most of the R genes were cloned using map-based cloning approach, which is time and labor consuming.In this study, candidate gene approach based on homology and comparative genomics was used to map the resistance gene against TMV in wild tobacco Nicotiana paniculata. A cross was made between a resistant (PI555550) and a susceptible (PI241769) genotypes of N. paniculata; the Fl plants were selfed to generate a F2population. Innoculation of the F2population with TMV sap identified206resistant individuals and100susceptible individuals. A total of180genetic markers were designed based on the NBS-LRR genes in Solanaceae. Of them,29CAPs markers and4presence/absence polymorphic markers have been identified. The CAPs marker F115-F15/R15Vsp Ⅰ co-segregates with the resistance phenotype in the F2population of N. paniculata. Using the sequenced genome of tomato as a reference.24markers flanking the N locus on chromosome11were developed. The two markers are polymorphic between the two parents and segregate in the F2population:one cosegregrates with the resistance trait and the other is highly linked with the trait, showing that in N. paniculata is located at the N locus on chromosome11. The only NBS-LRR encoding genes in this region are N homologues and therefore we predicted that the resistance in N. paniculata is encoded by a N homologue. Two full-length N homologues were obtained from the resistance genotype. One of them does not cosegregate with the resistance trait, while the other cosegregates with the trait but is predicted to be a pseudogene. Therefore, neither of the two full-length N homologues should encode the resistance against TMV. A third N homologue, which cosegregates with the resistance trait, has been identified, and cloning of its full-length sequence is in progress. The fine mapping of TMV resistance in N.paniculata will facilitate the cloning of the corresponding resistance gene. The cloning of another resistance gene against TMV in future will further our understanding of R gene evolution and the origin of new resistance.