![]() In flax, while the L locus comprises a single copy of a Toll/interleukin-1 receptor (TIR)–NBS–LRR R gene with ∼13 alleles, the M locus encompasses ∼15 copies in tandem, but only one of these is responsible for resistance to a particular race of the rust pathogen ( A nderson et al. For instance, in Arabidopsis thaliana the Rpm1 locus has one copy of a coiled-coil–nucleotide binding site–leucine rich repeat (CC–NBS–LRR) R gene in plants that are resistant to Pseudomonas syringae, but this copy is deleted in susceptible plants ( G rant et al. In addition, genetic and molecular analysis of R loci have shown varying degrees of complexity. Analysis of the deduced amino acid sequence of R genes has revealed structural motifs that are indicative of a role in signal transduction ( H ammond-K osack and J ones 1997 E llis and J ones 1998 D angl and J ones 2001) and that suggest the existence of conserved pathways used by plants to trigger defense responses ( B ent 1996 D angl and J ones 2001 S taskawicz et al. MOLECULAR and genetic characterization of a number of disease resistance (R) genes has increased our understanding not only of their structure and possible mode of action, but also of their mode of evolution. ![]() RNA gel blot analysis indicated that the TIR family of genes plays a role in the response to inoculations with BCMV or BCMNV. A survey of wild accessions from Argentina to Mexico showed that this multigene family has expanded significantly during evolution and domestication. In contrast, beans with a recessive allele display simpler and variable haplotypes. Bean cultivars from the Andean or Mesoamerican gene pool that contain the dominant allele share the same haplotypes as revealed by gel blot hybridizations with a TIR probe. We have identified a large cluster of TIR–NBS–LRR sequences associated within this locus, which extends over a distance >425 kb. These clones were obtained from a library of the cultivar “Sprite,” which carries the dominant allele at the I locus. We have generated a high-resolution linkage map around this locus and have aligned it with a physical map constructed with BAC clones. The I locus of the common bean, Phaseolus vulgaris, controls the development of four different phenotypes in response to inoculation with Bean common mosaic virus, Bean common mosaic necrosis virus, several other related potyviruses, and one comovirus.
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