Genetic and Molecular Characterization of the I Locus of Phaseolus vulgaris

Abstract

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. We have generated a high-resolution linkage map around this locus and have aligned it with a physical map constructed with BAC clones. These clones were obtained from a library of the cultivar ''Sprite,'' which carries the dominant allele at the I locus. We have identified a large cluster of TIR-NBS-LRR sequences associated within this locus, which extends over a distance .425 kb. 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. In contrast, beans with a recessive allele display simpler and variable haplotypes. A survey of wild accessions from Argentina to Mexico showed that this multigene family has expanded significantly during evolution and domestication. RNA gel blot analysis indicated that the TIR family of genes plays a role in the response to inoculations with BCMV or BCMNV.

Molecular Analysis of a Large Subtelomeric Nucleotide-Binding-Site–Leucine-Rich-Repeat Family in Two Representative Genotypes of the Major Gene Pools of Phaseolus vulgaris

“…In the common bean genome, most of the well-characterized large R gene clusters are located at the end of linkage groups, suggesting that the location at the end of a linkage group, and by inference a subtelomeric location, is favorable for R gene proliferation. For example, the Co-x, I, and Co-2 loci have been mapped to the ends of LG B1, LG B2, and LG B11, respectively (Geffroy et al , 2008Creusot et al 1999;Vallejos et al 2006). Furthermore, the Co-4 anthracnose resistance locus was also confirmed to be in a subtelomeric position of bean chromosome 8 (Melotto et al 2004).…”

The Blast Resistance Gene Pi37 Encodes a Nucleotide Binding Site–Leucine-Rich Repeat Protein and Is a Member of a Resistance Gene Cluster on Rice Chromosome 1

“…This pattern is reminiscent of the maize rp1 gene complex. Suppression of recombination has also been noted in some other R gene regions, such as Mi (Van Daelen et al 1993), Mla (Wei et al 1999), Pita 2 (Nakamura et al 1997), Pi-CO39(t) (Chauhan et al 2002), Pi5 ( Jeon et al 2003), and I loci (Vallejos et al 2006). Recombination frequency is severely reduced in the hemizygous state (Ozias-Akins et al 1998;Goel et al 2003).…”

“…Recombination frequency is severely reduced in the hemizygous state (Ozias-Akins et al 1998;Goel et al 2003). The Pi37 region lies within a segment introgressed from indica into japonica rice (Yunoki et al 1970), and the dominant mode of inheritance of the four markers (see FPSM4, FSTS4, FSTS1, and FSTS3 in Figure 1A) cosegregating with Pi37 is suggestive that hybrids between Pi37 carriers and noncarriers may well be effectively hemizygous for the introgression segment Vallejos et al 2006). Thus the absence of localized recombination between resistant and susceptible haplotypes may have driven the evolution of diversity at this R gene locus.…”

Stepwise artificial evolution of a plant disease resistance gene

“…In addition, there may be a cost to carrying NB-LRR R genes. The disease-resistant plants may have reduced fitness in competition with susceptible plants (24), there may be a tradeoff with plant growth (25), the plants may exhibit hybrid incompatibility (26), or they may exhibit a partial resistance against some strains of pathogen so that there is a spreading necrosis that kills the plant (27)(28)(29)(30).…”