Characterization of expressed NBS-LRR resistance gene candidates from common bean.

A complex ancestral resistance (R) gene cluster, localized at the end of linkage group B4, and referred to as the B4 R gene cluster, has been previously genetically characterized. The B4 R gene cluster existed prior to the separation of the two major gene pools of cultivated common bean and contains several resistance specificities effective against the fungus Colletotrichum lindemuthianum. In this paper we report the molecular analysis of four expressed resistance gene candidates (RGCs) that map at the B4 R-cluster and co-localize with R-specificities or R-QTLs effective against C. lindemuthianum. These RGCs have been isolated from two genotypes that are representative of the two major gene pools of common bean: the BA8 and BA11 RGCs originating from the Mesoamerican BAT93 genotype, and the JA71 and JA78 RGCs originating from the Andean JaloEEP558 genotype. These RGCs encode NBS-LRR resistance-like proteins that are closely similar to the tomato I2 R-protein. Based upon sequence comparisons and genetic localization, we established that these four bean RGCs belong to two different subfamilies of R-sequences independently of their gene pool of origin. No feature discriminating the four RGCs according to their gene pool of origin has been observed yet. Comparative sequence analyses of the full-length RGCs and their flanking genomic sequences confirmed the ancestral origin of the B4 R-cluster.