Resistance to Bean Common Mosaic Virus Conferred by the bc-u Gene Does Not Affect Replication or Systemic Spread of the Virus in Common Bean but Ameliorates the Strain-Specific Systemic Necrosis

Resistance to bean common mosaic virus (BCMV) in common bean is governed by four strain-specific, recessive (bc-u, bc-1, bc-2, and bc-3) and one strain non-specific, dominant (I) genes. BCMV exists as a complex of strains defined biologically based on host range and on their molecular characteristics. An isolate of BCMV phylogenetically close to a blackeye cowpea strain of BCMV was found in the rattlepod Crotalaria micans, it was biologically isolated and named BCMV-B71. Here, the role of bc-u in infection by BCMV-B71 in common bean was examined using a panel of bean differential cultivars harboring different combinations of resistance genes. BCMV-B71 was able to multiply in inoculated leaves of all bean cultivars lacking bc-3 and I-genes, inducing necrotic local lesions, and was capable of rapid systemic spread in three cultivars harboring no known resistance genes, concomitantly inducing whole plant necrosis and plant death in 10 to 20 days. In bean cultivars harboring only bc-u, BCMV-B71 was able to establish systemic infection in upper, non-inoculated leaves inducing mosaic and limited vein necrosis. The presence of the bc-u alone apparently did not restrict the systemic spread of BCMV-B71 in common beans but modulated symptom expression by ameliorating the systemic necrotic response. The data obtained provides the first direct evidence of a possible mode of action for the recessive gene bc-u in BCMV infection process.