Abstract
Bean common mosaic virus (BCMV) is an important pathogen affecting common bean
and other legumes. BCMV is an aphid-transmitted virus, which can also be seed-transmissible in common bean with efficiencies of up to 80%, depending on virus strain and bean cultivar. The virus belongs to the genus Potyvirus (family Potyviridae), having an approximately 10-kb single-stranded, positive-sense RNA genome. BCMV resistance in common bean is governed by one dominant (I) gene and four recessive (bc-u, bc-1, bc-2, and bc-3) genes which are deployed by breeders in different combinations to protect commercial bean cultivars against the virus. The dominant I gene confers extreme resistance or immunity against all strains of BCMV when the temperature stays below 30°C, and variable types of local and systemic necrosis when temperature exceeds 30°C. The bc-3 gene was identified as an eIF4E translation initiation factor gene mediating resistance through disruption of the interaction between this protein and the VPg protein of the virus. The mode of action of bc-1 and bc-2 in expression of BCMV resistance was unknown until this work. To investigate the possible role of both bc-1 and bc-2 genes in replication, cell-to-cell, and long distance movement of BCMV in P. vulgaris, we tested virus spread of eight BCMV isolates representing pathogroups I, IV, VI, VII, and VIII, in a set of bean differentials expressing different combinations of six resistance alleles including bc-u, bc-1, bc-12, bc-2, bc-22, and bc-3. The data suggest that bc-1 and bc-2 recessive resistance genes have no effect on the replication and cell-to-cell movement of BCMV, but affect systemic spread of BCMV in common bean. The BCMV resistance conferred by bc-1 and bc-2 and affecting systemic spread was found only partially effective when these two genes were expressed singly. The efficiency of the restriction of the systemic spread of the virus was greatly enhanced when the alleles of bc-1 and bc-2 genes were combined together.
Beside common bean, there are other crop legumes that are susceptible to BCMV, such as peanut, soybean, and azuki bean. In this thesis, I described a new strain of BCMV from lima bean with distinct biological and molecular characteristics found in Honolulu, HI. This new BCMV strain, BCMV-A1, was able to partially overcome resistance to the virus conferred by bc-1 and bc-2 alleles in common bean, establishing an asymptomatic systemic infection in some common bean cultivars. Later, two BCMV isolates were found in commercial common bean samples in Idaho exhibiting 99% identity to BCMV-A1 in partial genome sequence. This new BCMV strain from lima bean, BCMV-A1, may present a threat for the common bean production.