| Bean dwarf mosaic virus (BDMV) is a single-stranded DNA virus (Genus Begomovirus, Family Geminiviridae ) that infects common bean (Phaseolus vulgaris L.) and causes stunted growth and mosaic symptoms. BDMV shows differential pathogenicity in common bean, infecting beans of the Andean (A) but not the Middle American (MA) gene pool. In the MA cultivar (cv.) Othello, BDMV resistance involves a blockage of long-distance movement and a vascular hypersensitive response (HR). Here, it was established that germplasm representing the four MA races (i.e., Durango, Guatemala, Jalisco, and Mesoamerica) and the parents of cv. Othello were resistant to BDMV, but that HR was not correlated with resistance. Crosses were made between the BDMV-resistant cv. Othello and the BDMV-susceptible cv. Topcrop, and segregation data of F1, F2, and F 3 progeny indicated that a single dominant allele, Bdm, conferred BDMV resistance.;To identify genes involved in BDMV resistance in cv. Othello, two approaches were used: (i) PCR-based cDNA subtraction and (ii) reverse transcription (RT)-PCR with degenerate primers derived from conserved sequences of known plant resistance (R) genes. One gene (Fu) had a unique double Toll/interleukin-1 receptor (TIR) motif, and a co-agroinfiltration assay indicated that this gene mediated BDMV resistance in the BDMV-susceptible cv. Topcrop. However, transgenic Nicotiana benthamiana plants expressing Fu were not resistant to BDMV; interestingly, these Fu transgenic plants developed a severe symptom phenotype when infected by the potyvirus Bean yellow mosaic virus (BYMV). Another gene, RT4-4, obtained via the RT-PCR and degenerate primer approach, was a TIR-nucleotide binding site-leucine rich repeat-type R gene. Transgenic N. benthamiana plants expressing RT4-4 were not resistant to BDMV, but developed a systemic necrosis response to all the Cucumber mosaic virus (CMV) isolates tested, except for a bean-infecting strain. Agrobacterium-mediated transient expression experiments revealed that RT4-4 recognized the CMV 2a protein. Thus, RT4-4 is a common bean CMV R gene that acts in a gene-for-gene manner. |
