Characterization of a new Agrobacterium tumefaciens strain from alfalfa

Agrobacterium tumefaciens strain 1D1609 is reported here as the first field isolate from alfalfa (Medicago sativa L.). Unlike well-characterized A. tumefaciens strains such as C58 and Ach5, strain 1D1609 is highly virulent on alfalfa and has a distinctive host range. Interestingly, strain 1D1609 is naturally resistant to kanamycin and spectinomycin. The Ti plasmid in strain 1D1609 is an octopine-type; thus, tumors formed by strain 1D1609 synthesize octopine, which is utilized by the bacterium as a sole carbon source. Reciprocal exchange of Ti plasmids between strains 1D1609 and C58 showed that both chromosomal and Ti plasmid genes in strain 1D1609 contribute specifically to tumor formation on alfalfa. In addition, the nondormant CUF101 alfalfa cultivar from which strain 1D1609 was isolated was significantly more susceptible to all Agrobacterium strains tested than was the dormant Agate cultivar.