STRAIN SPECIFICITY IN TRANSFORMATION OF ALFALFA BY AGROBACTERIUM-TUMEFACIENS

Production of transgenic alfalfa plants by Agrobacterium-mediated tran sformation requires Agrobacterium infection and regeneration from tiss ue culture. Variation in alfalfa (Medicago sativa L.) germplasm for re sistance to oncogenic and disarmed strains of A. tumefaciens (Smith & Townsend) Conn was tested in plant populations representing the nine d istinct sources of alfalfa germplasm introduced into North America and used to develop modern varieties. For each of the virulent strains th ere was a positive correlation (p=0.05) of resistance to tumorigenesis with the trait for fall dormancy. There was also a significant correl ation between plants selected for ineffective nodulation and resistanc e to tumorigenesis suggesting that the genetic loci required for succe ssful symbiosis are also involved in tumorigenesis. Tissue explants of seedlings from the nine diversity groups were tested for transformati on by three disarmed strains containing a plasmid with the scorable ma rker beta-glucuronidase. The strong correlation between dormancy and r esistance to oncogenic strains was not observed with disarmed strains. However, there was a strong germplasm-strain interaction or transform ation and embryogenesis in a highly embryogenic genotype. Thus, transf ormation at the whole plant level is germplasm dependent while in tiss ue culture the bacterial strain used is the critical variable for succ essful transformation.