Ti- and cryptic-plasmid-borne virulence of wild-type Agrobacterium tumefaciens strain CNI5 isolated from chrysanthemum (Dendranthema grandiflora Tzvelev)

Abstract.

The genetic similarity between pTiBo542 and pTiCNI5, which are harbored, respectively by the supervirulent Agrobacterium tumefaciens strain A281 and by the highly tumorigenic wild-type strain CNI5 isolated from chrysanthemum was investigated by Southern hybridization. pTiCNI5 and pTiBo542 exhibited highly similar hybridization patterns in both BamHI- and EcoRI-digested plasmids by using four vir-region-specific probes, whereas similarity in these two plasmids was not observed by probing with five TL-DNA-specific probes. The characteristics related to tumor formation of cryptic plasmids carried by strain CNI5 were investigated by using single-plasmid-cured derivatives. pTiCNI5-cured derivatives predictably failed to utilize agropine and mannopine and failed to induce tumors on chrysanthemum and tobacco leaf explants, while pAtCNI5a-, pAtCNI5c- and pAtCNI5d-cured derivatives could utilize these opines similar to the parent strain CNI5. Interestingly, pAtCNI5c- and pAtCNI5d-cured derivatives showed low tumorigenicity in comparison with strain CNI5 or with the pAtCNI5a-cured derivative. These results suggest that the highly virulent phenotype of strain CNI5 may be due to one or more vir genes, which exhibit cartographic similarity to those of pTiBo542. The results also suggest that the gene(s) related to tumor formation of strain CNI5 may exist not only on pTiCNI5 but also on cryptic plasmids pAtCNI5c and pAtCNI5d.