Transformed cell clones as a tool to study T-DNA integration mediated by Agrobacterium tumefaciens

A large number of tobacco SR1 cell clones transformed by the wild-type Agrobacterium C58 have been analysed for the presence of screenable markers such as tumour morphology, opine synthesis and hormone dependence. Distinct phenotypic classes were observed depending upon whether the cell clones were isolated from primary tumours or were obtained via cocultivation of protoplasts. These classes of tobacco SR1-C58 transformants appear to arise from errors in the Ti plasmid (T-DNA) transfer and integration mechanism itself rather than from subsequent T-DNA rearrangements, since 900 subclones, obtained by recloning a wild-type SR1-C58-transformed cell clone, yielded no variation in the phenotypes. A detailed genomic T-DNA analysis showed the presence of characteristic, abnormally short T-DNAs in the teratoma-forming, Acs- class and also in the Nos- class. The abnormal right border in two Nos- clones ends close to a sequence that resembles the normal T-DNA terminus and lies adjacent to the nos promoter, suggesting that this sequence could have functioned as a recognition site directing these particular T-DNA transfers. On the basis of the phenotypic and genomic blotting data it is clear that the short T-DNAs are characteristic of the cocultivation method. Other phenomena causing phenotypic variation, such as the loss of the T-DNA, and the gradual repression of T-DNA gene expression by methylation, are the main causes of aberrations in primary tumours. Moreover, the physical data suggest that early in the transformation cycle of Agrobacterium a replication step of a preselected T-DNA occurs before integration into the plant genome.