Reversion of Aberrant Plants Transformed with Agrobacterium rhizogenes Is Associated with the Transcriptional Inactivation of the TL-DNA Genes

Transgenic plants harboring the left transfer DNA (TL-DNA) of the root inducing plasmid of Agrobacterium rhizogenes show many developmental abnormalities. We observed frequent appearance of normal looking lateral (revertant) shoots from such aberrant plants. Unlike aberrant shoots of the plant, revertant shoots exhibited a very high growth rate and set viable seeds. Sexual and vegetative reproduction studies showed inheritance of the revertant phenotype. Southern hybridization experiments demonstrated that the T-DNA pattern was identical in aberrant and revertant shoots, indicating that the revertant phenotype was not due to deletion or rearrangement of the T-DNA genes. Specific T-DNA transcripts were not expressed in revertant shoots. Thus, the revertant phenotype appears to result from the transcriptional inactivation of T-DNA genes. We propose that similar events in the past may have mediated horizontal acquisition of TL-DNA genes by ancestors of the genus Nicotiana, which are still found as silent endogenous T-DNA in present day untransformed Nicotiana species.