Structure and expression of Ri T-DNA from Agrobacterium rhizogenes in Nicotiana tabacum. Organ and phenotypic specificity.

The incorporation of transferred DNA (T-DNA) from the Ri plasmid of Agrobacterium rhizogenes into the chromosomal DNA of higher plants is correlated with the appearance of a complex phenotype. The transformed genotype and phenotype undergo Mendelian inheritance. Through studies of Ri T-DNA content and transcription in Nicotiana tabacum, we have delineated a particular part of this foreign DNA as the likely source of the transformed phenotype. One inducible/repressible aspect of the transformed phenotype is termed T' and is correlated with the presence of a supplementary Ri T-DNA-encoded transcript. This transcript is found specifically in leaves, whereas most of the other T-DNA transcripts are more abundant in roots. The T' phenotype does not appear to be due to structural changes in the Ri T-DNA. It is inherited in a dominant Mendelian fashion. We propose that the T' phenotype is caused by heritable changes in the regulation of Ri T-DNA expression. We comment on the potential of this system as a model for studying eukaryotic gene expression.