Expression of the rolB gene of A. rhizogenes T-DNA triggers root differentiation in transformed plant cells. In order to study the regulation of this morphogenetic gene, the GUS reporter gene was placed under the control of several deleted fragments of the rolB 5' non-coding region: carrot disc transformations and the analysis of transgenic tobacco plants containing these constructions identified the presence of distinct regulatory domains in the rolB promoter. Two regions (located from positions -623 to -471 and from -471 to -341, from the translation start codon) control the level but not the tissue specificity of rolB expression: progressive deletions of the rolB promoter starting from position -1185 to -341, although at different levels, maintained the same pattern of GUS expression-maximal in root meristems and less pronounced in the vascular tissue of aerial organs. Further deletion of 35 bp, from -341 to -306, drastically affected tissue specificity: GUS activity was still clearly detectable in the vascular tissue of the aerial organs while expression in the root meristem was totally suppressed. Analysis of transgenic embryos and seedlings confirmed that distinct promoter domains are responsible for meristematic (root) and differentiated (vascular) expression of rolB. Finally, we present data concerning the effects of plant hormones on the expression of rolB-GUS constructions.