EXPRESSION IN DIFFERENT POPULATIONS OF CELLS OF THE ROOT-MERISTEM IS CONTROLLED BY DIFFERENT DOMAINS OF THE ROLB PROMOTER

Selective gene expression in different populations of cells of the roo t apex of transgenic tobacco could be evidenced by means of GUS constr ucts with deletions of the rolB promoter and fusions with the CaMV 35S minimal promoter. Five regulatory regions have been broadly identifie d in the rolB 5' noncoding region. The presence of all five domains (A to E) directs gene expression in the root cap, in the protoderm and i n the different tissues within the root meristematic region: the derma tocalyptrogen, the cortex and the vascular cylinder. Deletion of domai n A (-623 to -471) selectively suppresses expression in non-meristemat ic cells, i.e. the root cap and the protoderm. Deletion of either doma in B (-341 to -306) or E (80 bp around the TATA box) causes loss of ex pression in all cells of the.root apex: constructs C + D + E, B + C D, B + C are inactive. Domain D (70 bp around the CAAT box) is necessa ry for gene expression in the dermatogen and in meristematic cells of the cortex but not in the innermost meristematic layer: construct B C + E is active only in vascular meristematic cells. Domain C (-216 to -158) seems to have a double regulatory role as construct B + E is no longer expressed in meristematic cells of the vascular cylinder but i s very active in the protoderm. Constructs allowing gene expression in meristematic cells are also inducible by auxin in leaf protoplasts, w hile activation of the regulatory elements necessary for gene expressi on in the non-meristematic cells of the root apex do not seem to depen d upon the hormone. The connection between auxin induction and meriste matic expression is discussed.