Subdomains of the Octopine Synthase Upstream Activating Element Direa Cell-Specific Expression Transgenic Tobacco Plants

Previous work has shown that the octopine synthase (ocs) gene encoded by the Agrobacterium tumefaciens Ti-plasmid contains an upstream activating sequence necessary for its expression in plant cells. This sequence is composed of an essential16-bp palindrome and flanking sequences that modulate the level of expression of the ocs promoter in transgenic tobacco calli. In this study, we have used RNA gel blot analysis of RNA extracted from transgenic tobacco plants to show that the octopine synthase gene is not constitutively expressed in all plant tissues and organs. This tissuespecific pattern of expression is delermined, to a large extent, by the 16-bp palindrome. Histochemical analysis, using an ocs-lacZ fusion gene, has indicaled that the 16-bp palindrome directs the expression of the ocs promoter in specific cell types in the leaves, stems, and roots of transgenic tobacco plants. This expression is especially strong in the vascular tissue of the leaves, leaf mesophyll cells, leaf and stem guard cells, and the meristematic regions of the shoots and roots. Sequences surrounding the palindrome in the upstream activating sequence restrict the expression of the ocs promoter to fewer cell types, resulting in a reduced level of expression of P-galactosidase activity in the central vascular tissue of leaves, certain types of leaf trichomes, and the leaf primordia.