TISSUE-SPECIFIC EXPRESSION OF THE ROLA GENE MEDIATES MORPHOLOGICAL-CHANGES IN TRANSGENIC TOBACCO

The spatial and temporal activity of the entire and individual promote r domains of the rolA gene of Agrobacterium rhizogenes was investigate d and correlated with the distinctive features of the phenotypes of tr ansgenic tobacco plants. The GUS assay was performed in the presence o f an oxidative catalyst during the development of transgenic plants ex pressing chimeric genes containing the beta-glucuronidase coding seque nce under the control of the different promoter domains. In situ hybri dization was also used an transgenic plants harbouring rolA under the control of the entire or deleted promoter. This paper demonstrates for the first time that the entire rolA promoter, composed of domains, A, B and C, is silent in seeds, then activated at the onset of germinati on in the cotyledons and in the elongation zone of the radicle and is finally expressed throughout the vegetative and floral phases. Domains B + C, which were sufficient to induce wrinkled leaves and short inte rnodes, were active in all the stem tissues, but only in the companion cells of the phloem strands of the leaves. Domain C, which specified a dwarf phenotype with normal leaves, was weakly expressed in the stem vascular bundles and in the leaf internal phloem. These results indic ate that the vascular bundles are the primary targets for the generati on of the short internode phenotype. Furthermore, the local expression of rolA in the stem vascular bundles induced a size reduction of the surrounding parenchyma cells, suggesting the existence of some diffusi ble factor(s) associated with the expression of the rolA gene.