New plant growth-modifying properties of the Agrobacterium T-6b oncogene revealed by the use of a dexamethasone-inducible promoter.

Agrobacterium 6b oncogenes induce tumours on Nicotiana glauca and enations and associated modifications in transgenic N. tabacum plants. 2x35S-AB-6b tobacco rootstocks produced a graft-transmissible factor that induced enations in wild-type scions; the nature of this enation factor remains to be identified. Here, we report on the properties of tobacco plants carrying a dexamethasone-inducible T-6b gene (dex-T-6b). Induction with dex led to complex growth modifications, many of which have not been reported previously. Modifications were only found in growing tissues; mature tissues remained unaffected. Growth could be either stimulated or inhibited. Dex induction of young plants led to morphogenetic gradients that included enations, tubular leaves and fragmented leaf primordia. Root elongation was increased or slowed down, while radial root growth was strongly enhanced. Additional cell divisions were found in the root pericycle and vasculature. Enation factor import from mature tissues did not have the same effects on growing tissues as local T-6b synthesis: normal scions grafted on induced dex-T-6b rootstocks formed enations, whereas local dex-T-6b induction at the shoot apex led to numerous dark-green spots on the abaxial side of the leaves. In leaf patch assays, the 23-kDa T-6b protein was found to move through leaves and to enter the vascular system. This and the fact that rootstocks of spontaneous tobacco enation mutants did not modify wild-type scions contrary to 6b plants indicate that the 6b protein might be the enation factor.