The T-DNA ORF8 of the cucumopine-type Agrobacterium rhizogenes Ri plasmid is involved in auxin response in transgenic tobacco

In A. rhizogenes, various Ri plasmids are found that show some differences in terms of genetic organization as compared to the reference plasmid pRiA4. On this plasmid as well on the pRi2659 and pRi8196, the T-DNA geries rolA, rolB, and rolC (ORFs 10, 11, and 12) appear to sufice to induce root formation in most assayed plant species. However, the most conserved sequences of the A. rhizogenes T-DNAs are not of the rol genes but those defining ORF8, 13 and 14. Sequence comparisons have suggested that the gene product of ORF8 might be involved in mediating auxin-related effects since it has certain significant similarities with: (i) the Tmsl protein encoded by tmsl gene located on the A. tumefaciens T-DNA; (ii) the A. rhizogenes rolB products and (iii) the putative protein encoded by A. tumefaciens T-DNA gene 5. Hormone responses were analyzed using transgenic tobacco plants expressing the ORF8 of pRi2659 under the control of the cauliflower mosaic virus (CaMV) 35S promoter (35S-ORF8). Transgenic seedlings expressing 35S-ORF8 appeared to be resistant to auxin concentrations that inhibited germination and growth of wild-type seedlings. Cotyledons of transgenic seedlings exhibited a thin and lanceolated shape but adult tobacco plants expressing ORF8 were not affected in then, growth pattern and morphology. A fusion of the ORF8 promoter with uidA, encoding beta-glucuronidase (GUS), revealed that ORF8 transcription is auxin-induced. Moreover, pRi2659 ORF8 promoter sequences harbor motif's corresponding to auxin-related genes from plants. (C) 2003 Elsevier Ireland Ltd. All rights reserved.