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SIGNAL TRANSDUCTION AND HORMONE ACTION

The orf13 T-DNA Gene of Agrobacterium rhizogenes Confers Meristematic Competence to Differentiated Cells¹

Institute of Plant Sciences, University of Bern, CH–3013 Bern, Switzerland (P.A.S., P.K., I.N., M.B., C.K.); Institut des Sciences Végétales, Centre National de la Recherche Scientifique, 91198 Gif-sur-Yvette, France (A.D.M.); and Friedrich Miescher Institut, CH-4058 Basel, Switzerland (C.F.)

Plant infections by the soil bacterium Agrobacterium rhizogenes result in neoplastic disease with the formation of hairy roots at the site of infection. Expression of a set of oncogenes residing on the stably integrated T-DNA is responsible for the disease symptoms. Besides the rol (root locus) genes, which are essential for the formation of hairy roots, the open reading frame orf13 mediates cytokinin-like effects, suggesting an interaction with hormone signaling pathways. Here we show that ORF13 induced ectopic expression of KNOX (KNOTTED1-like homeobox) class transcription factors, as well as of several genes involved in cell cycle control in tomato (Lycopersicon esculentum). ORF13 has a retinoblastoma (RB)-binding motif and interacted with maize (Zea mays) RB in vitro, whereas ORF13, bearing a point mutation in the RB-binding motif (ORF13^*), did not. Increased cell divisions in the vegetative shoot apical meristem and accelerated formation of leaf primordia were observed in plants expressing orf13, whereas the expression of orf13^* had no influence on cell division rates in the shoot apical meristem, suggesting a role of RB in the regulation of the cell cycle in meristematic tissues. On the other hand, ectopic expression of LeT6 was not dependent on a functional RB-binding motif. Hormone homeostasis was only altered in explants of leaves, whereas in the root no effects were observed. We suggest that ORF13 confers meristematic competence to cells infected by A. rhizogenes by inducing the expression of KNOX genes and promotes the transition of infected cells from the G1 to the S phase by binding to RB.

² Present address: Physiologie Végétale University of Neuchâtel, Rue Emile Argand 13, CH–2007 Neuchâtel, Switzerland.

³ Present address: Advolis Life Science Concepts, Oetlingerstrasse 10, CH–4057 Basel, Switzerland.

⁴ Present address: Novartis Pharma AG, CH–4002 Basel, Switzerland.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.040899.

^* Corresponding author; e-mail cris.kuhlemeier{at}ips.unibe.ch; fax 41–31–631–49–42.

Received February 12, 2004; returned for revision April 26, 2004; accepted May 2, 2004.

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