Role of T-Region Borders in Agrobacterium Host Range. François Paulus. Department of Molecular Plant Pathology, Institute of Plant Molecular Biology, Strasbourg 67084, France. Brigitte Huss, Bruno Tinland, Anette Herrmann, Jean Canaday, and Léon Otten. Department of Molecular Plant Pathology, Institute of Plant Molecular Biology, Strasbourg 67084, France.. MPMI 4:163-172. Accepted 3 December 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991.

The limited host range AB3 strain of Agrobacterium tumefaciens induces tumors by transferring two T-regions, TA and TB. TA is a deleted version of the well-known biotype I octopine TL-region that lacks the iaa and ipt genes, but carries an intact oncogene, gene 6b, and typical left and right border sequences. TB carries two iaa genes that together code for the synthesis of indoleacetic acid. Gene 6b and the iaa gene act synergistically when transferred in a coinoculation experiment. The TA-region of the limited host range isolate Ag57 is related to the TA-region of AB3, but differs from it at several positions. The most significant difference is the absence of the right border region. In spite of this, Ag57 and the exconjugant strain C58C9(pTiAg57) induce normal tumors on Nicotiana rustica and Vitis vinifera. Various experiments indicate that gene 6b of the Ag57 TA-region is active and transferred in spite of the absence of the right border. On N. tabacum, C58C9(pTiAg57) is nononcogenic but becomes oncogenic when the pTiAg57 TA-region is restored by the right TA border sequence of pTiAB3. Thus, the right TA border sequence of the biotype III limited host range strains is required for tumor induction on some hosts, but not on others.