Chapter 15 : Diverse Roles of Agrobacterium Ti Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors

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This chapter describes the various steps in plant colonization, including binding of the bacteria to host cells, recognition of diffusible host-released chemical signals, and processing and transfer of oncogenic DNA (T-DNA). It includes a discussion on the utilization of tumor-released compounds called opines and on the horizontal transfer of the Ti plasmid. Species of have been classified primarily by their phytopathogenic properties, which are largely due to differences in plasmid content. With respect to chromosomally encoded properties, and are well-defined separate taxa whereas , , and are not. The most thoroughly studied strains contain either octopine-type or nopaline-type Ti plasmids. Virtually all genes found on Ti plasmids play direct or indirect roles in some aspect of crown gall tumorigenesis or tumor colonization. VirD4 is absolutely required for transfer and pilus formation. Some members of the regulon are not essential for tumorigenesis and may play other roles in pathogenesis. Since Ti plasmids encode both TraI and TraR, each conjugal donor takes a census of other donors rather than of recipients. It is a challenge even to speculate about the adaptive value of such a system. The observation that two opines regulate conjugation in opposite ways is equally perplexing.

Citation: Winans S, Kalogeraki V, Jafri S, Akakura R, Xia Q. 1999. Diverse Roles of Agrobacterium Ti Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors, p 289-307. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch15

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Image of Figure 1
Figure 1

Genetic map of an octopine-type Ti plasmid, pTiΔ6. TL-DNA and TR-DNA, two DNA fragments transferred to plant nuclei; ,and . VirG-regulated operons that are essential for efficient plant tumorigenesis; ,and VirG-regulated operons that are not required for tumorigenesis; and , operons required for Ti plasmid conjugal transfer; ,and ,regulatory genes that control the expression of the and operons; and , genes required for catabolism of octopine; , an ATP-binding cassette-type transporter for an unknown substrate; , , , and,operons required for catabolism of mannopine, agropine, agropinic acid, and mannopinic acid, respectively; ,gene required for vegetative plasmid replication.

Citation: Winans S, Kalogeraki V, Jafri S, Akakura R, Xia Q. 1999. Diverse Roles of Agrobacterium Ti Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors, p 289-307. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch15
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Image of Figure 2
Figure 2

Two-way exchange of chemical signals between agrobacteria and host plants. Host-released chemical signals (including phenolic compounds, monosaccharides, and acidity) are perceived by the VirA to VirG proteins, which transcribe promoters. T-DNA is nicked by VirD2, and single-stranded, linear T-strands are formed by strand displacement. T-strands and VirE2 separately exit the bacteria via a conjugal pore encoded by the operon, and they form a T-complex within the plant cytoplasm. T-complexes are transported into the nucleoplasm via two classes of host transport proteins, and the T-DNA is integrated into genomic DNA. For the sake of clarity, the relative orientations of genes and T-DNA have been inverted.

Citation: Winans S, Kalogeraki V, Jafri S, Akakura R, Xia Q. 1999. Diverse Roles of Agrobacterium Ti Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors, p 289-307. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch15
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Image of Figure 3
Figure 3

Cell density-dependent expression of the Ti plasmid regulon. The gene regulator TraR is synthesized in response to host-released opines. At high cell densities, TraR binds the pheromone 3-oxo-C-HSL and activates the transcription of other and promoters. TraR activity is antagonized by TraM and by TrlR. is part of the regulon, resulting in negative autoregulation, while is induced in response to host-released mannopine. The role of MocR in the regulation of and is speculative.

Citation: Winans S, Kalogeraki V, Jafri S, Akakura R, Xia Q. 1999. Diverse Roles of Agrobacterium Ti Plasmid-Borne Genes in the Formation and Colonization of Plant Tumors, p 289-307. In Kaper J, Hacker J (ed), Pathogenicity Islands and Other Mobile Virulence Elements. ASM Press, Washington, DC. doi: 10.1128/9781555818173.ch15
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