Ti Plasmid and Chromosomally Encoded Two-Component Systems Important in Plant Cell Transformation by Agrobacterium Species

Joe Don Heath; Eugene W. Nester; Trevor C. Charles

doi:10.1128/9781555818319.ch23

Chapter 23 : Ti Plasmid and Chromosomally Encoded Two-Component Systems Important in Plant Cell Transformation by Agrobacterium Species

Authors: Joe Don Heath¹, Eugene W. Nester¹, Trevor C. Charles²

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Affiliations: 1: Department of Microbiology, University of Washington 98195; 2: Department of Natural Resource Sciences, McGill University, Macdonald Campus, Ste. Anne de Bellevue, Quebec, Canada H9X 3V9

Content Type: Monograph

Publication Year: 1995

Category: Microbial Genetics and Molecular Biology

Book DOI: 10.1128/9781555818319

Chapter DOI: 10.1128/9781555818319.ch23

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Abstract:

Tumorigenic Agrobacterium strains incite the formation of crown gall tumors at wound sites on a wide variety of dicotyledonous plants as well as some monocotyledonous species. The second region of the Ti plasmid essential for tumor formation is the virulence (vir) region. The plant signals are recognized and transduced by the products of two vir genes, virA and virG. These two genes are members of the highly conserved class of two-component sensory transduction systems, virA coding for the sensor protein and virG for the response regulator. In addition to the Ti plasmid-encoded virulence genes, several chromosomal loci are important for tumor formation. Thus, genes on both the chromosome and the Ti plasmid are required for tumorigenesis, and two-component regulatory systems that are involved in virulence are located on each replicon. The VirA/VirG system of Agrobacterium is one of the few two-component systems in which the signal compounds are known. For two reasons, this class of constitutively vir-expressing virA mutant may be difficult to isolate. First, according to the model, this mutant VirA would have to harbor mutations that bypass both the Off and the Standby modes to become constitutively activated. Second, a mutant strain that is constitutively expressing its vir genes is likely to be less fit, and therefore revertants would arise at a high frequency. The pleiotropic nature of the phenotype suggests that any relation to virulence may be indirect.

Citation: Heath J, Nester E, Charles T. 1995. Ti Plasmid and Chromosomally Encoded Two-Component Systems Important in Plant Cell Transformation by Agrobacterium Species, p 367-385. In Hoch J, Silhavy T (ed), Two-Component Signal Transduction. ASM Press, Washington, DC. doi: 10.1128/9781555818319.ch23

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Ti Plasmid and Chromosomally Encoded Two-Component Systems Important in Plant Cell Transformation by Agrobacterium Species

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FIGURE 1

Overview of crown gall tumorigenesis. Schematic representation of the steps involved in the interaction of Agrobacterium with its plant host. (1) In response to plant wound-released sugars and a variety of other substituents of plant wound exudate, Agrobacterium moves toward the wound site and attaches to a plant cell. (2) In the acidic environment of the wound, Agrobacterium induces expression of its vir genes via the VirA/VirG regulatory system. (3) Transcription and translation of the vir genes lead to (4) T-DNA processing and (5) T-DNA transfer to the plant cell. (6) The T-DNA and bound vir gene products are targeted to the plant nucleus where (7) the T-DNA is integrated into the genome and the T-DNA oncogenes are expressed. The expression of the T-DNA encoded oncogenes leads to axenic tumor proliferation and (8) the production of opines that are used as carbon and nitrogen sources by the infecting Agrobacterium.

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FIGURE 1

/content/10.1128/9781555818319.chap23.fig23-2

FIGURE 2

Functional domains of the sensor molecule VirA. The VirA protein can be divided into three major functional domains with a body plan of ITR, according to the nomenclature of Parkinson and Kofoid (1992) (the input domain, the transmitter domain, and the receiver domain). The input domain is further divided into the periplasmic domain, harboring the ChvE-responsive region, and the linker domain, which senses phenolic compounds and acidity. The transmitter (or kinase) domain contains the conserved histidine that is phosphorylated. The receiver domain is similar in sequence to the VirG protein, contains a conserved aspartate residue, and may play a regulatory role.

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FIGURE 2

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FIGURE 3

Models for vir gene induction in Agrobacterium. (A) Model for the sensing and transmission of signals by VirA in conjunction with ChvE. The model is consistent with phenolic compounds detected directly by VirA or through the action of a phenolic binding protein. For simplicity, the model is illustrated with VirA as the direct phenolic sensor. (B) Model for the activation of VirG by VirA and activation of vir gene transcription by activated VirG.

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FIGURE 3

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