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Odyssey of Agrobacterium T-DNA.

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Ziemienowicz A.

2001

vol. 48

issue 3

623-635

Agrobacterium tumefaciens, a plant pathogen, is characterized by the unique feature of interkingdom DNA transfer. This soil bacterium is able to transfer a fragment of its DNA, called T-DNA (transferred DNA), to the plant cell where T-DNA is integrated into the plant genome leading to "genetic colonization" of the host. The fate of T-DNA, its processing, transfer and integration, resembles the journey of Odysseus, although our hero returns from its long trip in a slightly modified form.The soil bacterium, Agrobacterium tumefaciens, is a plant pathogen responsible for tumor induction on dicotyledonous plants due to its ability to transfer DNA to the plant cell (reviewed in: de la Cruz & Lanka, 1998; Gelvin, 2000; Hansen & Chilton, 1999; Lartey & Citovsky, 1997; Rossi et al., 1998; Zupan & Zambryski, 1997). In biotechnology this ability is widely used for plant transformation. During tumor induction Agrobacterium attaches to plant cells and then transfers part of its DNA to some of these cells. The transferred DNA (T-DNA) which resides on a large Ti (tumor inducing) plasmid, is processed within the bacterium and is exported to the plant where it becomes integrated into the plant genome (reviewed in: Sheng & Citovsky, 1996; Tinland & Hohn, 1995; Tinland, 1996). Proteins encoded by the virulence (vir) region of the Ti plasmid regulate T-DNA processing and transfer. Phenolic compounds derived from a wounded plant cell wall induce expression of the vir region genes. Virulence proteins recognize 25 bp imperfect direct repeats (border sequences) that define the T-DNA. In the presence of VirD1 protein, VirD2 cleaves the border sequence in a site- and strand-specific manner and subsequently becomes covalently attached to the 5' end of the nicked DNA. The nicked DNA is then displaced from the plasmid producing single-stranded T-DNA. The T-DNA-VirD2 complex and the VirE2 protein are believed to be transferred to the plant through a pilus-like structure containing VirB and VirD4 proteins. In the plant cell, T-DNA becomes coated with the single-stranded DNA-binding protein, VirE2. The T-DNA-protein complex is imported into the nucleus where the T-DNA is integrated into the nuclear genome. Expression of genes located on T-DNA leads to the formation of proteins involved in the production of auxins and cytokinins. These plant hormones cause the tumorous phenotype that is characterized by the ability of the plant cells to proliferate limitlessly and autonomously even in the absence of added phytohormones. Crown gall tumors are characterized by the production of opines (amino-acid derivatives). The biosynthesis of opines is catalyzed by opine synthases, which are encoded by the T-DNA. Opines formed in the tumors can be metabolized by the tumorigenic agrobacteria, but not by most of the other soil organisms. Thus, Agrobacterium creates for itself a favorable niche by genetic modification of plant cells, a process called "genetic colonization". All stages of this colonization, including chemotaxis, attachment, induction of virulence region, processing of T-DNA, T-DNA transfer, T-DNA integration, expression of T-DNA genes and changes in the plant phenotype, will be discussed in the following chapters. This will be an odyssey of T-DNA that leaves the Agrobacterium cell in the form of nucleic acid and returns from its journey in the form of opines, derivatives of amino acids (Fig. 1).

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1 Acta Biochimica Polonica

1 2001

1 Ziemienowicz A.

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Odyssey of Agrobacterium T-DNA.