Agrobacterium Tumefaciens - Introduction and Working

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Agrobacterium tumefaciens

Agrobacterium tumefaciens (updated scientific name: Rhizobium radiobacter) is a gram-negative rod-shaped bacteria closely related to nitrogen-fixing bacteria which dwell at root nodules in legumes and unlike most other soil-dwelling bacteria, it infects the roots of plants to cause Crown Gall Disease of the family Rhizobiaceae, which includes the nitrogen fixing legume symbionts. Unlike the nitrogen fixing symbionts, tumor producing Agrobacterium are pathogenic and do not benefit the plant. The wide variety of plants affected by Agrobacterium makes it of great concern to the agriculture industry. Various remediation methods, including utilization of a strain of closely related bacteria controls and limits its damage, but it is also useful as a genetic engineering tool in plants. It is famous for taking advantage of its host by injecting DNA derived from its Ti (tumor inducing) plasmid into its host, causing the plant to create galls which excrete opines that the bacteria use as an energy source. A. tumefaciens have emerged as an important molecular tool for manipulating plants and creating genetically modified crops for research and agriculture.

Genome structure

A. tumefaciens is one of the few bacterium that has both a linear and a circular chromosome. Its genome has a total of 5.7 million base-pairs, with 2.8 million residing on its circular chromosome and 2.1 million residing on its linear chromosome and although most of the genes essential for its survival are located on the circular chromosome; through evolution some essential genes have migrated to the linear chromosome. Based on sequence analysis, it was determined that the linear chromosome was derived from a plasmid that was transformed into the bacteria.

The ends of the linear chromosomes are protected by a telomere that forms a covalently closed hairpin, like in other bacteria which contain a linear chromosome. In addition to the two chromosomes, strain C58 also contains two plasmids, pTiC58 (generically called Ti) and pAtC58 (also called the "cryptic plasmid"). pTiC58 contains genes necessary for its pathogenicity against plants, including the T-DNA(Transfer DNA) which is injected into the plant genome at a semi-random location and causes it to produce opines, along with accessory proteins which helps the T-DNA enter and transform the plant cell into a tumour cell.

Pathology (Method of infection)

A. tumefaciens infects plants usually through an open wound. It seeks out phenolic compounds which spills from a plant wound and chemo-tactically moves toward its source. Once it enters its plant host, it injects a section of its DNA called the T-DNA which is derived from its Ti (tumor inducing) plasmid into its host. The T-DNA is then integrated into the plant's genome, and has two effects on the plant host. The T-DNA first directs the plant cells to make auxins and cytokinins, which causes the cells to become irregularly shaped and form a visible tumor called a gall. The T-DNA then directs the plant cell to start making opines (usually nopaline or agropine) which A. tumefaciens use as an energy source

Although it is clear that A. tumefaciens integrates its own DNA into that of its hosts and induces them to make opine-producing tumor cells, the details of this process are just being unraveled. When the T-DNA is copied from the Ti plasmid a protein called VirD2 complexes with it to protect it from host cell nucleases and help its transport into the nucleus where it can act. In order for the T-DNA to cross from the bacteria into the plant cell, a protein called VirE2 forms a channel between the membrane of the plant and bacterial cell. Another protein, VirE3, has been discovered to be injected into the plant cell and it has been shown that VirE3 is translocated into the plant nucleus where it interacts with native transcription factors to cause a deregulation of plant cell proliferation and cause the growth of galls. The...
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