Agrobacterium

Agrobacterium is a genus of Gram-negative bacteria that uses horizontal gene transfer to cause tumors in plants. Agrobacterium tumefaciens is the most commonly studied species in this genus. Agrobacterium is well known for its ability to transfer DNA between itself and plants, and for this reason it has become an important tool for plant improvement by genetic engineering.

The Agrobacterium genus is quite heterogeneous. Recent taxonomic studies have reclassified all of the Agrobacterium species in to new genera, such as Ruegeria, Pseudorhodobacter and Stappia, but most species have been reclassified as Rhizobium species.

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Plant pathogen

A. tumefaciens causes crown-gall disease in plants. The disease is characterised by a tumour like growth or gall on the infected plant, often at the junction between the root and the shoot. Tumors are incited by the conjugative transfer of a DNA segment (T-DNA) from the bacterial tumour-inducing (Ti) plasmid. The closely related species, A. rhizogenes, induces root tumors, and carries the distinct Ri (root-inducing) plasmid. Although the taxonomy of Agrobacterium is currently under revision it can be generalised that 3 biovars exist within the genus, A. tumefaciens or biovar 1, A. rhizogenes or biovar 2, and A. vitis or biovar 3. Strains within biovars 1 and 2 are known to be able to harbour either a Ti or Ri-plasmid, whilst strains of biovar 3, generally restricted to grapevines, can harbour a Ti-plasmid. Non-Agrobacterium strains have been isolated from environmental samples which harbour a Ri-plasmid whilst laboratory studies have shown that non-Agrobacterium strains can also harbour a Ti-plasmid. Many environmental strains of Agrobacterium do not possess either a Ti or Ri-plasmid. These strains are avirulent.

The plasmid T-DNA is integrated semi-randomly into the genome of the host cell (Francis and Spiker, 2005. Plant Journal. 41(3): 464.), and the virulence (vir) genes on the T-DNA are expressed, causing the formation of a gall. The T-DNA carries genes for the biosynthetic enzymes for the production of unusual amino acids, typically octopine or nopaline. It also carries genes for the biosyntheis of the plant hormones, auxin and cytokinins. By altering the hormone balance in the plant cell, the division of those cells cannot be controlled by the plant, and tumors form. The ratio of auxin to cytokinin produced by the tumor genes determines the morphology of the tumor (root-like, disorganized or shoot-like).

Agrobacterium in humans

Although generally seen as an infection in plants, Agrobacterium can be responsible for opportunistic infections in humans with weakened immune systems,^[1][2] but has not been shown to be a primary pathogen in otherwise healthy individuals. A 2000 study published by the National Academy of Sciences suggested that Agrobacterium attaches to and genetically transforms several types of human cells by integrating its T-DNA into the human cell genome. The study was conducted under laboratory conditions and states that it does not draw any conclusions regarding related biological activity in nature.^[3]

There is a conjectured connection with Morgellons syndrome. Dr. Stricker, along with Dr. Citovsky, MRF board member from the State University of New York at Stony Brook and an expert on plant pathogens, reported in January, 2007, that Morgellons skin fibers appear to contain cellulose. Five skin samples of Morgellons patients contained evidence of DNA from Agrobacterium.^[4]

Uses in biotechnology

See also: horizontal gene transfer

The ability of Agrobacterium to transfer genes to plants is used in biotechnology, in particular, genetic engineering for plant improvement. A modified Ti or Ri plasmid can be used. The plasmid is 'disarmed' by deletion of the tumor inducing genes, the only essential parts of the T-DNA are its two small (25 base pair) border repeats, at least one of which is needed for plant transformation. Marc Van Montagu and Jozef Schell at the University of Ghent (Belgium) discovered the gene transfer mechanism between Agrobacterium and plants, which resulted in the development of methods to alter Agrobacterium into an efficient delivery system for gene engineering in plants. A team of researchers led by Dr Mary-Dell Chilton were the first to demonstrate that the virulence genes could be removed without adversely affecting the ability of Agrobacterium to insert its own DNA into the plant genome (1983).

The genes to be introduced into the plant are cloned into a plant transformation vector that contains the T-DNA region of the disarmed plasmid, together with a selectable marker (such as antibiotic resistance) to enable selection for plants that have been successfully transformed. Plants are grown on media containing antibiotic following transformation, and those that do not have the T-DNA integrated into their genome will die. An alternative method is agroinfiltration.

  Transformation with Agrobacterium can be achieved in two ways. Protoplasts, or leaf-discs can be incubated with the Agrobacterium and whole plants regenerated using plant tissue culture. A common transformation protocol for Arabidopsis is the floral-dip method: the flowers are dipped in an Agrobacterium culture, and the bacterium transforms the germline cells that make the female gametes. The seeds can then be screened for antibiotic resistance (or another marker of interest), and plants that have not integrated the plasmid DNA will die.

Agrobacterium does not infect all plant species, but there are several other effective techniques for plant transformation including the gene gun.

See also

• Agroinfiltration
• Marc Van Montagu

References

1. ^ Hulse, M; Johnson S, Ferrieri P. (1993 Jan). "Agrobacterium infections in humans: experience at one hospital and review." 16(1):112-7. Clinical infectious diseases. ISSN 1058-4838. PMID 8448285.
2. ^ Dunne,, W M; Jr, J Tillman, and J C Murray (1993 September). "Recovery of a strain of Agrobacterium radiobacter with a mucoid phenotype from an immunocompromised child with bacteremia." 31(9): 2541–2543. Journal of Clinical Microbiology.
3. ^ Kunik, Talya; Tzvi Tzfira, Yoram Kapulnik, Yedidya Gafni, Colin Dingwall, and Vitaly Citovsky (2001 February 13). "Genetic transformation of HeLa cells by Agrobacterium" 98(4): 1871–1876. Proceedings of the National Academy of Sciences.
4. ^ Stricker RB, Savely VR, Zaltsman A, Citovsky V. Contribution of Agrobacterium to morgellons disease. J Invest Med. 2007. 55 (1): S123-S123 Suppl. S. (Abstract)