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Agrobacterium

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Agrobacterium
Scientific classification
Kingdom:
Bacteria
Phylum:
Proteobacteria
Class:
Alpha Proteobacteria
Order:
Rhizobiales
Family:
Rhizobiaceae
Genus:
Agrobacterium
Species

Agrobacterium is a genus of Gram-negative bacteria discovered by Nicholas Davidson from Glasgow[citation needed] 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 genetic engineering.

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

Plant pathogen

The large growths on these roots are galls induced by Agrobacterium sp.

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, A. rhizogenes, and A. vitis. Strains within A. tumefaciens and A. rhizogenes are known to be able to harbour either a Ti or Ri-plasmid, whilst strains of A. vitis, 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,[1] 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 biosynthesis of the plant hormones, auxin and cytokinins. These plant hormones produce opines, providing a carbon and nitrogen source for the bacteria that most other micro-organisms can't use, giving Agrobacterium a selective advantage.[2] 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,[3][4] but has not been shown to be a primary pathogen in otherwise healthy individuals. One of the earliest associations of human disease caused by Agrobacterium radiobacter was reported by Dr. J. R. Cain in Scotland (1988).[5] A later study 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 using cultured human tissue and did not draw any conclusions regarding related biological activity in nature.[6]

There is a conjectured connection with Morgellons syndrome. Dr Raphael Stricker from New Morgellons Order along with Dr. Citovsky, an expert on plant pathogens State University of New York at Stony Brook, reported in January, 2007, that Morgellons skin fibers appear to contain cellulose. Five skin samples of Morgellons patients contained evidence of DNA from Agrobacterium. However, Dr. Randy Wymore from Oklahoma State University repeated This Agrobacterium Study in 2010 and did not find DNA evidence from agrobacterium on any Morgellons patients. Dr. Randy Wymore stated this at a 2010 "New Morgellons Order" Conference in Austin, TX.[7]

Uses in biotechnology

See also: horizontal gene transfer

The ability of Agrobacterium to transfer genes to plants and fungi 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.[8][9] 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.

Plant (S. chacoense) transformed using Agrobacterium. Transformed cells start forming calluses on the side of the leaf pieces

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.

Agrobacterium is listed as being the original source of genetic material that was transferred to these USA GMO foods [10]:

  • Soybean
  • Cotton
  • Corn
  • Sugar Beet
  • Alfalfa
  • Wheat
  • Rapeseed Oil (Canola)
  • Creeping bentgrass (for animal feed)
  • Rice (Golden Rice)

Genomics

The sequencing of the genomes of several species of Agrobacterium has permitted the study of the evolutionary history of these organisms and has provided information on the genes and systems involved in pathogenesis, biological control and symbiosis. One important finding is the possibility that chromosomes are evolving from plasmids in many of these bacteria. Another discovery is that the diverse chromosomal structures in this group appear to be capable of supporting both symbiotic and pathogenic lifestyles. The availability of the genome sequences of Agrobacterium species will continue to increase, resulting in substantial insights into the function and evolutionary history of this group of plant-associated microbes.[11]

See also

References

  1. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1365-313X.2004.02312.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1365-313X.2004.02312.x instead.
  2. ^ Andrea Pitzschke and Heribert Hirt (2010). "New insights into an old story: Agrobacterium-induced tumour formation in plants by plant transformation". The EMBO Journal. 29: 1021–103. doi:10.1038/emboj.2010.8. PMID 20150897.
  3. ^ Hulse M, Johnson S, Ferrieri P (1993). "Agrobacterium infections in humans: experience at one hospital and review". Clin. Infect. Dis. 16 (1): 112–7. PMID 8448285. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  4. ^ Dunne WM, Tillman J, Murray JC (1 September 1993). "Recovery of a strain of Agrobacterium radiobacter with a mucoid phenotype from an immunocompromised child with bacteremia". J. Clin. Microbiol. 31 (9): 2541–3. PMC 265809. PMID 8408587.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Cain, JR (1988) A case of septicaemia caused by Agrobacterium radiobacter. Journal of Infection Vol 16 (2): 205-206.
  6. ^ Kunik T, Tzfira T, Kapulnik Y, Gafni Y, Dingwall C, Citovsky V (2001). "Genetic transformation of HeLa cells by Agrobacterium". Proc. Natl. Acad. Sci. U.S.A. 98 (4): 1871–6. doi:10.1073/pnas.041327598. PMC 29349. PMID 11172043. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  7. ^ 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)
  8. ^ Schell J, Van Montagu M (1977). "The Ti-plasmid of Agrobacterium tumefaciens, a natural vector for the introduction of nif genes in plants?". Basic Life Sci. 9: 159–79. PMID 336023.
  9. ^ Joos H, Timmerman B, Montagu MV, Schell J (1983). "Genetic analysis of transfer and stabilization of Agrobacterium DNA in plant cells". Embo J. 2 (12): 2151–60. PMC 555427. PMID 16453483.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ The FDA List of Completed Consultations on Bioengineered Foods
  11. ^ Setubal JC; et al. (2009). "The Genomics of Agrobacterium: Insights into its Pathogenicity, Biocontrol, and Evolution". Plant Pathogenic Bacteria: Genomics and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-37-0. {{cite book}}: Explicit use of et al. in: |author= (help)
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