Agrobacterium tumefaciens

Agrobacterium tumefaciens
A. tumefaciens attaching itself to a carrot cell
Scientific classification
Kingdom:	Bacteria
Phylum:	Proteobacteria
Class:	Alphaproteobacteria
Order:	Rhizobiales
Family:	Rhizobiaceae
Genus:	Agrobacterium
Species:	A. tumefaciens
Binomial name
Agrobacterium tumefaciens Smith & Townsend, 1907
Synonyms
Bacterium tumefaciens Smith and Townsend 1907 Pseudomonas tumefaciens (Smith and Townsend 1907) Duggar 1909 Phytomonas tumefaciens (Smith and Townsend 1907) Bergey et al. 1923 Polymonas tumefaciens (Smith and Townsend 1907) Lieske 1928

Agrobacterium tumefaciens (updated scientific name: Rhizobium radiobacter)^[1][2] is the causal agent of crown gall disease (the formation of tumours) in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium (Smith et al., 1907). Symptoms are caused by the insertion of a small segment of DNA (known as the T-DNA, for 'transfer DNA') into the plant cell,^[3] which is incorporated at a semi-random location into the plant genome.

Agrobacterium tumefaciens (or A. tumefaciens) is an alphaproteobacterium 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.^[4]

Economically, A. tumefaciens is a serious pathogen of walnuts, grape vines, stone fruits, nut trees, sugar beets, horse radish and rhubarb.

Conjugation

In order to be virulent, the bacterium must contain a tumour-inducing plasmid (Ti plasmid or pTi), of 200 kb, which contains the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of A. tumefaciens do not contain a pTi.

Since the Ti plasmid is essential to cause disease, pre-penetration events in the rhizosphere occur to promote bacterial conjugation - exchange of plasmids amongst bacteria. In the presence of opines, A. tumefaciens produces a diffusible conjugation signal called 30C8HSL or the Agrobacterium autoinducer. This activates the transcription factor TraR, positively regulating the transcription of genes required for conjugation.

Method of infection

The Agrobacterium tumefaciens infects the plant through its Ti plasmid. The Ti plasmid integrates a segment of its DNA, known as T-DNA, into the chromosomal DNA of its host plant cells.

A. tumefaciens have flagella that allow them to swim through the soil towards photoassimilates that accumulate in the rhizosphere around roots. Some strains may chemotactically move towards chemical exudates from plants, such as acetosyringone and sugars. The former is recognised by the VirA protein, a transmembrane protein encoded in the virA gene on the Ti plasmid. Sugars are recognised by the chvE protein, a chromosomal gene-encoded protein located in the periplasmic space.^[5]

At least 25 vir genes on the Ti plasmid are necessary for tumor induction. In addition to their perception role, virA and chvE induce other vir genes. The virA protein has kinase activity: it phosphorylates itself on a histidine residue. Then the virA protein phosphorylates the virG protein on its aspartate residue. The virG protein is a cytoplasmic protein produced from the virG Ti plasmid gene. It is a transcription factor, inducing the transcription of the vir operons. The chvE protein regulates the second mechanism of the vir genes' activation. It increases VirA protein sensibility to phenolic compounds.^[5]

Attachment is a two-step process. Following an initial weak and reversible attachment, the bacteria synthesize cellulose fibrils that anchor them to the wounded plant cell to which they were attracted. Four main genes are involved in this process: chvA, chvB, pscA and att. It appears that the products of the first three genes are involved in the actual synthesis of the cellulose fibrils. These fibrils also anchor the bacteria to each other, helping to form a microcolony.

After production of cellulose fibrils a calcium-dependent outer membrane protein called rhicadhesin is produced, which also aids in sticking the bacteria to the cell wall. Homologues of this protein can be found in other Rhizobia species.

Possible plant compounds that initiate Agrobacterium to infect plant cells:^[6]

• Acetosyringone: Phenolic compound
• alpha-Hydroxyacetosyringone
• Catechol
• Ferulic acid
• Gallic acid
• p-Hydroxybenzoic acid
• Protocatechuic acid
• Pyrogallic acid
• Resorcylic acid
• Sinapinic acid
• Syringic acid
• Vanillin

Formation of the T-pilus

In order to transfer the T-DNA into the plant cell A. tumefaciens uses a Type IV secretion mechanism, involving the production of a T-pilus. When acetosyringone and other substances are detected, a signal transduction event activates the expression of 11 genes within the VirB operon which are responsible for the formation of the T-pilus.

The pro-pilin is formed first. This is a polypeptide of 121 amino acids which requires processing by the removal of 47 residues to form a T-pilus subunit. The subunit is circularized by the formation of a peptide bond between the two ends of the polypeptide.

Products of the other VirB genes are used to transfer the subunits across the plasma membrane. Yeast two-hybrid studies provide evidence that VirB6, VirB7, VirB8, VirB9 and VirB10 may all encode components of the transporter. An ATPase for the active transport of the subunits would also be required.

Transfer of T-DNA into the plant cell

A: Agrobacterium tumefaciens
B: Agrobacterium genome
C: Ti Plasmid  : a: T-DNA , b: Vir genes , c: Replication origin , d: Opines catabolism genes
D: Plant cell
E: Mitochondria
F: Chloroplast
G: Nucleus

The T-DNA must be cut out of the circular plasmid. A VirD1/D2 complex nicks the DNA at the left and right border sequences. The VirD2 protein is covalently attached to the 5' end. VirD2 contains a motif that leads to the nucleoprotein complex being targeted to the type IV secretion system (T4SS).

In the cytoplasm of the recipient cell, the T-DNA complex becomes coated with VirE2 proteins, which are exported through the T4SS independently from the T-DNA complex. Nuclear localization signals, or NLS, located on the VirE2 and VirD2 are recognised by the importin alpha protein, which then associates with importin beta and the nuclear pore complex to transfer the T-DNA into the nucleus. VIP1 also appears to be an important protein in the process, possibly acting as an adapter to bring the VirE2 to the importin. Once inside the nucleus, VIP2 may target the T-DNA to areas of chromatin that are being actively transcribed, so that the T-DNA can integrate into the host genome.

Genes in the T-DNA

Hormones

In order to cause gall formation, the T-DNA encodes genes for the production of auxin or indole-3-acetic acid via the IAM pathway. This biosynthetic pathway is not used in many plants for the production of auxin, so it means the plant has no molecular means of regulating it and auxin will be produced constitutively. Genes for the production of cytokinins are also expressed. This stimulates cell proliferation and gall formation.

Opines

The T-DNA contains genes for encoding enzymes that cause the plant to create specialized amino acids which the bacteria can metabolize, called opines.^[7] Opines are a class of chemicals that serve as a source of nitrogen for A. tumefaciens, but not for most other organisms. The specific type of opine produced by A. tumefaciens C58 infected plants is nopaline (Escobar et al., 2003).

Two nopaline type Ti plasmids, pTi-SAKURA and pTiC58, were fully sequenced. A. tumefaciens C58, the first fully sequenced pathovar, was first isolated from a cherry tree crown gall. The genome was simultaneously sequenced by Goodner et al.^[8] and Wood et al.^[9] in 2001. The genome of A. tumefaciens C58 consists of a circular chromosome, two plasmids, and a linear chromosome. The presence of a covalently bonded circular chromosome is common to Bacteria, with few exceptions. However, the presence of both a single circular chromosome and single linear chromosome is unique to a group in this genus. The two plasmids are pTiC58, responsible for the processes involved in virulence, and pAtC58, coined the “cryptic” plasmid.^[8][9]

The pAtC58 plasmid has been shown to be involved in the metabolism of opines and to conjugate with other bacteria in the absence of the pTiC58 plasmid.^[10] If the pTi plasmid is removed, the tumor growth that is the means of classifying this species of bacteria does not occur.

Beneficial uses

Plants that have undergone transformation with Agrobacterium.

The DNA transmission capabilities of Agrobacterium have been extensively exploited in biotechnology as a means of inserting foreign genes into plants. Marc Van Montagu and Jeff Schell, (University of Ghent and Plant Genetic Systems, Belgium) discovered the gene transfer mechanism between Agrobacterium and plants, which resulted in the development of methods to alter the bacterium into an efficient delivery system for genetic engineering in plants.^[11] The plasmid T-DNA that is transferred to the plant is an ideal vehicle for genetic engineering.^[12] This is done by cloning a desired gene sequence into the T-DNA that will be inserted into the host DNA. This process has been performed using firefly luciferase gene to produce glowing plants. This luminescence has been a useful device in the study of plant chloroplast function and as a reporter gene.^[13] It is also possible to transform Arabidopsis thaliana by dipping their flowers into a broth of Agrobacterium: the seed produced will be transgenic. Under laboratory conditions the T-DNA has also been transferred to human cells, demonstrating the diversity of insertion application.^[14]

The mechanism by which Agrobacterium inserts materials into the host cell by a type IV secretion system is very similar to mechanisms used by pathogens to insert materials (usually proteins) into human cells by type III secretion. It also employs a type of signaling conserved in many Gram-negative bacteria called quorum sensing. This makes Agrobacterium an important topic of medical research as well.

See also

• SuhB

References

1. ^ "Rhizobium radiobacter (Agrobacterium tumefaciens) (Agrobacterium radiobacter)". UniProt Taxonomy. http://pir.uniprot.org/taxonomy/358. Retrieved 2010-06-30. 
2. ^ Young J.M., Kuykendall L.D., Martínez-Romero E., Kerr A. and Sawada H. 2001. A revision of Rhizobium Frank 1889, with an emended description of the genus, and the inclusion of all species of Agrobacterium Conn 1942 and Allorhizobium undicola de Lajudie et al. 1998 as new combinations: Rhizobium radiobacter, R. rhizogenes, R. rubi, R. undicola and R. vitis. International Journal of Systematic and Evolutionary Microbiology, 51:89-103
3. ^ Chilton MD, Drummond MH, Merio DJ, Sciaky D, Montoya AL, Gordon MP, Nester EW., Stable incorporation of plasmid DNA into higher plant cells: the molecular basis of crown gall tumorigenesis, Cell. 1977 Jun;11(2):263-71.
4. ^ Moore LW, Chilton WS, Canfield ML. 1997. Diversity of Opines and Opine-Catabolizing Bacteria Isolated from Naturally Occurring Crown Gall Tumors. App. Environ. Microbiol. 63:201-207.
5. ^ ^{a b} Stanton B. Gelvin,Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, Agrobacterium-Mediated Plant Transformation: the Biology behind the “Gene-Jockeying” Tool, http://mmbr.asm.org/cgi/reprint/67/1/16
6. ^ U.S. Patent 6483013
7. ^ Zupan J, Muth TR, Draper O, Zambryski P. 2000. The transfer of DNA from Agrobacterium tumefaciens into plants: a feast of fundamental insights. Plant J. 23:11-28.
8. ^ ^{a b} Goodner B, Hinkle G, Gattung S, Miller N, et al. 2001. Genome Sequence of the Plant Pathogen and Biotechnology Agent Agrobacterium tumefaciens C58. Science. 294:2323-2328.
9. ^ ^{a b} Wood DW, Setubal JC, Kaul R, Monks DE, et al. 2001. The Genome of the Natural Genetic Engineer Agrobacterium tumefaciens C58. Science. 294:2317-2323.
10. ^ Vaudequin-Dransart V, Petit A, Chilton WS, Dessaux Y. 1998. The cryptic plasmid of Agrobacterium tumefaciens cointegrates with the Ti plasmid and cooperates for opine degradation. Molec. Plant-microbe Interact. 11:583-591.
11. ^ Schell J, Van Montagu M., The Ti-plasmid of Agrobacterium tumefaciens, a natural vector for the introduction of nif genes in plants?, Basic Life Sci. 1977;9:159-79.
12. ^ Zambryski P. et al. 1983. Ti plasmid vector for introduction of DNA into plant cells without alteration of their normal regeneration capacity. EMBO J. 2:2143-2150.
13. ^ Root M. 1988. Glow in the dark biotechnology. Bioscience. 38:745-747.
14. ^ Kunik T, Tzfira T, Kapulnik Y, Gafni Y, Dingwall C, Citovsky V. 2001. Genetic transformation of HeLa cells by Agrobacterium. Proc. Natl. Acad. Sci. 98:1871-1876.

• Dickinson, M. (2003). Molecular Plant Pathology. BIOS Scientific Publishers.
• Lal, Erh-Min and Kado, Clarence I. (2000). The T-Pilus of Agrobacterium tumefaciens. Trends in Microbiology, Vol. 8, Issue 8.
• Ward, Doyle V., Zupan, John R and Zambryski, Patricia C. (2002). Agrobacterium VirE2 gtes the VIP1 treatment in plant nuclear import. Trends in Plant Science, Vol. 7 Issue 1.

External links

• Agrobacterium tumefacien C58 Genome Page — as sequenced by Cereon Genomics/University of Richmond
• Agrobacterium tumefacien C58 Genome Page — as sequenced by DuPont/Univ of Washington/Univ of Campinas