Agrobacterium tumefaciens
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Agrobacterium tumefaciens |
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| Agrobacterium tumefaciens | ||||||||||||
| ( Smith & Townsend 1907) Conn 1942 |
Agrobacterium tumefaciens (Latin for "tumor-making arable bacterium "; recently also called Rhizobium radiobacter ) is a plant-pathogenic , gram-negative soil bacterium . It belongs to the alpha class of proteobacteria .
A. tumefaciens is a model organism and has the ability to transfer DNA into plant cells . This process was first described by Jozef Schell and Marc Van Montagu in 1983.
Pathogenicity
Galls on the roots of the pecan nut ( Carya illinoinensis ) caused by A. tumefaciens .
Bile caused by A. tumefaciens on a branch of forsythia ( Forsythia × intermedia )
Under natural conditions, the bacterium can only infect injured, dicotyledonous plants . The infection usually occurs at the base of the stem, but other parts of the plant above ground can also be infected. The recognition between bacterial and plant cells takes place with the help of certain signal molecules ( pectins and glucans ) on their surface. Infection of monocots is also possible under laboratory conditions .
The growths that A. tumefaciens triggers by interfering with the plant hormone balance often have specific names in useful plants. When the grapevine is infested , one speaks of mucous membrane disease , with the sugar beet it is called root goiter . In other plants, such as horseradish and rhubarb , the tumors are also known as crown-galls ( root neck gall tumor ).
Course of the infection
Pathogenic agrobacteria always carry a plasmid . This Ti plasmid (Ti stands for tumor inducing) contains genes that are necessary for later DNA transfer into the plant cell. The transcription of these so-called "virulence" or vir genes is activated by secondary plant substances ( 1 ) that emerge from wounded parts of the plant ( 2 ). On the part of the bacterium, one also speaks of positive chemotaxis , as the bacteria move specifically towards the source of the messenger substance.
Possible plant substances that cause Agrobacterium to become infected:
- Acetosyringone
- Vanillin
- alpha-hydroxyacetosyringone
- Sinapic acid
- Syringic acid
- Ferulic acid
- Catechol
- 4-hydroxybenzoic acid
The activated vir genome of the bacterium ( 3 ) is located on its Ti plasmid and codes for proteins ( 4 ) that penetrate the plant ( 5 ) and can install foreign genes in the plant. With the help of the vir gene products, another part of the DNA of the Ti plasmid can be transferred into the plant cell as a so-called “transfer or t- DNA”. The t -DNA is introduced into the plant host cell as a single-stranded DNA molecule with the help of the bacterial type IV secretion system. This t -DNA is covalently bound at its 5 'end to the bacterial VirD2 protein, which, through a certain motif , helps the DNA to be transported into the plant cell nucleus ( 6 , nuclear localization signal ). Furthermore, the t -DNA is bound by VirE2 proteins and now forms the t -complex. It is known from Arabidopsis thaliana that VirE2 can interact with the plant VIP1 protein or its homologues after it has been phosphorylated by the plant MAP kinase 3 . The activated VIP1 protein now functions as an adapter of the plant core import machinery (especially Importin alpha) and the bacterial VirE2 protein and now also contributes to the nuclear transport of the t -complex. In the cell nucleus, the t- strand has to be freed from the VirE2 and VIP1 proteins. This is done by the bacterial VirF protein, which feeds the proteins to the proteasome-mediated breakdown. The t -DNA can now integrate into the host genome. In eukaryotic cells, this is the only place where DNA is protected from degradation and can be replicated and transcribed.
Once in the cell nucleus, the DNA is integrated into the plant genome ( 7 ). The place of insertion in the host genome is purely coincidental. Promoters , which are recognized by the plant cell, ensure that the genes in the plant are also active. The transferred genes therefore have a typical eukaryotic structure, although they come from a bacterium. The transferred genes trigger tumors and induce the plant cells to produce opines (octopine and nopaline), which are used as food for the bacteria but are worthless for the plant. The name Agrobacterium tumefaciens is derived from the tumor-inducing property.
Use as a vector
Activation of the vir genes can be used to introduce desired genes into plants, so-called transformation by agrobacteria . Agrobacteria can therefore genetically modify plant cells in a targeted manner. Because of this property, it has been an important vector for transferring genes to plants since the beginning of genetic engineering . Under laboratory conditions, this works with many plants and even some fungi. For this purpose, the genes of the t -DNA, which are normally necessary for the metabolism of opin and tumor induction, are replaced by others. In addition to dicotyledonous plants such as potatoes, rape, soy and tobacco, transformation goals are also monocotyledonous plants from the sweet grass family . Most of the most important cereal crops, such as corn, wheat, rye, rice, and barley, come from this family. Successful transformations have been described for all of the monocots mentioned.
Genome
The genome of A. tumefaciens was sequenced in 2001 .
Another well-known representative of the genus is Agrobacterium rhizogenes , which infects plants mainly at the roots and leads to massive growths of the root hairs . The equivalent of the Ti plasmid is here the Ri plasmid.
Individual evidence
- ↑ US Patent 6483013
- ↑ Cheng, M., JE Fry, et al. ( 1997 ). "Genetic transformation of wheat mediated by Agrobacterium tumefaciens." Plant Physiology 115 (3): 971-980.
- ^ Frame, BR, HX Shou, et al. ( 2002 ). "Agrobacterium tumefaciens-mediated transformation of maize embryos using a standard binary vector system." Plant Physiology 129 (1): 13-22.
- ↑ Ishida, Y., Y. Hiei, et al. ( 2007 ). "Agrobacterium-mediated transformation of maize." Nature Protocols 2 (7): 1614-1621.
- ↑ Ishida, Y., H. Saito, et al. ( 1996 ). "High efficiency transformation of maize (Zea mays L) mediated by Agrobacterium tumefaciens." Nature Biotechnology 14 (6): 745-750.
- ↑ Mooney, PA, PB Goodwin, et al. ( 1991 ). "AGROBACTERIUM-TUMEFACIENS-GENE TRANSFER INTO WHEAT TISSUES." Plant Cell Tissue and Organ Culture 25 (3): 209-218.
- ^ Popelka, JC and F. Altpeter ( 2003 ). "Agrobacterium tumefaciens-mediated genetic transformation of rye (Secale cereale L.)." Molecular Breeding 11 (3): 203-211.
- ↑ Raineri, DM, P. Bottino, et al. ( 1990 ). "AGROBACTERIUM-MEDIATED TRANSFORMATION OF RICE (ORYZA-SATIVA-L)." Bio-Technology 8 (1): 33-38.
- ↑ Sidorov, V., L. Gilbertson, et al. ( 2006 ). "Agrobacterium-mediated transformation of seedling-derived maize callus." Plant Cell Reports 25 (4): 320-328.
- ↑ Tingay, S., D. McElroy, et al. ( 1997 ). "Agrobacterium tumefaciens-mediated barley transformation." Plant Journal 11 (6): 1369-1376.
