Agrobacterium tumefaciens
Agrobacterium tumefaciens
Agrobacterium tumefaciens (Latin for " tumor -making bacteria Acker ") is a plant pathogenic gram-negative soil bacterium. It belongs to the class of alpha - 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
By A. tumefaciens induced gall on a branch of forsythia ( Forsythia x intermedia)
Under natural conditions, the bacteria can only injured, infecting dicotyledonous plants. The infection usually occurs at the stem base, but there can be other above-ground plant parts are infected. Recognition between bacteria and plant cells is carried out with the help of certain signaling molecules ( pectin and glucans) on its surface. Under laboratory conditions, the infection of monocots is possible.
The growths that A. tumefaciens causes by affecting the plant hormone balance in crops often have specific names. When infestation of grapevine is called Maukekrankheit, in sugar beet root crop. In other plants such as horseradish and rhubarb tumors are also referred to as crown galls - ( crown gall tumor).
Infection
Pathogenic agrobacteria always carry a plasmid. This Ti plasmid (Ti stands for inducing tumor - tumor inducing) contains genes that are necessary for subsequent DNA transfer into the plant cell. The transcription of these so-called " virulence " or vir genes is secondary plant substances (1), which emerge from wounded plant parts, enabled ( 2). Man talking on the part of the bacterium also of positive chemotaxis, as the bacteria move targeted to the source of the neurotransmitter back.
Possible plant substances that cause Agrobacterium infection:
- Acetosyringone
- Vanillin
- Alpha- Hydroxyacetosyringon
- Sinapinic
- Syringic
- Ferulic acid
- Catechol
- 4-hydroxybenzoic acid
The activated vir genome of the bacterium (3) is at its Ti - plasmid and encodes for proteins (4) which penetrate into the plant (5) and in the plant can be installed foreign genes. With the aid of the vir gene products can be transferred into the plant cell a different part of the DNA of the Ti plasmid as a so-called " transfer or T-DNA ." The T- DNA is introduced as a single-stranded DNA molecule with the aid of the bacterial type IV secretion system in the plant host cell. This T-DNA is covalently bonded at its 5 ' end to the bacterial VirD2 protein, which contribute through a specific subject that the DNA is transported into the plant cell nucleus (6, nuclear localization signal). Further, the T-DNA bound by VirE2 proteins and now forms the T- complex. Arabidopsis thaliana is known 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 acts as an adapter to the plant nuclear import machinery (in particular, importin alpha) and the bacterial VirE2 protein and now also contributes to the nuclear transport of the T- complex. In the nucleus of the T- strand of the VirE2 VIP1 proteins and must be removed. This is done by VirF bacterial protein, which supplies the proteins to the proteasome -mediated degradation. Now the T- DNA can integrate into the host genome. In eukaryotic cells, DNA is protected from degradation and only here can be replicated and transcribed.
Once in the nucleus, the DNA is integrated into the plant genome (7) of the insertion site in the host genome is purely coincidental. Promoters that are recognized by the plant cell, ensure that the genes in the plant are also active. Thus, the transferred genes possess a typical eukaryotic structure, although they come from a bacterium. The transferred genes trigger tumors and induce the plant cell, opines (octopine and nopaline ) to produce that serve the bacteria as food for the plant are but worthless. The name is derived from Agrobacterium tumefaciens tumor-inducing of the property.
Benefits as a vector
Activation of vir genes can be used to introduce desired genes in plants, so-called transform Agrobacteria. Agrobacteria can genetically modify plant cells specifically so. Because of this property, it is more important since the genetic vector to transfer genes to plants. Under laboratory conditions, this is achieved in many plants and even some fungi. For this purpose the genes of T-DNA, which are usually necessary for opine metabolism and tumor induction, replaced. Transformation goals, in addition to dicotyledonous plants such as potatoes, canola, soybean and tobacco and monocotyledonous plants of the family Gramineae. In this family are most of the major cereal crops such as maize, wheat, rye, rice and barley. Successful transformations have been described for all of the above monocots.
Genome
The genome of A. tumefaciens was sequenced in 2001.
Another well-known representative of the genus Agrobacterium rhizogenes, plants mainly infects the roots and leads to massive growths of root hairs. The equivalent of the Ti plasmid is the Ri plasmid here.