Agrobacterium tumefaciens is a phytopathogenic bacterium belonging to the Rhizobiales order. The most important characteristic of this bacterium is that it introduces part of its DNA into plant cells and transforms these from normal cells to tumor cells in a short time, causing the crown gall.
This bacterium is a gram-negative bacillus that forms whitish or yellowish colonies and produces a mucilaginous polysaccharide in culture media with carbohydrates. It is displaced by peritrichous flagella, lives in the soil and infects plant cells through wounds.
Agrobacterium tumefaciens cells beginning to infect a root. Source: AG Matthysse, KV Holmes, RHG Gurlitz
The symptoms caused by Agrobacterium in its hosts are not due to the pathogenic species, but to the type of plasmid (circular DNA fragment) they have. Accordingly, bacteria containing Ti plasmids are inducers of tumors that produce crown gall, and bacteria containing Ri plasmids induce the formation of hairy roots.
This bacterial species, together with certain viruses as vectors of genetic material to transform plant species, has opened an era in the cultivation of transgenic plants with high productive potential. Furthermore, the study of the crown galls produced by Agrobacterium tumefaciens constituted an important part of the applications of in vitro plant tissue cultures.
Currently, biotechnologists use this bacterium also to transform other organisms such as insects and to transfer genes between related and unrelated plants.
Species: Rhizobium radiobacter (Beijerinck and van Delden, 1902) Young et al., 2001 (valid name)
Some synonyms are Agrobacterium radiobacter (Beijerinck and van Delden, 1902) Conn, 1942, and Agrobacterium tumefaciens (Smith and Townsend, 1907) Conn, 1942.
Morphology
Agrobacterium tumefaciens is a bacillary-shaped bacterium that has scattered lateral flagella and grows in white and sometimes yellowish colonies in culture.
The bacillary shape of a bacterium indicates that its appearance is rod-shaped. The dimensions of A. tumefaciens are 0.8 μm long by 1.5 to 3 μm wide.
Bacteria of the Rhizobiaceae family are gram-negative bacteria that present between 1 and 6 flagella. Specifically, A. tumefaciens travels through 1 or 4 peritrichous flagella. In the case of having a single flagellum, it is lateral and not polar.
With respect to its growth in culture media, if the medium contains a carbohydrate source, the bacterium produces an abundant polysaccharide-type mucilage, similar to that produced by its relatives, rhizobial bacteria. Colonies generally have a smooth appearance.
Diseases
Crown gall
This disease is produced in more than a hundred infected plants in which a gall or tumor forms in structures such as roots, petioles and stems.
Tumors develop after bacteria enter newly made wounds on a susceptible host.
Once the bacteria recognize a wound and vice versa, the cells that are closest to it begin to divide. Agrobacterium binds to the cell walls of its hosts, but does not invade their cells.
The crown gall produced by Agrobacterium tumefaciens can at some point cause the stem to drop. Source: jacilluch
Two or three days after infection, a conditioning occurs in plant cells that makes them sensitive to a DNA fragment of the bacterial plasmid, known as Ti DNA, since it is a sequence that induces tumors.
This fragment of bacterial DNA integrates with the nuclear DNA of the host plant cell and induces a transformation of plant cells from normal to tumor cells.
Later transformed cells divide and grow uncontrollably independently of the bacteria and the plant.
The gall formed either in the stem or in the roots of the plants, causes that the elongated cells that are near the xylem or around it, produce pressure on the xylem vessels and these are compressed and dislocated, becoming less efficient to transport the water inside the plant.
At the beginning of the disease, the tumors are almost spherical, white and soft in texture. At first they can be confused with a callus product of the wound. The tissues then darken due to peripheral cell death and rot.
Some tumors can be woody, and others can be spongy. Its size can be up to 30 cm.
Hairy root
It is a disease produced by the Agrobacterium tumefaciens species and by its relative Agrobacterium rhizogenes. Both present Ri plasmids and induce the formation of hairy roots in their hosts, which shows a very particular phenotype in the roots of infected plants.
The roots develop abundantly and look like hair or roots with many hairs. This occurs once the bacterial DNA is integrated into the plant DNA, and the synthesis of indole acetic acid is stimulated, which promotes the differentiation from normal roots to hairy roots.
Biologic control
The crown gall caused by Agrobacterium tumefaciens can be biocontrolled by a bacterium of the same genus (Agrobacterium radiobacter), which is not pathogenic.
For this biocontrol, the seeds, seedlings and plant cuttings are treated with a suspension of the K84 strain of A. radiobacter, thanks to the production of a bacteriocin known as agrocin 84, which functions as an antibiotic against bacteria taxonomically related to it.
This substance selectively inhibits the phytopathogenic bacteria that reach the surface of the plant tissues impregnated with the non-pathogenic bacteria. However, it is known that in several countries, there are strains of A. tumefaciens resistant to agrocin 84.
Chemical control
In the case of cherry, which is susceptible to infection by A. tumefaciens, it is usually treated preventively with dichlone (dichloro naphthoquinone).
The Ti plasmid from Agrobacterium tumefaciens is a bacterial DNA fragment useful in genetic engineering. Source: Ti plasmid.svg: Mouagipderivative work: Miguelferig
References
- Ruggiero, MA, Gordon, DP, Orrell, TM, Bailly, N., Bourgoin, T., Brusca, RC, et al. 2015. A Higher Level classification of All Living Organisms. PLoS ONE 10 (4): e0119248.
- Agrios, GN 2005. Plant Pathology. 5 th ed. Elsevier Academic Press. United States of America. 922 p.
- Catalog of Life: 2019 Annual Checklist. Species details: Rhizobium radiobacter (Beijerinck and van Delden, 1902) Young et al., 2001. Taken from: catalogueoflife.org
- Echeverrigaray, S. 1995. Changes in peroxidase and polypeptide profiles in Nicotiana tabacum L. transformed with Agrobacterium rhizogenes. Rural Science, Santa Maria 25 (2): 229-232.
- De la Isla, L. Phytopathology. 1994. Phytopathology. Postgraduate College, UTEHA Noriega Editores. 384 p.