Characteristics of Agrobacterium tumefaciens
Molecular Biology
Agrobacterium tumefaciens is a gram negative, motile, rod shaped bacterium
which is non sporing, and is closely related to the N-fixing rhizobium bacteria
which form root nodules on leguminous plants. The bacterium is surrounded by a
small number of peritricious flagella. Virulent bacteria contain one or more large
plasmids, one of which carries the genes for tumour induction and is known as the
Ti (tumour inducing) plasmid. The Ti plasmid also contains the genes that determine
the host range and the symptoms, which the infection will produce. Without this Ti plasmid,
the bacterium is described as being non virulent and will not be able to cause
disease on the plant.
Crown Galls first appear as small, white, soft protrusions, initially found at the base of the plant stem. As the tumours enlarge, the surface takes on a mottled dark brown appearance due to the death and decay of the peripheral cells. The tumour usually appears either as a swelling of the plant tissue, or as a separate mass of tissue close to the plant surface, joined only by a narrow neck of tissue. Tumours can either be soft and spongy and may crumble on touch, or can be hard and appear knobbly or knotty. Some tumours can reach up to 30cm in diameter, though 5-10cm is more common (see cross section below). The tumours may rot away in the autumn, only to re-appear again the following spring.
When infected with the bacterium, plants may also become stunted, produce small chlorotic leaves, and are more susceptible to extreme environmental conditions such as winter cold and wind.
A. tumefaciens is most well known for its ability to integrate a small part of the Ti plasmid into the host plant genome, which causes the plant cells to become cancer cells and produce specific compounds called Opines, which the bacterium utilise as a carbon source. This property means that many textbooks class A. Tumefaciens as a genetic parasite, since the bacterium redirects the metabolic activities of the plant to produce compounds specific to the bacterium. It is this process which gives A. Tumefaciens its potential to be used as a tool for plant transformation.
Disease Control
As A. tumefaciens is so ubiquitous in the soil, there is currently much
interest in ways to try and prevent the bacterium from infecting susceptible
plants, either through biocontrol or through the use of pesticides. As the bacterium
can only enter through wounds, the most logical solution would be to prevent the
plants from becoming wounded. However, this is not practical in the long term as
even wind abrasion can cause enough damage on the surface of the plant for the
bacterium to enter.
Agrobacterium radiobacter is a non-pathogenic, soil inhabiting bacterium which is related to A. tumefaciens. A. radiobacter produces the antibiotic agrocin, and for many years the K84 strain of this bacterium was used as a biocontrol agent against certain strains of A. tumefaciens. Seedlings of a plant were dipped into a suspension containing K84, which would then colonise the soil surrounding the plant, thus providing a degree of resistance against the gall. However, as the large plasmid of K84 contains a transfer region, there was a concern that the agrocin resistance gene could be transferred to A. tumefaciens, thus leading to agrocin resistance. A new strain, K1026 was thus engineered in the 1980's without this transfer region, and has been sold commercially as a biocontrol agent in Australia, and the USA, with no obvious detrimental effect on the rhizosphere. K1026 is deemed to be entirely safe in the environment, and cannot harm humans or animals as it will not grow at 37 Degrees Centigrade.
Crown Gall can also be eradicated using creosote based chemical compounds, copper based solutions and strong oxidants such as sodium hypochlorite. However, these are costly to apply both in terms of labour rate, and the cost of buying the product. They are also very harmful to the surrounding environment, and accumulating large quantities of copper in the soil can have a disastrous impact on other plants in the area. Therefore chemical controls are rarely used against A. tumefaciens.