AGROBACTERIUM: CHARACTERISTICS, MORPHOLOGY AND DISEASES - BIOLOGY - 2025

Agrobacterium is a genus of Gram-negative bacteria capable of causing disease in plants through the transfer of DNA. DNA transfer allows modification of the recipient plant to allow the expression of the genetic information of the bacterium. Because of this, bacteria of this genus are sometimes called "nature's genetic engineers."

The genus Agrobacterium is currently considered invalid and the species that contained it have been relocated, for the most part, to the genus Rhizobium. The latter genus was originally erected to contain plant endosymbiotic bacteria. These bacteria help nitrogen fixation by associated plants, mainly legumes.

Mechanism of transmission of genetic information of Agrobacterium. Taken from commons.wikimedia.org

characteristics

They do not form spores, they are Gram-negative, aerobic. They produce an acid reaction in the presence of mannitol. They do not produce acid or gas in a glucose-peptone medium.

They are capable of inducing the self-proliferation of tumors in plants. This capacity is due to the genetic transfer of a small region of DNA carried in tumor inducer (Ti) or root inducer (Ri) genes.

Agrobacterium species invade through wounds, the crown, roots, and stems of many dicots and some gymnosperm plants. Gene transfer results in the expression in the recipient plant of particular properties of the bacteria.

Morphology

Bacteria of this genus are small, short rod-shaped (0.5-1.0 x 1.2-3.0 μm). They are mobile due to the presence of 1-4 flagella located laterally. If they present a single flagellum, their fixation can be lateral or polar.

Taxonomy and systematics

The genus Agrobacterium was proposed by Conn (1942) to include two pathogenic species previously assigned to Phytomonas: A. tumefaciens and A. rhizogenes and a non-pathogenic species, A. radiobacter.

Subsequently, the Agrobacterium rubi, A. vitis and A. larrymoorei species were added due to their ability to cause plant diseases.

Genetic studies of various Agrobacterium species showed that the disease-causing ability of A. tumefaciens (tumor producer) or A. rhizogenes (root producer) could be transferred between Agrobacterium strains, or lost. Later it was shown that this ability to produce diseases comes from the transfer of plasmids.

The Agrobacterium and Rhizobium species are very similar to each other. The only systematic difference recorded between these genera is their pathogenic interaction, in the case of Agrobacterium, or symbiotic (those of the Rhizobium genus) with plants.

This and the fact that the ability of Agrobacterium to produce diseases can be lost or transferred, led many authors to unite both genera into one (Rhizobium).

Diseases it causes

Agrobacterium species can have a high capacity to produce diseases on plants. They produce two main types of diseases.

Agrobacterium tumefaciens (currently Rhizobium radiobacter) produces tumors or galls on roots and trunk of numerous species of gymnosperm, monocotyledonous and dicotyledonous plants, including at least 40 species of commercial interest.

Agrobacterium rhizogenes (now Rhizobium rhizogenes), on the other hand, causes unusual root growth in some dicotyledonous plants (hairy root disease or hairy root disease).

Gall disease in uvero. Taken from commons.wikimedia.org

Forms of contagion

The spread of diseases can occur both through soils with pathogenic strains and through the spread of contaminated material. For the strains to have the ability to produce diseases, they must possess particular plasmids. These plasmids are called Ti plasmids (tumor inducers) or Ri plasmids (inducers of root growth).

During the infection process, a segment of the Ti or Ri plasmid, called T-DNA (transfer DNA) is transported from the bacteria to the recipient plant.

The bacterial T-DNA penetrates the nucleus of the plant's cells and integrates with the plant's DNA. As a result, plant cells are genetically transformed, which allows the expression of genetic information from the bacteria's T-DNA. Expression of bacterial DNA leads to tumor growth or abnormal rooting.

The tumors or galls produced by A. tumefaciens in some cases do not have harmful effects on plants. In other cases they can cause growth reduction and even death of the infected plant.

This disease has proliferated in recent years due to the exchange and commercialization of plants with the disease but without visible signs of it.

The effect of hairy root disease on the infected plant is poorly understood. Some authors have shown that the formation of secondary roots induced by A. rhizogenes can have beneficial effects on the infected plant.

Treatment

Treatment of gill disease should be preventive. In case of infection, the development of the disease will sometimes progress regardless of the presence of the bacteria causing the disease.

The application of antibacterial products made from copper and bleach can reduce populations of A. tumefaciens on the surface of plants. Another preventive treatment mechanism is the application of non-pathogenic strains of the bacteria that compete with the pathogenic strains.

Creosote-based chemicals, copper-based solutions, and strong oxidants can be used for curative treatment of crown gall disease.

Since there is no evidence of the harmful effects of hairy root disease on the infected plant, there is no specific treatment against it.

Pathogenicity in humans

Although Agrobacterium is known primarily to be pathogenic to plants, it can eventually affect humans. In humans it is considered a polluting organism or with a low capacity to produce disease.

However, A. tumefaciens may be responsible for nosocomial infections in patients with weakened immune systems. Among the diseases caused by this bacterium are infections associated with central venous catheters, peritonitis, infections of the blood, inflammation of the endocardium, inflammation of the gallbladder and urinary tract infections.

Agrobacterium can be resistant to multiple antibiotics including cotrimoxazole and tetracycline. The only successful therapy to date is cefotaxime for the treatment of gallbladder inflammation.

Agrobacterium's ability to transfer genes to plants and fungi has been used as a tool in genetic engineering to make genetic improvements in plants.

However, this ability to transform host organisms is not restricted to plants. Many other eukaryotic and even prokaryotic organisms can be manipulated under laboratory conditions to be genetically transformed by Agrobacterium.

Many species of yeast and fungi have been transformed in the laboratory using Agrobacterium. Researchers have also succeeded in transforming algae, mammalian cells, and the Gram-positive bacterium Streptomyces lividans.

References

1. Agrobacterium. On Wikipedia. Retrieved on September 13, 2018 from wikipedia.org.
2. T. Tzfira, V. Citovsky, Eds (2008). Agrobacterium: From Biology to Biotechnology. Springer, New York. 1-735.
3. R. Cain (1988). A case of septicaemia caused by Agrobacterium radiobacter. The Journal of Infection.
4. M. Hulse, S. Johnson, P. Ferrieri (1993). Agrobacterium infections in humans: experience at one hospital and review. Clinical Infectious Diseases.
5. A. Ziemienowicz (2001). Odyssey of Agrobacterium T-DNA. Acta Biochimica Polonica.
6. H. Hwang, SB Gelvin, EM Lai (2015). Editorial: "Agrobacterium biology and its application to transgenic plant production" Frontiers in Plant Science.
7. W. Nester (2015). Agrobacterium: nature's genetic engineer. Frontiers in Plant Science.