Agrobacterium characteristics, morphology and diseases

2889
Abraham McLaughlin

Agrobacterium It 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 gender Agrobacterium it 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

Article index

  • 1 Features
  • 2 Morphology
  • 3 Taxonomy and systematics
  • 4 Diseases caused
  • 5 Forms of contagion
  • 6 Treatment
  • 7 Pathogenicity in humans
  • 8 Agrobacterium and its uses in biotechnology
  • 9 References

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..

Species of Agrobacterium They 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 have the shape of small, short rods (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 gender Agrobacterium was proposed by Conn (1942) to include two pathogenic species previously assigned to Phytomonas: A. tumefaciens Y A. rhizogenes and a non-pathogenic species, A. radiobacter.

Later the species were added  Agrobacterium rubi, A. vitis Y A. larrymoorei due to its ability to cause plant diseases.

Genetic studies of various species of Agrobacterium showed that the ability to produce diseases of A. tumefaciens (producer of tumors) or A. rhizogenes (root-producing) could be transferred between strains of Agrobacterium, or get lost. It was later shown that this ability to produce diseases comes from the transfer of plasmids.

The species of Agrobacterium Y Rhizobium they 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 genus Rhizobium) with plants.

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

Diseases it causes

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

Agrobacterium tumefaciens (at the moment 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 (at the moment Rhizobium rhizogenes), meanwhile, 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 (root growth inducers)..

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 bacterium's 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 bacterium's T-DNA. Bacterial DNA expression leads to tumor growth or abnormal rooting.

Tumors or galls produced by A. tumefaciens in some cases they 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 secondary root formation induced by A. rhizogenes can have beneficial effects on the infected plant.

Treatment

Treatment of gall 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

Even if Agrobacterium It is known mainly to be pathogenic for plants, it can eventually affect humans. In humans it is considered a contaminating organism or with a low capacity to produce disease.

However, A. tumefaciens it 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, blood infections, inflammation of the endocardium, inflammation of the gallbladder and urinary tract infections..

Agrobacterium it 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 and its uses in biotechnology

The capacity of Agrobacterium to transfer genes to plants and fungi has been used as a tool in genetic engineering to make genetic improvements in plants.

However, this abilityto 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 yeasts and fungi have been transformed in the laboratory using Agrobacterium. Researchers have also achieved the transformation of algae, mammalian cells, and the Gram-positive bacteria. 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, S.B. Gelvin, E.M. 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.

Yet No Comments