Agrobacterium tumefaciens

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gall caused by Agrobacterium tumefaciens on Forsythia (click on image to enlarge it)
Source: Wikimedia Commons

Agrobacterium tumefaciens (Smith and Townsend 1907) Conn 1942

The bacterium causes crown gall on fruit trees and vegetables. It is soil-borne and infects plants through small wounds. The galls are typically caused on roots or at the crown (the border area between stems and roots). Some hosts are infected systemically, causing tumors along the vascular system of the trunk or stems.

The disease has a very large host range, but fruit and nut trees, as well as woody ornamentals are most frequently attacked. The bacterium spreads by the galls which are easily detached from the host. It will also disperse via contaminated soil and farm equipment.

Management options include crop rotation with cereals, disease-free planting material and biological control using non-pathogenic strains (see antagonistic Rhizobium radiobacter). Chemical control is usually not considered economical.

During the infection process, the bacterium transfers one or more DNA fragments to the nuclei of the plant host, using a "tumor-inducing" (Ti) plasmid. The plant cells then produce excessive mitogenic phytohormones as well as nutrient sources for the bacterium. The latter are called opines.

A. tumefaciens is a Gram-negative, aerobic and bacilliform bacterium, approximately 3 x 1 µm large. It has become very important as a tool for inserting new or altered genes into plants and thus modifying them genetically.

Vernacular names
• Deutsch: Wurzelkrebs der Obstbäume
• English: crown gall
• Español: agalla de la corona de los frutales
• Français: tumeur du collet

Taxonomic note:
Agrobacterium tumefaciens is regarded by taxonomists as being identical to or at least very closely related to Rhizobium radiobacter which has been described first and should have priority. However, the issue of the correct genus name remains unresolved. A. tumefaciens is the type species of the genus Agrobacterium and has been declared a conserved name. On the other hand, the genus Rhizobium has been also established before the genus Agrobacterium. During taxonomic studies of the issue (see Young et al., 2001) it was, therefore, suggested to use Rhizobium radiobacter instead of Agrobacterium tumefaciens.

Synonyms:
Agrobacterium radiobacter var. tumefaciens

For additional literature on this species, see Rhizobium radiobacter.

For details see the respective page in Wikipedia.