Agrobacterium tumefaciens is the causal agent of crown gall disease (the formation of tumours) in over 140 species of dicot. It is rod shaped. Symptoms are caused by the insertion of a small segment of DNA into the plant cell. It is an alphaproteobacterium of the family Rhizobiaceae which includes the nitrogen fixing legume symbionts. They are pathogenic and provide no benefit to the plant. It also affects a wide variety of plants.
In an economical sense it affects walnuts, grape vines, stone fruits, nut trees, sugar beets, horse radish, and rhubarb. To be virulent the bacterium must contain a tumor-inducing plasmid which contains the T-DNA and all the genes necessary to transfer it to the plant cell.
It infects the plant through its Ti plasmid which integrates a segment of its DNA into the chromosomal DNA of its host plant cells. A. tumefaciens have flagella that allow them to swim through the soil towards photo assimilates that accumate in the rhizosphere around roots. There are at least 25 vir genes on the Ti plasmid that are necessary for tumor induction.
After cellulose fibrils are produced a calcium-dependent outer membrane protein called rhicadhesin is produced that aids in sticking the bacteria to the cell wall. A. tumefaciens uses a Type IV secretion mechanism in order to transfer the T-DNA into the plant cell. In order to cause gall formation the T-DNA encodes genes for the production of auxin or indole-3-acetic acid via the IAM pathway. Typically a plant has no means of regulating auxin which allows it to be produced constitutively. T-DNA also contains genes that encode enzymes causing the plant to create specialized amino acids called opines. They are a class of chemicals that serve as a source of nitrogen for A. tumefaciens but not for most other organisms.