Agrobacteriumwas identified as the agent causing the plant tumor, crown gall over 100 years ago.
In the United States, two plant pathologists, Smith and Townsend (1907) reported that the causative agent of the disease, crown gall, was a bacterium that they named Bacterium tumefaciens.
Armin Braun demonstrated that Agrobacteriuminfected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease.
30(Thirty ) years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease.
The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex.
A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex.
The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium.
The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant.
Plant signals are recognized by a two-component regulatory system which activates vir Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun’s old observations and also explain why Agrobacteriumis nature’s genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.