Agrobacterium is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants.

Tumors are incited by the conjugative transfer of a DNA segment (T-DNA) from the bacterial tumour-inducing (Ti) plasmid.

The closely related species, Agrobacterium rhizogenes, induces root tumors, and carries the distinct Ri (root-inducing) plasmid.

The T-DNA carries genes for the biosynthetic enzymes for the production of unusual amino acids, typically octopine or nopaline.

The study was conducted using cultured human tissue and did not draw any conclusions regarding related biological activity in nature.

Genomes of plants and fungi can be engineered by use of Agrobacterium for the delivery of sequences hosted in T-DNA binary vectors.

The plasmid is 'disarmed' by deletion of the tumor inducing genes; the only essential parts of the T-DNA are its two small (25 base pair) border repeats, at least one of which is needed for plant transformation.

[27] The sequencing of the genomes of several species of Agrobacterium has permitted the study of the evolutionary history of these organisms and has provided information on the genes and systems involved in pathogenesis, biological control and symbiosis.

The availability of the genome sequences of Agrobacterium species will continue to increase, resulting in substantial insights into the function and evolutionary history of this group of plant-associated microbes.

[18][19] A team of researchers led by Mary-Dell Chilton were the first to demonstrate that the virulence genes could be removed without adversely affecting the ability of Agrobacterium to insert its own DNA into the plant genome (1983).