Agrobacterium

Agrobacterium is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. Agrobacterium tumefaciens is the most commonly studied species in this genus. Agrobacterium is well known for its ability to transfer DNA between itself and plants, and for this reason it has become an important tool for genetic engineering.

The Agrobacterium genus is quite heterogeneous. Recent taxonomic studies have reclassified all of the Agrobacterium species into new genera, such as Ahrensia, Pseudorhodobacter, Ruegeria, and Stappia, but most species have been controversially reclassified as Rhizobium species.

Plant pathogen

A. tumefaciens causes crown-gall disease in plants. The disease is characterised by a tumour-like growth or gall on the infected plant, often at the junction between the root and the shoot. Tumors are incited by the conjugative transfer of a DNA segment (T-DNA) from the bacterial tumour-inducing (Ti) plasmid. The closely related species, A. rhizogenes, induces root tumors, and carries the distinct Ri (root-inducing) plasmid. Although the taxonomy of Agrobacterium is currently under revision it can be generalised that 3 biovars exist within the genus, A. tumefaciens, A. rhizogenes, and A. vitis. Strains within A. tumefaciens and A. rhizogenes are known to be able to harbour either a Ti or Ri-plasmid, whilst strains of A. vitis, generally restricted to grapevines, can harbour a Ti-plasmid. Non-Agrobacterium strains have been isolated from environmental samples which harbour a Ri-plasmid whilst laboratory studies have shown that non-Agrobacterium strains can also harbour a Ti-plasmid. Many environmental strains of Agrobacterium possess neither a Ti nor Ri-plasmid. These strains are of course avirulent.

Agrobacterium tumefaciens

Agrobacterium tumefaciens (updated scientific name Rhizobium radiobacter, synonym Agrobacterium radiobacter) is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram-negative soil bacterium. Symptoms are caused by the insertion of a small segment of DNA (known as the T-DNA, for 'transfer DNA'), from a plasmid, into the plant cell, which is incorporated at a semi-random location into the plant genome.

A. tumefaciens is an alphaproteobacterium of the family Rhizobiaceae, which includes the nitrogen-fixing legume symbionts. Unlike the nitrogen-fixing symbionts, tumor-producing Agrobacterium species are pathogenic and do not benefit the plant. The wide variety of plants affected by Agrobacterium makes it of great concern to the agriculture industry.

Economically, A. tumefaciens is a serious pathogen of walnuts, grape vines, stone fruits, nut trees, sugar beets, horse radish, and rhubarb.

Conjugation