Agrobacterium - Plant Pathogen

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 avirulent.

The plasmid T-DNA is integrated semi-randomly into the genome of the host cell, and the tumor morphology genes on the T-DNA are expressed, causing the formation of a gall. The T-DNA carries genes for the biosynthetic enzymes for the production of unusual amino acids, typically octopine or nopaline. It also carries genes for the biosynthesis of the plant hormones, auxin and cytokinins), and for the biosynthesis of opines, providing a carbon and nitrogen source for the bacteria that most other micro-organisms can't use, giving Agrobacterium a selective advantage. By altering the hormone balance in the plant cell, the division of those cells cannot be controlled by the plant, and tumors form. The ratio of auxin to cytokinin produced by the tumor genes determines the morphology of the tumor (root-like, disorganized or shoot-like).