Editing Agrobacterium (section)

==Plant pathogen==
[[File:Agrobacteriumgall.jpg|left|thumb|The large growths on these roots are [[gall]]s induced by ''Agrobacterium'' sp.]]
''[[Agrobacterium 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 [[bacterial conjugation|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. Although the taxonomy of ''Agrobacterium'' is currently under revision it can be generalised that 3 biovars exist within the genus, ''Agrobacterium tumefaciens'', ''Agrobacterium rhizogenes'', and ''Agrobacterium vitis''. Strains within ''Agrobacterium tumefaciens'' and ''Agrobacterium rhizogenes'' are known to be able to harbour either a Ti or Ri-[[plasmid]], whilst strains of ''Agrobacterium 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. Some environmental strains of ''Agrobacterium'' possess neither a Ti nor Ri-plasmid. These strains are avirulent.<ref>{{cite journal | vauthors = Sawada H, Ieki H, Oyaizu H, Matsumoto S | title = Proposal for rejection of ''Agrobacterium tumefaciens'' and revised descriptions for the genus ''Agrobacterium'' and for ''Agrobacterium radiobacter'' and ''Agrobacterium rhizogenes'' | journal = International Journal of Systematic Bacteriology | volume = 43 | issue = 4 | pages = 694–702 | date = October 1993 | pmid = 8240952 | doi = 10.1099/00207713-43-4-694 | doi-access = free }}</ref>

The plasmid T-DNA is integrated semi-randomly into the [[genome]] of the host cell,<ref>{{cite journal | vauthors = Francis KE, Spiker S | title = Identification of ''Arabidopsis thaliana'' transformants without selection reveals a high occurrence of silenced T-DNA integrations | journal = The Plant Journal | volume = 41 | issue = 3 | pages = 464–77 | date = February 2005 | pmid = 15659104 | doi = 10.1111/j.1365-313X.2004.02312.x | doi-access = free }}</ref> 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 acid]]s, 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 [[Selection (biology)|selective advantage]].<ref>{{cite journal | vauthors = Pitzschke A, Hirt H | title = New insights into an old story: ''Agrobacterium''-induced tumour formation in plants by plant transformation | journal = The EMBO Journal | volume = 29 | issue = 6 | pages = 1021–32 | date = March 2010 | pmid = 20150897 | pmc = 2845280 | doi = 10.1038/emboj.2010.8 }}</ref> 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).