Agrobacterium tumefaciens is quite
possibly the most important bacterium to plant geneticists whether they are trying to understand how a gene or proteins works i'n vivo or trying to introduce resistance genes into a cropline.
Agrobacterium tumefaciens is the only organism of interkingdom gene transfer. This in itself is very interesting and a topic of research for many.
Smith and Townsend characterized Agrobacterium tumefaciens in 1907 as the bateria that caused crown gall disease in plants. This is characterized by tumor formation on the roots.
1923-1941 Riker lab starts to develop the system as a possible biological tumor inducing tool
1940's-1970's Braun at The Rockefeller Institute (later University) discovered that the tumors did not require the continuous introduction of the bacteria for infection to remain.
1912 Smith et al added that tumors far from the infection site occured, suggesting a systemic response to the bacteria.
1942 Braun and Laskaris (1942) suggest two phases: 1) Inception 2) Stimulation
There are two key parts to the Agrobacterium mechanism:
1. Vir Proteins supress the plant's innate immune system
2. T-DNA integrates into the host cell's chromosomes.
Researchers clone their gene of interest into the tumor-inducing (Ti) plasmid which they then transform the Agrobacterium with. After the transformation, plants are then exposed to the Agrobacterium.
The intereaction is broken into 5 major steps:
2. Virulence (Vir) gene expression
-Vir A and Vir G function in a phosphorelay to activate VirG as a transcription factor to activate other Vir)
3. Attachment to host cell
-chvA, chvB, and pscA proteins allow the attachment of the bacterial cell to the host plant cell
4. Vir and T-DNA get into host cell
-Requires VirB complex of 12 proteins, VirB1-11 and VirD
5. T-DNA inserts itself in host chromosomes
T-DNA codes for:
-opines and agrocinopines (novel plant metabolites)
-VirD2 and VirE2 have nuclear localization signals (NLS) sequences that target them for the host nucleus.
-Some use GALLS protein complex instead If the blacterium is grown at its maximum temperature range then it loses it tumor inducing ability and thus its virulance. It can still survive, but cannot infect plant cells. The different strains of Agrobacterium are based on the different plasmids they carry. All have generally the same method of entry but slightly changed proteins.
A link to Agrobacterium.org which gives excellent genomic information about Agrobacterium and the annual Crown Gall Disease meeting.
OTHER RESOURCES USEFUL
A link to the MicrobeWiki
See an introductory video on the mechanism here
Pitzschke, A., & Hirt, H. (2010). New insights into an old story: Agrobacterium-induced tumour formation in plants by plant transformation. The EMBO journal, 29(6), 1021–32. doi:10.1038/emboj.2010.8