Agrobacterium tumefaciens - Uses
Usefulness as a gene delivery system
Agrobacterium tumefaciens is seen as such a useful gene delivery system because it is able to carry any gene of interest within the T-complex, and insert the gene into the target plants DNA with a high degree of success. The reason for this is because unlike other mobile genetic elements such as transposons and retroviruses, the T-DNA strand does not encode functions required for movement and integration of the DNA. Therefore the T-DNA strand can be replaced by a gene of interest which will be inserted automatically into the host plant nucleus with a high degree of success and with little human intervention. This process is usually much more efficient than traditional methods of genetic modification. Follow the link below to the Purdue Agricultural Biotechnology Website, to see an animated demonstration of how A. tumefaciens can be used to genetically modify plants.
A. tumefaciens mediated transformation is relatively efficient for many species, and a low copy number of intact, unmonified transgenes are frequently integrated successfully into the plant genome. However, transformation of many crop species has, in the past, been relatively ineficient, although recent advances in transformation technology is set to change that.
There are however a few species of dicotyledonous plants and most species of monocotyledonous which are recalcitrant to transformation by A. tumefaciens. Ke et al. (2001) investigated whether synthesis at a high level of a T-strand DNA intermediate could improve the transformation efficiency of plants. It was found that a mutation in the gene regulator virG & VirGN54D when combined to produce a strain producing high level of T-strand DNA did indeed have a positive effect on the efficiency of transformation.
Despite the many recent developments in the world of plant genetic manipulation, A. tumefaciens still remains a major method of choice for transforming plant cells, despite the development of sophisticated alternative gene transfer methods. Work is still ongoing to try and improve our understanding of the gene transfer mechanism. A number of economically important cereals have now been transformed using A. tumefaciens (Newell, 2000), working alongside other, more traditional gene transfer methods.
Applications of the Technology
Agrobacterium tumefaciens may prove to be the breakthrough needed in order
to successfully insert foreign DNA into plant genomes for genetic modification.
Bacterial vectors such as Eschericia coli have already been used successfully
as vectors in microbiology (Kikkert et al., 1999) ; this same technology can now be applied to the field
of botany. Several different plant species have already been successfully transformed,
including Lettuce (Curtis, 1995), Rice (Hiei, 1997) and Tomato (Tzfira et al., 2002).
This proves that direct gene transfer methods are no longer the only avenue of
approach for transforming important crop plants (Newell, 2000). One of the main
reasons for favouring transformation by A. tumefaciens is that it allows
delivery of a well defined piece of DNA into the plant genome, although the success
rate is not 100% (Gheysen et al., 1998).
There are however some valid arguments against the validity of A. tumefaciens
mediated transformation.