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independent gene-transfer method using electric discharge particle bombardment.
Later, Cao et al., (1992) reported the production of transgenic rice plants using
PDS1000IHe biolistic method with embryogenic cell suspensions. Since then, this
method has been used in the production of transgenic rice plants with dise;ise
resistance (Datta et al.,
2001).
insect resistance (Sudhnkar et ol.. 1998) and herbicide
resistance (Oard eta!., 1996) genes. More significantly, biolistic method is claimed to
be genotype independent and transformation frequency as high as in dicots has been
reported in some cases (Tyagi et al., 1999).
2.5.2.
Agrobacterium-mediated transformation
The search for microorganisms that could be used as vehicles for DNA lead
researchers to the genus Agrobacterium.
A.
tumefaciens is capable of infecting almost
any plant tissue. Because of its uniqueness, A. tumefaciens has become the primary
choice for many plant transformation experiments. A. tumefaciens is a soil dwelling
pathogenic bacterium first described in 1943 (Braun. 1943). Some strains of
Agrobacterium are capable of infecting almost all wounded dicotyledonous plants,
which makes it a perfect candidate as a transformation vector.
2.5.2.1. Mechanism of
T-DNA
transfer
Agrobacterium tumefaciens, a Gram-negative soil bacterium considered as a
natural genetic engineer, transfers a portion of its Ti (tumor inducing) plasmid called
T-DNA into the host genomes through a well developed system. The T-DNA transfer
initiates when Agrobacterium perceives certain phenolic and sugar compounds from
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