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wounded plant cells. These phenolic compounds serve as inducer or coinducers of the
bacterial vir genes. Phenolic chemicals such as acetosyringone and related compounds
are perceived via the VirA sensory protein. Then. VirA protein undcrgoes
autophosphorylation followed by the tmnsphosphorylntion of VirG protein resulting
in the activation of vir gene transcription. Most of the induced vir proteins nre directly
involved in T-DNA processing from the Ti plasmid and subsequent transfer of
T-
DNA from bacterium to the plant.
VirD2 protein accompanied by VirD1 is directly involved in processing the T-
DNA from the Ti plasmid. VirD2 nicks the Ti plasmid at 25 bp directly repeated
sequences, called T-DNA borders that flank the T-DNA. Nicked T-DNA is
transferred to plants as a single stranded DNA molecule called T-strand. Thus. T-
DNA enters the plant as a protein/nucleic acid complex composed of a single VirD2
molecule attached to a single-stranded T-DNA. Apart from this, several other vir
genes and host-coded proteins are also involved in the T-DNA transfer process. Once
the T-DNA enters the host nucleus, T-DNA genes encode enzymes involved in the
production of plant growth regulators (auxin and cytokinins) resulting in increased
cell division leading to crown gall formation. Apart from growth regulators. T-DNA
genes encode enzymes involved in the production of opines which are consumed
specifically by the infectious bacteria (Gelvin, 2000).
To use this naturally occurring gene transfer mechanism for plant genetic
engineering, all the genes that are naturally present in T-DNA are inactivated or
replaced with other genes (gene of interest). A number of sophisticated plant-
transformation vectors were designed on the basis of this naturally occurring gene
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