Agrobacterium is a gram-negative bacteria involved in causing root gall formation disease in plant species. The crown gall formation is due to the transfer of a segment of oncogene DNA into plant cell at wounded sites. This DNA segment i.e. T-dna or transfer DNA is present on a large plasmid called tumor-inducing plasmid in the bacterium. The T-dna is integrated into the plant chromosome by recombination. I series of vir genes are involved in directing this infection process. So when the plant root or stem is wounded it gives off certain signals. In response to those signals, A.T becomes activated and directs a series of events required for the transfer of T-dna from ti plasmid to the plant chromosome. The function of different vir gene includes a copy of T-dna followed by attachment of product to the copied T-dna strand, subsequently add proteins along with the length of T-dna, and possibly act as a protective mechanisms. These eventually open a channel in the bacterial cell membrane, through which the T-dna passes. It enters the plant then through the Wounds. To use these bacteria as vector its T-dna region is removed except the border region and vir genes. The transgene is then inserted between the T-dna regions where it is transferred to the plant and becomes integrated into the plant chromosome. The T-dna is cloned in ti plasmid which is cut to size and replicated in E.coli to facilitate further manipulation. These vectors are mobilized in agrobacterium host strains and used to infect plant tissues. These infected plant tissues are subsequently grown on media containing specific chemicals, growth regulators to facilitate the regeneration of transformed cells.
TYPES OF VIR GENES
vir A– senses actinosyringone
vir b– conjugational ores between plant cell and bacteria,
vir b11– ATPase activity,
vir c– helps in dna transfer-
vir d1– essential for cleavage of super coiled stranded substrate, respond for gene transfer protein.
vir g– transcriptional activator of vir box