[SagarikaGhosh]

Plant cells do not have any endogenous plasmids. The plasmid vectors used for plant cell transformation are mostly based on pTi (tumor inducing plasmid) of A.tumifaciens, but some are derived from pRi (root inducing plasmid) of A.rhizogenes. these are plant pathogenic, gram negative soil bacteria known for more than 90 years to cause crown gall (A.tumifaciens) and hairy root (A.rhizogenes) disease of dicot plants. They infect plant cells near wounds, usually at the crown of roots at the soil surface.

Mechanism of Agrobacterium Infection:

The molecular mechanisms involved in Agrobacterium infection of plant cells became known only recently during the 1980s. the process of infection by A.tumifaciens culminates in the transfer of a small part of pTi into the plant cell genome; this DNA sequence is called T-DNA (transferred DNA). The infection process is governed by both chromosomal and the plasmid-borne genes of A.tumifaciens. Infection begins when Agrobacterium cells become attached to plant cells; this step determines the host range of bacterium, is a function of host –parasite interaction, and is governed by bacterial chromosomal genes, generally the chv (chromosomal virulence) genes. Most of these genes, e.g., chvB, exo genes, cel genes, are concerned with the biosynthesis of cell attachment polysaccharides due to which the bacterial cells become firmly adhered to plant cells. But at least two genes, viz., chvD and chvE, are needed for an optimal expression of pTi vir genes. These chromosomal genes are expressed constitutively, i.e. expressed in all bacterial cells at all the times.

Vectors derived from pTi:

The use of wild type pTi as a vector presents the following three problems:
1. presence of oncogenes (iaaM, iaaH and ipt) in T DNA which causes a disorganized growth and a loss of regeneration potential of the cells having T DNA in their genomes.
2. Their large size makes the handling procedures during cloning tedious and cumbersome.
3. A general lack of unique cloning sites within the T DNA, which are needed for the insertion of DNA segments to be cloned.

These problems have been resolved by deleting the oncogenes from the T-DNA (disarming and by developing intermediate vectors and binary vectors to facilitate gene cloning procedures.