Plasmid Vectors - Plant cells do not have any endogenous plasmids. The plasmid vectors used for plant cell transformation are mostly based on pTi (tumour-inducing plasmid) of A. tumefaciens, but some are derived from pRi (root-inducing plasmid) of A. rhizogenes.

These are plant pathogenic, gram negative soil bacteria known for about 90 years to cause crowngall (A. tumefaciens) and hairy root diseases of dicot plants. They infect plant cells near wounds, usually at the crown of roots at the soil surface.