text: Bacteria related to the nitrogen fixing Rhizobia are an interesting case of inter-kingdom conjugation. For example, the tumor-inducing (Ti) plasmid of Agrobacterium and the root-tumor inducing (Ri) plasmid of A. rhizogenes contain genes that are capable of transferring to plant cells. The expression of these genes effectively transforms the plant cells into opine-producing factories. Opines are used by the bacteria as sources of nitrogen and energy. Infected cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.
  The Ti and Ri plasmids can also be transferred between bacteria using a system (the tra, or transfer, operon) that is different and independent of the system used for inter-kingdom transfer (the vir, or virulence, operon). Such transfers create virulent strains from previously avirulent strains.