The Ti-Plasmids of Agrobacterium tumefaciens

Abstract

Many species of higher plants, in fact most dicotyledonous plants, are susceptible to crown gall disease (De Cleene and De Ley 1976). This disease is characterized by the formation of tumors, often at the crown separating stem from roots, as a result of infection of wounded sites by gram-negative soil bacteria (Agrobacterium tumefaciens). Although these bacteria are required for tumor induction and are mostly found associated with crown gall harboring plants in nature, they are not necessary for tumor maintenance and growth (BRAUN 1953). In fact, sterile crown gall tissues can readily be cultured indefinitely on simple media lacking any added growth hormones. Crown gall plant cells therefore are persistently altered cells with a capacity for autonomous growth in the absence of any outside Stimulation by the inciting bacteria. The normal mechanisms Controlling growth and differentiation of plant cells do not operate properly in these transformed cells and, as such, crown gall cells can be compared with animal tumor cells.