Ti-Plasmids: Genetic Engineering of Plants

Abstract

Crown gall is a neoplastic disease of most dicotyledonous plants and is caused by the soil bacterium Agrobacterium tumefaciens. A large extra-chromosomal plasmid in these bacteria was found to be responsible for its tumor-inducing capacity and was, therefore, called Ti-plasmid (1). Bacteria-free crown gall cells can be cultured in the absence of phytohormones and this hormone-independent growth defines tumor cells in plants (2). Sterile tumor tissues have been shown to contain a DNA segment (called T-DNA) which is homologous and colinear with a defined fragment of the Ti-plasmid, and it is covalently linked to plant DNA (3–9). The T-DNA has been localized in the nucleus (10, 11) and is directly responsible for the hormone-independent growth of the tumor cells. It is also responsible for the synthesis of low molecular weight compounds, called opines, which are not found in normal plant tissue. The opine produced defines crown galls as octopine, nopaline or agropine type tumors (12). Opines can be utilized by A. tumefaciens selectively as sources for carbon, nitrogen and energy, and, thus, the interaction between these bacteria and plants can be seen as a special parasitic relationship which benefits the bacteria (4).