USE OF TI-PLASMIDS FOR THE GENETIC ENGINEERING OF PLANTS.

Crown gall is a neoplastic disease of many dicotyledonous plants and is caused by the soil bacterium Agrobacterium tumefaciens. A large extrachromosomal plasmid in these bacteria was found to be responsible for its tumor-inducing capacity and was, therefore, called Ti-plasmid. Bacteria-free crown gall cells can be cultured in the absence of phytohormones like auxines and cytokinines, and this hormone-independent growth defines tumorous growth in plants. 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. The T-DNA has been localized in the nucleus and is directly responsible for the hormone-independent growth of the cells. It is also responsible for the synthesis of a number of low molecular weight compounds, called opines, which are not found in normal plant tissues. The opine produced defines crown galls as octopine, nopaline, or agropine type tumors. Opines can be utilized by Agrobacterium tumefaciens selectively as sources for carbon, nitrogen, and energy; thus, the interaction between these bacteria and plants can be seen as a special parasitic relationship which benefits the bacteria.