The Use of Ti-Plasmids for the Genetic Engineering of Plants

Abstract

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 [36]. 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 [1]. 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 [4,23,29,33,34,44,46]. The T-DNA has been localized in the nucleus [5,37] 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 [15](Fig. 1). 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 [29].