Most dicotyledonous plants are susceptible to tumour formation as a result of infection of wounded sites by the Gram-negative bacterium Agrobacterium tumefaciens (for a general review see Caplan et al. 1983). The bacterium is necessary for tumour induction but not for tumour maintenance and growth. The ability of the bacteria to induce tumour formation is strictly limited to those Agrobacteria species harbouring a large extrachromosomal DNA element, the so-called Ti-(tumour-inducing) plasmid, which has a size of 120-180 kb. The molecular basis underlying the neoplastic transformation is the transfer and stable integration of a well-defined part of the Ti-plasmid, the so-called T-DNA (T=Transfer or Tumour) into the plant nuclear DNA, leading to the formation of a gall. The Ti-plasmid therefore represents a natural gene vector to plant cells. As it is essential for the practical use of this system for gene transfer to obtain normal plants; mutants of the Ti-plasmid have been constructed with all the T-DNA genes leading to tumour formation deleted. Normal plants which are fertile and which sexually transmit the introduced genes as a single, dominant Mendelian locus (Zambryski et al. 1983) have been obtained.
The main limitation at the present time concerning the use of Ti-plasmid mediated gene transfer is the limited host range of Agrobacterium species.