Crown gall: tumor as a result of oncogenic DNA transfer

It has been well documented that some viruses can cause neoplasmic transformations in animal cells because particular segments of the virus genome integrated in the animal cell DNA alter the differentiated state of these transformed cells. Therefore, the DNA of these viruses is oncogenic. Recent studies on so-called crown gall tumors in plants have shown that oncogenic DNA's need not always be of viral origin. Indeed, it was shown that these plant tumors result from the integration, maintenance, and expression in the plant cell nucleus of a specific DNA segment introduced in the plant cells by some pathogenic bacteria. Thus crown gall neoplastic transformation is a natural example of "genetic engineering" because it was shown that the growth of the tumor-inducing bacteria is stimulated by organic compounds (so-called "opines") excreted by the transformed plant cells. The bacterial DNA transferred to and expressed in the plant cells is directly responsible for the synthesis of the opines and for the tumorous character of the transformed plant cells. The bacterial oncogenic DNA (T-DNA) is a segment of an extrachromosomal element (Ti-plasmid) carried by the tumor-inducing bacteria. A complicated set of genes on the Ti-plasmid control the various steps involved in the tumorous transformation and are thus responsible for the transfer of the oncogenic T-DNA. A molecular study of the products of the T-DNA will allow us to have a better understanding of the control of growth and differentiation in eukaryotic cells. Furthermore, the Ti-plasmid can be used as a gene vector and thus allow the transfer of selected genes into plants.