The Crown Gall Breakthrough
As the events described in chapters 1 and 2 were unfolding, another avenue to introduce foreign DNA into plants was being discovered: the now classical Agrobactrerium-mediated gene transfer process. Thus, the first unequivocal demonstration that DNA can be transferred to plant cells came in fact from the study of a seemingly mundane plant pathogen, the soil bacterium Agrobacterium tumefaciens. Although fundamentally different from the notion that plants could conceivably be genetically transformed by uptake of isolated DNA, this new gene transfer process gave confidence that DNAmediated transformation might one day become a reality as well. However, the study of Agrobacterium and crown gall was not originally intended to lead to the genetic transformation of plants; this became a goal only after the nature of crown gall disease had been partially understood.
It had been known since the turn of the century that A. tumefaciens was the causative agent of crown gall, a neoplastic disease of dicotyledonous plants, characterized by undifferentiated tumor formation, usually at the site of a wound (fig. 3.1). The work of Armin Braun of Rockefeller University in the 1940s clearly demonstrated that tumor tissue could be propagated in vitro under axenic conditions, meaning that once tumorigenesis has been incited, the causative agents, the bacteria, are no longer needed. Thus, A. tumefaciens is necessary for tumor initiation but not for tumor maintenance and proliferation. These cells are thus permanently [transformed.] Further, crown gall tumors can be propagated on defined culture medium without phytohormones, auxins, and cytokinins, conditions under which normal plant tissue (except habituated tissue, a very rare occurrence) will simply not survive. Also, tumors cannot be forced to redifferentiate into morphologically normal plants through the action of externally added plant hormones. Somehow, they are [stuck] in a permanent undifferentiated or semidifferentiated state.