clarify parts of the story. If one of the hormone biosynthesis genes was disrupted and the
bacterium was inoculated onto tobacco plants, the tumor would produce a mass of mis-
shapen roots. If the other hormone biosynthesis gene were disrupted, a shooty tumor
would result from tobacco inoculation. If both genes were disrupted, no tumor would
form (Fig. 10.3). This hormone effect was suspiciously similar to results obtained with
tobacco callus in tissue culture, and these different tumor phenotypes were correctly ident-
ified as resulting from an altered hormone balance in this tissue. Much of the research that
showed the transfer of DNA from the bacterium to the plant cell and even speculation on the
use of this process to improve plants was put forward by the “AgrobacteriumQueen,”
Mary-Dell Chilton (see Life Box 1.2). Other contributions from Monsanto scientists and
Marc van Montague and colleagues from Ghent University were also very significant.
10.3.2 Use of the T-DNA Transfer Process for Transformation
The transition from forming tumors on tobacco stems to routinely transforming wheat and
corn with specific genes of interest resulted from multiple advances in the understanding of
both the T-DNA process and the interaction of bacteria with plant cells. Since there are
Figure 10.3.Sunflower seedling hypocotyls inoculated withAgrobacteriumwithout (a) and with (b)
hormone biosynthesis genes.
Figure 10.2.Agrobacteriumgrowing on soybean tissue.10.3. AGROBACTERIUM 251