notably roses, grapes, and euonymus. The main symptom of the disease is a gall or tumor
that forms on the cut stem (Fig. 10.1) or crown of a plant. Thecrownis the part of the plant
that lies at the soil–air interface. This disease was a mystery to plant pathologists for many
years as it does not always follow Koch’s postulates, which specify that an extract from an
infected organism should cause the disease when reinoculated on a healthy plant. Also, the
tumors that were formed on plants would continue to grow in the absence of any microor-
ganisms. For some time, the plant tumors were thought to be similar to some types of
human cancer, but this was an incorrect assumption. Why do plant cells infected with
wild-typeAgrobacteriumgrow as a tumor?
Unraveling the mystery of the disease is a fascinating story in itself and has led to the use
of the bacterium for genetic engineering research. For crown gall disease, wild-type
Agrobacteriuminvade wounded tissues of dicotyledonous plants. The crown is a suitable
entry point as the stem is often split or torn here. The bacteria may either colonize dead
and dying cells or simply attach themselves to the outside of a wounded living cell
(Fig. 10.2). Through a series of chemical signals that are sent from the plant cell to the bac-
terium,virulencegenes are activated in the bacterium that cause the bacterium to enter its
virulence mode. Some of the more important mechanisms are outlined in the next section.
In the end, the T-DNA is excised from the bacterium and delivered to the genome of the
target plant cell.
For wild-type bacteria, the T-DNA contains only a few genes, which encode for
enzymes leading to the production of plant hormones and an opine, which is a nitrogen-
rich organic compound that is a suitable food source for the bacterium. Tumors are
formed as a result of hormone production in the plant cells, and the opines that are produced
in the tumor are used by bacteria on the tumor or in the soil after being washed from the
tumor by rain. The bacteria do not colonize living, dividing tumor cells, and these tumor
cells can be grown in tissue culture without added hormones. Generation and analysis of
someAgrobacteriummutants that contained disrupted hormone synthesis genes helped
Figure 10.1.Agrobacterium-induced tumor formation on tobacco stem.250 TRANSGENIC PLANT PRODUCTION