(Picture
to the left of Dr. E. F. Smith from An Outline of the History of Phytopathology
by Herbert Hice Whetzel)
In 1890, Erwin F. Smith (1854-1927) showed that crown gall is a disease caused by bacteria. He was fascinated by the etiology and control of the disease, as well as its morphology. Smith considered it to be similar to cancerous tumors of humans and animals. In 1913, he received a certificate of honor from the American Medical Association for his work on "Cancer in Plants".
So
what is crown gall and why was it similar to tumor in animals and plants?
This question could not be answered further until 1980. Crown gall is a
disease caused by the bacterium Agrobacterium tumefaciens. The symptoms,
galls or tumors (called plant cancers initially) are found most often at
or near the soil line on the "crown" of the plant. The bacterium
lives in the intercellular spaces of the gall tissue, or undifferentiated
mass of plant tissue. This pathogen can live in the soil as a saprophyte
for many years. It enters the plants through wounds and has a very wide
host range. Galls can be removed for aesthetic purposes but this does not
remove infection.
But why was it considered to be similar to cancer and why is it considered a genetic engineer? The tumors or callus that is formed on a plant infected with Agrobacterium tumefaciens resemble tumors seen in animal cancers, both grow uncontrollably.
As for being a genetic engineer it was not until 1980 that this disease was finally explained. Crown galls were found to be induced by excessive hormones produced by uncontrollable expression of certian genes present in a piece of DNA introduced by the bacterium into the plant genome. So what does that mean?
The bacterium carries a plasmid (small chromosomelike bodies composed of circular double-stranded DNA), the plasmid contains a gene for tumor induction (called Ti for tumor inducing). The plasmid is often called the Ti-plasmid. Once the bacterium has infected the host, it attatches to host cells. The Ti-plasmid processes T-DNA and then through conjugation transfers the T-DNA into the plant cell. This T-DNA is then intergrated into the plant cell genome. The plant begins to overproduce growth regulators which lead to the tumors or gall formation.
To see a slide show of how Agrobacterium genetically modifies plants check out this page..... Agrobacterium--The First Genetic Engineer
To get the story behind the Fischer-Smith Debates click here
