Overview
The laboratory is interested in the biology and the molecular biology of the plant
pathogen, Agrobacterium tumefaciens. This organism causes cancerous tumors, called crown
galls, on susceptible plants. The tumors result from the transfer of a small piece of DNA,
called the T-DNA, from the bacterium to the plant cell. The T-DNA becomes integrated into
plant cell nuclear DNA and expression of genes on this segment causes the normal plant cell
to differentiate into a tumor cell. Expression of additional T-DNA genes causes the tumor
cells to produce and secrete novel small carbon compounds called opines. In turn,
Agrobacterium cells can utilize opines as sole carbon and energy sources. Thus, the
bacterium has developed a way to genetically transform plants to produce a carbon source that
only it can utilize. The ability of the bacterium to specifically utilize the opines that
the T-DNA instructs the plant tumor to produce led to the idea that these compounds play a
central role in the biology of Agrobacterium. According to the Opine Concept, these
molecules serve to foster the relationship between the bacterium and the plant by providing
the bacterium with a source of carbon that only it can utilize. This, then provides a
selective advantage to Agrobacterium in the face of competition by other microorganisms
present in the soil. Agrobacterium tumefaciens then, is Nature's own genetic
engineer. It has evolved the ability to genetically alter plants such that the tumors produce a
nutritional source that only the bacterium can utilize.
We focus on two general areas of the Agrobacterium-Plant interaction. Both have as a
common theme the roles that opines play in the biology of this microbe-host relationship. In
the bacterium, the T-DNA and the genes for opine catabolism reside on a large,
extrachromosomal virulence element called the Ti plasmid. Thus, this plasmid codes for most
of the machinery necessary for the bacterium to transform the plant host into an
opine-producing factory, as well as to confer on the bacterium the ability to utilize the
opines produced by the plant tumor. Seen another way, the Ti plasmid contains genes that,
when transferred to the plant, instruct the host to synthesize novel metabolites and also
genes that confer on the bacterium the capacity to take up and degrade these novel
metabolites supplied by the transformed plants. Opines play a second role in the life of
Agrobacterium. The Ti plasmids are conjugal elements, being transmissible by direct
contact mating between bacterial donor and recipient cells. However conjugation normally is strongly
repressed. Induction is dependent upon the donor bacteria receiving a proper set of chemical
signals. These signals are a subset of the opines produced by the crown gall tumors. Thus
these metabolites also act as signal molecules to instruct the bacterium to initiate conjugal
transfer, and hence horizontal transfer of the Ti plasmids.