Agrobacterium tumefaciens (A.t.) is a soil-inhabiting bacterium that causes a disease known as crown gall in many plant species. Roots naturally exude chemicals into the rhizosphere that can be detected by microbes in the soil. A wound can increase the flow of exudates from a plant and specific compounds, such as precursor molecules for lignin (produced by the plant as wound tissue) can stimulate the process of pathogenesis. An organic matrix is created by the bacteria as they attach to the surface of the plant. This matrix facilitates the chance of successful colonization. The cytoplasm of A.t. contains two types of DNA: (1.) a chromosome form and (2.) a smaller, circular piece of DNA called a plasmid. Basically, the plasmid has a short segment (T-DNA) that has genes for hormone production and opine synthesis. Opines are a carbon compound that the bacteria can utilize. This T-DNA is transferred from the bacterial cell through the cell walls of the plant and into the plant cell nucleus. In the nucleus, the T-DNA integrates into the plant chromosome. The cellular processes of the plant treat the T-DNA as it's own and production of the hormones indoleacetic acid and cytokinin begins. Plant cells proliferate undifferentiated tissue, forming a gall. Opines are also produced that are metabolized only by the bacteria. The bacterium may colonize the roots, crown, and other parts of the plant. A.t. has evolved to genetically colonize its host. This is an amazing feat of cross-kingdom genetic engineering by a common soil dwelling bacterium. A computer animation (21 MB mov) illustrates the natural pathogenesis. A second computer animation (16 MB mov) illustrates how this phenomenon is utilized in the lab for Agrobacterium mediated transformation.

Agrobacterium tumefaciens has Gram-negative cell walls.

Left: A 3-D model of Gram-negative (left)
and Gram-positive (right) cell walls. 

Click the image to enlarge.

Flagellar movement of bacteria.
A wounded plant cell will begin
to synthesize lignin in order to heal the wound.
Specific plant lignin precursors are chemotactically
sensed by the pathogen. Flagellar
movement is a series of tumbles and runs counterclockwise up
the gradient and cells have been
reported to move as fast as
60 um/second.

A. tumefaciens exhibits polar attachment to the plant cell.
The production of cellulose fibrils serve to anchor the bacteria to the plant
as well as trap other bacteria.  Once the concentration of lignin precursors
reaches approximately 10^-5 M, the virulence genes of the Ti plasmid are induced
and the T-DNA is processed.

Plant cell with nucleus and endoplasmic reticulum

Plant cell nucleus with nuclear pores, the entry site for T-DNA

The model at left shows in an approximate manner how the T-DNA moves into the plant cell and is passed
through the nuclear pores. The bacterium forms an external pilus (type IV secretion system) for the transfer of T-DNA into the plant cell. The assembly of this secretion system has been studied. A draft animation of this process is available. 
Type IV Secretion System