Transformation by Agrobacterium: A Natural Instance of Plant Genetic Engineering
Stephen Winans

Agrobacterium tumefaciens is a plant pathogenic bacterium that is able to cause tumerous growths on infected plants. It does this by transferring a piece of DNA into the nuclei of plant cells. This DNA becomes integrated into plant DNA. This DNA contains about fifteen genes, some of which cause the cell either to overproduce the growth hormones auxin and cytokinin or to become hypersensitive to the normal amounts of these hormones. This causes the cells to proliferate into a tumor. Other transferred genes cause the plant cell to synthesize and release a variety of novel compounds that the bacterium is able to use as food sources. This ability to deliver specific genes to plants is at its essence an example of genetic engineering, and so far as we know, it is unique to two species of Agrobacterium.

A second theme that we explore is the ability of these bacteria to communicate with each other inside or near plant tumors. They do this by synthesizing, releasing, and detecting low molecular weight compounds called autoinducers. By exchanging these compounds, the bacteria can estimate the size of the bacterial population, and carry out certain behaviors only at high population density.

In this exploration, students will inoculate plants with a variety of A. tumefaciens strains to investigate the growth of plant tumors. Students will take the plants home with them to watch the course of tumor development. We also will have a simple exercise demonstrating the release and detection of autoinducers by these bacteria.