16.5F: Agrobacterium and Crown Gall Disease

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Argobacterium causes Crown Gall Disease by transferring a DNA plasmid to the host plant, causing the host to make nutrients for it.

Learning Objectives

Summarize the symbiotic relationship between plants and agrobacterium

Key Points

Crown Gall Disease is caused by Agrobacterium tumefaciens, a bacteria that infects plants. The bacteria causes tumors on the stem of its host.
Agrobacterium tumefaciens manipulates its hosts by transferring a DNA plasmid to the cells of its host. Plasmids are normally used to transfer DNA from bacteria to bacteria.
Once in the host cell, the plasmid integrates itself into the host plant cell’s genome and forces the host to produce unique amino acids and other substances which nourish the bacteria. These compounds are unusable by most bacteria, so Argobacteria can out-compete other species.

Key Terms

plasmid: A circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa.
pilus: A hair-like appendage found on the cell surface of many bacteria.

Crown Gall Disease is caused by a bacteria called Agrobacterium tumefaciens. The disease manifests as a tumor-like growth usually at the junction of the root and shoot. A. tumefaciens can transfer part of its DNA to the host plant, through a plasmid – a bacterial DNA molecule that is independent of a chromosome. The new DNA segment causes the plant to produce unusual amino acids and plant hormones which provide the bacteria with carbon and nitrogen.

Figure: **Smart plants cue farmers to nutrient deficiencies**: A. tumefaciens attaching itself to a plant cell

Bacteria normally use plasmids for horizontal gene transfer, so they can share genes with related bacteria to help them cope with stressful environments. For example, plasmids can confer on bacteria the ability to fix nitrogen, or to resist antibiotic compounds. Typically bacteria transfer plasmids through conjugation: a donor bacteria creates a tube called a pilus that penetrates the cell wall of the recipient bacteria and the plasmid DNA passes through the tube. The other bacteria either integrates the plasmid into its chromosomes, or it remains free-floating in the cytoplasm. In either case, the recipient bacteria receives new genetic material.

In the case of Crown Gall Disease, A. tumefaciens transfers a plasmid containing T-DNA into the cells of its host plant through conjugation, as it would with another bacteria. However, once inside the plant cell, the DNA integrates semi-randomly into the genome of the plant and changes the behavior of the celll.

The new plasmid genes are expressed by the plant cells, and cause them to secrete enzymes that produce the amino acids octopine or nopaline. It also carries genes for the biosynthesis of the plant hormones, auxin and cytokinins, and for the biosynthesis of opines, providing a carbon and nitrogen source for the bacteria.

These opines can be used by very few other bacteria and give A. tumefaciens a competitive advantage.