Crown galls, a type of tumor, on plants usually form at the "crown," were the stem emerges from the soil. Crown galls are caused by the bacterium Agrobacterium tumefaciens. The bacteria attach to a wounded plant and insert a piece of DNA (called Ti) that genetically transforms a plant cell. The new genes cause the plant to make amino acids that the bacteria can use. Additionally, the genes code for substances that cause the plant cell to reproduce uncontrolled -- producing a tumor.

Researchers use the bacterium's Ti genes to transfer desirable genes to plant cells to get crops that are resistant to herbicides or are more nutritious.