Gall is an abnormal growth on plants that is often seen as a rounded swelling. Galls can form on roots, stems, and leaves, and even on flowers and seed pods. Galls range in size from tiny to enormous.
Most galls are caused by parasites (organisms that feed and live on the plant). These parasites include animals, such as certain wasps and worms; fungi, such as smuts and some rusts; plants, including certain mistletoes; and various bacteria and viruses. The gall-causing agents produce chemical substances called phytohormones, which stimulate plant cells around the agent to multiply and become abnormally enlarged.
Abnormal growths resembling galls also may form as the result of a genetic defect in the plant. Such galls or tumors develop in many plants produced by crossbreeding, including apricot, lily, tobacco, and tomato hybrids. These tumors are thought to result from an imbalance of phytohormones.
Some galls and the agents that cause them can seriously damage plants. For example, a species of worm called the rootknot nematode causes galls in potato, corn, and many other plants. The worm feeds on the plant roots, and the galls divert nutrients from the rest of the plant.
Other galls benefit plants. For example, galls or nodules formed by Rhizobium bacteria on the roots of legumes (plants in the pea family) help supply life-giving nitrogen. Rhizobium converts nitrogen gas from the air to nitrogen compounds the plant can use.
Studies of crown gall have led to the development of improved plants. This gall is caused by Agrobacterium tumefaciens, a bacterium that infects many kinds of plants. Upon infection, the bacterium transfers some of its genes to chromosomes of the plant. Using genetic engineering, scientists can alter the bacterium so it transfers genes that give the plant more favorable traits. In this way, researchers have developed disease-resistant varieties of crop plants.
