PLANT GALLS

• A gall is a plant structure formed by abnormal growth within plant tissues
• The growth is a reaction to a parasitic attack on/in the plant's cells
• Galls are numerous, widespread, and come in a variety of shapes and colors
• By looking at a gall's shape, color, and location on the plant, you can determine what organism caused it

What causes a gall?

• Excluding viruses, there are 5 major gall-causers: Bacteria, Fungi, Nematoda, Acarina and Insecta
• Each species of gall-causer attacks a specific plant structure, such as the leaves or the roots
• Because the gall-causers do not destroy the plant, but slowly uses its' nutrients, they can be called parasites
• The parasites can move inside or remain outside of the plant structures
• In either case, the parasites act only as a growth stimulus
• The gall is made up entirely of the plant's (the host's) tissues
• The plant reacts to the parasite's intrusion by increasing the number of plant cells or enlargening the cell's size
• Gall tissues form a "bubble" around the gall-causers in order to separate the parasites and any of their harmful by-products from the rest of the plant
• Therefore, the gall is the plant's way of protecting itself from intruders

Gall forms

• The gall's form depends on what organism is attacking the plant and where it is attacking
• There is a classifying system of gall forms based on where the gall-causers attack
• Galls can be unilocular- having one cavity, or plurilocular- having one or more cavities
• Galls can be unilarval- having one parasite in each cavity, or multilarval- having one or more parasite in each cavity

Parasite-plant relationship

There must be a balanced relationship between the parasite and the plant in order for them to both continue their existance. If too many gall-causers infect one plant, the plant would not be able to protect itself properly. Also, if there are too many gall-causers on one plant, for reasons not completely clear, it has been shown that the amount of parasitic offspring is decreased. It therefore benefits both parasite and plant to have a limited amount of parasites on each structure. The system is dependent on the evolved preference of parasites to not attack structures with other parasites nearby. This belief is supported by the fact that after a certain number of galls arise per structure, it is rare to see an increase in gall number.

What benefits do gall-causers get out of the relationship?

Is food, shelter, and a place to breed enough? The gall provides a microclimate for the organism. A microclimate is a small area characterized by uniformity in environmental conditions. In this case, the microclimate is protected from wind, storms, and temperature extremes. A protein rich inner wall lines the parasite's "home" with plenty of food. Not a bad deal. With these conditions, the parasites are able to breed very successfully, and breeding success is the name of the game.

What do plants get out of it?

Usually nothing, but in most cases the plants are not harmed either. There are two rare exceptions: Andricus quercus-calicis and Rhizobium radicola and R. beyerinckii. Rhizobium radicola and R. beyrinckii are bacteria which have been shown to be beneficial to leguminous plants- plants within the pea family. The bacteria are located within root galls and assist the legumes in nitrogen uptake. On the other hand, Andricus quercus-calicis hinders reproduction in certain oaks since it destroys their acorns and therefore their seeds. For the most part, however, the only harm to the plant is that it is forced to use energy and materials for gall formation instead of for improving other structures.

Who cares about galls anyway?

There is a lot still unknown about the mechanisms that cause gall formation and the life cycles of the organisms that initiate the gall growth. Since most galls do not cause any economic damage to crop plants, little research funding is available in this area. However, the insect cycles and gall structures should be of interest because they are amazing examples of the complexity of nature and of co-evolution. Co-evolution is when closely associated species undergo complementary evolution. Over generations, characteristics of a species can mutate. If these mutations are favorable to the species, meaning they can better survive and reproduce in its environment, then this mutated species will flourish. An example of this is the insect that inherits and passes on characteristics for the behavior to burrow into plant tissues. A closely associated species, in this example, the plant being burrowed into, can also mutate. If these mutations enable the plant to survive and reproduce even with the damage that the insect causes, then the mutated form will flourish. The mutated plant which can form galls can survive the insects intrusion into its' tissues. Each organism has evolved based on the other's evolution. They have both changed in ways to increase their survival and reproduction rate. Insects have obtained a means of receiving room and board at no cost, while the plant has learned to deal with this in a way that makes living a normal life and reproduction possible.

For an example of a plant gall see oak apple gall.