Why is the formation of a gall a defense response of the tree?

Answer

Dear Didier,

What an interesting question! Did you know that there are hundreds of different types of galls, even on stamens and roots of plants? The study of galls even has a separate name: cecidology (or cecidology in English).

A gall results from a certain kind of interaction between an insect, bacterium or fungus on the one hand, and a plant on the other. Usually these insects and microorganisms will eat plants, but sometimes they go a little further and cause the formation of a gall. As a result, they are nicely surrounded by food and they are protected. You rightly wonder why a plant contributes to this. Usually, the plant does not benefit at all from this interaction and the gall former is considered a parasite.

When a plant ‘co-operates’ in the formation of a gall, it is often in response to damage caused by the insect (or other organism). Just as a wound in a person prefers to close as quickly as possible so that no infections can develop, the plant will protect itself by letting cells ‘proliferate’ and thus repair the damage as quickly as possible. The insect itself then ‘molds’ these cells into its desired shape by injecting certain chemicals. Thus, each of the organisms has its own bile form. In gall wasps these are the typical gall apples on oak leaves.

Although the bile-forming organisms are in most cases the only ones that benefit from this interaction and cause harm to the host (=parasitism), there are cases known in which the host, the plant, also benefits from it (=symbiosis ). For example, there is the Rhizobium bacterium that causes nodules (galls) on the roots of leguminous plants (for example, bean plants). These bacteria absorb nitrogen from the air, which they then pass on to the plant via the roots. For example, beans can grow on nutrient-poor soil without additional nitrogen fertilization!

In conclusion, we can say that the initiation and development of the bile is regulated by the insect or micro-organism, and that the plant usually only cooperates in quickly closing a wound and thus limiting and repairing damage. And sometimes, very occasionally, there are cases where a plant even benefits from a gall…