Galls are abnormal tumorous plant growths caused by modifications of plant cells and can be caused by various parasites, from fungi and bacteria, to insects and mites. Galls are often very organised structures and because of this, the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite galls.
Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf and twig galls are most noticeable. Galls come in an astounding array of colors, shapes and sizes.
Insect galls develop under the influence of gall-inducing insects. Insect galls are usually induced by the chemicals injected by the larvae or the adults in the plants, either causing mechanical damage or not. After the galls are formed, the larvae develop inside until fully grown, at which time they leave, sometimes as adults. In order to form galls, the insects must seize the time when plant cell division occurs at a high speed, the growing season, usually spring in temperate climates, but which can be extended in tropical latitudes. Also, the specific places where plant cell division occurs are needed to induce galls, that is, the meristems. Although insect galls can be found on a variety of parts of the plant, such as the leaves, stalks, branches, buds, roots or even flowers and fruits, gall-inducing insects are usually species-specific and sometimes tissue-specific on the plants they gall. Exceptions to this rule induce galls on plants similar to each other, frequently within genera or family.
Gall-inducing insects include gall wasps, gall midges, aphids, and psyllids.
Some gall tumors can cause big damage to plant tissue. For example, crown galls induced by the bacterium Agrobacterium tumefaciens can be very destructive to trees. Those galls are very interesting because they are a result of Agrobacterium’s ability to transfer plasmids (A plasmid is a DNA molecule separate from the chromosomal DNA and capable of autonomous replication) between itself and plants, and for this reason it has become an important tool for plant improvement by genetic engineering. Those plasmids transfer genes which are responsible for the host cells to divide uncontrollably and this is the reason that these plasmids are called tumor-inducing plasmids or in short - Ti plasmids. This explains why these "plant cancers" continue to grow even when the bacteria are destroyed.
Agrobacterium tumefaciens Ti plasmids are a powerful tool for genetic engineers because the tumor-inducing genes can be replaced with beneficial ones and thus obtain transgenic plants with improved traits.
In most cases, insect galls do not pose serious threats to plant life. However, galls caused by bacterium like Agrobacterium tumefaciens, mentioned above, or fungi may present a more serious problem, requiring professional intervention.
Other uses of plant galls. Galls are rich in resins and tannic acid and have been used in the manufacture of permanent inks and astringent ointments, in dyeing, and in tanning. A high-quality ink has long been made from the Aleppo gall, found on oaks in the Middle East; it is one of a number of galls resembling nuts and called gallnuts or nutgalls.
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