Crown gall bacteria
Throughout the nature trail are trees with tumor-like growths called galls. Those at the base of trees are probably caused by crown gall bacteria, Agrobacterium. This bacterium is interesting in that it contains a special fragment of DNA called the Ti plasmid in addition to its regular chromosome (Ti stands for Tumor inducing). When the bacterium infects a plant, the plasmid inserts itself into one of the host’s chromosomes and is replicated whenever that cell divides. The plasmid contains the genes that code for the production of the gall, which is essentially a food resource for the bacterium. Agrobacterium has received much attention because of its role in genetic engineering in plants. Molecular biologists are able to remove the Ti plasmids from the bacterium and insert bits of DNA from plants into the plasmid, the plasmid is then inserted into another plant where it inserts the new DNA into the chromosomes of the second plant. This allows the genetic engineer to transfer genes among totally unrelated species. In
essence, the Ti plasmid is used as a vector to transfer genes. Although there are other possible vectors, the Ti plasmid is one of the easiest to use and one of the most effective.
Other galls higher up on the trunks of the trees are probably insect galls or possibly old wounds that have been healed with scar tissue. There are a number of different insect species that can form galls on plants (e.g., some species of wasps, flies and moths). The insects do not manipulate the genetic information of the host as does Agrobacterium, but instead synthesize analogues of plant hormones to induce the plant to form the gall.
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