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PERSPECTIVE 8.2 Bacteria Can Conjugate with Plants: A Natural Case of Genetic Engineering

For more than 50 years, scientists have known that DNA can be transferred between bacteria.Twenty-five years ago, it was shown that a bacterium can even transfer its genes into plant cells, such as tobacco, carrots, and cedar trees, through a process analogous to conjugation.What led to this discovery started almost 100 years ago in the laboratory of the plant pathologist, Dr. Erwin Smith. He showed that the causative agent of a common plant disease, termed crown gall, is a bacterium,Agrobacterium tumefaciens.This disease is characterized by large galls or swellings that occur on the plant at the site of infection, usually near the soil line, the crown of the plant. When other investigators cultured the diseased plant tissue on agar plates containing nutrients necessary for the growth of plant tissue, it had properties that differed from normal plant tissue.Whereas normal tissue requires several plant hormones for growth, crown gall tissue grows in the absence of these added hormones. In addition, crown gall tissue synthesizes large amounts of a compound termed an opine, which neither normal plant tissue nor Agrobacterium synthesizes.The most surprising observation was that the plant cells maintained their altered nutritional requirements and the ability to synthesize opine even after the bacteria were killed by penicillin. Investigators concluded that the crown gall plant cells are permanently transformed. Although Agrobacterium is required to start the infection, they are not necessary to maintain the altered nutritional requirements and biosynthetic capabilities of the plant cells.

The explanation of the process by which Agrobacterium causes crown gall tumors and transforms plant cells was established in 1977 following a report from a team of Belgian investigators that all strains of Agrobacterium capable of causing crown gall tumors contained a large plasmid termed the tumor inducing or Ti plasmid. A group of microbiologists at the University of Washington then showed that a specific piece of the Ti plasmid, termed the transferred DNA, or T-DNA, is transferred from the bacterial cell to the plant cell, where it becomes incorporated into the plant chromosome (figure 1). Like conjugation between bacteria, a pilus is required for DNA transfer.

The studies of investigators from around the world yielded many surprising observations.This bacterial DNA acts like plant DNA because the promoters of the transferred genes resemble those of plants rather than those of bacteria. Therefore, the genetic information in the T-DNA is expressed in the plants but not in Agrobacterium.This DNA encodes enzymes for the synthesis of the plant hormones as well as for the opine.The expression of these genes supplies the plant cells with the plant hormones, explaining why the transformed plant cells can grow in the absence of added hormones and are able to synthesize opine.Thus once incorporated into the plant chromosome, the DNA provides the transformed cell with additional genetic information that confers new properties on the plant cell.This is the only certain case in nature in which prokaryotic DNA is transferred and integrated into the genome of a eukaryotic cell. It seems like a good bet that other examples will follow. ■ promoter, p. 174

Why does Agrobacterium transform plants? This bacterium has the ability to use the opine as a source of carbon and energy, whereas most other bacteria in the soil, as well as plants, cannot.Therefore,Agrobacterium subverts the metabolism of the plant to produce food that only Agrobacterium can use.Thus,Agrobacterium is a natural genetic engineer of plants.

The Agrobacterium—crown gall system is of great interest for several reasons. First, it shows that DNA can be transferred from prokaryotes to eukaryotes. Many people believed that such transfer would be impossible in nature and could only occur in the laboratory. Second, this system has spawned an industry of plant biotechnology dedicated to improving the quality of higher plants.Thus, it is possible to replace the genes of hormone synthesis in the Ti plasmid with any other genes, which will then be transferred and incorporated into the plant.With this technology, genes conferring resistance to bacteria, viruses, insects, and different herbicides have been incorporated into a wide variety of plants. Rice has been transformed to synthesize high levels of ^-carotene, the precursor of vitamin A. Edible vaccines are being synthesized in bananas following transformation by Agrobacterium.

Genetic engineering of plants became a reality once scientists learned how a common soil bacterium caused a well-recognized and serious plant disease.This system serves as a beautiful example of how solving a riddle in basic science can lead to major industrial applications.

Chromosome T-DNA

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