Bacterially Induced Tumors in Plants
Plants appear to be simpler than mammals and yet they are susceptible to tumors just like mammals. Therefore, some of the lessons that we learn about plant tumors may be helpful in the analysis of animal tumors. One feature that makes the study of plant cells and plant tumors valuable in research on oncogenesis is that cell cultures from some plants can be maintained indefinitely in cell culture, but when desired, these undifferentiated cells can be induced to differentiate back into normal plants that reproduce sexually. Such techniques permit detailed analysis of the causes of oncogenesis. In addition, these approaches have the potential for yielding valuable mutants by using techniques similar to those used with bacteria for the isolation of single mutant cells from large cultures and then regenerating complete mutant plants.
The bacterium Agrobacterium tumefaciens can induce the growth of masses of undifferentiated cells, called crown galls, in susceptible plants. This transformation to the undifferentiated state requires a 200-Kb plasmid carried by the bacterium. In the transformation process at least 8 to 10 Kb of DNA from the plasmid are transferred from the bacterium into the plant cells. There the DNA is integrated into the chromosome of the plant, where it is replicated along with the cellular DNA. As a result, all cells of a crown gall contain fragments of DNA originating from the plasmid. Part of the integrated plasmid DNA directs the plant to synthesize and secrete the compounds octopine or nopaline (Fig. 23.1). In turn, these compounds can be catabolized by the Agrobacter bacteria in the crown gall. Few other bacteria or parasites can utilize these compounds for growth. Thus, one Agrobacterium subverts part of the plant to produce nutrients for a large bacterial colony.
Figure 23.1 Structures of octopine and nopaline.
In contrast to the nontransformed plant cells, the cells from crown galls do not require the growth factors auxin and cytokinin for their continued growth in culture (Fig. 23.2). Ordinarily, medium for plant cells requires the presence of these two small-molecule growth factors in addition to a variety of other metabolites. With the ratio of auxin and cytokinin at one value, plant cells in culture remain undifferentiated, but if the ratio of auxin to cytokinin is increased, stems and leaves tend to develop. Conversely, if the auxin to cytokinin ratio is decreased, the cells become root-like. Without either auxin or cytokinin present, the cells do not grow. In the whole plant, auxin is synthesized in the stem tips and cytokinin in the root tips. A concentration gradient in these molecules from the top to bottom of the plant helps cells identify their positions and develop appropriately. Plant cells transformed with Agro-bacter do not require either auxin or cytokinin for growth. It is likelythat the DNA that was acquired from the plasmid by the transformed cells directs or induces synthesis of auxin and cytokinin-like substances that substitute for these chemicals both in the crown gall and in cell culture. By analogy to the crown galls in plants, it was predicted and found that many types of animal cancers would involve alterations in the cell’s synthesis of, or their response to, growth factors.
Figure 23.2 Indole 3-aceticacid, an auxin and 6-(4-hy-droxy-3-methyl-trans-2-buten-ylamino)purine, a cytokinin.
The DNA transfer mechanism utilized by Agrobacter can be utilized for genetic engineering. The DNA to be introduced to the plant cell can be included between the DNA recombination sequences of the transfor-mation plasmid and transformation then occurs much like lambda phage integration.
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