In recent years, powerful new tools have been added to the toolboxes of plant breeders. Most breeding progress today is made using crosses among crop plants and their relatives, as described in the previous section, "Breeding Methods Using Sexually Compatible Germplasm." However, in many breeding programs, species boundaries no longer exist. Through techniques such as protoplast fusion and gene splicing (discussed in sections that follow), genes from virtually any living organism can be incorporated into crop plants. Everything, therefore, from an antifreeze gene in North Atlantic fish to a pesticide gene in bacteria is now accessible to breeding programs.
There are times when a breeder would like to tap into the genetic diversity of a plant species that is distantly related to a crop plant. However, it is impossible to make a cross between the two species and obtain viable seeds. An alternative strategy involves protoplast fusion. With this method, cells of each species are grown in a liquid nutrient solution. Then, their cell walls are chemically stripped off to produce protoplasts. The protoplasts of the two species are mixed together and stimulated, with the aid of an electric current or chemical solution, to fuse with each other. When screening the fusion products, a scientist must distinguish between autofusions, where two protoplasts of the same species have fused together, and hybrid fusions, in which the protoplast of one species has fused with that of the other species. One way of doing this involves dyeing the protoplasts of the each species with a different color before the fusion event. Then, the scientist can examine the fusions with a microscope and identify protoplasts that show both colors. The selected protoplasts are then carried through a series of tissue culture events (see "Micropropagation" on page 268) that cause them to grow into whole plants. These new plants carry genes from two distantly related species. They are called somatic hybrids because they resulted from combining (hybridizing) somatic, or body, cells from two different plants. In contrast, plants usually reproduce when gamete cells (eggs and sperm) are combined, producing sexual hybrids.
Although protoplast fusion appears to have value for the incorporation of new genes into crop plants, few success stories are currently available. Somatic hybrids have been created between potato cultivars and nontuber-bearing wild relatives. Some are highly resistant to a number of bacterial and fungal diseases, but varieties have not yet been created from the hybrids or their offspring. Tobacco is probably the best example of a commercially successful somatic hybrid. A common variety produced in Canada is the result of a fusion between cultivated tobacco and a wild relative containing a novel form of disease resistance.
Genes from virtually any organism, from viruses to humans, can now be inserted into plants, creating transgenic plants. The technologies that have been developed to support this work arose from basic research to understand DNA structure and function. Recombinant DNA technology is based on the knowledge that DNA behaves the same no matter what organism carries it. Because the DNA code is nearly universal, a plant cell can read a DNA sequence from almost any other organism and produce a foreign gene product.
Stern-Jansky-Bidlack: I 14. Plant Breeding and I Text I I © The McGraw-Hill
Introductory Plant Biology, Propagation Companies, 2003
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