Fields of Fingerprints: DNA Testing for Crops

Crop and Soil Environmental News, February 1998

Charles Hagedorn
Extension Specialist
Biotechnology

DNA testing, the technique which has helped solve high-profile murder cases, may now help to solve crop crimes. Several organizations have started offering DNA testing to the North American plant breeding and seed industry. Most often, the test will be used by plant breeders and research scientists to identify important genes. But sometimes, DNA testing will come in handy when police are trying to solve crimes that involve grain theft. While it is very difficult to tell the differences in a crop variety just by looking at the seeds, DNA fingerprinting will make it possible for police investigators or researchers to pinpoint specific plant traits and accurately identify seed varieties. Easy to use DNA test kits for certain crops should be on the market within the next few years. Specialized computer-based analysis programs identify the fingerprint, or specific genes carried in the seed of individual crop varieties.

Producing a Print
A DNA fingerprint can be called a genetic photograph of an individual, whether that individual is a plant, animal or person. The technique of DNA fingerprinting has been developed using the science of genetics. Genetics is the study of genes, tiny units of deoxryribonucleic acid, or DNA. This chemical is located in the nucleus of every cell. An organism's DNA contains the blueprint of its characteristics --in the case of plants, that would include features like yield, drought resistance and starch content. Making a DNA fingerprint involves several steps as follows:

1. To obtain the DNA necessary for the test, a small sample of the plant cells is required.

2. The sample is treated with chemicals to extract DNA from the cells.

3. Enzymes (proteins which promote chemical reactions) are added to the DNA. The enzymes act like scissors. They are used to cut the DNA into fragments of various lengths.

4. The fragments are placed on a bed of gel. Next, an electrical current is applied. The current sorts the fragments by length and organizes them into a pattern. This process is similar to placing sand in a series of sieves to sort the particles by size.

5. The DNA pattern is transferred to a nylon sheet by placing the gel and the nylon next to each other.

6. A probe of radioactive DNA is introduced to the pattern on the nylon sheet. The probe, which is a short strand of DNA treated to make it radioactive, is designed to bind to specify DNA fragments.

7. Finally, X-ray film is exposed to the nylon sheet containing the radioactive probes. Dark bands, which resemble consumer product bar codes, develop at the probe sites in a pattern unique to the organism. The bands indicate the site where a probe has bound to the DNA fragments. The DNA of each individual is unique, producing a unique set of fragments. This makes each pattern of probe-binding unique.

Simplifying the Search
DNA fingerprinting can be of use to plant breeders to simplify their work and reduce the amount of time it takes to produce crops with desirable new traits. For example, once a scientist isolates a specific gene that expresses a certain crop trait, a batch of seed is then produced which the scientist hopes carries the trait. At one time, the researcher would have to grow the crop to see if the trait is present. But now, the DNA of the seed batch can be tested to determine if the seeds contain the sought-after gene. The DNA test can also be used to identify and keep track of genes as they are isolated and transferred into crops. As well, it can become a tool to simplify the more traditional methods of selective breeding, by identifying what are known as "markers." Since DNA fingerprints are taken from the same DNA that carries the entire genetic blueprint for the plant, pieces of DNA that are close together tend to be passed on together from one generation to the next. If one particular band of a DNA fingerprint is found to be inherited along with a useful trait, that band serves as a marker for that trait. This marker shows which offspring may carry the trait, without having to search for the specific genetic material.

Guaranteeing Crops
Protecting plant breeders's rights (the breeders' patents on specific types of seed), is another use for the DNA test. Disputes over the true identify of seed varieties can be easily resolved, since the test will be able to isolate the specific traits that distinguish one seed variety from another. The ability to identify seed varieties will make the test important to guaranteeing the authenticity of a crop being purchased. Often it is very important to the buyer that the crop being purchased is of a particular type. For example, millers want wheat which produces a high quality milling flour that can be made into bread. Pasta producers are looking for wheat which produces a soft, doughy flour the kind that makes a good noodle. As well, new international rules are requiring crops which are genetically altered to be separated from ordinary crops. Crops may have specific genes inserted which, for example, make the plants resistant to a certain type of herbicide. This herbicides resistance reduces farmers' input costs by reducing the amount of chemical they use to control weeds.

Until recently, a commodity buyer had to rely on the seller for assurance that the crop was exactly what the buyer wanted. With DNA fingerprinting, a buyer no longer has to simply accept the sellers' word. Another way that DNA fingerprints can be used is if a farmer grows a crop and its performance does not match the claims made for it. A fingerprint could be taken to show whether the seed which the farmer planted was in fact the variety that was chosen. DNA fingerprinting may also be used in the future to identify disease infection in crops. Each disease-causing agent, such as a fungus, bacteria or virus, has a unique DNA fingerprint. If a DNA test indicated the presence of a disease organism, infection might be detected at an early stage, and a farmer could take appropriate preventive steps.

Building a Library
In order for the seed industry or others to effectively use DNA tests, both private and government labs are working on building a library of crop DNA profiles. As new samples are analyzed, a computer scan can produce matches between the samples and DNA profiles already recorded. These labs plan to offer the testing service at a reasonable cost. That means DNA testing will likely becomes as commonplace as other agricultural testing services, like soil sampling and seed germination testing.

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