Science Behind Clearfield Lentils
VANCOUVER - Jan 21/04 - STAT -- The technology behind the development of the Clearfield lentils which are expected to be approved for use by the Canadian government this year dates back to 1922 and efforts by scientists to force the DNA in seeds to mutate in unexpected ways.
However, the first commercial applications of the technique did not appear until the 1950s. Researchers appeared to originally place more emphasis on using radioactive materials -- neutrons, gamma rays, or X-rays -- to force the DNA within seeds to mutate in unexpected ways. Now, chemical agents seem to be more often used. The technique is called mutagenesis.
In the case of the Clearfield lentil, seed from unspecified lentil varieties were exposed to a chemical which triggered unexpected mutations within the seed. Researchers were searching for mutations would result in a plant which is resistant to the imidazolinone family of herbicides. The herbicides used for this project are manufactured by BASF.
In simple terms, the seeds are then grown. They are sprayed to see if any mutated to become resistant to the target herbicide. The survivors are then crossed with a commercial lentil variety. The seed was planted and the plants again tested for herbicide resistance. The process continued until the breeders had a seed which was commercially acceptable and which was resistant to the herbicide.
Rick Holm, director of the University of Saskatchewan's Crop Development Centre, offers a more detailed explanation, "Clearfield crops have been developed using mutagenesis and back-crossing. Both techniques have been used in tandem for many decades and are universally accepted as conventional plant variety development techniques for improving a variety of traits. These techniques are particularly popular in Europe.
"Seeds are exposed to a mutagen, either radiation or a chemical, usually a chemical.
"The treated seeds are grown out and the breeder looks for the desired trait. In the case of herbicide resistance, the plants would be sprayed with the herbicide in question to determine if any are resistant.
"Plants that exhibit tolerance to the herbicide (if any - there is no guarantee that the desired mutation will occur and most mutations that do occur are deleterious and so the plants are destroyed) are selected and increased. Usually these plants do not possess the agronomic traits, disease resistance, yield potential, etc required of a commercial variety and so they are used as a parent in a back-crossing program to transfer the herbicide resistance trait into varieties that have all the traits required for commercial acceptance.
"At each generation of the back-crossing program the plants are exposed to the herbicide to ensure that only plants possessing the herbicide tolerance trait are retained.
"After a sufficient number of back-crosses (typically 5 or 6) you end up with a variety that is essentially the same as the recurrent parent except that it carries the herbicide tolerance trait."
Dr. Graham Scoles, associate dean for the Department of Plant Sciences, University of Saskatchewan, explained the process in a paper, "Mutagenesis is a term that refers to the deliberate production of genetic variability through the use of various forms of energy (neutrons, gamma rays, X-rays) or various chemical treatments. Either of these treatments, at appropriate levels, will cause changes in the DNA of an organism.
"With plants, it has also been shown that passage through tissue-culture can cause similar effects. The integrity of any organism's DNA is critical. As these changes are random, any gene (and any number of genes) could be disrupted with consequences that will depend on their function.
"Usually seeds are exposed to the treatment and the effects will not be exhibited until the second generation after treatment. Many seeds may not germinate and of those that do, many may carry deleterious or lethal mutations. Because of this a mutagenesis program will usually screen very large populations (10,000+) for the best plants.
"While this may be done with the hope of producing a plant with a particular trait, mutagenesis programs that had no specific objective in mind have produced valuable lines. Because mutagenic effects will occur throughout the organism's DNA, lines produced through mutagenesis may carry inconsequential changes in their DNA in addition to the desired mutation(s)."
There are several plant varieties grown in Canada and elsewhere, which were developed using mutagenesis.
Scoles said, "Since the 1950's FAO has documented the number of cultivars that have been derived directly from mutagenesis programs. In 1994 the FAO list included 1,800 cultivars world-wide and it would be higher now. Many of these cultivars are no longer grown. However lines developed by mutagenesis programs appear in the pedigrees of many of the cultivars growing in the world today and in many crops they have played a very significant role."
These varieties include Regina II rapeseed, released in Canada in 1953; Redwood 65 flax, registered in 1965; Pursuit Smart canolas; low linolenic traits flaxseed and canola.
Scoles said, "(T)he FAO estimated in 1994 that almost 70% of the durum wheat in Italy was mutant varieties and that there were 200 rice cultivars derived from mutagenesis programs. Two barley cultivars that are in the pedigrees of many barley cultivars grown today derive from mutation programs (Diamant and Golden Promise).
"Michelite, an x-ray induced white bean mutant with altered plant type is in the pedigrees of most of the white beans grown in north America. In summary, mutagenesis programs have been widely used by plant breeders and many of our food crops derive either directly or indirectly from such programs."
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