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Breeding out glycoalkaloids

Researchers at Agriculture and Agri-Food Canada in Charlottetown are analyzing potato seedlings and tubers to find the ones carrying genes that result in very low SGA  production. Photo courtesy of Dr. Bourlaye Fofana.

Depending on the variety, environmental conditions and handling practices, potatoes may contain high levels of toxic and bitter substances called steroidal glycoalkaloids (SGAs). SGAs are naturally produced in both foliage and tubers (in the green parts of the potato) as a defence against animals, insects and fungi attack. The common glycoalkaloids found in potato plants are solanine and chaconine, with solanine being the more toxic of the two. The production of solanine in potatoes is thought to provide protection from the Colorado potato beetle, potato leafhopper and wireworms.

Of the major food products, potatoes are of most concern in terms of human food safety because glycoalkaloids as a residual level of these compounds are almost always present. SGAs are not destroyed during cooking or frying. In fact, the Lenape potato variety was removed as an option for commercial cultivation in Canada and the United States, as it contained unacceptably high levels of SGAs.

These substances affect the human central nervous system and have disruptive effects on cell membrane integrity, affecting the digestive system and more. Most people who ingest SGAs at a level to cause health concerns experience only mild gastrointestinal effects, beginning within about eight to 12 hours of consumption and disappearing within a day or two. Effects can range from nausea and vomiting and headaches to hallucinations and paralysis. Some studies have also linked SGAs to birth defects in humans and breeding problems in animals.

Some sources state that potato vines (which contain solanine) can be a valuable ruminant livestock feed when fed just after harvest, as they are not toxic at that point. Harvesting the vines also helps to control disease, but the costs involved in harvest may not make it a worthwhile practice.

Although breeding efforts over the years have resulted in the release of potato varieties that result in the lowest-ever production levels of SGAs, a great deal of effort, time, energy and resources are still deployed to minimize SGA formation during storage – and in turn, in processed potato end-products. Thus, a new Canadian project using cutting-edge genetic tools is aiming to develop, in a short period of time, new cultivars to contain levels not exceeding 20 mg/100 g fresh weight, which is the current internationally accepted maximum.

The research is headed by Dr. Bourlaye Fofana, a geneticist specialized in plant molecular physiology and genomics at the Agriculture and Agri-Food Canada (AAFC) Crops and Livestock Research Centre (CLRC) in Charlottetown, P.E.I., and includes Dr. Benoit Bizimungu (potato breeder, AAFC-Fredericton, N.B.), Dr. Jason McCallum (phytochemist, AAFC-Charlottetown), David Main (biologist, AAFC-Charlottetown), Dr. Helen Tai (molecular biologist, AAFC-Fredericton), and Dr. Lawrence Kawchuk (plant pathologist, AAFC-Lethbridge, Alta.).

“We are working on complementary aspects of potato crop improvement (genetics and genomics, chemistry, pathology, breeding, and agronomy) using innovative approaches for accelerating the breeding of potato cultivars free of glycoalkaloid content,” Fofana explains.

Using genetic techniques and tools, the scientists are analyzing thousands of potato seedlings and tubers generated from true seeds to find the ones carrying genes that result in very low SGA production. “The approach involves inducing mutations in genes of true potato seeds (harvested from potato fruits, which look like tomatoes) obtained from selected germplasm,” Fofana explains. “We’ve treated approximately 4,000 true potato seeds with ethyl methanesulfonate, a compound that induces random mutations in the genetic material.”

The team is then characterizing the 2,500 surviving mutagenized lines for their ability to produce tubers and their SGA production levels. Next-generation sequencing technologies and extensive bioinformatics analysis are being used to identify lines carrying mutations in targeted genes.

The selected lines will be evaluated later for other useful traits such as disease resistance and agronomic traits related to yield, tuber shape, flesh and skin colour, maturity date and so on. If necessary, the team will treat more true potato seeds with ethyl methanesulfonate. “We expect to pinpoint about 100 candidates for the breeding program that within five years, will result in potatoes low or free of glycoalkaloids,” says Fofana. To his knowledge, no similar studies using the same approach to breed out glycoalkaloids are occurring elsewhere in the world.

Fofana was born and grew up in Cote d’Ivoire, Africa. He was trained in agricultural sciences and biological engineering at the Gembloux Agricultural University in Belgium and obtained graduate degrees in plant molecular genetics. After his Ph.D. graduation, Fofana joined a research group at Laval University in Quebec, holding a postdoctoral fellow position and working on cucumber defence mechanisms. In 2002, he obtained a National Research Council postdoctoral position at the AAFC Cereal Research Centre in Winnipeg, working on flax and wheat functional genomics. From 2006 to 2007, Fofana worked as a term researcher at AAFC-Winnipeg in wheat genomics. Since 2008, he has been at CLRC in Charlottetown, focusing on wild rose, flax and potato genomics. Fofana’s current research group is composed of a postdoctoral fellow, a graduate student and a co-op student.

“Diversity of life on Earth has fascinated me since elementary school,” he says. “The question I used to ask my brothers was, ‘Why are there so many different insects, animals and plants?’ The response lies in genetics, of course. It is fascinating to study how a change of a ‘single letter’ in the genetic code of a living organism can make huge morphological difference, or an important difference in the ability of a plant to produce a particular metabolite, even if the plant is not visually different from its counterparts.”

Fofana says the challenge for him in genetics is “to understand the factors leading naturally to changes of genetic material, and how these factors can be controlled to human advantage.” His work in meeting this challenge by producing glycoalkaloid-free potatoes is sure to provide significant advantage for the Canadian potato industry, at home and worldwide.

January 12, 2015  By Treena Hein