WEST LAFAYETTE, Ind. – Too much metal in the soil and you have a contaminated brownfield. Too little metal in the diet and you see disease and malnutrition.
The solution to both problems may be metal-loving plants that take up large amounts of metal in their tissues, says David E. Salt, professor of plant molecular physiology at Purdue University.
There has been much scientific interest in recent years in using metal hyperaccumulating plants to clean up hazardous materials sites, a process called phytoremediation. But Salt says metal-accumulating plants have a broader potential as a way to improve people's diets or even to create foods that fight cancer.
"It's just two sides of the same coin," he says. "One of the things about metals that many people don't realize is that a lot of them are essential micronutrients that we need. They play a crucial role in certain enzymes that our bodies use to function."
Salt recently announced that he has identified and cloned the genes from a species of these plants that store metals in their tissue.
Humans need a variety of metals in their diets, including iron, copper, manganese and zinc. They are needed in tiny amounts, however, which is why they are called micronutrients. According to the World Health Organization, the lack of proper micronutrients causes health problems in many underdeveloped nations, particularly in children and pregnant women.
"For example, iron and zinc deficiencies have been termed the hidden hunger in the world," Salt says. "Many people suffer disease from the lack of zinc or iron in their diets; they're not just suffering from the lack of food. So we're interested in making foods that are enriched in these essential micronutrients."
The metal selenium is known to be a potent anti-carcinogen, and there are wild plants that accumulate selenium naturally. If these genes could be moved into crop plants, Salt says, new foods could be created that have anti-cancer properties.
In the Western United States there is a plant known as locoweed (Astragalus bisulcatus), which accumulates selenium. The plant gets its descriptive name because livestock that eat it can become disoriented and stumble about after ingesting too much selenium. There is even a legend – most likely untrue – that Gen. George Armstrong Custer faltered against the Sioux at the Battle of Little Bighorn because his horses had eaten too much locoweed.
But locoweed hyperaccumulates selenium, and Salt says that it may be possible to create functional foods that have cancer-fighting properties from this plant.
"Fortuitously, one of the most potent and easily absorbed selenium compounds is the compound that is hyperaccumulated by this plant," Salt says. "So this plant has the very extraordinary ability to make this anticarcinogenic form of selenium."
Salt, in collaboration with NuCycle Therapies Inc., has a grant from the National Cancer Institute to clone the gene from locoweed that causes the plant to pull selenium from the soil. Salt says these genes could be used to create plants that could be used as nutritional supplements.
Although selenium supplements are available already, Salt says most of these are of little use because the human body can only absorb and use selenium if it is in certain chemical forms.
"If you go into the health food store right now, you'll see that there are many different selenium supplements. Most of those are actually sodium selenide, or sodium selenate, which is a chemical form that our bodies can't use very well. It has been shown to not be very effective," he says. "The other common supplements are yeast which has been fed selenium, and that has been shown to contain about 40 percent elemental selenium, which is completely unavailable to the human body."
Salt says the first products to market would be dried plant material that is enriched in bioavailable forms of selenium.
"In the long term we'd probably like to try to engineer a vegetable crop so that we would take the selenium that's in the crops right now and move it into a more anticarcinogenic form in the foods we already eat," he says.
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