Scientists Develop Plant That Produces Potential Anti-carcinogen

By introducing a gene that makes plants tolerate selenium, David Salt, professor of plant molecular physiology, has developed plants capable of building up in their tissues unusually high levels of a selenium compound. His interest in selenium stems in part from recent research sponsored by the National Institutes of Health showing that selenium can reduce the risk of developing prostate cancer by 60 percent.

"We now know how to genetically modify plants so they will make this anti-carcinogenic selenium compound," Salt said. "This research gives us the genetic means to manipulate the amount of this material that's produced in any plant."

Selenium, a mineral that occurs naturally in the soil in some parts of the world, is an essential micronutrient for animals, including humans, but is toxic to animals and most plants at high levels.

However, a few plant species have the ability to build up high levels of selenium in their tissues with no ill effects. These plants, called selenium hyper-accumulators, convert selenium taken up from the soil into a non-toxic form called methylselenocysteine, or MSC.

By inserting the gene responsible for this conversion into Arabidopsis thaliana, a model lab plant that does not tolerate selenium, Salt and his colleagues produced plants that not only thrive in a selenium-enriched environment but also amass high levels of the selenium-containing MSC in their tissues.

"We now know that this gene works," Salt said. "If you put it into another plant, it will make MSC, and we didn't know that before. So now we're in a comfortable position to say, 'okay, let's put this gene into a plant that we can use to make into a nutritional supplement, knowing that we have a very, very high likelihood of it working and producing this compound.'"

The plants that naturally hyper-accumulate selenium would not be good candidates for use as a supplement because they often produce other compounds that may have toxic effects in humans, Salt said.

Salt and his colleagues used two different methods to verify the production of MSC in the engineered Arabidopsis. The first method, called mass spectroscopy, relies on extracting compounds from the plant tissue using a variety of solvents, then running those compounds through a type of machine that identifies their chemical nature.

The other method they used is called x-ray absorbance spectroscopy, or XAS. This technique identifies the various forms of selenium in living plant tissue and can also provide a spatial map of where in the plant these selenium compounds are located.

Both techniques confirmed the presence of MSC in the engineered plants, Salt said.

Other lab studies involving selenium have shown MSC to be the most effective selenium-containing compound in reducing cancer risk in animal models, making it an attractive prospect for eventual use in a nutritional supplement, Salt said.

However, he said the effectiveness of MSC in humans has not yet been tested, because to date there hasn't been a good commercial source of it that could be used in human trials.

"We would be very interested in knowing the efficacy of MSC in humans, clearly. The problem has been there's no material to run such an experiment, and that will be an important piece of this story down the road."

Another very different aspect of the research is the possibility of developing plants that remove contaminants from the environment. Selenium contamination, for example, is a major problem in certain parts of the world, including the agricultural region of California's San Joaquin valley, Salt said. Selenium occurs naturally in the soil in that part of the country, but agricultural practices build that selenium to hazardous levels, he said.

"The central valley of California is a multi-billion dollar agricultural zone, but the intensive irrigation there leaches selenium out of the soil. It's a major problem for California," he said.

A possible solution, he said, lies in the potential to engineer fast-growing plants capable of removing large quantities of selenium from the soil. Now that he and his colleagues have successfully produced a selenium-hyper-accumulating Arabidopsis, they have the tools to start to develop a plant that would be a good candidate for removing selenium from the soil.

Natural hyper-accumulators process environmental selenium in a series of steps culminating in the production of MSC, and what Salt and his colleagues have re-created in Arabidopsis is the last step in that process.

"Imagine planting something like a cornfield, but with the ability to remove contaminants from the soil," he said. "We're not yet at that point, but we're stepping towards that, and that's a sensible approach. We've made the first step by starting with the end product."

Salt's research is part of collaboration between Purdue and NuCycle Therapy, a small biotechnology company that develops and sells plant-based nutritional supplements. This partnership was funded through a Small Business Technology Transfer grant through the National Institutes of Health National Cancer Institute. The research is published in the current issue of BMC Plant Biology.

Also collaborating in this research were Danielle Ellis, visiting scientist with NuCycle Therapy currently working in the Purdue Center for Plant Environmental Stress Physiology; Thomas Sors, Dennis Brunk, Carrie Albrecht and Brett Lahner with the Purdue Center for Plant Environmental Stress Physiology; Cindy Orser consulting for NuCycle Therapy; Karl Wood with the Purdue chemistry department; H.H. Harris with the Stanford Synchrotron Radiaion Laboratory at the Stanford Linear Accelerator Center (currently at the University of Sydney, Australia); and Ingrid Pickering, also with the Stanford Synchrotron Radiation Laboratory (currently at the University of Saskatchewan).