April 1, 2008

Electrical Stimulation Increases Chemical Yields in Plants

University of Arizona scientists have discovered that stimulating plants with electricity can induce them to produce certain chemicals -- including some that are commercially useful -- at a much higher rate than they normally would, and more efficiently than current methods allow.

These chemicals, called secondary metabolites, are used in a huge variety of ways. Food, dyes, poisons, perfumes and medicines come from them. Latex, herbs like ginseng and chamomile, medicines such as morphine, digitalis and steroids as well as nicotine and cocaine are all secondary metabolites whose production by plants could be increased by electricity.

"What you're doing is activating the plant to synthesize the chemical," said Hans VanEtten, professor of plant pathology in the Department of Plant Sciences and one of the main researchers on the experiment.

When plants experience different kinds of stimuli, they produce chemicals in response. For instance, many plants are attractive to parasitic fungi and so have evolved to produce anti-fungal chemicals when attacked by those fungi. These chemicals act as "plant antibiotics," VanEtten said.

"Most diseases on plants are fungi," he said.

Chemicals and radiation also can cause plants to produce useful chemicals, a fact known and used in commercial and industrial applications. "Elicitors," including heavy metals, or the chemicals in fungi, are used to create a higher concentration of a desired chemical than occurs naturally in plants.

The problem is that this is a "laborious and inconvenient process," and the elicitors are "hard to remove from a culture" once they are added, said Joel Cuello, associate professor of agricultural and biosystems engineering at UA.

Using electricity is a "more convenient way of stressing the cells" and inducing them to produce the desired chemicals, he said. The methods used commercially are trade secrets, but Cuello said the team has applied for a provisional patent through the UA. The seed money for the experiment came from the Bio5 Institute at the university.

In the experiment, the roots of a pea plant in a liquid solution were stimulated with a small amount of electricity, and after one to two days, the fluid was drained and the anti-fungal chemical pisatin was extracted. The plants produced 13 times more pisatin when stimulated electrically than in the control group where the plants were not stimulated electrically.

Hydroponically grown whole pea plants also had their roots tested to prove the principle applied to the whole plant and produced 10 times more pisatin in a day than the control plants. This incidentally proved that the pisatin is released from the roots of the pea plant.

After the experiment on pea plants, other plants were used, including fenugreek, barrel medic, Arabidopsis, red clover, chickpea, Japanese pagoda and sorghum.

All except the sorghum responded to the electricity, and VanEtten said that with experimentation, they can "probably make it work, too."

It is more than one chemical affected, as alfalfa at least doubled its output of more than 55 chemicals, in one case a "168-fold increase," VanEtten said.

Electricity has benefits over other elicitors because using electricity and then extracting the chemicals from the nutrient solution means there can be a continual harvest, whereas using heavy metals and fungal chemicals "interferes with growth," VanEtten said.

The results will be published this week in Biotechnology Progress, a bimonthly journal published by the American Chemical Society.

The next step in the experiment is to "make mutants," VanEtten said. By using transgenic technology to "silence" certain genes, the process of producing the end chemicals can be interrupted.

Stopping the process will add valuable insight into the system of production, as well as create "a way to make new compounds" or "to synthesize the chemicals artificially," without needing the plants, he said.

One of the other things scientists don't understand is how plants perceive signals and why they react the way they do to stimuli.

"We're trying to understand how these chemicals are synthesized," VanEtten said.

The experiment "contributes to basic science of how the pathway works," he said. But the commercial applications will also be explored.

Electricity definitely looks to be the "strategy to maximize the output of chemicals," Cuello said.

--Contact NASA Space Grant intern Eric Schwartz at 807-8012 or at [email protected]