As gardeners know, potassium is a basic element in plant fertilizers. Plants fall flat without it. They need potassium to control the water content of their stems and leaves. Without enough potassium, plant leaves become limp and cells can't elongate.

Now University of Wisconsin-Madison scientists have shown that plants can create an electric potential of almost a quarter of a volt across the cell membrane. That differential allows root cells to draw the positively charge potassium ions from soil water into the negatively charged cells. The nutrient moves through special membrane proteins called potassium channels.

The findings may lead to crops that need less potassium fertilizer, according to molecular biologist Michael Sussman and plant electrophysiologist Edgar Spalding. Cell membranes have hundreds of different proteins that plants use to move compounds across cell boundaries. Understanding these mechanisms also might lead to crops that can concentrate other compounds, such as iron, which could help counter anemia in some countries.

Sussman, Spalding and graduate researchers Rebecca Hirsch and Bryan Lewis published their findings in the May 8 issue of Science magazine, which featured a cover photo of one of their plant's roots. The research is a collaboration between Sussman and Hirsch, from the Department of Horticulture in the College of Agricultural and Life Sciences, and Spalding and Lewis from the Department of Botany in the College of Letters and Science.

"The most startling aspect of the study was how well the uptake mechanism worked at very low potassium concentrations," Spalding says. "The magnitude of the electrical gradient allows the channels to take up potassium at concentrations much lower than scientists believed possible." Scientists are more familiar with the electrical gradient and potassium channels that drive nerve conduction in animals and people. "The electrical gradient in plants is several times greater than the one in animal and human nerves," says Sussman. "It allows plants to take up and concentrate enormous amounts of potassium from soils that have relatively little of the element."

"Scientists have been unsure just how plants concentrate potassium from surrounding soils," Sussman says. "Plants have another type of membrane protein called a high affinity transporter that researchers thought was more important in taking in potassium. No one thought potassium channels could function at very low soil potassium levels." But the UW-Madison researchers showed they could.

Working with Arabidopsis thaliana, a small mustard relative, the scientists created 14,000 mutant plants. Using a new strategy called reverse genetics, the team found one plant that contained a defective potassium channel gene. By comparing the mutant with plants with a functioning gene, the researchers could test just how well the potassium channel really worked in plants.

"Important crops may not have the exact same potassium channel gene as the one in Arabidopsis," Sussman says, "but crops will have a very similar gene." Plants have a family of potassium channel genes, he says. By identifying and testing each separately, researchers can evaluate all of them with an eye toward developing crops that grow well with less potassium fertilizer.

Scientists have begun similar studies on other important plant nutrients, according to Sussman.

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Writer: George Gallepp