WEST LAFAYETTE, Ind. --Spurred by predictions that we may have only 90 years of high-quality rock phosphate fertilizer left, Purdue University researchers have taken a step toward helping plants get the nutrient out of soil.
They were the first to isolate genes that help plant roots take up phosphate, a common form of phosphorus. Their work was reported in the Proceedings of the National Academy of Science.
"Lack of phosphorus fertilizer is going to be a serious problem in the future in certain parts of the United States and especially in the tropics, unless we find another source of phosphorus in the world or unless we create plants that are more efficient phosphorus users," says K.G. Raghothama (RAG-oh-TOM-uh), Purdue assistant professor of horticulture.
Based on currently known reserves, rock phosphate mines will be depleted by 2090, according to calculations by a Canadian researcher in 1996.
Among the big three nutrients -- nitrogen, phosphorus and potassium -- phosphorus is the hardest for plants to get out of soil. The degree of phosphorus availability varies from place to place, but many soils jealously guard their phosphorus supplies.
The very acid soils of the tropics contain many molecules of iron and aluminum, which latch onto and tie up nearly all available phosphorus.
"We also have problems in the Southeastern United States and on calcareous soils in the Great Plains of the American West," says Purdue agronomist Dave Mengel. "In alkaline soils of the West, calcium reacts with the phosphorus and essentially fixes it."
Midwestern soils hold the mineral less tightly, but generally still require annual applications of phosphorus to keep crops healthy, Mengel says. Even in the Midwest, soil phosphorus is the least available of the big three nutrients.
When soil phosphorus is sparse and plants can't get what they need, they make some internal changes to bring in more of the mineral, according to Raghothama. Some plants develop more roots. Some produce and release organic acids and enzymes that can pry the nutrient away from the attraction of the soil clay and organic matter.
And in some plants, Raghothama says, phosphorus starvation flips a genetic switch that changes certain molecules in roots and makes plants better at acquiring phosphate.
In collaboration with Jose Pardo from Instituto de Recursos Naturales y Agrobiologia in Spain, Raghothama decided to concentrate on plants' genetic and molecular responses to phosphorus deficiency. Perhaps, he thought, he could find out what mechanism makes plants better at phosphorus uptake, then track down the genes that turn on that mechanism. He predicts that once they understand it, researchers might enhance a plant's natural ability to compete for the few phosphate molecules found free in soil solution.
Other scientists working with yeast and other fungi already had identified protein molecules called "phosphate transporters" that actively take up phosphate. They also located the DNA that told cells to produce those proteins.
"The phosphate transporter sits on the cell membrane and transfers phosphate through the membrane along with hydrogen ions," Raghothama says. "We've known about this for a long time in yeast and fungi, but before our work, phosphate transporter genes were not isolated from higher plants."
Raghothama and postdoctoral researcher U. Muchhal starved Arabidopsis plants (a member of the mustard family often used as a model system for research) for a week, figuring that this would cause the plants to beef up their phosphate uptake mechanisms. It did.
Then they probed the DNA libraries of the starved plants for genes that produce phosphate transporter proteins. They found the genes there, isolated them, and decoded them. They also noted that the phosphate-starved plants sent out significantly more messages calling for production of phosphate transporter proteins.
"Now we are in a better position to understand how phosphorus is taken up by plants, to make changes to the genes involved, and to create plants that are efficient acquirers of phosphorus," Raghothama says.
Journal
Proceedings of the National Academy of Sciences