"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, Purdue assistant professor of horticulture.

Nitrogen, phosphorus and potassium are the big three of plant nutrition. Of these, 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. For example, 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. Even in the Midwest, soil phosphorus is the least available of the big three nutrients."

The current approach to this problem is to amend soils with phosphorus extracted from rock phosphate mines. Canadian researchers recently dtermined that known reserves of phosphate in these mines will be depleted by 2090. Researchers at Purdue University have made an important discovery, the isolation of the  genes that help plant roots take up phosphate.

The researchers began by studying the ways plants adapt to phosphorus-poor conditions. Some plants make internal changes to bring in more of the mineral, while others develop more roots. Still other plants produce and release rganic acids and enzymes that can pry the nutrient away from the attraction of the soil clay and organic matter. And in some cases, phosphorus starvation flips a genetic switch that changes certain molecules in roots and makes plants better at acquiring phosphate.

The Purdue team. in collaboration with researchers at the Instituto de Recursos Naturales y Agrobiologia in Spain, concentrated on plants' genetic and molecular responses to phosphorus deficiency. They hoped to find out what mechanism makes plants better at phosphorus uptake, then track down the genes that turn on that mechanism.

The researchers 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.

"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. 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.

The work was reported in the Proceedings of the National Academy of Science.