Researchers Discover Brain Protein at Work in Plants

RALEIGH, N.C., Dec. 19--The same kinds of natural proteins that regulate human hormone secretion and blood vessel constriction also have a hand in determining how much nitrogen fertilizer crops can use, U.S. Department of Agriculture scientists have discovered.

Plants contain an enzyme called nitrate reductase. This enzyme can turn nitrate from the soil into nitrite. During daylight hours, the plant uses the sun’s energy to convert the nitrite into amino acids the plant needs for survival. But at night, without the energy source, the nitrite could continue to accumulate in the plant and kill it.

Nature protects the plant by using a process known as protein phosphorylation to bind a phosphate to the nitrate reductase. The phosphate makes it possible for 14.3.3 proteins to also bind to the nitrate reductase and shut down its nitrate-to-nitrite conversion activities at night. When the sun comes up, photosynthesis triggers the removal of the phosphate and 14.3.3 protein so the nitrate reductase can go back to work.

“What’s interesting is a 14.3.3 protein shuts the conversion process down very quickly at night, but when the nitrate reductase starts back to work, it does so very slowly,” said Huber. ”This could be further evidence that the plant is protecting itself from concentrations of potentially toxic chemicals. “

The importance of the discovery of 14.3.3. proteins’ role reaches beyond nitrogen efficiency in plants.

“Scientists have long dreamed of increasing crop yields in nutrient-poor soils or enhancing the carbohydrate content of harvested grain. Huber’s findings on 14.3.3 proteins and other metabolites are giving researchers a blueprint to make these things possible,” said Rich Wilson, who oversees the ARS Soybean and Nitrogen Fixation Research Unit where Huber works.

For example, Huber and colleagues were able to show protein phosphorylation regulated another key enzyme, sucrose phosphate synthase or SPS, which determines how fast sugar is made and distributed in the plant. These sugars contribute to a crop’s carbohydrate content.

Huber and his colleagues hope to use the findings on SPS and 14.3.3 proteins to study metabolic processes in plants and perhaps alter them to boost crops’ nutritional value.

The discoveries on the role of 14.3.3. proteins aren’t the first time Huber’s research team has broken new ground.

“We were able to show conclusively that nitrogen reductase was regulated by protein phosphorylation in 1992, but it became clear that the phosphate alone didn’t inactivate the enzyme,” said Huber. “Scientists have wondered what 14.3.3 proteins were doing in plants. It has been thought that they play a role in gene expression, but we now know at least one additional function--the control of nitrate reductase activity.”

Scientific contact: Steve Huber, Plant Science Research Unit, Agricultural Research Service, USDA, Raleigh, N.C. 27607; phone (919) 515-3906, fax (919) 856-4598.