Researchers Discover Brain Protein at Work in
By Jill Lee
December 19, 1996
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.
Understanding this process in plants may allow us to increase a
plants ability to take up nitrogen, said plant physiologist Steve
Huber with USDAs Agricultural Research
Service. This could mean less man-made fertilizer would wind up in
groundwater. These proteins, called 14.3.3 proteins, represent an important
on-off switch in plants.
Plants contain an enzyme called nitrate
reductase. This enzyme can turn nitrate from the soil into nitrite. During
daylight hours, the plant uses the suns 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.
Whats 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
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.
Hubers 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
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 crops 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 arent the first time
Hubers 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 didnt 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.