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Robust plants' secret? Rubisco activase! (Friday, Nov. 22, 2002 -- CropChoice news) -- The following article comes from the USDA Agricultural Research Service.
Green plants can't grow without an enzyme called rubisco. Found in the cells of
plant leaves, rubisco is a major player in photosynthesis. This is the process by
which plants use sunlight, water from the soil, and carbon dioxide from the air to
make the food that they need for growth.
Rubisco's role in photosynthesis has been known for decades. Now, however,
findings from the meticulous investigations of ARS scientists shed new light on
the pivotal role of another enzyme, known as rubisco activase.
Rubisco activase's main assignment is to serve as an activator and regulator of
rubisco. Specifically, rubisco activase helps convert rubisco from its inactive form
to its active state. That's essential. Only the active form of rubisco can help with
photosynthesis. If plant cells have more inactive than active rubisco, for example,
photosynthesis slows. The result: Plants don't grow as fast and our harvests aren't
as bountiful.
Pinpointing Problematic Conditions
A plant could end up with more inactive than active rubisco if rubisco activase
were to somehow become impaired. In fact, the hard-working rubisco activase can
be undermined by certain environmental conditions, namely high temperatures
and high carbon dioxide. That's what plant physiologists Steven J.
Crafts-Brandner and Michael E. Salvucci have shown. Until now, no one had
identified rubisco activase as the culprit that limits photosynthesis under these
climatic conditions. Crafts-Brandner and Salvucci did the work at the ARS
Western Cotton Research Laboratory in Phoenix, Arizona.
In the arid Southwest, where Crafts-Brandner and Salvucci are based, damaging
high temperatures can occur. That's also true for the South and Southeast as well.
What's more, worldwide records of the past 100 years indicate a gradual warming
trend and an increase in atmospheric carbon dioxide levels. Some experts predict
both trends will continue as our global climate changes.
Of the two enzymes, rubisco is more heat-tolerant than rubisco activase. And
rubisco actually functions better with an increase in atmospheric carbon dioxide,
the scientists report. Crafts-Brandner and Salvucci learned of rubisco activase's
vulnerabilities by studying it in cotton leaves. Their analyses clarified that rubisco
activase, not rubisco, is the more vulnerable of the two essential enzymes when
temperatures and carbon dioxide levels increase.
Making Math-Based Models Better
These new discoveries about the nature of rubisco activase offer a novel
explanation of why photosynthesis slows under those adverse regimens. Too, the
findings may enhance the precision and accuracy of today's math-based models
of how plants will react to climate change. Data from the Arizona investigations
can be factored into the models, adding an important new dimension that may
improve the reliability of model-derived projections. The research also opens the
door to new strategies that could help crop plants sidestep the unwanted,
climate-driven influences on rubisco activase and the negative impacts on rubisco
and photosynthesis.
Crafts-Brandner and Salvucci determined that heat and carbon dioxide work in
different ways to thwart rubisco activase. Heat literally unravels the enzyme, a
process known as denaturing. Unraveling renders rubisco activase unable to fit
correctly onto rubisco. Lacking the correct fit, denatured rubisco activase can't
efficiently convert inactive rubisco to the necessary active form. Denaturing of
rubisco activase can occur at temperatures as low as 89.6°F. But rubisco
continues to function effectively until temperatures reach 131°F, Crafts-Brandner
and Salvucci found.
Key Energy Source Strongly Affected
High carbon dioxide impedes rubisco activase by altering levels of its favorite
energy source, a high-energy compound called adenosine triphosphate, or ATP
for short. Carbon dioxide levels cause a shift in the ratio of ATP to a
lower-energy compound called adenosine diphosphate, or ADP. ATP and ADP
occur in all living cells, including leaf cells. In the lab experiments,
higher-than-normal carbon dioxide makes ADP more plentiful than ATP. This
shift in the ratio of the two compounds significantly impairs rubisco activase.
So what can be done to forestall the unwanted effects on rubisco activase?
Crafts-Brandner and Salvucci hope to find or construct genes that could cue
plants to synthesize a more heat-stable rubisco activase. And they plan to explore
other biotech-based options that could enable rubisco activase to overcome the
unfavorable shift in the ATP to ADP ratio that high carbon dioxide instigates.
They're collaborating with Pioneer Hi-Bred International under the terms of a
research and development agreement.
Taking Earlier Discoveries Forward
The Arizona studies build on breakthroughs made by the team of William L.
Ogren, formerly with ARS in Urbana, Illinois, and now retired; Archie R. Portis,
Jr., with ARS at Urbana; and Salvucci, who worked with Ogren and Portis as a
postdoctoral researcher. In 1985, Ogren, Portis, and Salvucci discovered the
existence of rubisco activase and proved that it activates rubisco—short for
ribulose-1, 5-bisphosphate carboxylase/oxygenase. Ogren's group was the first to
show that rubisco is activated and regulated by rubisco activase.
In 1990, Ogren received the coveted International Alexander Von Humboldt
Foundation Award. In addition, he was selected for the ARS Hall of Fame and
named to the National Academy of Sciences.
The newer work by Crafts-Brander and Salvucci about the effects of temperature
and carbon dioxide on rubisco activase was documented in the Proceedings of
the National Academy of Sciences, one of the world's leading scientific
journals.—By Marcia Wood, Agricultural Research Service Information Staff.
This research is part of Plant Biological and Molecular Processes, an ARS
National Program (#302) described on the World Wide Web at
http://nps.ars.usda.gov.
Steven J. Crafts-Brandner and Michael E. Salvucci are with the USDA-ARS
Western Cotton Research Laboratory, 4135 E. Broadway Rd., Phoenix, AZ
85040; phone (602) 437-0121, fax (602) 437-1274.
"Robust Plants' Secret? Rubisco Activase!" was published in the November
2002 issue of Agricultural Research magazine. http://www.ars.usda.gov/is/AR/archive/nov02/plant1102.htm | |
