Robust Plants' Secret? Rubisco Activase!
Technician Donald Brummett
(left) and plant physiologist
Steve Crafts-Brandner measure
photosynthesis in an Arizona
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
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.
Crafts-Brandner and technician
Laura Crews inspect a cotton
plant that will be used in
a heat-stress experiment.
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.
Plant physiologist Mike
Salvucci prepares an extract
from a cotton leaf. He will
analyze the heat tolerance
of key photosynthetic enzymes
in the extract.
| 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 rubiscoshort 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.
"Robust Plants' Secret? Rubisco Activase!" was published in the November 2002 issue of Agricultural Research magazine.