January 20, 2005 news releases | receive our news
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Key Molecule in Plant Photo-Protection Identified
Contact: Lynn Yarris (510) 486-5375, [EMAIL PROTECTED]
BERKELEY, CA – Another important piece to the
photosynthesis puzzle is now in place. Researchers
with the U.S. Department of Energy’s Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University
of California at Berkeley have identified one of the
key molecules that help protect plants from oxidation
damage as the result of absorbing too much light.
The researchers determined that when chlorophyll
molecules in green plants take in more solar energy
than they are able to immediately use, molecules of
zeaxanthin, a member of the carotenoid family of
pigment molecules, carry away the excess energy.
(teks foto)
>From left, Graham Fleming, Nancy Holt and Kris Niyogi,
of Berkeley Lab's Physical Biosciences Division, have
identified a key molecule in the photo-protection
mechanism of green plants.
This study was led by Graham Fleming, director of
Berkeley Lab’s Physical Biosciences Division and a
chemistry professor with UC Berkeley, and Kris Niyogi,
who also holds joint appointments with Berkeley Lab
and UC Berkeley. Its results are reported in the
January 21, 2005 issue of the journal Science.
Co-authoring the paper with Fleming and Niyogi were
Nancy Holt, plus Donatas Zigmantas, Leonas Valkunas
and Xiao-Ping Li.
Through photosynthesis, green plants are able to
harvest energy from sunlight and convert it to
chemical energy at an energy transfer efficiency rate
of approximately 97 percent. If scientists can create
artificial versions of photosynthesis, the dream of
solar power as a clean, efficient and sustainable
source of energy for humanity could be realized.
A potential pitfall for any sunlight-harvesting system
is that if the system becomes overloaded with absorbed
energy, it will likely suffer some form of damage.
Plants solve this problem on a daily basis with a
photo-protective mechanism called feedback
de-excitation quenching. Excess energy, detected by
changes in pH levels (the feedback mechanism), is
safely dissipated from one molecular system to
another, where it can then be routed down relatively
harmless chemical reaction pathways.
Said Fleming, “This defense mechanism is so sensitive
to changing light conditions, it will even respond to
the passing of clouds overhead. It is one of Nature’s
supreme examples of nanoscale engineering.”
The light harvesting system of plants consists of two
protein complexes, Photosystem I and Photosystem II.
Each complex features antennae made up of chlorophyll
and carotenoid molecules that gain extra “excitation”
energy when they capture photons. This excitation
energy is funneled through a series of molecules into
a reaction center where it is converted to chemical
energy. Scientists have long suspected that the
photo-protective mechanism involved carotenoids in
Photosystem II, but, until now, the details were
unknown.
Said Holt, “While it takes from 10 to 15 minutes for a
plant’s feedback de-excitation quenching mechanism to
maximize, the individual steps in the quenching
process occur on picosecond and even femtosecond
time-scales (a femtosecond is one millionth of a
billionth of a second). To identify these steps, we
needed the ultrafast spectroscopic capabilities that
have only recently become available.”
The Berkeley researchers used femtosecond
spectroscopic techniques to follow the movement of
absorbed excitation energy in the thylakoid membranes
of spinach leaves, which are large and proficient at
quenching excess solar energy. They found that intense
exposure to light triggers the formation of zeaxanthin
molecules which are able to interact with the excited
chlorophyll molecules. During this interaction, energy
is dissipated via a charge exchange mechanism in which
the zeaxanthin gives up an electron to the
chlorophyll. The charge exchange brings the
chlorophyll’s energy back down to its ground state and
turns the zeaxanthin into a radical cation which,
unlike an excited chlorophyll molecule, is a
non-oxidizing agent.
Green plants use photosynthesis to convert sunlight
to chemical energy, but too much sunlight can result
in oxidation damage.
To confirm that zeaxanthin was indeed the key player
in the energy quenching, and not some other
intermediate, the Berkeley researchers conducted
similar tests on special mutant strains of Arabidopsis
thaliana, a weed that serves as a model organism for
plant studies. These mutant strains were genetically
engineered to either over express or not express at
all the gene, psbS, which codes for an eponymous
protein that is essential for the quenching process
(most likely by binding zeaxanthin to chlorophyll).
“Our work with the mutant strains of Arabidopsis
thaliana clearly showed that formation of zeaxanthin
and its charge exchange with chlorophyll were
responsible for the energy quenching we measured,”
said Niyogi. “We were surprised to find that the
mechanism behind this energy quenching was a charge
exchange, as earlier studies had indicated the
mechanism was an energy transfer.”
Fleming credits calculations performed on the
supercomputers at the National Energy Research
Scientific Computing Center (NERSC), under the
leadership of Martin Head-Gordon, as an important
factor in his group’s determination that the mechanism
behind energy quenching was an electron charge
exchange. NERSC is a U.S. Department of Energy
national user facility hosted by Berkeley Lab.
Head-Gordon is a UC Berkeley faculty chemist with
Berkeley Lab’s Chemical Sciences Division.
“The success of this project depended on several
different areas of science, from the greenhouse to the
supercomputer,” Fleming said. “It demonstrates that to
understand extremely complex chemical systems, like
photosynthesis, it is essential to combine
state-of-the-art expertise in multiple scientific
disciplines.”
There are still many pieces of the photosynthesis
puzzle that have yet to be placed for scientists to
have a clear picture of the process. Fleming likens
the on-going research effort to the popular board
game, Clue.
“You have to figure out something like it was Colonel
Mustard in the library with the lead pipe,” he says.
“When we began this project, we didn't know who did
it, how they did it, or where they did it. Now we know
who did it and how, but we don't know where. That's
next!”
Berkeley Lab is a U.S. Department of Energy national
laboratory located in Berkeley, California. It
conducts unclassified scientific research and is
managed by the University of California. Visit our
Website at www.lbl.gov.
Additional Information
For additional information visit the Website at
http://www.lbl.gov/pbd/photosynthesis/default.htm
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