San Diego, California and
Helsinki, Finland
February 27, 2008
By Kim McDonald
Biologists at the University of
California, San Diego, working with collaborators at the
University of
Helsinki in Finland and two other European institutions,
have elucidated the mechanism of a plant gene that controls the
amount of atmospheric ozone entering a plant’s leaves.
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Colored guard cells surround a stomatal pore.
Credit: University of California, San Diego |
Their finding helps explain why
rising concentrations of carbon dioxide in the atmosphere may
not necessarily lead to greater photosynthetic activity and
carbon sequestration by plants as atmospheric ozone pollutants
increase. And it provides a new tool for geneticists to design
plants with an ability to resist droughts by regulating the
opening and closing of their stomata—the tiny breathing pores in
leaves through which gases and water vapor flow during
photosynthesis and respiration.
“Droughts, elevated ozone levels and other environmental
stresses can impact crop yields,” said Jean Chin, who oversees
membrane protein grants at the National Institute of General
Medical Sciences, which partially funded the research. “This
work gives us a clearer picture of how plants respond to these
kinds of stresses and could lead to new ways to increase their
resistance.”
The discovery is detailed in this week’s advance online
publication of the journal Nature by biologists at UCSD,
University of Helsinki in Finland, University of Tartu in
Estonia and the University of the West of England. Last year,
the journal published another study by British researchers that
found that ozone generated from the nitrogen oxides of vehicle
emissions would significantly reduce the ability of plants to
increase photosynthesis and store the excess carbon in the
atmosphere projected from rising levels of carbon dioxide.
“When ozone enters the leaf through the stomatal pores, it
damages the plants photosynthetic machinery and basically causes
green leaves to lose their color, a process called chlorosis,”
said Julian Schroeder, a professor of biological sciences at UC
San Diego and one of the principal authors of the recent study.
“Plants have a way to protect themselves and they do that by
closing the stomatal pores when concentrations of ozone
increase.”
While this protective mechanism minimizes the damage to plants,
he adds, it also minimizes their ability to photosynthesize when
ozone levels are high, because the stomatal pores are also the
breathing holes in leaves through which carbon dioxide enters
leaves. The result is diminished plant growth or at least less
than one might expect given the rising levels of carbon dioxide.
Some scientists assessing the impacts of rising greenhouse gases
had initially estimated that increased plant growth generated
from extra carbon dioxide in the atmosphere could sequester much
of the excess atmospheric carbon in plant material. But in a
paper published last July in Nature, researchers from Britain’s
Hadley Centre for Climate Prediction and Research concluded that
the damage done to plants by increasing ozone pollution would
actually reduce the ability of plants to soak up carbon from the
atmosphere by 15 percent which corresponds to about 30 billion
tons of carbon per year on a global scale---a dire prediction
given that humans are already putting more carbon into the
atmosphere than plants can soak up.
The discovery of the ozone-responsive plant gene was made when
Jaakko Kangasjarvi and his collaborators at the University of
Helsinki in Finland found a mutant form of the common mustard
plant, Arabidopsis, that was extremely sensitive to ozone. They
next found that this mutant does not close its stomatal pores in
response to ozone stress.
“When the mutant plant is exposed to ozone, the leaves lose
their dark green color and eventually become white,” said
Kangasjarvi, who is also one of the principal authors of the
study. “This is because the stomatal pores in the leaves stay
open even in the presence of high ozone and are unable to
protect the plant.”
The scientists found that the gene responsible for the mutation
is essential for the function of what they called a “slow or
S-type anion channel.” Anions are negatively charged ions and
these particular anion channels are located within specialized
cells called guard cells that surround the stomatal pores. The
gene was therefore named SLAC1 for “slow anion channel 1.”
Guard cells close stomatal pores in the event of excess ozone or
drought. When this gene is absent or defective, the mutant plant
fails to close its stomatal pores.
In 1989, Schroeder discovered these slow anion channels in guard
cells by electrical recordings from guard cells using tiny
micro-electrodes. He predicted that these anion channels would
be important for closing the stomatal breathing pores in leaves
under drought stress.
“The model we proposed back then was that the anion channels are
a kind of electrical tire valve in guard cells, because our
studies suggested that closing stomatal pores requires a type of
electrically controlled deflation of the guard cells,” he said.
“But finding the gene responsible for the anion channels has
eluded many researchers since then.”
The latest study shows that the SLAC1 gene encodes a membrane
protein that is essential for the function of these anion
channels. “We analyzed a lot of mechanisms in the guard cells
and, in the end, the slow anion channels were what was missing
in the mutant,” said Yongfei Wang, a post doctoral associate in
Schroeder’s lab and co-first author of the paper.
The scientists showed that the SLAC1 gene is required for
stomatal closing to various stresses, including ozone and the
plant hormone abscisic acid, which controls stomatal closing in
response to drought stress. Elevated carbon dioxide in the
atmosphere also causes a partial closing of stomatal pores in
leaves. By contrast, the scientists found, the mutant gene does
not close the plants’ stomatal pores when carbon dioxide levels
are elevated.
“We now finally have genetic evidence for the electric tire
valve model and the gene to work with,” said Schroeder.
Because the opening and closing of stomatal pores also regulates
water loss from plants, Schroeder said, understanding the
genetic and biochemical mechanisms that control the guard cells
during closing of the stomatal pores in response to stress can
have important applications for agricultural scientists seeking
to genetically engineer crops and other plants capable of
withstanding severe droughts.
“Plants under drought stress will lose 95 percent of their water
through evaporation through stomatal pores, and the anion
channel is a central control mechanism that mediates stomatal
closing, which reduces plant water loss,” he said.
The study was financed by grants from the National Science
Foundation and the National Institute of General Medical
Sciences.
Breakthrough in plant research -
Gene discovery provides new tool to develop drought-tolerant
crops
The research groups of the
Department of Biological and Environmental Sciences of the
University of
Helsinki and the University of
California in San Diego have discovered a gene that is
centrally involved in the regulation of carbon dioxide uptake
for photosynthesis and water evaporation in plants. The
discovery can aid the development of drought-tolerant crops. The
article is published online ahead of print in Nature’s Advance
Online Publication (AOP) on 27 February 2008.
Stomata are tiny pores on the plant leaf surface, through which
the leaves absorb carbon dioxide necessary for photosynthesis
and release moisture into the air. The plasma membranes of the
guard cells that surround the stomatal pore contain several
types of ion channels which control the opening and closing of
the circular guard cells when the plant encounters a stressful
situation, such as increased ozone in the air or drought. The
regulation of stomata is an intensively-studied topic and
several ion channel types that control their activity have been
discovered earlier. However, an anion channel, which is of
central importance in the regulation of stomatal activity, was
identified only recently by Finnish and American scientist. A
measuring device developed at the University of Tartu, Estonia,
was of great help in the process.
Professor Jaakko Kangasjärvi and his research group from the
University of Helsinki identified the anion channel using a
mutation of Arabidopsis thaliana commonly known as thale cress.
The mutant does not react by closing its stomata as a response
to high ozone or carbon dioxide concentration in the air like a
healthy plant does. Scientist at the University of California
demonstrated with electrophysiological measurements that the
gene identified encodes an anion channel involved in the
regulation of stomatal activities.
The gene discovered is of central importance for the mechanisms
of stomatal regulation. Unlike the ion channels detected
previously, this newly discovered anion channel takes part in
the regulation of all the main stomatal activities.
Climate change makes it all the more important to know about the
mechanisms involved in stomata regulation. Aridity is on the
increase across the globe, as is the world population.
Increasingly dry areas should be taken into cultivation to
ensure food production. When developing crops that thrive in dry
areas, it is important to know well the mechanisms that regulate
stomata, through which plants evaporate moisture.
The effects of climate change, which increases atmospheric ozone
and carbon dioxide concentrations, cause another challenge for
plants. Plants protect themselves against high ozone by closing
the stomata on their leaves. While this protection mechanism
minimises damage to the plant, it also reduces carbon dioxide
uptake for photosynthesis and thus could reduce the sequestering
of the excess atmospheric carbon in plant material. A different
kind of plant, however, could grow better in the new conditions.
This research will provide a new tool for geneticists in the
development of drought-resistant plants.
Other news
from the University of Helsinki |
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