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.
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.


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