Biologists
have identified plant enzymes that may help to engineer plants that
take advantage of elevated carbon dioxide to use water more
efficiently.

Plants take in the carbon dioxide they
need for photosynthesis through microscopic breathing pores in the
surface of leaves. But for each molecule of the gas gained, they lose
hundreds of water molecules through these same openings. The pores can
tighten to save water when CO2 is abundant, but scientists didn't know
how that worked until now.

A team led by Julian Schroeder,
professor of biology at the University of California, San Diego, has
identified the protein sensors that control the response. Enzymes that
react with CO2 cause cells surrounding the opening of the pores to
close down they report in the journal Nature Cell Biology online Dec. 13.

The
discovery could help to boost the response in plants that do not take
full advantage of elevated levels of the gas, Schroeder says. "A lot of
plants have a very weak response to CO2. So even though atmospheric CO2
is much higher than it was before the industrial age and is continuing
to increase, there are plants that are not capitalizing on that.
They're not narrowing their pores, which would allow them to take in
CO2, while losing less water," he said. "It could be that with these
enzymes, you can improve how efficiently plants use water, while taking
in CO2 for photosynthesis. Our data in the lab suggest that the CO2
response can be cranked up."

Plants lose 95 percent of the
water they take in to evaporation through these pores, also called
stoma. Modifying crops to be more responsive to CO2  could help farmers
meet demand for food as competition for water increases. In California,
for example, 79 percent of water diverted from streams and rivers or
pumped from the ground is used for agriculture according to the
California Department of Water Resources.

Schroeder's team identified a pair of proteins that are required for the CO2  response in Arabidopsis, a
plant commonly used for genetic analysis. The proteins, enzymes called
carbonic anhydrases, split CO2  into bicarbonate and protons. Plants
with disabled genes for the enzymes fail to respond to increased CO2
concentrations in the air, losing out on the opportunity to conserve
water.

Several types of cells in plant leaves contain
carbonic anhydrases, including those responsible for photosynthesis,
but Schroeder's team showed that the enzymes work directly within a
pair of cells, called guard cells, that control the opening of each
breathing pore. By adding normal carbonic anhydrase genes designed to
work only in guard cells they were able to restore the CO2-triggered
pore-tightening response in mutant plants.

Adding extra
copies of the genes to the guard cells actually improved water
efficiency, the researchers found. "The guard cells respond to CO2 more
vigorously," said Honghong Hu, a post doctoral researcher in
Schroeder's lab and co-first author of the report. "For every molecule
of CO2 they take in, they lose 44 percent less water."

The
action of carbonic anhydrases is specific to changes in CO2, the
researchers found. Mutant plants still open their pores in response to
blue light, a sign that photosynthesis can begin. And their pores also
shut when water is scarce, a response mediated by a plant
drought-stress hormone.

Photosynthesis continued normally
in the mutants as well, suggesting that altering CO2 sensitivity
wouldn't stunt growth – good news if the goal is to engineer
drought-resistant crops with robust yields.

But saving
water and surviving heat involves a tradeoff for plants: Evaporation of
water through the pores also cools the plant, just like sweat cools
human beings. If future growing conditions are hotter and drier, as
they are predicted to be in some parts of the world, then modifications
to the CO2 response will need to be carefully calibrated.

Lead
authors in Schroeder's lab are Honghong Hu, Aurelien Boisson-Dernier
and Maria Israelsson-Nordstrom, who contributed equally to the work.
Additional co-authors include Maik Bohmer, Shaowu Xue, Amber Ries, Jan
Godoski and Josef M. Kuhn. The National Science Foundation, National
Institute of General Medical Sciences and U.S. Department of Energy
supported this research.

SOURCE: University of California-San Diego.