Urbana, Illinois
September 15, 2008
A giant perennial grass used as a
biofuels source has a much longer growing season than corn, and
researchers think they've found the secret of its success. Their
findings offer a promising avenue for developing cold-tolerant
corn, an advance that would significantly boost per-acre yields.
The new study, from researchers at
University of Illinois, appears this month in Plant
Physiology Preview.
Miscanthus x giganteus is one of the most productive grasses
known. It is able to capture the sun's energy even as cool
temperatures shut down photosynthesis in other plants.
In Illinois, green Miscanthus leaves emerge up to six weeks
before corn can be planted. Miscanthus thrives into late
October, while corn leaves wither at the end of August.
Corn and Miscanthus are C4 plants,
which are more efficient than C3 plants in converting sunlight
into leaves and stalks. (C3 and C4 simply refer to the number of
carbon atoms in a molecule critical to photosynthesis.)
"The C4 process differs from C3 in having just four extra steps
in its metabolism," said Stephen Long, a professor of crop
sciences and principal investigator on the study. "There are
four extra proteins in this process, so we assumed that these
proteins are related to low temperature tolerance."
When they compared the levels of these proteins in plants grown
in warm and cold conditions, the researchers noticed that one of
the proteins, pyruvate phosphate dikinase (PPDK), was present at
much higher levels in the Miscanthus leaves grown at cool
temperatures than in the leaves of either corn or Miscanthus
grown in warmer conditions.
Although photosynthesis declined in both plants when they were
first subjected to cool temperatures, after two days,
photosynthesis rebounded in the Miscanthus.
The increase corresponded to the upsurge in PPDK in its leaves.
"After seven days PPDK was 10 times the level it was in the warm
conditions," Long said.
In C4 plants, PPDK catalyzes a chemical reaction in the leaf
critical to the cascade of reactions that convert carbon from
carbon dioxide into starches that form the plant's tissues.
Previous studies had shown that PPDK is generally not very
stable in cold conditions. The protein is made up of four
subunits, which tend to come apart at low temperatures, Long
said.
To test how cold temperatures affect the protein when it is
expressed in cells at high concentrations, post-doctoral fellow
Dafu Wang cloned the PPDK gene into E. coli bacteria to produce
large quantities of the protein.
"What he showed in the test tube was that if you concentrate the
protein, then it becomes more resistant to cold," Long said. "At
higher concentration the protein creates its own
microenvironment where in the cold it doesn't come apart. This
appears to be the secret of success for Miscanthus at low
temperature: Expressing more of the protein allows it to
photosynthesize at low temperature where corn can't."
The next step for the researchers is to develop a corn plant in
which this gene is expressed at high levels to determine if that
will make the corn more tolerant of low temperatures, Long said.
Cold weather after emergence of corn in the spring or in late
summer during grain-filling can limit photosynthesis, he said.
"This change should make corn more resistant to these cold
weather events." |
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