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Novel mechanism protects plants from freezing

August 31, 2010 – New
ground broken by Michigan State University biochemists helps explain how plants
protect themselves from freezing temperatures and could lead to discoveries
related to plant tolerance for drought and other extreme conditions.

August 31, 2010 – New
ground broken by Michigan State University biochemists helps explain how plants
protect themselves from freezing temperatures and could lead to discoveries
related to plant tolerance for drought and other extreme conditions.

“This brings together two
classic problems in plant biology,” said Christoph Benning, MSU professor of
biochemistry and molecular biology. “One is that plants protect themselves
against freezing and that scientists long thought it had something to do with
cell membranes, but didn’t know exactly how.

“The other is the search
for the gene for an enigmatic enzyme of plant lipid metabolism in the
chloroplasts;” in other words, how lipids, which are membrane building blocks,
are made for the plant cell organelles responsible for converting solar energy
into chemical energy by photosynthesis.

In an article published
online in the journal Science, Benning and his then-doctoral degree candidate
Eric Moellering and technical assistant Bagyalakshmi Muthan describe how a
particular gene leads to the formation of a lipid that protects chloroplast and
plant cell membranes from freeze damage by a novel mechanism in Arabidopsis
thaliana, common mustard weed. Working on his dissertation project under
Benning, Moellering identified a mutant strain of Arabidopsis that can't
manufacture the lipid and linked this biochemical defect to work done by others
who originally described the role of the gene in freeze tolerance, but did not
find the mechanism.

“One of the big problems
in freezing tolerance or general stress in plants is that some species are
better at surviving stress than others,” Moellering said. “We are only
beginning to understand the mechanisms that allow some plants to survive while
others are sensitive.”

There is no single
mechanism involved in plant freezing tolerance, Moellering added, so he can't
say that his findings will lead any time soon to genetic breakthroughs making
citrus or other freezing-intolerant plants able to thrive in northern climates.
But it does add to our understanding of how plants survive temperature
extremes.

Much plant damage in
freezing temperatures is due to cell dehydration, in which water is drawn out
as it crystallizes and the organelle or cell membrane shrivels as liquid volume
drops. Lipids in the membranes of tolerant plants are removed and converted to
oil that accumulates in droplets, the researchers said, retaining membrane
integrity, keeping membranes from fusing with one another and conserving the
energy by storing oil droplets. With rising concern globally about water
supplies and climate change, scientists see additional reasons to understand
the ways hardy plants survive.

The research, funded by
the U.S. Department of Energy Office of Science Basic Energy Sciences and the
Michigan Agricultural Experiment Station, also leads to speculation that
freezing itself can prompt cell proteins directly to change the composition of
the membrane, without activation by gradual acclimation. That has been a major
focus in the plant freezing tolerance field, the researchers said.

“This opens a huge door
now for people to do this kind of research, and to redirect researchers,”
Benning said. “There are lots of them out there trying to understand cold, salt
and drought tolerance in plants, and we've given them a new idea about how they
can approach this problem mechanistically.”