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People aren't the only ones in need of
antioxidants to neutralize free radicals. Scientists
have long known that plants use their own vitamin C to
reduce oxidative damage. Now,
Agricultural Research
Service scientists are looking into ways that plants
use vitamin C to defend against ozone, which damages
more plants than all other air pollutants combined.
Stratospheric, or upper-level, ozone
protects Earth from harmful ultraviolet radiation. But
tropospheric, or ground-level, ozone, is a pollutant.
Tropospheric ozone results when air pollutants react
with oxygen in the presence of sunlight to form a
molecule with three highly charged oxygen atoms (O3).
Tropospheric ozone enters plants through their leaves
and decomposes into unstable molecules called reactive
oxygen intermediates (ROIs). If not neutralized by an
antioxidant, ROIs injure plants.
At the Air Quality-Plant Growth and
Development Research Unit in Raleigh, North Carolina,
plant physiologist Kent Burkey is studying how plants
transport vitamin C out of their leaf cells and into a
complex of adjoining cell walls. This outer cellular
space is called the apoplast—an interconnected liquid
layer surrounding the cells. "We've found that plants
that are able to move greater quantities of vitamin C
into the leaf apoplast have a better chance of
detoxifying ozone," says Burkey.
He has evidence that ozone tolerance in
snap beans is associated with elevated vitamin C in the
leaf apoplast. He has also found that plants vary widely
in terms of how much vitamin C they make inside their
cells. "But that doesn't seem to be related to how
tolerant they are," says Burkey. While some plants make
lots of vitamin C in their cells, they are not capable
of transporting it into the apoplast, where it could
provide protection against ozone injury.
After vitamin C neutralizes ROIs, the
vitamin C itself becomes oxidized into dehydroascorbic
acid (DHA). The plant then moves the DHA back into the
cell where it is reduced, or revitalized, into vitamin
C, which is once again available for transport back into
the apoplast to fight ozone.
Questions remain about the protective
importance of vitamin C stored in the apoplast before
ozone exposure versus vitamin C that is pumped into the
apoplast in response to ozone stress. But Burkey's most
recent tests on snap beans suggest that the presence of
vitamin C in the apoplast before ozone enters the leaf
is critical.
He will next look more closely at how
vitamin C and DHA are transported between the cell and
the apoplast. And he will look for other antioxidant
compounds in the leaf apoplast that could protect
against ozone injury.
Burkey hopes the research will lead to
finding genes associated with a plant's ability to pump
vitamin C into the leaf apoplast. "You could potentially
develop plants with greater ozone tolerance," he says.
"Once you have the gene, you could express it in other
plants using molecular techniques."—By
Rosalie Marion Bliss, Agricultural Research
Service Information Staff.
This research is part of Air Quality
(#203) and Global Change (#204), two ARS National
Programs described on the World Wide Web at
http://www.nps.ars.usda.gov.
Kent O. Burkey
is with the USDA-ARS
Air
Quality-Plant Growth and Development Research Unit,
3127 Ligon St., Raleigh, NC 27607; phone (919) 515-1620,
fax (919) 856-4598.
"Vitamin C Protects Stressed-Out Plants" was
published in the
January 2003 issue of Agricultural
Research magazine.
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