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 apoplastan
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
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.