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 enn State research has shown, for the first time,
that ozone, a major smog constituent, has a direct effect on
the genes associated with the aging process in plants.
 Dr.
Jennifer Miller, who produced the finding as part of her doctoral
thesis, says, "Plant scientists have long known that ozone
accelerates the process through which the leaves of a plant age,
eventually die and drop. However, our work provides the first
evidence that the genetic program that controls the aging process
is directly affected by ozone exposure."
She notes
that the findings may help other researchers find a way to bolster
plants' resistance to ozone, a critical component of smog, which
causes an estimated $3 billion in agricultural losses in the
United States each year.
Miller detailed
her findings in her doctoral dissertation, "Scenescence-Associated
Gene Expression in Ozone-Stressed Arabi-dopsis Leaves,"
which she defended in June. She is currently an assistant professor
of biology at Southwestern College, Winfield, Kansas. Some of
her findings were also detailed earlier in a paper, Senescence-Associated
Gene Expression during Ozone-Induced Leaf Senescence in Arabidopsis,
published in the journal Plant Physiology. Coauthors are
Dr. Richard N. Arteca, professor of horticulture and plant physiology,
and Dr. Eva J. Pell, the Steimer professor of agricultural sciences,
who was Miller's thesis adviser.
Miller conducted
her experiments with Arabidopsis, a plant with a short 6- to
8-week life span and a small gene pool that is often used in
plant studies. In one experiment, she grew the plants from seed
and exposed one group of plants to low levels of ozone for six
hours a day while leaving another group untreated.
"The
ozone treatment was higher than ambient levels but not high enough
to cause visible signs of damage immediately following the exposure.
Ozone concentrations in polluted areas can reach the levels used
in these experiments, but not usually for six hours straight,"
Miller explains.
The plants
received ozone exposure a few days after they had produced a
fifth leaf. As the leaves aged, yellowing was accelerated and
growth retarded on the ozone-treated plants. Examining the plants'
aging-related genes every other day during the 14 day treatment
period showed that some of them were turned on earlier in the
ozone-stressed plants, indicating a direct effect on the plants'
genetic program.
In another
experiment, Miller used mutant plants that were unable to perceive
ethylene produced by the plant. Ethylene production has long
been thought to be an important cue that signals the timing of
aging. However, even in the plants in which ethylene perception
was disrupted, ozone treatment produced early aging and early
activation of some aging genes.
"Although
we don't know exactly what all of the aging-related genes do,
we now know that ethylene production is not the primary signal
for gene expression," Miller says.
She proposes
that oxidative products, the same free radicals that are thought
to influence produce aging in people, also may be a possible
aging cue for plants. She says, as others have shown, it may
be possible to enhance ozone resistance in plants simply by selecting
plant varieties with higher levels of antioxidants. 
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