University Park, Pa. — Penn 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."
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, she notes.
Miller detailed her findings in her doctoral dissertation, "Scenescence-Associated Gene Expression in Ozone-Stressed Arabidopsis 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. Co-authors 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 anti-oxidants.
The research was supported by grants from the Environmental Protection Agency and the U.S. Department of Agriculture.
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