Scientists have found that elevated CO2 may negatively impact the relationship between some plants and insects. Elevated CO2 is considered to be a serious catalyst of global change. Its effects can be felt throughout the ecosystem, including the insect-plant food chain link. Safeguarding highly-usable crops is of great importance to many local and national economies.
Many plants have inherent enzyme-based defenses that are released during insect attack. This study found that when soybeans (Glycine max) were exposed to elevated amounts of CO2 the plants became more susceptible to attack by Japanese beetles (Popillia japonica). Furthermore, as these beetles consumed the weakened soybeans, the insect's invasive abilities were intensified.
Dr. Jorge Zavala, Sr. of the Institute for Genomic Biology at the University of Illinois, and his colleagues conducted tests in which they evaluated this herbivorous attack-defense cycle. They studied soybeans grown in traditional field conditions but with additional exposure to ambient CO2. The results showed that the amount of cysteine proteinase inhibitors (CystPls) expressed in the genes decreased when soybeans were exposed to elevated CO2. CystPls is naturally produced by soybeans when they are under insect attack. It inhibits further attack once the invader has ingested it.
"Our results suggest that elevated CO2 increased the susceptibility of soybean to invasive insects by down-regulating the expression of hormones related with defense, which down-regulate the important defense CystPls against beetles," Zavala said.
Zavala also explained, "Under natural field conditions, elevated CO2 not only increased susceptibility of soybean to herbivory by the invasive species Japanese beetle, but also enhanced the performance of these beetles."
The investigation also determined the expression of genes that regulate two of the soybean's defensive hormones, jasmonic acid and ethylene. The results showed that exposure to increased CO2 lowers the plant's ability to regulate its defensive hormones.
The relevancy of this study is far-reaching since soybeans are an important crop to many different economies. Furthermore, while this study is specific to one plant and insect pairing, these findings will help inform the on-going debate on CO2 exposure and global change.
This research was presented at the annual meeting of the American Society of Plant Biologists in Chicago (July 7-11, 2007).
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