Champaign, Illinois
March 25, 2008
Atmospheric carbon dioxide levels
are rising at an alarming rate, and new research indicates that
soybean plant defenses go down as carbon dioxide goes up.
Elevated carbon dioxide impairs a key component of the plant's
defenses against leaf-eating insects, according to the report.
The University of Illinois
study appears this week online in the
Proceedings of the National
Academy of Sciences.
Deforestation and the burning of fossil fuels have significantly
increased carbon dioxide levels since the late 18th century,
said plant biology professor and department head Evan DeLucia,
an author of the study.
"Currently, carbon dioxide in the atmosphere is about 380 parts
per million," DeLucia said. "At the beginning of the Industrial
Revolution it was 280 parts per million, and it had been there
for at least 600,000 years – probably several million years
before that."
Current predictions are that atmospheric carbon dioxide will
reach 550 parts per million by the year 2050, DeLucia said, and
the rapid industrialization of India and China may even
accelerate that timetable.
The new study, led by entomology professor and department head
May Berenbaum, used the Soybean Free Air Concentration
Enrichment (Soy FACE) facility at Illinois. This open-air
research lab can expose the plants in a soybean field to a
variety of atmospheric carbon dioxide and ozone levels – without
isolating the plants from other environmental influences, such
as rainfall, sunlight and insects.
High atmospheric carbon dioxide is known to accelerate the rate
of photosynthesis. It also increases the proportion of
carbohydrates relative to nitrogen in plant leaves.
The researchers wanted to know how this altered
carbon-to-nitrogen ratio affected the insects that fed on the
plants. They predicted the insects would eat more leaves to meet
their nitrogen needs.
When they exposed the soybean field to elevated carbon dioxide
levels, the researchers saw the expected effect: Soybeans in the
test plot exhibited more signs of insect damage than those in
nearby plots. A closer inspection showed that soybeans grown at
elevated carbon dioxide levels attracted many more adult
Japanese beetles, Western corn rootworms and, during outbreaks
of Asian soybean aphids, more of these than soybeans in other
plots.
Caterpillars and other insect larvae need nitrogen to grow and
build new tissues, but adult insects can survive and reproduce
on a high carbohydrate diet. So it made sense that more adults
would migrate to the high carbon dioxide plants, DeLucia said.
But did the higher sugar levels in the leaves explain the whole
effect? To find the answer, the team allowed beetles to live out
their lives in one of three conditions: on a high carbon dioxide
plant, on a low carbon dioxide plant outside the Soy FACE plot,
or on a low carbon dioxide plant grown outside the test plot but
which had its sugar content artificially boosted.
"What we discovered was startling," DeLucia said.
The beetles on the high carbon dioxide soybean plants lived
longer, and as a result produced more offspring, than those
living outside the Soy FACE plot. Even those fed a supplemental
diet of sugars did not see their life span extended.
"So here we were thinking that sugars were the main thing
causing the beetles to feed more on these high carbon dioxide
leaves," DeLucia said. "And that still may be true, but sugars
aren't what's causing them to live longer and have more breeding
events and offspring."
The team turned its attention to the hormonal signaling pathways
of the plants, focusing on a key defensive chemical the plants
produced to ward off an insect attack. When insects eat their
leaves, soybeans and other plants produce a hormone, jasmonic
acid, that starts a chain of chemical reactions in the leaves
that boost their defenses. Normally this cascade leads to the
production of high levels of a compound called a protease
inhibitor. When the insects ingest this enzyme, it inhibits
their ability to digest the leaves.
"What we discovered is that leaves grown under high carbon
dioxide lose their ability to produce jasmonic acid, and that
whole defense pathway is shut down," Delucia said. "The leaves
are no longer adequately defended."
The higher carbohydrate content of the leaves and the lack of
chemical defenses allowed the adult insects to feast and live
longer and produce more offspring.
"This study demonstrates that global environmental change is
multifaceted," Berenbaum said. "The impact of elevated carbon
dioxide on crippling the capacity of the plant to respond to
insect damage is exacerbated by the presence of invasive insect
pests in soybean fields. The Japanese beetle, as the name
suggests, is a relatively recent arrival in Illinois soybean
fields. It is causing considerable damage now but this study
suggests that its ability to inflict damage will only increase
over time."
The researchers, both of whom also are affiliated with the
university's Institute for Genomic Biology, will now seek to
determine whether the same process occurs in other plants. |
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