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A novel enzyme in corn helps the plants defend themselves from voracious
caterpillars by disrupting the insects' ability to digest food, and
ultimately killing them, according to researcher s. The enzyme could be used
in tandem with other biological pesticides such as the Bt toxin to prevent
the pests from developing resistance and making the toxin more effective,
April 2006.

"The enzyme is found in insect-resistant strains of corn, and it breaks
down proteins and peptides in the insects' gut. It is a unique active
defense against herbivory," says Dawn Luthe, professor of plant stress
biology at Penn State University.

Luthe and researchers at Mississippi State University have since developed
several lines of corn resistant to multiple pests, using conventional plant
breeding and insect-resistant strains of corn from Antigua.

Researchers have found that when caterpillars fed on the insect-resistant
plants, one enzyme -- Mir1-CP or maize insect resistance cysteine protease,
in particular --accumulated at the feeding site within an hour of the
caterpillar's feeding and continued to accumulate at the site for several
days.

"Upon isolation and purification of the enzyme, we found that Mir1-CP binds
to chitin, a major component of insects and fungi," says Luthe.
"Physiological tests show that caterpillars have impaired nutrient
utilization when they eat the enzyme. They just can't convert what they eat
into body mass."

Luthe presented the findings at the annual meeting of the American Chemical
Society March 30 in Atlanta.

With the help of antibodies specific to the enzyme, the researchers were
able to determine that Mir1-CP is made in the vascular bundles, or strands
of conducting vessels in the stem and leaves of a plant.

Luthe thinks that when an insect starts feeding, the enzyme is probably
transported to vascular tissue that conducts sugars and other metabolic
products upward from the leaves, as well as to the soft tissue found in
leaves and stem.

Though it is still unclear whether the transport of Mir1-CP is a specific
response to the insect feeding, studies show that maize tissue that
naturally expresses Mir1-CP causes a 50 percent inhibition in caterpillar
growth. Transgenic black Mexican sweet corn cells that express Mir1-CP
inhibit caterpillar growth by 70 percent, Luthe says.

Mir1-CP is harmful to caterpillars mainly because of its damaging effect on
their peritrophic matrix. This is a membrane that lines the gut of most
insects and aids digestion. It also protects the insects from being invaded
by microorganisms and parasites through the food they eat.

At the heart of the matrix is a protein called the insect intestinal mucin,
or IIM. It is very similar to the mucus layer in animals and is vital for
nutrient utilization because it helps the flow of nutrients into the food
gut.

The researchers tested the permeability of the matrix using blue dextran, a
fermented sugar solution commonly used as a molecular size marker. Results
showed that Mir1-CP created holes in the matrix.

To replicate the test in vivo, the researchers fed caterpillars with plants
susceptible to the insects and those resistant to them. Results indicate
that after seven days, the level of both IIM and IIM messenger RNA in
insects that were feeding on the resistant plants had fallen significantly.

"If the IIM is being degraded by the enzyme, pieces of it should not appear
in the fecal pellets of the insect," notes the Penn State researcher.

When used in conjunction with the Bt-toxin, a low dose of Mir1-CP was able
to achieve a very high mortality rate in the insects, as well as an
extremely low growth rate.

"In the long run, the enzyme degrades the insect's peritrophic matrix and
retards the caterpillar's ability to generate a new one," says Luthe.

The research has potential global implications in generating a cheap and
highly effective way of controlling crop pests.

Other authors of the paper include Tibor Pechan, Srinidi Mohan, Renuka
Shivaji, Lorena Lopez, Alberto Camas, Erin Bassford, Seval Ozkan, Peter Ma,
all at Mississippi State University; and W. Paul Williams, U.S.D.A.

The U.S. Department of Agriculture and the National Science Foundation
funded this study.

Graphics are at: [www.psu.edu]

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