In the ongoing bout with plant pests, scientists at the Boyce Thompson Institute for Plant Research Inc. at Cornell have found a one-two punch. The researchers have discovered and cloned a protein that, when delivered into an insect's gut by way of a "Trojan horse," attacks the pest's intestines, rendering the pest helpless against a deadly companion virus.
"No one has really studied the biochemistry and molecular biology of an insect's peritrophic matrix before," said Robert R. Granados, the Charles E. Palm Distinguished Scientist and virologist at Boyce Thompson Institute (BTI) and Cornell adjunct professor of entomology. "In our opinion, the peritrophic matrix [PM -- the interior lining of the insect's intestine], is an important component in the insect's immune system against biological control agents. We've finally defined the site of the action of enhancin on the peritrophic matrix."
Enhancin, an enzyme, binds to a major mucinous protein -- or mucin -- of the peritrophic matrix, destroying the matrix structure, allowing the baculovirus to pass through this relatively impermeable barrier where it can kill the host, Granados explained.
Granados and colleagues reported their findings, "An intestinal mucin is the target substrate for a baculovirus enhancin," in Proceedings of the National Academy of Sciences (June 24, 1997), authored by Granados and Ping Wang, Cornell graduate student in entomology. Another research report, "Molecular cloning and sequencing of a novel invertebrate intestinal mucin cDNA," is scheduled for publication in the Journal of Biological Chemistry (June 27, 1997), also authored by Wang and Granados.
In graphic A, viruses cannot pass through the intestine wall to kill the insect. In B, enhancin protein attacts the immune system, allowing viruses to pass through. Graphic courtesy of Robert R. Granados
The inside lining of the insect's intestine is made mostly of chitin and proteins. It was previously believed that the lining provided a barrier against certain microbial attacks. What this discovery allows: The enhancin, when consumed by the insect, will break down the mucin of the lining, facilitating the passage of a virus and resulting in the rapid death of the insect.
"Mucins from mammals have been extensively studied," Granados said. "In contrast, knowledge about invertebrate mucins is very limited."
Wang and Granados discovered and cloned the first invertebrate intestinal mucin gene from the cabbage looper larvae (Trichoplusia ni). The researchers looked at seven types of pests and found the method they had discovered to be effective against the fall armyworm, beet armyworm, the cotton bollworm, the American budworm, the velvetbean caterpillar and the common armyworm.
In the future, growers may use polyhedron crystals, which contain enhancin and a pest-specific baculovirus. A caterpillar, for example, will eat part of a leaf containing the polyhedron. The crystal will be dissolved in the insect's intestines, releasing the gut-wrenching enhancin and the baculovirus simultaneously.
Neither the protein nor the baculovirus will harm humans or animals, Granados said. Acidity, such as that in human stomachs and intestines, renders the protein ineffective against vertebrates. Invertebrate species -- such an insects -- do not have highly acidic intestinal contents.
The BTI researchers are seeking to genetically engineer the enhancin gene into plants, where it has considerable potential, Granados said.