Corn Defenses: Bitten plants deploy gut-rotting enzyme

Some corn varieties that arose on the Caribbean island of Antigua defend themselves with chemical attacks that leave insect gut linings in tatters.

When armyworm caterpillars make the mistake of chewing on some of this corn, they don’t grow well, reaching only half the weight of counterparts that consume less gut-wrenching corn, says Dawn S. Luthe of Mississippi State University. Now, she and her colleagues propose at least one reason why.

Corn plants under attack quickly accumulate a cysteine protease–a protein-slicing enzyme–surrounding the location where the caterpillars are chewing. In an upcoming Proceedings of the National Academy of Sciences, the researchers report microscope observations of the sorry state of the innards of insects that had digested enzyme-laced corn tissue.

“That’s pretty novel,” comments Clarence Ryan of Washington State University in Pullman, another specialist in built-in plant weaponry. Although chemical defenses are common in the plant world, Ryan says he hadn’t heard of an enzyme of this particular class being deployed that way.

Luthe explains that the toxins from Bacillus thuringiensis (Bt), now widely engineered into commercial crops, also attack insect guts. But Bt toxins do their damage differently. She says that the new enzyme doesn’t knock out insects fast enough to substitute for Bt toxins. However, she speculates that combining the protease with other defenses could make a pesticide to which it would be hard for insects to develop resistance.

Corn breeders have long known about the caterpillar-stunting power of the Antigua lineages. These plants don’t express the enzyme in their kernels. A coauthor of the new paper, W. Paul Williams of a U.S. Department of Agriculture laboratory at Mississippi State, has used the corn in traditional, nontransgenic corn-breeding projects.

To see what causes the stunting, Luthe, Williams, and their colleagues studied the chemicals released by the Antigua corn varieties when caterpillars start chewing on them. Other corn-defense chemicals typically show up in 8 hours, but the cysteine protease surges in about an hour and remains at high concentrations for at least a week.

They identified the gene that encodes this enzyme and inserted it into another corn variety. They grew masses of the transgenic corn tissue, called callus, which they then fed to armyworm caterpillars. Under a scanning electron microscope, the guts of caterpillars that ate enzyme-enhanced callus had many little rips. Innards of caterpillars that ate nontransgenic callus looked smooth.

Another scientist who studies plants defenses, Gary Felton of Pennsylvania State University in State College, calls the work “an elegant demonstration of this new mechanism.”

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