Caterpillars feeding on corn plants can actually trick the plant into lower its defenses, chemical ecologists in Penn State's College of Agricultural Sciences say, leaving the corn plant to lose out and the caterpillar to eat more and grow faster.
Because corn plants are under constant attack by both insects and pathogens, it must determine which poses the most severe threat and switch on defenses against herbivores or pathogens, but not both at once.
In this case, researchers studied the substances that can manipulate the plant's response, finding that the caterpillar's feces, or "frass" is what stimulates corn to defend against a pathogen, rather than fighting off the herbivore.
"It turns out that the caterpillar frass tricks the plant into sensing that it is being attacked by fungal pathogens and mounting a defense against them, thereby suppressing the plant's defenses against herbivores," says Dawn Luthe, professor of plant stress biology.
"It would be disadvantageous for the insect to deposit cues that could enhance plant defenses against it, so we investigated what chemical compounds in the frass were signaling the plant."
For fall armyworms, which feed on leaves in the confined whorls of corn plants, leave huge amounts of frass in leaf crevices on corn plants – and the frass remains there for a long period of time.
The research, recently published in the Journal of Chemical Ecology, may lead to the isolation of specific components of the frass that can be incorporated into a compound to be sprayed on crops.
Such an organic, ecologically sustainable pesticide could enhance plant defenses against pathogens, Luthe said. Or perhaps plants may be genetically modified to incorporate the proteins from the frass to boost a crop's native resistance to pathogens.
Caterpillar frass is composed of molecules derived from the host plant, the insect itself and associated microbes, and hence it provides abundant cues that may alter plant defense responses, explained lead researcher Swayamjit Ray.
He pointed out that proteins from fall armyworm caterpillar frass initially induced wound-responsive defense genes in corn; however, a pathogenesis-related defense gene was induced shortly after.
The creation of pathogen defenses by frass proteins correlated to more herbivore growth and less fungal pathogen prevalence over time.
These responses differ from the typical plant response to oral secretions of the fall armyworm caterpillar. The results pave the way for identification of a protein molecule from herbivore feces that boosts pathogen defense response and suppresses herbivore defenses in plants.
To test their hypothesis, researchers applied frass extract to the leaves of corn plants and compared the growth of fall armyworm caterpillars that fed on the leaves to the growth of caterpillars that fed on untreated leaves. They also measured the performance of a fungal pathogen in response to frass treatment of corn leaves. They inoculated the leaves with spores of a fungus that causes leaf blight in corn (Cochliobolus heterostrophus).
"The plant perceives that it is being attacked by a pathogen and not an insect, so it turns on its defenses against pathogens, leaving the caterpillar free to continue feeding on the plant. It is an ecological strategy that has been perfected over thousands of years of evolution," Ray said.
The research was supported by the U.S. Department of Agriculture.
Source: Penn State