Feb. 9, 2016

Herbivore dilemma: How corn plants fight simultaneous attacks

aphids feed on corn leaf
Meena Haribal
Aphids feed on a corn leaf that was previously nibbled by caterpillars.

Corn seedlings are especially susceptible to hungry insect herbivores, such as caterpillars and aphids, because they lack woody stems and tough leaves. So what’s a tender young corn plant to do?

A recent study by professor Georg Jander’s group at the Cornell-affiliated Boyce Thompson Institute (BTI) finds that corn plants may make serious trade-offs when defending themselves against multiple types of insects. Some corn varieties make themselves more vulnerable to aphids after generating defensive compounds against nibbling caterpillars. The results, which appear in the journal Molecular Ecology, may lead to the development of corn plants that are naturally resistant to certain insects.

“It’s like a metabolic dilemma,” said Vered Tzin, a first author and postdoctoral scientist in the Jander laboratory. “When caterpillars are feeding, there’s a change in the metabolic pathway that makes chemical defense compounds that protects the plants from caterpillars. But when we studied aphids, it seems like the same compounds that make the plants caterpillar-resistant have the potential to make them aphid-susceptible.”

Corn plants face an onslaught of herbivorous insects that chew on leaves, suck out sap, bore into stems or consume the roots. Researchers estimate that insects consume 6 to 19 percent of the world corn crop each year, while also spreading bacteria and viruses between plants.

To defend against these attacks, corn plants have physical and chemical defense mechanisms. To ward off aphids, plants make callose, a carbohydrate that can seal off openings between cells and stop aphids from drawing out the sap. Callose formation is triggered by a defensive compound called DIMBOA. In the event of a caterpillar attack, plants produce a compound called MBOA that deters their feeding. MBOA and DIMBOA are in the same metabolic pathway and come from the same parent molecule.

Due to the shared pathway, the researchers suspected that feeding by one group of insects, such as chewing caterpillars, might affect the plant’s ability to fight off another group, like aphids.

To test this idea, the researchers grew corn seedlings of a common variety, called B73, and exposed some to caterpillars. They then seeded them with aphids and counted the number of offspring that the aphids produced on pristine plants, compared to previously nibbled ones. The aphids consistently produced more offspring on corn that had been pre-chewed by caterpillars.

But when the researchers tested other corn varieties, results varied. They repeated the experiment with 17 different lines of corn from around the world. Some varieties supported more aphid offspring after a caterpillar feeding, while others had fewer aphids or there was no effect.

The variation they saw is likely due to the evolutionary history of the different corn varieties. Aphids tend to be more common in temperate areas, such as the Midwest, while caterpillars are a larger problem in tropical areas. Different varieties likely arose from breeding programs aimed at fighting the threats that corn faces in different environments.

Using a genetic approach, they identified three regions on chromosomes 1, 7 and 10 that appear to have a significant impact on a corn plant’s susceptibility to aphids. By breeding for specific genetic variations that naturally reduce caterpillar or aphid damage, scientists can develop new crop varieties that may require fewer pesticide applications.

In the future, the Jander group plans to use a similar approach to see how corn plants respond to simultaneous attacks from other insect pests.

Patricia Waldron is the staff science writer for the Boyce Thompson Institute.