By Gillian Klucas
IANR News Service

For years, University of Nebraska-Lincoln entomologist Leon Higley cautioned his students against researching aphids. Despite 50 years of research, no one had figured out how the tiny, agricultural pests harmed plants. Fortunately for Higley, one of his graduate students didn't take his advice.

Doctoral student Fikru Haile's initial findings launched Higley and several Institute of Agriculture and Natural Resources colleagues into a new line of study that is debunking old assumptions about aphids.

Smaller than a pumpkin seed, aphids attack wheat, soybeans, corn and other crops, transmit diseases and cause more crop damage than any other insect. "As a group, aphids are probably the single most important agricultural pest worldwide," Higley said.

Because aphids cause plants to yellow, scientists have long assumed they produce a toxin that affects plant cells' chloroplasts, where photosynthesis happens. But the toxin had never been found.

Normally in photosynthesis, the energy in sunlight charges, or excites, molecules inside the chloroplasts. This energy is passed along in a series of reactions and eventually leaves the chloroplast as carbohydrates. In the process, the excited molecules lose their energy.

By looking at what happens to aphid-infested plants over time, instead of after yellowing as researchers had done in the past, Higley found abnormalities before visible signs of injury emerged.

"As silly as it is, that's probably the biggest thing we did to help reveal what was going on," says Higley. "We started to see things that people hadn't seen before."

Researchers also used fluorometry, which measures plants' energy status. The combination of early inspection and fluorometry revealed that aphids block energy from leaving the chloroplasts. It is the buildup of excited molecules, not a toxin, that eventually chews up the cells and causes visible damage.

Though Higley hasn't determined how aphids do this -- that's the next step -- the discovery seems to hold true for most, if not all, types of aphids.

The scientific implications are exciting, he said. It suggests a single evolutionary event that may shed light on how aphids and plants adapted to each other.

He's collaborating with colleague Tiffany Heng-Moss and others who envision a single solution to agricultural losses across a variety of crops and aphid species.

Heng-Moss is studying peroxidases, enzymes plants produce to neutralize peroxides, which are created from excess energy in the chloroplast. Because aphids block energy from leaving, abnormally large amounts of peroxides are created. Most plants can't sustain peroxidase production long enough to ward off an aphid infestation and eventually perish.

But some can, and after aphids leave, these resistant plants resume normal function. Finding the gene or genes that regulate peroxidase production could be the answer to transferring resistance to other plants, Heng-Moss said.

Scientists elsewhere have sequenced the peroxidase genes. Heng-Moss is researching whether those genes are turned on in response to insect feeding, as she suspects.

"If we find more activity of those genes in the resistant plants than in the susceptible ones, then that would provide evidence that they contribute to the resistance," she said.

If so, the next step will be transferring those genes into susceptible plants.

Giving plants the ability to withstand aphids is a better solution than killing the aphids, both entomologists said. Over time, insects can develop resistance to chemical controls. But allowing aphids to feed on but not kill the plant maintains a natural balance.

Though they are years away from engineering resistant plants, Higley and Heng-Moss believe they have found a simple solution to a problem that had proved too complex to decipher for decades.

The Nebraska soybean and sorghum boards, and the USDA regional research funds support this IANR Agricultural Research Division research.