Backup digestive enzyme rescues insects.

Many plants, after insects have munched on them for a day or so, produce a protein that can weaken or kill their predators by interfering with digestive enzymes called proteases. Exploiting this ability, scientists have genetically engineered experimental crops to churn out these proteins full-time. That way, insects get a dose with their first bite.

But the bugs found a way to turn the tables. Many insects survive the transgenic and normal plants, protein defenses, known as protease inhibitors.

A team of Dutch researchers say they know the survivors, secret. In response to the protease inhibitors, these insects produce a second type of enzyme that can withstand the inhibitors, assert Maarten A. Jongsma and his colleagues at the Agricultural Research Department-Center for Plant Breeding and Reproduction Research in Wageningen.

"With our transgene, we gave plants more of what they already have, but that turned out in the end not to be too clever," says coauthor Dirk Bosch.

The researchers inserted a gene from potatoes into tobacco plants, they report in the Aug. 15 Proceedings of the National Academy of Sciences. The potato gene caused the tobacco to produce protease inhibitor II (P12) continuously. P12 closely resembles the protease inhibitor that the tobacco plant produces when under attack.

The team then fed leaves from the transgenic plant, a normal plant, and a damaged one that was producing natural protease inhibitors to beet army worms, a foe of tobacco.

To find out how well the different protease inhibitors worked, the researchers extracted enzymes from the insects, guts and tested the enzymes, ability to break down a protein outside the plants. Overall, the enzymes from all the insects acted similarly, says Bosch, no matter which leaf diet they had fed on. Although the protease inhibitors curtailed the activity of one type of enzyme, the insects produced over twice as much of a second class and largely made up their losses.

The scientists also found that Colorado potato beetles reared on potato plants that produce large amounts of a protease inhibitor make enzymes that are insensitive to the inhibitor.

The findings suggest that genetically engineering plants for insect resistance may prove more difficult than originally thought, the authors and others assert. Researchers will need to take a closer look at what chemicals the insects, not just the plants, produce, notes Bosch. Other scientists have found that factors such as temperature can boost insects, resistance to plant toxins (SN: 4/9/94, p.230).

"We are naive newcomers to this game that plants and insects have been playing for a millennium," says Matthew P. Ayres of Dartmouth College in Hanover, N.H.

The inhibitor-induced enzyme may enable the insects to break down the plant protein to use as a nutrient, speculates Frank Slansky of the University of Florida in Gainesville.