Chemical SOS not just for farm, lab plants

The chemical screams for help that scientists have detected from agricultural plants under attack by pests in lab settings have now been heard in the wild.

In Utah, a wild tobacco releases certain chemicals when attackers start chewing, report André Kessler and Ian T. Baldwin of the Max-Planck-Institute for Chemical Ecology in Jena, Germany. When the researchers dabbed some of these defensive chemicals onto test plants, bugs that prey on the pests came to the rescue. The researchers report their findings in the March 16 Science.

“This has two firsts,” Baldwin explains. Other studies of chemical SOS signals used farm crops in lab setups. Kessler and Baldwin studied a desert population of Nicotiana attenuata, a relative of smoking tobacco. Also novel, says Baldwin, is the demonstration that specific signaling chemicals attracted pest-killers.

Jack Schultz, a pioneer in plant signaling at Pennsylvania State University in State College, comments that the new report gives “the most complete story we have” on these defenses.

Whether such calls for help work in the wild has come into question in at least two recent reviews. Plant-to-predator signaling in crops makes sense to scientists because a single bodyguard insect can make a big difference in expanses of uniform plants attacked by uniform pests. Plants vary much more in the wild, however, so chemical emissions might not prove reliable as a signal.

Also, most of the rescuing insects that have responded in tests have been highly specialized parasites of particular species. Researchers have wondered whether predators of more general tastes–the most readily available bodyguards in the wild–respond to such signals.

To address such qualms, Baldwin and Kessler developed a low-tech system for measuring scents the plants gave off. They covered each test plant with an open-top dome and attached a charcoal filter that trapped volatile chemicals from the plants. Attacks by flea beetles, hawkmoth caterpillars, and true bugs elicited a series of plant chemicals, starting soon after the attack with compounds that signal most kinds of wounding. Then came more-specialized substances, such as cis-alpha-bergamotene, known to be triggered by chemicals in caterpillar spit.

The researchers tested the effects of these substances by treating plants with one of the six chemicals in their study. To monitor the consequences, the researchers glued hawkmoth eggs underneath plant leaves and then waited to see what egg predators arrived. Several of the chemicals, including the cis-alpha-bergamotene, drew significant numbers of a common predatory bug, Geocoris pallens, which can account for 95 percent of hawkmoth deaths. Also, the researchers found that a chemical cocktail reduced egglaying in hawkmoths.

A commentary in the same issue of Science by Maurice W. Sabelis and his colleagues at the University of Amsterdam calls the experiments “elegant.”

Schultz, too, praises the work but notes that “it still falls a little short of the complete enchilada.” He wonders whether the artificially treated plants come close to mimicking real plants under attack.

Baldwin himself acknowledges there are unanswered questions. Still, he says, “I hope this will give this field a big push.”