Insect-damaged sagebrush has a novel way of broadcasting to nearby plants that a predator is in the area: It releases a bouquet of airborne odors and perfumes.

If wild tobacco is growing nearby, it will "eavesdrop" on these chemical signals, and in response, fortify its defenses against such plant-eaters as caterpillars.

In a study published in a recent issue of Oecologia, Cornell University researchers say they have found that the release of chemicals called volatile organic compounds (VOCs) from a wounded sagebrush (Artemisia tridentata) primes the defenses of wild tobacco (Nicotiana attenuata) to prepare for herbivore attacks of its own.

But the tobacco plant holds off actually creating its defenses until it is attacked. That's because the plant pays a price for deploying its arsenal.

Most of the proteins and compounds used for defense contain nitrogen and carbon, which also are needed to produce seeds. So there is a fitness cost for the tobacco. The defenses are only advantageous to the plant if an herbivore actually attacks, because production of proteins and compounds for defense results in fewer seeds.

"By priming its defense response the plant is not investing resources before it is actually attacked," said Andre Kessler, the paper's lead author and an assistant professor of ecology and evolutionary biology at Cornell. "This could be a crucial mechanism of plant-plant communication."

The researchers have observed how plants prime their defense responses after receiving VOCs from damaged neighboring plants under controlled conditions in the laboratory and field; but they have yet to define the ecological relevance of this process in nature. For starters, tobacco and sagebrush plants rarely share the same habitat, except on roadsides and other disturbed habitats. Also, with the exception of some grasshopper species, the same herbivores do not eat both plants.

"The ecological relevance of the phenomena still has not been fully proven," said Rayko Halitschke, a co-author and a Cornell postdoctoral researcher in ecology and evolutionary biology. "We are short on data from plant-plant communications in a natural setting."

Tobacco hornworm, Manduca sexta, feeding on wild tobacco. Copyright © Cornell University

Still, priming may be the mechanism that allows plants to eavesdrop on a neighbor's VOC signal without paying a price in overall fitness.

In both greenhouse and field experiments, the researchers spread sagebrush clippings with their perfumes and odors around wild tobacco plants. Each experiment included an isolated tobacco plant as a control, a tobacco plant exposed to clipped sagebrush, a tobacco plant loaded with tobacco hornworms (the caterpillar of the tobacco hawkmoth) that feed on tobacco, and a plant exposed to both sagebrush and hornworms.

The damage and compounds from the hornworms' saliva trigger plants to produce defensive proteins called trypsin proteinase inhibitors (TPI). The TPIs function as a plant defense and make it difficult for caterpillars to digest proteins, which stunts their growth.

The researchers found that the undamaged and unexposed controls showed little difference from the plants exposed only to sagebrush. But plants that were exposed to both hornworms and sagebrush clippings produced more TPIs on the third day than plants just damaged by hornworms. By the fourth day, however, the TPI levels of the hornworm-damaged plants and those eaten by hornworms and primed with sagebrush were comparable, indicating that exposure to sagebrush gave the tobacco a head start in defending itself.

At the same time, the researchers tested levels of the cell's genetic machinery (mRNA) that readies the production of TPIs without actually making the protein, in plants both before and after exposure to clipped sagebrush. For both field and greenhouse tests, they found TPI-generating mRNA reached much higher values 24 hours after exposure to sagebrush, compared with plants that were not exposed to clippings.

These results suggest that when a tobacco plant senses that a neighboring sagebrush is being damaged, it primes itself for an attack without actually investing resources that would affect its overall fitness.