The animal kingdom lives off the plant kingdom, an arrangement that can be devastating to individual plants. While some plants, such as stinging nettles or latex laden oleander, have evolved obvious means of deterring foliage-eaters, the majority appear to be defenseless. Why haven't all plants acquired protection against herbivores, especially the most voracious ones, slugs and bugs?
It turns out that many plants are not as helpless as they seem. Instead, they have quiescent defense systems that kick in only when something starts munching their leaves. In response to such wounding, the plant produces a cocktail of chemicals that makes its foliage less palatable. This form of resistance, known as "induced defense", is scientifically intriguing as well as holding promise for agriculture.
Professors Richard Karban, Sean Duffy and other researchers from their labs in the Department of Entomology and the Center for Population Biology at UC, Davis, have been studying the phenomenon in what might be the most familiar plant around Davis - the tomato. When a caterpillar first nibbles on a tomato leaf, a chemical known as jasmonic acid is released. This chemical acts as a "signal" to the plant to start producing certain proteins that will make its leaves less attractive as food. Scientists wondered if they could induce this resistance in the absence of caterpillars simply by applying the signal chemical to the leaves.
In one experiment conducted by Jennifer Thaler and Michael Stout, young tomato plants grown in a greenhouse were sprayed with jasmonic acid. Another group of plants were unsprayed but subjected to wounding by caterpillars (corn earworm larvae). To ensure that the worms did some serious munching, researchers starved them for a couple of hours and then confined them in tiny cages to single leaves. After two days, all leaves were puréed and analyzed for defensive proteins. Sure enough, the tomato's chemical responses to jasmonic acid alone mirrored closely the responses to caterpillar feeding. Apparently, the spraying had "fooled" the tomato plants into thinking they were being eaten. Not only that, but spraying cut out a link in the chain of defense and gave the plants a head start in developing resistence.
The next step was to see if the artificially induced defense deterred the caterpillars. Larvae raised on leaves sprayed with jasmonic acid did indeed have smaller appetites than those raised on unsprayed leaves, and as a result, they grew 25 - 80% more slowly. It looks as though it is possible to stimulate a plant's natural resistance to bugs before the bugs arrive.
This is all very well within the controlled environment of greenhouse and lab. But would it also work in a tomato field? During the summer young tomato plants in large field plots were sprayed with jasmonic acid, but protected from caterpillars. The leaves were periodically harvested and chemically analyzed to reveal similar responses to those found in the lab. Future experiments will explore the extent to which jasmonic acid treatment in a large field deters caterpillars.
According to graduate student, Anurag Agrawal, it is not clear why plants evolve a system of induced defense rather than just being unpalatable all the time. An obvious explanation is that permanent resistance uses up too much of the plant's energy and resources, or has other harmful side effects. Surprisingly, Agrawal says, the evidence for this suggestion is weak. While other theories abound, none are yet satisfactory and many still need to be tested. The whole question is important to our understanding of how plants are adapted to cope with their enemies, an intriguing problem in the study of the evolutionary arms' race between the consumers and the consumed. In addition, the work has a practical side.
Researchers have previously proposed the use of induced resistance
to control agricultural pests. There is an obvious advantage in
using a plant's natural defenses rather than the insecticides
that pollute our environs at present. The UCD study reported here
is the first to show that such natural resistance to insects can
be artificially stimulated on a large, agricultural scale. It
provides hope for the development of more benign methods of pest
control in the future.
Department of Anthropology, UCD, Davis, CA 95616.
email: kjstewart@ucdavis.edu