These inducible immune-like responses are common throughout the plant kingdom and are being studied to demonstrate their adaptive nature and potential as an alternative to chemical pesticides in agricultural crops.
The UC Davis study, which will appear in the Feb. 20 issue of the journal Science, confirms the benefit of these responses and raises the question of why such defensive mechanisms evolved in such a way that they can be switched on and off, rather than remaining constantly active.
"Inducible responses have been observed in plants for more than 100 years, but were experimentally confirmed only in the last two decades," said Anurag Agrawal, the study author and a population biology doctoral candidate in the UC Davis entomology department. "Researchers in this field have predicated much of their work on the presumption that plants do benefit from induced responses. This study confirms those assumptions."
"Learning how and when plants benefit from inducible responses is absolutely essential if we are going to try to use similar responses for pest management in agriculture," says Richard Karban, a Davis professor of entomology and an authority on the interactions between insects and their host plants.
The study was conducted using wild radish [Raphanus sativus L. (brassicacea)] as an experimental model. The plant is a member of the mustard family, members of which are found in most parts of the world.
Agrawal gathered wild radish seeds and planted them in an experimental plot. When the young plants grew to the four-leaf stage he caged a cabbage worm -- the larval stage of the cabbage white butterfly -- onto one leaf of each plant in such a way that the caterpillar could feed on the leaf.
He also grew two sets of control plants: one group in which a leaf on each plant was clipped off at the base of the leaf stem or petiole and another group whose plants received neither caterpillar nor clipping damage.
Naturally occurring plant-eating insects were plucked from the plants in all groups so that only the test group plants received feeding-induced signals to trigger an immune response -- those signals provided by the caged caterpillars.
As the growing season proceeded, all of the plants were colonized by the normal insect pests, primarily earwigs, green peach aphids and flea beetles.
Agrawal observed the following effects of these pests:
- Plants initially treated with the caged caterpillars received only half as much leaf damage from earwigs as did plants in the two control groups.
- There were 30 percent more aphids on the control plants than on test plants initially treated with the caged caterpillars.
- Flea beetles killed 28 percent fewer plants among the caterpillar-induced group than among the control plants.
The immune-like response of the caterpillar-damaged wild radish plants was evidenced by characteristic biochemical changes in the plant tissue. (Previous research indicates that potential insect pests are deterred by chemical changes in the mustard oils that make the plant less tasty and an accompanying increase in numbers and stiffness of tiny leaf hairs, Agrawal noted.)
Still, Agrawal needed to determine not only that such a response was triggered, but also that the response actually enhanced plant fitness.
As an indicator of plant fitness, Agrawal measured seed production in terms of seed number multiplied by mean seed mass. He found that the plants in which the caterpillars had triggered a response yielded 60 percent more seed than did the control group.
Interestingly, plants in the leaf-clipped control group, which lost the same amount of leaf tissue as did plants in the caterpillar-bearing group, actually had a 38 percent lower seed yield than did the control plants.
"It's not clear exactly why clipping the plants didn't produce a resistance response equal to that of the caterpillar-damaged plants, since both groups of plants lost equal amounts of leaf area," said Agrawal. "It is suspected, however, that the saliva from the caterpillar is important in triggering the induced defense. Furthermore, a much smaller area of plant tissue was actually damaged by clipping off the leaf than was by the caterpillars eating the leaf bit by bit."
Agrawal and Karban are hopeful that this study will provide further information about the evolution and ecology of plant defenses and for development of non-chemical pest-control systems. Early research elsewhere has identified a hormone that appears to trigger the biochemical chain reaction that constitutes an immune response in plants. One product that makes use of this chemical to trigger an immune response against disease-causing agents is already on the market. The UC Davis researchers suspect similar products aimed at plant-eating insects will be commercialized in the next few years.
The wild radish study was funded by the National Science Foundation and the UC Davis Center for Population Biology.
-- Anurag Agrawal, Entomology, (530) 752-7525, firstname.lastname@example.org; or (805) 643-6000 Feb. 23-26
-- Richard Karban, Entomology, (530) 752-2800, email@example.com
-- Patricia Bailey, News Service, (530) 752-9843, firstname.lastname@example.org