Early ecologists thought plants were wimps. Sure, some plants have toxins or thorns, but most just sit there like wild salad bars. Plant passivity is the dark side of The Very Hungry Caterpillar. In a conflict between plant and herbivore, the caterpillar gets fat and the plant gets dead.
Researchers at UC Davis, however, are studying plant responses to enemies like viruses and insects. It turns out plants are far from defenseless. They can fight off disease and even call in reinforcements. In the struggle for survival, every leaf is a battlefield.
Dr. Bryce Falk, professor of plant pathology at UC Davis, said that viral infections are easy to see in a garden. Find a plant where the leaves look like a dark green/light green “mosaic.” This odd coloring is often the result of a virus. The light green patches are where the virus is winning, causing the plant to produce less green-colored chlorophyll. The dark green patches are where the plant’s immune system is winning. Many plants build up natural immunity to viruses over multiple generations.
Naturally, humans arrive and eff that up.
California’s famous orange trees and grapevines are losing a battle against foreign viruses that cause diseases called citrus tristeza and grapevine leafroll.
“When these grapes and trees were brought here, someone brought a virus with them,” Falk said.
Not only did humans give plants viruses, but we also took away their weapons. Farmers don’t usually plant grapevines or trees using seeds. Instead, they use a clipping from a live vine or tree. The result is a plant that is genetically identical to its parent. The fields of genetic copies have no chance of evolving immunity to disease because we keep them from reproducing naturally.
“In many cases, they’ve lost the natural resistance to pathogens,” Falk said.
Ian Pearse, an entomology graduate student at UC Davis, said we can thank plant defenses against insect predators for some of our favorite foods.
Pearse said that researchers discovered long ago that basil grown in a greenhouse is less tasty than basil grown outside. They found that basil leaves naturally release chemicals to ward off predators. When grown indoors, the predator threat is reduced and leaves don’t produce the chemicals.
Insects might hate the taste of basil, but many humans enjoy plant-defense-chemicals in spaghetti sauce.
“A lot of the flavor is a result of defenses against herbivores,” Pearse said.
Pearse added that caffeine and nicotine are also produced by plants as protection against insects.
Plants may be sedentary, but they have allies that can launch an attack. Pearse said many “clever” plants react to insect herbivores by releasing chemicals that attract herbivore-killing critters. Plants munched on by caterpillars can call on parasitoid wasps to fly down and take out enemies. This same strategy has been observed in corn, tomatoes and tobacco plants.
A recent study out of the University of Florida found that citrus tree roots attacked by pests called citrus root weevils (Diaprepes abbreviates) emit compounds that attract weevil-munching worms called nematodes. Scientists call this a “tritrophic interaction.” I call it the circle of death!
Pearse said that an important plant defense is simply being hard to find. Many plants have specific predators out to get them. So if there are a few grapevines scattered in an ecosystem, it’s harder for insects to locate and travel between them.
In gardening folklore, they say you can stop pests by placing gross-tasting marigolds between plants in a garden. Pearse doesn’t know if marigolds are anything special, but he thinks the real trick is having a variety of plants.
“By having a diverse garden, it’s likely that you’ll cut down on some degree of herbivory,” Pearse said.
Of course, people found a way to mess up that defense, too. We like to grow crops of the same kind very close together. This system is called “monoculture,” and insects see it as a feast. We screw up plant defenses against pests and then spend bushels of money on pesticides. Plants have spent millennia evolving protection against natural enemies, but they have little way of stopping the very hungry Homo sapiens.
As Falk put it, “Humans are the worst.”
MADELINE McCURRY-SCHMIDT wants to share a plant joke that her little brother made up. Q: What do plants do in the Shire? A: They Frodo-synthesize. E-mail her your favorite science jokes at memschmidt@ucdavis.edu.
