U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Date of this Version


Document Type



Agricultural Research, January 2012


Unlike in animals, the “fight or flight” response isn’t an option for imperiled plants—including those grown as crops for our food, fuel, and fiber needs. A plant, for example, cannot simply uproot itself and sprint to safety from an approaching caterpillar.

But plants can defend themselves, and they’ve done so for millennia, by using an abundance of potent secondary compounds broadly referred to as “phytochemicals.” Some, like glyceollins in soybeans, are produced only in response to a specific threat—similar to the adrenaline surge a person experiences from a life-threatening event. For plants, this threat can be from a grazing animal, the first few chomps of a hungry insect, or the germination of a fungal spore. Other phytochemicals are “constitutive,” meaning they occur continuously to protect vital plant parts. An example is capsaicin, which occurs in chili pepper seeds and gives the fruit its tongue-torching “heat.”

Both phytochemical types have been the focus of scientific attention and formal study for well over 100 years. Agricultural Research Service scientists have been on the forefront of such studies and today conduct projects investigating the defensive responses of plants at laboratories in Peoria, Illinois; Gainesville, Florida; Oxford, Mississippi; Prosser, Washington; and other locations. All of these projects ask the question, “If plants can make their own natural chemical defenses, why the need for synthetic pesticides?”

In part, the answer has to do with the thousands of years of agriculture that preceded our modern-day understanding of plant genetics and advanced breeding technologies. The result of this nascent period was the loss of many plant-beneficial traits in favor of a few desirable ones singled out by humans. In some cases, where phytochemicals imparted a bitter or undesirable taste (such as alkaloids, a group of compounds believed to function mostly as plant protectants), selecting for plants with lower levels of those phytochemicals may have been deliberate. In either case, the result was a greater need for human intervention in protecting the plants from pests that they once may have been able to defend against on their own.