All plants produce a diverse group of chemicals whose main function is to protect the plant against herbivores and diseases; these are the plant's chemical defenses. Many of these compounds seem to have no role in such core plant functions as growth and reproduction, and they are synthesized in unique pathways in the plant. As a result, they are often called secondary compounds or secondary metabolites. Others, including many enzymes , also have functions in growth, reproduction, and acquiring light or nutrients. Even though humans have exploited these plant products for thousands of years, it was not until the 1950s that scientists reasoned that these chemicals might be produced as defenses. While many plants have physical defenses such as thorns or spines, and some are just too tough to chew, those traits block feeding by large animals only, and do nothing against diseases. Chemical defenses are potentially effective against all of a plant's enemies. The study of how plants make, deploy, and benefit from chemical defenses is an important branch of chemical ecology.
Types of Chemical Defenses
Defensive chemicals are grouped into classes based on their structures and how the plant makes them. Some classes are very large and occur in all plants, while others are smaller and may occur in only one or two plant families or a few species.
It has been estimated that, overall, plants synthesize several hundreds of thousands of different secondary compounds. New ones are reported every day. Five groups are most common, diverse, or widespread: the alkaloids, cyanogenic glycosides, terpenoids, phenolics, and glucosinolates.
There are more than ten thousand different alkaloids and relatives known from plants. Alkaloids are cyclic nitrogen-containing compounds. They are widely distributed among many higher plant families, where they are often produced in roots. Their activity in animals is diverse, but many interfere with neurotransmitters . When consumed, many alkaloids are addictive . Examples include caffeine (coffee), morphine (poppy), tomatine (tomato), nicotine (tobacco), and lupine alkaloids (legumes ).
Cyanogenic glycosides also contain nitrogen, bound with other carbons to a sugar. Certain plant and animal enzymes can remove the sugar, freeing hydrogen cyanide, which poisons the energy-producing mitochondria in all cells. Probably all plants can produce cyanogenic compounds, but they are most common in legumes (for example, bird's-foot trefoil) and the fruits of plants in the rose/apple family (Rosaceae); they have the odor of almonds.
Terpenoids are the second-largest group of secondary compounds (fifteen-to twenty-thousand known). They are incredibly diverse in structure and activity, even though they all originally derive from a simple molecule called isoprene. Most monoterpenoids are volatile and comprise the characteristic odor of conifers and mints. Some volatile terpenes are produced only when the plant is wounded by an herbivore. They attract predators or parasites to the plant, which then kill the damaging herbivore; this is an indirect chemical defense because it acts via a third party (the predator or parasite). Sesquiterpenoids are the largest subgroup of terpenes and include gossypol from cotton and the sticky sap of plants in the family Aster-aceae, such as lettuce and goldenrod. Triterpenoids include many extremely bitter compounds, including cucurbitacin from squashes and cucumbers; some are used as insecticides. Insects cannot synthesize cholesterol, the basis of their growth hormones, and must obtain the basic terpenoid skeleton from plants. But some plants produce steroids that act as insect hormones, disrupting insect development as a defensive ploy.
The most diverse and common secondary compounds are phenolics. Defined by possessing a benzene ring with one attached hydroxyl , an enormous number of structures can be called phenolic. Most of the more than twenty-five thousand known types of phenolics are good antioxidants and are frequently used as preservatives; in plants they prevent membrane oxidation and other types of oxidative damage. As defenses, various phenolics are distasteful, toxic, and inhibit digestion. When activated by light, coumarins in such plants as carrots and celery cross-link deoxyribonucleic acid (DNA) strands and halt cell division. The blue and red colors of most flowers are provided by flavonoids , oaks and tea are rich in phenolic polymers called tannins , and the odor of wintergreen is a phenolic acid (methyl salicylate).
Glucosinolates comprise a small (one hundred) group of compounds containing both nitrogen and sulfur. They, too, are good antioxidants, but they are best known as repellents. They occur primarily in the cabbage family, where they provide the distinctive odor of those foods.
Many plant enzymes serve a defensive function. Some (such as oxidases) oxidize and activate secondary compounds; many phenolics and glucosinolates are much more defensive when this has occurred. Others produce toxic ionic forms of various molecules, called reactive oxygen species (ROS), which damage membranes, proteins, and DNA. Others produce signals that coordinate defense responses. Chitinases digest fungal hyphae , and other defensive enzymes can digest bacterial cells.
While all plants produce some chemical defenses all the time, they also increase or alter chemical defenses when attacked by microbes or animals. These are called inducible defenses. Many things can induce chemical defenses, including wounding (for example, tearing), insect chewing, pathogen attack, and wind motion.
Induced responses to microbes can be very specific. A given plant geno-type (e.g., variety) can recognize and respond with specific defenses against particular microbe genotypes (e.g., bacterial strain) but may not respond to others. This ability has been exploited in breeding resistant crop plants and is called gene-for-gene resistance. Plants recognize potentially deadly microbes by detecting unique proteins or other molecules on the pathogens' surface. The plant's recognition device, usually a protein, eventually activates plant genes that encode the enzymes necessary to produce defensive chemicals. Phenolics and reactive oxygen species are produced at the point of infection, blocking microbial enzymes, killing microbes and the plant's own cells, and strengthening cell walls to prevent the spread of the infection. These steps comprise a strategy for stopping the infection called programmed cell death or apoptosis. The results appear as brown spots, or necrotic lesions, and the entire process from detection to lesion is called the hypersensitive response (HR). This approach to defense is shared with animal systems.
As HR proceeds, nearby plant cells produce signals that spread through the plant and generate systemic acquired resistance (SAR). A plant exhibiting SAR is resistant to the original pathogen as well as others. Even tissues not yet produced when the plant was first attacked are resistant once they appear. This effect superficially resembles immune responses in animals. Scientists do not know with certainty what the signal is that circulates through the plant, but evidence indicates that it is probably not a protein, as it would be in animals. Candidates include carbohydrate cell wall fragments, phenolics (salicylic acid), plant growth hormones (e.g., abscisic acid), and electrical impulses. Many defenses are induced when genes' encoding defense-production mechanisms are activated in response to a complicated web of signals.
Plant responses to herbivores are not as well understood. Even small amounts of damage can induce plantwide defenses and systemic resistance. HR is not a usual component of wound responses, but signals emanating from the site of damage do produce systemic resistance. There is good evidence that a fatty acid product, jasmonic acid (JA), circulates through a wounded plant, inducing chemical defenses. In tomatoes a peptide, systemin, plays a similar role. Plant responses to herbivores is often less specific than to microbes, although different insects can induce the production of different volatile defenses from the same plant. A molecule (volicitin) related to JA has been found in the regurgitant of insects and triggers induced responses. It appears that at least some plants can recognize their attacker via regurgitant or saliva chemistry, and induced defenses against herbivores are also probably often activated by altered gene expression.
Effectiveness of Plant Defenses
While the physiological action of some plant chemical defenses is well established, and it is relatively easy to find plant chemicals that repel or poison animals or microbes, it is more difficult to demonstrate that chemical defenses benefit plants in nature, for three reasons. First, there has been repeated evolution of microbes and herbivores that can tolerate or detoxify plant defenses. Many of these plant pests can attack only the few plant species or even tissues to which they are adapted, but no plant species is totally protected; there has been an evolutionary response to every plant defense. This dynamic process, in which two species (for example, plant and insect) influence each other's evolution reciprocally, is called coevolution.
Second, many scientists believe that producing defenses may be costly. Costs may include using materials (for example, sugars) for defense at the expense of growth or reproduction, a risk of poisoning one's self, or incompatibility with other life functions (for example, some nectars contain defensive chemicals and are toxic to bees). If so, then selection to reduce costs may counter selection for defense, leaving some plants vulnerable.
Third, the ability to produce defenses is constrained by the plant's genes and environment. If the mutations necessary to permit the development of a class of defenses (for example, alkaloids) has never arisen in a plant's evolutionary lineage , that defense—no matter how effective it might be—is not an option. And producing some defenses may be more difficult under some environmental circumstances. For example, low soil nitrogen can limit alkaloid production, and low light can restrict phenolic synthesis.
No plant is perfectly protected in nature, even if it is deadly to some enemies. Total plant protection must involve other forces acting together with the plant's own defenses. Employing extremely effective chemical defenses (pesticides) as the only plant protection produces resistant pests that can tolerate and overcome them. This does not happen in nature; most natural plant systems are rarely decimated by pests. Many ecologists believe that in nature, plant protection derives from chemical defenses acting in combination with each other and with other pest control agents. For example, plant chemistry may help parasites or predators find and kill pests (indirect defense), or make pests more susceptible to their pathogens. Using more than one chemical defense at a time, and varying them through time, slows the rate at which a pest can evolve resistance. The complexity of nature is an important component of plant protection.
Human Use of Defensive Chemicals
Humans have exploited plant chemicals for thousands of years. Many uses derive directly from their defensive action. Nicotine was among the earliest of insecticides developed by humans, a practice that continues today with the isolation of antimicrobial and antiherbivore chemicals and eventual synthesis of analogs . Citrus chemistry is exploited as a mosquito repellent. Many antiseptic and antibiotic agents are derived from plants (e.g., terpenes in pine-scented cleaners).
The nervous system activity of some alkaloids has been exploited for recreational and religious drug use (opium, cocaine, nicotine, caffeine, and mescaline) and medicine (opium and codeine). The ability of some to block signal transmission at neuromuscular junctions makes them important in surgery as well as hunting tools (e.g., curare). Polyphenols have broad antimicrobial activity; they inhibit oxidative enzymes (e.g., cyclooxygenases) that cause disease, and their antioxidant characteristics are thought to prevent aging and some cancers. Much the same has been claimed for glucosinolates (cabbage family). More than 90 percent of the medicines prescribed in the twentieth century were originally plant derived, mostly involving presumed defensive chemicals. Human medicinal use of plants is based almost entirely on the action of defensive chemicals.
The quality of many foods derives from plant chemical defenses. Tannins are an essential flavor and color component of ripening fruits, wines, and chocolates. The distinctive flavors and aromas of plants in the citrus, cabbage, cucumber, and tomato families, among others, derive from their defensive chemistries. Apart from basic nutrition, food chemistry is largely the chemistry of plant defenses.
The cultural and industrial applications of plant defense chemistry are too numerous to list. Tannins preserve leather and were the original ink; plant phenols and carotenoids are important dyes; rubber and latex began as defenses against wood-boring beetles in rubber trees; jasmonic acid, a defensive signal, is the "queen of aromas" and is crucial to perfume formulation.
see also Alkaloids; Allelopathy; Coevolution; Defenses, Physical; Flavonoids; Interactions, Plant-Fungal; Interactions, Plant-Insect; Interactions, Plant-Plant; Interactions, Plant-Vertebrate; Physiology; Terpenes; Trichomes.
Jack C. Schultz
Agosta, William C. Bombardier Beetles and Fever Trees. New York: Addison-Wesley, 1996.
Karban, Rick, and Ian T. Baldwin. Induce Responses to Herbivores. Chicago: University of Chicago Press, 1998.
Price, Peter W. Insect Ecology. New York: John Wiley & Sons, 1997. Schoonhoven, L. M., T. Jeremy, and J. J. A. van Loon. Insect-Plant Biology. London: Chapman and Hall, 1998.
Salicylic acid is the chemical from which aspirin is made.