Plant chemicals

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The chemicals in plants include a range of nutrient compounds, such as sugars, proteins, and lipids. In addition, plants produce a great variety of chemicals that are not derived from primary metabolic pathways and that have some ecological signaling function. These secondary compounds may be volatile and therefore may affect animals at some distance from the plant. In contrast, other compounds are nonvolatile and are not detected until the animal makes contact with (e.g., bites into) a plant. The compounds belong to several different chemical classes, including alkaloids, nonprotein amino acids, cyanogenic glycosides, terpenoids, glucosinolates, and phenolic compounds. Many thousands of these compounds are known, and their distribution among plants is often limited to particular taxa. For example, the cabbage family is characterized by glucosinolates and their breakdown products, which include the volatile thiocyanates; lupines contain quinolizidine alkaloids; onions contain thiosulfinates; and mint contains suites of monoterpenes. These compounds are largely responsible for the odors and flavors of these plants that are perceived by humans and other animals. Often a particular chemical is found only in a genus or a particular species of plant. Because these chemicals affect the behavior and fitness of animals that encounter them, they are of major importance in determining the range of plants eaten by an animal.

Plant-feeding animals may be polyphagous, if they eat plants from several different families, oligophagous, if they are restricted to feeding on members of one plant family, or monophagous, if they feed on only one genus or one species of plant. These differences depend on plant chemistry and, to a very large extent, on what the animal smells and tastes.

Attraction

Plant odors are usually complex mixtures, but often they are characterized by particular chemicals. For example, the characteristic odor (to humans) of plants in the cabbage family is produced by sulfur-containing compounds called isothiocyanates. The odors of mint, lavender, and pine are dominated by different terpenoids. Since most insects are monophagous or oligophagous, the distinctness of the odors of different plants often enables them to locate their specific hosts from a distance. Flight toward host-plant odors is known to occur in a number of different butterflies, moths, flies, beetles, and aphids. The insect has receptor cells on its antennae that respond to the appropriate compounds. For example, the antennal sensory system of cabbage root flies responds strongly to isothiocyanates from cabbage but weakly to most of the disulfides produced by onions. In contrast, the antennal sensory system of the closely related onion fly responds strongly to disulfides from onions and weakly to isothiocyanates from cabbage. These differences in antennal response mediate the differences in movement by these insects toward their respective host plants.

Many flowers produce characteristic odors that attract pollinators. These odors are blends of volatile secondary compounds, including terpenes, derivatives of fatty acids, and aromatic phenolic compounds. Receptors on the insects’ antennae respond to these compounds. However, while the response to odors of foliage is often innate, the response to odors of specific flowers is for the most part learned, since most insect pollinators are not specific to particular plants. In order to enhance the probability that an insect will visit plants of the same species, the insect must associate the presence of nectar with the odor of the flower that it last visited. This improves the foraging efficiency of the pollinator and increases the chance of cross pollination within a plant species. Thus, the odors of flowers are distinct and are species-specific mixtures of compounds. Some flowers, such as arums and carrion flowers, have qualitatively different odors that, to humans, are unpleasant. These flowers are pollinated by flies, moths, or beetles that are normally attracted to carrion, on which they lay their eggs.

Phagostimulation

All plants contain carbohydrates, proteins, amino acids, and various lipids that are potential nutrients for animals. Some of these compounds can be tasted by animals and generally stimulate feeding and thus are called phagostimulants (based on the Greek phagein, meaning “to eat”). In general, the taste of nutrient compounds is often essential for feeding and is used to adjust the amount eaten so that an organism maintains a suitable balance of nutrients. However, phagostimulants do not play a major role in determining the range of plants an animal will eat. Instead, the range of plants that animals feed on is determined to a very large extent by plant secondary compounds.

Although most secondary compounds are deterrent to the vast majority of species, there are some cases in which these compounds act as essential sign stimuli for an animal, indicating that it has the correct food. This is true for many insects that are oligophagous or monophagous on plants that contain characteristic chemicals. For example, plants in the cabbage family contain sulfur-containing compounds that act as sign stimuli for insects that habitually feed on only these plants. In the absence of the compounds, these insects will not feed. This is not because the compounds contain a chemical that provides some essential nutrient. In a few cases, it is known that the insects have receptor cells in the sensilla on their mouthparts or tarsi that are specifically sensitive to the sulfur-containing compounds, and this may be common in insects with chemically defined host-plant ranges. These same chemicals may be deterrents for insects that do not feed on these plants, as well as for insects that do feed on them.

Deterrents and repellents

Many secondary compounds have low volatility and usually serve to reduce or completely inhibit feeding by most plant-feeding insects. Secondary compounds only affect an animal when it makes contact with the plant, which generally occurs when the animal bites into the plant. Quinine and other alkaloids are examples of deterrents, as are glucosinolates and iridoid glycosides. In mammals these compounds are detected by the bitter taste receptors. Grasshoppers, butterflies, and moths also have cells that respond to a range of secondary compounds. The activity of these cells correlates with aversive behavior, and they are usually called deterrent cells. Phytophagous beetles may not have these cells, and their host plant choice may depend on the indirect effect of secondary chemicals on the activity of sensory cells that signal acceptability.

Only a few instances are known in which a plant odor causes an insect to move away from the source. Linalool, a very common component of flower odors, is known to have a repellent effect on the carrot aphid, Cavariella. This may be a common phenomenon, but it has been little studied.

Feeding decisions

Whether or not an animal eats a plant depends on phagostimulatory effects, mainly caused by nutrient compounds and sign stimulants, and on deterrent effects, caused by a variety of secondary chemicals. Polyphagous insects eat many plants that are unpalatable to oligophagous or monophagous species, even though all these insects may receive the same sensory information about plant chemistry. In the polyphagous species, deterrent compounds are less important in the interpretation of information by the central nervous than is true for selective feeders. An insect that is deprived of food or water tends to place less emphasis on deterrent signals and thus will eat a wide range of plants. The longer the period of deprivation, the greater the variety of plants that will be eaten.

Blood-feeding insects

Insects that feed on vertebrate blood, such as mosquitoes and tsetse flies, employ similar responses when locating and identifying their hosts. However, the chemical signals they use are different. Host odors cause takeoff, followed by upwind flight or, as in some tsetse flies, by visually oriented flight. Lactic acid from human sweat is an important attractant for some mosquitoes, and octenol and acetone from cattle breath odors are also attractants. Blood-feeding insects have receptors on their antennae that are sensitive to these compounds. Carbon dioxide is also an activator and attractant for several species of bloodsucking insects. Receptors for carbon dioxide have been demonstrated in not only insects that feed on blood but also several other kinds of insects, and these receptors are often on the maxillary palps (sensory structures associated with the mouthparts), rather than on the antennae. When an insect arrives at a potential host, chemicals on the host’s skin likely influence the insect’s behavior, although the role of these chemicals is poorly understood. Once the insect starts to probe host tissues, adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are released from red blood cells and may act as phagostimulants, causing the insects to gorge on blood. While many bloodsucking insects have receptors that are sensitive to ADP and ATP, others have receptors that are sensitive to different compounds.