Plants are preyed on by a host of animals from elephants, which are herbivores that eat massive amounts of plants, to microbes that infest, infect, and kill plants. Plant defenses may be molecular, cellular, or tissue-level. A plant has preexisting defenses in its structural growth, beginning with the stem and leaves. A specific hormone within leaves and stems, called a jasmonate, triggers a reaction when a plant's surface is wounded. This reaction is critical in helping the plant ward off predators so that its structures can heal.
An example of a defense at the molecular level is the production of chemical compounds that ward off herbivores, such as tannins, which taste bad to many plant eaters and may slow digestive processes once they are ingested. When their bark is cut, rubber trees ooze latex as a protector. Other molecular compounds mimic insect hormones. When insects eat a plant containing these hormones, their bodies molt (shed their skin or shell) too soon and the organism dies.
Plant Defense Against Pathogens
A pathogen is a disease-causing organism or virus that causes harm to a host organism. Leaves often have a waxy coating called a cuticle that prevents pathogens from entering the plant structures. The wax encourages water to roll off leaf surfaces, and the pathogen does not have enough water to multiply on the leaf's surface.
The structures of epidermal cell walls in plants are thick, strong, and pathogen-repellent. Within those walls, hormones (such as salicylic acid) and specific defense proteins act as natural defenses against both microbial pathogens and some insect pests.
Although plant leaves have multiple stomata (openings on the surface of plant cells that facilitate gas exchange), these openings do not allow pathogens to enter leaf structures. Often, the stomata's size and the guard cells' shape serve to ward off bacteria trying to enter leaves. An example of this is the extremely small stomata and raised guard cells of citrus trees (such as oranges, lemons, grapefruit, and limes) that thwart the entry of the citrus canker bacterium, an infection present in water droplets that causes lesions on the leaves, stems, and fruit of these trees.
Internal structures can also protect plants from disease. For example, if left unchecked, adult cereal rust mites feed on and destroy the plants they infect and can cause a loss in crop yield of 30–70%. However, the leaf veins in many types of grain crops are too narrow to allow rust pathogens, such as cereal rust mites, from progressing through the leaves. In another example, tiny hairs on nectarines block openings by which invading pathogens could enter.
Cells also have defenses to prevent disease. Cell walls provide blockading structures by developing additional cellulose (thick fibers) to slow down or thwart entry into cells, producing fibrous material to trap bacteria, and callose material (a plant polysaccharide) that encases pathogens that have attacked cell walls. Within a cell, cytoplasm surrounds an attacking pathogen. The cytoplasm becomes thick and grainy and causes specific pathogens to disintegrate.
Plant Structural Defenses
A structural defense is a protective plant part, such as a spine or thorn. Most are designed to prevent herbivores from feeding on plant structures. Surface structures that deter herbivores include thorns, prickles, spines, and trichomes.
Thorns are nonvascular tissue that are highly modified shoots: sharp, pointed branches or stems. Thorns grow from buds and may grow as individual thorns or in clusters. Common trees that support thorn growth include firethorn, hawthorn, blackthorn, and citrus.
Prickles, pointed growths from plant epidermal layers, are most often found on leaf structures or stems. Roses have prickles (not thorns) protruding from their stems, while holly has prickles on its leaves.Spines are adapted leaves, stipules (protective leaf parts at the base of a leaf stalk), or parts of leaves; spines have vascular tissue carrying fluid through them. The most common plants with spines are cactus, on which spines act to thwart herbivores and also provide shade from the desert sun. Acacia, the preferred food of giraffes, also has spines, although giraffes and various antelope are not deterred from eating the tender leaves by the presence of sharp acacia spines.
Structural Defense of Plants
Plant Chemical Defenses
Many plants react to the stress of attack from herbivores eating the plant. Other stresses might include bacterial or viral infections or environmental conditions such as drought. Some plants in these threatening situations release a toxic or damaging substance called a volatile organic compound (VOC) that easily vaporizes and is used for communication in plants. VOCs include terpenoids (naturally occurring organic chemicals) and green-leaf volatiles (compounds that are released when plants suffer tissue damage). The release of these chemical compounds alerts nearby plants as to the current situation, and those plants may also release foul-smelling or repelling substances that fend off predation or scents to attract animals that prey on the insects or creatures eating the victim plant.
Other plants produce actual toxins, which are poisonous either by ingesting them or by contact with them. Some plants have both toxic and edible parts. Rhubarb stalks are edible, but the leaves contain toxins that cause kidney disorders. All members of the nightshade plant family exude a foul scent that warns animals not to eat them, and this includes tomatoes, of which only the fruit is edible. In some plants, the fruit is poisonous to some animals but not to others. American bittersweet, juniper, holly, and yew produce berries that are poisonous to humans, yet bluebirds eat bittersweet berries, robins and mockingbirds eat juniper berries, and warblers and waxwings eat holly berries. The toxins ward off larger predators, but the plants need birds to eat the fruit for seed dispersal through fecal matter to other locations.
At times, animals take advantage of plant toxins for their own benefit. Monarch butterfly caterpillars eat toxic milkweed leaves. The toxin is kept in their bodies and makes the caterpillars less appealing to predators. That toxin survives through metamorphosis, and Monarch butterflies also carry the same toxin. The African variegated grasshopper also feeds on toxic plants, using the toxin as its own defense mechanism.
Another defense mechanism for plants is producing toxic oils that cause skin rashes. These plants, such as poison oak and poison ivy, can leave itchy, painful blisters on any skin surface that comes in contact with the plant's natural oil. Few animals are willing to take a bite, knowing that their tongues and mouths will suffer as a result. Other plants with this type of defense mechanism include wild poison hemlock, poison sumac, cow parsnip, giant hogweed, stinging nettle, and the plant group called spurges.