Plant defense against herbivory

Plants alter their appearance by changing their size or quality in a way that prevents overconsumption by large herbivores, reducing the rate at which they are consumed.

Another approach diverts herbivores toward eating non-essential parts or enhances the ability of a plant to recover from the damage caused by herbivory.

[3] An important antiparasitic action is caused by the blockage in the transport of iodide of animal cells, inhibiting sodium-iodide symporter (NIS).

[30] The release of unique VOCs and extrafloral nectar (EFN) allow plants to protect themselves against herbivores by attracting animals from the third trophic level.

Jasmonic acid induces the release of VOCs and EFN which attract parasitic wasps and predatory mites to detect and feed on herbivores.

The volatile compounds emitted by plants are easily detected by third trophic level organisms as these signals are unique to herbivore damage.

They become toxic when herbivores eat the plant and break cell membranes allowing the glycosides to come into contact with enzymes in the cytoplasm releasing hydrogen cyanide which blocks cellular respiration.

[40] Glucosinolates are activated in much the same way as cyanogenic glucosides, and the products can cause gastroenteritis, salivation, diarrhea, and irritation of the mouth.

Plant steroids and sterols are also produced from terpenoid precursors, including vitamin D, glycosides (such as digitalis) and saponins (which lyse red blood cells of herbivores).

Phenolics range from simple tannins to the more complex flavonoids that give plants much of their red, blue, yellow, and white pigments.

Complex phenolics called polyphenols are capable of producing many different types of effects on humans, including antioxidant properties.

[50] The synthesis of fluoroacetate in several plants is an example of the use of small molecules to disrupt the metabolism of herbivores, in this case the citric acid cycle.

Structural defenses can be described as morphological or physical traits that give the plant a fitness advantage by deterring herbivores from feeding.

[56] A plant's leaves and stem may be covered with sharp prickles, spines, thorns or trichomes- hairs on the leaf often with barbs, sometimes containing irritants or poisons.

[59][60] Trees such as palms protect their fruit by multiple layers of armor, needing efficient tools to break through to the seed contents.

These mechanically reduce the digestibility of plant tissue, causing rapid wear to vertebrate teeth and to insect mandibles,[62] and are effective against herbivores above and below ground.

The leaves of the sensitive plant, Mimosa pudica, close up rapidly in response to direct touch, vibration, or even electrical and thermal stimuli.

The proximate cause of this mechanical response is an abrupt change in the turgor pressure in the pulvini at the base of leaves resulting from osmotic phenomena.

This response lowers the surface area available to herbivores, which are presented with the underside of each leaflet, and results in a wilted appearance.

One group of semiochemicals are allelochemicals; consisting of allomones, which play a defensive role in interspecies communication, and kairomones, which are used by members of higher trophic levels to locate food sources.

The subsequent reduction in the number of herbivores confers a fitness benefit to the plant and demonstrates the indirect defensive capabilities of semiochemicals.

Orre Gordon et al 2013 tests several methods of artificially restoring the plant-predator partnership, by combining companion planting and synthetic predator attractants.

[75] Plants sometimes provide housing and food items for natural enemies of herbivores, known as "biotic" defense mechanisms, to maintain their presence.

Similarly, several Acacia tree species have developed stipular spines (direct defenses) that are swollen at the base, forming a hollow structure that provides housing for protective ants.

They communicate and have dependent relationships through connections below the soil called underground mycorrhiza networks, which allows them to share water/nutrients and various signals for predatory attacks while also protecting the immune system.

[89] Examples of apparent plants that produce generalized protections include long-living trees, shrubs, and perennial grasses.

Riley, with J. E. Planchon, helped save the French wine industry by suggesting the grafting of the susceptible but high quality grapes onto Vitis labrusca root stocks.

[25][118] The use of botanical pesticides is widespread, including azadirachtin from the neem (Azadirachta indica), d-Limonene from Citrus species, rotenone from Derris, capsaicin from chili pepper, and pyrethrum from Chrysanthemum.

[120] Many currently available pharmaceuticals are derived from the secondary metabolites plants use to protect themselves from herbivores, including opium, aspirin, cocaine, and atropine.

[124] One of the best-known medicinally valuable terpenes is an anticancer drug, taxol, isolated from the bark of the Pacific yew, Taxus brevifolia, in the early 1960s.