Tritrophic interactions in plant defense
They may also be called multitrophic interactions when further trophic levels, such as soil microbes, endophytes, or hyperparasitoids (higher-order predators) are considered.
[1][2] Tritrophic interactions join pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals.
[3] Natural enemies—predators, pathogens, and parasitoids that attack plant-feeding insects—can benefit plants by hindering the feeding behavior of the harmful insect.
This recruitment of natural enemies functions to protect against excessive herbivory and is considered an indirect plant defense mechanism.
Rather than participating in basic metabolic processes, they mediate interactions between a plant and its environment, often attracting, repelling, or poisoning insects.
[7] In a tritrophic system, volatiles, which are released into the air, are superior to surface chemicals in drawing foraging natural enemies from afar.
Plants also produce root volatiles which will drive tritrophic interactions between below-ground herbivores and their natural enemies.
Predators and parasitoids exploit the specificity of volatile profiles to navigate the complex infochemical signals presented by plants in their efforts to locate a particular prey species.
However, volatile chemicals may not have evolved initially for this purpose; they act in within-plant signaling,[9] attraction of pollinators,[10] or repulsion of herbivores that dislike such odors.
The inducibility gives rise to the idea that plants are sending out a "distress call" to the third trophic level in times of herbivore attack.
The presence of herbivore saliva or regurgitant mediates this differentiation, and the resulting chemical pathway leads to a stronger natural enemy response than mechanical damage could.
Locally induced defenses aid parasitoids in targeting their foraging behaviors to the exact location of the herbivore on the plant.
Therefore, they not only ensure better survival, but eliminate the time required for natural enemies to locate and travel to the damaged plant.
Domatia may be as well-developed as acacia tree thorns or as simple and incidental as a depression or crevice in a leaf stem, but they are distinguishable from galls and other similar structures in that they are not induced by the insect but formed constitutively by the plant.
[1] As long as natural enemies have some potential to be omnivorous, plants can provide food resources to encourage their retention and increase the impact they have on herbivore populations.
[17] The field of chemical ecology has elucidated additional types of plant multitrophic interactions that entail the transfer of defensive compounds across multiple trophic levels.
[26][27][28] In some cases, populations of predators and parasitoids are decimated, necessitating even greater use of insecticide because the ecological service they provided in controlling herbivores has been lost.
Even when pesticides are not widely used, monocultures often have difficulty support natural enemies in great enough numbers for them to diminish pest populations.
[citation needed] A lack of diversity in the first trophic level is linked to low abundance in the third because alternative resources that are necessary for stable, large natural enemy populations are missing from the system.
Specific plant features such as the hairiness or glossiness of vegetation can have mixed effects on different natural enemies.
[24] Two ways the release of volatile organic compounds (VOCs) may benefit plants are the deterrence of herbivores and the attraction of natural enemies.
Chinese citrus farmers have capitalized on this mutualistic relationship for many years by incorporating artificial ant nests into their crops to suppress pests.
The extent of the influence largely depends on the evolutionary history shared between the two and the pathogens' method of infection and survival duration outside of a host.
Plants can also influence pathogen efficacy indirectly, and this typically occurs either by increasing the susceptibility of the herbivore hosts or by changing their behavior.
This finding offers the possibility of injecting crops with compatible entomopathogenic viruses to defend against susceptible insect pests.
A high degree of specificity is involved; species that make up these tritrophic interactions have evolved with one another over a long period of time and as a result have close interrelationships.
[48] The bacterium does not deter insect herbivory; it actually increases weight gain and leaf consumption in the caterpillar Spodoptera littoralis.
[48] However, the parasitic wasp Cotesia marginiventris is attracted more readily to maize plants grown in soil cultures containing either the volatile-producing bacterium or pure 2,3-butanediol.
Research thus far has had a relatively narrow focus, which may be suitable for controlled environments such as greenhouses but which has not yet addressed multi-generational plant interactions with dynamic communities of organisms.
