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Tritrophic Interactions in Plant Defense
Tritrophic interactions as they relate to plant defense against herbivory describe the ecological impacts of three trophic levels on each other: the plant, the herbivore, and its natural enemies. They may also be called multitrophic interactions when further trophic levels, such as hyperparasitoids or higher-order predators, are considered. Tritrophic interactions join pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals.
Predators, pathogens, and parasitoids that attack plant-feeding insects, called natural enemies in a tritrophic context, can benefit plants by removing or hindering the feeding behavior of the harmful insect. It is thought that many plant traits have evolved in response to this mutualism to make themselves more attractive to natural enemies, and so the enlisting of natural enemies to protect against excessive herbivory is considered an indirect plant defense mechanism.〔 Traits attractive to natural enemies can be physical, in the case of flower structure and color patterns, or chemical, in the case of induced plant volatile chemicals used by natural enemies to pinpoint a food source.
== Chemical Mechanisms of Enemy Attraction ==
Plants universally produce secondary metabolites, also called allelochemicals, which serve no purpose in basic metabolic processes. Instead they persist because they mediate the interactions between a plant and its environment, often attracting, repelling, or poisoning insects. In a tritrophic system, volatiles, which readily escape from plants into the air, are superior to surface chemicals in drawing foraging natural enemies from afar. Root volatiles also exist, driving tritrophic interactions among below-ground herbivores and their natural enemies. A very small fraction of plant volatiles are detectable by humans, giving plants like basil, eucalyptus, and pine trees their distinctive odors.〔 The mixture and ratios of individual volatiles emitted by a plant under given circumstances is referred to as a volatile profile. These are highly specific to the plant species and detectable within several meters of the source. Predators and, more commonly, 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.〔
The production of volatiles is likely to be beneficial to a plant as long as they are effective in inviting visitation by natural enemies and as long as those natural enemies are effective in removing or impeding herbivores. However, volatile chemicals may not have evolved initially for this purpose, but for within-plant signaling, to attract pollinators, or to repel herbivores that dislike such odors.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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