Tritrophic interactions in plant defense

Tritrophic interactions in 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 soil microbes, endophytes, or hyperparasitoids (higher-order predators) are considered. Tritrophic interactions join pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals. Natural enemies—predators, pathogens, and parasitoids that attack plant-feeding insects—can benefit plants by 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. This recruitment of natural enemies functions to protect against excessive herbivory and is considered an indirect plant defense mechanism. Traits attractive to natural enemies can be physical, as in the cases of domatia and nectaries; or chemical, as in the case of induced plant volatile chemicals that help natural enemies pinpoint a food source. Humans can take advantage of tritrophic interactions in the biological control of insect pests.

Chemical mechanisms of enemy attraction Plants produce secondary metabolites known as allelochemicals. Rather than participating in basic metabolic processes, they mediate interactions between a plant and its environment, often attracting, repelling, or poisoning insects. They also help produce secondary cell wall components such as those that require amino acid modification. 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. Some plant volatiles can be smelled by humans and give plants like basil, eucalyptus, and pine their distinctive odors. The mixture and ratios of individual volatiles emitted by a plant under given circumstances (also referred to as synomones in the context of natural enemy attraction) is referred to as a volatile profile. These are highly specific to certain plant species and are detectable meters from the source. 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. The production of volatiles is likely to be beneficial given two conditions: that they are effective in attracting natural enemies and that the natural enemies are effective in removing or impeding herbivores. However, volatile chemicals may not have evolved initially for this purpose; they act in within-plant signaling, attraction of pollinators, or repulsion of herbivores that dislike such odors.

Induced defenses

When an herbivore starts eating a plant, the plant may respond by increasing its production of volatiles or changing its volatile profile. This plasticity is controlled by either the jasmonic acid pathway or the salicylic acid pathway, depending largely on the herbivore; these substances are often called herbivore-induced plant volatiles (HIPVs). The plant hormone jasmonic acid increases in concentration when plants are damaged and is responsible for inducing the transcription of enzymes that synthesize secondary metabolites. This hormone also aids in the production of defensive proteins such as α-amylase inhibitors, as well as lectins. Since α-amylase breaks down starch, α-amylase inhibitors prevent insects from deriving nutrition from starch. Lectins likewise interfere with insect nutrient absorption as they bind to carbohydrates. Though volatiles of any kind have an attractive effect on natural enemies, this effect is stronger for damaged plants than for undamaged plants, perhaps because induced volatiles signal definitive and recent herbivore activity. 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. Natural enemies can distinguish between mechanical tissue damage, which might occur during events other than herbivory, and damage that is the direct result of insect feeding behavior. 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. The reliability of HIPVs in broadcasting the location of prey means that, for many foraging enemies, induced plant volatiles are more attractive than even the odors emitted by the prey insect itself. Plants are able to determine what types of herbivore species are present, and will react differently given the herbivore's traits. If certain defense mechanisms are not effective, plants may turn to attracting natural enemies of herbivore populations. For example, wild tobacco plants use nicotine, a neurotoxin, to defend against herbivores. However, when faced with nicotine-tolerant herbivores, they will attract natural enemies.

Local and systemic signals When herbivores trigger an inducible chemical defense pathway, the resulting HIPVs may be emitted either from the site of feeding damage (local induction) or from undamaged tissues belonging to a damaged plant (systemic induction). For example, when an herbivore feeds on a single corn seedling leaf, the plant will emit volatiles from all its leaves, whether or not they too have been damaged. Locally induced defenses aid parasitoids in targeting their foraging behaviors to the exact location of the herbivore on the plant. Systemic defenses are less spatially specific and may serve to confuse the enemy once the source plant is located. A plant might employ both local and systemic responses simultaneously.

Morphological mechanisms of enemy attraction

Domatia

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