Plant defense against herbivory
Plant defense against herbivory or host-plant resistance is a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Many plants produce secondary metabolites, known as allelochemicals, that influence the behavior, growth, or survival of herbivores. These chemical defenses can act as repellents or toxins to herbivores or reduce plant digestibility. Another defensive strategy of plants is changing their attractiveness. Plants can sense being touched, and they can respond with strategies to defend against herbivores. 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.
Other defensive strategies used by plants include escaping or avoiding herbivores at any time in any placefor example, by growing in a location where plants are not easily found or accessed by herbivores or by changing seasonal growth patterns. Another approach diverts herbivores toward eating non-essential parts or enhances the ability of a plant to recover from the damage caused by herbivory. Some plants support the presence of natural enemies of herbivores, which protect the plant. Each type of defense can be either constitutive or induced.
Historically, insects have been the most significant herbivores, and the evolution of land plants is closely associated with the evolution of insects. While most plant defenses are directed against insects, other defenses have evolved that are aimed at vertebrate herbivores, such as birds and mammals. The study of plant defenses against herbivory is important from an evolutionary viewpoint; for the direct impact that these defenses have on agriculture, including human and livestock food sources; as beneficial 'biological control agents' in biological pest control programs; and in the search for plants of medical importance.
Evolution of defensive traits
The earliest land plants evolved from aquatic plants around in the Ordovician period. Many plants have adapted to an iodine-deficient terrestrial environment by removing iodine from their metabolism; in fact, iodine is essential only for animal cells. An important antiparasitic action is caused by the blockage in the transport of iodide of animal cells, inhibiting sodium-iodide symporter. Many plant pesticides are glycosides and cyanogenic glycosides that liberate cyanide, which, by blocking cytochrome c oxidase and NIS, is poisonous only for a large part of parasites and herbivores and not for the plant cells, in which it seems useful in the seed dormancy phase. Iodide is not itself a pesticide, but is oxidized by vegetable peroxidase to iodine, which is a strong oxidant able to kill bacteria, fungi, and protozoa.The Cretaceous period saw the appearance of more plant defense mechanisms. The diversification of flowering plants at that time is associated with the sudden burst of speciation in insects. This diversification of insects represented a major selective force in plant evolution and led to the selection of plants that had defensive adaptations. Early insect herbivores were mandibulate and bit or chewed vegetation, but the evolution of vascular plants led to the co-evolution of other forms of herbivory, such as sap-sucking, leaf mining, gall forming, and nectar-feeding.
The relative abundance of different species of plants in ecological communities including forests and grasslands may be determined in part by the level of defensive compounds in the different species. Since the cost of replacing damaged leaves is higher in conditions where resources are scarce, it may be that plants growing in areas where water and nutrients are scarce invest more resources into anti-herbivore defenses, resulting in slower plant growth.
Records of herbivores
Knowledge of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defense or herbivory-related damage; the observation of plant debris in fossilised animal feces; and the structure of herbivore mouthparts.Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils can show it. As previously discussed, the first land plants emerged around 450 million years ago; however, herbivory, and therefore the need for plant defenses, undoubtedly evolved among aquatic organisms in ancient lakes and oceans. Within 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around, there is evidence that plants were being consumed. Animals fed on the spores of early Devonian plants, and the Rhynie chert provides evidence that organisms fed on plants using a "pierce and suck" technique.
During the ensuing 75 million years, plants evolved a range of more complex organs, from roots to seeds. There was a gap of 50 to 100 million years between each organ's evolution and its being eaten. Hole feeding and skeletonization are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.
File:Plain tiger moat.JPG|thumb|A plain tiger Danaus chrysippus caterpillar making a moat to block defensive chemicals of Calotropis before feeding
Co-evolution
Herbivores are dependent on plants for food and have evolved mechanisms to obtain this food despite the evolution of a diverse arsenal of plant defenses. Herbivore adaptations to plant defense have been likened to offensive traits and consist of adaptations that allow increased feeding and use of a host plant. Relationships between herbivores and their host plants often result in reciprocal evolutionary change, called co-evolution. When an herbivore eats a plant, it selects for plants that can mount a defensive response. In cases where this relationship demonstrates specificity and reciprocity, the species are thought to have co-evolved.The "escape and radiation" mechanism for co-evolution presents the idea that adaptations in herbivores and their host plants have been the driving force behind speciation and have played a role in the radiation of insect species during the age of angiosperms. Some herbivores have evolved ways to hijack plant defenses to their own benefit by sequestering these chemicals and using them to protect themselves from predators. Plant defenses against herbivores are generally not complete, so plants tend to evolve some tolerance to herbivory.
Types
Constitutive and induced defenses
Plant defenses can be classified as constitutive or induced. Constitutive defenses are always present, while induced defenses are produced or mobilized to the site where a plant is injured. There is wide variation in the composition and concentration of constitutive defenses; these range from mechanical defenses to digestibility reducers and toxins. Many external mechanical defenses and quantitative defenses are constitutive, as they require large amounts of resources to produce and are costly to mobilize. A variety of molecular and biochemical approaches are used to determine the mechanisms of constitutive and induced defensive responses.Induced defenses include secondary metabolites and morphological and physiological changes. An advantage of inducible, as opposed to constitutive defenses, is that they are only produced when needed, and are therefore potentially less costly, especially when herbivory is variable. Modes of induced defence include systemic acquired resistance and plant-induced systemic resistance.
Chemical defenses
The evolution of chemical defenses in plants is linked to the emergence of chemical substances that are not involved in the essential photosynthetic and metabolic activities. These substances, secondary metabolites, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms, and often produced as by-products during the synthesis of primary metabolic products. Examples of these byproducts include phenolics, flavonoids, and tannins. Although these secondary metabolites have been thought to play a major role in defenses against herbivores, a meta-analysis of recent relevant studies has suggested that they have either a more minimal or more complex involvement in defense.Plants can communicate through the air. Pheromone release and other scents can be detected by leaves and regulate plant immune response. In other words, plants produce volatile organic compounds to warn other plants of danger and change their behavioral state to better respond to threats and survival. These warning signals produced by infected neighboring trees allow the undamaged trees to provocatively activate the necessary defense mechanisms. Within the plant itself, it transmits warning, nonvolatile signals as well as airborne signals to surrounding undamaged trees to strengthen their defense/immune system. For instance, poplar and sugar maple trees demonstrated that they received tannins from nearby damaged trees. In sagebrush, damaged plants send out airborne compounds, such as methyl jasmonate, to undamaged plants to increase proteinase inhibitor production and resistance to herbivory.
The release of unique VOCs and extrafloral nectar allow plants to protect themselves against herbivores by attracting animals from the third trophic level. For example, caterpillar-damaged plants guide parasitic wasps to prey on victims through the release of chemical signals. The sources of these compounds are most likely from glands in the leaves which are ruptured upon the chewing of an herbivore. The injury by herbivores induces the release of linolenic acid and other enzymatic reactions in an octadecanoid cascade, leading to the synthesis of jasmonic acid, a hormone which plays a central role in regulating immune responses. Jasmonic acid induces the release of VOCs and EFN which attract parasitic wasps and predatory mites to detect and feed on herbivores. These volatile organic compounds can also be released to other nearby plants to be prepared for the potential threats. The volatile compounds emitted by plants are easily detected by third trophic level organisms as these signals are unique to herbivore damage. An experiment conducted to measure the VOCs from growing plants shows that signals are released instantaneously upon the herbivory damage and slowly dropped after the damage stopped. It was also observed that plants release the strongest signals during the time of day which animals tend to forage.
Since trees are sessile, they have established unique internal defense systems. For instance, when some trees experience herbivory, they release compounds that make their vegetation less palatable. The herbivore saliva left on the leaves of the tree sends a chemical signal to the tree's cells. The tree cells respond by increasing the concentration of salicylic acid production. Salicylic acid is a phytohormone that is one of the essential hormones for regulating plants' immune systems. This hormone then signals to increase the production of tree chemicals called tannins within its leaves.