Plant defense against herbivory

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Poison ivy produces a chemical, urushiol, that protects the plant from herbivores by producing severe pain in the digestive tract, when eaten.
Poison ivy produces a chemical, urushiol, that protects the plant from herbivores by producing severe pain in the digestive tract, when eaten.
Digitalis plants (Foxglove) produce several deadly chemical defenses, namely cardiac and steroidal glycosides. Ingestion causes a range of symptoms including nausea, vomiting, hallucinations, convulsions, and death.
Digitalis plants (Foxglove) produce several deadly chemical defenses, namely cardiac and steroidal glycosides. Ingestion causes a range of symptoms including nausea, vomiting, hallucinations, convulsions, and death.

Plant defense against herbivory include a range of adaptations evolved by plants to improve their survival and reproduction by reducing the impact of animals that eat them. Plants have evolved an enormous array of mechanical and chemical defenses against herbivores.

These defenses include mechanical protections on the surface of the plant, production of complex polymers that reduce plant digestibility to animals, and the production of toxins that kill or repel herbivores. Defenses can either be constitutive, always present in the plant, or induced, produced or translocated by the plant following damage or stress. The term host plant resistance is also used by plant breeders to refer to these mechanisms.

Plants have also evolved features that enhance the probability of attracting natural enemies to herbivores. Specifically, they emit semiochemicals, odors that attract natural enemies, and provide food and housing to maintain the natural enemies’ presence.

A given plant species often has many types of defensive mechanisms, mechanical or chemical, constitutive or induced, which additively serve to protect the plant, and allow it to escape from herbivores.

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[edit] Mechanical defenses

The thorns on the stem of this raspberry plant, serve as a mechanical defense against herbivory.
The thorns on the stem of this raspberry plant, serve as a mechanical defense against herbivory.

Plants have many external structural defenses that discourage herbivory. Depending on the herbivore’s physical characteristics (i.e. size and defensive armor), plant structural defenses on stems and leaves can deter, injure, or kill the grazer. Some defensive compounds are produced internally but are released onto the plant’s surface; for example, resins, lignins, silica, and wax cover the epidermis of terrestrial plants and alter the texture of the plant tissue. A plant’s leaves and stem can be covered with sharp spines or trichomes. Plant structural features like spines and thorns reduce feeding by large ungulate herbivores (e.g. kudu, impala, and goats) by restricting the herbivores’ feeding rate.[1]

Some plants prevent the laying of eggs by insects species by mimicking the presence of insect eggs on their leaves. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some species of neotropical vines of the genus Passiflora (Passion flowers) containing physical structures resembling the yellow eggs of Heliconius butterflies on their leaves, and preventing oviposition (the process of laying eggs).[2]

[edit] Chemical defenses

[edit] Secondary metabolic products

Plants contain a wide variety of chemicals known as secondary metabolites, that are not essential to plant metabolism. These chemicals are often by-products produced during the synthesis of primary metabolic products.[3] Secondary metabolic products produced by a plant that influence the behavior, growth, and survival of another species are known as allelochemicals.[4] These secondary metabolic products are often produced in large quantity and are metabolically expensive and therefore must be serving a valuable purpose. Observations of selective feeding by herbivorous insects suggest that the presence of these secondary metabolic products serves as repellents to herbivores.[5][6]

Persimmon, genus Diospyros, has a high tannin content which gives immature fruit, seen above, an astringent and bitter flavor.
Persimmon, genus Diospyros, has a high tannin content which gives immature fruit, seen above, an astringent and bitter flavor.

There are several alternative explanations for the presence of allelochemicals in plants. Evidence suggest that flavonoids and cuticle waxes absorb between 90 – 99% of incoming ultraviolet (UV) radiation in cucumbers and maize, effectively acting as UV shields.[7][8] Allelochemicals, particularly cuticle waxes, may also prevent undesired water loss and maintain sufficient amounts of water in plants during periods of plant stress and drought. Additionally, some allelochemicals act as storage compounds for essential plant nutrients such as nitrogen and phosphorus.[9]

[edit] Qualitative and quantitative chemical defenses

Allelochemicals can be characterized as either qualitative or quantitative. Qualitative allelochemicals are defined as toxins that interfere with an herbivore’s metabolism, often by blocking specific biochemical reactions. For instance; alkaloids, such as caffeine, inhibit DNA and RNA synthesis, cyanogenic glycosides produce hydrogen cyanide, which blocks cellular respiration, and cardenolides and glucosinolates are emetics (agents that cause vomiting) .[10] Quantitative allelochemicals are digestibility reducers that make plant cell walls indigestible to animals. Condensed tannins, polymers composed of 2 to 50 (or more) flavonoid molecules, inhibit herbivore digestion by binding to consumed plant proteins and making them more difficult for animals to digest, and by interfering with protein absorption and digestive enzymes.[11] Silica and lignins, which are completely indigestible to animals, grind down insect mandibles (appendages necessary for feeding). The effects of quantitative allelochemicals are dosage dependent and the higher these chemicals’ proportion in the herbivore’s diet, the less nutrition animals gain from ingesting plant tissues.

Quantitative allelochemicals are present in high concentration in plants (5 – 40% dry weight) and are equally effective against both specialists and generalist herbivores. Because they are typically large molecules, these defenses are energetically expensive to produce and maintain, and often take longer than smaller, qualitative allelochemicals to synthesize and transport.

Qualitative allelochemicals are present in plants in relatively low concentrations (often less than 2% dry weight), and are not dosage dependent. These defenses have morphological properties (i.e. water soluble, small molecules, and are energetically inexpensive) that facilitate rapid synthesis, transport, and storage. These chemicals are effective against non-adapted specialist and generalist herbivores.

[edit] Constitutive and induced defenses

Defenses can further be classified as induced or constitutive. Constitutive defenses are those that are always present in the plant species, while induced defenses are synthesized at and/or mobilized to the site of attack when a plant is injured. There are huge variations in the composition and concentration of constitutive defenses, ranging from mechanical defenses to digestibility reducers and toxins. Most external mechanical protections are built-in defenses, and large quantitative defenses are almost all constitutive, in part because they are expensive to produce and difficult to mobilize.[12]

Induced defenses include secondary metabolic products, and morphological and physiological changes. Some defenses induced by damage result in greater resistance to herbivores.[13] These defenses are thought to be inducible, rather than constitutive, because an increased defensive variability increases the effectiveness of defense.[13] Research has demonstrated, at least theoretically, that if herbivores can choose among different plants and plant tissues, they may avoid eating plants that have a variety of defenses (i.e. both constitutive and induced), thus providing an evolutionary advantage for defensive variability.[13]

[edit] Indirect defenses

The large thorn-like stipules of Acacia collinsii are hollow and afford shelter for ants, which in return protect the plant against herbivores.
The large thorn-like stipules of Acacia collinsii are hollow and afford shelter for ants, which in return protect the plant against herbivores.

Another category of plant defenses are those features that indirectly protect the plant by enhancing the probability of attracting the natural enemies of herbivores. One such feature are semiochemicals, given off by plants. Semiochemicals are a group of volatile chemicals involved in interactions between organisms. One group of semiochemicals are allelochemics; 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. When a plant is attacked it releases allelochemics containing a different than normal ratio of volatiles.[14] Predators sense these volatiles as food cues, attracting them to the damaged plant, and to feeding herbivores. The subsequent reduction in the number of herbivores confers a fitness benefit to the plant and demonstrates the indirect defensive capabilities of semiochemicals. Induced volatiles also have drawbacks, however; some studies have supported the idea that these volatiles also attract herbivores.[14]

Plants also provide housing and food items for natural enemies of herbivores, known as “biotic” defense mechanisms, as a means to maintain their presence. For example, trees from the genus Macaranga have adapted their thin stem walls to create ideal housing for an ant species (genus Crematogaster), which, in turn, protects the plant from herbivores.[15] In addition to providing housing, the plant also provides the ant with its exclusive food source; from the food bodies produced by the plant. Similarly, some Acacia tree species have developed thorns that are swollen at the base, forming a hollowing structure that acts as housing. Theses Acacia trees also produce nectar in extrafloral nectaries on their leaves as food for the ants.[16]

There have been suggestions that leaf shedding may be a response that provides protection against diseases and certain kinds of pests such as leaf miners and gall forming insects.[17] Other responses such as the change of leaf colours prior to fall have also been suggested as adaptations that may help undermine the camouflage of herbivores.[18]

[edit] Study of defense strategies

It has been known since the late 17th century that plants contain noxious chemicals which are used to protect against herbivory. These chemicals have also been used by man as early insecticides; in 1690 nicotine was extracted from tobacco and used as a contact insecticide. In 1773, insect infested plants were treated with nicotine fumigation by heating tobacco and blowing the smoke over the plants.[19] The important role of secondary plant substances in plant defense against herbivores was described in the late 1950s by Vincent Dethier and G.S. Fraenkel.[20][5]

In the latter half of the 20th century the focus of plant-herbivore research shifted to understanding the phenotypic, genetic, and geographic variations in plant defense. In 1964, Paul R. Ehrlich and Peter H. Raven investigated patterns of interaction between butterflies and their food plants in an attempt to document coevolution between two organisms with a close ecological relationship. Patterns of food plant utilization within the plant families of Araliaceae and Umbelliferae led the two to conclude that secondary plant substances played a leading role in determining the plant’s palatability and use.[6]

Later, the optimal defense hypothesis (OD) was developed to explain the pattern and variation in plant defenses in relation to the plant’s risk of attack, value, and cost of production.[21][22] The OD theory was later broadened to include the newer theories that now serve as a foundation for understanding patterns in plant defense; including the Carbon:Nutrient Balance Hypothesis, the Growth Rate Hypothesis, and the Growth-Differentiation Balance Hypothesis.[23]

[edit] Optimal Defense Hypothesis

The optimal defense hypothesis attempts to explain how the kinds of defenses a particular plant might use, reflect the threats that each individual plant faces.[23] The pattern of OD can be explained by three main factors, namely risk of attack, value of plant part, and cost of defense.[22]

The first determining factor of defining optimal defense is risk: how likely is it that a plant or certain plant parts will be attacked? The related Plant Apparency Hypothesis dictates that a plant will invest heavily in broadly effective defenses when the plant is easily found by herbivores.[21] Examples of apparent plants that impart generalized protections include long-living trees, shrubs, and perennial grasses.[21] Unapparent plants, such as short-lived plants of early successional stages, on the other hand, preferentially invest in small amounts of qualitative toxins that are effective against all but the most specialized herbivores.[21]

The second factor is the value of protection: would the plant be less able to survive and reproduce if it lost of a given quantity of tissue to an herbivore? Not all plant parts are of equal value to the plant, and thus the parts that are the most valuable to the plant would tend to contain a larger proportion of a plant’s defenses. A plant’s stage of development at the time of organ loss also affects the resulting change in fitness. Experimentally, the fitness value of a plant is determined by removing a part of the plant and observing the subsequent fitness effect.[24] In general, reproductive parts are not as easily replaced as vegetative parts, terminal leaves have greater value than basal leaves, and the loss of plant parts mid-season has a greater negative effect on the fitness of the plant than removal at the beginning or end of the season.[25][26]

The final tenet is cost: how much will a particular defensive strategy cost a plant when the amount of energy and materials required are considered? This tenet centers on the fact that defense has a cost and the materials and that the energy spent on defense cannot be used for other functions, such as reproduction and growth. The optimal defense hypothesis predicts that plants will allocate more energy towards defense when the benefits of protection outweigh the costs, specifically in situations where there is high herbivore pressure[27]

[edit] Carbon:Nutrient Balance Hypothesis

The Carbon:Nutrient Balance (CNB) Hypothesis, also known as the Environmental Constraint Hypothesis, attempts to explain that variation in plant defense is based on the availability of nutrients in the environment.[28][29] The CNB hypothesis predicts that plants will use defensive compounds, prepared from the most abundant nutrient available. For example, plants growing in nitrogen-poor soils will use carbon-based defenses (mostly digestibility reducers), while those growing in low carbon environments (such as shady conditions) are more likely to produce nitrogen-based toxins. The hypothesis further predicts that if plants are grown in low-nutrient conditions, then these plants will implement a defensive strategy composed of constitutive carbon-based defenses. If these plants are then exposed to nutrients (e.g. through the addition of fertilizers) the carbon-based defenses will be decreased.

[edit] Growth Rate Hypothesis

The Growth Rate Hypothesis (GR), also known as the Resource Availability Hypothesis, states that defense strategies are determined by the inherent growth rate of the plant, which is in turn determined by the resources available to the plant. A major assumption, made by the GR hypothesis, is that available resources are the limiting factor in determining the maximum growth rate of a plant species, and predicts that the level of defense investment will increase as the potential of growth decreases.[30] Additionally, plants in resource-poor areas, with inherently slow-growth rates, tend to have long-lived leaves and twigs, and the loss of plant appendages may result in a loss of necessary nutrients that are not easily replaced .[31]

In recent support of this hypothesis, Fine and colleagues conducted a reciprocal transplant study of seedlings of 20 species of trees between clay soils (nutrient rich) and white sand (nutrient poor) to to determine whether trade-offs between growth rate and defense against herbivory restricts species to one habitat. Seedlings originating from the nutrient poor white sand had higher levels of constitutive carbon-based defenses, but when they were transplanted into nutrient rich clay soils they experienced higher mortality from herbivory, leading the group to conclude that defensive strategies limit the habitats of some plants.[32]

[edit] Growth-Differentiation Balance Hypothesis

The Growth-Differentiation Balance (GDB) Hypothesis explains that patterns of plant defense are a result of the energy being divided between “growth-related processes” and “differentiation-related processes” in different environments.[33] Differentiation-related processes are defined as “those processes that enhance the structure or function of existing cells (i.e. maturation and specialization).”[23] This hypothesis explains that a plant will produce chemical defenses only when the resources become available, as a result of the net gain of energy from photosynthesis; plants with the highest concentrations of secondary metabolites are the ones with an intermediate level of available resources.[33] Support for this hypothesis was shown by examining the phenolic content in tomatoes when grown at 4 nitrate levels. The highest concentration of phenolics were measured when the tomatoes were grown at an intermediate nitrate level.[34]

[edit] See also

[edit] References

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  2. ^ Williams, K. S., and L. E. Gilbert. 1981. Insects as selective agents on plant vegetative morphology – egg mimicry reduces egg-laying by butterflies. Science 212:467 – 469.
  3. ^ Whittaker, R. H. 1970. The biochemical ecology of higher plants. Chemical ecology edited by Ernest Sondheimer and John B. Simeone Academic Press, Boston pages 43 – 70. ISBN 0-12-654750-5.
  4. ^ Whittaker, R. H. 1975. Communities and ecosystems, 2 edition. Macmillan, New York, USA.
  5. ^ a b Fraenkel G (1959). "The raison d'ĕtre of secondary plant substances; these odd chemicals arose as a means of protecting plants from insects and now guide insects to food". Science 129 (3361): 1466-70. PMID 13658975. 
  6. ^ a b Ehrlich, P. R. and P. H. Raven. 1964. Butterflies and plants: a study of coevolution. Evolution 18:586 - 608.
  7. ^ Caldwell, M. M., R. Robberecht, and S. D. Flint. 1983. Internal filters: prospects for UV-acclimation in higher plants. Physiologia Plantarum 58:445 – 450.
  8. ^ Stapleton A, Walbot V (1994). "PDF Flavonoids can protect maize DNA from the induction of ultraviolet radiation damage". Plant Physiol 105 (3): 881-9. PMID 8058838. 
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  10. ^ Rhoades, D. F. 1979. Evolution of plant chemical defense against herbivores. Pages 1– 55 in G. A. Rosenthal and D. H. Janzen, editors. Herbivores: Their interaction with secondary plant metabolites. Academic Press, New York, USA ISBN 0-12-597184-2.
  11. ^ Van Soest, P. J. 1982. Nutritional ecology of the ruminant. O & B Books, Corvallis, Oregon, USA.
  12. ^ Traw, M. D., and T. E. Dawson. 2002. Differential induction of trichomes by three herbivores of black mustard. Oecologia 131:526 – 532.
  13. ^ a b c Karban, R., A. A. Agrawal, and M. Mangel. 1997. The benefits of induced defenses against herbivores PDF. Ecology 78:1351 - 1355
  14. ^ a b Dicke, M., and J. J. A. van Loon. 2000. Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context. Entomologia Experimentalis et Applicata 97:237 - 249.
  15. ^ Heil, M., B. Fiala, K. E. Linsenmair, G. Zotz, P. Menke, and U. Maschwitz. 1997. Food body production in Macaranga triloba (Euphorbiaceae): A plant investment in anti-herbivore defense via symbiotic ant partners. Journal of Ecology 85:847 – 861. et al. 1997).
  16. ^ Young, T. P., C. H. Stubblefield, and L. A. Isbell. 1997. Ants on swollen-thorn acacias: species coexistence in a simple system. Oecologia 109:98 – 107.
  17. ^ Alan G. Williams, Thomas G. Whitham (1986) Premature Leaf Abscission: An Induced Plant Defense Against Gall Aphids. Ecology. 67(6):1619-1627
  18. ^ Lev-Yadun, S. , Amots Dafni, Moshe A. Flaishman, Moshe Inbar, Ido Izhaki, Gadi Katzir, and Gidi Ne’eman (2004) Plant coloration undermines herbivorous insect camouflage. PDF BioEssays 26:1126–1130
  19. ^ Ware, G., and D. Whitaker, editors. 2004. The pesticide book, 6 edition. Thomson Publications, Fresno, California, USA.
  20. ^ Dethier, V. G. 1954. Evolution of feeding preferences in phytophagous insects. Evolution 8:33 – 54.
  21. ^ a b c d Feeny, P. 1976. Plant apparency and chemical defense. Pages 1 – 40 in J. W. Wallace and R. L. Mansell, editors. Recent advances in phytochemistry, Volume 10. Plenum Press, New York, USA.(1976)
  22. ^ a b Rhoades, D. F., and R. G. Cates. 1976. Towards a general theory of plant antiherbivore chemistry. Pages 168 – 213 in J. W. Wallace and R. L. Mansell, editors. Recent advances in phytochemistry. Plenum Press, New York, USA.
  23. ^ a b c Stamp N (2003). "Out of the quagmire of plant defense hypotheses". Q Rev Biol 78 (1): 23-55. PMID 12661508. 
  24. ^ McKey, D. 1979. The distribution of secondary compounds within plants. Pages 55 – 133 in G. A. Rosenthal and D. H. Janzen, editors. Herbivores: Their interaction with secondary plant metabolites. Academic Press, New York, USA.
  25. ^ Krischik, V. A., and R. F. Denno. 1983. Individual, population, and geographic patterns in plant defense. Pages 463 – 512 in R. F. Denno and M. S. McClure, editors. Variable plants and herbivores in natural and managed systems. Academic Press, New York, USA.
  26. ^ Zangerl, A. R., and C. E. Rutledge. 1996. The probability of attack and patterns of constitutive and induced defense: A test of optimal defense theory. The American Naturalist 147:599 – 608.
  27. ^ Pennings, S. C., E. L. Siska, and M. D. Bertness. 2001. Latitudinal differences in plant palatability in Atlantic coast salt marshes. Ecology 82:1344 – 1359.
  28. ^ Bryant, J. P., F. S. Chapin, III, and D. R. Klein. 1983. Carbon/nutrient balance of boreal plants in relation to vertebrate herbivory. Oikos 40:357–368.
  29. ^ Tuomi, J., P. Niemela, F. S. Chapin, III, J. P. Bryant, and S. Siren. 1988. Defensive responses of trees in relation to their carbon/nutrient balance. Pages 57–72 in W. J. Mattson, J. Levieux, and C. Bernard-Dagan, editors. Mechanisms of woody plant defenses against insects: Search for pattern. Springer, New York, USA.
  30. ^ Coley, P. D., J. P. Bryant, and F. S. Chapin, III. 1985. Resource availability and plant antiherbivore defense. Science 230:895 – 899.
  31. ^ Chapin, III, F. S. 1980. Nutrition of wild plants. Annual Review of Ecological Systematics 11: 233-255.
  32. ^ Fine P, Mesones I, Coley P (2004). "Herbivores promote habitat specialization by trees in Amazonian forests". Science 305 (5684): 663-5. PMID 15286371. 
  33. ^ a b Loomis, W. E. 1953. Growth and differentiation—an introduction and summary. Pages 1–17 in W. E. Loomis, editor. Growth and differentiation in plants. Iowa State College Press, Ames, Iowa, USA.
    Herms, D. A., and W. J. Mattson. 1992. The dilemma of plants: to grow or defend. Quarterly Review of Biology 67:283 – 335.
  34. ^ Wilkens, R. T., J. M. Spoerke, and N. E. Stamp. 1996. Differential Responses of Growth and Two Soluble Phenolics of Tomato to Resource Availability Ecology 77:247–258.

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