Plant defense against herbivory 4189740 220163379 2008-06-18T15:55:53Z Discospinster 82432 Reverted edits by [[Special:Contributions/60.52.96.246|60.52.96.246]] ([[User talk:60.52.96.246|talk]]) to last version by Fratrep <!-- What form of English is the article supposed to be using? --> [[Image:Toxicodendron radicans.jpg|thumb|240px|[[Poison ivy]] produces [[urushiol]] to protect the plant from herbivores. In humans this chemical produces an allergic skin rash, known as urushiol-induced contact dermatitis.]] [[Image:Foxglove2.jpg|thumb|240px|([[Foxglove]]) produce several deadly chemicals, namely [[cardiac glycosides|cardiac]] and [[steroid]]al [[glycoside]]s. Ingestion can cause [[nausea]], [[vomit]]ing, [[hallucination]]s, [[convulsion]]s, or [[death]].]] '''Plant defense against herbivory''' or '''host-plant resistance''' (HPR) includes a range of [[adaptation]]s evolved by [[plants]] that improve their [[fitness (biology)|survival and reproduction]] by reducing the impact of [[herbivore]]s. There are four basic strategies plants use to reduce damage by herbivores. One strategy is to escape or avoid herbivores in time or in place, for example by growing in a location where plants are not easily found or accessed by herbivores or by repelling herbivores chemically (also termed non-preference or antixenosis). Another approach is the plant tolerates herbivores, by diverting the herbivore to eat non-essential parts of the plant, or developing an enhanced ability to recover from the damage caused by herbivory. Some plants encourage the presence of [[predator|natural enemies]] of herbivores, which in turn protect the plant from herbivores. Finally, plants protect themselves by confrontation; the use of chemical or mechanical defenses, such as [[toxins]] that kill herbivores or reduce plant [[digestion|digestibility]] (also called [[Amensalism|antibiosis]]).<ref name=Painter>{{cite book| last=Painter| first=Reginald Henry| year=1951| title=Insect Resistance in Crop Plants| publisher=University of Kansas Press| location=Lawrence|oclc=443998}}</ref> These defenses can either be ''constitutive'', always present in the plant, or ''induced'', produced in reaction to damage or stress caused by herbivores. 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 [[bird]]s and [[mammal]]s. The study of plant defenses against herbivory is important, not only from an evolutionary view point, but also in the direct impact that these defenses have on [[agriculture]], including human and livestock food sources, as well as the in the search for [[medicinal plants|plants of medical importance]]. ==Evolution== ===Plant evolution=== {{main|Evolutionary history of plants}} [[Image:InsectPlantEvol.svg|thumb|Time line of plant evolution and the beginnings of different modes of insect herbivory]] The earliest land plants evolved from aquatic plants around {{Ma|450}} (Ma) in the [[Ordovician]] period. These early land plants had no vascular system and required free water for their reproduction. [[Vascular plant]]s appeared later and their diversification began in the [[Devonian]] era (about 400 Ma). Their reduced dependence on water resulted from adaptations such as protective coatings to reduce evaporation from their tissues. Reproduction and dispersal of vascular plants in these dry conditions was achieved through the evolution of specialized seed structures. The diversification of flowering plants ([[angiosperm]]s) during the [[Cretaceous]] period is associated with the sudden burst of [[speciation]] in insects.<ref name= Ehrlich>{{cite journal|last=Ehrlich| first=Paul R.| coauthors=Peter H. Raven| year=1964 | month=December| title=Butterflies and plants: a study of coevolution.|journal=Evolution| volume=18| issue=4| pages=586–608|doi=10.2307/2406212}}</ref> This diversification of insects represented a major selective force in plant evolution, and led to selection of plants that had defensive adaptations. Early insect herbivores were [[mandibulate]] and bit or chewed vegetation; but the evolution of vascular plants lead to the co-evolution of other forms of herbivory, such as sap-sucking, [[Leaf miner|leaf mining]], [[gall]] forming and nectar-feeding.<ref name=Labandeira>{{cite journal |last=Labandeira|first= C.C.|coauthors= D.L. Dilcher, D.R. Davis, D.L. Wagner |title=Ninety-seven million years of angiosperm-insect association: paleobiological insights into the meaning of coevolution |url=http://www.pnas.org/cgi/reprint/91/25/12278.pdf |journal=Proceedings of the National Academy of Science of the U.S.A. |volume=91 |issue=25 |pages=12278–82 |year=1994 |pmid=11607501 |doi=10.1073/pnas.91.25.12278}}</ref> ===Records of herbivory=== Our understanding of herbivory in geological time comes from three sources: fossilised plants, which may preserve evidence of defence (such as spines), or herbivory-related damage; the observation of plant debris in fossilised [[coprolite|animal faeces]]; and the construction of herbivore mouthparts.<ref name=Labandeira1998>{{cite journal | author = Labandeira, C.C. | year = 1998 | title = Early History Of Arthropod And Vascular Plant Associations 1 | journal = Annual Reviews in Earth and Planetary Sciences | volume = 26 | issue = 1 | pages = 329–377 | doi = 10.1146/annurev.earth.26.1.329 }}</ref> Long thought to be a [[Mesozoic]] phenomenon, evidence for herbivory is found almost as soon as fossils which could show it. Within under 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around {{Ma|{{period start|devonian|-1}}}}, there is evidence that they were being consumed.<ref name=Labandeira2007>{{cite journal | author = Labandeira, C. | year = 2007 | title = The origin of herbivory on land: Initial patterns of plant tissue consumption by arthropods | journal = Insect Science | volume = 14 | issue = 4 | pages = 259–275 | doi = 10.1111/j.1744-7917.2007.00152.x }}</ref> Animals fed on the spores of early Devonian plants, and the [[Rhynie chert]] also provides evidence that organisms fed on plants using a "pierce and suck" technique.<ref name=Labandeira1998/> Many plants of this time are preserved with spine-like enations, which may have performed a defensive role before being co-opted to develop into leaves. 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 evolving, and it being fed upon.<ref name=Labandeira2007/> Hole feeding and skeletonisation are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.<ref name=Labandeira1998/> ===Co-evolution=== [[Image:Plain tiger moat.JPG|right|thumb|200px|A Plain Tiger ''[[Danaus chrysippus]]'' caterpillar making a moat to block defensive chemicals of ''[[Calotropis]]'' before feeding.]] Herbivores depend 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.<ref name= Karban>{{cite journal |last=Karban |first=Richard |coauthors= Anurag A. Agrawal |year=2002 |month=November |title=Herbivore offense |journal=Annual Review of Ecology and Systematics |volume=33 |pages=641–664 |doi= 10.1146/annurev.ecolsys.33.010802.150443}}</ref> Relationships between herbivores and their host plants often results in reciprocal evolutionary change, called [[co-evolution]]. When an herbivore eats a plant it [[evolutionary pressure|selects]] for plants that can mount a defensive response. In cases where this relationship demonstrates ''specificity'' (the evolution of each trait is due to the other), and ''reciprocity'' (both traits must evolve), the species are thought to have co-evolved.<ref name=Futuyma>{{cite book |last=Futuyma|first=Douglas J.|coauthors=Montgomery Slatkin |title=Coevolution |publisher=Sinauer Associates |location=Sunderland, Massachusetts |year=1983 |isbn=0-87893-228-3}}</ref> 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]],<ref name= Ehrlich/><ref name= Thompson>{{cite book |last=Thompson |first=J. |editor= H. Olff, V. K. Brown, R. H. Drent |title=Herbivores: between plants and predators; the 38th symposium of the British Ecological Society in cooperation with the Netherlands Ecological Society held at the Wageningen Agricultural University, The Netherlands, 1997 |publisher=Blackwell Science |location=Oxford |year=1999 |pages=7–30 |chapter=What we know and do not know about coevolution: insect herbivores and plants as a test case.|isbn=0-632-05155-8 |oclc= |doi=}}</ref> and have played a role in the radiation of insect species during the age of [[angiosperm]]s.<ref>{{cite journal |last=Farrell |first=Brian D. |coauthors=Charles Mitter |year=1994 |title=Adaptive Radiation in Insects and Plants: Time and Opportunity |journal=American Zoologist |volume=34 |issue=1 |pages=57–69 |doi=10.1093/icb/34.1.57}}</ref> Some herbivores have evolved ways to hijack plant defenses to their own benefit, by [[sequester]]ing these chemicals and using them to protect themselves from predators.<ref name= Ehrlich/> ==Types== Plant defenses can be classified generally as induced or constitutive. Constitutive defenses are always present in the plant species, while induced defenses are synthesized or mobilized to the site where a plant is injured. There are wide variations in the composition and concentration of constitutive defenses and these range from mechanical defenses to digestibility reducers and toxins. Most external mechanical defenses and large quantitative defenses are constitutive, as they require large amounts of resources to produce and difficult to mobilize.<ref>{{cite journal |last=Traw |first=Brian M. |coauthors=Todd E. Dawson |year=2002 |month=May |title=Differential induction of trichomes by three herbivores of black mustard |journal=[[Oecologia]] |volume=131 |issue=4 |pages=526–532 |doi=10.1007/s00442-002-0924-6| url=http://www.pitt.edu/~biohome/Dept/pdf/1723.pdf| accessdate=2007-05-27}}</ref> Induced defenses include secondary metabolic products, as well as morphological and physiological changes.<ref name=Karban97>{{cite journal |last=Karban |first=Richard |coauthors=Anurag A. Agrawal, Marc Mangel |year=1997 |month=July |title=The benefits of induced defenses against herbivores |journal=Ecology |volume=78 |issue=5 |pages=1351–1355 |doi=10.2307/2266130| url=http://www.botany.utoronto.ca/ResearchLabs/AgrawalLab/publications/papers/karban-et-al-97.pdf| accessdate=2007-05-27}}</ref> An advantage of inducible, rather than constitutive defenses, is that increased variability increases the effectiveness of the defenses.<ref name=Karban97 /> This advantage comes from the suggestion that if herbivores can choose among different plants and plant tissues, they may avoid eating plants that have both constitutive and induced defenses.<ref name=Karban/> ===Chemical defenses=== [[Image:Diospyros kaki fruit.jpg|thumb|210px|[[Persimmon]], genus ''[[Diospyros]]'', has a high [[tannin]] content which gives immature fruit, seen above, an [[astringent]] and [[bitter]] [[flavor]].]] 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 metabolite]]s, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms,<ref name= Fraenkel>{{cite journal |last=Fraenkel|first= G. |title=The raison d'être of secondary plant substances|journal=Science |volume=129 |issue=3361 |pages=1466–70 |year=1959 |pmid= 13658975| doi=10.1126/science.129.3361.1466}}</ref> and often produced as by-products during the synthesis of primary metabolic products.<ref name="Whittaker1970">{{cite book |last=Whittaker|first= Robert H.| editor= Ernest Sondheimer and John B. Simeone |title=Chemical ecology |publisher=Academic Press |location=Boston |year=1970 |pages=43–70 | chapter= The biochemical ecology of higher plants | isbn=0-12-654750-5}}</ref> These secondary metabolites play a major role in defenses against herbivores.<ref>{{cite book |first=Robert H.|last= Whittaker |title=Communities and ecosystems |publisher=Macmillan |location=New York |year=1975 |pages= |isbn=0-02-427390-2}}</ref><ref name= Fraenkel/><ref name= Ehrlich/> Secondary metabolites are often characterized as either [[qualitative]] or [[quantitative]]. Qualitative metabolites are defined as [[toxins]] that interfere with an herbivore’s metabolism, often by blocking specific biochemical reactions. Qualitative chemicals 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 [[generalist and specialist species|specialists and generalist]] herbivores. Quantitative chemicals are those that are present in high concentration in plants (5 – 40% dry weight) and are equally effective against all specialists and generalist herbivores. Most quantitative metabolites are digestibility reducers that make plant [[cell wall]]s indigestible to animals. The effects of quantitative metabolites are dosage dependent and the higher these chemicals’ proportion in the herbivore’s diet, the less nutrition the herbivore can gain from ingesting plant tissues. Because they are typically large molecules, these defenses are energetically expensive to produce and maintain, and often take longer than smaller, qualitative chemicals to synthesize and transport, therefore these chemicals are expected to serve an important purpose within the plant.<ref name="theis">{{cite journal |last=Theis |first=Nina |coauthors=Manuel Lerdau |year=2003 |title=The evolution of function in plant secondary metabolites |journal=International Journal of Plant Science |volume=164 |issue=3 Suppl. |pages=S93–S102 |url=http://life.bio.sunysb.edu/ee/mlerdau/Papers/Theis2003.pdf| accessdate=2007-05-27 |doi=10.1086/374190}}</ref> ====Types of chemical defenses==== Plants have developed many secondary metabolites involved in plant defense, which are collectively known as antiherbivory compounds and can be classified into three sub-groups: [[nitrogen]] compounds (including ''alkaloids'', ''cyanogenic glycosides'' and ''glucosinolates''), ''terpenoids'', and ''phenolics''.<ref>{{cite web |url=http://www2.mcdaniel.edu/Biology/botf99/herbnew/aprodbc.htm |title=Biochemical defenses: secondary metabolites: |accessdate=2007-05-21 |work=Plant Defense Systems & Medicinal Botany}}</ref> [[Alkaloid]]s are derived from various [[amino acid]]s. Over 3000 known alkaloids exist, examples include [[nicotine]], [[caffeine]], [[morphine]], [[colchicine]], [[ergoline]]s, [[strychnine]], and [[quinine]].<ref>{{cite web |url=http://www2.mcdaniel.edu/Biology/botf99/herbnew/alkaloids.htm|title=Alkaloids: contain a N-containing heterocycle |accessdate=2007-06-26 |work=Plant Defense Systems & Medicinal Botany}}</ref> Alkaloids have [[pharmacological]] effects on humans and other animals. Some alkaloids can inhibit or activate [[enzyme]]s, or alter [[carbohydrate]] and fat [[storage]] by inhibiting the formation [[phosphodiester]] bonds involved in their breakdown.<ref name= Roberts>{{cite book |first= Margaret F. |last=Roberts |coauthors= Michael Wink |title=Alkaloids: biochemistry, ecology, and medicinal applications |url= http://books.google.com/books?id=bMCzyrAtrvYC&dq|publisher=Plenum Press |location=New York |year=1998 |isbn=0-306-45465-3}}</ref> Certain alkaloids bind to [[nucleic acid]]s and can inhibit synthesis of proteins and affect [[DNA repair]] mechanisms. Alkaloids can also affect [[cell membrane]] and [[cytoskeletal]] structure causing the cells to weaken, collapse, or leak, and can affect [[nerve]] transmission.<ref>{{cite web |first= Albert T.| last= Sneden |url=http://www.people.vcu.edu/~asneden/alkaloids.htm |title=Alkaloids |accessdate=2007-05-21 |format= |work=Natural Products as Medicinally Useful Agents}}</ref> [[Cyanogenic glycoside]]s become toxic when they are broken down by enzymes in the herbivore's [[digestive tract]] and release [[hydrogen cyanide]] or [[prussic acid]], which blocks cellular respiration. Glucosinolates can cause [[gastroenteritis]], salivation, diarrhea, and irritation of the mouth.<ref>{{cite book |last=Rhoades|first= D. F. |editor= Gerald A. Rosenthal, Daniel H. Janzen |title=Herbivores, their interaction with secondary plant metabolites |publisher=Academic Press |location=Boston |year=1979 |pages= 1–55|chapter= Evolution of plant chemical defense against herbivores|isbn=0-12-597180-X}}</ref> The [[terpenoids]], sometimes referred to as isoprenoids, are organic chemicals similar to [[terpene]]s, derived from five-carbon [[isoprene]] units. There are over 10,000 known types of terpenoids.<ref>{{cite web |url=http://www2.mcdaniel.edu/Biology/botf99/herbnew/aterpenes.htm|title=Terpenoids |accessdate=2007-06-26 |work=Plant Defense Systems & Medicinal Botany}}</ref> Most are multicyclic structures which differ from one another in both functional groups, and in basic carbon skeletons.<ref name=Terp>{{cite book |last=Gershezon |first=Jonathan |coauthors= Wolfgang Kreis |editor= Michael Wink |title=Biochemistry of plant secondary metabolism |publisher=Sheffield Academic Press |location=London |year=1999 |pages=222-279 |isbn=0-8493-4085-3 |chapter=Biochemistry of terpinoids}}</ref> Monoterpenoids, continuing 2 isoprene units, are [[volatile]] [[essential oil]]s such as [[citronella]], [[limonene]], [[menthol]], [[camphor]], and [[pinene]]. Diterpenoids, 4 isoprene units, are widely distributed in [[latex]] and [[resins]], and can be quite toxic. Diterpenes are responsible for making [[Rhododendron]] leaves poisonous. Plant [[steroid]]s and [[sterol]]s are also produced from terpenoid precursors, including [[vitamin D]], [[glycosides]] (such as [[digitalis]]) and [[saponin]]s (which lyse [[red blood cell]]s of herbivores).<ref>{{cite web|first= Albert T.|last= Sneden |url=http://www.people.vcu.edu/~asneden/terpenes.htm |title=Terpenes |accessdate=2007-05-21 |format= |work=Natural Products as Medicinally Useful Agents}}</ref> [[phenols|Phenolic]]s, sometimes called phenols, consist of an [[aromatic]] 6-carbon ring bonded to a [[hydroxy]] group. Some phenols have [[antiseptic]] properties, while others disrupt [[endocrine]] activity. Phenolics range from simple [[tannins]] to the more complex [[flavonoids]] that give plants much of their red, blue, yellow, and white pigments. Complex phenolics called [[polyphenols]] are capable of producing many different types of effects on humans, including [[antioxidant]] properties. Some examples of phenolics used for defense in plants are: [[lignin]], [[silymarin]] and [[cannabinoid]]s.<ref name= Phenols>{{cite web |url=http://www2.mcdaniel.edu/Biology/botf99/herbnew/aphenols.htm |title=Phenols |accessdate=2007-05-21 |format= |work=Plant Defense Systems & Medicinal Botany}}</ref> [[Proanthocyanidin|Condensed tannin]]s, 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 enzyme]]s.<ref>{{cite book |first=Peter J.|last= Van Soest |title=Nutritional ecology of the ruminant: ruminant metabolism, nutritional strategies, the cellulolytic fermentation, and the chemistry of forages and plant fibers |publisher=O & B Books |location=Corvallis, Oregon |year=1982 |isbn=0-9601586-0-X}}</ref> [[Silica]] and [[lignin]]s, which are completely indigestible to animals, grind down insect [[mandible]]s (appendages necessary for feeding). In addition to the three larger groups of substances mentioned above, [[fatty acid]] derivates, [[amino acid]]s and even [[peptide]]s<ref>{{cite journal | author = John W. Hylin | title = Toxic peptides and amino acids in foods and feeds | year = 1969 | journal = Journal of Agricultural and Food Chemistry | volume = 17 | issue = 3 | pages = 492–496 | doi = 10.1021/jf60163a003}}</ref> are used as defence. The [[cholinergic|cholinergic toxine]], [[cicutoxin]] of [[water hemlock]], is an [[polyyne]] derived from the fatty acid metabolism.<ref>{{cite journal | author = E. Anet, B. Lythgoe, M. H. Silk, S. Trippett | title = Oenanthotoxin and cicutoxin. Isolation and structures | journal = [[Journal of the Chemical Society]] | year = 1953 | volume = | issue = | pages = 309–322 | doi = 10.1039/JR9530000309 }}</ref> β-N-Oxalyl-L-α,β-diaminopropionic acid as simple amino acid is used by the [[sweet pea]] which leads also to intoxication in humans.<ref>{{cite journal | author = Mark V. Barrow; Charles F. Simpson; Edward J. Miller | title = Lathyrism: A Review | year = 1974 | journal = The Quarterly Review of Biology | volume = 49 | issue = 2 | pages = 101–128 | url= http://links.jstor.org/sici?sici=0033-5770%28197406%2949%3A2%3C101%3ALAR%3E2.0.CO%3B2-%232 | doi = 10.1086/408017 }}</ref> The synthesis of [[fluoroacetate]] in several plants is an example for the use of small molecules to disturb the metabolism of the herbivore, in this case the [[citric acid cycle]].<ref>{{cite journal | author = Donald A. Levin | title = The Impact of Fluoroacetate-Bearing Vegetation on Native Australian Fauna: A Review | year = 1991 | journal = Oikos | volume = 61 | issue = 3 | pages = 412–430 | url = http://links.jstor.org/sici?sici=0030-1299%28199109%2961%3A3%3C412%3ATIOFVO%3E2.0.CO%3B2-2 | doi = 10.2307/3545249}}</ref> ===Mechanical defenses=== [[Image:Raspberry cane - closeup in winter - P.2005.03.jpg|thumb|right|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 [[Plant stem|stem]]s 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]], [[lignin]]s, [[silica]], and wax cover the [[Epidermis (botany)|epidermis]] of [[terrestrial plant]]s and alter the texture of the plant tissue. The leaves of [[holly]] plants, for instance, are very smooth and slippery making feeding difficult. Some plants produce [[gummosis]] or sap that traps insects. A plant's leaves and stem may be covered with sharp spines or [[trichomes]]- hairs on the [[leaf]] often with barbs, sometimes containing [[irritants]] or [[poisons]]. Plant structural features like spines and thorns reduce feeding by large ungulate herbivores (e.g. [[kudu]], [[impala]], and [[Capra (genus)|goats]]) by restricting the herbivores' feeding rate, or by wearing down the molars as in [[pear]]s.<ref>{{cite journal |last=Cooper |first=Susan M. |coauthors=Norman Owen-Smith |year=1986 |month=September |title=Effects of plant spinescence on large mammalian herbivores |journal=Oecologia |volume=68 |issue=3 |pages=446–455 |doi=10.1007/BF01036753}}</ref> The structure of a plant, its branching and leaf arrangement may also be evolved to reduce herbivore impact. The shrubs of New Zealand have evolved special wide branching adaptations believed to be a response to browsing birds such as the [[moa]]s.<ref>{{cite journal|author=Bond W, Lee W & Craine J|year=2004|title=Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas.|journal=Oikos|volume=104|issue=3|pages=500–508|doi=10.1111/j.0030-1299.2004.12720.x}}</ref> Similarly, [[Acacia]]s have dense thorns on the outside, but none in the middle of the crown, which is safe from herbivores such as [[giraffe]]s.<ref name=Attenborough>[[Attenborough, D.]] (1995) ''[[The Private Life of Plants]]'' BBC.</ref> ====Thigmonasty==== {{Further|[[Thigmonasty]]}} [[Thigmonasty|Thigmonastic movements]], those that occur in response to touch, are used as a defense in some plants. The leaves of the [[sensitive plant]], ''Mimosa pudica'', close up rapidly in response to direct touch, vibration, or even electrical and thermal stimuli. The [[proximate cause]] of this mechanical response is an abrupt change in the [[turgor]] pressure in the [[pulvini]] at the base of leaves resulting from [[osmosis|osmotic]] phenomena. This is then spread via both electrical and chemical means through the plant; only a single leaflet need be disturbed.<ref name="Raven 2005" /> This response lowers the surface area available to herbivores, which are presented with the underside of each leaflet. It may also physically dislodge small herbivores, such as insects.<ref name=Attenborough /> Thigmonasty is not only useful in discouraging herbivores, however. For instance the [[venus flytrap]] makes use of it to catch its own food. ===Mimicry and camouflage=== Some plants [[mimic]] the presence of insect eggs on their leaves, dissuading insect species from laying their eggs there. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some species of [[neotropical]] [[vine]]s of the [[genus]] ''[[Passiflora]]'' (Passion flowers) contain physical structures resembling the yellow eggs of ''[[Heliconius]]'' [[butterflies]] on their leaves, which discourage [[oviposition]] by butterflies.<ref>{{cite journal |last=Williams |first=Kathy S. |coauthors=Lawrence E. Gilbert |year=1981 |month=April |title=Insects as selective agents on plant vegetative morphology: egg mimicry reduces egg-laying by butterflies |journal=Science |volume=212 |issue=4493 |pages=467–469 |doi=10.1126/science.212.4493.467 |pmid=17802547}}</ref> ===Indirect defenses=== ====Mutualists==== [[Image:Acacia-collinsii.jpg|thumb|right|200px|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 enemy|natural enemies]] of herbivores. Such an arrangement is known as [[mutualism]], in this case of the "[[the enemy of my enemy is my friend|enemy of my enemy]]" variety. One such feature are [[semiochemical]]s, given off by plants. Semiochemicals are a group of [[volatile organic compounds]] involved in interactions between organisms. One group of semiochemicals are [[allelochemics]]; consisting of [[allomone]]s, which play a defensive role in [[interspecies communication]], and [[kairomone]]s, which are used by members of higher [[trophic]] levels to locate food sources. When a plant is attacked it releases allelochemics containing an abnormal ratio of volatiles.<ref name= Dicke>{{cite journal |last=Dicke |first=Marcel |coauthors=Joop J.A. van Loon |year=2000 |month=December |title=Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context |journal=Entomologia Experimentalis et Applicata |volume97= |issue=3 |pages=237–249 |doi=10.1046/j.1570-7458.2000.00736.x |volume=97}}</ref> 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 (biology)|fitness]] benefit to the plant and demonstrates the indirect defensive capabilities of semiochemicals. Induced volatiles also have drawbacks, however; some studies have suggested that these volatiles also attract herbivores.<ref name= Dicke/> 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.<ref name= Heil>{{cite journal |last=Heil |first=Martin |coauthors=Brigitte Fiala, K. Eduard Linsenmair, Gerhard Zotz, Petra Menke |year=1997 |month=December |title=Food body production in Macaranga triloba (Euphorbiaceae): A plant investment in anti-herbivore defense via symbiotic ant partners |journal=Journal of Ecology |volume=85 |issue=6 |pages=847–861 |doi=10.2307/2960606}}</ref> 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.<ref>{{cite journal |last=Young |first=T. P. |coauthors=Cynthia H. Stubblefield, Lynne A. Isbell |year=1997 |month=January |title=Ants on swollen-thorn acacias: species coexistence in a simple system |journal=Oecologia |volume=109 |issue=1 |pages=98–107 |doi=10.1007/s004420050063}}</ref> Most plants have [[endophyte]]s, microbial organisms that live within them. While some cause disease, others protect plants from herbivores and [[pathogen]]ic microbes. Endophytes can help the plant by producing toxins harmful to other organisms that would attack the plant, such as alkaloid producing [[fungi]] which are common in grasses such as [[tall fescue]] (''Festuca arundinacea'').<ref name="Raven 2005">{{cite book| last=Raven| first=Peter H.| coauthors=Ray F. Evert, & Susan E. Eichhorn| year=2005| title=Biology of Plants| publisher=W. H. Freeman and Company| location=New York|isbn=0-7167-1007-2}}</ref> ===Leaf shedding and colour=== There have been suggestions that [[abscission|leaf shedding]] may be a response that provides protection against diseases and certain kinds of pests such as [[leaf miner]]s and [[gall]] forming insects.<ref>{{cite journal |last=Williams |first=Alan G. |coauthors=Thomas G. Whitham |year=1986 |month=December |title=Premature Leaf Abscission: An Induced Plant Defense Against Gall Aphids |journal=Ecology |volume=67 |issue=6 |pages=1619–1627 |doi=10.2307/1939093}}</ref> 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.<ref>{{cite journal |last=Lev-Yadun |first=Simcha |coauthors=Amots Dafni, Moshe A. Flaishman, Moshe Inbar, Ido Izhaki, Gadi Katzir, Gidi Ne'eman |year=2004 |month=October |title=Plant coloration undermines herbivorous insect camouflage |journal=BioEssays |volume=26 |issue=10 |pages=1126–1130 |doi=10.1002/bies.20112| url=http://harvardforest.fas.harvard.edu/research/leaves/pdf/bIOeSSAYS%20PLANT%20COLOR.pdf| accessdate=2007-05-27}}</ref> [[Autumn leaf color]] has also been suggested to act as an [[Handicap principle|honest warning signal]] of defensive commitment towards insect pests that migrate to the trees in autumn.<ref>{{cite journal|last=Archetti, M.|year=2000 |url=http://users.ox.ac.uk/~zool0643/papers/JTB_2000_autumn.pdf|title=The origin of autumn colours by coevolution.|journal=J. Theor. Biol. |volume=205|issue=4|pages=625–630|doi=10.1006/jtbi.2000.2089}}</ref><ref>{{cite journal|author=Hamilton, W. D.|coauthors=Brown, S. P.|year=2001|url=http://cluster3.biosci.utexas.edu/research/meyers/SamB/HamiltonBrown01.pdf|title=Autumn tree colours as a handicap signal.|journal=Proc. R. Soc. B|volume=268|issue=1475|pages=1489–1493|doi=10.1098/rspb.2001.1672}}</ref> ==Costs and benefits== Defensive structures and chemicals are costly as they require resources that could otherwise be used by plants to maximize growth and reproduction. Many models have been proposed to explore how and why some plants make this investment in defenses against herbivores. ===Optimal defense hypothesis=== The optimal defense hypothesis attempts to explain how the kinds of defenses a particular plant might use reflect the threats each individual plant faces.<ref name= Stamp>{{cite journal |last=Stamp|first= Nancy |title=Out of the quagmire of plant defense hypotheses |journal=Quarterly Review of Biology |volume=78 |issue=1 |pages=23–55 |year=2003|month=March |pmid= 12661508 |doi=10.1086/367580}}</ref> This model considers three main factors, namely: risk of attack, value of the plant part, and the cost of defense.<ref name= Rhoades>{{cite book |last=Rhoades|first= D. F.|coauthor= R. G. Cates.| editor=V. C. Runeckles and E. E. Conn |title=Recent advances in phytochemistry: proceedings of the annual meeting of the Phytochemical society of North America |publisher=Academic Press |location=Boston |year=1974 |pages=168–213|chapter= Towards a general theory of plant antiherbivore chemistry|isbn=0-12-612408-6}}</ref><ref>{{cite journal |last=Wilf |first=Peter |coauthors=Conrad C. Labandeira, Kirk R. Johnson, Phyllis D. Coley, and Asher D. Cutter |year=2001 |title=Insect herbivory, plant defense, and early Cenozoic climate change |journal=Proceedings of the National Academy of Science |volume=98 |issue=11 |pages=6221–6226 |url=http://www.pnas.org/cgi/reprint/111069498v1.pdf| accessdate=2007-05-27 |doi=10.1073/pnas.111069498 |pmid=11353840}}</ref> The first factor determining optimal defense is risk: how likely is it that a plant or certain plant parts will be attacked? This is also related to the ''plant apparency hypothesis'', which states that a plant will invest heavily in broadly effective defenses when the plant is easily found by herbivores.<ref name= Feeny>{{cite book |last=Feeny|first= P.|editor=James W. Wallace and Richard L. Mansell |title=Biochemical interaction between plants and insects: proceedings of the fifteenth annual meeting of the Phytochemical Society of North America|publisher=Plenum Press |location=New York |year=1976 |pages=1–40 |chapter=Plant apparency and chemical defense. |isbn=0-306-34710-5}}</ref> Examples of apparent plants that produce generalized protections include long-living [[tree]]s, [[shrub]]s, and [[Perennial plant|perennial]] [[grasses]].<ref name= Feeny/> Unapparent plants, such as short-lived plants of early [[Ecological succession|successional]] stages, on the other hand, preferentially invest in small amounts of qualitative toxins that are effective against all but the most specialized herbivores.<ref name= Feeny/> The second factor is the value of protection: would the plant be less able to survive and reproduce after removal of part of its structure by a herbivore? Not all plant parts are of equal evolutionary value, thus valuable parts contain more defenses. A plant’s stage of development at the time of feeding also affects the resulting change in fitness. Experimentally, the fitness value a plant structure is determined by removing that part of the plant and observing the effect.<ref name=McKey1>{{cite book |first=McKey|last= D. |editor= Gerald A. Rosenthal, Daniel H. Janzen|title=Herbivores, their interaction with secondary plant metabolites |publisher=Academic Press |location=Boston |year=1979 |pages=55–133|chapter= The distribution of secondary compounds within plants. |isbn=0-12-597180-X}}</ref> In general, [[Biological reproduction|reproductive]] parts are not as easily replaced as [[vegetative]] parts, terminal leaves have greater value than [[anatomical terms of location|basal]] leaves, and the loss of plant parts mid-season has a greater negative effect on fitness than removal at the beginning or end of the season.<ref>{{cite book |last= Krischik|first= V. A.|coauthors= R. F. Denno.|editor= Robert F. Denno, Mark S. McClure |title=Variable plants and herbivores in natural and managed systems |publisher=Academic Press |location=Boston |year=1983 |pages=463–512|chapter= Individual, population, and geographic patterns in plant defense. |isbn=0-12-209160-4}}</ref><ref>{{cite journal |last=Zangerl |first=Arthur R. |coauthors=Claire E. Rutledge |year=1996 |month=April |title=The probability of attack and patterns of constitutive and induced defense: A test of optimal defense theory |journal=The American Naturalist |volume=147 |issue=4 |pages=599–608 |url=http://links.jstor.org/sici?sici=0003-0147(199604)147%3A4%3C599%3ATPOAAP%3E2.0.CO%3B2-D| accessdate=2007-05-27 |doi=10.1086/285868}}</ref> Seeds in particular tend to be very well protected. For example, the seeds of many edible fruits and nuts contain cyanogenic glycosides such as amygdalin. This results from the need to balance the effort needed to make the fruit attractive to animal dispersers while ensuring that the seeds are not destroyed by the animal.<ref>{{cite journal |last=Swain|first= Elisabeth|coauthors= Chun Ping Li, Jonathan E. Poulton |title=Development of the Potential for Cyanogenesis in Maturing Black Cherry (''Prunus serotina'' Ehrh.) Fruits |url= http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16668810|journal=Plant Physiology |volume=98 |issue=4 |pages=1423–1428 |year=1992 |pmid=16668810}}</ref><ref>{{cite journal |last=Witmer|first= M.C. |title=Ecological and evolutionary implications of energy and protein requirements of avian frugivores eating sugary diets |journal=Physiological Zoology |volume=71 |issue=6 |pages=599–610 |year=1998 |pmid=9798248}}</ref> The final consideration is cost: how much will a particular defensive strategy cost a plant in energy and materials? This is particularly important, as 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.<ref>{{cite journal |last=Pennings |first=Steven C. |coauthors=Erin L. Siska, Mark D. Bertness |year=2001 |month=May |title=Latitudinal differences in plant palatability in Atlantic coast salt marshes |journal=Ecology |volume=82 |issue=5 |pages=1344–1359 |doi=10.2307/2679994}}</ref> ===Carbon:nutrient balance hypothesis=== The carbon:nutrient balance hypothesis, also known as the ''environmental constraint hypothesis'', states that the various types of plant defenses are responses to variations in the levels of [[nutrient]]s in the environment.<ref name=Bryant>{{cite journal |last=Bryant |first=John P. |coauthors=Stuart Chapin, III, David R. Klein |year=1983 |month=May |title=Carbon/nutrient balance of boreal plants in relation to vertebrate herbivory |journal=Oikos |volume=40 |issue=3 |pages=357–368 |doi=10.2307/3544308}}</ref><ref>{{cite book |last=Tuomi|first= J.|coauthrs= P. Niemela, F. S. Chapin, III, J. P. Bryant, and S. Siren. |editor= William J. Mattson, Jean Levieux, C. Bernard-Dagan|title=Mechanisms of woody plant defenses against insects: search for pattern |publisher=Springer-Verlag |location=Berlin |year=1988 |pages=57–72 |chapter=Defensive responses of trees in relation to their carbon/nutrient balance. |isbn=0-387-96673-0 }}</ref> This hypothesis predicts that plants will use defensive compounds constructed from the most abundant nutrient available. For example, plants growing in [[nitrogen]]-poor [[soil]]s 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 plants can change their defences in response to changes in nutrients. For example, if plants are grown in low-nitrogen conditions, then these plants will implement a defensive strategy composed of constitutive carbon-based defenses. If nutrient levels subsequently increase, by for example the addition of [[fertilizer]]s, these carbon-based defenses will decrease. ===Growth rate hypothesis=== The growth rate hypothesis, 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 is that available resources are the [[limiting factor]] in determining the maximum growth rate of a plant species. This model predicts that the level of defense investment will increase as the potential of growth decreases.<ref name= Coley>{{cite journal |last=Colley |first=Phyllis D. |coauthors=John P. Bryant, and F. Stuart Chapin III |year=1985 |title=Resource availability and plant antiherbivore defense |journal=Science |volume=230 |issue=4728 |pages=895–899 |doi= 10.1126/science.230.4728.895 |pmid=17739203}}</ref> 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 scarce and valuable nutrients.<ref>{{cite journal |last=Chapin |first=F. Stuart, III |year=1980 |month= |title=The Mineral Nutrition of Wild Plants |journal=Annual Review of Ecological Systematics |volume=11 |pages=233–260 |url=http://links.jstor.org/sici?sici=0066-4162(1980)11%3C233%3ATMNOWP%3E2.0.CO%3B2-P| accessdate=2007-05-27|doi=10.1146/annurev.es.11.110180.001313}}</ref> A recent test of this model involved a reciprocal transplants of seedlings of 20 species of trees between [[clay]] [[soil]]s (nutrient rich) and white [[sand]] (nutrient poor) to determine whether trade-offs between growth rate and defenses restrict species to one habitat. Seedlings originating from the nutrient-poor 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. These finding suggest that defensive strategies limit the habitats of some plants.<ref>{{cite journal |last=Fine|first= Paul V. A.| coauthors= Italo Mesones, Phyllis D. Coley |title=Herbivores promote habitat specialization by trees in Amazonian forests |journal=Science |volume=305 |issue=5684 |pages=663–5| month=July |year=2004 |pmid= 15286371| doi=10.1126/science.1098982}}</ref> ===Growth-differentiation balance hypothesis=== The growth-differentiation balance hypothesis states that plant defenses are a result of the energy being divided between “growth-related processes” and “differentiation-related processes” in different environments.<ref name= Loomis>{{cite book |last=Loomis|first= W. E.| editor=P. F. Wareing and I. D. J. Phillips |title=Growth and differentiation in plants |publisher=Pergamon Press |location=New York |year=1981 |pages=1–17 |chapter= Growth and differentiation—an introduction and summary. |isbn=0-08-026351-8}}<br/>{{cite journal |last=Herms |first=Daniel A. |coauthors=William J. Mattson |year=1992 |month=September |title=The dilemma of plants: to grow or defend |journal=Quarterly Review of Biology |volume=67 |issue=3 |pages=283–335 |url=http://links.jstor.org/sici?sici=0033-5770(199209)67%3A3%3C283%3ATDOPTG%3E2.0.CO%3B2-A| accessdate=2007-05-27 |doi=10.1086/417659}}</ref> [[Cellular differentiation|Differentiation]]-related processes are defined as “processes that enhance the structure or function of existing cells (i.e. maturation and specialization).”<ref name= Stamp/> A plant will produce chemical defenses only when energy is available from [[photosynthesis]] and plants with the highest concentrations of secondary metabolites are the ones with an intermediate level of available resources.<ref name= Loomis/> Support for this hypothesis came from studies of the [[phenolic]] content in [[tomatoes]] when grown at four different nitrate levels. The highest concentration of phenolics were measured when the tomatoes were grown at an intermediate nitrate level.<ref>{{cite journal |last=Wilkens |first=Richard T. |coauthors=Jill M. Spoerke, Nancy E. Stamp |year=1996 |month=January |title=Differential Responses of Growth and Two Soluble Phenolics of Tomato to Resource Availability |journal=Ecology |volume=77 |issue=1 |pages=247–258 |doi=10.2307/2265674}}</ref> == Importance to humans == === Agriculture === The variation of plant susceptibility to pests was probably known even in the early stages of agriculture in humans. In historic times, the observation of such variations in susceptibility have provided solutions for major [[socio-economic]] problems. The [[phylloxera|grape phylloxera]] was introduced from North America to [[France]] in 1860 and in 25 years it destroyed nearly a third (100,000 km²) of the French [[grape]] yards. [[Charles Valentine Riley]] noted that the American species ''Vitis labrusca'' was resistant to ''Phylloxera''. Riley, with J. E. Planchon, helped save the [[French wine]] industry by suggesting the [[grafting]] of the susceptible but high quality grapes onto ''Vitis labrusca'' root stocks.<ref name= Rutgers>{{cite web |first= Sridhar| last= Polavarapu |url=http://aesop.rutgers.edu/~hamilton/lecture12.htm |title=Plant Resistance to insects| accessdate=2007-05-16 |date= 2001 |work=Agricultural Entomology & Pest Management |publisher= [[Rutgers University]]}}</ref> The formal study of plant resistance to herbivory was first covered extensively in 1951 by Reginald (R.H.) Painter, who is widely regarded as the founder of this area of research, in his book ''Plant Resistance to Insects''.<ref name=Painter/> While this work pioneered further research in the US, the work of Chesnokov was the basis of further research in the USSR.<ref>{{cite book |last=Chesnokov |first=Pavel G. |title=Methods of Investigating Plant Resistance to Pests |year=1953 |publisher=Israel Program for Scientific Translations |location=Jerusalem |oclc=3576157}}</ref> Fresh growth of grass is sometimes high in [[prussic acid]] content and can cause poisoning of grazing [[livestock]]. The production of [[cyanogenic]] chemicals in grasses is primarily a defense against herbivores.<ref>{{cite journal |last=Gleadow|first= Roslyn M.|coauthors= Ian E. Woodrow |title=Constraints on effectiveness of cyanogenic glycosides in herbivore defense |journal=Journal of Chemical Ecology |volume=28 |issue=7 |pages=1301–13 |year=2002 |pmid=12199497 |doi=10.1023/A:1016298100201}}</ref><ref>{{cite web| last=Vough|first= Lester R.|coauthors= E. Kim Cassel| year=2002| month=July|url=http://agbiopubs.sdstate.edu/articles/ExEx4016.pdf| title= Prussic Acid Poisoning of Livestock: Causes and Prevention (ExEx 4016)| work= Extension Extra | publisher=South Dakota State University Extension Service}}</ref> The human innovation of cooking may have been particularly helpful in overcoming many of the defensive chemicals of plants. Many [[enzyme inhibitor]]s in [[cereal grain]]s and [[Pulse (legume)|pulse]]s, such as [[trypsin]] inhibitors prevalent in pulse crops, are denatured by cooking, making them digestible.<ref>{{cite journal |last=Grant|coauthors=Linda J. More, Norma H. McKenzie, Arpad Pusztai |title=The effect of heating on the haemagglutinating activity and nutritional properties of bean (Phaseolus vulgaris) seeds |journal= Journal of the Science of Food and Agriculture |volume=33 |issue=12 |pages=1324–6 |year=1982 |pmid=7166934 |doi=10.1002/jsfa.2740331220 |unused_data=|first George}}</ref><ref>{{cite web |author=Tu|last= Jean-Louis |url= http://www.beyondveg.com/tu-j-l/raw-cooked/raw-cooked-1g.shtml |title=Natural Toxins in Raw Foods and How Cooking Affects Them |date= 1999|accessdate=2007-05-22 |publisher=Beyond Vegetarianism |work=Is Cooked Food Poison?}}</ref> It has been known since the late 17th century that plants contain [[wiktionary:noxious|noxious]] chemicals which are avoided by insects. These chemicals have been used by man as early [[insecticide]]s; 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.<ref>{{cite book |last=George W.|firat Ware |title=The Pesticide Book |publisher=MeisterPro |location=Willoughby |year=2004 |isbn=1-892829-11-8}}</ref> The flowers of ''[[Chrysanthemum]]'' species contain [[pyrethrin]] which is a potent insecticide. In later years, the applications of plant resistance became an important area of research in [[agriculture]] and [[plant breeding]], particularly because they can serve as a safe and low-cost alternative to the use of [[pesticide]]s.<ref name=smith>{{cite book |first=C.|last= Michael Smith |title=Plant Resistance to Arthropods: Molecular and Conventional Approaches |publisher=Springer |location=Berlin |year= 2005 |isbn=1-4020-3701-5}}</ref> The important role of secondary plant substances in plant defense was described in the late 1950s by [[Vincent Dethier]] and G.S. Fraenkel.<ref>{{cite journal |last=Dethier |first=V. G. |year=1954 |month=March |title=Evolution of feeding preferences in phytophagous insects |journal=Evolution |volume=8 |issue=1 |pages=33–54 |doi=10.2307/2405664}}</ref><ref name= Fraenkel/> The use of botanical pesticides is widespread and notable examples include Azadirachtin from the [[neem]] (''Azadirachta indica''), d-Limonene from [[Citrus]] species, Rotenone from ''[[Derris]]'', Capsaicin from [[Chili Pepper]] and Pyrethrum.<ref>{{cite web|last=Russ| first=Karen|title=Less toxic insecticides|url=http://hgic.clemson.edu/factsheets/HGIC2770.htm|publisher=Clemson University Home & Garden Information Center| accessdate=2007-05-27}}</ref> The selective breeding of crop plants often involves selection against the plant's intrinsic resistance strategies. This makes crop plant varieties particularly susceptible to pests unlike their wild relatives. In breeding for host-plant resistance, it is often the wild relatives that provide the source of resistance [[gene]]s. These genes are incorporated using conventional approaches to plant breeding, but have also been augmented by [[recombinant]] techniques, which allow introduction of genes from completely unrelated organisms. The most famous [[transgenic]] approach is the introduction of genes from the bacterial species, ''[[Bacillus thuringiensis]]'', into plants. The bacterium produces proteins that, when ingested, kill [[lepidoptera]]n [[caterpillar]]s. The gene encoding for these highly toxic proteins, when introduced into the host plant genome, confers resistance against caterpillars, when the same toxic proteins are produced within the plant. This approach is controversial, however, due to the possibility of [[ecological]] and [[toxicological]] side effects.<ref>{{cite journal |last=van Emden |first=H.F. |year=1999 |month=November |title=Transgenic Host Plant Resistance to Insects—Some Reservations |journal=Annals of the Entomological Society of America |volume=92 |issue=6 |pages=788–797 |url=http://www.ingentaconnect.com/content/esa/aesa/1999/00000092/00000006/art00002| accessdate=2007-05-27}}</ref> === Pharmaceutical === [[Image:Mandragora Tacuinum Sanitatis.jpg|thumb|Illustration from the 15th century manuscript ''[[Tacuinum Sanitatis]]'' detailing the beneficial and harmful properties of [[Mandrake (plant)|Mandrake]]s.]] Many currently available [[pharmaceutical]]s are derived from the secondary metabolites plants use to protect themselves from herbivores, including [[opium]], [[aspirin]], [[cocaine]], and [[atropine]].<ref name="pmid11395950">{{cite journal |last=Ghosh|first= B. |title=Polyamines and plant alkaloids |journal=Indian Journal of Experimental Biology |volume=38 |issue=11 |pages=1086–91 |year=2000 |pmid=11395950}}</ref> These chemicals have evolved to affect the biochemistry of insects in very specific ways. However, many of these biochemical pathways are conserved in vertebrates, including humans, and the chemicals act on human biochemistry in ways similar to that of insects. It has therefore been suggested that the study of plant-insect interactions may help in [[bioprospecting]].<ref>{{cite journal |last=Eisner |first=Thomas |year=1990 |month=March |title=Prospecting for nature's chemical riches |journal=Chemoecology |volume=1 |issue=1 |pages=38–40 |doi=10.1007/BF01240585}}</ref> There is evidence that humans began using plant alkaloids in medical preparations as early as 3000 [[Before Christ|B.C.]]<ref name=Roberts/> Although the active components of most medicinal plants have been isolated only recently (beginning in the early 19th century) these substances have been used as drugs throughout the human history in potions, medicines, teas and as [[poison]]s. For example, to combat herbivory by the larvae of some Lepidoptera species, [[Cinchona]] trees produce a variety of alkaloids, the most familiar of which is [[quinine]]. Quinine is extremely bitter, making the bark of the tree quite unpalatable, it is also an anti-[[fever]] agent, known as [[Jesuit's bark]], and is especially useful in treating [[malaria]].<ref>{{cite web |url=http://www.people.vcu.edu/~asneden/The%20Quinine%20Alkaloids.pdf |title=The Quinine Alkaloids |accessdate=2007-05-23| author= Albert T. Sneden|format=pdf |work= Medicinal Chemistry and Drug Design}}</ref> Throughout history [[Mandrake (plant)|mandrake]]s (''Mandragora officinarum'') have been highly sought after for their reputed [[aphrodisiac]] properties. However, the roots of the mandrake plant also contain large quantities of the alkaloid [[scopolamine]], which, at high doses, acts as a [[central nervous system]] [[depressant]], and makes the plant highly toxic to herbivores. Scopolamine was later found to be medicinal use in pain management before and during [[childbirth|labor]]; in smaller doses it is used to prevent [[motion sickness]].<ref>{{cite web |url=http://www.people.vcu.edu/~asneden/tropane%20alkaloids.pdf |title=The Tropane Alkaloids |accessdate=2007-05-23| author= Albert T. Sneden| |format=pdf |work=Medicinal Chemistry and Drug Design}}</ref> One of the most well-known medicinally valuable [[terpene]]s is an [[antineoplastic|anticancer]] drug, [[taxol]], isolated from the bark of the [[Pacific yew]], ''Taxus brevifolia'', in the early 1960s.<ref>{{cite web |url=http://www.people.vcu.edu/~asneden/Taxol.pdf |title= Taxol (Paclitaxe)|accessdate=2007-05-23 |author= Albert T. Sneden| format=pdf |work=Medicinal Chemistry and Drug Design}}</ref> ==See also== *[[Alarm signal#Other alarm signals|Alarm signals in plants]] *[[Aposematism]] *[[Chemical ecology]] *[[Canavanine]] *[[Frugivore]] *[[Laticifer]] *[[Lectin]] *[[Raphides]] *[[Pollination]] *[[Phytoalexin]] *[[Rapid plant movement]] *[[Seed predation]] ==References== {{Reflist|2}} ==Further reading== ''Titles with links are available in the form of a [[Google Books]] "limited preview".'' *{{cite book |author= Robert S. Fritz and Ellen L. Simms (editors)|title=Plant resistance to herbivores and pathogens: ecology, evolution, and genetics |url= http://books.google.com/books?vid=ISBN0226265544&id=NkF0xuz664gC&dq=Herbivores:+Their+interaction+with+secondary+plant| publisher=University of Chicago Press |location=Chicago |year=1992 |isbn=0-226-26553-6 |oclc= |doi=}} *Howe, H. F., and L. C. Westley. 1988. Ecological relationships of plants and animals. Oxford University Press, Oxford, UK. *{{cite book |author=Pierre Jolivet, |title=Interrelationship Between Insects and Plants|url= http://books.google.com/books?vid=ISBN1574440527&id=J5sTAB_mymwC&dq=Herbivores:+Their+interaction+with+secondary+plant |publisher=CRC |location=Boca Raton |year= |isbn=1-57444-052-7 |oclc= |doi=}} *{{cite book |author=Richard Karban and Ian T. Baldwin |title=Induced responses to herbivory |url= http://books.google.com/books?vid=ISBN0226424952&id=thHTfR3ICZAC&dq=Herbivores:+Their+interaction+with+secondary+plant| publisher=University of Chicago Press |location=Chicago |year=1997 |isbn=0-226-42495-2 |oclc= |doi=}} *{{cite book |author=Martin R. Speight, Mark D. Hunter, Allan D. Watt |title=Ecology of insects: concepts and applications |url= http://books.google.com/books?vid=ISBN0865427453&id=JiO_Nnk5Y8AC&pg=PP1&lpg=PP1&ots=EBsSbH3pMo&dq=%22Insect+ecology%22&sig=SJW1R1voFTsyclpSBdEkYlJRnPM#PRA5-PA344,M1| publisher=Blackwell Science |location=Oxford |year=1999 |isbn=0-86542-745-3 |oclc= |doi=}} *{{cite book |author=John N. Thompson |title=The coevolutionary process |url= http://books.google.com/books?vid=ISBN0226797597&id=InCAChmWM1QC&dq=Herbivores:+Their+interaction+with+secondary+plant| publisher=University of Chicago Press |location=Chicago |year=1994 |isbn=0-226-79759-7 |oclc= |doi=}} *{{cite journal |last=Wiens |first=D. |year=1978 |title=Mimicry in plants |journal=Evolutionary Biology |volume=11 |pages=365–403 |id= |url= |accessdate= 2008-03-24}} </div> <!--* {{cite journal | author = Ute Wittstock and Jonathan Gershenzon | title = Constitutive plant toxins and their role in defense against herbivores and pathogens | year = 2002 | journal = Current Opinion in Plant Biology | volume = 5 | issue = 4 | pages =300–307 | doi = 10.1016/S1369-5266(02)00264-9}} *{{cite journal | author = Donald A. Levin | title = The Chemical Defenses of Plants to Pathogens and Herbivores | year = 1976 | journal = Annual Review of Ecology and Systematics | volume = 7 | issue = 4 | pages =121–159 | url = http://links.jstor.org/sici?sici=0066-4162%281976%297%3C121%3ATCDOPT%3E2.0.CO%3B2-2 | doi = 10.1146/annurev.es.07.110176.001005}} *{{cite journal | author = R. J. Mead; A. J. Oliver; D. R. King; P. H. Hubach | title = The Co-Evolutionary Role of Fluoroacetate in Plant-Animal Interactions in Australia | year = 1983 | journal = Oikos Plant-Animal Interactions | volume = 44 | issue = 1 | pages = 55–60 | url = http://links.jstor.org/sici?sici=0030-1299%28198503%2944%3A1%3C55%3ATCROFI%3E2.0.CO%3B2-T}} --> ==External links== *[http://www2.biology.ualberta.ca/plantbiology/Welcome%20to%20PLAIG/PLAIG%20WEB3/PLAI%20research%20areas.htm Centre for Plant Animal Interaction studies] at The [[University of Alberta]] *Bruce A. Kimball [http://www.colostate.edu/Depts/Entomology/courses/en570/papers_1996/kimball.html Evolutionary Plant Defense Strategies Life Histories and Contributions to Future Generations] *[http://www2.mcdaniel.edu/Biology/botf99/herbnew/aintro.htm Plant Defense Systems & Medicinal Botany] *[http://www.sankey.ws/senecio.html Herbivore Defenses of Senecio viscosus L.] *[http://www.timeout.com/film/features/show-feature/4987/ten-films-in-which-plants-fight-back.html Ten Films In Which Plants Fight Back] [[Category:Herbivory]] [[Category:Plant physiology]]